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MODULE 1

INTRODUCTION

Air pollution occurs when harmful substances including particulates and biological

molecules are introduced into Earth's atmosphere. It may cause diseases, allergies or death of

humans; it may also cause harm to other living organisms such as animals and food crops, and

may damage the natural or built environment. Human activity and natural processes can both

generate air pollution.

Indoor air pollution and poor urban air quality are listed as two of the world's worst toxic

pollution problems in the 2008 Blacksmith Institute World's Worst Polluted Places

report.[1] According to the 2014 World Health Organization report, air pollution in 2012 caused

the deaths of around 7 million people worldwide,[2] an estimate roughly echoed by one from

the International Energy Agency.[3][4]

An air pollutant is a substance in the air that can have adverse effects on humans and the

ecosystem. The substance can be solid particles, liquid droplets, or gases. A pollutant can be of

natural origin or man-made. Pollutants are classified as primary or secondary. Primary pollutants

are usually produced from a process, such as ash from a volcanic eruption. Other examples

include carbon monoxide gas from motor vehicle exhaust, or the sulfur dioxide released from

factories. Secondary pollutants are not emitted directly. Rather, they form in the air when

primary pollutants react or interact. Ground level ozone is a prominent example of a secondary

pollutant. Some pollutants may be both primary and secondary: they are both emitted Substances

emitted into the atmosphere by human activity include:

Carbon dioxide (CO2) - Because of its role as a greenhouse gas it has been described as "the

leading pollutant"[5] and "the worst climate pollution".[6] Carbon dioxide is a natural

component of the atmosphere, essential for plant life and given off by the human respiratory

system.[7] This question of terminology has practical effects, for example as determining

whether the U.S. Clean Air Act is deemed to regulate CO2 emissions. CO2 currently forms

about 405 parts per million (ppm) of earth's atmosphere, compared to about 280 ppm in pre-

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industrial times,[9] and billions of metric tons of CO2 are emitted annually by burning

of fossil fuels.[10] CO2 increase in earth's atmosphere has been accelerating.[11]

Sulfur oxides (SOx) - particularly sulfur dioxide, a chemical compound with the formula

SO2. SO2 is produced by volcanoes and in various industrial processes. Coal and petroleum

often contain sulfur compounds, and their combustion generates sulfur dioxide. Further

oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, and

thus acid rain.[2] This is one of the causes for concern over the environmental impact of the

use of these fuels as power sources.

Nitrogen oxides (NOx) - Nitrogen oxides, particularly nitrogen dioxide, are expelled from

high temperature combustion, and are also produced during thunderstorms by electric

discharge. They can be seen as a brown haze dome above or a plume downwind of cities.

Nitrogen dioxide is a chemical compound with the formula NO2. It is one of several nitrogen

oxides. One of the most prominent air pollutants, this reddish-brown toxic gas has a

characteristic sharp, biting odor.

Carbon monoxide (CO) - CO is a colorless, odorless, toxic yet non-irritating gas. It is a

product of incomplete combustion of fuel such as natural gas, coal or wood. Vehicular

exhaust is a major source of carbon monoxide.

Volatile organic compounds (VOC) - VOCs are a well-known outdoor air pollutant. They are

categorized as either methane (CH4) or non-methane (NMVOCs). Methane is an extremely

efficient greenhouse gas which contributes to enhanced global warming. Other hydrocarbon

VOCs are also significant greenhouse gases because of their role in creating ozone and

prolonging the life of methane in the atmosphere. This effect varies depending on local air

quality. The aromatic NMVOCs benzene, toluene and xylene are suspected carcinogens and

may lead to leukemia with prolonged exposure. 1,3-butadiene is another dangerous

compound often associated with industrial use.

Particulates, alternatively referred to as particulate matter (PM), atmospheric particulate

matter, or fine particles, are tiny particles of solid or liquid suspended in a gas. In contrast,

aerosol refers to combined particles and gas. Some particulates occur naturally, originating

from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray.

Human activities, such as the burning of fossil fuels in vehicles, power plants and various

industrial processes also generate significant amounts of aerosols. Averaged worldwide,

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anthropogenic aerosols—those made by human activities—currently account for

approximately 10 percent of our atmosphere. Increased levels of fine particles in the air are

linked to health hazards such as heart disease,[12] altered lung function and lung cancer.

Persistent free radicals connected to airborne fine particles are linked to cardiopulmonary

disease.[13][14]

Toxic metals, such as lead and mercury, especially their compounds.

Chlorofluorocarbons (CFCs) - harmful to the ozone layer; emitted from products are

currently banned from use. These are gases which are released from air conditioners,

refrigerators, aerosol sprays, etc. On release into the air, CFCs rise to the stratosphere. Here

they come in contact with other gases and damage the ozone layer. This allows harmful

ultraviolet rays to reach the earth's surface. This can lead to skin cancer, eye disease and can

even cause damage to plants.

Ammonia (NH3) - emitted from agricultural processes. Ammonia is a compound with the

formula NH3. It is normally encountered as a gas with a characteristic pungent odor.

Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving

as a precursor to foodstuffs and fertilizers. Ammonia, either directly or indirectly, is also a

building block for the synthesis of many pharmaceuticals. Although in wide use, ammonia is

both caustic and hazardous. In the atmosphere, ammonia reacts with oxides of nitrogen and

sulfur to form secondary particles.[15]

Odours — such as from garbage, sewage, and industrial processes

Radioactive pollutants - produced by nuclear explosions, nuclear events, war explosives, and

natural processes such as the radioactive decay of radon.

Secondary pollutants include:

Particulates created from gaseous primary pollutants and compounds in photochemical

smog. Smog is a kind of air pollution. Classic smog results from large amounts of coal

burning in an area caused by a mixture of smoke and sulfur dioxide. Modern smog does not

usually come from coal but from vehicular and industrial emissions that are acted on in the

atmosphere by ultraviolet light from the sun to form secondary pollutants that also combine

with the primary emissions to form photochemical smog.

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Ground level ozone (O3) formed from NOx and VOCs. Ozone (O3) is a key constituent of the

troposphere. It is also an important constituent of certain regions of the stratosphere

commonly known as the Ozone layer. Photochemical and chemical reactions involving it

drive many of the chemical processes that occur in the atmosphere by day and by night. At

abnormally high concentrations brought about by human activities (largely the combustion

of fossil fuel), it is a pollutant, and a constituent of smog.

Peroxyacetyl nitrate (C2H3NO5) - similarly formed from NOx and VOCs.

Minor air pollutants include:

A large number of minor hazardous air pollutants. Some of these are regulated in USA under

the Clean Air Act and in Europe under the Air Framework Directive

A variety of persistent organic pollutants, which can attach to particulates

Persistent organic pollutants (POPs) are organic compounds that are resistant to environmental

degradation through chemical, biological, and photolytic processes. Because of this, they have

been observed to persist in the environment, to be capable of long-range transport,

bioaccumulate in human and animal tissue, biomagnify in food chains, and to have potentially

significant impacts on human health and the environment.

d directly and formed from other primary pollutants.

Anthropogenic (man-made) sources:

Stationary sources include smoke stacks of power plants, manufacturing facilities

(factories) and waste incinerators, as well as furnaces and other types of fuel-burning heating

devices. In developing and poor countries, traditional biomass burning is the major source of

air pollutants; traditional biomass includes wood, crop waste and dung.[16][17]

Mobile sources include motor vehicles, marine vessels, and aircraft.

Controlled burn practices in agriculture and forest management. Controlled or prescribed

burning is a technique sometimes used in forest management, farming, prairie restoration or

greenhouse gas abatement. Fire is a natural part of both forest and grassland ecology and

controlled fire can be a tool for foresters. Controlled burning stimulates the germination of

some desirable forest trees, thus renewing the forest.

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Fumes from paint, hair spray, varnish, aerosol sprays and other solvents

Waste deposition in landfills, which generate methane. Methane is highly flammable and

may form explosive mixtures with air. Methane is also an asphyxiant and may displace

oxygen in an enclosed space. Asphyxia or suffocation may result if the oxygen concentration

is reduced to below 19.5% by displacement.

Military resources, such as nuclear weapons, toxic gases, germ warfare and rocketry

Dust from natural sources, usually large areas of land with little or no vegetation

Methane, emitted by the digestion of food by animals, for example cattle

Radon gas from radioactive decay within the Earth's crust. Radon is a colorless, odorless,

naturally occurring, radioactive noble gas that is formed from the decay of radium. It is

considered to be a health hazard. Radon gas from natural sources can accumulate in

buildings, especially in confined areas such as the basement and it is the second most

frequent cause of lung cancer, after cigarette smoking.

Smoke and carbon monoxide from wildfires

Vegetation, in some regions, emits environmentally significant amounts of Volatile organic

compounds (VOCs) on warmer days. These VOCs react with primary anthropogenic

pollutants—specifically, NOx, SO2, and anthropogenic organic carbon compounds — to

produce a seasonal haze of secondary pollutants.[18] Black gum, poplar, oak and willow are

some examples of vegetation that can produce abundant VOCs. The VOC production from

these species result in ozone levels up to eight times higher than the low-impact tree

species.[19]

Volcanic activity, which produces sulfur, chlorine, and ash particulates

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MODULE 2

METEOROLOGY

Lapse rate is the rate at which Earth's atmospheric temperature decreases with an increase in

altitude, or increases with the decrease in altitude. [1][2] Lapse rate arises from the word lapse,

in the sense of a gradual change.

Environmental lapse rate and stability

The environmental lapse rate (ELR), is the rate of decrease of temperature with altitude in

the stationary atmosphere at a given time and location. As an average, the International Civil

Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a

temperature lapse rate of 6.49 K/km[15] (3.56 °F or 1.98 °C/1,000 ft) from sea level to

11 km (36,090 ft or 6.8 mi). From 11 km up to 20 km (65,620 ft or 12.4 mi), the constant

temperature is −56.5 °C (−69.7 °F), which is the lowest assumed temperature in the ISA.

The standard atmosphere contains no moisture. Unlike the idealized ISA, the temperature of the

actual atmosphere does not always fall at a uniform rate with height. For example, there can be

an inversion layer in which the temperature increases with altitude.

The atmospheric lapse rate ( ) refers to the change of an atmospheric variable with a change

of altitude, the variable being temperature unless specified otherwise (such

as pressure, density or humidity).[1][2][3] While usually applied to Earth's atmosphere, the concept

of lapse rates can be extended to atmospheres (if any) that exist on other planets.

Lapse rates are usually expressed as the amount of temperature change associated with a

specified amount of altitude change, such as 9.8 K per kilometre, 0.0098 K per metre or the

equivalent 5.4 °F per 1000 feet. If the atmospheric air cools with increasing altitude, the lapse

rate may be expressed as a negative number. If the air heats with increasing altitude, the lapse

rate may be expressed as a positive number.

The lapse rate is most often denoted by the Greek capital letter Gamma, or Γ, but not always.

For example, the U.S. Standard Atmosphere uses L to denote lapse rates: A few others use the

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Greek lower case letter gamma, , which is an unfortunate choice since gamma is also used for

the specific heat ratio.

Atmospheric stability is a term used to qualitatively describe the amount of vertical motion of the

air in the lower atmosphere (the troposphere). In broad general terms, the atmospheric stability

can be characterized by these four categories:

A very stable atmosphere is one that has very little, if any, vertical motion of the air.

A stable atmosphere is one that discourages vertical motion but does have some

motion of the air.

An unstable atmosphere is one that encourages continual vertical motion of the air,

upwards or downwards.

A neutral atmosphere is one that neither discourages nor encourages vertical motion

of the air and is often referred to as conditionally stable.

The numerical value of the environmental lapse rate determines the stability category of the

atmospheric air. Referring to the adjacent diagram:

If the environmental lapse rate (i.e., the actual ambient temperature gradient) is

greater than zero as for the rate marked 1 in the diagram, then an inversion layer is

present and the atmospheric temperature increases with altitude. There is essentially

no vertical turbulence and the atmosphere is said to be very stable or extremely

stable.

If the environmental lapse rate is greater than – 5.5 K/km as for the rate marked 2 in

the diagram, then there is some small amount of vertical turbulence and the

atmosphere is said to be stable. It is also referred to as being sub-adiabatic.

If the environmental lapse rate lies between the wet adiabatic lapse rate and the dry

adiabatic lapse rate as for the rate marked 3 in the diagram, then the atmosphere is

said to be neutral. That designation would apply to the U.S. Standard Atmosphere of

– 6.5 K/km in most cases.

If the environmental lapse rate is less than the dry adiabatic lapse rate as for the line

marked 4 in the diagram, then there turbulence in the atmosphere and it is said to

be unstable. It is also referred to as being super-adiabatic.

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Although not shown in the diagram, if the environmental lapse rate were zero

(perfectly vertical), then the atmosphere would be in an isothermal condition (no

change of temperature with altitude) and would be also be said to be very stable.

Importance of understanding atmospheric stability

An understanding and knowledge of atmospheric stability is important for many reasons.

What follows is a brief discussion of some of those reasons:

Probably one of the most important reasons is that atmospheric turbulence and mixing

plays a major role in air pollution dispersion modeling. Turbulence and mixing is

provided by an unstable atmosphere and thus enhances the dispersion of air pollutant,

while a stable atmosphere inhibits turbulence and results in very poor dispersion of air

pollutants.[11]

A stable atmosphere inhibits rain fall, while an unstable atmosphere encourages rainfall

and thunder storms. A stable atmosphere also inhibits forest fire activity and an

understanding of atmospheric stability helps explain certain aspects of forest fire

behavior.

A certain amount of atmospheric instability is important for glider pilots, since without it

the thermals needed for glider flight would not form. Understanding of atmospheric

stability is also important for the safety of glider pilots because high atmospheric

instability may lead to thunderstorms. The atmospheric stability has a large impact on the

deposition and drift of aerially applied sprays of various farm crop protection materials.

wind rose is a graphic tool used by meteorologists to give a succinct view of

how windspeed and direction are typically distributed at a particular location. Historically,

wind roses were predecessors of the compass rose (found on charts), as there was no

differentiation between a cardinal direction and the wind which blew from such a direction.

Using a polar coordinate system of gridding, the frequency of winds over a time period is

plotted by wind direction, with color bands showing wind speed ranges. The direction of the

longest spoke shows the wind direction with the greatest frequency.

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Plume behaviour The mixing or dispersion of the waste gases and products into the

atmosphere is called plume behaviour. In stable air, and where the vertical movement of the

plume is slow, a fanning plume is produced. This wide, shallow, spreading plume is very

common after calm clear nights. A temperature inversion limits the rise of the plume into the

upper atmosphere. The following diagram shows normal air movements, and a temperature

inversion. A layer of warm air limits the rise of the plume into the upper atmosphere, and

creates a higher concentration of polluted air at lower levels. This plume exists for several

hours

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In windy conditions the plume can swirl up and down. This is a looping plume, and is

common in the afternoon. Moderate and strong winds are formed on sunny days creating

unstable conditions. This plume exists for several hours. A Looping Plume2 With moderate

winds and overcast days, the plume may become a coning plume. This plume is wider than it

is deep, and is elliptical in shape. This plume exists for several hours

The fumigating plume is short-lived (fraction of an hour), but reaches the earth's surface.

Fumigating plumes occur when the conditions move from stable to more unsettled. A fanning

plume might have developed overnight under stable conditions. As the day heats up, unstable air

is produced. This unstable air affects the fanning plume, causing the plume to move vertically up

and down. These plumes can cause localised pollution. Fumigating plumes become looping or

coning plumes as the air conditions stabilise.

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Module 3

Sampling

Air Sampling Techniques As is the case with aquatic systems, it is always better if possible

to evaluate pollutant levels in the atmosphere in situ. On some occasions however, it may

become necessary to obtain an ambient air sample. Any sampling process has limitations, but

provided standard techniques are followed the data obtained from analysis of air samples

should be a reasonable reflection of ambient air. Most air pollution monitoring equipment

performs the act of sampling and analysis in one action. This is because most modern

equipment is capable of so called real time measurement – whilst older equipment had to

perform intermittent sampling where there was a time lag between when the sample was

obtained and when data from it was available. This type of system provides average data (not

real time). Today almost all gaseous pollutants are monitored by real time analysis.

Particulate pollutants on the other hand are still mostly monitored by intermittent sampling,

even though real time methods are available. When obtaining a sample for air pollution

analysis a number of considerations should be kept in mind. • The sample should be of

sufficient quantity to allow analysis. Most pollutants are present in very low levels and

require a large volume of gas for accurate measurement. • When pollutants are present in

very small quantities it is very easy to contaminate them. For this reason great care should be

taken to purge sampling containers if grab samples are used. • Collection and analysis

limitations may require collection over extended periods which means that air quality data

may often only be a 24 hour average (depends on sampling and analysis time). This is not

necessarily as great a limitation as it might first seem as many real time measuring

instruments produce so much data that they are often set to give hourly averages to make the

data more understandable. Air pollutant sampling systems require gases or particles to be

drawn to the surface of a collecting medium or a sensor. These functions are usually

accomplished by sampling trains, which may include a vacuum pump, vacuum trap, a flow

regulator and a collecting device or sensing unit. A bubbler-type gas sampling train used to

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collect samples on an intermittent basis is illustrated in Figure 4.1. Sampling trains for gases

may also utilize filters to present particles from entering the collection unit

Sampling procedures

Sampling Procedures Depending on the type of information required and limitations of

collection and analysis, sampling may be conducted by static, grab, intermittent or

continuous procedures. These sampling procedures provide air quality data representing a

range of averaging times, from instantaneous (for continuous monitoring instruments) to a

30-day average (for some static samplers). Static or passive sampling may involve the

collection of contaminants by the diffusion of gases to a collection medium, the

sedimentation of heavy particles into a container, or the impact of particles on sticky paper.

In the early days of air pollution monitoring static sampling was commonly used. It had the

advantage of simplicity and low cost. However, because of the long averaging times (a week

to 30 days), static sampling data were limited in use for ambient air pollution control. Some

examples of static sampling devices include deposit gauges and lead candles. Static sampling

is widely employed for personal monitoring in industrial environments and for measurement

of indoor air quality parameters such as HCHO (methanal) and radon.

Selection of Locations for Monitoring Equipment

In order to obtain meaningful and useful data it is not only important to use the correct

sampling procedures, but also to choose an appropriate position to sample from. In order to

do this the objectives of the air monitoring program must be considered. For example a site

next to a high stack would not be suitable to determine the level of stack emissions as the

emissions would be carried far away from the sampling point. Such an analysis would

require in stack monitoring. General Requirements for Site Selection A number of factors

such as the purpose of monitoring, number and type of instruments required, and duration of

measurements ultimately determine the choice of a suitable site2 . There are few reliable

guidelines for selecting suitable monitoring sites. Probably the best available general guide

comes from AS29223 . Monitoring sites should be easily accessible (as difficult access lead

to data loss), but it should not be easily vandalised – as this is a major source of data loss.

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Specific Requirements for Site Selection For all air quality measurements it is recommended

that the following steps be taken when determining a suitable monitoring site: • determine

the purpose(s) of the site(s) and ensure that data obtained will be: - relevant to the stated

objectives or goals, and - representative of the local, urban or regional air • optimise location

choice based on published guideline criteria to generally suit the purpose(s) of the

monitoring site(s) • use scientific judgement, past experience and common sense in precisely

siting instrumentation or monitoring stations ensuring that there are no local interferences

(chemical or physical) which could cast doubt on the validity of the data obtained2 .

The Importance of Meteorological Monitoring Meteorological conditions play a large part in

determining air quality at any particular place and time. Emission levels may be fairly

constant in most ambient air, but

changing weather conditions can produce dramatic changes in air quality and ambient

pollution levels. Factors such as: • wind dispersion rates (velocity and direction) •

temperature inversions • photochemical reactions, and • rain. Wind Direction and Velocity

Measurements These should be available whenever possible for each sampling site. Wind

sensors (anemometers) should be installed to measure the general winds of the local urban or

regional area under investigation and should not be influenced by physical obstructions. The

standard exposure of anemometers over level open terrain is 10 metres above the ground.

Any obstructions to the airflow, such as buildings, hills or tall trees, should be distant by at

least five, and preferably ten, times their height. In some cases rough guides to general wind

direction can be provided by double sided adhesive tapes on the outside of devices such as

deposit gauges. Ambient Air Monitoring Choice of Monitoring Equipment For almost every

type of air pollutant there are several different acceptable methods of analysis. The type of

equipment and methodology used for analysis may be determined by many factors such as: •

cost • the number of data points required • the purpose for which the data are being used • the

time interval required between data points • the devices power requirements • the type of air

pollutant, and • the environment in which the monitoring equipment is being placed. Cost is

probably the most important factor for many monitoring programs. Obviously everyone

would prefer to have all monitoring stations of the real time continuous analysis type – but

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the cost of this would in many cases be prohibitive. Hence less expensive alternatives must

be found. For example a deposit gauge may be used instead of a gravity microbalance for

particulate matter. If data are being used for a rough estimate then much less expensive (and

reliable) methods may be used. If data are being used for EPA monitoring or legal purposes

to determine emission levels, then much more expensive accredited methods are required. If

only a few data points per week are required, then a different type of monitoring may be used

than if continuous monitoring is required. For example a high volume particulate sampler

may be used instead of a microbalance

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MODULE-4

CONTROL TECHNIQUES

PM stands for particulate matter (also called particle pollution): the term for a mixture of solid

particles and liquid droplets found in the air. Some particles, such as dust, dirt, soot, or smoke,

are large or dark enough to be seen with the naked eye. Others are so small they can only be

detected using an electron microscope.

Particle pollution includes:

PM10 : inhalable particles, with diameters that are generally 10 micrometers and smaller; and

PM2.5 : fine inhalable particles, with diameters that are generally 2.5 micrometers and smaller.

o How small is 2.5 micrometers? Think about a single hair from your head. The average human

hair is about 70 micrometers in diameter – making it 30 times larger than the largest fine particle.

Sources of PM

These particles come in many sizes and shapes and can be made up of hundreds of different

chemicals.

Some are emitted directly from a source, such as construction sites, unpaved roads, fields,

smokestacks or fires.

Most particles form in the atmosphere as a result of complex reactions of chemicals such as

sulfur dioxide and nitrogen oxides, which are pollutants emitted from power plants, industries

and automobiles.

What are Gaseous Pollutants?

The major gaseous pollutants include sulfur dioxide (SO2), carbon monoxide (CO), and

nitrogen oxides (NOx) as well as ozone (O3). The primary source of these gases is the

combustion of fossil fuels in power plants, various industrial processes, and motor vehicles and

equipment. Each of these pollutants, in their gaseous form, can cause harm to human health and

the environment. They are also essential ingredients in chemical and physical transformations

which can result in further damages (see sections on acid deposition and visibilit

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Gravity Settling Chambers:

This is a simple particulate collection device using the principle of gravity to settle the

particulate matter in a gas stream passing through its long chamber. The primary requirement of

such a device would be a chamber in which the carrier gas velocity is reduced so as to allow the

particulate matter to settle out of the moving gas stream under the action of gravity. This

particulate matter is then collected at the bottom of the chamber. The chamber is cleaned

manually to dispose the waste.

The gas velocities in the settling chamber must be sufficiently low for the particles to settle

due to gravitational force. Literature indicates that gas velocity less than about 3 m/s is needed

to prevent re-entrainment of the settled particles. The gas velocity of less than 0.5 m/s will

produce good results.

Curtains, rods, baffles and wire mesh screens may be suspended in the chamber to minimize

turbulence and to ensure uniform flow. The pressure drop through the chamber is usually low

and is due to the entrance and exit losses.

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2. Cyclones:

Settling chambers discussed above are not effective in removing small particles. Therefore,

one needs a device that can exert more force than gravity force on the particles so that they can

be removed from the gas stream. Cyclones use centrifugal forces for removing the fine

particles. They are also known as centrifugal or inertial separators.

The cyclone consists of a vertically placed cylinder which has an inverted cone attached to

its base. The particulate laden gas stream enters tangentially at the inlet point to the cylinder. The

velocity of this inlet gas stream is then transformed into a confined vortex, from which

centrifugal forces tend to drive the suspended particles to the walls of the cyclone. The vortex

turns upward after reaching at the bottom of the cylinder in a narrower inner spiral. The clean gas

is removed from a central cylindrical opening at the top, while the dust particles are collected at

the bottom in a storage hopper by gravity.

The efficiency of a cyclone chiefly depends upon the cyclone diameter. For a given

pressure drop, smaller the diameter, greater is the efficiency, because centrifugal action increases

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with decreasing radius of rotation. Centrifugal forces employed in modern designs vary from 5 to

2500 times gravity depending on the diameter of the cyclone. Cyclone efficiencies are greater

than 90% for the particles with the diameter of the order of 10 µ. For particles with diameter

higher than 20 µ, efficiency is about 95%.

The efficiency of a cyclone can be increased by the use of cyclones either in parallel or in

series. A brief explanation of both arrangements is given below:

a)MultipleCyclones:

A battery of smaller cyclones, operating in parallel, designed for a constant pressure drop in

each chamber. The arrangement is compact, with convenient inlet and outlet arrangements. They

can treat a large gas flow, capturing smaller particles.

b)Cyclonesinseries:

Two cyclones are used in series. The second cyclone removes the particles that were not

collected in the first cyclone, because of the statistical distribution across the inlet, or accidental

re-entrainment due to eddy currents and re-entrainment in the vortex core, thus increasing the

efficiency.

Typical applications of cyclones are:

i) For the control of gas borne particulate matter in industrial operations such as cement

manufacture, food and beverage, mineral processing and textile industries.

ii) To separate dust in the disintegration operations, such as rock crushing, ore handling and sand

conditioning in industries.

iii) To recover catalyst dusts in the petroleum industry.

iv) To reduce the fly ash emissions.

The operating problems are:

i) Erosion: Heavy, hard, sharp edged particles, in a high concentration, moving at a high velocity

in the cyclone, continuously scrape against the wall and can erode the metallic surface.

ii) Corrosion: If the cyclone is operating below the condensation point, and if reactive gases are

present in the gas stream, then corrosion problems can occur. Thus the product should be kept

above the dew point or a stainless steel alloy should be used.

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iii) Build - up: A dust cake builds up on the cyclone walls, especially around the vortex finder, at

the ends of any internal vanes, and especially if the dust is hygroscopic. It can be a severe

problem.

3. Electrostatic Precipitators:

Electrostatic precipitators (ESP) are particulate collection devices that use electrostatic force to

remove the particles less than 5 micron in diameter. It is difficult to use gravity settlers and

cyclones effectively for the said range of particles. Particles as small as one-tenth of a

micrometer can be removed with almost 100% efficiency using electrostatic precipitators.

The principle behind all electrostatic precipitators is to give electrostatic charge to particles in a

given gas stream and then pass the particles through an electrostatic field that drives them to a

collecting electrode.

The electrostatic precipitators require maintenance of a high potential difference between the two

electrodes, one is a discharging electrode and the other is a collecting electrode. Because of the

high potential difference between the two electrodes, a powerful ionizing field is formed. Very

high potentials - as high as 100 kV are used. The usual range is 40- 60 kV. The ionization

creates an active glow zone (blue electric discharge) called the 'corona' or 'corona glow'. Gas

ionization is the dissociation of gas molecules into free ions.

As the particulate in the gas pass through the field, they get charged and migrate to the

oppositely charged collecting electrode, lose their charge and are removed mechanically by

rapping, vibration, or washing to a hopper below.

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The ESP is made of a rectangular or cylindrical casing. All casings provide an inlet and outlet

connection for the gases, hoppers to collect the precipitated particulate and the necessary

discharge electrodes and collecting surfaces. There is a weatherproof, gas tight enclosure over

the precipitator that houses the high voltage insulators.

Electrostatic precipitators also usually have a number of auxiliary components, which include

access doors, dampers, safety devices and gas distribution systems. The doors can be closed and

bolted under normal conditions and can be opened when necessary for inspection and

maintenance. Dampers are provided to control the quantity of gas. It may either be a guillotine, a

louver or some such other device that opens and closes to adjust gas flow.

The safety grounding system is extremely important and must always be in place during

operation and especially during inspection. This commonly consists of a conductor, one end of

which is grounded to the casing, and the other end is attached to the high voltage system by an

insulated operating lever.

The precipitator hopper is an integral part of the precipitator shell and is made of the same

material as the shell. Since ESPs require a very high voltage direct current source of energy for

operation, transformers are required to step up normal service voltages to high voltages.

Rectifiers convert the alternating current to unidirectional current.

Types of electrostatic precipitators:

There are many types of ESPs in use throughout the world. A brief description of three different

types is given below:

A) Single stage or two stage:

In a single stage ESP, gas ionization and particulate collection are combined in a single step. An

example is the "Cottrell" single-stage precipitator. Because it operates at ionizing voltages from

40,000 to 70,000 volts, DC, it may also be called a high voltage precipitator. It is used

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extensively for heavy duty applications such as utility boilers, large industrial boilers and cement

kilns.

In the two-stage precipitator particles are ionized in the first chamber and collected in the second

chamber. For example, "Penny"- the two stage precipitator uses DC voltages from 11,000 to

14,000 volts for ionization and is referred to as a low voltage precipitator. Its use is limited to

low inlet concentration, normally not exceeding 0.025 grains per cubic feet. It is the most

practical collection technique for many hydrocarbon applications, where the initial clear exhaust

stack turns into a visible emission as vapor condenses.

B) Pipe type or Plate type:

In the pipe type electrostatic precipitators, a nest of parallel pipes form the collecting electrodes,

which may be round, or square. Generally the pipe is about 30 cm in diameter or less. Most

commonly a wire with a small radius of curvature, suspended along the axis of each pipe, is

used. The wires must be weighted or supported to retain proper physical tension and location,

electrically insulated from the support grid and strong enough to withstand rapping or vibration

for cleaning purpose. The gas flow is axial from bottom to top.

The pipe electrodes, may be 2-5 m high. Spacing between the discharge electrode and collecting

electrode ranges from 8-20 cm. Precipitation of the aerosol particles occurs on the inner pipe

walls, from which the material can be periodically removed by rapping of pipes or by flushing

water. The pipe type precipitator is generally used for the removal of liquid particles.

In the plate type precipitators the collection electrodes consist of parallel plates. The discharge

electrodes are again wires with a small curvature. Sometimes square or twisted rods can be used.

The wires are suspended midway between the parallel plates and usually hang free with a weight

suspended at the bottom to keep them straight. Discharge electrodes are made from non-

corrosive materials like tungsten, and alloys of steel and copper. The gas flow is parallel to the

plates.

The plates may be 1-2 m wide and 3-6 m high. The parallel plates should be at equally spaced

intervals (between 15 and 35 cm). The collection of the aerosols takes place on the inner side of

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the parallel plates. The dust material can be removed by rapping either continuously or

periodically. The dust particles removed fall into the hopper at the base of the precipitator.

Collection electrodes should have a minimum amount of collection surface, bulking resistance,

resistance to corrosion and a consistent economic design.

Plate type precipitators are horizontal or vertical, depending on the direction of the gas flow. Gas

velocities are maintained at 0.5-0.6 m/s in these precipitators. They're used for collection of solid

particulate.

C) Dry and Wet Precipitators:

If particulate matter is removed from the collecting electrodes, by rapping only, it is known as a

dry precipitator. If, on the other hand, water or any other fluid is used for removal of the solid

particulate matter, then it is known as a wet precipitator. In general, wet precipitators are more

efficient. However, it is the dry type plate precipitators that are predominantly used.

Efficiency:

Generally, the collection efficiency of the electrostatic precipitator is very high, approaching

100%. Many installations operate at 98 and 99% efficiency. Some materials ionize more readily

than others and are thus more adapted to removal by electrostatic precipitation.

Acid mists and catalyst recovery units have efficiencies in excess of 99%. However, for

materials like carbon black, which have very low efficiencies due to very low collection

capacity, by proper combination of an ESP with a cyclone, very high efficiencies can be

achieved. The gas entering the ESP may be pre-treated (i.e., removing a portion of particulate)

by using certain mechanical collectors or by adding certain chemicals to the gas to change the

chemical properties of the gas to increase their capacity to collect on the discharge electrode and

thus increase the efficiency.

The factors affecting the efficiency of electrostatic precipitators are particle resistivity and

particle re-entrainment. Both are explained below:

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A) Particle Resistivity :

Dust resistivity is a measure of the resistance of the dust layer to the passage of a current. For

practical operation, the resistivity should be 107 and 1011 ohm-cm. At higher resistivities,

particles are too difficult to charge. Higher resistivity leads to a decrease in removal efficiency.

At times, particles of high resistivity may be conditioned with moisture to bring them into an

acceptable range.

If the resistivity of the particles is too low,(<10 ohm-cm), little can be done to improve

efficiency. This is due to the fact that the particles accept a charge easily, but they dissipate it so

quickly at the collector electrode, that the particles are re- entrained in the gas stream. This

results in low efficiency.

Particle resistivity depends upon the composition of the dust and the continuity of the dust layer.

Resistivity is also affected by the ESP operating temperature and by the voltage gradient that

exists across the dust layer.

B) Particle re-entrainment:

This is a problem associated with particle charging. It occurs primarily in two situations - due to

either inadequate precipitator area, or inadequate dust removal from the hopper. Re-entrainment

reduces the precipitator performance, because of the necessity of recollecting the dust that had

been previously removed from the carrier gas. The problem can be overcome by a proper design

of the ESP and necessary maintenance.

iv) Precautions are necessary to maintain safety during operation. Proper gas flow

distribution, particulate conductivity and corona spark over rate must be carefully maintained.

v) The negatively charged electrodes during gas ionization produce the ozone.

4. Scrubbers:

Scrubbers are devices that remove particulate matter by contacting the dirty gas stream with

liquid drops. Generally water is used as the scrubbing fluid. In a wet collector, the dust is

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agglomerated with water and then separated from the gas together with the water.

The mechanism of particulate collection and removal by a scrubber can be described as a four-

step process.

i) Transport : The particle must be transported to the vicinity of the

water droplets which are usually 10 to 1000 times larger.

ii) Collision : The particle must collide with the droplet.

iii) Adhesion : This is promoted by the surface tension property.

iv) Precipitation: This involves the removal of the droplets, containing

the dust particles from the gas phase.

The physical principles involved in the operation of the scrubbers are: i) impingement, ii)

interception, iii) diffusion and iv) condensation. A brief description is given below:

i)Impingement :

When gas containing dust is swept through an area containing liquid droplets, dust particles

will impinge upon the droplets and if they adhere, they will be collected by them. If the liquid

droplet is approximately 100 to 300 times bigger than the dust particle, the collection efficiency

of the particles is more, because the numbers of elastic collisions increase.

ii)Interception:

Particles that move with the gas stream may not impinge on the droplets, but can be

captured because they brush against the droplet and adhere there. This is known as interception.

iii)Diffusion:

Diffusion of the particulate matter on the liquid medium helps in the removal of the

particulate matter.

iv)Condensation:

Condensation of the liquid medium on the particulate matter increases the size and weight

of the particles. This helps in easy removal of the particles.

The various types of scrubbers are:

i)Spraytowers.

ii)Venturiscrubbers.

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iii)Cyclonescrubbers.

iv)Packedscrubbers.

v) Mechanical scrubbers.

The simpler types of scrubbers with low energy inputs are effective in collecting particles above

5 - 10 µ in diameter, while the more efficient, high energy input scrubbers will perform

efficiently for collection of particles as small as 1 - 2 µ in diameter.

The advantages of scrubbers are:

i)Lowinitialcost.

ii)Moderately high collection efficiency for small particles.

iii)Applicable for high temperature installations.

iv) They can simultaneously remove particles and gases.

v) There is no particle re- entrainment.

The disadvantages of scrubbers are:

i) High power consumption for higher efficiency.

ii) Moderate to high maintenance costs owing to corrosion and abrasion.

iii) Wet disposal of the collected material.

The scrubbers are used in a variety of applications. Some of the situations are:

i) They're particularly useful in the case of a hot gas that must be cooled for some reason.

ii) If the particulate matter is combustible or if any flammable gas is present, even in trace

amounts, in the bulk gas phase, a scrubber is preferred to an electrostatic precipitator.

iii) Scrubbers can be used when there are waste water treatment systems available on the site,

with adequate reserve capacity to handle the liquid effluent.

iv) Scrubbers are also used when gas reaction and absorption are required simultaneously with

particulate control.

5. Fabric Filters:

Fabric filtration is one of the most common techniques to collect particulate matter from

industrial waste gases. The use of fabric filters is based on the principle of filtration, which is a

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reliable, efficient and economic methods to remove particulate matter from the gases. The air

pollution control equipment using fabric filters are known as bag houses.

Bag Houses:

A bag house or a bag filter consists of numerous vertically hanging, tubular bags, 4 to 18 inches

in diameter and 10 to 40 feet long. They are suspended with their open ends attached to a

manifold. The number of bags can vary from a few hundreds to a thousand or more depending

upon the size of the bag house. Bag houses are constructed as single or compartmental units. In

both cases, the bags are housed in a shell made of rigid metal material. Occasionally, it is

necessary to include insulation with the shell when treating high temperature flue gas. This is

done to prevent moisture or acid mist from condensing in the unit, causing corrosion and rapid

deterioration of the bag house.

Hoppers are used to store the collected dust temporarily before it is disposed in a landfill or

reused in the process. Dust should be removed as soon as possible to avoid packing which would

make removal very difficult. They are usually designed with a 60 degrees slope to allow dust to

flow freely from the top of the hopper to the bottom discharge opening. Sometimes devices such

as strike plates, poke holes, vibrators and rappers are added to promote easy and quick discharge.

Access doors or ports are also provided. Access ports provide for easier cleaning, inspection and

maintenance of the hopper.

A discharge device is necessary for emptying the hopper. Discharge devices can be manual

(slide gates, hinged doors and drawers) or automatic trickle valves, rotary airlock valves, screw

conveyorsorpneumaticconveyors)

Filter Media:

Woven and felted materials are used to make bag filters. Woven filters are used with low energy

cleaning methods such as shaking and reverse air. Felted fabrics are usually used with low

energy cleaning systems such as pulse jet cleaning.

While selecting the filter medium for bag houses, the characteristics and properties of the carrier

gas and dust particles should be considered. The properties to be noted include:

a)Carriergastemperature

b)Carriergascomposition

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c)Gasflowrate

d) Size and shape of dust particles and its concentration

The abrasion resistance, chemical resistance, tensile strength and permeability and the cost of the

fabric should be considered. The fibers used for fabric filters can vary depending on the

industrial application. Some filters are made from natural fibers such as cotton or wool. These

fibers are relatively inexpensive, but have temperature limitations (< 212 F) and only average

abrasion resistance. Cotton is readily available making it very popular for low temperature

simple applications. Wool withstands moisture very well and can be made into thick felts easily.

Synthetic fibers such as nylon, orlon and polyester have slightly higher temperature

limitations and chemical resistance. Synthetic fibers are more expensive than natural fibers.

Polypropylene is the most inexpensive synthetic fiber and is used in industrial applications such

as foundries, coal crushers and food industries. Nylon is the most abrasive resistant synthetic

fiber making it useful for applications filtering abrasive dusts. Different types of fibers with

varying characteristics are available in the market.

Fabric Treatment:

Fabrics are usually pre-treated, to improve their mechanical and dimensional stability. They can

be treated with silicone to give them better cake release properties. Natural fibers (wool and

cotton) are usually preshrunk to eliminate bag shrinkage during operation. Both synthetic and

natural fabrics usually undergo processes such as calendering, napping, singeing, glazing or

coating. These processes increase the fabric life and improve dimensional stability and ease of

bag cleaning.

a) Calendering:

This is the high pressure pressing of the fabric by rollers to flatten, smooth, or

decorate the material. Calendering pushes the surface fibers down on to the body of the filter

medium. This is done to increase surface life, dimensional stability and to give a more uniform

surface to bag fabric.

b) Napping:

This is the scraping of the filter surface across metal points or burrs on a revolving

cylinder. Napping raises the surface fibers, that provides a number of sites for particle collection

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by interception or diffusion. Fabrics used for collecting sticky or oily dusts are occasionally

napped to provide good collection and bag cleaning ease.

c) Singeing:

This is done by passing the filter material over an open flame, removing

any straggly surface fibers. This provides a more uniform surface.

d) Glazing:

This is the high pressure pressing of the fiber at elevated temperatures. The fibers

are fused to the body of the filter medium. Glazing improves the mechanical stability of the filter

and helps reduce bag shrinkage that occurs from prolonged use.

e) Coating:

Coating or resin treating involves immersing the filter material in natural or

synthetic resin such as polyvinyl chloride, cellulose acetate or urea - phenol. This is done to

lubricate the woven fibers or to provide high temperature durability or chemical resistance for

various fabric material.

Operation of a bag house:

The gas entering the inlet pipe strikes a baffle plate, which causes larger particles to fall into a

hopper due to gravity. The carrier gas then flows upward into the tubes and outward through

the fabric leaving the particulate matter as a "cake" on the insides of the bags.

Efficiency during the pre-coat formation is low, but increases as the pre-coat (cake) is

formed, until a final efficiency of over 99% is obtained. Once formed, the pre-coat forms part of

the filtering medium, which helps in further removal of the particulate. Thus the dust becomes

the actual filtering medium. The bags in effect act primarily as a matrix to support the dust cake.

The cake is usually formed within minutes or even seconds.

The accumulation of dust increases the air resistance of the filter and therefore filter bags

have to be periodically cleaned. They can be cleaned by rapping, shaking or vibration, or by

reverse air flow, causing the filter cake to be loosened and to fall into the hopper below. The

normal velocities at which the gas is passed through the bags at 0.4-1m/min. There are many

types of "filter bags" depending on the bag shape, type of housing and method of cleaning the

fabric.

Efficiency:

The efficiency of bag filters may decrease on account of the following factors:

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a) Excessive filter ratios - 'Filter ratio' is defined as the ratio of the carrier gas volume to gross

filter area, per minute flow of the gas. Excessive filter ratios lower particulate removal efficiency

and result in increased bag wear. Therefore, low filter ratios are recommended. Therefore, low

filter ratios are recommended for high concentration of particulate.

b) Improper selection of filter media - While selecting filter media, properties such as

temperature resistance, resistance to chemical attack and abrasion resistance should be taken into

consideration.

Operating Problems:

Various problems during the operation of a bag house are:

a)Cleaning -

At intervals the bags get clogged up with a covering of dust particles that the gas can

no longer pass through them. At that point, the bags have to be cleaned by rapping, shaking or by

reverse air flow by a pulse jet.

b) Rupture of the cloth -

The greatest problem inherent in cloth filters is the rupture of cloth, which results

from shaking. It is often difficult to locate ruptures and when they're found the replacement time

isoften onsiderable.

c)Temperature -

Fabric filters will not perform properly if a gross temperature overload occurs. If

the gas temperature is expected to fluctuate, a fiber material that will sustain the upper

temperaturefluctuationmustbeselected.

Also, whenever the effluent contains a reactive gas like SO2 which can form an acid whenever

the temperature in the bag house falls below the dew point it can create problems. Sometimes it

may even be necessary to provide an auxiliary heater to make sure that the temperature in the

bag house does not fall below acid gas dew point.

d)Bleeding -

This is the penetration of the fabric by fine particles, which is common in fabric filtration.

It can occur if the weave is too open or the filter ratio is very high. The solution is to use a

double layer material or a thick woven fabric.

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e)Humidity -

This is a common and important problem, especially if the dust is hygroscopic. It

would therefore be advisable to maintain moisture free conditions within the bag house, as a

precautionarymeasure.

f)Chemicalattack -

This is another problem associated with fabric filters. The possibility of chemical attack

due to corrosive chemicals present in the effluent. A proper choice of fabric filter will avoid this

problem.

Filter cleaning mechanisms:

The following mechanisms are used for cleaning the filters in a bag

house:

i) Rapping

ii) Shaking

iii) Reverseairflow(backwash)

iv) Pulse jet

Multi-Compartment Type Bag House:

If the requirements of the process being controlled are such that continuous operation is

necessary, the bag filter must be of a multi-compartment type to allow individual units of the bag

filter to be successively off-stream during shaking. This is achieved either manually in small

units or by programming control in large, fully automatic units. In this case, sufficient cloth area

must be provided to ensure that the filtering efficiency will not be reduced during shaking off

periods, when any one of the units is off-stream.

The advantages of a fabric filter are:

i) High collection efficiencies for all particle sizes, especially for particles smaller than 10

micronindiameter.

ii)Simpleconstructionandoperation.

iii)Nominalpowerconsumption.

iv) Dry disposal of collected material.

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The disadvantages of a fabric filter are:

i) Operating limits are imposed by high carrier gas temperatures, high humidity and other

parameters.

ii) High maintenance and fabric replacement costs. Bag houses are difficult to maintain because

of the difficulty in finding and replacing even a single leaking bag. Also as general rule, about

1/4th of the bags will need replacement every year.

iii)Largesizeofequipment.

iv) Problems in handling dusts which may abrade, corrode, or blind the cloth.