Sample Case :
Let us calculate Boiler efficiency of coal fired boiler. Ambient temp is 80 F and Back End Temperature (Exh gas temp) is 302 F. The percent composition of Coal is as under:
Carbon , C - 76.0 ; Hydrogen, H2 - 4.1 ; Nitrogen , N2 - 1.0 ; Oxygen, O2 - 7.6 ; Suphur, S - 1.3 ; Moisture, H2O - 3.0 ; Ash - 7.0 ;
The Combustion calculations of the above fuel is already explained in detail in the other article.
From the above calculations, Unit Wet Gas, Kg / Kg of fuel = Unit Wet Air + (1-Ash)
= 13.12 + (1-0.007)
= 14.05
Unit Dry Gas, Kg / Kg of fuel = Unit Wet Gas – (Moisture in Air + Water produced during combustion)
= 13.484
Higher Heating Value, HHV or Gross Calorific Value, GCV in BTU/Lb
= 14600.C + 62000 (H2-O2/8) + 4050.S
Lower Heating Value, LHV or Lower Calorific Value, LCV or Net Calorific Value, NCV, BTU/lb
= HHV – 1030(9.H2 + Moisture)
Let us use HHV and LHV notation.
HHV = (14600 x 76 +62000 (4.1-7.6/8) + 4050 x 1.3 )/100
= 13101.65 BTU/lb (7278.7 Kcal/kg )
LHV = 13101.65 – 1030(9*4.1+3)/100
= 12690.6 BTU/lb (7050 Kcal/kg)
Calculations of the Losses based on Higher Heating Value:
a) Dry gas losses:
Exhaust gases always leave the boiler at a higher temp than ambient. Heat thus carried away by hot exhaust gases is called Dry gas losses
Heat Losses, La = UnitDryGas x Cp x (Tg-Ta) x 100/HHV
= 13.478 x 0.24 x (302 -80) x 100 / 13101.65
= 5.48 %
Loss due to Moisture in fuel :
The moisture present in the fuel absorbs heat to evaporate and get superheated to exit gas temperature.
Lb = MoistureInFuel x (1089-Ta+0.46xTg)x100/HHV
= 0.03 x (1089 – 80 +0.46 x 302) x100 / 13101.6
= 0.263 %
c) Loss due to Moisture Produced during combustion :
Lc = MoistureProduced x (1089-Ta+0.46xTg)x100/HHV
= 0.369 x (1089 – 80 +0.46 x 302) x100 / 13101.6
= 3.23 %
d) Loss due to Moisture in air :
Ld = MoistureInAir x Cp of Steam x (Tg-Ta) x 100/HHV
= 0.0132 x 12.95 x 0.46 x (302 - 80) x100 / 13101.6
= 0.133 %
Here, Moisture in Air = 0.0132 lb/ lb of dry air at 60% Relative Humidity
Cp of steam = 0.46
e) Unburnt fuel loss :
This is purely based on experience. Unburnt fuel loss depends up on type of Boiler , grate, grate loading and type of fuel. For Bio-Mass fuels, it ranges from 1.5 to 3 %, for oils from 0-0.5 and almost nil for gaseous fuels.
Let us consider Unburnt fuel loss, Le = 2.5 % for Coal.
f) Radiation Loss:
Radiation Loss is because of hot boiler casing loosing heat to atmosphere. ABMA chart gives approximate radiation losses for fired boilers.
Let us take a radiation Loss , Lf = 0.4 % in this case.
g) Manufacturer’s margin :
This is for all unaccounted losses and for margin. Unaccounted losses are because of incomplete combustion carbon to CO, heat loss in ash ..etc. This can be 0.5 to 1.5 % depending up on fuel and type of boiler.
In this case, let us take, Manufacturer’s margin Lg = 1.5%.
Total Losses = La + Lb + Lc + Ld + Le + Lf + Lg
= 5.48 + 0.263 + 3.23 + 0.4 +0.133 +2.5 + 1.5
= 13.506 %
Therefore, Efficiency of the boiler on HHV basis = 100 – Total Losses
= 100 – 13.506
= 86.494 %
Efficiency based on LHV:
Efficiency based on LHV = EfficiencyOnHHV x HHV/LHV
= 86.494 x 13101.6/12690.6
= 89.29 %
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