training modules in esp

Lesson 1Electrostatic Precipitator Operation

Goal

To familiarize you with the operation of electrostatic precipitators (ESPs).

Objectives

At the end of this lesson, you will be able to do the following:

1. Describe the theory of precipitation

2. Describe how an ESP operates to collect particulate matter

3. Describe the two ESP designs for particle charging and collection: high voltage single-stageand low voltage two-stage

4. Distinguish between cold-side and hot-side ESPs

5. Briefly describe wet ESP operation

Introduction

As you may know, particulate matter (particles) is one of the industrial air pollution problemsthat must be controlled. It's not a problem isolated to a few industries, but pervasive across awide variety of industries. That's why the U.S. Environmental Protection Agency (EPA) hasregulated particulate emissions and why industry has responded with various control devices.Of the major particulate collection devices used today, electrostatic precipitators (ESPs) areone of the more frequently used. They can handle large gas volumes with a wide range of inlettemperatures, pressures, dust volumes, and acid gas conditions. They can collect a wide rangeof particle sizes, and they can collect particles in dry and wet states. For many industries, thecollection efficiency can go as high as 99%. ESPs aren't always the appropriate collectiondevice, but they work because of electrostatic attraction (like charges repel; unlike chargesattract). Let's see how this law of physics works in an ESP.

Theory of Precipitation

Every particle either has or can be given a charge—positive or negative. Let's suppose weimpart a negative charge to all the particles in a gas stream. Then suppose we set up agrounded plate having a positive charge. What would happen? The negatively charged particlewould migrate to the grounded collection plate and be captured. The particles would quicklycollect on the plate, creating a dust layer. The dust layer would accumulate until we removed

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it, which we could do by rapping the plate or by spraying it with a liquid. Charging, collecting,and removing—that's the basic idea of an ESP, but it gets more complicated. Let's look at atypical scenario using a common ESP construction.

Particle Charging

Our typical ESP as shown in Figure 1-1 has thin wires called discharge electrodes, whichare evenly spaced between large plates called collection electrodes, which are grounded.Think of an electrode as something that can conduct or transmit electricity. A negative,high-voltage, pulsating, direct current is applied to the discharge electrode creating a neg-ative electric field. You can mentally divide this field into three regions (Figure 1-2). Thefield is strongest right next to the discharge electrode, weaker in the areas between the dis-charge and collection electrodes called the inter-electrode region, and weakest near thecollection electrode. The region around the discharge electrode is where the particle charg-ing process begins.

Figure 1-1. Typical dry electrostatic precipitator

Figure 1-2. ESP electric field

WeakestWeakest Strongest

Inter-electroderegion

Electric field strength

Electrostatic Precipitator Operation

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Corona Discharge: Free Electron GenerationSeveral things happen very rapidly (in a matter of a millisecond) in the small areaaround the discharge electrode. The applied voltage is increased until it produces acorona discharge, which can be seen as a luminous blue glow around the dischargeelectrode. The free electrons created by the corona are rapidly fleeing the negativeelectric field, which repulses them. They move faster and faster away from the dis-charge electrode. This acceleration causes them to literally crash into gas molecules,bumping off electrons in the molecules. As a result of losing an electron (which isnegative), the gas molecules become positively charged, that is, they become positiveions (Figure 1-3). So, this is the first thing that happens—gas molecules are ionized,and electrons are liberated. All this activity occurs very close to the discharge elec-trode. This process continues, creating more and more free electrons and more posi-tive ions. The name for all this electron generation activity is avalanchemultiplication (Figure 1-4).

Figure 1-3. Corona generation

Figure 1-4. Avalanche multiplication of gas molecules

The electrons bump into gas molecules and create additional ionized molecules. Thepositive ions, on the other hand, are drawn back toward the negative discharge elec-trode. The molecules are hundreds of times bigger than the tiny electrons and move

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slowly, but they do pick up speed. In fact, many of them collide right into the metaldischarge electrode or the gas space around the wire causing additional electrons to beknocked off. This is called secondary emission. So, this is the second thing that hap-pens. We still have positive ions and a large amount of free electrons.

Ionization of Gas MoleculesAs the electrons leave the strong electrical field area around the discharge electrode,they start slowing down. Now they're in the inter-electrode area where they are stillrepulsed by the discharge electrode but to a lesser extent. There are also gas moleculesin the inter-electrode region, but instead of violently colliding with them, the electronskind of bump up to them and are captured (Figure 1-5). This imparts a negative chargeto the gas molecules, creating negative gas ions. This time, because the ions are nega-tive, they too want to move in the direction opposite the strong negative field. Now wehave ionization of gas molecules happening near the discharge electrode and in theinter-electrode area, but with a big difference. The ions near the discharge electrodeare positive and remain in that area. The ions in the middle area are negative and moveaway, along the path of invisible electric field lines, toward the collection electrode.

Figure 1-5. Negative gas ions formed in the inter-electrode region

Charging of ParticlesThese negative gas ions play a key role in capturing dust particles. Before the dustparticles can be captured, they must first acquire a negative charge. This is when andwhere it happens. The particles are traveling along in the gas stream and encounternegative ions moving across their path. Actually, what really happens is that the parti-cles get in the way of the negatively charged gas ions. The gas ions stick to the parti-cles, imparting a negative charge to them. At first the charge is fairly insignificant asmost particles are huge compared to a gas molecule. But many gas ions can fit on aparticle, and they do. Small particles (less than 1 µm diameter) can absorb “tens” ofions. Large particles (greater than 10 µm) can absorb "tens of thousands" of ions(Turner et al. 1992). Eventually, there are so many ions stuck to the particles, the par-ticles emit their own negative electrical field. When this happens, the negative fieldaround the particle repulses the negative gas ions and no additional ions are acquired.This is called the saturation charge. Now the negatively-charged particles are feelingthe inescapable pull of electrostatic attraction. Bigger particles have a higher satura-tion charge (more molecules fit) and consequently are pulled more strongly to the col-lection plate. In other words, they move faster than smaller particles. Regardless of

Tocollectionplate

Negativegas ion

GasmoleculeElectron

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size, the particles encounter the plate and stick, because of adhesive and cohesiveforces.

Let's stop here and survey the picture. Gas molecules around the discharge electrodeare positively ionized. Free electrons are racing as fast as they can away from thestrong negative field area around the discharge electrode. The electrons are capturedby gas molecules in the inter-electrode area and impart a negative charge to them.Negative gas ions meet particles and are captured (Figure 1-6). And all this happens inthe blink of an eye. The net result is negatively charged particles that are repulsed bythe negative electric field around the discharge electrode and are strongly attracted tothe collection plate. They travel toward the grounded collection plate, bump into it,and stay there.

More and more particles accumulate, creating a dust layer. This dust layer builds untilit is somehow removed. Charging, collecting, and removing—isn't that what we saidit's all about?

Figure 1-6. Particle charging

Particle Charging MechanismsParticles are charged by negative gas ions moving toward the collection plate by oneof these two mechanisms: field charging or diffusion charging. In field charging (themechanism described above), particles capture negatively charged gas ions as the ionsmove toward the grounded collection plate. Diffusion charging, as its name implies,depends on the random motion of the gas ions to charge particles.

Negativegas ion

Negativelychargedparticle

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In field charging (Figure 1-7), as particles enter the electric field, they cause a localdislocation of the field. Negative gas ions traveling along the electric field lines col-lide with the suspended particles and impart a charge to them. The ions will continueto bombard a particle until the charge on that particle is sufficient to divert the electriclines away from it. This prevents new ions from colliding with the charged dust parti-cle. When a particle no longer receives an ion charge, it is said to be saturated. Satu-rated charged particles then migrate to the collection electrode and are collected.

Figure 1-7. Field charging

Diffusion charging is associated with the random Brownian motion of the negativegas ions. The random motion is related to the velocity of the gas ions due to thermaleffects: the higher the temperature, the more movement. Negative gas ions collidewith the particles because of their random thermal motion and impart a charge on theparticles. Because the particles are very small (submicrometer), they do not cause theelectric field to be dislocated, as in field charging. Thus, diffusion charging is the onlymechanism by which these very small particles become charged. The charged parti-cles then migrate to the collection electrode.

Each of these two charging mechanisms occurs to some extent, with one dominatingdepending on particle size. Field charging dominates for particles with a diameter>1.0 micrometer because particles must be large enough to capture gas ions. Diffusioncharging dominates for particles with a diameter less than 0.1 micrometer. A combina-tion of these two charging mechanisms occurs for particles ranging between 0.2 and1.0 micrometer in diameter.

A third type of charging mechanism, which is responsible for very little particle charg-ing is electron charging. With this type of charging, fast-moving free electrons thathave not combined with gas ions hit the particle and impart a charge.

b.) Saturated particle migrates towardcollection electrode

Saturatedcharged particle

a.) Field lines distorted by particle

negativegas ion

Collectionelectrode

particle


Electric Field StrengthIn the inter-electrode region, negative gas ions migrate toward the grounded collectionelectrode. A space charge, which is a stable concentration of negative gas ions, formsin the inter-electrode region because of the high electric field applied to the ESP.Increasing the applied voltage to the discharge electrode will increase the fieldstrength and ion formation until sparkover occurs. Sparkover refers to internal spark-ing between the discharge and collection electrodes. It is a sudden rush of localizedelectric current through the gas layer between the two electrodes. Sparking causes animmediate short-term collapse of the electric field (Figure 1-8.)

For optimum efficiency, the electric field strength should be as high as possible. Morespecifically, ESPs should be operated at voltages high enough to cause some sparking,but not so high that sparking and the collapse of the electric field occur too frequently.The average sparkover rate for optimum precipitator operation is between 50 and 100sparks per minute. At this spark rate, the gain in efficiency associated with increasedvoltage compensates for decreased gas ionization due to collapse of the electric field.

Figure 1-8. Spark generation profile

Particle Collection

When a charged particle reaches the grounded collection electrode, the charge on the par-ticle is only partially discharged. The charge is slowly leaked to the grounded collectionplate. A portion of the charge is retained and contributes to the inter-molecular adhesiveand cohesive forces that hold the particles onto the plates (Figure 1-9). Adhesive forcescause the particles to physically hold on to each other because of their dissimilar surfaces.Newly arrived particles are held to the collected particles by cohesive forces; particles areattracted and held to each other molecularly. The dust layer is allowed to build up on theplate to a desired thickness and then the particle removal cycle is initiated.

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Figure 1-9. Particle collection at collection electrode

Particle Removal

Dust that has accumulated to a certain thickness on the collection electrode is removed byone of two processes, depending on the type of collection electrode. As described ingreater detail in the next section, collection electrodes in precipitators can be either platesor tubes, with plates being more common. Tubes are usually cleaned by water sprays,while plates can be cleaned either by water sprays or a process called rapping.

Rapping is a process whereby deposited, dry particles are dislodged from the collectionplates by sending mechanical impulses, or vibrations, to the plates. Precipitator plates arerapped periodically while maintaining the continuous flue-gas cleaning process. In otherwords, the plates are rapped while the ESP is on-line; the gas flow continues through theprecipitator and the applied voltage remains constant. Plates are rapped when the accumu-lated dust layer is relatively thick (0.08 to 1.27 cm or 0.03 to 0.5 in.). This allows the dustlayer to fall off the plates as large aggregate sheets and helps eliminate dust reentrainment.Most precipitators have adjustable rappers so that rapper intensity and frequency can bechanged according to the dust concentration in the flue gas. Installations where the dustconcentration is heavy require more frequent rapping.

Dislodged dust falls from the plates into the hopper. The hopper is a single collection binwith sides sloping approximately 50 to 70° to allow dust to flow freely from the top of thehopper to the discharge opening. Dust should be removed as soon as possible to avoid(dust) packing. Packed dust is very difficult to remove. Most hoppers are emptied by sometype of discharge device and then transported by a conveyor.

In a precipitator using liquid sprays to remove accumulated liquid or dust, the sludge col-lects in a holding basin at the bottom of the vessel. The sludge is then sent to settlingponds or lined landfills for proper ultimate disposal.

Spraying occurs while the ESP is on-line and is done intermittently to remove the col-lected particles. Water is generally used as the spraying liquid although other liquids couldbe used if absorption of gaseous pollutants is also being accomplished.

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Types of Electrostatic Precipitators

ESPs can be grouped, or classified, according to a number of distinguishing features in theirdesign. These features include the following:

• The structural design and operation of the discharge electrodes (rigid-frame, wires orplate) and collection electrodes (tubular or plate)

• The method of charging (single-stage or two-stage)

• The temperature of operation (cold-side or hot-side)

• The method of particle removal from collection surfaces (wet or dry)

These categories are not mutually exclusive. For example, an ESP can be a rigid-frame, sin-gle-stage, cold-side, plate-type ESP as described below.

Tubular and Plate ESPs

TubularTubular precipitators consist of cylindrical collection electrodes (tubes) with dis-charge electrodes (wires) located in the center of the cylinder (Figure 1-10). Dirty gasflows into the tubes, where the particles are charged. The charged particles are thencollected on the inside walls of the tubes. Collected dust and/or liquid is removed bywashing the tubes with water sprays located directly above the tubes. The tubes maybe formed as a circular, square, or hexagonal honeycomb with gas flowing upward ordownward. A tubular ESP is tightly sealed to minimize leaks of collected material.Tube diameters typically vary from 0.15 to 0.31 m (0.5 to 1 ft), with lengths usuallyvarying from 1.85 to 4.0 m (6 to 15 ft).

Figure 1-10. Gas flow through a tubular precipitator

Tubular precipitators are generally used for collecting mists or fogs, and are mostcommonly used when collecting particles that are wet or sticky. Tubular ESPs havebeen used to control particulate emissions from sulfuric acid plants, coke oven by-product gas cleaning (tar removal), and iron and steel sinter plants.

Dischargeelectrode

Collectionelectrodes

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PlatePlate electrostatic precipitators primarily collect dry particles and are used more oftenthan tubular precipitators. Plate ESPs can have wire, rigid-frame, or occasionally,plate discharge electrodes. Figure 1-11 shows a plate ESP with wire discharge elec-trodes. Dirty gas flows into a chamber consisting of a series of discharge electrodesthat are equally spaced along the center line between adjacent collection plates.Charged particles are collected on the plates as dust, which is periodically removed byrapping or water sprays. Discharge wire electrodes are approximately 0.13 to 0.38 cm(0.05 to 0.15 in.) in diameter. Collection plates are usually between 6 and 12 m (20and 40 ft) high. For ESPs with wire discharge electrodes, the plates are usually spacedfrom 15 to 30 cm (6 to 12 in.) apart. For ESPs with rigid-frame or plate discharge elec-trodes, plates are typically spaced 30 to 38 cm (12 to 15 in.) apart and 8 to 12 m (30 to40 ft) in height.

Plate ESPs are typically used for collecting fly ash from industrial and utility boilersas well as in many other industries including cement kilns, glass plants and pulp andpaper mills.

Figure 1-11. Gas flow through a plate precipitator

Single-stage and Two-stage ESPs

Another method of classifying ESPs is by the number of stages used to charge and removeparticles from a gas stream. A single-stage precipitator uses high voltage to charge theparticles, which are then collected within the same chamber on collection surfaces ofopposite charge. In a two-stage precipitator, particles are charged by low voltage in onechamber, and then collected by oppositely charged surfaces in a second chamber.

Single StageMost ESPs that reduce particulate emissions from boilers and other industrial pro-cesses are single-stage ESPs (these units will be emphasized in this course). Single-stage ESPs use very high voltage (50 to 70 kV) to charge particles. After beingcharged, particles move in a direction perpendicular to the gas flow through the ESP,

Dischargeelectrode

Collectionplate

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and migrate to an oppositely charged collection surface, usually a plate or tube. Parti-cle charging and collection occurs in the same stage, or field; thus, the precipitatorsare called single-stage ESPs. The term field is used interchangeably with the termstage and is described in more detail later in this course. Figure 1-10 shows a single-stage tubular precipitator. A single-stage plate precipitator is shown in Figure 1-11.

Two StageThe two-stage precipitator differs from the single-stage precipitator in both design andamount of voltage applied. The two-stage ESP has separate particle charging and col-lection stages (Figure 1-12). The ionizing stage consists of a series of small, positivelycharged wires equally spaced 2.5 to 5.1 cm (1 to 2 in.) from parallel grounded tubes orrods. A corona discharge between each wire and a corresponding tube charges the par-ticles suspended in the air flow as they pass through the ionizer. The direct-currentpotential applied to the wires is approximately 12 to 13 kV.

Figure 1-12. Representation of gas flow in a two-stage precipitator

The second stage consists of parallel metal plates less than 2.5 cm (1 in.) apart. Theparticles receive a positive charge in the ionizer stage and are collected at the negativeplates in the second stage. Collected smoke or liquids drain by gravity to a pan locatedbelow the plates, or are sprayed with water mists or solvents that remove the particlesand cause them to fall into the bottom pan.

Two-stage precipitators were originally designed for air purification in conjunctionwith air conditioning systems. (They are also referred to as electronic air filters). Two-stage ESPs are used primarily for the control of finely divided liquid particles. Con-trolling solid or sticky materials is usually difficult, and the collector becomes ineffec-tive for dust loadings greater than 7.35 x 10-3g/m3 (0.4 gr/dscf). Therefore, two-stageprecipitators have limited use for particulate-emission control. They are used almostexclusively to collect liquid aerosols discharged from sources such as meat smoke-houses, pipe-coating machines, asphalt paper saturators, high speed grindingmachines, welding machines, and metal-coating operations.

Ionizer(to charge particles)

Collection plate

Baffle(to distributeair uniformly)

Cleanair

Precipitated(collected)particles

Positivelychargedparticles

Unchargedparticles

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Cold-side and Hot-side ESPs

Electrostatic precipitators are also grouped according to the temperature of the flue gasthat enters the ESP: cold-side ESPs are used for flue gas having temperatures of approxi-mately 204°C (400°F) or less; hot-side ESPs are used for flue gas having temperaturesgreater than 300°C (572°F).

In describing ESPs installed on industrial and utility boilers, or municipal waste combus-tors using heat recovery equipment, cold side and hot side also refer to the placement ofthe ESP in relation to the combustion air preheater. A cold-side ESP is located behind theair preheater, whereas a hot-side ESP is located in front of the air preheater. The air pre-heater is a tube section that preheats the combustion air used for burning fuel in a boiler.When hot flue gas from an industrial process passes through an air preheater, a heatexchange process occurs whereby heat from the flue gas is transferred to the combustionair stream. The flue gas is therefore "cooled" as it passes through the combustion air pre-heater. The warmed combustion air is sent to burners, where it is used to burn gas, oil,coal, or other fuel including garbage. APTI Course SI:428A Introduction to Boiler Opera-tion describes boilers and heat recovery equipment in greater detail.

Cold SideCold-side ESPs (Figure 1-13) have been used for over 50 years with industrial andutility boilers, where the flue gas temperature is relatively low (less than 204°C or400°F). Cold-side ESPs generally use plates to collect charged particles. Becausethese ESPs are operated at lower temperatures than hot-side ESPs, the volume of fluegas that is handled is less. Therefore, the overall size of the unit is smaller, making itless costly. Cold-side ESPs can be used to remove fly ash from boilers that burn high-sulfur coal. As explained in later lessons, cold-side ESPs can effectively remove flyash from boilers burning low-sulfur coal with the addition of conditioning agents.

Figure 1-13. Cold-side ESP

Boiler

ESP

Fan

Combustionair preheater

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Hot SideHot-side ESPs (Figure 1-14) are placed in locations where the flue gas temperature isrelatively high. Their collection electrodes can be either tubular or plate. Hot-sideESPs are used in high-temperature applications, such as in the collection of cement-kiln dust or utility and industrial boiler fly ash. A hot-side precipitator is locatedbefore the combustion air preheater in a boiler. The flue gas temperature for hot-sideprecipitators is in the range of 320 to 420°C (608 to 790°F).

The use of hot-side precipitators help reduce corrosion and hopper plugging. How-ever, these units (mainly used on coal-fired boilers) have some disadvantages.Because the temperature of the flue gas is higher, the gas volume treated in the ESP islarger. Consequently, the overall size of the precipitator is larger making it morecostly. Other major disadvantages include structural and mechanical problems thatoccur in the precipitator shell and support structure as a result of differences in ther-mal expansion.

For years, cold-side ESPs were used successfully on boilers burning high-sulfur coal.However, during the 1970s when utilities switched to burning low-sulfur coal, cold-side ESPs were no longer effective at collecting the fly ash. Fly ash produced fromlow sulfur coal-fired boilers has high resistivity (discussed in more detail later in thecourse), making it difficult to collect. As you will learn later, high temperatures canlower resistivity. Consequently, hot-side ESPs became very popular during the 1970sfor removing ash from coal-fired boilers burning low sulfur coal. However, many ofthese units did not operate reliably, and therefore, since the 1980s, operators have gen-erally decided to use cold-side ESPs along with conditioning agents when burning lowsulfur coal.

Hot-side ESPs are also used in industrial applications such as cement kilns and steelrefining furnaces. In these cases, combustion air preheaters are generally not used andhot side just refers to the high flue gas temperature prior to entering the ESP.

Figure 1-14. Hot-side ESP

Boiler

ESP

Fan

Combustionair preheater

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Wet and Dry ESPs

Wet ESPsAny of the previously described ESPs can be operated with a wet spray to remove col-lected particles. Wet ESPs are used for industrial applications where the potential forexplosion is high (such as collecting dust from a closed-hood Basic Oxygen Furnacein the steel industry), or when dust is very sticky, corrosive, or has very high resistiv-ity. The water flow may be applied continuously or intermittently to wash the col-lected particles from the collection electrodes into a sump (a basin used to collectliquid). The advantage of using a wet ESP is that it does not have problems with rap-ping reentrainment or with back corona which are discussed in more detail in Lesson3.

Figures 1-15 and 1-16 show two different wet ESPs. The casing of wet ESPs is madeof steel or fiberglass and the discharge electrodes are made of carbon steel or specialalloys, depending on the corrosiveness of the flue gas stream.

In a circular-plate wet ESP, shown in Figure 1-15, the circular collection plates aresprayed with liquid continuously. The liquid provides the electrical ground for attract-ing the particles and for removing them from the plates. These units can handle gasflow rates of 30,000 to 100,000 cfm. Preconditioning sprays located at the inletremove some particulate matter prior to the charging stage. The operating pressuredrop across these units is typically 1 to 3 inches of water.

Figure 1-15. Circular-plate wet EPSReproduced with permission of Fluid Ionics Systems, adivision of Dresser Industries, Inc.

Gas inlet

Water distributor

Insulator Concentriccollection surfaces

Emittingelectrodes

Venturi/draingutters

Straighteningvanes

Preconditionersprays

Clean gas discharge

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Rectangular flat-plate wet ESPs, shown in Figure 1-16, operate similarly to circular-plate wet ESPs. Water sprays precondition the gas stream and provide some particleremoval. Because the water sprays are located over the top of the electrical fields, thecollection plates are continuously irrigated. The collected particulate matter flowsdownward into a trough that is sloped to a drain.

Figure 1-16. Flat-plate modular wet ESPReproduced with permission of Fluid Ionics Systems, adivision of Dresser Industries, Inc.

Dry ESPsMost electrostatic precipitators are operated dry and use rappers to remove the col-lected particulate matter. The term dry is used because particles are charged and col-lected in a dry state and are removed by rapping as opposed to water washing which isused with wet ESPs. The major portion of this course covers dry ESPs that are usedfor collecting dust from many industries including steel furnaces, cement kilns andfossil-fuel-fired boilers.

Water manifolds

Gas outlet

Water Inlet

Discharge electrode

Water outlet

Gas inlet

Access manway

Turning vanes

Perforated plates

Collectionplate

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Summary

All ESPs, no matter how they are grouped, have similar components and operate by chargingparticles or liquid aerosols, collecting them, and finally removing them from the ESP beforeultimate disposal in a landfill or reuse in the industrial process.

ESPs are occasionally referred to as cold-side, tubular, or by some other descriptor. ESPdesigns usually incorporate a number of ESP features into one unit. For example, a typicalESP used for removing particulate matter from a coal-fired boiler will be a cold-side, single-stage, plate ESP. On the other hand, a hot-side, single-stage, tubular ESP may be used to cleanexhaust gas from a blast furnace in a steel mill.

Remember that an ESP is specifically designed to collect particulate matter or liquids for anindividual industrial application. Vendors use those features, i.e., tubes, plates, etc., that mostreadily enhance the removal of the pollutants from the flue gas. These features are described inmore detail in the remaining lessons.

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Review Exercise

1. In an electrostatic precipitator, the ____________________ electrode is normally a small-diametermetal wire or a rigid frame containing wires.

2. The charged particles migrate to the ____________________ ____________________.

3. In a single-stage, high-voltage ESP, the applied voltage is increased until it produces a(an)

a. Extremely high alternating current for particle chargingb. Corona discharge, which can be seen as a blue glow around the discharge electrodec. Corona spark that occurs at the collection electrode

4. True or False? Particles are usually charged by negative gas ions that are migrating toward the col-lection electrode.

5. True or False? Large particles move more slowly towards the collection plate than small particles.

6. The average sparkover rate (in sparks per minute) for optimum precipitator operation is between:

a. 1 - 25b. 50 - 100c. 100 - 150d. 500 - 1,000

7. As dust particles reach the grounded collection electrode, their charge is:

a. Immediately transferred to the collection plateb. Slowly leaked to the grounded collection electrodec. Cancelled out by the strong electric field

8. Particles are held onto the collection plates by:

a. A strong electric force fieldb. A high-voltage, pulsating, direct currentc. Intermolecular cohesive and adhesive forcesd. Electric sponsors

9. Dust that has accumulated on collection electrodes can be removed either by____________________ ____________________ or a process called ____________________.

10. True or False? During the rapping process, the voltage is turned down to about 50% of the normaloperating voltage to allow the rapped particles to fall freely into the hopper.

11. ____________________ electrostatic precipitators are used for removing particulate matter fromflue gas that usually has a temperature range of 320 to 420° C (608 to 790° F).

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12. In a boiler, hot-side ESPs are located ____________________ air preheaters, whereas cold-sideESPs are located ____________________ air preheaters.

a. In front of, behindb. Behind, in front of

13. True or False? Wet electrostatic precipitators are used when collecting dust that is sticky or hashigh resistivity.

14. ____________________ ESPs are units where particle charging occurs in the first stage, followedby collection in the second stage.

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Review Exercise Answers

1. DischargeIn an electrostatic precipitator, the discharge electrode is normally a small-diameter metal wire or arigid frame containing wires.

2. Collection electrodeThe charged particles migrate to the collection electrode.

3. b. Corona discharge, which can be seen as a blue glow around the discharge electrodeIn a single-stage, high-voltage ESP, the applied voltage is increased until it produces a corona dis-charge, which can be seen as a blue glow around the discharge electrode.

4. TrueParticles are usually charged by negative gas ions that are migrating toward the collection elec-trode.

5. FalseLarge particles move faster towards the collection plate than small particles. Large particles have ahigher saturation charge than small particles; consequently, large particles are pulled more stronglyto the collection plate.

6. b. 50 - 100The average sparkover rate for optimum precipitator operation is between 50 - 100 sparks perminute.

7. b. Slowly leaked to the grounded collection electrodeAs dust particles reach the grounded collection electrode, their charge is slowly leaked to thegrounded collection electrode.

8. c. Intermolecular cohesive and adhesive forcesParticles are held onto the collection plates by intermolecular cohesive and adhesive forces.

9. Water spraysRappingDust that has accumulated on collection electrodes can be removed either by water sprays or a pro-cess called rapping.

10. FalseDuring the rapping process, the voltage is NOT turned down. Rapping occurs while the ESPremains on-line.

11. Hot-sideHot-side electrostatic precipitators are used for removing particulate matter from flue gas that usu-ally has a temperature range of 320 to 420°C (608 to 790°F).

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12. a. In front of, behindIn a boiler, hot-side ESPs are located in front of air preheaters, whereas cold-side ESPs are locatedbehind air preheaters. Recall that flue gas is cooled as it passes through the combustion air pre-heater.

13. TrueWet electrostatic precipitators are used when collecting dust that is sticky or has high resistivity.

14. Two-stageTwo-stage ESPs are units where particle charging occurs in the first stage, followed by collectionin the second stage.

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Bibliography

Beachler, D. S., J. A. Jahnke, G. T. Joseph and M. M. Peterson. 1983. Air Pollution Control Systems forSelected Industries-Self-Instructional Guidebook. (APTI Course SI:431). EPA 450/2-82-006. U.S.Environmental Protection Agency.

Bethea, R. M. 1978. Air Pollution Control Technology-an Engineering Analysis Point of View. NewYork: Van Nostrand Reinhold.

Katz, J. 1979. The Art of Electrostatic Precipitators. Munhall, PA: Precipitator Technology.

Nichols, G. B. 1976, September. Electrostatic Precipitation. Seminar presented to the U.S. Environ-mental Protection Agency. Research Triangle Park, NC.

Richards, J.R. 1995. Control of Particulate Emissions-Student Manual. (APTI Course 413). U.S. Envi-ronmental Protection Agency.

Turner, J. H., P. A. Lawless, T. Yamamoto, D. W. Coy, G. P. Greiner, J. D. McKenna, and W. M. Vata-vuk. 1992. Electrostatic precipitators. In A. J. Buonicore and W. T. Davis (Eds.), Air PollutionEngineering Manual (pp. 89-113). Air and Waste Management Association. New York: Van Nos-trand Reinhold.

U.S. Environmental Protection Agency. 1973. Air Pollution Engineering Manual. 2d ed. AP-40.

U.S. Environmental Protection Agency. 1985. Operation and Maintenance Manual for ElectrostaticPrecipitators. EPA 625/1-85/017.

White, H. J. 1977. Electrostatic precipitation of fly ash. APCA Reprint Series. Journal of Air PollutionControl Association. Pittsburgh, PA.

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1-22
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Lesson 2Electrostatic Precipitator Components

Goal

To familiarize you with the components of an ESP.

Objectives


1. Identify six major components of an ESP2. Describe typical discharge electrode designs3. Describe typical collection electrode designs4. Identify how discharge electrodes and collection plates are installed in an ESP5. List three types of rappers and briefly describe how they operate6. Describe how the high-voltage equipment operates7. Describe two factors that are important in hopper design8. Identify two discharge devices used to remove dust from hoppers, and three types of conveyors9. State the purpose for installing insulation on an ESP

Video Presentation (optional): If you have acquired the video titled, Electrostatic Precipitators:Operating Principles and Components, please view it at the end of this lesson.

Precipitator Components

All electrostatic precipitators, regardless of their particular designs, contain the followingessential components:

• Discharge electrodes

• Collection electrodes

• High voltage electrical systems

• Rappers

• Hoppers

• Shell

Discharge electrodes are either small-diameter metal wires that hang vertically (in the electro-static precipitator), a number of wires attached together in rigid frames, or a rigid electrode-made from a single piece of fabricated metal. Discharge electrodes create a strong electricalfield that ionizes flue gas, and this ionization charges particles in the gas.

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2-2

Collection electrodes collect charged particles. Collection electrodes are either flat plates ortubes with a charge opposite that of the discharge electrodes.

High voltage equipment provides the electric field between the discharge and collection elec-trodes used to charge particles in the ESP.

Rappers impart a vibration, or shock, to the electrodes, removing the collected dust. Rappersremove dust that has accumulated on both collection electrodes and discharge electrodes.Occasionally, water sprays are used to remove dust from collection electrodes.

Hoppers are located at the bottom of the precipitator. Hoppers are used to collect and tempo-rarily store the dust removed during the rapping process.

The shell provides the base to support the ESP components and to enclose the unit.

Figure 2-1 shows a typical ESP with wires for discharge electrodes and plates for collectionelectrodes. This ESP is used to control particulate emissions in many different industries.

Figure 2-1. Typical dry electrostatic precipitator

Discharge Electrodes

The discharge electrodes in most U.S. precipitator designs (prior to the 1980s) are thin,round wires varying from 0.13 to 0.38 cm (0.05 to 0.15 in.) in diameter. The most com-mon size diameter for wires is approximately 0.25 cm (0.1 in.). The discharge electrodesare hung vertically, supported at the top by a frame and held taut and plumb by a weight atthe bottom. The wires are usually made from high-carbon steel, but have also been con-structed of stainless steel, copper, titanium alloy, and aluminum. The weights are made ofcast iron and are generally 11.4 kg (25 lb) or more.

Discharge wires are supported to help eliminate breakage from mechanical fatigue. Thewires move under the influence of aerodynamic and electrical forces and are subject tomechanical stress. The weights at the bottom of the wire are attached to guide frames tohelp maintain wire alignment and to prevent them from falling into the hopper in the eventthat the wire breaks (Figure 2-2).

Dischargeelectrodes

Flue gas in

Rappers

Hoppers

Cleangasout

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Electrostatic Precipitator Components

Weights that are 11.4 kg (25 lb) are used with wires 9.1 m (30 ft) long, and 13.6 kg (30 lb)weights are used with wires from 10.7 to 12.2 m (35 to 40 ft) long. The bottom and top ofeach wire are usually covered with a shroud of steel tubing. The shrouds help minimizesparking and consequent metal erosion by sparks at these points on the wire.

Figure 2-2. Guide frames and shrouds fordischarge wires

The size and shape of the electrodes are governed by the mechanical requirements for thesystem, such as the industrial process on which ESPs are installed and the amount andproperties of the flue gas being treated. Most U.S. designs have traditionally used thin,round wires for corona generation. Some designers have also used twisted wire, squarewire, barbed wire, or other configurations, as illustrated in Figure 2-3.

Figure 2-3. Typical wire dischare electrodes

European precipitator manufacturers and most of the newer systems (since the early1980s) made by U.S. manufacturers use rigid support frames for discharge electrodes. Theframes may consist of coiled-spring wires, serrated strips, or needle points mounted on asupporting strip. A typical rigid-frame discharge electrode is shown in Figure 2-4. The

Upperguide frame

Top shroud

Bottom shroud

Guide loop

Weight

Lowerguide frame

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2-4

purpose of the rigid frame is to eliminate the possible swinging of the discharge wires.Another type of discharge electrode is a rigid electrode that is constructed from a singlepiece of fabricated metal and is shown in Figure 2-5. Both designs are occasionallyreferred to as rigid-frame electrodes. They have been used as successfully as the olderU.S. wire designs. One major disadvantage of the rigid-frame design is that a broken wirecannot be replaced without removing the whole frame.

Figure 2-4. Rigid frame discharge electrode design

Figure 2-5. Typical rigid discharge electrode

Dischargeelectrode

Dischargeelectrode

Supportinsulator

Rapperanvil

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One U.S. manufacturer (United McGill) uses flat plates instead of wires for dischargeelectrodes. The flat plates, shown in Figure 2-6, increase the average electric field that canbe used for collecting particles and provide an increased surface area for collecting parti-cles, both on the discharge and collection plates. The corona is generated by the sharp-pointed needles attached to the plates. These units generally use positive polarity forcharging the particles. The units are typically operated with low flue gas velocity to pre-vent particle reentrainment during the rapping cycle (Turner, et al. 1992).

Figure 2-6. Flat-plate discharge electrode(United McGill design)

Collection Electrodes

Most U.S. precipitators use plate collection electrodes because these units treat large gasvolumes and are designed to achieve high collection efficiency. The plates are generallymade of carbon steel. However, plates are occasionally made of stainless steel or an alloysteel for special flue-gas stream conditions where corrosion of carbon steel plates wouldoccur. The plates range from 0.05 to 0.2 cm (0.02 to 0.08 in.) in thickness. For ESPs withwire discharge electrodes, plates are spaced from 15 to 30 cm apart (6 to 12 in.). Normalspacing for high-efficiency ESPs (using wires) is 20 to 23 cm (8 to 9 in.). For ESPs usingrigid-frame or plate discharge electrodes, collection plates are typically spaced 30 to 38cm (12 to 15 inches) apart. Plates are usually between 6 and 12 m (20 to 40 ft) high.

Dischargeelectrodeplate

Collectionplate

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Collection plates are constructed in various shapes, as shown in Figure 2-7. These platesare solid sheets that are sometimes reinforced with structural stiffeners to increase platestrength. In some cases, the stiffeners act as baffles to help reduce particle reentrainmentlosses. This design minimizes the amount of excess rapping energy required to dislodgethe dust from the collection plates, because the energy is distributed evenly throughout theplate. The baffles also provide a "quiet zone" for the dislodged dust to fall while mini-mizing dust reentrainment.

Figure 2-7. Typical collection plates

As stated in Lesson 1, tubes are also used as collection electrodes, but not nearly as often.Tubes are typically used to collect sticky particles and when liquid sprays are used toremove the collected particles.

High-Voltage Equipment

High-voltage equipment determines and controls the strength of the electric field gener-ated between the discharge and collection electrodes. This is accomplished by usingpower supply sets consisting of three components: a step-up transformer, a high-voltagerectifier, and control metering and protection circuitry (automatic circuitry). The powersystem maintains voltage at the highest level without causing excess sparkover betweenthe discharge electrode and collection plate. These power sets are also commonly calledtransformer-rectifier (T-R) sets.

In a T-R set, the transformer steps up the voltage from 400 volts to approximately 50,000volts. This high voltage ionizes gas molecules that charge particles in the flue gas. Therectifier converts alternating current to direct current. Direct (or unidirectional current) isrequired for electrical precipitation. Most modern precipitators use solid-state silicon rec-tifiers and oil-filled, high-voltage transformers. The control circuitry in a modern precipi-tator is usually a Silicon-controlled Rectifier (SCR) automatic voltage controller with alinear reactor in the primary side of the transformer. Meters, also included in the control

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circuitry, monitor the variations in the electrical power input. A simplified drawing of thecircuitry from the primary control cabinet to the precipitator field is shown in Figure 2.8

Figure 2-8. Schematic diagram of circuitry associated with precipitators

The most commonly used meters are the following:

Primary voltmeter. This meter measures the input voltage, in a.c. volts, coming intothe transformer. The input voltage ranges from 220 to 480 volts; however, most mod-ern precipitators use 400 to 480 volts. The meter is located across the primary windingof the transformer.Primary ammeter. This meter measures the current drawn acrossthe transformer in amperes. The primary ammeter is located across the primary wind-ing (wires wound in the coil) of the transformer. The primary voltage and currentreadings give the power input to a particular section of the ESP.

Secondary voltmeter. This meter measures, in d.c. volts, the operating voltage deliv-ered to the discharge electrodes. The meter is located between the output side of therectifier and the discharge electrodes.

Secondary ammeter. This meter measures the current supplied to the discharge elec-trodes in milliamperes. The secondary ammeter is located between the rectifier outputand the automatic control module. The combination of the secondary voltage and cur-rent readings gives the power input to the discharge electrodes.

Sparkmeter. This meter measures the number of sparks per minute in the precipitatorsection. Sparks are surges of localized electric current between the discharge elec-trodes and the collection plate.

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Lesson 2

2-8

The terms primary and secondary refer to the side of the transformer being monitored bythe meter. Figure 2-9 shows the typical meters used on each ESP field and are located inthe control cabinet.

Figure 2-9. Typical gauges (meters) installed on control cabinet foreach precipitator field

The transformer-rectifier set ios connected to the discharge electrodes by a bus line. A busline is electric cable that carries high voltage from the transformer-rectifier to the dis-charge electrodes (Figure 2-10). The bus line is encased in a pipe, or bus duct, to protectthe high-voltage line from the environment and to prevent the line from becoming a poten-tial hazard to humans. The high-voltage bus lines are separated, or isolated, from the ESPframe and shells by insulators. The insulators are made of nonconducting plastic orceramic material.

Figure 2-10. High-voltage system

90100

8070605040302010

0

D.C. Kilovolts

7525100

50

0

Sparks/Minute

21.51.5

0

D.C. Amps

20015010050

0

A.C. Amps500

400300200

100

0

A.C. Volts

Secondary Voltage

AlarmPower

OffPower

On

On

StartOff

Spark meterSecondary Current

Primary CurrentPrimary Voltage

Bus lineSupportinsulatorhousing

High voltage bus duct

Transformerrectifier

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Rappers

Dust that has accumulated on collection and discharge electrodes is removed by rapping.Dust deposits are generally dislodged by mechanical impulses, or vibrations, imparted tothe electrodes. A rapping system is designed so that rapping intensity and frequency canbe adjusted for varying operational conditions. Once the operating conditions are set, thesystem must be capable of maintaining uniform rapping for a long time.

Collection electrodes are rapped by hammer/anvil or magnetic impulse systems. Rigidframe discharge electrodes are rapped by tumbling hammers and wires are rapped byvibrators. As stated previously, liquid sprays are also used (instead of rapping) to removecollected particles form both tubes and plates.

Hammer/AnvilCollection plates are rapped by a number of methods. One rapper system uses ham-mers mounted on a rotating shaft, as shown in Figure 2-11. As the shaft rotates, thehammers drop (by gravity) and strike anvils that are attached to the collection plates.Rappers can be mounted on the top or on the side of collection plates. European pre-cipitator manufacturers use hammer and anvil rappers for removing particles fromcollection plates.

Rapping intensity is controlled by the weight of the hammers and the length of thehammer mounting arm. The frequency of rapping can be changed by adjusting thespeed of the rotating shafts. Thus, rapping intensity and frequency can be adjusted forthe varying dust concentration of the flue gas.

Figure 2-11. Typical hammer/anvil rappers forcollection plates

Hammer

Anvil

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Lesson 2

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Magnetic ImpulseAnother rapping system used for many U.S. designs consists of magnetic-impulse rap-pers to remove accumulated dust layers from collection plates. A magnetic-impulserapper has a steel plunger that is raised by a current pulse in a coil. The raised plungerthen drops back, due to gravity, striking a rod connected to a number of plates withinthe precipitator as shown in Figure 2-12. Rapper frequency and intensity are easilyregulated by an electrical control system. The frequency could be one rap every fiveminutes or one rap an hour with an intensity of 10 to 24 g's (Katz 1979). Magnetic-impulse rappers usually operate more frequently, but with less intensity, than rotatinghammer and anvil rappers.

Figure 2-12. Typical magnetic-impulse rappersfor collection plates

Tumbling Hammersfor Rigid Frame Discharge Electrodes

Rigid frame discharge electrodes are rapped by tumbling hammers. The tumblinghammers operate similarly to the hammers used to remove dust from collection elec-trodes. The hammers are arranged on a horizontal shaft. As the shaft rotates, thehammers hit an impact beam which transfers the shock, or vibration, to the centertubes on the discharge system, causing the dust to fall (Figure 2-13).

Electric VibratorWire discharge (or corona) electrodes must also be rapped to prevent excessive dustdeposit buildup that will interfere with corona generation. This is usually accom-plished by the use of air or electric vibrators that gently vibrate the discharge wires.Vibrators are usually mounted externally on precipitator roofs and are connected byrods to the high-tension frames that support the corona electrodes (Figure 2-14). Aninsulator, located above the rod, electrically insulates the rapper while mechanicallytransmitting the rapping force.

Rapper rods

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Figure 2-13. Tumbling hammers for rigid-framedischarge electrode

Figure 2-14. Typical electric-vibrator rappers usedfor wire discharge electrodes

Impactbeam

Tumblinghammer

Dischargewire

Centertube

Rapper

Highvoltage

frame

Rapperinsulator

Wiresupportchannel

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2-12

Hoppers

When the electrodes are rapped, the dust falls into hoppers and is stored temporarilybefore it is disposed in a landfill or reused in the process. Dust should be removed as soonas possible to avoid packing, which would make removal very difficult. Hoppers are usu-ally designed with a 50 to 70° (60° is common) slope to allow dust to flow freely from thetop of the hopper to the bottom discharge opening.

Some manufacturers add devices to the hopper to promote easy and quick discharge.These devices include strike plates, poke holes, vibrators, and rappers. Strike plates aresimply pieces of flat steel that are bolted or welded to the center of the hopper wall. If dustbecomes stuck in the hopper, rapping the strike plate several times with a mallet will freethis material. Hopper designs also usually include access doors, or ports. Access portsallow easier access for cleaning, inspection, and maintenance of the hopper (Figure 2-15).

Figure 2-15. Hopper

Hopper vibrators are occasionally used to help remove dust from the hopper walls. Hoppervibrators are electrically operated devices that cause the side walls of the hopper tovibrate, thereby removing the dust from the hopper walls. These devices must be care-fully designed and chosen so that they do not cause dust to be firmly packed against thehopper walls, and thereby plug the hopper. Before installing vibrators to reduce hopperplugging, make sure they have been successfully used in other, similar industrial applica-tions.

Hopper Discharge DevicesA discharge device is necessary for emptying the hopper and can be manual or auto-matic. The simplest manual discharge device is the slide gate, a plate held in place bya frame and sealed with gaskets (Figure 2-16). When the hopper needs to be emptied,the plate is removed and the material is discharged. Other manual discharge devices

Accessport

Strikeplate

Dischargedevice

Conveyor

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include hinged doors and drawers. The collector must be shut down before openingany manual discharge device. Thus, manual discharge devices are used only on verysmall units that operate on a periodic basis.

Figure 2-16. Slide-gate

Automatic continuous discharge devices are installed on ESPs that operate continuously.Some devices include double-dump valves (also called double flap or trickle valves), androtary airlock valves. Double-dump valves are shown in Figure 2-17. As dust collects inthe hopper, the weight of the dust pushes down the counterweight of the top flap and dustdischarges downward. The top flap then closes, the bottom flap opens, and the materialfalls out. This type of valve is available in gravity-operated and motorized versions.

Figure 2-17. Double-dump discharge device

Rotary airlock valves are used on medium or large-sized ESPs. The valve is designedwith a paddle wheel that is shaft mounted and driven by a motor (Figure 2-18). Therotary valve is similar to a revolving door; the paddles or blades form an airtight sealwith the housing, and the motor slowly moves the blades to allow the dust to dischargefrom the hopper.

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Lesson 2

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Figure 2-18. Rotary airlock discharge device

After the dust leaves the discharge device it is transported to the final disposal destina-tion by screw, drag, or pneumatic conveyers. Screw conveyors can be used as dis-charge devices when located in the bottom of the hopper as shown in Figure 2-19 oras a separate conveyor to move dust after it is discharged. Screw conveyers employ arevolving screw feeder to move the dust through the conveyor. Drag conveyors usepaddles, or flaps, that are connected to a drag chain to pull the dust through the con-veyor trough (Figure 2-20). Drag conveyors are used frequently for conveying stickyor hygroscopic dusts such as calcium chloride dust generated from municipal wastecombustors (collected fly ash/acid gas products). Pneumatic conveyers use blowersto blow or move the dust through the conveyor (Figure 2-21). Pneumatic conveyorscan be positive pressure (dust is moved by a blower) or vacuum type systems (dust ispulled by a vacuum).

In large ESPs, dust is usually discharged from hoppers by using a combination ofdevices. Either rotary airlock or double dump valves empty dust into screw, drag, orpneumatic conveyers that move dust for final disposal into trucks or storage bins.

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Figure 2-19. Screw conveyor

Figure 2-20. Drag conveyor

Figure 2-21. Pneumatic conveyor for transporting dust from ESP

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Lesson 2

2-16

Shell

The shell structure encloses the electrodes and supports the precipitator components in arigid frame to maintain proper electrode alignment and configuration (Figure 2-22). Thesupport structure is especially critical for hot-side precipitators because precipitator com-ponents can expand and contract when the temperature differences between the ESP(400°C or 752°F) and the ambient atmosphere (20°C or 68°F) are large. Excessive temper-ature stresses can literally tear the shell and hopper joints and welds apart. The outer sheetor casing wall is usually made of low-carbon or mild-grade steel that is 0.5 to 0.6 cm (3/16to 1/4 in.) thick.

Figure 2-22. ESP shell

Collection plates and discharge electrodes are normally attached to the frame at the top sothat the elements hang vertically due to gravity. This allows the elements to expand orcontract with temperature changes without binding or distorting.

Shells, hoppers, and connecting flues should be covered with insulation to conserve heat,and to prevent corrosion resulting from water vapor and acid condensation on internal pre-cipitator components. If the ESP is installed on a coal-fired boiler, the flue gas temperatureshould be kept above 120°C (250°F) at all times to prevent any acid mists in the flue gasfrom condensing on ESP internal components. Insulation will also help minimize temper-ature-differential stresses, especially on hot-side precipitators. Ash hoppers should beinsulated and heated because cold fly ash has a tendency to cake, making it extremely dif-ficult to remove. Insulation material is usually 10 to 15 cm (4 to 6 in.) thick.

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Summary

The precipitator should be designed to provide easy access to strategic points of the collectorfor internal inspection of electrode alignment, for maintenance, and for cleaning electrodes,hoppers, and connecting flues during outages. Vendors typically design the ESPs for a spe-cific particulate emission removal efficiency. The overall design, including the specific com-ponents, is based on engineering specifications and/or previous experience with the industrialapplication. These components have an effect on the overall performance and ease of opera-tion of the ESP. These topics are discussed in more detail in the following lessons.

Please view the video titled Electrostatic Precipitators: Operating Principles and Componentsbefore proceeding to the next lesson.

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Lesson 2

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Review Exercise

1. List the six major components of an ESP.

___________________________ ___________________________

___________________________ ___________________________

___________________________ ___________________________

2. In many U.S. precipitators, the discharge electrodes are thin wires that are approximately____________________ in diameter.

a. 2.0 in.b. 0.1 in.c. 0.01 in.d. 15.0 in.

3. The discharge wires are hung vertically in the ESP and are held taut and plumb at the bottom by:

a. A 25-lb weightb. Two 25-lb weightsc. A 50-lb weightd. A 5-lb weight

4. True or False? Accumulated dust can be removed from discharge and collection electrodes by rap-ping.

5. European precipitators and most new U.S.-designed ESPs use ____________________ for dis-charge electrodes.

a. Wiresb. Rigid framesc. Plates with stiffeners

6. Normal spacing for plates used on high-efficiency wire/plate ESPs is generally:

a. 0.2 to 0.8 in.b. 2 to 4 in.c. 8 to 9 in.d. 24 to 36 in.

7. Normal spacing for plates used on high-efficiency rigid-frame ESPs is generally:

a. 2-4 in.b. 5-7 in.c. 8-9 in.d. 12-15 in.

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2-20

8. In ESPs, plates are usually between ____________________ high.

a. 4 to 12 in.b. 20 to 40 ftc. 40 to 60 ft

9. Collection electrodes can be constructed in two general shapes: ____________________ and____________________.

10. Collected dust is removed from tubular ESPs using:

a. Magnetic impulse rappersb. Water spraysc. Hammer and anvil rappersd. Electric vibrator rappers

11. ESPs control the strength of the electric field generated between the discharge and collection elec-trodes by using:

a. Transformer-rectifier setsb. Metersc. Capacitorsd. Insulators

12. In a T-R set, the transformer ____________________ while the rectifier____________________.

a. Steps down the voltage, converts direct current into alternating currentb. Converts alternating current into direct current, steps up the voltagec. Steps up the voltage, converts alternating current into direct current

13. In the control circuitry on an ESP, the primary voltmeter measures the:

a. Number of sparksb. Input voltage (in a.c. volts) coming into the transformerc. Output voltage from the rectifierd. Operating d.c. voltage delivered to the discharge electrodes

14. The combination of the ____________________ voltage and current readings gives the powerinput to the discharge electrodes.

a. Primaryb. Sparkingc. Secondaryd. Tertiary

15. An electric cable that carries high voltage from the T-R set to the discharge electrode is calleda(an):

a. Bus lineb. Pipec. Ductd. Electric vibrator

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16. Most precipitators use ____________________ or ____________________ to remove accumu-lated dust from collection plates.

a. Air-vibrator rappers (or) water spraysb. Hammer and anvil (or) magnetic-impulse rappersc. Electric-vibrator (or) magnetic-impulse rappers

17. Which rappers are commonly used for removing dust from discharge electrodes?

a. Hammerb. Electric-vibrator and tumbling-hammerc. Magnetic-impulsed. Water-spray

18. The dust is temporarily stored in a ____________________.

19. A ____________________ ____________________ discharge device works similarly to arevolving door.

20. A ____________________ ____________________ uses a screw feeder located at the bottom ofthe hopper to remove dust from the bin.

21. A ____________________ ____________________ uses a blower or compressed air to removedust from the hopper.

22. A ____________________ ____________________ uses paddles or flaps connected to a dragchain to move dust from the ESP to its final destination.

23. In a precipitator, shells and hoppers should be covered with ____________________ to conserveheat and prevent corrosion.

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Lesson 2

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1. discharge electrodescollection electrodeshigh voltage electrical systemsrappershoppersshellThe six major components of an ESP are discharge electrodes, collection electrodes, high voltageelectrical systems, rappers, hoppers, and the shell.

2. b. 0.1 in.In many U.S. precipitators, the discharge electrodes are thin wires that are approximately 0.1 inchin diameter.

3. a. A 25-lb weightThe discharge wires are hung vertically in the ESP and are held taut and plumb at the bottom by a25-lb weight.

4. TrueAccumulated dust can be removed from discharge and collection electrodes by rapping.

5. b. Rigid framesEuropean precipitators and most new U.S.-designed ESPs use rigid frames for discharge elec-trodes.

6. c. 8 to 9 in.Normal spacing for plates used on high-efficiency wire/plate ESPs is generally 8 to 9 inches.

7. d. 12 to 15 in.Normal spacing for plates used on high-efficiency rigid-frame ESPs is generally 12 to 15 inches.

8. b. 20 to 40 ftIn ESPs, plates are usually between 20 to 40 ft high.

9. PlatesTubesCollection electrodes can be constructed in two general shapes: plates and tubes.

10. b. Water spraysCollected dust is removed from tubular ESPs using water sprays.

11. a. Transformer-rectifier setsESPs control the strength of the electric field generated between the discharge and collection elec-trodes by using transformer-rectifier sets.

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Lesson 2

2-24

12. c. Steps up the voltage, converts alternating current into direct currentIn a T-R set, the transformer steps up the voltage while the rectifier converts alternating currentinto direct current.

13. b. Input voltage (in a.c. volts) coming into the transformerIn the control circuitry on an ESP, the primary voltmeter measures the input voltage (in a.c. volts)coming into the transformer.

14. c. SecondaryThe combination of the secondary voltage and current readings gives the power input to the dis-charge electrodes.

15. a. Bus lineAn electric cable that carries high voltage from the T-R set to the discharge electrode is called abus line.

16. b. Hammer and anvil (or) magnetic-impulse rappersMost precipitators use hammer and anvil or magnetic-impulse rappers to remove accumulated dustfrom collection plates.

17. b. Electric-vibrator and tumbling-hammerFor removing dust from discharge electrodes, electric-vibrator rappers (for wires) and tumbling-hammer rappers (for rigid frames) are commonly used.

18. HopperThe dust is temporarily stored in a hopper.

19. Rotary airlockA rotary airlock discharge device works similarly to a revolving door.

20. Screw conveyorA screw conveyor uses a screw feeder located at the bottom of the hopper to remove dust from thebin.

21. Pneumatic conveyorA pneumatic conveyor uses a blower or compressed air to remove dust from the hopper.

22. Drag conveyorA drag conveyor uses paddles or flaps connected to a drag chain to move dust from the ESP to itsfinal destination.

23. InsulationIn a precipitator, shells and hoppers should be covered with insulation to conserve heat and preventcorrosion.

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Bibliography

Beachler, D. S., J. A. Jahnke, G. T. Joseph and M. M. Peterson. 1983. Air Pollution Control Systemsfor Selected Industries, Self-Instructional Guidebook. (APTI Course SI:431). EPA 450/2-82-006.U.S. Environmental Protection Agency.

Bethea, R. M. 1978. Air Pollution Control Technology-an Engineering Analysis Point of View. NewYork: Van Nostrand Reinhold.

Cheremisinoff, P. N., and R. A. Young. (Eds.) 1977. Air Pollution Control and Design Handbook,Part 1. New York: Marcel Dekker.

Gallaer, C. A. 1983. Electrostatic Precipitator Reference Manual. Electric Power Research Institute.EPRI CS-2809, Project 1402-4.

Hall, H. J. 1975. Design and application of high voltage power supplies in electrostatic precipitation.Journal of Air Pollution Control Association. 25:132.

Hesketh, H. E. 1979. Air Pollution Control. Ann Arbor: Ann Arbor Science Publishers.


Richards, J.R. 1995. Control of Particulate Emissions, Student Manual. (APTI Course 413). U.S.Environmental Protection Agency.

Szabo, M. F., Y. M. Shah, and S. P. Schliesser. 1981. Inspection Manual for Evaluation of Electro-static Precipitator Performances. EPA 340/1-79-007.

Turner, J. H., P. A. Lawless, T. Yamamoto, D. W. Coy, G. P. Greiner, J. D. McKenna, and W. M. Vata-vuk. 1992. Electrostatic precipitators. In A. J. Buonicore and W. T. Davis (Eds.), Air PollutionEngineering Manual (pp. 89-113). Air and Waste Management Association. New York: Van Nos-trand Reinhold.


White, H. J. 1963. Industrial Electrostatic Precipitation. Reading, MA: Addison-Wesley.White, H. J.1977. Electrostatic precipitation of fly ash. APCA Reprint Series. Journal of Air Pollution Con-trol Association. Pittsburgh, PA.

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Lesson 3ESP Design Parameters andTheir Effects on Collection Efficiency

Goal

To familiarize you with the variables used by vendors to optimally design ESP systems.

Objectives


1. Define the term migration velocity2. Explain the difference between the Deutsch-Anderson equation and the Matts-Ohnfeldt equa-

tion for estimating collection efficiency3. Define the term resistivity4. List three ways to reduce high resistivity and two ways to combat low resistivity5. Explain how sectionalization and increasing corona power improves collection efficiency6. Define aspect ratio and specific collection area and describe their importance for achieving

collection efficiency7. Calculate the aspect ratio and specific collection area of an ESP given a set of design informa-

tion

Introduction

Because of legislation such as the Clean Air Act and the 1977 and 1990 Clean Air Act Amend-ments, ESPs have been carefully designed to collect more than 99.5% of particles in the fluegas from many industries. ESPs efficiently collect particles of various sizes: large particles of3 to 10 µm in diameter, and smaller particles of less than 1 µm in diameter.

An ESP is designed for a particular industrial application. Building an ESP is a costlyendeavor, so a great deal of time and effort is expended during the design stage. Manufacturersuse various methods to design ESPs. They also consider a variety of operating parameters thataffect collection efficiency including resistivity, electrical sectionalization, specific collectionarea, aspect ratio, gas flow distribution, and corona power. This lesson focuses on these meth-ods and operating parameters.

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Lesson 3

3-2

Design Methods

Manufacturers use mathematical equations to estimate collection efficiency or collection area.In addition, they may build a pilot-plant to determine the parameters necessary to build thefull-scale ESP. They may also use a mathematical model or computer program to test thedesign features and operating parameters in a simulation of the final design. Once the basis ofthe ESP design is completed, the vendor can design the unit using various individual parame-ters that are appropriate for each specific situation.

Using Estimates of Collection Efficiency

Collection efficiency is the primary consideration of ESP design. The collection effi-ciency and/or the collection area of an ESP can be estimated using several equations.These equations give a theoretical estimate of the overall collection efficiency of the unitoperating under ideal conditions. Unfortunately, a number of operating parameters canadversely affect the collection efficiency of the precipitator. A discussion of collection-efficiency equations and operating parameters affecting collection-efficiency equationsfollows.

Particle-Migration VelocityBefore determining the collection area and the collection efficiency, the designer mustestimate or measure (if possible) the particle-migration velocity. This is the speed atwhich a particle, once charged, migrates toward the grounded collection electrode.Variables affecting particle velocity are particle size, the strength of the electric field,and the viscosity of the gas. How readily the charged particles move to the collectionelectrode is denoted by the symbol, w, called the particle-migration velocity, or driftvelocity. The migration-velocity parameter represents the collectability of the particlewithin the confines of a specific ESP. The migration velocity is expressed in Equation3-1.

(3-1)

Where: dp = diameter of the particle, µmEo = strength of field in which particles are charged

(represented by peak voltage), V/m (V/ft)Ep = strength of field in which particles are collected

(normally the field close to the collecting plates), V/m (V/ft)µ = gas viscosity, Pa • s (cp)π = 3.14

As shown in Equation 3-1, migration velocity depends on the voltage strength of boththe charging and collection fields. Therefore, the precipitator must be designed usingthe maximum electric field voltage for maximum collection efficiency. The migrationvelocity also depends on particle size; larger particles are collected more easily thansmaller ones.

w =d E Ep o p

4πµ

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ESP Design Parameters and Their Effects on Collection Efficiency

Particle-migration velocity can also be determined by Equation 3-2.

(3-2)

Where: q = particle charge(s)Ep = strength of field in which particles are collected, V/m (V/ft)µ = gas viscosity, Pa • s (cp)r = radius of the particle, µmπ = 3.14

The particle-migration velocity can be calculated using either Equations 3-1 or 3-2,depending on the information available on the particle size and electric field strength.However, most ESPs are designed using a particle-migration velocity based on fieldexperience rather than theory. Typical particle migration velocity rates, such as thoselisted in Table 3-1, have been published by various ESP vendors.

Table 3-1. Typical effective particle-migrationvelocity rates for various applications

ApplicationMigration velocity

(ft/sec) (cm/s)

Utility fly ashPulverized coal fly ashPulp and paper millsSulfuric acid mistCement (wet process)Cement (dry process)GypsumSmelterOpen-hearth furnaceBlast furnaceHot phosphorousFlash roasterMultiple-hearth roasterCatalyst dustCupola

0.13-0.670.33-0.440.21-0.310.19-0.250.33-0.370.19-0.230.52-0.640.060.16-0.190.20-0.460.090.250.260.250.10-0.12

4.0-20.410.1-13.46.4-9.55.8-7.6210.1-11.36.4-7.015.8-19.51.84.9-5.86.1-14.02.77.67.97.63.0-3.7

Sources: Theodore and Buonicore 1976; U.S. EPA 1979.

w =qE

rp

6πµ

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Lesson 3

3-4

Deutsch-Anderson EquationProbably the best way to gain insight into the process of electrostatic precipitation isto study the relationship known as the Deutsch-Anderson equation. This equation isused to determine the collection efficiency of the precipitator under ideal conditions.The simplest form of the equation is given below.

(3-3)

Where: η = collection efficiency of the precipitatore = base of natural logarithm = 2.718w = migration velocity, cm/s (ft/sec)A = the effective collecting plate area of the precipitator, m2 (ft2)Q = gas flow through the precipitator, m3/s (ft3/sec)

_____________________Source: Deutsch 1922; Anderson 1924.

This equation has been used extensively for many years to calculate theoretical collec-tion efficiencies. Unfortunately, while the equation is scientifically valid, a number ofoperating parameters can cause the results to be in error by a factor of 2 or more. TheDeutsch-Anderson equation neglects three significant process variables. First, it com-pletely ignores the fact that dust reentrainment may occur during the rapping process.Second, it assumes that the particle size and, consequently, the migration velocity areuniform for all particles in the gas stream. As stated previously, this is not true; largerparticles generally have higher migration velocity rates than smaller particles do.Third, it assumes that the gas flow rate is uniform everywhere across the precipitatorand that particle sneakage (particles escape capture) through the hopper section doesnot occur. Particle sneakage can occur when the flue gas flows down through the hop-per section instead of through the ESP chambers, thus preventing particles from beingsubjected to the electric field. Therefore, this equation should be used only for makingpreliminary estimates of precipitator collection efficiency.

More accurate estimates of collection efficiency can be obtained by modifying theDeutsch-Anderson equation. This is accomplished either by substituting the effectiveprecipitation rate, we, in place of the migration velocity, w, or by decreasing the cal-culation of collection efficiency by a factor of k, which is constant (Matts-Ohnfeldtequation). These calculations are used in establishing preliminary design parametersof ESPs.

Modified Deutsch-Anderson EquationUsing the Effective-Precipitation Rate

To make the Deutsch-Anderson equation more accurate in cases where all particlesare not uniform in size, a parameter called the effective precipitation rate (we) can besubstituted for the migration velocity in the equation. Therefore, Dr. Harry White pro-posed modifying the Deutsch-Anderson equation by using the term we instead of w inthe Deutsch-Anderson equation (White 1982).

η 1 e w A Q⁄( )––=

2.0-2/98


(3-4)

Where: η = collection efficiency of the precipitatore = base of natural logarithm = 2.718we = effective migration velocity, calculated from field experienceA = collecting area, m2 (ft2)Q = gas flow rate, m3/s (ft3/sec)

In contrast to the migration velocity (w), which refers to the speed at which an indi-vidual charged particle migrates to the collection electrode, the effective precipitationrate (we) refers to the average speed at which all particles in the entire dust mass movetoward the collection electrode. The variable, we, is calculated from field experiencerather than from theory; values for we are usually determined using data banks accu-mulated from ESP installations in similar industries or from pilot-plant studies. Insummary, the effective precipitation rate represents a semi-empirical parameter thatcan be used to determine the total collection area necessary for an ESP to achieve aspecified collection efficiency required to meet an emission limit.

Using the Deutsch-Anderson equation in this manner could be particularly usefulwhen trying to determine the amount of additional collection area needed to upgradean existing ESP to meet more stringent regulations or to improve the performance ofthe unit. However, other operating parameters besides collection area play a majorrole in determining the efficiency of an ESP.

Matts-Ohnfeldt EquationAnother modification to the Deutsch-Anderson equation that accounts for non-idealeffects was devised by Sigvard Matts and Per-Olaf Ohnfeldt of Sweden (SvenskaFlaktfabriken) in 1964. The Matts-Ohnfeldt equation is

(3-5)

Where: η = collection efficiency of the precipitatore = base of natural logarithm = 2.718wk = average migration velocity, cm/s (ft/sec)k = a constant, usually 0.4 to 0.6A = collection area, m2 (ft2)Q = gas flow rate, m3/s (ft3/sec)

The term, wk, the average migration velocity in equation 3-5, is determined frominformation obtained from similar installations. The terms wk and we (in equations 3-5and 3-4 respectively) are similar in that both are average migration velocities. Theconstant, k, in the equation is usually between 0.4 and 0.6, depending on the standarddeviation of the particle size distribution and other dust properties affecting collectionefficiency. However, most people who have used this equation report that a value of kequal to 0.5 gives satisfactory results (Gallaer 1983 and U.S. EPA 1985). In an Elec-tric Power Research Institute (EPRI) study, a table was constructed to show the rela-tionship of predicting collection efficiency using the Deutsch-Anderson and Matts-Ohnfeldt equations. This information is given in Table 3-2.

η 1 ewe A Q⁄( )–

–=

η 1 ewk A Q⁄( )k–

–=

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Lesson 3

3-6

When k = 1.0, the Matts-Ohnfeldt equation is the same as the Deutsch-Andersonequation. To predict the collection efficiency of an existing ESP when the collectionarea or gas flow rate is varied, using lower values for k gives more conservativeresults. From Table 3-2, you can see that the efficiency estimates calculated using theMatts-Ohnfeldt equation are more conservative than those estimated using theDeutsch-Anderson equation, and may more likely predict how efficiently the ESP willactually operate.

Using Pilot Plants

Probably the most reliable method for designing ESPs is to construct and operate a pilotplant. However, time limitations and the expense of construction may make this impossi-ble; a pilot plant can easily cost one million dollars or more. A pilot ESP project can beconstructed on an existing industrial process. In this case, a side stream of flue gas is sentto the small pilot ESP. Flue gas sampling gives valuable information such as gas tempera-ture, moisture content, and dust resistivity. Relating these parameters to the measured col-lection efficiency of the pilot project will help the design engineers plan for scale-up to afull-sized ESP.

Using Computer Programs and Models

Engineers can also use mathematical models or computer programs to design precipita-tors. A mathematical model that relates collection efficiency to precipitator size and vari-ous operating parameters has been developed by Southern Research Institute (SoRI) forEPA. The (SoRI/EPA) model is used to do the following:

• Design a full-scale ESP from fundamental principles or in conjunction with a pilot-plant study·

• Evaluate ESP bids submitted by various manufacturers

• Troubleshoot and diagnose operating problems for existing ESPs

• Evaluate the effectiveness of new ESP developments and technology, such as flue gasconditioning and pulse energizing.

Table 3-2. Collection-efficiency estimationsusing the Deutsch-Anderson andMatts-Ohnfeldt equations

Relative sizeof ESP (A/Q)

Deutschk = 1.0

Matts-Ohnfeldt

k = 0.4 k = 0.5 k = 0.6

12345

909999.999.9999.999

9095.197.298.198.7

9096.298.19999.6

9097.298.899.599.76

Source: Gallaer 1983.

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Details of this model are given in EPA publications A Mathematical Model of ElectrostaticPrecipitation (Revision 1), Volumes I and II.

Table 3-3 lists the input data used in the SoRI/EPA Model. Assuming that accurate inputdata are available for use, the model usually can estimate emissions within ± 20 percent ofmeasured values (U.S. EPA 1985). The computer model goes through an iterative compu-tational process to refine its predictions of emission levels for a particular ESP. First, themodel uses secondary voltage and current levels (corona power) to predict emission levelsleaving the ESP. Then, actual emission levels are measured and compared to the predictedemission levels. Empirical factors are then adjusted and the process repeats itself until thepredicted emission levels of the model agree with the actual, measured levels. This modelcan be used to obtain reasonable estimates of emission levels for other ESP operating con-ditions (U.S. EPA 1985). For example, once you create a good, working computer modelfor a particular ESP design under one set of operating conditions, you can run the modelfor different scenarios by altering one or more of the parameters (precipitator length, num-ber of fields, etc.) to obtain reasonably accurate emission level predictions.

Table 3-3. Input data for EPA/SORI ESP computermodel

ESP Specifications Gas/particulatespecifications

Estimated efficiencyPrecipitator lengthSuperficial gas velocityFraction of sneakage/reentrainmentNormalized standard deviation of gas velocity

distributionNumber of stages for sneakage/reentrainmentNumber of electrical sections in direction of gas

flowFor each electrical section

LengthAreaApplied voltageCurrentCorona wire radiusCorona wire lengthWire-to-wire spacing (1/2)Wire-to-plate spacingNumber of wires per linear section

Gas flow rateGas pressureGas temperatureGas viscosityParticulate concentrationParticulate resistivityParticulate densityParticle size distributionDielectric constantIon speed

Source: U.S. EPA 1985.

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Lesson 3

3-8

Another model, the EPA/RTI model, has been developed by the Research Triangle Insti-tute (RTI) for EPA (Lawless 1992). The EPA/RTI model is based on the localized electricfield strengths and current densities prevailing throughout the precipitator. These data canbe input based on actual readings from operating units, or can be calculated based on elec-trode spacing and resistivity. The data are used to estimate the combined electrical charg-ing on each particle size range due to field-dependent charging and diffusional charging.Particle size-dependent migration velocities are then used in a Deutsch-Anderson typeequation to estimate particle collection in each field of the precipitator. This model takesinto account a number of the site specific factors including gas flow maldistribution, parti-cle size distribution, and rapping reentrainment.

These performance models require detailed information concerning the anticipated config-uration of the precipitator and the gas stream characteristics. Information needed to oper-ate the EPA/RTI model is provided below. It is readily apparent that all of these parametersare not needed in each case, since some can be calculated from the others. The followingdata is data utilized in the EPA/RTI computerized performance model for electrostatic pre-cipitators.

ESP Design

• Specific collection area

• Collection plate area

• Collection height and length

• Gas velocity

• Number of fields in series

• Number of discharge electrodes

• Type of discharge electrodes

• Discharge electrode-to-collection plate spacing

Particulate Matter and Gas Stream Data

• Resistivity

• Particle size mass median diameter

• Particle size distribution standard deviation

• Gas flow rate distribution standard deviation

• Actual gas flow rate

• Gas stream temperature

• Gas stream pressure

• Gas stream composition

2.0-2/98


Design Parameters

Once the basis of the ESP design has been set, the vendor will complete the design by incorpo-rating a number of parameters that can be adjusted for each specific industrial application.However, before starting this design phase, the vendor must take into account the effect thatparticle resistivity can have on the actual collection efficiency.

Resistivity

Resistivity, which is a characteristic of particles in an electric field, is a measure of a parti-cle's resistance to transferring charge (both accepting and giving up charges). Resistivity isa function of a particle's chemical composition as well as flue gas operating conditionssuch as temperature and moisture. Particles can have high, moderate (normal), or lowresistivity.

In an ESP, where particle charging and discharging are key functions, resistivity is animportant factor that significantly affects collection efficiency. While resistivity is animportant phenomenon in the inter-electrode region where most particle charging takesplace, it has a particularly important effect on the dust layer at the collection electrodewhere discharging occurs. Particles that exhibit high resistivity are difficult to charge. Butonce charged, they do not readily give up their acquired charge on arrival at the collectionelectrode. On the other hand, particles with low resistivity easily become charged andreadily release their charge to the grounded collection plate. Both extremes in resistivityimpede the efficient functioning of ESPs. ESPs work best under normal resistivity condi-tions.

Resistivity is the electrical resistance of a dust sample 1.0 cm2 in cross-sectional area, 1.0cm thick, and is recorded in units of ohm-cm. A method for measuring resistivity will bedescribed later in this lesson. Table 3-4 gives value ranges for low, normal, and high resis-tivity.

Dust Layer ResistivityLet’s take a closer look at the way resistivity affects electrical conditions in the dustlayer. A potential electric field (voltage drop) is formed across the dust layer as nega-tively charged particles arrive at the dust layer surface and leak their electrical chargesto the collection plate. At the metal surface of the electrically grounded collectionplate, the voltage is zero. Whereas at the outer surface of the dust layer, where newparticles and ions are arriving, the electrostatic voltage caused by the gas ions can bequite high. The strength of this electric field depends on the resistivity and thicknessof the dust layer.

Table 3-4. Low, normal, and high resistivity

Resistivity Range of measurement

Low

Normal

High

between 104 and 107 ohm • cm

between 107 and 1010 ohm • cmabove 1010 ohm • cm

(usually between 1010 and 1014 ohm • cm)

2.0-2/98 3-9

Lesson 3

3-10

In high resistivity dust layers, the dust is not sufficiently conductive, so electricalcharges have difficulty moving through the dust layer. Consequently, electricalcharges accumulate on and beneath the dust layer surface, creating a strong electricfield. Voltages can be greater than 10,000 volts. Dust particles with high resistivitiesare held too strongly to the plate, making them difficult to remove and causing rappingproblems.

In low resistivity dust layers, the corona current is readily passed to the grounded col-lection electrode. Therefore, a relatively weak electric field, of several thousand volts,is maintained across the dust layer. Collected dust particles with low resistivity do notadhere strongly enough to the collection plate. They are easily dislodged and becomereentrained in the gas stream.

The following discussion of normal, high, and low resistivity applies to ESPs operatedin a dry state; resistivity is not a problem in the operation of wet ESPs because of themoisture concentration in the ESP. The relationship between moisture content andresistivity is explained later in this lesson.

Normal ResistivityAs stated above, ESPs work best under normal resistivity conditions. Particles withnormal resistivity do not rapidly lose their charge on arrival at the collection electrode.These particles slowly leak their charge to grounded plates and are retained on the col-lection plates by intermolecular adhesive and cohesive forces. This allows a particu-late layer to be built up and then dislodged from the plates by rapping. Within therange of normal dust resistivity (between 107 and 1010 ohm-cm), fly ash is collectedmore easily than dust having either low or high resistivity.

High ResistivityIf the voltage drop across the dust layer becomes too high, several adverse effects canoccur. First, the high voltage drop reduces the voltage difference between the dis-charge electrode and collection electrode, and thereby reduces the electrostatic fieldstrength used to drive the gas ion - charged particles over to the collected dust layer.As the dust layer builds up, and the electrical charges accumulate on the surface of thedust layer, the voltage difference between the discharge and collection electrodesdecreases. The migration velocities of small particles are especially affected by thereduced electric field strength.

Another problem that occurs with high resistivity dust layers is called back corona.This occurs when the potential drop across the dust layer is so great that corona dis-charges begin to appear in the gas that is trapped within the dust layer. The dust layerbreaks down electrically, producing small holes or craters from which back coronadischarges occur. Positive gas ions are generated within the dust layer and are acceler-ated toward the "negatively charged" discharge electrode. The positive ions reducesome of the negative charges on the dust layer and neutralize some of the negativeions on the "charged particles" heading toward the collection electrode. Disruptions ofthe normal corona process greatly reduce the ESP's collection efficiency, which insevere cases, may fall below 50% (White 1974).

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The third, and generally most common problem with high resistivity dust is increasedelectrical sparking. When the sparking rate exceeds the "set spark rate limit," the auto-matic controllers limit the operating voltage of the field. This causes reduced particlecharging and reduced migration velocities toward the collection electrode.

High resistivity can generally be reduced by doing the following:

• Adjusting the temperature

• Increasing moisture content

• Adding conditioning agents to the gas stream

• Increasing the collection surface area

• Using hot-side precipitators (occasionally)

Figure 3-1 shows the variation in resistivity with changing gas temperature for six dif-ferent industrial dusts (U.S. EPA 1985). For most dusts, resistivity will decrease as theflue gas temperature increases. However, as can be seen from Figure 3-1, the resistiv-ity also decreases for some dusts (cement and ZnO) at low flue gas temperatures.

Figure 3-1. Resistivity of six different dusts at varioustemperaturesSource: U.S. EPA 1985.

2.0-2/98 3-11

Lesson 3

3-12

The moisture content of the flue gas stream also affects particle resistivity. Increasingthe moisture content of the gas stream by spraying water or injecting steam into theduct work preceding the ESP lowers the resistivity. In both temperature adjustmentand moisture conditioning, one must maintain gas conditions above the dew point toprevent corrosion problems in the ESP or downstream equipment. Figure 3-2 showsthe effect of temperature and moisture on the resistivity of cement dust. As the per-centage of moisture in the dust increases from 1 to 20%, the resistivity of the dust dra-matically decreases. Also, raising or lowering the temperature can decrease cementdust resistivity for all the moisture percentages represented.

Figure 3-2. Effect of temperature and moisture on theresistivity of cement dustSources: Schmidt 1949, White 1977.

The presence of SO3 in the gas stream has been shown to favor the electrostatic pre-cipitation process when problems with high resistivity occur. Most of the sulfur con-tent in the coal burned for combustion sources converts to SO2. However,approximately 1% of the sulfur converts to SO3. The amount of SO3 in the flue gasnormally increases with increasing sulfur content of the coal. The resistivity of theparticles decreases as the sulfur content of the coal increases (Figure 3-3).

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Figure 3-3. Fly ash resistivity versus coal sulfur contentfor several flue gas temperature bandsSource: White 1977.

The use of low-sulfur western coal for boiler operations has caused fly ash resistivityproblems for ESP operators. For coal fly ash dusts, the resistivity can be loweredbelow the critical level by the injection of as little as 10 to 30 ppm SO3 into the gasstream. The SO3 is injected into the duct work preceding the precipitator. Figure 3-4shows the flow diagram of a sulfur-burning flue gas conditioning system used tolower resistivity at a coal-fired boiler.

Figure 3-4. Flow diagram of sulfur-burning flue gas conditioning systemCourtesy of Wahlco, Inc.

Liquidsulfur

storageMetering

pump

Ambientairin

Liquid sulfur

250° - 300°F

Airheater

Controlled to800° - 825°F

Sulfurburner

Converter

Air/SO3

800° - 1100°FBoiler

flue

Injectionprobes

Conditionedflue gas toprecipitator

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Lesson 3

3-14

Other conditioning agents, such as sulfuric acid, ammonia, sodium chloride, and sodaash, have also been used to reduce particle resistivity (White 1974). Therefore, thechemical composition of the flue gas stream is important with regard to the resistivityof the particles to be collected in the ESP. Table 3-5 lists various conditioning agentsand their mechanisms of operation (U.S. EPA 1985).

.

Two other methods that reduce particle resistivity include increasing the collectionsurface area and handling the flue gas at higher temperatures. Increasing the collectionarea of the precipitator will increase the overall cost of the ESP, which may not bedesirable. Hot-side precipitators, which are usually located in front of the combustionair preheater section of the boiler, are also used to combat resistivity problems. How-ever, the use of conditioning agents has been more successful and very few hot-sideESPs have been installed since the 1980s.

Table 3-5. Reaction mechanisms of majorconditioning agents

Conditioning agent Mechanism(s) of action

Sulfur trioxide andsulfuric acid

Ammonia

Ammonium sulfate1

Triethylamine

Sodium compounds

Compounds of transitionmetals

Potassium sulfate andsodium chloride

Condensation and adsorption on fly ash surfaces;may also increase cohesiveness of fly ash.

Reduces resistivity.

Mechanism is not clear; various ones proposed:Modifies resistivityIncreases ash cohesivenessEnhances space charge effect

Little is known about the actual mechanism; claimsare made for the following:Modifies resistivity (depends upon injection

temperature)Increases ash cohesivenessEnhances space charge effect

Experimental data lacking to substantiate which ofthese is predominant

Particle agglomeration claimed; no supporting data

Natural conditioner if added with coal.Resistivity modifier if injected into gas stream

Postulated that they catalyze oxidation of SO2 toSO3; no definitive tests with fly ash to verify thispostulation

In cement and lime kiln ESPs:Resistivity modifiers in the gas streamNaCl - natural conditioner when mixed with coal

1 If injection occurs at a temperature greater than about 600°F, dissociation into ammonia and sulfurtrioxide results. Depending upon the ash, SO2 may preferentially interact with fly ash as SO3

conditioning. The remainder recombines with ammonia to add to the space charge as well asincrease the cohesiveness of the ash.


2.0-2/98


Low ResistivityParticles that have low resistivity are difficult to collect because they are easilycharged (very conductive) and rapidly lose their charge on arrival at the collectionelectrode. The particles take on the charge of the collection electrode, bounce off theplates, and become reentrained in the gas stream. Thus, attractive and repulsive elec-trical forces that are normally at work at higher resistivities are lacking, and the bind-ing forces to the plate are considerably lessened. Examples of low-resistivity dusts areunburned carbon in fly ash and carbon black.

If these conductive particles are coarse, they can be removed upstream of the precipi-tator by using a device such as a cyclone. Baffles are often installed on the collectionplates to help eliminate this precipitation-repulsion phenomenon.

The addition of liquid ammonia (NH3) into the gas stream as a conditioning agent hasfound wide use in recent years. It is theorized that ammonia reacts with H2SO4 con-tained in the flue gas to form an ammonium sulfate compound that increases the resis-tivity of the dust. Ammonia vapor is injected into the duct leading to the precipitator atconcentrations of 15 to 40 ppm by volume. The injection of NH3 has improved theresistivity of fly ash from coal-fired boilers with low flue gas temperatures (Katz1979).

Table 3-6 summarized the characteristics associated with low, normal and high resis-tivity dusts.

Measuring ResistivityParticle resistivity is determined by measuring the leakage current through a dust layerto which a high voltage is applied using conductivity cells. A number of conductivitycells have been used in particle-resistivity measurements. For a good review of thedifferent kinds of cells employed, see White (1974). Resistivity can be measured by anumber of methods in either the laboratory or the field. In the lab method, dust sam-ples are first extracted from the flue gas leaving the industrial process and collected ona filter as described in EPA Reference Method 5. The samples are then taken back tothe laboratory and analyzed.

Resistivity measurements are made in the field using an in-situ resistivity probe. Theprobe is inserted into the duct leaving the industrial process and a dust sample isextracted into the probe. High voltage is applied across a point and plate electrode sys-tem inside the probe. Particles are charged and then collected on the plate. After a suf-ficiently thick layer of dust has collected on the plate, the power to the point is turnedoff and a disc is lowered onto the collected dust sample. The thickness of the dustlayer is first measured. Increasing voltages are then applied to the disc, and the corre-sponding current is recorded until the dust layer breaks down and sparkover occurs.The resistivity is calculated from the last set of voltage and current readings obtainedbefore sparkover occurs. Since these resistivity measurements are made at the indus-trial process conditions, these data are generally more useful than data obtained fromthe laboratory methods. A good review of in-situ resistivity measuring techniques isgiven by White (1974) and Gallaer (1983).

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Lesson 3

3-16

Electrical Sectionalization

Field SectionalizationAn electrostatic precipitator is divided into a series of independently energized bussections or fields (also called stages) in the direction of the gas flow. Precipitator per-formance depends on the number of individual bus sections, or fields, installed. Figure3-5 shows an ESP consisting of four fields, each of which acts as an independent pre-cipitator.

Table 3-6. ESP characterististics associated withdifferent levels of resistivity

Resistivity Level,ohm-cm ESP Characteristics

Less than 107

(Low Resistivity)1. Normal operating voltage and current levels

unless dust layer is thick enough to reduce plateclearances and cause higher current levels

2. Reduced electrical force component retainingcollected dust, vulnerable to high reentrainmentlosses

3. Negligible voltage drop across dust layer4. Reduced collection performance due to (2)

107 to 1010

(Normal Resistivity)

1011

1. Normal operating voltage and current levels2. Negligible voltage drop across dust layer3. Sufficient electrical force component retaining

collected dust4. High collection performance due to (1), (2), and

(3)

1. Reduced operating voltage and current levelswith high spark rates

2. Significant voltage loss across dust layer3. Moderate electrical force component retaining

collected dust4. Reduced collection performance due to (1) and

(2)

Greater than 1012

(High Resistivity)1. Reduced operating voltage levels; high operating

current levels if power supply controller is notoperating properly

2. Very significant voltage loss across dust layer3. High electrical force component retaining

collected dust4. Seriously reduced collection performance due to

(1), (2), and probable back corona

Typical values

Operating voltage: 30 to 70 kV, dependent on design factorsOperating current density: 5 to 50 nA/cm2

Dust layer thickness: 1/4 to 1 inch

Source: Adapted from U.S. EPA 1985.

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Figure 3-5. Field sectionalization

Each field has individual transformer-rectifier sets, voltage-stabilization controls, andhigh-voltage conductors that energize the discharge electrodes within the field. Thisdesign feature, called field electrical sectionalization, allows greater flexibility forenergizing individual fields to accommodate different conditions within the precipita-tor. This is an important factor in promoting higher precipitator collection efficiency.Most ESP vendors recommend that there be at least three or more fields in the precip-itator. However, to attain a collection efficiency of more than 99%, some ESPs havebeen designed with as many as seven or more fields. Previous experience with a par-ticular industry is the best factor for determining how many fields are necessary tomeet the required emission limits.

The need for separate fields arises mainly because power input requirements differ atvarious locations within a precipitator. The maximum voltage at which a given fieldcan be maintained depends on the properties of the gas and dust being collected. Theparticulate matter concentration is generally high at the inlet fields of the precipitator.High dust concentrations tend to suppress corona current, requiring a great deal ofpower to generate corona discharge for optimum particle charging. In the downstreamfields of a precipitator, the dust loading is usually lighter, because most of the dust iscollected in the inlet fields. Consequently, corona current flows more freely in down-stream fields. Particle charging will more likely be limited by excessive sparking inthe downstream than in the inlet fields. If the precipitator had only one power set, theexcessive sparking would limit the power input to the entire precipitator, thus reduc-ing the overall collection efficiency. The rating of each power set in the ESP will varydepending on the specific design of the ESP.

Modern precipitators have voltage control devices that automatically limit precipitatorpower input. A well-designed automatic control system keeps the voltage level atapproximately the value needed for optimum particle charging by the discharge elec-trodes. The voltage control device increases the primary voltage applied to the T-R setto the maximum level. As the primary voltage applied to the transformer increases, thesecondary voltage applied to the discharge electrodes increases. As the secondaryvoltage is increased, the intensity and number of corona discharges increase. The volt-age is increased until any of the set limits (primary voltage, primary current, second-ary voltage, secondary current, or spark rate limits) is reached. Occurrence of a sparkcounteracts high ESP performance because it causes an immediate, short-term col-lapse of the precipitator electric field. Consequently, power that is applied to capture

2.0-2/98 3-17

Lesson 3

3-18

particles is used less efficiently. There is, however, an optimum sparking rate wherethe gains in particle charging are just offset by corona-current losses from sparkover.

Measurements on commercial precipitators have determined that the optimum spark-ing rate is between 50 and 150 sparks per minute per electrical section. The objectivein power control is to maintain corona power input at this optimum sparking rate bymomentarily reducing precipitator power whenever excessive sparking occurs.

Besides allowing for independent voltage control, another major reason for having anumber of fields in an ESP is that electrical failure may occur in one or more fields.Electrical failure may occur as a result of a number of events, such as over-filling hop-pers, discharge-wire breakage, or power supply failure. These failures are discussed inmore detail later in this course. ESPs having a greater number of fields are less depen-dent on the operation of all fields to achieve a high collection efficiency.

Parallel SectionalizationIn field sectionalization, the precipitator is designed with a single series of indepen-dent fields following one another consecutively. In parallel sectionalization, theseries of fields is electrically divided into two or more sections so that each field hasparallel components. Such divisions are referred to as chambers and each individualunit is called a cell. A precipitator such as the one shown in Figure 3-6 has two paral-lel sections (chambers), four fields, and eight cells. Each cell can be independentlyenergized by a bus line from its own separate transformer-rectifier set.

Figure 3-6. Parallel sectionalization (with two parallelsections, eight cells, and four fields)

One important reason for providing sectionalization across the width of the ESP is toprovide a means of handling varying levels of flue gas temperature, dust concentra-tion, and problems with gas flow distribution. When treating flue gas from a boiler, anESP may experience gas temperatures that vary from one side of the ESP to the other,especially if a rotary air preheater is used in the system. Since fly ash resistivity is afunction of the flue gas temperature, this temperature gradient may cause variations inthe electrical characteristics of the dust from one side of the ESP to the other. The gasflow into the ESP may also be stratified, causing varying gas velocities and dust con-centrations that can also affect the electrical characteristics of the dust. Buildingnumerous fields and cells into an ESP design can provide a means of coping with vari-

Chamber 1

CellChamber 2

2.0-2/98


ations in the flue gas. In addition, the more cells provided in an ESP, the greater thechance that the unit will operate at its designed collection efficiency.

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Lesson 3

3-20

Specific Collection Area

The specific collection area (SCA) is defined as the ratio of collection surface area to thegas flow rate into the collector. This ratio represents the A/Q relationship in the Deutsch-Anderson equation and consequently is an important determinant of collection efficiency.The SCA is given in Equation 3-6.

(3-6)

Expressed in metric units,

Expressed in English units,

For example, if the total collection area of an ESP is 600,000 ft2 and the gas flow ratethrough the ESP is 1,000,000 ft3/min (acfm), the SCA is 600 ft2 per 1000 acfm as calcu-lated below.

Increases in the SCA of a precipitator design will, in most cases, increase the collectionefficiency of the precipitator. Most conservative designs call for an SCA of 20 to 25 m2

per 1000 m3/h (350 to 400 ft2 per 1000 acfm) to achieve collection efficiency of more than99.5%. The general range of SCA is between 11 and 45 m2 per 1000 m3/hr (200 and 800ft2 per 1000 acfm), depending on precipitator design conditions and desired collectionefficiency.

Aspect Ratio

The aspect ratio, which relates the length of an ESP to its height, is an important factor inreducing rapping loss (dust reentrainment). When particles are rapped from the electrodes,the gas flow carries the collected dust forward through the ESP until the dust reaches thehopper. Although the amount of time it takes for rapped particles to settle in the hoppers isshort (a matter of seconds), a large amount of "collected dust" can be reentrained in the gasflow and carried out of the ESP if the total effective length of the plates in the ESP is smallcompared to their effective height. For example, the time required for dust to fall from thetop of a 9.1-m plate (30-ft plate) is several seconds. Effective plate lengths must be at least10.7 to 12.2 m (35 to 40 ft) to prevent a large amount of "collected dust" from being car-ried out of the ESP before reaching the hopper.

SCAtotal collection surface

gas flow rate------------------------------------------------------=

SCAtotal collection surface in m2

1000 m3 h⁄---------------------------------------------------------------------=

SCAtotal collection surface in ft2

1000 ft3 min⁄--------------------------------------------------------------------=

SCA600,000 ft2

1000 (1000 acfm)-------------------------------------------=

600 ft2

1000 acfm---------------------------=

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The aspect ratio is the ratio of the effective length to the effective height of the collectorsurface. The aspect ratio can be calculated using Equation 3-7.

(3-7)

The effective length of the collection surface is the sum of the plate lengths in each con-secutive field and the effective height is the height of the plates. For example, if an ESPhas four fields, each containing plates that are 10 feet long, the effective length is 40 feet.If the height of each plate is 30 feet, the aspect ratio is 1.33 as shown below:

Aspect ratios for ESPs range from 0.5 to 2.0. However, for high-efficiency ESPs (thosehaving collection efficiencies of > 99%), the aspect ratio should be greater than 1.0 (usu-ally 1.0 to 1.5) and in some installations may approach 2.0.

Gas Flow Distribution

Gas flow through the ESP chamber should be slow and evenly distributed through theunit. Gas velocity is reduced by the expansion, or diverging, section of the inlet plenum(Figure 3-7). The gas velocities in the duct leading into the ESP are generally between 12and 24 m/s (40 and 80 ft/sec). The gas velocity into the ESP must be reduced to0.6-2.4 m/s (2-8 ft/sec) for adequate particle collection. With aspect ratios of 1.5, the opti-mum gas velocity is generally between 1.5 and 1.8 m/s (5 and 6 ft/sec).

Figure 3-7. Gas inlet with perforated diffuser plates

AReffective length, m (ft)effective height, m (ft)------------------------------------------------------=

AR10 ft 10 ft 10 ft 10 ft+ + +

30 ft---------------------------------------------------------------=

40 ft30 ft-----------=

1.33=

Perforateddiffuserplates

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Lesson 3

3-22

In order to use all of the discharge and collection electrodes across the entire width of theESP, the flue gas must be evenly distributed. The inlet plenum contains perforated open-ings, called diffuser plate openings to evenly distribute the gas flow into the chambersformed by the plates in the precipitator.

Corona Power

As stated previously, a strong electric field is needed for achieving high collection effi-ciency of dust particles. The strength of the field is based on the rating of the T-R set. Thecorona power is the power that energizes the discharge electrodes and thus creates thestrong electric field. The corona power used for precipitation is calculated by multiplyingthe secondary current by the secondary voltage and is expressed in units of watts. In ESPdesign specifications, the corona power is usually given in units of watts per 1000 m3/h(watts per 1000 acfm). Corona power expressed in units of watts/1000 acfm is also calledthe specific corona power. Corona power for any bus section of an ESP can be calculatedby the following approximate relation:

(3-8)

Where: Pc = corona power, wattsVp = peak voltage, voltsVm = minimum voltage, voltsIc = average corona current, amperes

As you can see, corona power increases as the voltage and/or current increases. The totalcorona power of the ESP is the sum of the corona power for all of the individual T-R sets.In an ESP, the collection efficiency is proportional to the amount of corona power suppliedto the unit, assuming the corona power is applied effectively (maintains a good sparkingrate).

(3-9)

Where: η = collection efficiencye = base of natural logarithm = 2.718k = a constant, usually between 0.5 and 0.7Pc/Q = corona power density in units of watts per 1000 m3/hr

(watts per 1000 acfm)

From equation 3-9, you can see that for a given exhaust flow rate, the collection efficiencywill increase as the corona power is increased. This efficiency will depend on the operat-ing conditions of the ESP and on whether the amount of power has been applied effec-tively. For high collection efficiency, corona power is usually between 59 and 295 wattsper 1000 m3/h (100 and 500 watts per 1000 acfm). Recent ESP installations have beendesigned to use as much as 470 to 530 watts per 1000 m3/h (800 to 900 watts per 1000acfm).

Pc 1 2⁄ Vp Vm+( )Ic=

η 1 ekPc Q⁄–

–∝

2.0-2/98


The terms current density and power density are also used to characterize the design of theESP. Current density is the secondary current supplied by the T-R set for the given platearea and expressed in units of mA/ft2 of plate area. Power density is the corona powersupplied to the plate area and is expressed in units of watts per ft2 of plate area.

The size of the individual power sets (T-R sets) in the ESP will vary depending on theirspecific location and the conditions of the flue gas such as particle size, dust concentra-tion, dust resistivity, and flue gas temperature. In an ESP, the T-R sets are selected to pro-vide lower current density at the inlet sections, where the dust concentration will tend tosuppress the corona current, and to provide higher current density at the outlet sections,where there is a greater percentage of fine particles.

Summary

ESPs can be designed using a number of techniques including mathematical equations, pilotplant studies, and computer modeling programs. The use of pilot plant studies is very effectivebut is not often used because of time limitations and the expense of construction. Use of com-puter models is therefore becoming more common for both the initial design and for trouble-shooting of existing ESPs.

During this lesson we covered a number of equations. The equation for particle migrationvelocity depends on the voltage strength of both the charging and collection fields and on theparticle size. The Deutsch-Anderson and Matts-Ohnfeldt equations can be used to estimatecollection efficiency in an ESP. The Deutsch-Anderson equation assumes that all particles inthe flue gas have the same migration velocity, and that particles do not become reentrained ordo not sneak through the hopper sections. The Deutsch-Anderson equation can be modified byusing field data to determine the effective migration velocity.

The Matts-Ohnfeldt equation also uses information obtained from similar ESP field installa-tions. Use of both the modified Deutsch-Anderson and the Matts-Ohnfeldt equations will typi-cally yield more accurate estimates for collection efficiency.

We also covered operating parameters that affect the collection efficiency of the ESP includingthe following:

• Resistivity

• Sectionalization

• Corona power

• Aspect ratio

• Specific collection area (SCA)

These parameters will be discussed in more detail in Lessons 4 and 6.

Careful design of the ESP involves consideration of the important operating parameters tokeep the unit operating efficiently and effectively. Not only will this help an industry complywith air pollution regulations, but a good design up-front will also reduce plant downtime andkeep maintenance problems to a minimum.

2.0-2/98 3-23

Lesson 3

3-24

Review Exercise

1. A charged particle will migrate toward an oppositely charged collection electrode. The velocity atwhich the charged particle moves toward the collection electrode is called the____________________ ____________________ and is denoted by the symbol w.

2. What is the name of the equation given below?

a. Johnstone equationb. Matts-Ohnfeldt equationc. Deutsch-Anderson equationd. Beachler-Joseph equation

3. The symbol η in the Deutsch-Anderson equation is the:

a. Collection areab. Migration velocityc. Gas flow rated. Collection efficiency

4. The Deutsch-Anderson equation does not account for:

a. Dust reentrainment that may occur as a result of rappingb. Varying migration velocities due to various-sized particles in the flue gasc. Uneven gas flow through the precipitatord. All of the above

5. True or False? Using the Matts-Ohnfeldt equation to estimate the collection efficiency of an ESPwill give less conservative results than using the Deutsch-Anderson equation.

6. Resistivity is a measure of a particle’s resistance to ____________________ and____________________ charge.

7. Dust resistivity is a characteristic of the particle in the flue gas that can alter the____________________ of an ESP.

a. Gas flow rateb. Collection efficiencyc. Gas velocity

8. Dust particles with ____________________ resistivity are difficult to remove from collectionplates, causing rapping problems.

a. Lowb. Normalc. High

( )η = −1 e-w A/Q

2.0-2/98


9. High dust resistivity can be reduced by:

a. Adjusting the flue gas temperatureb. Increasing the moisture content of the flue gasc. Injecting SO3 into the flue gasd. All of the above

10. True or False? Fly ash that results from burning high-sulfur coal generally has high resistivity.

11. A precipitator is divided into a series of independently energized bus sections called:

a. Hoppersb. Fieldsc. Stagesd. b and c, above

12. In the following figure there are ____________________ fields and ____________________cells.

a. Two, fourb. Four, eightc. Eight, twod. Eight, four

13. A precipitator should be designed with at least ____________________ field(s) to attain a highcollection efficiency.

a. Oneb. Twoc. Three or fourd. Ten

14. Electrical sectionalization improves collection efficiency by:

a. Improving resistivity conditionsb. Allowing for independent voltage control of different fieldsc. Allowing continued ESP operation in the event of electrical failure in one of the fieldsd. b and c, above

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Lesson 3

3-26

15. If the design of the precipitator states that 500,000 ft2 of plate area is used to remove particles fromflue gas flowing at 750,000 ft3/min, what is the SCA of the unit?

a. 0.667 ft2/1000 acfmb. 667 ft2/1000 acfmc. 667 acfm/1000 ft2

d. 1.5 acfm/ft2

16. To achieve a collection efficiency greater than 99.5%, most ESPs have a SCA:

a. Less than 250 ft2/1000 acfmb. Between 350 and 400 ft2/1000 acfmc. Always greater than 800 ft2/1000 acfm

17. To improve the aspect ratio of an ESP design, the ____________________ of the collection sur-face should be increased relative to the ____________________ of the plate.

a. Height; lengthb. Length; height

18. Given an ESP having a configuration as shown below, what is the aspect ratio of this unit?

a. 0.33b. 1.5c. 0.75d. 1.33

19. What should the aspect ratio be for high-efficiency ESPs?

a. Less than 0.8b. Greater than 1.0c. Always greater than 1.5

20. In a properly designed ESP, the gas velocity through the ESP chamber will be:

a. Between 2 and 8 ft/secb. Greater than 20 ft/secc. Approximately between 20 and 80 ft/secd. At least 400 ft2/1000 acfm

10 ft 15 ft 15 ft

30 ft30 ft

2.0-2/98


21. In an ESP, the collection efficiency is proportional to the amount of ________________________________________ supplied to the unit.

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Lesson 3

3-28
2.0-2/98


Review Exercise Answers1. Migration velocity (or drift velocity)

The velocity at which the charged particle moves toward the collection electrode is called themigration velocity (or drift velocity) and is denoted by the symbol w.

2. c. Deutsch-Anderson equation

The following equation, , is the Deutsch-Anderson equation.

3. d. Collection efficiencyThe symbol η in the Deutsch-Anderson equation is the collection efficiency.

4. d. All of the aboveThe Deutsch-Anderson equation does not account for the following:

• Dust reentrainment that may occur as a result of rapping

• Varying migration velocities due to various-sized particles in the flue gas

• Uneven gas flow through the precipitator

5. FalseUsing the Matts-Ohnfeldt equation to estimate the collection efficiency of an ESP will give moreconservative results than using the Deutsch-Anderson equation because the Matts-Ohnfeldt equa-tion accounts for non-ideal effects.

6. AcceptingReleasingResistivity is a measure of a particle’s resistance to accepting and releasing charge.

7. b. Collection efficiencyDust resistivity is a characteristic of the particle in the flue gas that can alter the collection effi-ciency of an ESP.

8. c. HighDust particles with high resistivity are difficult to remove from collection plates, causing rappingproblems.

9. d. All of the aboveHigh dust resistivity can be reduced by the following:

• Adjusting the flue gas temperature

• Increasing the moisture content of the flue gas

• Injecting SO3 into the flue gas

10. FalseFly ash that results from burning high-sulfur coal generally has low resistivity. SO3, which lowersthe resistivity of fly-ash, normally increases as the sulfur content of the coal increases.

( )η = −1 e-w A/Q

2.0-2/98 3-29

11. d. b and c, aboveA precipitator is divided into a series of independently energized bus sections called fields orstages.

12. b. Four, eight

In the above figure there are four fields and eight cells.

13. c. Three or fourA precipitator should be designed with at least three or four fields to attain a high collection effi-ciency.

14. d. b and c, aboveElectrical sectionalization improves collection efficiency by allowing the following:

• Independent voltage control of different fields• Continued ESP operation in the event of electrical failure in one of the fields

15. b. 667 ft2/1000 acfmIf the design of the precipitator states that 500,000 ft2 of plate area is used to remove particles fromflue gas flowing at 750,000 ft3/min, the SCA of the unit is as follows:

16. b. Between 350 and 400 ft2/1000 acfmTo achieve a collection efficiency greater than 99.5%, most ESPs have a SCA between 350 and400 ft2/1000 acfm.

17. b. Length; heightTo improve the aspect ratio of an ESP design, the length of the collection surface should beincreased relative to the height of the plate.

SCA500,000 ft2( )

750 1000 acfm( )-----------------------------------------=

667 ft2 1000 acfm⁄=

2.0-2/98 3-30

3-31

Bibliography

18. d. 1.33

An ESP with the above configuration has the following aspect ratio:

19. b. Greater than 1.0The aspect ratio for high-efficiency ESPs should be greater than 1.0.

20. a. Between 2 and 8 ft/secIn a properly designed ESP, the gas velocity through the ESP chamber will be between2 and 8 ft/sec, and most often between 4 and 6 ft/sec.

21. Corona powerIn an ESP, the collection efficiency is proportional to the amount of corona power supplied to theunit.

10 ft 15 ft 15 ft

30 ft30 ft

AR10 15 15+ +

30------------------------------=

4030------=

1.33=

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Lesson 3

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Bibliography

Anderson, E. 1924. Report, Western Precipitator Co., Los Angeles, CA. 1919. Transactions of theAmerican Institute of Chemical Engineers. 16:69.

Beachler, D. S., J. A. Jahnke, G. T. Joseph and M. M. Peterson. 1983. Air Pollution Control Systems forSelected Industries, Self-Instructional Guidebook. (APTI Course SI:431). EPA 450/2-82-006. U.S.Environmental Protection Agency.

Deutsch, W. 1922. Annals of Physics. (Leipzig) 68:335.


Hall, H. J. 1975. Design and application of high voltage power supplies in electrostatic precipitation.Journal of Air Pollution Control Association. 25:132.

Hesketh, H. E. 1979. Air Pollution Control. Ann Arbor: Ann Arbor Science Publishers.


Lawless, P. 1992. ESPVI 4.0, Electrostatic Precipitator V-I and Performance Model: Users’ Manual.EPA 600/R-29-104a.

Matts, S., and P. O. Ohnfeldt. 1964. Efficient Gas Cleaning with SF Electrostatic Precipitators. Flak-ten.

Richards, J. R. 1995. Control of Particulate Emissions, Student Manual. (APTI Course 413). U.S.Environmental Protection Agency.

Rose, H. E., and A. J. Wood. An Introduction to Electrostatic Precipitation in Theory and Practice.London: Constable and Company.

Schmidt, W. A. 1949. Industrial and Engineering Chemistry. 41:2428.

Theodore, L., and A. J. Buonicore. 1976. Industrial Air Pollution Control Equipment forParticulates. Cleveland: CRC Press.

U.S. Environmental Protection Agency. 1978, June. A Mathematical Model of Electrostatic Precipita-tion (Revision 1). Vol. 1, Modeling and Programming. EPA 600/7-78-llla.

U.S. Environmental Protection Agency. 1978, June. A Mathematical Model of Electrostatic Precipita-tion (Revision 1). Vol. II, User Manual. EPA 600/7-78-lllb.

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U.S. Environmental Protection Agency. 1979. Particulate Control by Fabric Filtration on Coal-FiredIndustrial Boilers. EPA 625/2-79-021.

U.S. Environmental Protection Agency. 1980, May. TI-59 Programmable Calculator Programs for In-stack Opacity, Venturi Scrubbers, and Electrostatic Precipitators. EPA 600/8-80-024.


White, H. J. 1963. Industrial Electrostatic Precipitation. Reading, MA: Addison-Wesley.

White, H. J. 1974. Resistivity problems in electrostatic precipitation. Journal of Air Pollution ControlAssociation 24:315-338.


White, H. J. 1982. Review of the state of the technology. Proceedings of the International Conferenceon Electrostatic Precipitation. Monterey, CA, October 1981. Air Pollution Control Corporation,Pittsburgh, PA.

2.0-2/98

Lesson 4ESP Design Review

Goal

To familiarize you with the factors to be considered when reviewing ESP design plans for the per-mit process.

Objectives


1. Explain how each of the following dust properties affects ESP performance:

• Dust type (chemical composition)• Size• Concentration in gas stream• Resistivity

2. Explain how each of the following flue gas properties affects ESP performance:

• Gas flow rate• Temperature• Moisture content• Chemical properties (dew point, corrosiveness, and combustibility)

3. Identify important design considerations for discharge electrodes, collection electrodes, andhopper and discharge devices

4. Explain how each of the following factors contributes to good ESP design:

• Electrical sectionalization• Specific collection area• Aspect ratio• Distribution of gas flow

5. Estimate the collection area and the collection efficiency for a given process flow rate andmigration velocity

6. Estimate the capital and operating cost of an ESP using tables and figures

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Lesson 4

4-2

Introduction

As discussed in Lessons 2 and 3, finalizing the design of an electrostatic precipitator and itscomponents involves consideration of many factors. Air pollution control agency officers whoreview ESP design plans should consider these factors during the review process. Some ofthese factors relate to the properties of the dust and flue gas being filtered, while others applyto the specific ESP design:

• Type of discharge electrode

• Type of collection electrode

• Electrical sectionalization (number of fields and individual power supplied used


• Aspect ratio

Construction details, such as shell insulation, inlet location, hopper design, and dust dischargedevices are also important.

This lesson reviews the ESP design parameters, along with typical ranges for these variables.It also familiarizes you with cost information for various ESP designs so that you can be awareof cost when reviewing design plans and making recommendations.

Review of Design Variables

The principal design variables are the dust concentration, measured in g/m3 (lb/ft3 or gr/ft3)and the gas flow rate to the ESP, measured in m3/min (ft3/min or acfm). The gas volume anddust concentration (loading) are set by the process exhaust gas flow rate. Once these variablesare known, the vendor can begin to design the precipitator for the specific application. A thor-ough review of ESP design plans should consider the factors presented below.

Physical and chemical properties of the dust such as dust type, size of the dust particles, andaverage and maximum concentrations in the gas stream are important ESP design consider-ations. The type of dust to be collected in the ESP refers to the chemical characteristics of thedust such as explosiveness. For example, a dry ESP should not be used to collect explosivedust. In this case, it might be a better idea to use a baghouse or scrubber. Particle size is impor-tant; small particles are more difficult to collect and become reentrained more easily thanlarger particles. Additional fields may be required to meet regulatory limits. The dust loadingcan affect the operating performance. If the dust concentration is too high, the automatic volt-age controller may respond by totally suppressing the current in the inlet fields. Suppressedcurrent flow drives the voltage up, which can cause sparking. For this reason, it might be agood idea to install a cyclone or multicyclone to remove larger particles and reduce the dustconcentration from the flue gas before it enters the ESP. The facility could install a larger ESP(with more plate area), however, this technique would be more costly.

Resistivity is a function of the chemical composition of the dust, the flue gas temperature andmoisture concentration. For fly ash generated from coal-fired boilers, the resistivity dependson the temperature and moisture content of the flue gas and on the sulfur content of the coalburned; the lower the sulfur content, the higher the resistivity, and vice versa. If a boiler burnslow-sulfur coal, the ESP must be designed to deal with potential resistivity problems. As pre-viously stated in Lesson 3, high resistivity can be reduced by spraying water, SO3 or someother conditioning agent into the flue gas before it enters the ESP.

2.0-2/98

ESP Design Review

Predicting the gas flow rate and gas stream properties is essential for proper ESP design.The average and maximum gas flow rates through the ESP, the temperature, moisture content,chemical properties such as dew point, corrosiveness, and combustibility of the gas should beidentified prior to final design. If the ESP is going to be installed on an existing source, a stacktest should be performed to determine the process gas stream properties. If the ESP is beinginstalled on a new source, data from a similar plant or operation may be used, but the ESPshould be designed conservatively (with a large SCA, a high aspect ratio, and high coronapower). Once the actual gas stream properties are known, the designers can determine if theprecipitator will require extras such as shell insulation for hot-side ESPs, corrosion-proof coat-ings, and installation of heaters in hoppers or ductwork leading into and out of the unit.

The type of discharge electrodes and electrode support are important. Small-diameter wiresshould be firmly supported at the top and connected to a weight heavy enough (11.4-kgweights for 9.1-m wires) to keep the wires from swaying. The bottom and top of each wireshould be covered with shrouds to help minimize sparking and metal erosion at these points.Newer ESPs are generally using rigid-frame or rigid-electrode discharge electrodes.

Collection electrodes—type (either tube or plate), shape of plates, size, and mechanicalstrength—are then chosen. Plates are usually less than 9 m (30 ft) high for high-efficiencyESPs. For ESPs using wires, the spacing between collection plate electrodes usually rangesfrom 15 to 30 cm (6 to 12 in.). For ESPs using rigid-frame or rigid electrodes, the spacing istypically 30 to 38 cm (12 to 15 inches). Equal spacing must be maintained between platesthroughout the entire precipitator. Stiffeners may be used to help prevent the plates from warp-ing, particularly when hot-side precipitators are used.

Proper electrical sectionalization is important to achieve high collection efficiency in theESP. Electrical sectionalization refers to the division of a precipitator into a number of differ-ent fields and cells, each powered by its own T-R set. ESPs should have at least three to fourfields to attain a high collection efficiency. In addition, the greater the number of fields the bet-ter the chance that the ESP will achieve the designed collection efficiency. There should beapproximately one T-R set for every 930 to 2970 m2 (10,000 to 30,000 ft2) of collection-platearea.

The specific collection area (SCA) is the collection area, in m2 per 1000 m3/h (ft2 per 1000ft3/min), of flue gas through the precipitator. The typical range for SCA is between 11 and 45m2 per 1000 m3/h (200 and 800 ft2 per 1000 acfm). The SCA must be large enough to effi-ciently collect particles (99.5% collection efficiency), but not so large that the cost of the ESPis too high. If the dust has a high resistivity, vendors will generally design the ESP with ahigher SCA [usually greater than 22 m2 per 1000 m3/h (400 ft2 per 1000 acfm)] to help reduceresistivity problems.

Aspect ratio is the ratio of effective length to height of the collector surface. The aspect ratioshould be high enough to allow the rapped particles to settle in the hopper before they are car-ried out of the ESP by the gas flow. The aspect ratio is usually greater than 1.0 for high-effi-ciency ESPs. Aspect ratios of 1.3 to 1.5 are common, and they are occasionally as high 2.0.

Even distribution of gas flow across the entire precipitator unit is critical to ensure collectionof the particles. To assure even distribution, gas should enter the ESP through an expansioninlet plenum containing perforated diffuser plates (see Figure 3-7). In addition, the ducts lead-ing into the ESP unit should be straight as shown in Figure 4-1. For ESPs with straight-line

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Lesson 4

4-4

inlets, the distance of A should be at least as long as the distance of B in the inlet (Katz 1979).In situations where a straight-line inlet is not possible and a curved inlet must be used (see Fig-ure 4-2), straightening vanes should be installed to keep the flue gas from becoming stratified.The gas velocity through the body of the ESP should be approximately 0.6 to 2.4 m/s (2 to 8 ft/sec). For ESPs having aspect ratios of 1.5, the optimum gas velocity is usually between 1.5and 1.8 m/s (5 and 6 ft/sec). The outlet of the ESP should also be carefully designed to provideeven flow of the gas from the ESP to the stack without excessive pressure buildup. This can bedone by using an expansion outlet, as shown in Figure 4-3. Figures 4-1 and 4-2 also haveexpansion outlets.

Figure 4-1. Straight-line inlet

Figure 4-2. Straightening vanes in a curved inlet

Figure 4-3. ESP with expansion outlet

B

A

Expansioninlet

Straighteningvanes

Expansionoutlet

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ESP Design Review

The hopper and discharge device design including geometry, size, dust storage capacity,number, and location are important so that dust is removed on a routine basis. A well-designeddust hopper is sloped (usually 60°) to allow dust to flow freely to discharge devices. Itincludes access ports and strike plates to help move dust that becomes stuck. Dust should beonly temporarily stored in the hopper and removed periodically by the discharge devices toprevent it from backing up into the ESP where it can touch the plates, possibly causing a cellto short out. In addition to the amount of fly ash present, there are a couple of special consider-ations to keep in mind when ESPs are used on coal-fired boilers. First, the amount of fly ash inthe flue gas can vary depending on what type of coal is burned and the ash content of the coal.Coal having a higher ash content will produce more fly ash than coal having lower ash values.Consequently, the discharge device must be designed so that the operator can adjust the fre-quency of fly ash removal. Second, hoppers need to be insulated to prevent ash from "freez-ing," or sticking, in the hopper.

Finally, emission regulations in terms of opacity and dust concentration (grain-loading)requirements will ultimately play an important role in the final design decisions. Electrostaticprecipitators are very efficient; collection efficiency can usually be greater than 99% if theESP is properly designed and operated.

Typical Ranges of Design Parameters

While reviewing a permit for ESP installation, check whether the design specifications arewithin the range that is typically used by that industry. The ranges of basic design parametersfor fly ash precipitators are given in Table 4-1.

Table 4-1. Typical ranges of design parameters for fly ashprecipitators

Parameter Range (metric units) Range (English units)

Distance between plates(duct width)

Gas velocity in ESP

SCA

Aspect ratio (L/H)

Particle migration velocity

Number of fields

Corona power/flue gasvolume

Corona current/ft2 platearea

Plate area per electrical (T-R) set

20-30 cm (20-23 cm optimum)

1.2-2.4 m/s (1.5-1.8 m/s optimum)

11-45 m2/1000 m3/h(16.5-22.0 m2/1000 m3/h optimum)

1-1.5 (keep plate height less than9 m for high efficiency)

3.05-15.2 cm/s

4-8

59-295 watts/1000 m3/h

107-860 microamps/m2

465-7430 m2/T-R set(930-2790 m2/T-R set optimum)

8-12 in. (8-9 in. optimum)

4-8 ft/sec (5-6 ft/sec optimum)

200-800 ft2/1000 cfm(300-400 ft2/1000 cfm optimum)

1-1.5 (keep plate height less than30 ft for high efficiency)

0.1-0.5 ft/sec

4-8

100-500 watts/1000 cfm

10-80 microamps/ft2

5000-80,000 ft2/T-R set (10,000-30,000 ft2/T-R set optimum)

Source: White 1977.

2.0-2/98 4-5

Lesson 4

4-6

Estimating Collection Efficiency and Collection Area

The manufacturer designs and sizes the electrostatic precipitator. However, the operator (orreviewer) needs to check or estimate the collection efficiency and the amount of collectionarea required for a given process flow rate. You can compute these estimates by using theDeutsch-Anderson or Matts-Ohnfeldt equations (see Lesson 3). These equations are repeatedin Table 4-2.

Table 4-2. Equations used to estimate collection efficiencyand collection area

Calculation Deutsch-Anderson Matts-Ohnfeldt

Collection efficiency

Collection area (to meet arequired efficiency)

Where: η = collection efficiencyA = collection areaw = migration velocityQ = gas flow rateln = natural logarithm

η = collection efficiencyA = collection areawk = average migration

velocityk = constant (usually 0.5)ln = natural logarithm

η 1 e w A Q⁄( )––=

AQ–

w------- ln 1 η–( )[ ]=

η 1 ewk A Q⁄( )k

––=

AQwk

------ k

– ln 1 η–( )[ ]1 k⁄

=

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ESP Design Review

Example Estimation

The exhaust rate of the gas being processed is given as 1,000,000 ft3/min. The inlet dustconcentration in the gas as it enters the ESP is 8 gr/ft3. If the emission regulations statethat the outlet dust concentration must be less than 0.04 gr/ft3, how much collection area isrequired to meet the regulations? Use the Deutsch-Anderson equation for this calculationand assume the migration velocity is 0.3 ft/sec.

1. From Table 4-2, use this version of the Deutsch-Anderson equation to solve theproblem:

Where: A = collection area, ft2

Q = gas flow rate, ft3/secw = migration velocity, ft/secη = collection efficiencyln = natural logarithm

In this example,

Q = 1,000,000 ft3/min × 1 min/60 sec= 16,667 ft3/sec

w = 0.3 ft/sec

2. Calculate the collection efficiency, η.

3. Calculate the collection area, A, in ft2.

AQ–

w------- ln 1 η–( )[ ]=

ηdustin dustout–

dustin

-----------------------------------=

8 gr ft3⁄ 0.04 gr ft3⁄–

8 gr ft3⁄------------------------------------------------------=

0.995 or 99.5%=

A16,667– ft3 sec⁄

0.3 ft sec⁄--------------------------------------- 1 0.995–( )ln[ ]=

55,557 ft– 2( ) 5.2983–[ ]×=

294,358 ft2=

2.0-2/98 4-7

Lesson 4

4-8

Estimating Capital and Operating Costs

This section contains generalized cost data for ESP systems described throughout this guide-book. These data should be used only as an estimate to determine system cost. The total capitalinvestment (TCI) includes costs for the ESP structure, the internals, rappers, power supply,and auxiliary equipment, and the usual direct and indirect costs associated with installing orerecting new structures. These costs, given in second-quarter 1987 dollars, are described in thefollowing subsections.

ESP Equipment Cost

Most of the following cost discussion is taken from the EPA OAQPS Cost Control Manual(1990). Costs for rigid-electrode, wire and plate, and flat-plate ESPs can be estimatedusing Figure 4-4. Costs for two-stage precipitators are given later.

Figure 4-4 represents two cost curves (the two in the middle) along with their respectiveequations (outer lines with arrows). Each curve requires two equations for calculatingcost: one for total plate areas between 10,000 and 50,000 ft2 and another for total plateareas between 50,000 and 1,000,000 ft2. The lower curve shows the cost for the basic unitwithout the standard options. It represents the flange-to-flange, field-erected price for arigid-electrode design. The upper curve includes all of the standard options (listed in Table4-3) that are normally used in a modern system. All units (both curves) include the ESPcasing, pyramidal hoppers, rigid electrodes and internal collection plates, transformer-rec-tifier (T-R) sets and microprocessor controls, rappers, and stub supports (legs) for 4-footclearance below the hopper discharges. The costs are based on a number of actual quotesthat have been fitted to lines using the “least squares” method. Don’t be surprised if youobtain quotes that differ from these curves by as much as ±25%. (Significant savings canbe obtained by solicitating multiple quotes.) The equations should not be used to extrapo-late costs for total plates areas below 10,000 or above 1,000,000 ft2. The standard optionsincluded in the upper curve add approximately 45% to the basic cost of the flange-to-flange hardware. Insulation costs are for 3 inches of field-installed glass fiber encased in ametal skin and applied on the outside of all areas in contact with the exhaust gas stream.Calculate insulation for ductwork, fan casings, and stacks separately. To obtain more accu-rate results, solve the equations for the lines instead of reading the values from the graph.

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ESP Design Review

Figure 4-4. Dry-type rigid electrode ESP flange-to-flange purchase priceversus plate area

Impact of Alternative Electrode Designs

All three designs—rigid electrode, weighted wire, and rigid frame—can be employed inmost applications. Any cost differential between designs will depend on the combinationof vendor experience and site-specific factors that dictate equipment size factors. Therigid-frame design will cost up to 25% more than the wire and plate design if the plateheight is restricted to that used in wire/plate designs. Several vendors can now providerigid-frame ESPs with taller plates, and thus the cost differential can approach zero.

The weighted wire design uses narrower plate spacings and more internal discharge elec-trodes. This design is being used less; therefore, its cost is increasing and currently is

Table 4-3. Standard options for basic equipment

Item Cost Adder, %

1. Inlet and outlet nozzles and diffuser plates2. Hopper auxiliaries/heaters, level detectors3. Weather enclosure and stair access4. Structural supports5. InsulationTotal options 1 to 5

8 to 108 to 108 to 1058 to 101.37 to 1.45 × base

2.0-2/98 4-9

Lesson 4

4-10

approximately the same as that for the rigid electrode ESP. Below about 15,000 ft2 of platearea, ESPs are not normally field-erected (erected at the installation site), and the costswill probably be higher than values extrapolated from Figure 4-4.

Impact of Materials of Construction:Metal Thickness and Stainless Steel

Corrosive or other adverse operating conditions may require specifications of thickermetal sections in the precipitator. Metal thickness can be moderately increased with mini-mal cost increases. For example, collection plates are typically constructed of 18-gaugemild steel. Most ESP manufacturers can increase the section thickness by 25% withoutsignificant design changes or increases in manufacturing costs of more than a few percent.

Changes in the type of material can increase the purchase cost of the ESP significantly.Using type 304 stainless steel instead of 18-gauge mild steel for collection plates and pre-cipitator walls can increase costs 30-50%. Using even more expensive materials for allelements of the ESP can increase costs up to several hundred percent. Based on the carbonsteel 18-gauge cost, the approximate factors given below can be used for other materials.

Recent Trends

Most of today's market (1987) is in the 50,000 to 200,000 ft2 plate area size range. ESPselling prices have increased very little over the past 10 years because of more effectivedesigns, increased competition from European suppliers, and a shrinking utility market.

Design improvements have allowed wider plate spacings that reduce the number of inter-nal components and higher plates and masts that provide additional plate area at a lowcost. Microprocessor controls and energy management systems have lowered operatingcosts.

Few, if any, hot-side ESPs (those used upstream from an air preheater on a combustionsource) are being specified for purchase. Recognition that low-sodium coals tend to buildresistive ash layers on the collection plates, thus reducing ESP efficiency, has almost elim-inated sales of hot-side units. Of the 150 existing units, about 75 are candidates for con-version to cold-side units (using resistivity conditioning agents) over the next 10 years(U.S. EPA 1990).

Table 4-4. ESP costs using various materials

Factor Material

1.01.31.71.92.33.24.5

Carbon Steel, 18-gaugeStainless Steel, 304Stainless Steel, 316Carpenter 20 CB-3Monel-400Nickel-200Titanium


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ESP Design Review

Specific industry application has little impact on either ESP design or cost, with the fol-lowing three exceptions: paper mills, sulfuric acid manufacturing plants, and coke by-product plants. Because paper mills have dust that can be sticky and difficult to remove,paper mill ESPs use drag conveyer hoppers. These hoppers increase the cost by approxi-mately 10 percent of the base flange-to-flange equipment cost. For emissions control insulfuric acid plants and coke by-product ovens, wet ESPs are used. In sulfuric acid manu-facture, wet ESPs are used to collect acid mist. These precipitators usually are small anduse lead for all interior surfaces; hence, they normally cost $65 to $95/ft2 of collectingarea installed (mid-1987 dollars) and up to $120/ft2 in special situations. Using Figure 4-4,the standard cost for a rigid-frame ESP ranges from $7 to $14/ft2 of collecting area. Inaddition, a wet circular ESP is typically used to control emissions from a coke oven off-gas detarring operation. These precipitators are made from high-alloy stainless steels andtypically cost $90 to $120/ft2 installed. Because of the small number of sales, small size ofunits sold, and dependency of site-specific factors, more definitive costs are not available.

Retrofit Cost Factor

Retrofit installations increase the cost of an ESP because of the frequent need to removesomething to make way for the new ESP. Also, the ducting usually is much more expen-sive as a retrofit application because the ducting path is often constrained by existingstructures, additional supports are required, and the confined areas make erection morelabor intensive and lengthy. Costs are site-specific; however, for estimating purposes, aretrofit multiplier of 1.3 to 1.5 applied to the total capital investment can be used. Themultiplier should be selected within this range based on the relative difficulty of the instal-lation.

A special case is the conversion of a hot-side to a cold-side ESP for coal-fired boiler appli-cations. The magnitude of the conversion is very site-specific, but most projects will con-tain the following elements:

• Relocating the air preheater and the ducting to it

• Resizing the ESP inlet and outlet duct to the new air volume and rerouting it

• Upgrading the ID (induced draft) fan size or motor to accommodate the higher staticpressure and horsepower requirements

• Adding or modifying foundations for fan and duct supports

• Assessing the required SCA and either increasing the collecting area or installing anSO3 gas-conditioning system

• Adding hopper heaters

• Upgrading the analog electrical controls to microprocessor-type controls

• Increasing the number of collecting plate rappers and perhaps the location of rappers

In some installations, it may be cost-effective to gut the existing collector totally, utilizeonly the existing casing and hoppers, and upgrade the ESP using modern internal compo-nents. The cost of conversion is a multimillion dollar project typically running at least 25to 35 percent of the total capital investment of a new unit.

2.0-2/98 4-11

Lesson 4

4-12

Costs for Two-Stage Precipitators

Purchase costs for modular, two-stage precipitators should be considered separately fromlarge-scale, single-stage ESPs (see Figure 4-5). To be consistent with industry practice,costs are given as a function of flow rate through the system. The lower cost curve is for atwo-cell unit without a precooler, installed cell washer, and a fan. The upper curve is foran engineered package system with the following components: inlet diffuser plenum, pre-filter, cooling coils with coating, coil plenums with access, water-flow controls, triple-passconfiguration, system exhaust fan with accessories, outlet plenum, and in-place foamcleaning system with semiautomatic control and programmable controller. All equipmentis fully assembled mechanically and electrically, and it is mounted on a steel structuralskid.

Figure 4-5. Purchase costs for two-stage, two-cell precipitators

Total Purchase Cost

The total purchase cost of an ESP system is the sum of the costs of the ESP, options, aux-iliary equipment, instruments and controls, taxes, and freight. The last three items gener-ally are taken as percentages of the estimated total cost of the first three items. Typicalvalues are 10% for instruments and controls, 3% for taxes, and 5% for freight.

Costs of standard and other options can vary from 0% to more than 150% of ESP basecost, depending on site and application requirements. Other factors that can increase ESPcosts are given in Table 4-5.

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ESP Design Review

Total Capital Investment

Total capital investment (TCI) is estimated from a series of factors applied to the pur-chased equipment cost (PEC) to obtain direct and indirect costs for installation. The TCI isthe sum of the direct costs (equipment and installation) and indirect costs. The requiredfactors are given in Table 4-6. Because ESPs can vary from small units attached to exist-ing buildings to large, separate structures, specific factors for site preparation or for build-ings are not given. However, costs for buildings and materials may be obtained fromreferences such as Means Square Foot Costs 1987. Land, working capital, and off-sitefacilities are excluded from the table because they are required only for very large installa-tions. However, they can be estimated on an as-needed basis.

Note that the factors given in Table 4-6 are for average installation conditions, and forexample, include no unusual problems with site earthwork, access, shipping, or interferingstructures. Considerable variation may be seen with other-than-average installation cir-cumstances. For two-stage precipitators purchased as packaged systems, several of thecosts in Table 4-6 would be greatly reduced or eliminated. These include instruments andcontrols, foundations and supports, erection and handling, painting, and model studies. Aninstallation factor of 0.25 of the PEC (instead of 0.67 PEC) would be more nearly appro-priate for the two-stage ESPs.

Table 4-5. Items that increase ESP costs

Item Factor orTotal Cost

Applied to

Rigid-frame electrode with restricted plate height

Type 304 stainless-steel collector plates andprecipitator walls

All-stainless construction

ESP with drag conveyor hoppers (paper mill)

Retrofit installations

Wet ESPSulfuric acid mist

Sulfuric acid mist (special installation)

Coke oven off-gas

1.0-1.25

1.3-1.5

2-3

1.1

1.3-1.5

$65-$95/ft2

Up to $120/ft2

$90-$120/ft2

ESP base cost

ESP base cost

ESP base cost

ESP base cost

ESP base cost

-

-

-


2.0-2/98 4-13

Lesson 4

4-14

ExampleA basic, flat-plate, rigid-electrode ESP, requiring a plate area of 40,800 ft2, is pro-posed. The manufacturer recommends using 304 stainless steel for the dischargeelectrodes and collection plates due to the corrosive nature of the flue gas.Assume that the auxiliary equipment costs $10,000.

Using Figure 4-4 and Tables 4-4 and 4-6, estimate the following:

1. Equipment cost (EC)

2. Purchased equipment cost (PEC)

3. Total capital cost of purchasing and installing the ESP

Table 4-6. Capital cost factors for ESPs

Cost Item Factor

Direct CostsPurchased equipment costs

ESP + auxiliary equipmentInstrumentsSales taxesFreight

Purchased equipment cost, PEC

Direct installation costsFoundation and supportsHandling and erectionElectricalPipingInsulation for ductwork1

PaintingDirect installation costs

Site preparationBuildings

Total Direct Costs DC

Indirect Costs (installation)EngineeringConstruction and field expenseContractor feesStart-up feePerformance testModel studyContingencies

Total Indirect Costs IC

Total Capital Cost = DC + IC

As estimated, EC0.10 EC0.03 EC0.05 EC

PEC = 1.18 EC

0.04 PEC0.50 PEC0.08 PEC0.01 PEC0.02 PEC0.02 PEC0.67 PEC

As required, SPAs required, Bldg.

1.67 PEC + SP + Bldg.

0.20 PEC0.20 PEC0.10 PEC0.01 PEC0.01 PEC0.02 PEC0.03 PEC0.57 PEC

2.24 PEC + SP + Bldg.1If ductwork dimensions have been established, cost may be estimated based on $10 to $12/ft2 of

surface for field application.Source: U.S. EPA 1990.

2.0-2/98

ESP Design Review

1. Estimate the equipment cost. Because the ESP is a basic, rigid-frame ESPwithout the standard options, the lower line from Figure 4-4 is used to obtainthe capital cost. Using a collection area of 40,800 ft2, a cost of $470,000 canbe read from Figure 4-4. But this cost figure assumes that the ESP dischargeelectrodes and collection plates are made out of carbon steel material. Asstated in Table 4-4, the cost factor for 304 stainless steel is 1.3. The equipmentcost is:

$470,000 × 1.3 = $611,000

Auxiliary equipment cost = $10,000

Equipment cost (EC) = $621,000

2. Estimate the purchased equipment cost (PEC) using the cost factors inTable 4-6 (some calculations are rounded).

Equipment cost (EC) = $621,000

Instrumentation (0.10 × 621,000) = $62,100

Sales Tax (0.03 × 621,000) = $18,600

Freight (0.05 × 621,000) = $31,100

Purchased equipment cost (PEC) = $732,800

3. Estimate the total capital cost. Knowing the PEC and using the cost factorsin Table 4-6, you can estimate the remaining direct and indirect costs, whichmake up the total capital cost. A summary of these costs are provided in Table4-7.

2.0-2/98 4-15

Lesson 4

4-16

Summary

Some key factors that affect the design of an ESP include the following:

• Type of discharge electrode

• Type of collection electrode

• Electrical sectionalization


• Aspect ratio

We also covered how to estimate the cost of ESPs. These estimates can be used as budgetaryestimates by facilities planning to install an ESP or by agency engineers for reviewing permitapplications.

Table 4-7. Example case capital costs

Cost Item Factor Cost(s)

Direct CostsPurchased equipment costs

ESP + auxiliary equipmentInstrumentsSales taxesFreight

Purchased equipment cost, PEC

Direct installation costsFoundation and supportsHandling and erectionElectricalPipingInsulation for ductwork1

PaintingDirect installation costs

Site preparationBuildings

Total Direct Cost, DC

Indirect Costs (installation)EngineeringConstruction and field expenseContractor feesStart-up feePerformance testModel studyContingencies

Total Indirect Cost, IC

Total Capital Cost = DC + IC

As estimated, EC0.10 EC0.03 EC0.05 EC

PEC = 1.18 EC

0.04 PEC0.50 PEC0.08 PEC0.01 PEC0.02 PEC0.02 PEC0.67 PEC

As required, SPAs required, Bldg.


0.20 PEC0.20 PEC0.10 PEC0.01 PEC0.01 PEC0.02 PEC0.03 PEC0.57 PEC


$621,00062,10018,60031,100

$732,800

$29,300367,00058,600

7,33014,70014,700

$491,630

$1,224,430

$147,000147,00073,300

7,3307,330

14,70022,000

$418,660

$1,643,0901If ductwork dimensions have been established, cost may be estimated based on $10 to $12/ft2 of surface for

field application.

2.0-2/98

ESP Design Review

Review Exercise

1. Two important process variables to consider when designing an ESP are the gas____________________ ____________________ and the dust ____________________.

2. In an ESP, the amount of dust coming into the ESP is important. If the dust loading is very high itwill:

a. Suppress the current in the inlet field and cause the controller to drive up the voltageb. Increase the current in the inlet field and cause the controller to decrease the voltagec. Cause an increase in the dust resistivityd. Have no effect on the ESP performance

3. If coal burned in a boiler has a low sulfur content, the resulting dust will usually have____________________ resistivity.

a. Highb. Low

4. Which of the drawings below shows a good design of an inlet into the ESP?

a.

b.

c.

2.0-2/98 4-17

Lesson 4

4-18

5. True or False? Dust can be stored in hoppers for any length of time without causing problems.

6. An ESP has a collection area of 750,000 ft2 and filters fly ash from flue gas flowing at 1,500,000ft3/min. The migration velocity of the dust is 0.25 ft/sec. Estimate the collection efficiency of theESP using the Deutsch-Anderson equation.

7. The design plan states that an ESP will filter fly ash from flue gas that has a dust loading of 2 gr/ft3

and a flow rate of 2,000,000 acfm (ft3/min). The dust migration velocity is 0.3 ft/sec. If the regula-tions state that the emissions must be less than 0.02 gr/ft3, what is the total collection area neededfor the ESP design? Use the Deutsch-Anderson equation.

η 1 e w A Q⁄( )––=

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ESP Design Review


1. Flow rateConcentrationTwo important process variables to consider when designing an ESP are gas flow rate and dustconcentration.

2. a. Suppress the current in the field and cause the controller to drive up the voltageIn an ESP, the amount of dust coming into the ESP is important. If the dust loading is very high itwill suppress the current in the inlet field and cause the controller to drive up the voltage.

3. a. HighIf coal burned in a boiler has a low sulfur content, the resulting dust will usually have high resistiv-ity.

4. c.

The figure in option “c” shows the best inlet design because it has a straight-on inlet and an inletplenum with a distance of A as long as (or longer than) B. Option "b" is fine if there are straighten-ing vanes in the duct.

5. FalseDust can NOT be stored in hoppers for any length of time without causing problems. Dust shouldbe stored temporarily in the hopper and removed periodically by the discharge device to preventthe dust from backing up into the ESP.

6. 99.94%Solution:Calculate the collection efficiency using the Deutsch-Anderson equation:

Where: w = 0.25 ft/sec × 60 sec/min = 15 ft/minA = 750,000 ft2

Q = 1,500,000 ft3/min

A

B

η 1 e w A Q⁄( )––=

η 1 e 15ft min⁄ 750,000 ft2 /1,500,000 ft3 /min( )––=

1 0.00055–=

0.9994 or 99.94%=

2.0-2/98 4-19

Lesson 4

4-20

7. 512,000 ft2

Solution:1. Using equation 4-1, calculate the collection efficiency required to meet emissions regulations.

2. Calculate the total collection area needed, using the following form of the Deutsch-Andersonequation:

Where: w = 0.3 ft/sec × 60 sec/min = 18 ft/minQ = 2,000,000 ft3/minη = 0.99

A =

= 512,000 ft2

η 2gr ft3 0.02gr ft3⁄–⁄2gr ft3⁄

---------------------------------------------------=

0.99 or 99%=

AQ–

w------- ln 1 η–( )[ ]=

2,000,000 ft– 3 min⁄18 ft min⁄

------------------------------------------------ ln 1 0.99–( )[ ]

2.0-2/98

Bibliography

Beachler, D. S., J. A. Jahnke, G. T. Joseph and M. M. Peterson. 1983. Air Pollution Control Systems forSelected Industries, Self-instructional Guidebook. (APTI Course SI:431). EPA 450/2-82-006. U.S.Environmental Protection Agency.



Neveril, R. B., J. U. Price, and K. L. Engdahl. 1978. Capital and operating costs of selected air pollu-tion control systems - I. Journal of Air Pollution Control Association. 28:829-836.


U.S. Environmental Protection Agency. 1990, January. OAQPS Cost Control Manual. 4th ed. EPA450/3-90-006.

U.S. Environmental Protection Agency. 1991. Control Technology for Hazardous Air PollutantsHandbook. EPA 625/6-91/014.


2.0-2/98 4-21

Lesson 4

4-22
2.0-2/98

Lesson 5Industrial Applications of ESPs

Goal

To familiarize you with the many ways ESPs are used by various industries to reduce emissions.

Objectives


1. List five major industries that use ESPs to reduce particulate emissions2. Describe how ESPs are used with dry flue gas desulfurization systems to reduce SO2 emis-

sions from boilers3. Identify two operating problems that can occur when using ESPs on cement kilns4. List two operating problems associated with ESPs in the steel industry5. Briefly describe how ESPs are used along with acid gas control systems to control particulate

and acid gas emissions from municipal solid waste and hazardous waste incinerators6. Identify two processes in the lead, zinc and copper smelting industries that use ESPs to control

particulate emissions

Introduction

Because ESPs can collect dry particles, sticky or tarry particles, and wet mists, they are usedby many different industries, as diverse as chemical production and food processing. This les-son reviews the following industries that use ESPs to reduce air pollutant emissions: fossil-fuel-fired boilers, cement plants, steel mills, petroleum refineries, municipal waste incinera-tors, hazardous waste incinerators, kraft pulp and paper mills, and lead, zinc, and coppersmelters.

Boilers

Particulate Matter Control System

ESPs are most widely used for the control of fly ash from industrial and utility boilers andhave been used on coal-fired boilers for over 50 years. Particulate matter is generated fromboilers when fossil fuels (coal and oil) are burned to generate steam for industrial pro-cesses or to produce electric power. Both hot-side and cold-side precipitators are used tocontrol particulate emissions. Other than some construction modifications to account forthe temperature difference of the flue gas handled, hot-side and cold-side ESPs are essen-tially the same. Cold-side ESPs are used most often for collecting fly ash from coal-fired

2.0-2/98 5-1

Lesson 5

5-2

boilers. If the dust has high resistivity, cold-side units are used along with a conditioningagent such as sulfur trioxide (see Lesson 3).

Dry Sulfur Dioxide (SO2) Control System

One technology for reducing sulfur dioxide (SO2) emissions from boilers is dry flue gasdesulfurization (FGD). In dry FGD, the flue gas containing SO2 is contacted with analkaline material to produce a dry waste product for disposal. This technology consists ofthree different FGD methods:

• Injection of wet alkaline material (slurry) into a spray dryer with collection of dry par-ticles in an electrostatic precipitator or baghouse,

• Injection of dry alkaline material into the flue gas stream with collection of dry parti-cles in an ESP or baghouse, or

• Addition of alkaline material to the fuel prior to combustion

Spray dryers used in dry FGD are similar to those that have been used for over 40 yearsin the chemical, food-processing, and mineral preparation industries. Spray dryers are ves-sels where hot flue gas is contacted with a finely atomized, wet alkaline spray (see Figure5-1). Flue gas enters the top of the spray dryer and is swirled by a fixed vane ring to causeintimate contact with the slurry spray. Sodium carbonate solutions and lime slurries are themost common alkaline material used. The slurry is atomized into extremely fine dropletsby rotary atomizers or two-fluid nozzles. In a rotary atomizer, slurry is broken into drop-lets by centrifugal force as the atomizer wheel spins at a very high speed. In two-fluid noz-zles, slurry is mixed with compressed air, which forms the very small droplets. The hightemperature of the flue gas, 120 to 204°C (250 to 400°F), evaporates the moisture from thewet alkaline sprays, leaving a dry, powdered product. The dry product is then collected inan ESP or baghouse (Joseph and Beachler 1981).

Figure 5-1. Spray dryer with ESP

ESPSpray dryer

absorber

Gas inlet

Fly ashhandlingsystem

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Industrial Applications of ESPs

A number of spray dryer FGD systems have been installed on industrial and utility boilers.They are particularly useful in meeting New Source Performance Standards (NSPS) thatrequire only 70% SO2 removal efficiency for utility boilers burning low-sulfur coal and asretrofit applications for units having to meet the standards required by the 1990 Clean AirAct Amendments (see Table 5-1).

Spray dryer absorbers systems can reduce SO2 emissions by 60 to 90%. They have beenused on boilers burning low-sulfur coal (usually less than 2% sulfur content) and areattractive alternatives to wet scrubbing technology, particularly in the arid western U.S.

In dry injection systems, a dry alkaline material (sorbent) is injected pneumatically intothe gas stream by nozzles located in the ductwork prior to the flue gas entering the ESP.Sodium-based sorbents are used more frequently than lime for industrial coal-fired boilersbut hydrated lime is prevalent for waste burning incinerators. Sodium bicarbonate is fre-quently used because it is highly reactive with SO2. Sodium carbonate (soda ash),although not as reactive as sodium bicarbonate, is also used (U.S. EPA 1980). SO2

removal efficiency for these systems is typically between 70 and 80%.

A third way to apply dry FGD is by adding alkaline material to the fuel (coal) prior tocombustion. In fluidized bed boilers, limestone or sometimes lime is added to the coal inthe fluidized burning bed. These systems are capable of removing more than 90% of SO2

from the boiler flue gas. Alkaline material can also be injected into the furnace throughports or directly into the fuel burners. The SO2 removal is typically greater than 70% inthese systems.

Table 5-1. Commercial spray dryer FGD systems using an ESP or a baghouse

Station or plant Size(MW)

Installationdate

System description Sorbent

Coalsulfur

content(%)

SO2

emissionremoval

efficiency(%)

Otter Tail PowerCompany: CoyoteStation No. 1,Beulah, ND

410 6/81 Rockwell/Wheelabrator-Frye system: four spraytowers in parallel with 3atomizers in each:reverse air-shakerbaghouse with Dacronbags

Soda ash(sodiumcarbonate)

0.78 70

Basin Electric:Laramie River StationNo. 3, Wheatland,WY

500 Spring1982

Babcock and Wilcox:four spray reactors with12 "Y-jet" nozzles ineach: electrostaticprecipitator

Lime 0.54-0.81

85-90

Strathmore Paper Co.:Woronco, MA

14 12/79 Mikropul: spray dryerand pulse jet baghouse

Lime 2-2.5 75

Celanese Corp.:Cumberland, MD

31 2/80 Rockwell/Wheelabrator-Frye system: one spraytower followed by abaghouse

Lime 1-2 85

Source: U.S. EPA February 1980.

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Lesson 5

5-4

Cement Plants

ESPs are used in cement plants to control particulate emissions from cement kilns and clinkercoolers. In a cement plant, raw materials are crushed, ground, blended, and fed into a kiln,where they are heated. The kiln is fired with coal, oil, or gas. The material is heated to a tem-perature above 1595°C (2900°F), which causes it to fuse. The fused material is called cementclinker. The temperature of the hot, marble-sized, glass-hard clinker is cooled by the clinkercooler. The cooled clinker is then sent to the final grinding mills.

ESPs are frequently used to control kiln emissions because of their ability to handle high-tem-perature gases. These ESPs are usually hot-side ESPs with collection plates that are rapped orsprayed with water to remove collected dust. The dust generated in the cement kiln frequentlyhas high resistivity. High resistivity can be reduced by conditioning the flue gas with moisture.Many of the newer cement plants send the high temperature kiln flue gas that contains particu-late matter through a cyclone and conditioning tower (uses water to cool the gas temperature)prior to ducting the flue gas to the ESP. The ESP is then operated at a temperature of approxi-mately 150°C (302°F).

A special problem arises during kiln startup due to the fact that the temperature of the kilnmust be raised slowly to prevent damage to the heat-resistant (refractory) lining in the kiln.While kilns (especially coal-fired ones) are warming up and temperatures are below those forsteady-state operating conditions, complete combustion of the fuels cannot occur, giving riseto combustible gases in the exhaust stream leading into the ESP. Electrostatic precipitatorscannot be activated in the presence of combustibles, because the internal arcing of the precipi-tator could cause a fire or explosion. Use of a cyclone preceding the precipitator helps to min-imize the excessive emissions during startup. Periods of excessive emissions during startup,malfunction, or shutdown are specifically exempted from the federal New Source Perfor-mance Standards for cement kilns.

ESPs can also be used on clinker coolers. However, the ESP must be carefully designed to pre-vent moisture in the flue gas from condensing. Condensed moisture can combine with clinkerdust to coat the ESP internals with cement. (A case history of an ESP used on a cement kiln isgiven in Szabo et al. 1981.)

Steel Mills

ESPs are used in steel mills for reducing particulate emissions from blast furnaces, basic oxy-gen furnaces, and sinter plants.

In a blast furnace, iron ore is reduced to molten iron, commonly called pig iron. Blast furnacesare large, refractory-lined steel shells. Limestone, iron ore, and coke are charged into the top ofthe furnace. The gases produced during the melting process contain carbon monoxide and par-ticulate matter. Particulate matter is removed from the blast furnace gas by wet ESPs or scrub-bers, so that the gas (CO) can be burned "cleanly" in blast furnace stoves or other processes.Both plate and tube-type ESPs having water sprays to remove dust from collection electrodesare commonly used for cleaning blast furnace gas.

Basic oxygen furnaces (BOFs) refine iron from the blast furnace into steel. A BOF is a pear-shaped steel vessel that is lined with refractory brick. The vessel is charged with molten ironand steel scrap. A water-cooled oxygen lance is lowered into the vessel, where oxygen isblown to agitate the liquid, add intense heat to the process, and oxidize any impurities still

2.0-2/98


contained in the liquid metal. The hot gases generated during the oxygen blow are approxi-mately 1090 to 1650°C (2000 to 3000°F). These are usually cooled by water sprays located inthe hood and ducting above the BOF. The cooled gases are then sent to an ESP or scrubber toremove the particulate matter (iron oxide dust). The iron oxide dust can have high resistivity,making the dust difficult to collect in an ESP. This problem can usually be reduced by condi-tioning the flue gas with additional moisture. Plate ESPs that are rapped or sprayed with waterto remove dust from collection plates are commonly installed on BOFs.

In a sinter plant, materials such as flue dusts, iron ore fines (small particles), coke fines, millscale (waste that occurs from various processing steps), and small scrap are converted into ahigh-quality blast furnace feed. These materials are first fed onto a traveling grate. The bed ofmaterials is ignited by burning gas in burners located at the inlet of the traveling grate. As thebed moves along the traveling grate, air is pulled down through the bed to burn it, forming afused, porous, red-hot sinter. The resulting gases are usually sent to an electrostatic precipita-tor to remove any particulate matter. If oily scrap is used as a feed material, care must be takento prevent ESP collection plates from being coated with tarry particulate matter. Controllingthe amount of oily mill scale and small scrap processed in the sinter plant can help alleviatethis problem. Plate ESPs are commonly used in sinter plants.

Petroleum Refineries

ESPs are used in petroleum refineries to control particulate emissions from fluid-catalyticcracking units and boilers. In a refinery, heavy crude is broken down into lighter componentsby various distilling, cracking, and reforming processes. One common process is to "crack"the high-molecular-weight, high-boiling-point compounds (heavy fuel oils) into smaller, low-molecular-weight, low-boiling-point compounds (gasoline). This is usually done in a fluid-catalytic cracking (FCC) unit.

In an FCC unit, the feed stream (heavy gas oils) is heated and then mixed with a hot catalystthat causes the gas oils to vaporize and crack into smaller hydrocarbon-chain compounds.During this process, the catalyst becomes coated with coke. The coke deposits are eventuallyremoved from the catalyst by a catalyst-regeneration step.

In the regenerator, a controlled amount of air is added to burn the coke deposits off the catalystwithout destroying it. The gases in the regenerator pass through cyclones to separate large cat-alyst particles. The gases can sometimes go to a waste heat boiler to burn any carbon monox-ide and organic emissions present in the gas stream. The boiler's exhaust gas still has a highconcentration of fine catalyst particles. This flue gas is usually sent to an electrostatic precipi-tator to remove the very fine catalyst particles.

ESPs can also reduce particulate emissions from boiler exhausts. Oil-fired and, occasionally,coal-fired boilers generate steam that is used in many processes in the refinery. The flue gasfrom boilers is frequently sent to ESPs to remove particulate matter before the gas is exhaustedinto the atmosphere. ESPs designed similarly to those used on industrial and utility boilers areused on FCC units and petroleum refinery boilers.

Municipal Waste Incinerators

Electrostatic precipitators have been successfully used for many years to reduce particulateemissions from municipal waste incinerators. Municipal incinerators, also commonly calledmunicipal waste combustors (MWCs) are used to reduce the volume of many different solid

2.0-2/98 5-5

Lesson 5

5-6

and liquid wastes. Generally, municipal wastes are composed of combustible materials (e.g.paper, wood, rags, food, yard clippings, and plastic and rubber materials) and noncombustiblematerials (e.g. rocks, metal, and glass). MWCs burn waste and produce ash residue that is dis-posed of in landfills.

Both dry and wet plate ESPs are commonly used on municipal incinerators. Collected dust canbe removed from collection plates by rapping or by using water sprays. Plate ESPs havingrigid frame discharge electrodes are currently being used on MWCs (installed after 1982). Thedesigned collection efficiency is usually in the range of 96 to 99.6%. Dust resistivity can be aproblem, particularly if the refuse contains a large quantity of paper products. The dust in theflue gas in this case usually has low resistivity. Resistivity can be adjusted by carefully con-trolling the temperature and the amount of moisture in the flue gas.

Since the mid-1980s a number of large MWCs (plants having a capacity of 250 to 3000 tonsper day) with heat recovery devices have been built. More recent installations have been builtwith acid gas control systems along with an ESP or baghouse. The ESP (or baghouse) collectsacid gas reaction products (mainly calcium chloride and calcium sulfate), unused sorbentmaterial, and fly ash. ESPs are typically designed with 3 to 5 fields and are capable of meetingparticulate emission limits of 0.015 gr/dscf and occasionally can achieve limits as low as 0.01gr/dscf. These units have successfully reduced SO2 by 80% (24 hr avg) and HCl by 90 to 95%.

The acid gas is removed by using dry sorbent injection or spray dryer absorbers. In dry injec-tion systems sorbent is injected (usually hydrated lime) into the furnace or into the ductingprior to the flue gas entering the ESP. Acid gas removal efficiencies of 50% for SO2 and 75%for HCl are routinely achieved (Beachler 1992).

A more commonly used acid gas control system is a spray dryer absorber placed ahead of theESP. These systems have been able to achieve 80% removal (24 hr avg) for SO2 and 90%removal for HCl. A wet calcium hydroxide slurry is injected into a spray dryer by a rotaryatomizer or two-fluid nozzle. The slurry is made by slaking pebble lime (CaO) with water in apaste or detention slaker. The heat of the flue gas evaporates the liquid slurry in the spray dryerand the dry acid gas reaction products along with the particulate matter are collected in theESP. Background information and data prepared as part of the promulgated NSPS and Emis-sion Guidelines (U.S. EPA 1991) shows very good acid gas removal and particulate emissioncontrol for these systems.

Hazardous Waste Incinerators

ESPs are used in combination with a number of other air pollution control (APC) devicesincluding wet scrubbers and dry scrubbers (also called spray dryer absorbers) to clean the fluegas generated by burning hazardous wastes. Some facilities have been designed to use spraydryers to remove the acid gases including HCl, HF, and SO2 followed by the ESP to removethe acid gas reaction salts, any unused sorbent, and particulate matter. Other facilities havebeen designed with an APC system consisting of a spray dryer, baghouse, wet scrubber, and awet ESP (Figure 5-2). The spray dryer cools the flue gas and reduces some of the acid gascomponents. The baghouse collects the particulate matter (including metals) and the wetscrubber removes HCl (> 99%) and other acid gases. The wet ESP collects any particulatematter not removed by the baghouse. The wet scrubbing system is a closed loop. The effluentproduced in the scrubbers is ultimately sent to the spray dryer to evaporate the liquid, therefore

2.0-2/98


Incineration Process

Gas Flow

Liquid Flow

eliminating the need for a waste water treatment system. A number of facilities using this APCsystem configuration are permitted to burn PCBs and other Toxic Substance Control Act(TSCA) and Resource Conservation and Recovery Act (RCRA) wastes.

Figure 5-2. APC system for a hazardous waste incinerator consisting of aspray dryer, baghouse, wet scrubbers, and wet ESPs

Kraft Pulp and Paper Mills

Plate or tube-type ESPs are used in the kraft pulp and paper industry to reduce particulateemissions from the recovery furnace. In the kraft process of making pulp and paper, chemicalsare recovered by using evaporators, recovery furnaces, and reaction tanks. As part of the pulp-ing process, a waste product, black-liquor, is produced. After it is concentrated, the black-liquor concentrate is burned in the recovery furnace to provide heat and steam to various pro-cesses in the plant. The recovery furnace is essentially a boiler designed to effectively burn theblack-liquor concentrate. The resulting flue gas contains particulate matter that is usuallyremoved by an ESP before it is exhausted into the atmosphere. Dust can be removed from col-lection electrodes by rapping or by using water sprays.

Lead, Zinc, and Copper Smelters

Plate ESPs are used to reduce particulate emissions from a number of processes in the smelt-ing of lead, zinc, and copper metals. Since lead, zinc, and copper are found in sulfide oredeposits, the release of sulfur compounds is a problem during the smelting process. Beforebeing smelted, ore concentrates are often treated, or prepared, by two processes called sinter-ing and roasting. Sintering changes the physical form of a material, usually by taking an oremixture of large and fine pieces and fusing them into strong, porous products that can be usedin the smelting processes. ESPs are commonly used to reduce emissions from lead and zincsinter plants. ESPs are also effective in reducing emissions from zinc and copper roasters.Roasters prepare zinc and copper ores by removing unwanted materials such as sulfur. Theroasted ore is then sent to other refining processes to produce zinc and copper metals.

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Lesson 5

5-8

Other Industries

ESPs are used in many other large and small industrial processes including glass melting, sul-furic acid production, food processing, and chemical manufacturing. Glass melting furnacesusually use hot-side ESPs because the flue gas temperature in this process is approximately230 to 260°C (450 to 500°F). Sulfuric acid plants usually use plate or tube-type ESPs to col-lect sulfuric acid mists. Collected mists are removed from collection electrodes by watersprays. Some smaller industries that produce coatings, resins, asphalt, rubber, textiles, plastics,vinyl, and carpet frequently use a small two-stage precipitator to control particles and smoke.The two-stage ESPs use liquid sprays to remove collected particles, smokes, and oils from thecollection plates.

Summary

Table 5-2 summarizes the information presented in this lesson for various industries that useESPs to reduce emissions.

Table 5-2. Summary of typical ESP applications (by industry)

Industry Process

MaterialCollected by

ESPESP Collection

Efficiency ESP FeaturesPotentialProblems

1. Industrial &utility boilers

Burning fossilfuels

Dry SO2controlsystems

Fly ash

Dry, alkalineproduct

> 99%

> 99%(particles);70-80% (SO2)

Hot-side andcold-sideESPs

Cold-side ESP(usually rigidelectrode orrigid frame)

Fly ash fromlow sulfurcoals hashighresistivity

2. Cement plants Cement kilns

Clinker coolers

Particulateemissions

Particulateemissions

> 99%

> 99%

Usually hot-side ESPswithcollectionplates.Rapped orsprayed withwater.

Hot-side orcold-sidedepending ongastemperature.

Dust often hashighresistivity.Combustiblegases arepresent whenkiln iswarming up.

Must preventmoisture influe gas fromcondensing

3. Steel mills Blast furnaces

Basic oxygenfurnaces

Carbonmonoxideandparticulatematter

Iron oxide dust

> 99%Particulatematter

> 99%

Wet ESPs.Both plateand tube withwater sprays.

Wet or dryplate ESPs

Iron oxide dustcan have highresistivity

Cont. on next page

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Table 5-2. (continued)Summary of typical ESP applications (by industry)

Industry Process

MaterialCollected by

ESPESP Collection

Efficiency ESP FeaturesPotentialProblems

3. Steel mills(cont’d)

Sinter plant Particulatematter

> 99% Wet or dryplate ESPs

Oily scrapused as feedmaterial cancoat plateswith tarrysubstance

4. Petroleumrefineries

Fluid-catalyticcracking

Boileroperations

Catalystparticles

Particulatematter

> 99%

> 99%

Usually dryESPs

Usually dryESPs

5. Municipal wasteincinerators

Incinerationand heatrecovery

Acid controlsystems(spray dryeralong withESP)

Particulatematter

Acid gasreactionproducts,unusedsorbents

96-99.6%

> 99.5 (0.015gr/dscf) forparticulatematter. SO2

and HClreduced 80and 90%respectively

Wet and dryplate ESPs

Usually rigid-electrodesystems(newerfacilities)

Low resistivityof dust frompaperproducts

6. Hazardouswasteincinerators

Acid controlsystems

(1) Spray dryerandbaghousefollowed bywet ESPs

(2) Spray dryerfollowed byan ESP

Acid gasreactionproducts,unusedsorbents

> 99% (0.015gr/dscf)HCl removalefficiency> 95%

Wet ESPs ordry ESPswhen usedwith spraydryer

7. Kraft pulp andpaper mills

Recoveryfurnaceboilers

Particulatematter

> 99% Wet or dryESPs

8. Lead, zinc,copper smelters

Sinter plants

Roasting

Particulatematter

Particulatematter

> 99%

> 99%

Usually plateESPs

Usually plateESPs

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Lesson 5

5-10

Suggested Reading

For more information about the specific industries discussed in this lesson see:

Beachler, D. S., J. A. Jahnke, G. T. Joseph and M. M. Peterson. 1983. Air Pollution Control Sys-tems for Selected Industries, Self-Instructional Guidebook. (APTI Course SI:431). EPA 450/2-82-006. U.S. Environmental Protection Agency.

U.S. Environmental Protection Agency. 1985. Operation and Maintenance Manual for Electro-static Precipitators. EPA 625/1-85/017.

U.S. Environmental Protection Agency. 1981. Inspection Manual for Evaluation of ElectrostaticPrecipitator Performance. EPA 340/1-79-007.

2.0-2/98


Review Exercise

1. ESPs reduce particulate emissions from which of the following industries?

a. Utility boilersb. Cement kilnsc. Steel furnaces (basic oxygen furnace) and sinter plantsd. Municipal waste incineratorse. All of the above

2. One technology for reducing both SO2 gas and particulate emissions involves the injection of a(an)____________________slurry in a spray ____________________ with dry particle collection inan electrostatic precipitator.

3. In a spray dryer, moisture is ____________________ from the wet alkaline sprays, leaving a____________________ powdered product.

4. Acid gas and particulate emissions can be controlled by using ____________________.

a. Spray dryer absorber and ESPb. Dry injection and ESPc. a and b, above

5. ESPs should not be activated during the startup of a(an) ________________________________________ because of the possibility of a fire or explosion.

6. In a steel mill, which of the following processes would not likely use an ESP to control particulateemissions?

a. Blast furnace meltingb. Sinter processc. Ingot pouringd. Basic oxygen furnace melting and tapping

7. In a municipal incinerator where the burned refuse contains a large quantity of paper products, theresulting dust usually has a ____________________ resistivity.

a. Highb. Low

8. True or False? ESPs are used in petroleum refineries to control particulate emissions from thefluid-catalytic-cracking unit and boiler exhausts.

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Lesson 5

5-12

9. When a spray dryer absorber is used with an ESP to control acid gas and particulate emissionsfrom municipal waste combustors, which of the following is (are) true?

a. The control system can reduce particulate emissions to a level of less than 0.015 gr/dscf.b. The control system can reduce SO2 by 80%.c. The dust collected in the ESP hoppers contains calcium chloride which is very hygroscopic

(sticky).d. All of the above.

10. True or False? Both wet and dry ESPs are used in the pulp and paper industries to remove greaterthan 99% of the particulate matter from recovery furnace.

2.0-2/98


Review Exercise Answers1. e. All of the above

ESPs reduce particulate emissions from the following industries:

• Utility boilers• Cement kilns• Steel furnaces (basic oxygen furnace) and sinter plants• Municipal waste incinerators

2. AlkalineDryerOne technology for reducing both SO2 gas and particulate emissions involves the injection of analkaline slurry in a spray dryer with dry particle collection in an electrostatic precipitator.

3. EvaporatedDryIn a spray dryer, moisture is evaporated from the wet alkaline sprays, leaving a dry powderedproduct.

4. c. a and b, aboveAcid gas and particulate emissions can be controlled by using either a spray dryer absorber andESP or dry injection and ESP.

5. Cement kilnESPs should not be activated during the startup of a cement kiln because of the possibility of a fireor explosion.

6. c. Ingot pouringIn a steel mill, ingot pouring would not likely use an ESP to control particulate emissions.

7. b. LowIn a municipal incinerator where the burned refuse contains a large quantity of paper products, theresulting dust usually has a low resistivity.

8. TrueESPs are used in petroleum refineries to control particulate emissions from the fluid-catalytic-cracking unit and boiler exhausts.

9. d. All of the aboveWhen a spray dryer absorber is used with an ESP to control acid gas and particulate emissionsfrom municipal waste combustors, the following are true:

• The control system can reduce particulate emissions to a level of less than 0.015 gr/dscf.• The control system can reduce SO2 by 80%.• The dust collected in the ESP hoppers contains calcium chloride which is very hygroscopic

(sticky).

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Lesson 5

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10. TrueBoth wet and dry ESPs are used in the pulp and paper industries to remove greater than 99% of theparticulate matter from recovery furnaces.

2.0-2/98


Bibliography

Beachler, D. S. 1992. Coming clean on waste-to-energy emissions. Chemical Processing TechnologyInternational. London.

Beachler, D. S., G. T. Joseph, and M. Pompelia. 1995. Fabric Filter Operation Review. (APTI CourseSI:412A). U.S. Environmental Protection Agency.

Beachler, D. S., J. A. Jahnke, G. T. Joseph and M. M. Peterson. 1983. Air Pollution Control Systems forSelected Industries, Self-Instructional Guidebook. (APTI Course SI:431). EPA 450/2-82-006. U.S.Environmental Protection Agency.

Kaplan, S. M., and K. Felsvang. 1979, April. Spray Dryer Absorption of SO2 from Industrial BoilerFlue Gas. Paper presented at 86th National AICHE Meeting. Houston, TX.

Pezze, J. 1983. Personal Communication. Pennsylvania Department of Environmental Resources,Pittsburgh, PA.


Richards, J. R. 1995. Control of Gaseous Emissions, Student Manual. (APTI Course 415). U.S. Envi-ronmental Protection Agency.

Szabo, M. F., Y. M. Shah, and S. P. Schliesser. 1981. Inspection Manual for Evaluation of ElectrostaticPrecipitator Performances. EPA 340/1-79-007.

U.S. Environmental Protection Agency. 1976. Capital and Operating Costs of Selected Air PollutionControl Systems. EPA 450/3-76-014.

U.S. Environmental Protection Agency. 1980, February. Survey of Dry SO2 Control Systems. EPA 600/7-80-030.

U.S. Environmental Protection Agency. 1991. Requirements for preparation, adoption, and submittalof implementation plans. In Code of Federal Regulations—Protection of the Environment. 40 CFR51. Washington, D.C.: U.S. Government Printing Office.

U.S. Environmental Protection Agency. 1991. Approval and promulgation of implementation plans. InCode of Federal Regulations—Protection of the Environment. 40 CFR 52. Washington, D.C.: U.S.Government Printing Office.

U.S. Environmental Protection Agency. 1991. Standards of performance for new stationary sources—general provisions. In Code of Federal Regulations—Protection of the Environment. 40 CFR 60.Washington, D.C.: U.S. Government Printing Office.

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2.0-2/98

Lesson 6ESP Operation and Maintenance

Goal

To familiarize you with typical operation and maintenance problems associated with ESPs.

Objectives


1. Identify typical ESP components which require inspection prior to startup2. Identify the major steps in ESP startup and shutdown procedures3. Explain the importance of monitoring each of the following parameters:

• Voltage/current

• Opacity

• Gas temperature

• Gas flow rate and distribution

• Gas composition and moisture

4. Describe the function of air-load and gas-load voltage-current curves5. Identify typical maintenance steps that ensure proper ESP functioning6. Identify and describe seven common problems that affect ESP performance7. Describe how evaluating the current, voltage, and spark rate trends from inlet to outlet fields

provides information about general resistivity conditions8. Identify important safety precautions to take when operating ESPs

Introduction

As with any air pollution control system, an ESP must be operated and maintained accordingto the manufacturer's recommendations. Plant personnel must be properly trained to performthese activities with confidence and efficiency. This lesson reviews some of the key functionsthat must be completed to keep the ESP operating as it was intended including installation,startup and shutdown procedures, performance monitoring, routine maintenance and record-keeping and problem evaluation.

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Lesson 6

6-2

ESP Installation

Depending on the electrostatic precipitator chosen, production, installation and operation star-tup may take from a few months to one or two years. In any case, proper installation proce-dures will save time and money, and will also help in future operation and maintenance(O&M) of the ESP.

Good coordination between the ESP designer (vendor) and the installation and maintenancecrews will help keep the ESP running smoothly for years. Occasionally this coordination isoverlooked. Because they are so large, ESPs are usually installed by skilled craftsmen who donot work for the ESP vendor, and, therefore, may not be informed of specific installationinstructions. Since all design tolerances are critical (especially those affecting discharge andcollection electrode alignment), it is imperative that information about the proper installationprocedures be transferred from designers to installers.

Some key considerations during installation are:

• Easy access to all potential maintenance areas—fans, motors, hoppers, discharge devices,dampers, flue gas flow rate and temperature monitors, insulators, rappers, T-R sets, anddischarge and collection electrodes

• Easy access to all inspection and test areas—stack testing ports and continuous emissionmonitors (opacity monitors)

• Weather conditions—the ESP must be able to withstand inclement weather such as rain orsnow

During installation, the customer purchasing the ESP should be responsible for checking thecriteria presented below. The regulatory agency review engineer also should review the pro-cess on which the ESP will be installed and verify that these items are being addressed.

1. Uniform flue gas distribution across the entire unit. Ductwork, turning vanes, baffleplates, and inlets with perforated diffuser plates all affect flue gas distribution. These itemsare usually installed in the field and should be checked visually. If improperly installed,they induce high airflow regions that decrease collection efficiency and cause reentrain-ment of collected dust, especially during rapping cycles.

2. Complete seal of ESP system from dust pickup to stack outlet. Air inleakage or outleakageat flanges or collector access points either adds additional airflow to be processed or forcesthe process gases to bypass the collector. Inleakage to a high-temperature system (hot-sideESP) is extremely damaging, as it creates cold spots which can lead to moisture or acidcondensation and possible corrosion. If severe, it can cause the entire process gas temper-ature to fall below the gas dew point, causing moisture or acid to condense on the hopperwalls, the discharge electrode, or collection plates. In addition, air inleakage and moisturecondensation can cause caking of fly ash in the hopper, making normal dust removal bythe discharge device very difficult. The best way to check for leaks is an inspection of thewalls from inside the system during daylight. Light penetration from outside helps to iso-late the problem areas.

3. Proper installation of discharge electrodes and collection plates. Collection electrodes areusually installed first, and the discharge wires or rigid frames are positioned relative tothem. Check each section of electrodes to ensure that the electrodes are plumb, level, andproperly aligned.

2.0-2/98

ESP Operation and Maintenance

4. Proper installation of rappers. Collection-plate rappers and discharge-electrode rappersshould be installed and aligned according to vendor specifications. Check magnetic-impulse rappers to see if they strike the support frame on the collection plates. Check ham-mer and anvil rappers to see if the hammers strike the anvils squarely. Check vibrator rap-pers installed on discharge wires to make sure they operate when activated. Rapperfrequency and intensity can be adjusted later when the unit is brought on-line.

5. Proper insulation. Most ESPs use some type of insulation to keep the flue gas temperaturehigh. This prevents any moisture or acids present in the flue gas from condensing on thehoppers, electrodes, or duct surfaces. Because most ESPs are installed in the field, checkthat all surfaces and areas of potential heat loss are adequately covered.

6. Proper installation and operation of discharge devices. It is important to check the opera-tion of the discharge devices before bringing the ESP on-line to see if they are properlyinstalled. Make sure that the discharge devices are moving in the right direction so theycan remove the dust freely from the hopper. A backward-moving screw conveyor can packdust so tightly that it can bend the screw.

Overfilled hoppers are common operating problems that can be avoided by proper installa-tion and maintenance of discharge devices. Installed as maintenance tools, dust-leveldetectors in the hoppers can help alert ESP operators that hoppers are nearly full.

7. Smoothly running fans. Check fans for proper rotation, drive component alignments, andvibration. Fans should be securely mounted to a component of sufficient mass to eliminateexcessive vibration.

In addition to the above items, each ESP installation should have its own checklist reflect-ing the unique construction features of that unit. The installation crew should prepare achecklist before beginning final inspection and initial startup. A prestartup checklist forthe initial startup suggested by Peter Bibbo (1982) is shown in Table 6-1.

2.0-2/98 6-3

Lesson 6

6-4

ESP Startup and Shutdown

A specific startup and shutdown procedure should be supplied by the ESP vendor. Improperstartup and shutdown can damage the collector. It is imperative for the operator (source) tohave a copy of these procedures. Review agency engineers may want to assure that these pro-

Table 6-1. Prestartup checklist for electrostaticprecipitators

Collecting plates1. Free of longitudinal and horizontal bows2. Free of burrs and sharp edges3. Support system square and level4. Spacer bars and corner guides free5. Free of excessive dust buildup6. Gas leakage baffles in place and not binding

Discharge electrodes1. No breaks or slack wires2. Wires free in guides and suspension weight free on pin3. Rigid frames square and level4. Rigid electrodes plumb and straight5. Free of excessive dust buildup and grounds6. Alignment within design specifications

Hoppers1. Scaffolding removed2. Discharge throat and poke holes clear3. Level detector unobstructed4. Baffle door and access door closed5. Heaters, vibrators, and alarms operational

Top housing or insulator compartments1. Insulators and bushing clear and dry with no carbon tracks2. All grounding chains in storage brackets3. Heaters intact, seal-air system controls, alarms, dampers, and filters

in place and operational4. Seal-air fan motor rotation correct, or vent pipes free5. All access doors closed

Rappers1. All swing hammers or drop rods in place and free2. Guide sleeves and bearings intact3. Control and field wiring properly terminated4. Indicating lights and instrumentation operational5. All debris removed from precipitator6. All personnel out of unit and off clearances7. All interlocks operational and locked out

a. No broken or missing keysb. Covers on all locks

Transformer-rectifier sets1. Surge arrestor not cracked or chipped and gap set2. Liquid level satisfactory3. High-voltage connections properly made4. Grounds on: precipitator, output bushings, bus ducts, conduits

Rectifier control units1. Controls grounded2. Power supply and alarm wiring properly completed3. Interlock key in transfer block

Source: Bibbo 1982.

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cedures exist at the sites and that the operators follow the procedures or document reasons fordeviations.

Startup

Startup of an electrostatic precipitator is generally a routine operation. It involves heatinga number of components such as support insulators and hoppers. If possible, the ESPshould not be turned on until the process reaches steady-state conditions. As described inLesson 5, this is particularly important for ESPs used on cement kilns burning coal as fuel.The internal arcing of the ESP could cause a fire or an explosion. When ESPs are used onoil-burning boilers, the boiler should be started with gas or #2 fuel oil. Heavy oil (#6 fueloil) is not a good fuel for startup because tarry particulate emissions can coat collectionplates and be difficult to remove. If an ESP is used on a coal-fired boiler, the ESP shouldnot be started until coal firing can be verified. This will help prevent combustible gasesfrom accumulating in the unit and causing explosive conditions. A typical startup proce-dure for an ESP used on a boiler is given in Table 6-2 (Bibbo 1982).

Table 6-2. Typical startup procedures forelectrostatic precipitators

Normal Operation

Startup (preoperational checks - at least 2 hours prior to gas load):1. Complete all maintenance/inspection items.2. Remove all debris from ESP.3. Safety interlocks should be operational and all keys accounted for.4. No personnel should be in ESP.5. Lock out ESP and insert keys in transfer blocks.

Prestart (at least one hour prior to gas load):6. Check hoppers.

a. Level-indicating system should be operational.b. Ash-handling system operating and sequence check - leave in

operational mode.c. Hopper heaters should be on.

7. Check top housing seal-air system.a. Check operation of seal-air fan—leave running.b. Bushing heaters should be on.

8. Check rappers.a. Energize control, run rapid sequence, ensure that all rappers are

operational.b. Set cycle time and intensity adjustments, using installed

instrumentation—leave rappers operating.9. Check T-R sets.

a. Check half-wave/full-wave operation (half-wave operation isrecommended for filtering fly ash when lignite is burned and acold-side ESP is used.)

b. Keys should be in all breakers.c. Test-energize all T-R sets and check local control alarm functions.d. Set power levels and de-energize all T-R controls.

e. Lamp and function-test all local and remote alarms.Continued on next page

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Lesson 6

6-6

Shutdown

When an industrial process is shut down temporarily, the ESP system should be de-ener-gized to save energy. The shutdown of the ESP is usually done by reversing the order ofthe startup steps. Begin with de-energizing the ESP fields starting with the inlet field tomaintain appropriate opacity levels from the stack. The rappers should be run for a shorttime after the ESP is de-energized so that accumulated dust from the collection plates anddischarge wires can be removed. All hoppers should be emptied completely before bring-ing the unit back on line. A typical shutdown and emergency shutdown procedure forESPs used on industrial sources is given in Table 6-3 (Bibbo 1982).

Table 6-2. (continued)Typical startup procedures forelectrostatic precipitators

Normal OperationGas load:

10. After gas at temperature of 200°F has entered ESP for 2 hours -a. Energize T-R sets.b. Check for normal operation of T-R control.c. Check all alarm functions in local and remote.d. Within 2 hours, check proper operation of ash removal system.e. De-energize bushing heaters after 2 hours (hopper heaters

optional).Cold start (when it is not possible to admit flue gas at 200°F for 2 hours prior

to energizing controls), proceed as follows:1. Perform steps 1-9 above. Increase rapping intensity 50%.2. Energize T-R sets, starting with inlet field, setting Powertrac voltage

to a point just below sparking.3. Successively energize successive field as load picks up to maintain

opacity, keeping voltage below normal sparking (less than 10flashes/min on spark indicator).

4. Perform step 10d above.5. After flue gas at 200°F has entered ESP for 2 hours, perform steps

10b, c, d, and e above.Set normal rapping.

Source: Bibbo 1982.

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Performance Monitoring

As with the operation of any piece of equipment, performance monitoring and recordkeepingare essential to establishing a good operation and maintenance program. The key to any moni-toring program is establishing an adequate baseline of acceptable ranges that is used as a refer-ence point. Then, by monitoring and recording key operating parameters, the operator canidentify performance problems, need for maintenance, and operating trends.

Typical parameters that can be monitored include:

• Voltage/current

• Opacity

• Gas temperature

• Gas flow rate and distribution

• Gas composition and moisture

In addition, site-specific data on process operating rates and conditioning system (if used)should also be documented. Operators should not rely on just one parameter as an indicator ofperformance—trends for a number of parameters gives a clearer picture. Let's briefly look atthe ways these parameters affect performance and the techniques used to measure them. Muchof this information was extracted from Operation and Maintenance Manual for ElectrostaticPrecipitators (U.S. EPA 1985).

Voltage and Current

Voltage and current values for each T-R set should be recorded; they indicate ESP perfor-mance more than any other parameter. Most modern ESPs are equipped with primary volt-age and current meters on the low-voltage (a.c.) side of the transformer and secondaryvoltage and current meters on the high-voltage rectified (d.c.) side of the transformer.When both voltage and current meters are available on the T-R control cabinet, these val-ues can be multiplied to estimate the power input to the ESP. (Note that the primary cur-

Table 6-3. Typical shutdown and emergency shutdownprocedures for electrostatic precipitators

Typical shutdown1. When boiler load drops and total ash quantity diminishes:

a. De-energize ESP by field, starting with inlet field to maintain opacitylimit.

b. De-energize outlet field when all fuel flow ceases and combustion airflow falls below 30% of rated flow.

c. Leave rappers, ash removal system, seal-air system, and hopperheaters operational.

d. Four hours after boiler shutdown, de-energize seal-air system andhopper heaters. Secure ash removal system.

e. Eight hours after boiler shutdown, de-energize rappers.Note: Normal shutdown is a convenient time to check operation of

alarms.

Emergency shutdown1. De-energize all T-R sets.2. Follow steps 1c, d, and e above (shutdown).

Source: Bibbo 1982.

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Lesson 6

6-8

rent reading is multiplied by the primary voltage reading and the secondary currentreading is multiplied by the secondary voltage reading). These values (current times volt-age) represent the number of watts being drawn by the ESP and is referred to as the coronapower input. In addition, whenever a short term spark occurs in a field it can be detectedand counted by a spark rate meter. ESPs generally have spark rate meters to aid in the per-formance evaluation.

The power input on the primary versus the secondary side of the T-R set will differbecause of the circuitry and metering of these values. The secondary power outlet (inwatts) is always less than the primary power input to the T-R. The ratio of the secondarypower to the primary power will range from 0.5 to 0.9 and average from 0.70 to 0.75 (U.S.EPA 1985).

Voltage and current values for each individual T-R set are useful because they inform theoperators how effectively each field is operating. However, the trends noted within theentire ESP are more important. T-R set readings for current, voltage, and sparking ratesshould follow certain patterns from the inlet to the outlet fields. For example, coronapower density should increase from inlet to outlet fields as the particulate matter isremoved from the gas stream.

The electrical meters on the T-R cabinets are always fluctuating. Normal sparking withinthe ESP causes these fluctuations in the meter readings. These short term movements ofthe gauges indicate that the automatic voltage controller is restoring the maximum voltageafter shutting down for several milliseconds to quench the spark. When recording valuesof the electrical data from the T-R meters it is important to note the maximum value that issustained for at least a fraction of a second.

Opacity

In many situations, ESP operation is evaluated in terms of the opacity monitored by atransmissometer (opacity monitor) on a real-time basis. Under optimum conditions theESP should be able to operate at some base-level opacity with a minimum of opacity spik-ing from rapper reentrainment. A facility can have one or more monitors that indicateopacity from various ESP outlet ducts and from the stack.

An opacity monitor compares the amount of light generated and transmitted by the instru-ment on one side of the gas stream with the quantity measured by the receiver on the otherside of the gas stream. The difference, which is caused by absorption, reflection, refrac-tion, and light scattering by the particles in the gas stream, is the opacity of the gas stream.Opacity is expressed as a percent from 0 to 100% and is a function of particle size, con-centration, and path length.

Most of the opacity monitors being installed today are double-pass monitors; that is, thelight beam is passed through the gas stream and reflected back across to a transceiver. Thisarrangement is advantageous for several reasons:

1. Automatic checking of the zero and span of the monitor is possible when the processis operational.

2. The monitor is more sensitive to slight variations in opacity because the path length islonger.

3. The entire electronics package is located on one side of the stack as a transceiver.

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Although single-pass transmissometers are available at a lower cost (and sensitivity), thesingle-pass monitors cannot meet the requirements in EPA Performance Specification 1,Appendix B, 40 CFR 60.

For many sources, dust concentration and opacity correlations can be developed to pro-vide a relative indication of ESP performance. These correlations are very site-specific,but can provide plant and agency personnel with an indication of relative performance forgiven opacity levels. In addition, site-specific opacity charts can be used to predict deteri-oration of ESP performance that requires attention by plant personnel. Readings fromopacity monitors can also be used to optimize spark rate, voltage/current levels, and rap-ping cycles, even though the conditions within the ESP are not static. In high-efficiencyESPs, reentrainment may account for 50 to 70% of the total outlet emissions. Therefore,optimization of the rapping pattern may prove more beneficial than trying to optimize thevoltage, current and sparking levels. Dust reentrainment from rapping must be observedby using the opacity monitor operating in a real-time or nonintegrating mode because rap-ping spikes tend to get smoothed out in integrated averages such as the 6-minute averagecommonly in use. However, the integrated average does provide a good indication of aver-age opacity and emissions.

When parallel ESPs or chambers are used, an opacity monitor is often placed in each out-let duct, as well as on the stack, to measure the opacity of the combined emissions.Although the stack monitor is commonly used to indicate stack opacity (averaging opaci-ties from different ducts can be difficult), the individual duct monitors can be useful inindicating the performance of each ESP or chamber and in troubleshooting. Although thisoption is often not required and represents an additional expense, it can be very useful,particularly on relatively large ESPs.

New systems, such as the digital microprocessor design, are available in which the opacitymonitor data can be used as input for the T-R controller. In this case, the data are used tocontrol power input throughout the ESP to maintain an opacity level preselected by thesource. If the opacity increases, the controller increases power input accordingly until theopacity limit, spark limit, current limit, or voltage limit is reached. This system (often soldas an energy saver because it uses only the power required) can save a substantial quantityof energy:

1. On large, high-efficiency ESPs

2. For processes operating at reduced gas loads.

In many cases, however, reduction of ESP power does not significantly alter ESP perfor-mance because dust reentrainment and gas sneakage constitute the largest sources of emis-sions; additional power often does not reduce these emissions significantly. In someobserved cases, reducing power by one-half did not change the performance. For unitstypically operated at 1000 to 1500 watts/1000 acfm, operating the ESPs at power levels of500 to 750 watts/1000 afcm still provide acceptable collection efficiencies.

Gas Temperature

Monitoring the temperature of the gas stream can provide useful information concerningESP performance. Temperature is measured using a thermocouple in conjunction with adigital, analog, or strip chart recorder. Temperature is usually measured using a single-point probe or thermocouple. This method has a major limitation in that the probe may be

2.0-2/98 6-9

Lesson 6

6-10

placed at an unrepresentative (stratified) point—one that is not representative of the bulkgas flow. Most ESPs are designed with a minimum of three fields. The gas temperature foreach field should be measured at both the inlet and outlet, if possible. Significant tempera-ture changes between the inlet and outlet values may indicate air inleakage problems thatshould be confirmed by measurement of gas composition.

Changes in gas temperature can have profound effects on ESP performance. The tempera-ture variation can be very small (in some cases as little as 15oF) and yet cause a significantchange in ESP power levels and opacity. Although gas temperature variations may havesome effect on corona discharge characteristics and physical characteristics of the ESP(corrosion, expansion/contraction), their most important effect is on particle resistivity.For sources with the potential for high resistivity, temperature changes can cause dramaticchanges in performance, even when all other parameters seem to be the same. The gastemperature should be checked once per shift for smaller sources and measured continu-ously on larger sources and on those sources with temperature-sensitive performance.

Temperature measurement can also be a useful tool in finding excessive inleakage orunequal gas flow through the ESP. Both of these conditions can affect localized gas veloc-ity patterns without noticeably affecting the average velocity within the ESP. Yet, local-ized changes in gas velocities can reduce ESP performance even though the average gasvelocity seems adequate.

Gas Flow Rate and Distribution

Gas flow rate determines most of the key design and operating parameters such as spe-cific collection area (ft2/1000 acfm), gas velocity (ft/sec) and treatment time within theESP, and specific corona power (watts/1000 acfm). The operator should calculate the fluegas flow rate if the ESP is not operating efficiently. For example, significant variations inoxygen may indicate large swings in the gas flow rate that may decrease ESP performanceand indicate the need to routinely determine ESP gas volume. Low SCA values, highvelocities, short gas treatment times (5 seconds or less), and much higher oxygen levels atnearly full load conditions are indicators that excess flue gas flow rate may be causingdecreased ESP performance.

Presently, most sources do not continuously measure gas velocities or flow rates. Gasvelocities are generally only measured during emission compliance testing or when thereis a perceived problem. Manual pitot tube traverses are normally used to measure gasvelocity (EPA Reference Methods 1 and 2). Because of new technologies and regulations,some of the larger sources are beginning to install continuous flow measurement systems.Multi-point pitot devices, ultrasonic devices, and temperature-based flow devices can beused to continuously measure gas velocity. These devices must be calibrated to the indi-vidual stack where they are installed. Most existing facilities currently use indirect indica-tors to estimate gas flow rate; these include fan operating parameters, production rates oroxygen/carbon dioxide gas concentration levels. However, EPA is now requiring largecoal-fired utility boilers to install and certify flow monitors (EPA Acid Rain Program, Part75 Regulations).

Another important parameter is gas flow distribution through the ESP. Ideally, the gasflow should be uniformly distributed throughout the ESP (top to bottom, side to side).Actually, however, gas flow through the ESP is not evenly distributed, and ESP manufac-turers settle for what they consider an acceptable variation. Standards recommended by

2.0-2/98


the Industrial Gas Cleaning Institute have been set for gas flow distribution. Based on avelocity sampling routine, 85% of the points should be within 15% of the average velocityand 99% should be within 1.4 times the average velocity. Generally, uneven gas flowthrough the ESP results in reduced performance because the reduction in collection effi-ciency in areas of high gas flow is not compensated for by the improved performance inareas of lower flow. Also, improper gas distribution can also affect gas sneakage throughthe ESP. As stated earlier, good gas distribution can be accomplished by using perforatedplates in the inlet plenum and turning vanes in the ductwork.

Measurement of gas flow distribution through the ESP is even less common than measur-ing flue gas flow rate. Because the flow measurements are obtained in the ESP rather thanthe ductwork (where total gas volumetric flow rates are usually measured), more sensitiveinstrumentation is needed for measuring the low gas velocities. The instrument typicallyspecified is a calibrated hot-wire anemometer. The anemometer test is usually performedat some mid-point between the inlet and outlet (usually between two fields). Care must betaken to assure that internal ESP structural members do not interfere with the samplingpoints.

Gas flow distribution tests are conducted when the process is inoperative, and the ESP andductwork are relatively cool. This often limits the amount of gas volume that can be drawnthrough the ESP to less than 50% of the normal operating flow; however, the relativevelocities at each point are assumed to remain the same throughout the normal operatingrange of the ESP. A large number of points are sampled by this technique. The actual num-ber depends upon the ESP design, but 200 to 500 individual readings per ESP are notunusual. By using a good sampling protocol, any severe variations should become readilyapparent.

Gas Composition and Moisture

The chemical composition of both the particulate matter and flue gas can affect ESP per-formance. In many applications, key indicators of gas composition are often obtained byusing continuous emission monitors. However, particle concentration and composition aredetermined by using intermittent grab sampling.

The operation of an ESP depends on the concentration of electronegative gases O2, H2O,CO and SO2/SO3 to generate an effective corona discharge. Often, sources use continuousmonitors to measure these gas concentrations to meet regulatory requirements, or in thecase of combustion sources to determine excess air levels (CO2 or O2).

Evaluating Air-Load/Gas-Load Voltage-Current (V-I) Curves

In addition to the routine panel meter readings, other electrical tests of interest to personnelresponsible for evaluating and maintaining ESPs include the air-load and gas-load V-I (volt-age-current) tests, which may be conducted on virtually all ESPs. Air-load and gas-load curvesare graphs of the voltage (kV) versus the current (mA) values obtained at a set condition (testpoint). These curves are developed to evaluate ESP performance by comparing the graphsfrom inlet field to outlet field and over periods in time. Deviation from the normal or previousresults can indicate that a problem exists.

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Lesson 6

6-12

Air-Load Curves

Air-load tests are generally conducted on cool, inoperative ESPs through which no gas isflowing. This test should be conducted when the ESP is new, after the first shutdown, andevery time off-line maintenance is performed on the ESP. These air-load V-I curves serveas the basis for comparison in the evaluation of ESP maintenance and performance. A typ-ical air-load curve is shown in Figure 6-1.

Figure 6-1. Typical air-load voltage and current readings

An air-load V-I curve can be generated with readings from either primary or secondarymeters. The following procedures can be used by the ESP operator to develop an air-loadcurve.

1. Energize a de-energized T-R set on manual control (but with zero voltage and current),and increase the power to the T-R set manually.

2. At corona initiation the meters should suddenly jump and the voltage and near zerocurrent levels should be recorded. It is sometimes difficult to identify this point pre-cisely, so the lowest practical value should be recorded.

3. After corona initiation is achieved, increase the power at predetermined increments[for example, every 50 or 100 milliamps of secondary current or every 10 volts of ACprimary voltage (the increment is discretionary)], and record the values for voltageand current.

4. Continue this procedure until one of the following occurs:

• Sparking

• Current limit is achieved

• Voltage limit is achieved

5. Repeat this procedure for each T-R set.

When the air-load tests have been completed for each field, plot each field's voltage/cur-rent curves. When ESPs are equipped with identical fields throughout, the curves for eachfield should be nearly identical. In most cases, the curves also should be similar to those

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generated when the unit was new, but shifted slightly to the right due to residual dust onthe wires (or rigid frames) and plates of older units. These curves should become part ofthe permanent record of the ESP.

The use of air-load curves enables plant personnel to identify which field(s) may not beperforming as designed. Also, comparison of air-load curves from test runs taken justbefore and after a unit is serviced will confirm whether the maintenance work correctedthe problem(s).

One major advantage of air-load tests is that they are performed under nearly identicalconditions each time, which means the curves can be compared. One disadvantage is thatthe internal ESP conditions are not always the same as during normal operation. Forexample, misalignment of electrodes may appear or disappear when the ESP is cooled(expansion/contraction), and dust buildup may be removed by rapping during ESP shut-down.

Gas-Load Curves

The gas-load V-I curve, on the other hand, is generated during the normal operation of theprocess while the ESP is energized. The procedure for generating the gas-load V-I curve isthe same as for the air load except that gas-load V-I curves are always generated from theoutlet fields first and move toward the inlet. This prevents the upstream flow that is beingchecked from disturbing the V-I curve of the downstream field readings. Although suchdisturbances would be short-lived (usually 2 minutes, but sometimes lasting up to 20 min-utes), working from outlet to inlet speeds up the process.

The curves generated under gas-load conditions will be similar to air-load curves. Gas-load curves will generally be shifted to the left however, because sparking occurs at lowervoltage and current when particles are present. The shape of the curve will be different foreach field depending on the presence of particulate matter in the gas stream (seeFigure 6-2).

Figure 6-2. Comparison of typical air-load and gas-loadV-I curves Source:EPA 1985

Gas-load

Air-load

I

V

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Lesson 6

6-14

Also, gas-load curves vary from day to day, even minute to minute. Curve positions maychange as a result of fluctuations in the following:

• Amount of dust on plates

• Gas flow

• Particulate chemistry loading

• Temperature

• Resistivity

Nonetheless, they still should maintain a characteristic pattern. Gas-load curves are nor-mally used to isolate the cause of a suspected problem rather than being used on a day-to-day basis; however, they can be used daily if necessary.

Routine Maintenance and Recordkeeping

While the overall performance of the ESP is continuously monitored by devices such as volt-age meters and transmissometers, the components of the ESP and their operation are periodi-cally inspected by plant personnel as part of a preventive maintenance program. In this way,problems are detected and corrected before they cause a major shutdown of the ESP. Ofcourse, good recordkeeping should be an integral part of any maintenance program.

The frequency of inspection of all ESP components should be established by a formal in-housemaintenance procedure. Vendors' recommendations for an inspection schedule should be fol-lowed. A listing of typical periodic maintenance procedures for an ESP used to collect fly ashis given in Table 6-4 (Bibbo 1982).

In addition to the daily monitoring of meters and the periodic inspection of ESP components,some operational checks should be performed every shift and the findings should be recordedon a shift data sheet. At the end of every shift, these shift data sheets should be evaluated formaintenance needs. These once-per-shift checkpoints include an inspection of rappers, dustdischarge systems, and T-R sets for proper functioning and an indication of which T-R sets arein the "off" position. Rappers that are not functioning should be scheduled for maintenance,particularly if large sections of rappers are out of service. Dust discharge systems should havehighest priority for repair; dust should not accumulate in the bottom of the ESP for long peri-ods of time because of the potential for causing severe plate misalignment problems. Hopperheaters can usually be repaired with little difficulty after removing weather protection andinsulation. Insulator heaters may be difficult to repair except during short outages. Hopperheaters keep condensation on the insulators to a minimum and help keep the dust warm andfree-flowing.

In addition to performing maintenance, keeping records of the actions taken is also important.For example, wire replacement diagrams should be kept. Although an ESP can operate effec-tively with up to 10% of its wires removed, care must be taken that no more than 5 to 10 wiresin any one gas lane are removed. The loss of wires down any one lane can result in a substan-tial increase in emissions. The only way to adequately track where wires have failed or slippedout of the ESP is with a wire replacement chart. Also, any adjustments to the rapper frequencyand intensity should be recorded along with any repairs. These same recordkeeping practicesshould be followed for any repairs or replacements made on T-R sets, insulator/heaters, align-ment, and the dust discharge systems.

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Problem Evaluation

Good site specific records of both the design and operating history will enable operating per-sonnel to better evaluate ESP performance. Design parameters built into the ESP include thefollowing: the specific collection area (SCA), number of fields, number of T-R sets, sectional-ization, T-R set capacity, design velocity and treatment time, aspect ratio and particle charac-teristics (resistivity). Design records indicate the specific conditions under which the ESP wasdesigned to operate. A comparison between design records and operating records indicatewhether operating parameters have changed significantly from the design conditions. Sec-

Table 6-4. Preventive maintenance checklist for a typicalfly ash precipitator

Daily1. Take and record electrical readings and transmissometer data.2. Check operation of hoppers and ash removal system.3. Examine control room ventilation system.4. Investigate cause of abnormal arcing in T-R enclosures and bus duct.

Weekly1. Check rapper operation.2. Check and clean air filter.3. Inspect control set interiors.

Monthly1. Check operation of standby top-housing pressurizing fan and thermostat.2. Check operation of hopper heaters.3. Check hopper level alarm operation.

Quarterly1. Check and clean rapper and vibrator switch contacts.2. Check transmissometer calibration.

Semiannual1. Clean and lubricate access-door dog bolt and hinges.2. Clean and lubricate interlock covers.3. Clean and lubricate test connections.4. Check exterior for visual signs of deterioration, and abnormal vibration, noise, leaks.5. Check T-R liquid and surge-arrestor spark gap.

Annual1. Conduct internal inspection.2. Clean top housing or insulator compartment and all electrical insulating surfaces.3. Check and correct defective alignment.4. Examine and clean all contactors and inspect tightness of all electrical connections.5. Clean and inspect all gasketed connections.6. Check and adjust operation of switchgear.7. Check and tighten rapper insulator connections.8. Observe and record areas of corrosion.

Situational1. Record air-load and gas-load readings during and after each outage.2. Clean and check interior of control sets during each outage of more than 72 hours.3. Clean all internal bushings during outages of more than 5 days.4. Inspect condition of all grounding devices during each outage over 72 hours.5. Clean all shorts and hopper buildups during each outage.6. Inspect and record amount and location of residual dust deposits on electrodes

during each outage of 72 hours or longer.7. Check all alarms, interlocks, and all other safety devices during each outage.

Source: Bibbo 1982.

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Lesson 6

6-16

ondly, maintaining proper operating records establishes good baseline information to bracketnormal ranges of operation.

Evaluating ESP operating problems can be difficult and no single parameter can identify allpotential problems; a combination of factors should be considered to accurately pinpoint prob-lems. For example, although most ESP problems are reflected in the electrical readings, manydifferent problems produce the same characteristics on the meters. In addition, an initial failureor problem can cause a "domino effect" bringing about even more problems and making it dif-ficult to identify the original cause. Table 6-5 contains a typical troubleshooting cycle (Szaboand Gerstle 1977) that is useful as a general guide.

The EPA (1985) categorized the major performance problems associated with electrostaticprecipitators into the following seven areas: resistivity, dust buildup, wire breakage, hopperpluggage, misalignment of ESP components, changes in particle size distribution, and airinleakage. These problems are related to design limitations, operational changes, and/or main-tenance procedures. The following discussion about the identification of these problems andtheir effect on ESP performance is excerpted from the EPA document titled Operation andMaintenance Manual for Electrostatic Precipitators (1985).

Problems Related to Resistivity

The resistivity of the collected dust on the collection plate affects the acceptable currentdensity through the dust layer, dust removal from the plates, and indirectly, the coronacharging process. High resistivity conditions in utility fly ash applications have receivedmuch attention. The optimum resistivity range for ESP operation is relatively narrow; bothhigh and low resistivity cause problems. Excursions outside the optimum resistivity rangeare particularly a problem when a unit is designed with a modest amount of plate area, sec-tionalization, and power-input capabilities. At industrial sources where resistivity changesare intermittent, modification of operating procedures may improve performance tempo-rarily. Expensive retrofitting or modifications may be required if the dust resistivity isvastly different than the design range.

High ResistivityHigh dust resistivity is a more common problem than low dust resistivity. Particleshaving high resistivity are unable to release or transfer electrical charge. At the collec-tion plate, the particles neither give up very much of their acquired charge nor easilypass the corona current to the grounded collection plates. High dust resistivity condi-tions are indicated by low primary and secondary voltages, suppressed secondary cur-rents and high spark rates in all fields. This condition makes it difficult for the T-Rcontroller to function adequately.

Severe sparking can cause excessive charging off-time, spark "blasting" of particulateon the plate, broken wires due to electrical erosion, and reduced average current lev-els. The reduced current levels generally lead to deteriorated performance. Becausethe current level is indicative of the charging process, the low current and voltage lev-els that occur inside an ESP operating with high resistivity dust generally reflectslower charging rates and lower particle migration velocities to the plate. Particle col-lection is reduced; consequently, the ESP operates as though it were "undersized." Ifhigh resistivity is expected to continue, the operating conditions can be modified or

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conditioning agents can be used to accommodate this problem and thereby improveperformance.

High resistivity also tends to promote rapping problems, as the electrical properties ofthe dust tend to make it very tenacious. High voltage drop through the dust layer andthe retention of electrical charge by the particles make the dust difficult to removebecause of its strong attraction to the plate. The greater rapping forces usually requiredto dislodge the dust may also aggravate or cause a rapping reentrainment problem.Important items to remember are (1) difficulty in removing the high-resistivity dust isrelated to the electrical characteristics, not to the sticky or cohesive nature of the dust;and (2) the ESP must be able to withstand the necessary increased rapping forceswithout sustaining damage to insulators or plate support systems.

Low ResistivityLow dust resistivity, although not as common, can be just as detrimental to the perfor-mance of an ESP as high resistivity. When particles with low resistivity reach the col-lection plate, they release much of their acquired charge and pass the corona currentquite easily to the grounded collection plate. Without the attractive and repulsive elec-trical forces that are normally present at normal dust resistivities, the binding forcesbetween the dust and the plate are considerably weakened. Therefore, particle reen-trainment is a substantial problem at low resistivity, and ESP performance appears tobe very sensitive to contributors of reentrainment, such as poor rapping or poor gasdistribution.

Since there is lower resistance to current flow for particles with low resistivity (com-pared to normal or high), lower operating voltages are required to obtain substantialcurrent flow. Operating voltages and currents are typically close to clean plate condi-tions, even when there is some dust accumulation on the plate. Low-resistivity condi-tions, are typically characterized by low operating voltages, high current flow, and lowspark rates.

Despite the large flow of current under low-resistivity conditions, the correspondinglow voltages yield lower particle migration velocities to the plate. Thus, particles of agiven size take longer to reach the plate than would be expected. When combined withsubstantial dust reentrainment, the result is poor ESP performance. In this case, thelarge flow of power to the ESP represents a waste of power.

Low-resistivity problems typically result from the chemical characteristics of the par-ticulate and not from flue gas temperature. The particulate may be enriched with com-pounds that are inherently low in resistivity, either due to poor operation of the processor to the inherent nature of the process. Examples of such enrichment include exces-sive carbon levels in fly ash (due to poor combustion), the presence of naturally occur-ring alkalis in wood ash, iron oxide in steel-making operations, or the presence ofother low-resistivity materials in the dust. Over-conditioning may also occur in someprocess operations, such as the burning of high-sulfur coals or the presence of highSO3 levels in the gas stream, which lower the inherent resistivity of the dust. In someinstances, large ESPs with SCAs greater than 750 ft2/1000 acfm have performedpoorly because of the failure to fully account for the difficulty involved in collecting alow-resistivity dust. Although some corrective actions for low resistivity are available,they are sometimes more difficult to implement than those for high resistivity.

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6-18
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Typical High, Normal and Low Resistivity CurvesEvaluating the current and spark rate trends from the inlet to the outlet fields providesa means of evaluating the general resistivity conditions. Moderate dust resistivity con-ditions, under which ESPs work very well, are indicated by low secondary currents inthe inlet field and progressively higher values going toward the outlet. Spark ratesunder moderate resistivity are moderate in the inlet fields and decrease to essentiallyzero in the outlet field. High resistivity conditions are indicated by low secondary cur-rents in all of the fields coupled with very high spark rates. Conversely, low resistivityhas very high currents and low spark rates in all the fields.

Figure 6-3 shows the typical trend lines for moderate (normal) and high resistivitydusts. As the resistivity goes from moderate to high, the currents decrease dramati-cally in all of the fields. This is due to the suppressing effect caused by the strong elec-trostatic field created on the dust layer, and to increased electrical sparking. Thedecrease in currents is most noticeable in the outlet fields which previously had rela-tively high currents. Spark rates increase dramatically during high resistivity. Oftenmost of the fields will hit the spark rate limits programmed in by the plant operators.Once the spark rate limit is sensed by the automatic voltage controllers, it no longerattempts to drive up the voltage. This causes a reduction in the operating voltages ofthese fields. The overall impact on the opacity is substantially increased emissions. Insome cases, puffing again occurs during rapping. This is due to reduced capability ofthe precipitator fields to collect the slight quantities of particles released during rap-ping of high resistivity dust.

Figure 6-4 shows the typical trend lines for moderate (normal) and low resistivitydusts in a four-field ESP. The moderate resistivity dust shows a steady increase of cur-rent from the first field to the fourth field, while the secondary current increases rap-idly for all fields when the dust exhibits low resistivity. This effect is especiallynoticeable in the inlet fields which previously had the lowest currents. This increase incurrent is due simply to the fact that the dust layer’s electrostatic field is too weak tosignificantly impede the charging field created by the discharged electrodes. At lowresistivity, the spark rates are generally very low or zero. The voltages in all of thefields are a little lower than normal since the automatic voltage controllers sense thatthe power supply is at its current limit; therefore, the controller does not attempt todrive the voltage up any further. While the low resistivity conditions persist, there canbe frequent and severe puffs (opacity increase) which occur after each collection platerapper activates.


Figure 6-3. Typical T-R set plots - high resistivity versus moderate (normal)resistivity

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Figure 6-4. Typical T-R set plots - low resistivity versus moderate (normal)resistivity

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Using the current, voltage, and spark rate plots is a very good way to use readily avail-able information to evaluate the impossible-to-directly monitor but neverthelessimportant resistivity conditions. It is possible to differentiate between problemscaused by mechanical faults in a single field (such as insulator leakage) and resistivityconditions which inherently affect all of the fields in varying degrees. However, thesetrend lines are not a perfect analysis tool for evaluating resistivity. A few precipitatorsnever display typical electrical trend lines since they have undersized T-R sets, under-sized fields, improperly set automatic voltage controllers, or severe mechanical prob-lems affecting most of the fields.

Dust Accumulation

There are three primary causes of dust accumulation on electrodes:

• Inadequate rapping system

• Sticky dust

• Operation at temperatures below the dew point level

The usual cause for buildup of dust on the collection plates or discharge wires is failure ofthe rapping system or an inadequate rapping system. The rapping system must providesufficient force to dislodge the dust without damaging the ESP or causing excessive reen-trainment. The failure of one or two isolated rappers does not usually degrade ESP perfor-mance significantly. The failure of an entire rapper control system or all the rappers in onefield, however, can cause a noticeable decrease in ESP performance, particularly withhigh-resistivity dust. Therefore, rapper operation should be checked at least once per day,or perhaps even once per shift. A convenient time to make this check is during routine T-Rset readings.

Rapper operation may be difficult to check on some ESPs because the time periodsbetween rapper activation can range from 1 to 8 hours on the outlet field. One method ofchecking rapper operation involves installing a maintenance-check cycle that allows acheck of all rappers in 2 to 5 minutes by following a simple rapping pattern. The cycle isactivated by plant personnel, who interrupt the normal rapping cycle and note any rappersthat fail to operate. After the check cycle, the rappers resume their normal operation.Maintenance of rapper operation is important to optimum ESP performance.

Excessive dust buildup also may result from sticky dusts or operation at gas dew pointconditions. In some cases, the dusts may be removed by increasing the temperature, but inmany cases the ESP must be entered and washed out. If sticky particulates are expected(such as tars and asphalts), a wet-wall ESP is usually used because problems can occurwhen large quantities of sticky particles enter a dry ESP.

Sticky particulates can also become a problem when the flue gas temperature falls belowthe dew point level. Although acid dew point is usually of greater concern in most applica-tions, moisture dew point is important, too. When moisture dew point conditions arereached, liquid droplets tend to form that can bind the particulate to the plate and wire.These conditions also accelerate corrosion. Carryover of water droplets or excessive mois-ture can also cause this problem (e.g., improper atomization of water in spray cooling ofthe gas or failure of a waterwall or economizer tube in a boiler). In some instances the dustlayer that has built up can be removed by increasing the intensity and frequency of the rap-ping while raising the temperature to "dry out" the dust layer. In most cases, however, it is

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Lesson 6

6-22

necessary to shutdown the unit and wash out or "chisel out" the buildup to clean the plates.Localized problems can occur where inleakage causes localized decreases in gas tempera-ture.

In an operating ESP, differences in the V-I curves can be used to evaluate if a dust buildupproblem exists. Buildup of material on the discharge electrodes often means an increase involtage to maintain a given operating current. The effect of dust buildup on discharge elec-trodes is usually equivalent to increasing the effective wire diameter. Since the coronastarting voltage is strongly a function of wire diameter, the corona starting voltage tends toincrease and the whole V-I curve tends to shift to the right (see Figure 6-5). Sparking tendsto occur at about the same voltage as it does without dust buildup, unless resistivity ishigh. This effect on corona starting voltage is usually more pronounced when straightwires are uniformly coated with a heavy dust, and less pronounced on barbed wires andrigid electrodes or when the dust layer is not uniform. Barbed wires and rigid electrodestend to keep the "points" relatively clean and to maintain a small effective wire diameterand, therefore, a low corona starting voltage. Nevertheless, a higher voltage would still berequired to spread the corona discharge over the wire when dust buildup occurs. Thus,buildup on the discharge electrodes would still be characterized by a higher voltage tomaintain a given current level.

Figure 6-5. V-I curve for a field with excessive wirebuildup

Wire Breakage

Some ESPs operate for 10 to 15 years without experiencing a single wire breakage.Whereas others experience severe wire breakage problems causing one or more sections tobe out of service nearly every day of operation. Much time and effort have been expended

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to determine the causes of wire breakage. One of the advantages of rigid-frame and rigid-electrode ESPs is their use of shorter wires or no wires at all. Although most new ESPshave either rigid frames or rigid electrodes, and some weighted-wire systems have beenretrofitted to rigid electrodes, the most common ESP in service today is still the weighted-wire.

Wires usually fail in one of three areas: at the top of the wire, at the bottom of the wire,and wherever misalignment or slack wires reduce the clearance between the wire andplate. Wire failure may be due to electrical erosion, mechanical erosion, corrosion, orsome combination of these. When wire failures occur, they usually short-out the fieldwhere they are located. In some cases, they may short-out an adjacent field as well. Thus,the failure of one wire can cause the loss of particle collection in an entire field or bus sec-tion. In some smaller ESP applications, this can represent one-third to one-half of thecharging/collecting area and thus substantially limit ESP performance. One of the advan-tages of higher sectionalization is that wire failure is confined to smaller areas so overallESP performance does not suffer as much. Some ESPs are designed to meet emissionstandards with some percentage of the ESP de-energized, whereas others may not haveany margin to cover downtime. Because they receive and remove the greatest percentageof particulate matter, inlet fields are usually more important to ESP operation than outletfields.

Electrical erosion is caused by excessive sparking. Sparking usually occurs at pointswhere there is close clearance within a field due to a warped plate, misaligned guidanceframes, or bowed wires. The maximum operating voltage is usually limited by these closetolerance areas because the spark-over voltage depends on the distance between the wireand the plate. The smaller the distance between the wire and plate, the lower the spark-over voltage. Under normal circumstances random sparking does little damage to the ESP.During sparking, most of the power supplied to energize the field is directed to the loca-tion of the spark, and the voltage field around the remaining wires collapses. The consid-erable quantity of energy available during the spark is usually sufficient to vaporize asmall quantity of metal. When sparking continues to occur at the same location, the wireusually "necks down" because of electrical erosion until it is unable to withstand the ten-sion and breaks. Misalignment of the discharge electrodes relative to the plates increasesthe potential for broken wires, decreases the operating voltage and current because ofsparking, and decreases the performance potential of that field in the ESP.

Although the breakage of wires at the top and bottom where the wire passes through thefield can be aggravated by misalignment, the distortion of the electrical field at the edgesof the plate tends to be the cause of breakage. This distortion of the field, which occurswhere the wire passes the end of the plate, tends to promote sparking and gradual electri-cal erosion of the wires.

Design faults and the failure to maintain alignment generally contribute to mechanicalerosion (or wear) of the wire. In some designs, the lower guide frame guides the wires ortheir weight hooks (not the weights themselves) into alignment with the plates. Whenalignment is good, the guide frame or grid allows the wires or weight hooks to float freelywithin their respective openings. When the position of the wire guide frame shifts, how-ever, the wire or weight hook rubs the wire frame within the particulate-laden gas stream.Failures of this type usually result from a combination of mechanical and electrical ero-sion. Corrosion may also contribute to this failure. Microsparking action between the

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Lesson 6

6-24

guide frame and the wire or weight hook apparently causes the electrical erosion. Thesame type of failure also can occur in some rigid frame designs where the wires ride in theframe.

Another mechanical failure that sometimes occurs involves crossed wires. When replac-ing a wire, maintenance personnel must make sure that the replacement wire does notcross another wire. Eventually, the resulting wearing action breaks one or both wires. Ifone of the wires does survive, it is usually worn down enough to promote greater sparkingat the point of contact until it finally does break. Any wires that are found to be exception-ally long and slack should be replaced; they should not be crossed with another wire toachieve the desired length.

Corrosion of the wires can also lead to wire failures. Corrosion, an electrochemical reac-tion, can occur for several reasons, the most common being acid dew point. When the rateof corrosion is slow and generally spread throughout the ESP, it may not lead to a singlewire failure for 5 to 10 years. When the rate of corrosion is high because of long periods ofoperating the ESP below the acid dew point, failures are frequent. In these cases the corro-sion problem is more likely to be a localized one (e.g., in places where cooling of the gasstream occurs, such as inleakage points and the walls of the ESP). Corrosion-related wirefailures can also be aggravated by startup-shutdown procedures that allow the gas streamsto pass through the dew point many times. Facilities have mainly experienced wire break-age problems during the initial process shakedown period when the process operation maynot be continuous. Once steady operation has been achieved, wire breakage problems tendto diminish at most plants.

Wire crimping is another cause of wire failure. Crimps usually occur at the top and bot-tom of the wires where they attach to the upper wire frame or bottle weight; however, acrimp may occur at any point along the wire. Because a crimp creates a residual stresspoint, all three mechanisms (electrical erosion, mechanical erosion, and corrosion) may beat work in this situation. A crimp can:

1. Distort the electric field along the wire and promote sparking;

2. Mechanically weaken the wire and make it thinner;

3. Subject the wire to a stress corrosion failure (materials under stress tend to corrodemore rapidly than those not under stress).

Wire failure should not be a severe maintenance problem or operating limitation in a well-designed ESP. Excessive wire failures are usually a symptom of a more fundamental prob-lem. Plant personnel should maintain records of wire failure locations. Although ESP per-formance will generally not suffer with up to approximately 10% of the wires removed,these records should be maintained to help avoid a condition in which entire gas lanes maybe de-energized. Improved sectionalization helps to minimize the effect of a broken wireon ESP performance, but performance usually begins to suffer when a large percentage ofthe ESP fields are de-energized.

Hopper Pluggage

Perhaps no other problem (except fire or explosion) has the potential for degrading ESPperformance as much as hopper pluggage. Hopper pluggage can permanently damage anESP and severely affect both short-term and long-term performance. Hopper pluggage is

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difficult to diagnose because its effect is not immediately apparent on the T-R set panelmeters. Depending on its location, a hopper can usually be filled in 4 to 24 hours. In manycases, the effect of pluggage does not show up on the electrical readings until the hopper isnearly full.

The electrical reaction to most plugged hoppers is the same as that for internal misalign-ment, a loose wire in the ESP, or excessive dust buildup on the plates. Typical symptomsinclude heavy or "bursty" sparking in the field(s) over the plugged hopper and reducedvoltage and current in response to the reduced clearance and higher spark rate. Inweighted-wire designs, high dust levels in the hopper may raise the weight and cause slackwires and increased arcing within the ESP. In many cases, this will trip the T-R set off-linebecause of overcurrent or undervoltage protection circuits. In some situations, the spark-ing continues even as the dust level exceeds hopper capacity and builds up between theplate and the wire; whereas in others, the voltage continues to decrease as the currentincreases and little or no sparking occurs. This drain of power away from corona genera-tion renders the field performance virtually useless. The flow of current also can cause theformation of a dust clinker (solidified dust) resulting from the heating of the dust betweenthe wire and plate.

The buildup of dust under and into the collection area can cause the plate or dischargeelectrode guide frames to shift. The buildup can also place these frames under enoughpressure to distort them or to cause permanent warping of the collection plate(s). If thishappens, performance of the affected field remains diminished by misalignment, evenafter the hopper is cleared.

Hopper pluggage can be caused by the following:

• Obstructions due to fallen wires and/or bottle weights

• Inadequately sized solids-removal equipment

• Use of hoppers for dust storage

• Inadequate insulation and hopper heating

• Air inleakage through access doors

Most dusts flow best when they are hot, therefore, cooling the dusts can promote a hopperpluggage problem.

Hopper pluggage can begin and perpetuate a cycle of failure in the ESP. For example,there was a case where a severely plugged hopper misaligned both the plates and the wireguide grid in one of the ESP fields. Because the performance of this field had decreased,the ESP was taken off-line and the hopper was cleared. But no one noticed the deterioratedcondition of the wire-guide grid. The misalignment had caused the wires and weighthooks to rub the lower guide and erode the metal. When the ESP was brought back on-line, the guide-grid metal eventually wore through. Hopper pluggage increased as weights(and sometimes wires) fell into the hopper, plugging the discharge opening and causingthe hopper to fill again and cause more misalignment. The rate of failure continued toincrease until it was almost an everyday occurrence. This problem, which has occurredmore than once in different applications, demonstrates how one relatively simple problemcan lead to more complicated and costly ones.

In most pyramid-shaped hoppers, the rate of buildup lessens as the hopper is filled due tothe geometry of the inverted pyramid. Hopper level indicators or alarms should provide

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Lesson 6

6-26

some margin of safety so that plant personnel can respond before the hopper is filled.When the dust layer rises to a level where it interferes with the electrical characteristics ofthe field, less dust is collected and the collection efficiency is reduced. Also, reentrainmentof the dust from the hopper can limit how high into the field the dust can go. Althoughbuildups as deep as 4 feet have been observed, they usually are limited to 12 - 18 inchesabove the bottom of the plates.

Misalignment

As mentioned several times in the previous sections, electrode misalignment is both a con-tributor to and a result of component failures. In general, most ESPs are not affected by amisalignment of less than about 3/16 inches. Indeed, some tolerance must be provided forexpansion and contraction of the components. Beyond this limit, however, misalignmentcan become a limiting factor in ESP performance and is visually evident during an internalinspection of the ESP electrodes. Whether caused by warped plates, misaligned or skeweddischarge electrode guide frames, insulator failure, or failure to maintain ESP "box-squareness," misalignment reduces the operating voltage and current required for spark-ing. The V-I curve would indicate a somewhat lower voltage to achieve a low current levelwith the sparking voltage and current greatly reduced. Since the maximum operating volt-age/current levels depend on the path of least resistance in a field, any point of close toler-ance will control these operating levels.

Changes in Particle Size

Unusually fine particles present a problem under the following circumstances:

1. When the ESP is not designed to handle them

2. When a process change or modification shifts the particle size distribution into therange where ESP performance is poorest.

A shift in particle size distribution tends to alter electrical characteristics and increase thenumber of particles emitted in the light-scattering size ranges (opacity).

As stated in Lesson 1, there are two principal charging mechanisms: field charging anddiffusion charging. Although field charging tends to dominate in the ESP and acts on par-ticles greater than 1 micrometer in diameter, it cannot charge and capture smaller particles.Diffusion charging, on the other hand, works well for particles smaller than 0.1 microme-ter in diameter. ESP performance diminishes for particulates in the range of 0.2 - 0.9micrometer because neither charging mechanism is very effective for particles in thisrange. These particles are more difficult to charge and once charged, they are easilybumped around by the gas stream, making them difficult to collect. Depending upon thetype of source being controlled, the collection efficiency of an ESP can drop from as highas 99.9% on particles sized above 1.0 micrometer or below 0.1 micrometer, to only 85 to90% on particles in the 0.2 - 0.9 micrometer diameter range. If a significant quantity ofparticles fall into this size range, the ESP design must be altered to accommodate the fineparticles.

When heavy loadings of fine particles enter the ESP, two significant electrical effects canoccur: space charge and corona quenching. At moderate resistivities, the space-chargeeffects normally occur in the inlet or perhaps the second field of ESPs. Because it takes a

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longer time to charge fine particles and to force them to migrate to the plate, a cloud ofnegatively charged particles forms in the gas stream. This cloud of charged particles iscalled a space charge. It interferes with the corona generation process and impedes theflow of negatively charged gas ions from the wire to the collection plate. The interferenceof the space charge with corona generation is called corona quenching. When this occurs,the T-R controller responds by increasing the operating voltage to maintain current flowand corona generation. The increase in voltage usually causes increased spark rates, whichmay in turn signal the controller to reduce the voltage and current in an attempt to main-tain a reasonable spark rate. Under moderate resistivity conditions, the fine dust particlesare usually collected by the time they reach the third field of the ESP which explains thedisappearance of the space charge in these later fields. The T-R controller responds to thecleaner gas in these later fields by decreasing the voltage level, but the current levels willincrease markedly. When quantities of fine particles being processed by the ESP increase,the space charging effect may progress further into the ESP.

Air Inleakage

Inleakage is often overlooked as an operating problem. In some instances, it can be benefi-cial to ESP performance, but in most cases its effect is detrimental. Inleakage may occurwithin the process itself or in the ESP and is caused by leaking access doors, leaking duct-work, and even open sample ports.

Inleakage usually cools the gas stream, and can also introduce additional moisture. Airinleakage often causes localized corrosion of the ESP shell, plates, and wires. The temper-ature differential also can cause electrical disturbances (sparking) in the field. Finally, theintroduction of ambient air can affect the gas distribution near the point of entry. The pri-mary entrance paths are through the ESP access and hopper doors. Inleakage through hop-per doors may reentrain and excessively cool the dust in the hopper, which can cause bothreentrainment in the gas stream and hopper pluggage. Inleakage through the access doorsis normally accompanied by an audible in-rush of air.

Inleakage is also accompanied by an increase in gas volume. In some processes, a certainamount of inleakage is expected. For example, application of Lungstrom regenerative airheaters on power boilers or recovery boilers is normally accompanied by an increase influe gas oxygen. For utility boilers the increase may be from 4.5% oxygen at the inlet to6.5% at the boiler outlet. For other boilers the percentage increase may be smaller whenmeasured by the O2 content, but 20 to 40% increases in gas volumes are typical and theESP must be sized accordingly. Excessive gas volume due to air inleakage, however, cancause an increase in emissions due to higher velocities through the ESP and greater reen-trainment of particulate matter. For example, at a kraft recovery boiler, an ESP that wasdesigned for a superficial velocity of just under 6 ft/s was operating at over 12 ft/s to han-dle an increased firing rate, increased excess air, and inleakage downstream of the boiler.Because the velocities were so high through the ESP, the captured material was blown offthe plate and the source was unable to meet emission standards.

Table 6-5 summarizes the problems associated with electrostatic precipitators, along withcorrective actions and preventive measures.

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Lesson 6

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Table 6-5. Summary of problems associated with electrostatic precipitators

Malfunction Cause

Effect onelectrostaticprecipitatorefficiency1 Corrective action

Preventivemeasures

1. Poor electrodealignment

1. Poor design2. Ash buildup on

frame hoppers3. Poor gas flow

Can drastically affectperformance andlower efficiency

Realign electrodesCorrect gas flow

Check hoppersfrequently for properoperation

2. Broken electrodes 1. Wire not rappedclean, causes anarc whichembroglios andburns through thewire

2. Clinkered wire.Causes:a. Poor flow area,

distributionthrough unit isuneven

b. Excess freecarbon due toexcess air abovecombustionrequirements orfan capacityinsufficient fordemand required

c. Wires notproperly centered

d. Ash buildup,resulting in bentframe, same as(c)

e. Clinker bridgesthe plates andwire shorts out

f. Ash buildup,pushes bottleweight upcausing sag inthe wire

g. "J" hooks haveimproperclearances to thehanging wire

h. Bottle weighthangs up duringcooling causing abuckled wire

i. Ash buildup onbottle weight tothe frame forms aclinker and burnsoff the wire

Reduction in efficiencydue to reducedpower input, bussection unavailability

Replace electrode Boiler problems;check spacebetween recordingsteam and air flowpens, pressuregauges, fouledscreen tubes

Inspect hoppers;check electrodesfrequently for wear;inspect rappersfrequently

3. Distorted or skewedelectrode plates

1. Ash buildup inhoppers

2. Gas flowirregularities

3. High temperatures

Reduced efficiency Repair or replaceplates

Correct gas flow

Check hoppersfrequently for properoperation; checkelectrode platesduring outages

4. Vibrating or swingingelectrodes

1. Uneven gas flow2. Broken electrodes

Decrease in efficiencydue to reducedpower input

Repair electrode Check electrodesfrequently for wear

Continued on next page

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Table 6-5. (continued)Summary of problems associated with electrostatic precipitators

Malfunction Cause


Preventivemeasures

5. Inadequate level ofpower input (voltagetoo low)

1. High dust resistivity2. Excessive ash on

electrodes3. Unusually fine

particle size4. Inadequate power

supply5. Inadequate

sectionalization6. Improper rectifier

and controloperation

7. Misalignment ofelectrodes

Reduction in efficiency Clean electrodes;gas conditioningor alterations intemperature toreduce resistivity;increasesectionalization

Check range ofvoltages frequentlyto make sure theyare correct; check in-situ resistivitymeasurements

6. Back corona 1. Ash accumulatedon electrodescauses excessivesparking requiringreduction in voltagecharge

Reduction in efficiency Same as above Same as above

7. Broken or crackedinsulator or flower potbushing leakage

1. Ash buildup duringoperation causesleakage to ground

2. Moisture gatheredduring shutdown orlow-load operation

Reduction in efficiency Clean or replaceinsulators andbushings

Check frequently;clean and dry asneeded; check foradequatepressurization of tophousing

8. Air inleakage throughhoppers

1. From dust conveyor Lower efficiency; dustreentrained throughelectrostaticprecipitator

Seal leaks Identify early byincrease in ashconcentration atbottom of exit toelectrostaticprecipitator

9. Air inleakage throughelectrostaticprecipitator shell

1. Flange expansion Same as above; alsocauses intensesparking

Seal leaks Check for large fluegas temperaturedrop across the ESP

10.Gas bypass aroundelectrostaticprecipitator• dead passage

above plates• around high

tension frame

1. Poor design;improper isolationof active portion ofelectrostaticprecipitator

Only few percent dropin efficiency unlesssevere

Baffling to directgas into activeelectrostaticprecipitatorsection

Identify early bymeasurement of gasflow in suspectedareas

11.Corrosion 1. Temperature goesbelow dew point

Negligible untilprecipitation interiorplugs or plates areeaten away; air leaksmay develop causingsignificant drops inperformance

Maintain flue gastemperatureabove dew point

Energize precipitatorafter boiler systemhas been on line forample period to raiseflue gas temperatureabove acid dew point

Continued on next page

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Lesson 6

6-30
2.0-2/98
Table 6-5. (continued)Summary of problems associated with electrostatic precipitators

Malfunction Cause


Preventivemeasures

12.Hopper pluggage 1. Wires, plates,insulators fouledbecause of lowtemperature

2. Inadequate hopperinsulation

3. Impropermaintenance

4. Boiler leaks causingexcess moisture

5. Ash conveyingsystem malfunction(gasket leakage,blower malfunction,solenoid valves)

6. Misjudgments ofhopper vibrators

7. Material droppedinto hopper frombottle weights

8. Solenoid, timermalfunction

9. Suction blower filternot changed

Reduction in efficiency Provide proper flowof ash

Frequent checks foradequate operationof hoppers. Provideheater thermalinsulation to avoidmoisturecondensation

13. Inadequate rapping,vibrators fail

1. Ash buildup2. Poor design3. Rappers

misadjusted

Resulting buildup onelectrodes mayreduce efficiency

Adjust rappers withoptical dustmeasuringinstrument inelectrostaticprecipitator exitstream

Frequent checks foradequate operationof rappers

14.Too intense rapping 1. Poor design2. Rappers

misadjusted3. Improper rapping

force

Reentrains ash,reduces efficiency

Same as above Same as above;reduce vibrating orimpact force

15.Control failures 1. Power failure inprimary systema. Insulation

breakdown intransformer

b. Arcing intransformerbetween high-voltage switchcontacts

c. Leaks or shorts inhigh-voltagestructure

d. Insulating fieldcontamination

Reduced efficiency Find source offailure and repairor replace

Pay close attention todaily readings ofcontrol roominstrumentation tospot deviations fromnormal readings

16.Sparking 1. Inspection door ajar2. Boiler leaks3. Plugging of hoppers4. Dirty insulators

Reduced efficiency Close inspectiondoors; repair leaksin boiler; unplughoppers; cleaninsulators

Regular preventivemaintenance willalleviate theseproblems

1The effects of precipitation problems can be discussed only on a qualitative basis. There are no known emission tests of precipitators to determineperformance degradation as a function of operational problems.

Sources: Szabo and Gerstle 1977, and Englebrecht 1980.


Safety

Persons who will be operating and maintaining an ESP must be well trained on all safetyaspects to avoid injury. One person at the plant should be assigned the responsibility of con-stantly checking safety standards and equipment and to train or procure safety training for allthose who will work with the ESP. A suggested list of important safety precautions is listed inTable 6-6 (Bibbo 1982).

Table 6-6. Important safety precautions

Wiring and controls1. Prior to startup, double-check that field wiring between controls and devices

(T-R sets, rapper prime motors, etc.) is correct, complete, and properly labeled.2. Never touch exposed internal parts of control system. Operation of the

transformer-rectifier controls involves the use of dangerous high voltage.Although all practical safety control measures have been incorporated into thisequipment, always take responsible precautions when operating it.

3. Never use fingers or metal screwdrivers to adjust uninsulated control devices.

Access1. Use a positive method to ensure that personnel are out of the precipitator, flues,

or controls prior to energization. Never violate established plant clearancepractices.

2. Never bypass the safety key interlock system. Destroy any extra keys. Alwayskeep lock caps in place. Use powdered graphite only to lubricate lock systemparts; never use oil or grease. Never tamper with a key interlock.

3. Use grounding chains whenever entering the precipitator, T-R switch enclosure,or bus ducts. The precipitator can hold a high static charge, up to 15 kV, after it isde-energized. The only safe ground is one that can be seen.

4. Never open a hopper door unless the dust level is positively below the door. Donot trust the level alarm. Check from the upper access in the precipitator. Hot dustcan flow like water and severely burn or kill a person standing below the door.Wear protective clothing.

5. Be on firm footing prior to entering the precipitator. Clear all trip hazards. Use theback of the hand to test for high metal temperatures.

6. Avoid ozone inhalation. Ozone is created any time the discharge electrodes areenergized. Wear an air-line mask when entering the precipitator, flues, or stackwhen ozone may be present. Do not use filters, cartridge, or canister respirators.

7. Never poke hoppers with an uninsulated metal bar. Keep safety and danger signsin place. Clean, bright signs are obeyed more than deteriorated signs.

Fire/explosion1. In case of boiler malfunction that could permit volatile gases and/or heavy carbon

carryover to enter the precipitator, immediately shut down all transformer-rectifiersets. Volatile gases and carbon carryover could be ignited by sparks in theprecipitator, causing fire or explosion, damaging precipitator internals.

2. If high levels of carbon are known to exist on the collecting surface or in thehoppers, do not open precipitator access doors until the precipitator has cooledbelow 52°C (125°F). Spontaneous combustion of the hot dust may be caused bythe inrush of air.

3. If a fire is suspected in the hoppers, empty the affected hopper. If unable to emptythe hopper immediately, shut down the transformer-rectifier sets above thehopper until it is empty. Use no other method to empty the hopper. Never usewater or steam to control this type of fire. These agents can release hydrogen,increasing the possibility of explosion.

Source: Bibbo 1982.

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Summary

Successful longtime operation of an ESP ultimately depends on effective inspection, startupand shutdown and operation and maintenance procedures. Regardless of how well the ESP isdesigned, if these procedures are not developed and routinely followed the ESP will deterio-rate resulting in a decrease of its particulate emission removal efficiency.

The lesson discusses the importance of monitoring key operating parameters including voltageand current readings of each T-R set, opacity, flue gas flow rate and flue gas composition andmoisture levels. We also covered how evaluating current, voltage and spark rate trends canhelp provide information on dust resistivity conditions. A change in dust resistivity can drasti-cally alter the performance of the ESP and will likely lead to emission compliance problems ifnot rectified.

Suggested Reading

Bibbo, P. P. 1982. Electrostatic precipitators. In L. Theodore and A. Buonicore (Eds.), Air PollutionControl Equipment-Selection, Design, Operation and Maintenance (pp.3-44). Englewood Cliffs,NJ: Prentice Hall.

Englebrecht, H. L. 1980. Mechanical and electrical aspects of electrostatic precipitator O&M. In R. A.Young and F. L. Cross (Eds.), Operation and Maintenance for Air Particulate Control Equipment(pp. 283-354). Ann Arbor, MI: Ann Arbor Science.


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Review Exercise

1. Air inleakage at flanges or collector access points in high-temperature systems (hot-side ESPs)may:

a. Allow dust to settle out quickly into hoppersb. Cause acids and moisture to condense on internal components of the ESPc. Increase the overall collection efficiency of the unit

2. Gas streams of high temperature should be maintained above the:

a. Ignition temperatureb. Gas dew pointc. Concentration limit

3. Since most ESPs are installed in the field, it is important to check that all surfaces and areas ofpotential heat loss are adequately covered with:

a. Paintb. Plastic coatingc. Insulationd. Aluminum siding

4. Before the ESP is started, the installation crew should prepare and use a____________________.

5. Which of the following ESP components should be checked before starting the collector?

a. Hoppers and discharge devicesb. Rappersc. Discharge and collection electrodesd. All of the above

6. Two very important parameters monitored by meters on T-R sets and used to evaluate ESP perfor-mance are ____________________ and____________________.

7. True or False? Individual T-R set values for voltage and current are important; however, the trendsfor voltage and current noted within an entire ESP are more valuable in assessing performance.

8. As particulate matter is removed from the gas stream, the ____________________ shouldincrease from the inlet to the outlet fields.

a. Opacityb. Current densityc. Rapper intensityd. Amperage

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9. An opacity monitor (transmissometer) measures:

a. Particle weightb. Particle sizec. Light differentiald. Primary current

10. True or False? Opacity monitors are useful tools to aid in optimization of spark rate, power levelsand rapping cycles in ESPs.

11. True or False? Changes in flue gas temperature generally have little or no effect on particle resis-tivity.

12. Operating parameters such as specific collection area, superficial velocity, and treatment time aredependent on the ____________________ ____________________ ____________________.

13. True or False? Because of their open design, gas flow distribution through ESPs are generally veryevenly distributed.

14. ____________________ tests are generally conducted on cool, inoperative ESPs through whichno gas is flowing.

a. Air Load V-I Curveb. Gas Load V-I Curvec. Complianced. All of the above

15. True or False? When ESPs are equipped with identical fields, the air-load curves for each fieldshould be very similar.

16. Air Load V-I curves for a given ESP field will generally shift to the ____________________ ifplates are dirty compared to previous tests.

a. Leftb. Rightc. a and b, above

17. Gas-load curves are similar to air-load curves except the gas-load curves are shifted to the____________________ compared to the air-load curves.

a. Leftb. Right

18. True or False? Gas-load curves generally are identical for a given ESP field on a day-to-day basis.

19. True or False? High dust resistivity is characterized by the tendency toward high spark rates at lowcurrent levels.

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20. Excessive dust buildup on the collecting plates or discharge wires can be caused by failure of the:

a. Primary and secondary voltageb. Rapping systemc. Back coronad. All the above

21. Wire failure can be caused by:

a. Electrical erosionb. Mechanical erosionc. Corrosiond. All of the above

22. True or False? Unlike baghouses, ESPs are not affected by operating temperatures falling belowthe acid or moisture dew point.

23. True or False? In general, a well-designed ESP can operate effectively with a small percentage(less than 10) of its wires out-of-service.

24. True or False? Dust discharge hopper pluggage is not a major concern for ESPs.

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1. b. Cause acids and moisture to condense on internal components of the ESPAir inleakage at flanges or collector access points in high-temperature systems (hot-side ESPs)may cause acids and moisture to condense on internal components of the ESP.

2. b. Gas dew pointGas streams of high temperature should be maintained above the gas dew point. When the temper-ature falls below the gas dew point, moisture or acid can condense on ESP components and possi-bly cause corrosion.

3. c. InsulationSince most ESPs are installed in the field, it is important to check that all surfaces and areas ofpotential heat loss are adequately covered with insulation.

4. ChecklistBefore the ESP is started, the installation crew should prepare and use a checklist.

5. d. All of the aboveThe following are some ESP components that should be checked before starting the collector:

• Hoppers and discharge devices

• Rappers

• Discharge and collection electrodes

6. VoltageCurrentTwo very important parameters monitored by meters on T-R sets and used to evaluate ESP perfor-mance are voltage and current.

7. TrueIndividual T-R set values for voltage and current are important; however, the trends for voltageand current noted within an entire ESP are more valuable in assessing performance. T-R set read-ings for current, voltage, and sparking should follow certain patterns from the inlet to the outletfields.

8. b. Current densityAs particulate matter is removed from the gas stream, the current density should increase from theinlet to the outlet fields. The dust concentration in the inlet sections will suppress the current.Increased current density is needed in the outlet sections where there is a greater percentage ofvery small particles.

9. c. Light differentialAn opacity monitor (transmissometer) measures light differential. An opacity monitor comparesthe amount of light generated and transmitted by the instrument on one side of the gas stream withthe quantity measured on the other side of the gas stream.

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10. TrueOpacity monitors are useful tools to aid in optimization of spark rate, power levels and rappingcycles in ESPs.

11. FalseChanges in flue gas temperature have an important effect on particle resistivity. In fact, while gastemperature variations may have some effect on corona discharge characteristics and physicalcharacteristics of the ESP (corrosion, expansion/contraction), their most important effect is on par-ticle resistivity. See Figure 3-1.

12. Gas flow rateOperating parameters such as specific collection area, superficial velocity, and treatment time aredependent on the gas flow rate.

13. FalseActually, gas flow through the ESP is not evenly distributed. ESP manufacturers settle for whatthey consider to be an acceptable variation.

14. a. Air Load V-I CurveAir-Load V-I Curve tests are generally conducted on cool, inoperative ESPs through which no gasis flowing.

15. TrueWhen ESPs are equipped with identical fields, the air-load curves for each field should be verysimilar.

16. b. RightAir Load V-I curves for a given ESP field will generally shift to the right if plates are dirty com-pared to previous tests. Dirty plates suppress the current. It takes a higher voltage to generate thesame amount of current as with a “clean plate” condition.

17. a. LeftGas-load curves are similar to air-load curves except the gas-load curves are shifted to the leftcompared to the air-load curves. Gas-load curves are generated while the unit is on-line. Thecurves are generally shifted to the left because sparking occurs at lower voltage and current whenparticles are present.

18. FalseGas-load curves for a given ESP field generally vary on a day-to-day basis. Curve positions canchange due to fluctuations in the amount of dust on the plates, gas flow, particulate loadings, tem-perature, and resistivity.

19. TrueHigh dust resistivity is characterized by the tendency toward high spark rates at low current levels.

20. b. Rapping systemExcessive dust buildup on the collecting plates or discharge wires can be caused by failure of therapping system.

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21. d. All of the aboveWire failure can be caused by the following:

• Electrical erosion

• Mechanical erosion

• Corrosion.

22. FalseLike baghouses, ESPs are affected by operating temperatures falling below the acid or moisturedew point. At temperatures below the acid or moisture dew point, acid or moisture can condenseon ESP components and cause corrosion.

23. TrueIn general, a well-designed ESP can operate effectively with a small percentage (less than 10) ofits wires out-of-service.

24. FalseDust discharge hopper pluggage is a major concern for ESPs. Hopper pluggage can permanentlydamage an ESP.

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Bibliography

Bibbo, P. P. 1982. Electrostatic precipitators. In L. Theodore and A. Buonicore (Eds.), Air PollutionControl Equipment-Selection, Design, Operation and Maintenance (pp.3-44). Englewood Cliffs,NJ: Prentice Hall.

Cross, F. L., and H. E. Hesketh. (Eds.) 1975. Handbook for the Operation and Maintenance of Air Pol-lution Control Equipment. Westport, CT: Technomic Publishing.

Englebrecht, H. L. 1980. Mechanical and electrical aspects of electrostatic precipitator O&M. In R. A.Young and F. L. Cross (Eds.), Operation and Maintenance for Air Particulate Control Equipment(pp. 283-354). Ann Arbor, MI: Ann Arbor Science.



Szabo, M. F., and R. W. Gerstle. 1977. Electrostatic Precipitator Malfunctions in the Electric UtilityIndustry. EPA 600/2-77-006.

Szabo, M. F., Y. M. Shah, and S. P. Schliesser. 1981. Inspection Manual for Evaluation of ElectrostaticPrecipitator Performances. EPA 340/1-79-007.


U.S. Environmental Protection Agency. 1987, August. Recommended Recordkeeping Systems for AirPollution Control Equipment. Part I, Particulate Matter Controls. EPA 340/1-86-021.

U.S. Environmental Protection Agency. 1993. Monitoring, Recordkeeping, and Reporting Require-ments for the Acid Rain Program. In Code of Federal Regulations - Protection of the Environment.40 CFR 75. Washington, D.C.; U.S. Government Printing Office.

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training modules in esp

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