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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