Calculating the LHV for Biomass and Coal
Thomas Reed, Biomass Energy Foundation, October 1, 2007 to the Biomass Cooking Stoves List
It is an embarrassment that we (in the US) use the high heating value for wood and coal and low heating value for liquid fuels and natural gas!
To calculate the LHV you must subtract the heat of vaporization of H2O from the HHV.
Long ago the heat of vaporization of water was 550 cal/g or (X the Mole wt of 18) 9900 cal/mole. Now it is (multiplying by 4.184 J/cal) 41,422J/mole or 41.4 kJ/mole (thanks to Mr. Metric).
In order to apply this accurately to a particular sample it is necessary to know the ultimate analysis values of C, H and O in order to determine how much water is made during combustion. This could cost you $100-$200 at a laboratory or $10,000 if you want to set up your own lab.
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Fortunately, most biomass has the C-ratio formula CH1.4O0.6 and burns according to
CH1.4O0.6 + 1.05 (O2 + 3.76N2) ==> CO2 + 0.7 H2O + 4 N2
It has a heating value of 21 kJ/g (bone dry, and minus the ash, Dry-Ashfree, DAF) and a molecular weight of 23. The hydrogen fraction of DAF biomass is 1.4/23 = 0.061, so the heat remaining in the
H2O(steam) will be (.061X41.4) 2.52 kJ/mole.
The LHV of biomass is then ~ 21-2.5 = 18.5 kJ/mole or 1.03 kJ/g, ~ 12% less than the HHV. good enough for a general rule and warning.
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In Europe they use the LHV for all fuels. As a consequence Swedish wood stoves have a 12% higher rating in Europe than in the US. They are also more energy conscious, and sometime condense and use the water during biomass combustion. So sometimes their efficiency exceeds 100% calculated on a LHV basis!
I hope this keeps us all honest here at stoves/gasification/REPP.
Yours truly,
TOM REED THE BIOMASS ENERGY FOUNDATION