Manual

CombuStioN effiCieNCy CalCulatioNS
 Thisidentifiesthreesourcesoflossassociatedwithfuelburning:
  • Lossesduetofluegasses:
   -DryFluegasloss,Moistureandhydrogen,
   -Sensibleheatofwatervapor,Unburnedgas
  •Lossesduetorefuse:
- Combustible in ash, riddling and dust
  •Otherlosses:
- Radiation, convection, conduction other unmeasured losses
Net efficiency calculations assume that the energy contained in the water vapor (formed as a product of
combustion and from wet fuel) is recovered and the wet loss term is zero. Gross efficiency calculations assume that
theenergycontainedinthewatervaporisnotrecovered.Sincethefuelairmixtureisneverconsistentthereisthe
possibility of unburned/partially unburned fuel passing through the flue. This is represented by the unburned carbon
loss.Lossesduetocombustiblematterinashes,riddling,dustandgrit,radiation,convectionandconductionarenot
included.
EfficiencyCalculation:
•KnownDataFuel:
 -Qgr=GrossCalorificValue(kJ/kg)
 -Qnet=NetCalorificValue(kJ/kg)
 -K1=ConstantbasedonGrossorNetCalorific••
•KnownDataValue:
 -K1g=(255x%Carboninfuel)/Qgr
 -K1n=(255x%Carboninfuel)/Qnet
 -K2=%maxtheoreticalCO2(drybasis)
 -K3=%WetLoss
 -H2=%Hydrogen
 -H2O=%Water
•MeasuredData:
 -Tf=FlueTemperature
-Ti = Inlet Temperature
-O2m = % Oxygen in flue gas
-O2r = Oxygen reference %
•Calculateddata:
- Tnet = Net Temperature
- % CO2 content in flue gas
 -%DryFlueGaslosses
-% Wet losses
-% Unburned carbon loss
 -%Efficiency
•Tnet=FlueTemperature-InletTemperature(orambi-
ent)
•Dryfluegasloss%
 =20.9xK1x(Tnet)/K2x(20.9-O2m)
•Wetloss% 
 =9xH2+H2O/Qgrx[2488+2.1Tf-4.2Ti]
•Simplified 
= [(9xH2+H2O)/Qgr]x2425x[1+0.001Tnet]
• Wetloss% 
 =K3(1+0.001xTnet)
•WhereK3 
 =[(9xH2+H2O)/Qgr]x2425
•NetEfficiency%
= 100 - dry flue gas losses
= 100-20.9xK1nx(Tnet)/K2x(20.9-O2m)
•GrossEfficiency%
= 100 - {dry flue gas losses + wet losses}
= 100–{[20.9xK1gx(Tnet)/K2x(20.9-O2m)]+
  [K3x(1+0.001xTnet)]}
•ExcessAir
 =[20.9/(20.9-O2m)-1]x100
•CO2%
 =[(20.9-O2m)xK2/20.9]
•Unburned
 =K4xCO/(CO+CO2)
 Note:COscaledin%fuelLoss% 
•WhereK4
= 70 for coke
 =65foranthracite
 =63forBituminouscoal
 =62forcoaltarfuel
 =48forliquidpetroleumfuel
= 32 for natural gas
TheformulaforK4isbasedonthegrosscalorificvalue
Qgr. To obtain the loss based on net calorific value multi-
plybyQgr/Qnet.Sincethislossisusuallysmallthiscon-
version has been ignored. This loss is subtracted from the efficiency.
Co air free
 Certainstandards(ANSIZ21.1)forCarbonMonoxidearestatedintermsofair-free.Air-freereferstothecon-
centration of CO in combustion gases undiluted with flue, or other gases containing little CO. This value is computed
usinganequationthattakesintoaccounttheO2concentrationofthefluegas.
 • If5%O2ismeasured(O2m)inthefluethentheCOgasvaluewillberecalculatedasif0%weremeasured. 
  Theequationforair-freeisasfollows:
  - COaf=COPPMx[(20.9)/(20.9-O2m)]
 • Inourexampleifareadingof325PPMweremeasuredthentheair-freevaluewouldbecalculatedasfollows:
  - COaf=325PPMx[(20.9)/(20.9-5)]COaf=325PPMx[(20.9)/(15.9)]COaf=427
We may be given a limit on our gas range by the local authority, which stated that we must not emit more than
400-PPMCarbonMonoxideair-free.Intheexamplewewouldbebreakingthelimitandcorrectiveactionshouldbe
taken to reduce the level of CO. Air-free values prevent false readings being submitted, e.g. allowing more air into the
boiler will increase the oxygen level in the flue and dilute any toxic gas reading. Air-free referencing gives readings as if
they were undiluted.
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