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B. COMBUSTION EFFICIENCY CALCULATION
The efficiency calculation is based upon British Standard BS845.
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 vapour
Unburned gas
Losses due to refuse: Combustible in ash
Combustible in riddlings
Combustible in dust
Other losses: radiation
convection
conduction
other unmeasured losses
Net efficiency calculations assume that the energy contained in the water vapour (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 vapour 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, riddlings, 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 Value:
K1g = ( 255 x %Carbon in fuel )/Q
gr
K1n = ( 255 x %Carbon in fuel )/Q
net
K2 = % max theoretical CO
2
(dry basis)
K3 = % Wet Loss
H
2
= % Hydrogen
H
2
O = % Water
Measured Data: Tf = Flue Temperature
Ti = Inlet Temperature
O
2
m = % Oxygen in flue gas
O
2
r = Oxygen reference %
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