boiler efficiency-an analysis

8/2/2019 Boiler Efficiency-An Analysis

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A PROJECT onBOILER EFFICIENCYAND PERFORMANCE

ANALYSIS


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INTRODUCTION

BOILER EFFICIENCYis defined as the heat addedto working fluid expressed as a percentage ofheat in the fuel being burnt.

The thermal efficiency of a boiler is the ratio ofusual energy output to the energy input.

Boiler efficiency depends solely on the boilersability to burn the fuel and transfer the resultingheat to water and steam.


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Methods of Efficiency

Measurement:-

Direct Method - By measuring input and output

Indirect Method - By measuring individuallosses , totaling them and deducting

sum from 100%.(also called losses method or

efficiency by difference)


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Direct Method Direct Method is used

as a check upon theIndirect Method.

By this method wedont get any idea

about the areas in

which losses areoccurring, which needsattention.


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HEAT INPUT - The measurement of heat input

requires knowledge ofCalorific value of fuel

Fuel flow rate (Tons/hr)

HEAT OUTPUT The measurement of heat outputrequires knowledge of

steam generated per hour (kg/hr)

working pressure (kg/cm2) and superheattemperature (C)

temperature of feed water (C)

By this method the Boiler efficiency is expressed as,


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Indirect Method The efficiency of a boiler can therefore be expressed

alternatively as 100% minus the sum of the lossesexpressed as a percentage of the input energy.

According to this method the Boiler efficiency isexpressed as,

By this method we get an idea about the differentareas in which losses are occurring.


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Advantages of Direct method:

Plant people can evaluate quickly the efficiencyof boilers.

Requires few parameters for computation.

Needs few instruments for monitoring.

Does not give clues to the operator as to whyefficiency of system is lower.

Does not calculate various losses accountable for

various efficiency levels.

Disadvantages of Direct method:


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Types of losses:-

1) Dry flue gas loss.2) Loss due to evaporationof water formed due tohydrogen in fuel.

3) Moisture present in fuel.

4) Moisture present in air.

5) Incomplete combustion.

6) Radiation and Convection

losses.7) Loss due to unburnt flyash.

8) Loss due to unburntbottom ash.


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1) Dry flue gas loss :-

This is the heat loss

from the boiler in thedry component of gasesto the stack.

This loss is typically ofthe order of4.5%.

The Dry Flue Gas Loss

Depends Upon TwoFactors:-

1) EXCESS AIR2) AIR HEATER OUTLET

GAS TEMPERATURE


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2) Loss due to Evaporation of Water

formed due to Hydrogen in Fuel :-

Coal containshydrogen, which

burns to form water. This loss is the latent

heat removed in fluegases by the water.

This loss depends onthe specific heat ofsuperheated steam.


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3) Loss due to Moisture present

in Fuel :-

This is the loss of heat from theboiler in the flue gases due to

water vapour which was presentinitially as moisture in the coalburnt.

This loss which cant be altered

as inherent moisture from thecoal cant be released by simply

heating it by hot air from PA fan.


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4) Loss due to Moisture present in

Air:

This loss is due to water vapour which waspresent initially as moisture in the actual air

supplied for combustion of fuel.

This type of loss depends upon the humidity

factor of the supplied air.


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5) Loss due to Incomplete Combustion

This is because of incomplete combustion ofcarbon, i.e. C to CO only.

Unburnt carbon contained in the residualsdischarged from ash and grit hoppers and thechimney.


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6) Radiation and Convection Loss

Occurs due to radiation

of heat from the outersurface of Boiler.

Remains almost sameover the whole loadrange.

Radiation loss increasesas load on boiler reduces,as this loss does not varymuch with varying load.

Economical to run boilersat full load.

Higher MWs boilerspreferred.


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7) Loss due to Unburnt Fly Ash :

Occurs due to incomplete combustion ofpulverized fuel (coal).

This loss is due to

High value of draught created

Lack of sufficient air for combustion

Improper ignition temperature

Very less time of exposure to flame

Results in loss of calorific value of unburnt carbon

particles.


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8) Loss due to Unburnt Bottom Ash :

Unburnt carbon particles settle down in the hopperbelow the furnace.

This occurs due to insufficient TTT (called 3Ts) i.e.TIME

TEMPERATURE

TURBULENCE

Loss of calorific value of unburnt coal particleswhich settle below the furnace in the hopper.


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CONCLUSION

As the dry flue gas loss is greatest, so attentionshould be given to harness the heat carried away byit.

About 1% efficiency gain, by reducing exit flue gas

temperature by 20C. Good quality coal should be used having less

moisture content.

To get higher Boiler efficiency proper draught

control should be made, with air having lessmoisture.

Proper TTT (Time, Temperature, Turbulence) tocontrol unburnt fuel loss.


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Thank you & anyquestions

boiler efficiency-an analysis

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