ByByImproving Boiler Efficiency Improving Boiler Efficiency

Utility and power plant Course

Assistant Chief EngineerQusay A.M.Saeed

Participant

for

How to Increase boiler Efficiency?

20 proven methods that save up to 30% in fuel

costs

Efficiency Equation

Boiler heat losses

Efficiency improvements

without Expenses

Method 1

Reduce Excess Air

Potential savingPotential saving :5%-10%Problem Problem :unneeded excess air is used, probably to dilute the flue gas so that

smoke is not seen.SolutionsSolutions: reduce excess air to the minimum 10%-15% requird.

Typical Excess Air Values

Excess Air

Normal combustion efficiencies for natural gas at different amounts of excess air and flue gas temperatures are indicated below

68.271.975.679.382.810.081.6

72.175.278.281.284.17.044.9

74.076.779.582.184.75.028.1

75.477.980.482.885.23.015

76.078.480.883.185.42.09.5

600500400300200OxygenAir

Net Stack Temperature1) (oF)Excess %

Combustion Efficiency (%)

Flue Gas Loss Combustion Oil

The relationship between temperature difference flue gas and supply air,

CO2 concentration in the flue gas, and the efficiency loss in the flue gas combustion oil, is expressed in the diagram below.

Method 2

Decrease Flue-Gas TemperaturePotential savingPotential saving :1% for every 40°F.the potential saving are 3%,since many

stacks run 120°F too high.ProblemProblem :Either excess air or fouling water and /or fire side tubes.Solutions:Solutions:1- If there is excess fuel,the fuel rate can be decreased to decease the

temperature .2- if the tube are fouling up, steam production will suffer ,and the only solution

is to shut down and clean them up or blow soot on fireside.2- An economizer may be economical (see method 15)

The TDS inside a boiler should be maintained at recommended levels else it can lead to scaling of the boiler tubes and eventually failure of the tubes which is a

safety hazard.

Energy Loss from Scale Deposits

CommerceIncreased Boiler Heating Time

due to Scale

Method 3

Reduce Boiler PressurePotential savingPotential saving :1%for every 70psig reductionProblem :the boiler is being operated at a pressure higher

than necessary.Solution :slowly reduce boiler pressure to a point where

the amount of steam produced is sufficient to fulfill plant requirements

Method 4

Increase Fuel Oil TemperaturePotential saving : 5%Problem :Atomize the fuel at the right

viscosity .Too high or too low a viscosity will yield poor atomization and poor efficiency .

Solution:Preheat the fuel at 212°F-230°F or more so

that the fuel viscosity will be 100-300 sus

Fuels oils - their viscosity versus temperature

Fuel Oil Heating System

Method 5

Optimize Fuel Atomization PressurePotential saving: 1%Problem :The fuel atomization pressure is

lower or higher than that specified by the nozzle or burner design

Solution: Adjust fuel pressure according to nozzle operating instructions

Method 6

Reduce Boiler blowdownPotential saving : 1%Problem :excessive blowdown due to poor water

treatment and /or poor operating practices .the hot blowdown stream has energy that is lost unless it is recovered (see Method 16)

Solution: control feed water quality with the appropriate water treatment; review operation

Procedures

Flash tank

Method 7

Optimize Single-boiler FiringPotential saving :5%-10%Problem: A boiler may come on for afewminutes and then be off for several minutes, resulting in large energy losses due to the removal of useful heat when the boiler is off; or a boiler may “hunt” i.e. the firing rate is continually adjusting ,resulting in much more excess air.

• Solution :for an on –off boiler ,fire the boiler at an intermediate rate or buy a smaller boiler ; for boiler that “hunt” adjust the firing so that larger steam – pressure fluctuation are allowed.

Method 8

Optimize multiple-boiler operationPotential saving :2%-5%problem: A plant have tow or more boilers

operates them without distributing the load according to the efficiency each boiler .

Solution : Obtain the efficiency of each boiler vs. load ; adjust each boiler to operate at peak efficiency.

boiler efficiency vs. load

Method 9

Stop Steam LeaksPotential :5%-10%Problem: piping leakSolution: plug leak as soon as they appear

• The biggest steam losses are caused by failing steam traps or leaks into the steam system net. These uncontrolled leaks can lead to enormous losses and also enormous costs. The leaking traps can cause problems with tracing of your equipment, problems with back pressure into the condensate lines (which causes failure of groups of steam traps), and make your cost for the production of steam much higher. Problems with tracing in one unit can be caused by leaking traps in a other unit on the other side of the plant which indicates that monitoring steam traps important is for the proper working of your steam system.

Flange Leak Repair

Piping System Leaks 90 ELL Enclosure

Flange Clamps

Valve Packing Leaks

Before - Steam Leak on Valve Packing After - Repaired Steam Leak on Valve Packing

Steam leak rates

Steam leak rates

In addition to costing energy, steam leaks waste boiler water and chemicals, and it can be dangerous to people

and equipment .

NOTE:One pound of 100 psi steam contains about 1,200 BTUs. If the steam is produced at 85% efficiency, the input energy is 1,200 / 85% = 1,411 BTUs per pound. Therefore, 1,000 pounds of steam requires at least 1.4 million BTUs to produce it. (1,411 BTUs per pound x 1,000)1 MCF of Natural Gas contains 1 million BTUsCost to produce 1,000 lbs of steam from natural gas = 1.4 x $ per MCF of Natural GasWhen natural gas costs $7.00 per MCF, 1,000 lbs of steam costs (1.4 x $7) = $9.80

Method 10

Stop Steam-Trap LeaksPotential saving :5%-10%Problem: steam trap malfunctioning and

leaking steam.Solution : check steam trap routinely and

repair them.

Steam trap leak

checking steam trap

Steam trap piping system

Efficiency Improvements with

some Expenses

Method 11

Reduce Deposits in BurnerPotential saving : 1%-5%Problem : organic and inorganic deposits

build in burner, reducing atomization efficiency and therefore reducing combustion efficiency .

Solution : use a fuel oil additive with detergent dispersant to keep burners clean

Burner

Clean burnerDeposit burner

Tips

Replacement and cleaning burners

Method 12

Reduce Scale and Soot Deposits on FiresidePotential saving : 2% - 9%Problem : soot and /or vanadium-based deposit

decrease the heat transfer rate. If all condition are constant ,this decrease is noticeable when the flue gas temperature increase with time.

Solution : treat fuel with additives to minimize either soot deposits or vanadium-based scale. Use soot blowers if available

Combustion products

resulting from burning of

fuels.

Layers of deposit associated with coal-ash corrosion.

The flue pipe over half filled with soot and deposit

Soot and scale do not only increase energy consumption but are as well a major cause of tube

failure

Fuel energy loss due to sootEffect of Soot on Fuel Consumption

0

2

4

6

8

10

0.5

1.5

2.5

3.5

Thickness of Soot Layer, mm

% F

uel E

nerg

y Lo

ss

Warning triangle - risk of soot fire

Boiler Sootblowers

Soot Blower

Method 13

Reduce Scale and Deposits on WatersidePotential saving : 2% - 4%

Problem : inorganic scale and deposits decrease heat transfer rate ; i.e. flue gas

temperature increase with time.Solution : treat feed water properly using guidelines of boiler manufacturer or water

boiler standardizing

Energy loss due to scale deposits

Fuel energy loss due to scale

Effect of Scale on Fuel Energy Losses

0

2

4

6

8

10

120

0.25 0.

5

0.75 1

1.25 1.

5

1.75 2

Scale Thickness,mm

% F

uel L

oss

High density

Medium Density

Low Density

Water tube with soot outside and scale inside

Fire tube with soot inside and scale outside

Method 14Increase Combustion Air

TemperaturePotential saving : 1%Problem : preheat combustion air .every 40 °F rise

yields a 1% gain in efficiency.Solution : there are several possible solutions:

relocate air intake duct so that a maximum air duct temperature is obtained ,or install an air preheater if possible and economically feasible.

Recoverable heat from boiler flue gases

Air HeatersAir heaters cool flue gases before they pass intothe atmosphere, increasing fuel-firing efficiencyand raising the temperature of the incoming airof combustion. In low pressure gas or oil-fired industrialboilers, air heaters function as gas coolersas there is no need to preheat the oil or gasin order for it to burn.

Air Heaters

Method 15Increase Feedwater Temperature

Potential saving :3%Problem : raise water temperatureSolution : the equipment needed is an

economizer that uses the heat from flue gases, if economically feasible. Also , the water can be preheated using the waste heat from blowdown(method 16).

10°F rise in feedwater temperature raiseaefficiency 1%

EconomizersEconomizers help to improve boiler efficiency byextracting heat from the flue gases dischargedfrom the final superheater section of aradiant/reheat unit or the evaporative bank of anon-reheat boiler. Heat is transferred to thefeedwater, which enters at a much lower temperaturethan saturated steam.

Economizes

Location of Air heaters and Economizers

Dew point of the flue gas

Method 16Recover Heat Energy From Blowdown

Potential saving : 1%Problem : preheat water by recovering energy from

blowdown.Solution : add flash tank to system .the blowdown is

flashed by lowering the pressure in the flash tank; the steam produced is then vented into the feed water to boiler .some 50% of the heat in the blowdown is recovered. Send blowdown at 220°F to wast.

• Also aflash tank may be added ,followed by aheat exchanger to extract one-third more energy from the blowdown before going to wast.

Recoverable heat from boiler blowdown

Boiler Blowdown Heat Recovery exchanger System

Install an Automatic BlowdownControl System

Saving through Installion of Automatic Blowdown

Control System

Method 17

Energy Recovery From Excessive Steam Pressure

Potential saving :variableProblem : use throttling or back pressure

turbine .Solution : the idea is to utilize the energy

from steam rather than to decrease its pressure though a throttling valve

Steam turbines

back pressure steam turbine Condensing Steam Turbine

Back-Pressure Steam Turbine

Condensing Steam Turbine

Replace Pressure-Reducing Valveswith Backpressure Turbogenerators

Many industrial facilities produce steam at a higher pressure than is demandedby process requirements. Steam passes through pressure-reducing valves (PRVs,also known as letdown valves) at various locations in the steam distributionsystem to let down or reduce its pressure. A non-condensing or backpressuresteam turbine can perform the same pressure-reducing function as a PRV, whileconverting steam energy into electrical energy.In a backpressure steam turbogenerator, shaft power is produced when a nozzledirects jets of high-pressure steam against the blades of the turbine’s rotor. Therotor is attached to a shaft that is coupled to an electrical generator. The steamturbine does not consume steam. It simply reduces the pressure of the steamthat is subsequently exhausted into the process header.

Consider Installing High-Pressure Boilerswith Backpressure Turbine-Generators

When specifying a new boiler, consider a high-pressure boiler with a backpressuresteam turbine-generator placed between the boiler and the steam distributionnetwork. A turbine-generator can often produce enough electricity to justify thecapital cost of purchasing the higher-pressure boiler and the turbine-generator.Since boiler fuel usage per unit of steam production increases with boiler pressure,facilities often install boilers that produce steam at the lowest pressure consistentwith end use and distribution requirements.In the backpressure turbine configuration, the turbine does not consume steam.Instead, it simply reduces the pressure and energy content of steam that is subsequentlyexhausted into the process header. In essence, the turbo-generatorserves the same steam function as a pressure-reducing valve (PRV)—it reducessteam pressure—but uses the pressure drop to produce highly valued electricityin addition to the low-pressure steam. Shaft power is produced when a nozzledirects jets of high-pressure steam against the blades of the turbine’s rotor. Therotor is attached to a shaft that is coupled to an electrical generator.

Method 18

Reduce Heat Losses in Boiler, Steam and Valves

Potential saving :5% - 8%Problem : heat is lost by radiation and convection

through the walls of uninsulated or poorly insulated boiler surfaces and piping.

Solution : use a surface thermometer and determine where heat losses are present ; then insulate.

Surface Thermometer

Digital Surface ThermometerInfrared (IR) Thermometer

Boiler Wall Insulation

Heat loss of uninsulated steam line

Pipe and Fitting Insulation

Tank Insulation

Valve Insulation

Jacket

Flue Duct

Method 19

Use Fuel Oil Rather Than Natural GasPotential saving : 2%Problem : If the price of natural gas is the

same or higher than that of fuel oil,which is more economical?

Solution: use fuel oil.even if both cost the sam per MMBtu,fuel oil gives about a 2% higher efficiency than natural gas ,the

• The reason for this is that natural gas has more hydrogen atoms per unit weight than fuel oil. therefore, more water is formed from the gas, the the latent heat of vaporization of water is lost when the water vapor leaves the stack.

• MMBtu=Million Metric British Thermal Units

Method 20

Change From Steam to Air AtomizationPotential saving : 1%Solution : use are atomization for fuel oils.The energy required to produce the are of

atomization is a small fraction of the energy required to produce the steam of atomization.

Air Atomization burner

References

• Manual for improving boiler and furnace performance –borras

• Improving Steam System Performance

U.S. Department of Energy