[Stoves] Calculating th LHV for Biomass and Coal

[Crispin Pemberton-Pigott]

Dear Friends

There is a really good discussion of carbon and emissions from Charcoal at
http://ecen.com/eee57/eee57e/carbon_content_in_biomass_fuel.htm

Eucalyptus Grandis firewood has an HHV of 18.879 MJ/Kg.

Making charcoal from it at 400 degrees C gives 82.1% Carbon and an HHV of
29.05 MJ/Kg.  As pure carbon has an HHV of 32.8, the remaining 17.9%
(volatiles) has an HHV of 11.85.

Because 82.1% x 32.8 MJ + 17.9% x 11.85 MJ = 29.05 MJ

If the volatiles portion of the charcoal is 6% H2 like the original wood (a
big 'if'), then the H2O production from combustion is:

1000 gm x 6% H2 x 17.9% volatiles x 9 gm of water per gm of H2 = 96.67 gm of
water produced during combustion of 1 Kg of charcoal.

Latent heat of condensation which cannot be recovered:

96.67 gm x 2257 J = 218,161 J  lost

so the ordinary LHV is 29.05 - 0.218 = 28.832 MJ/Kg

Calculating the LHV for 'average charcoal' at 29.77 MJ/Kg, the result is
29.77 - 0.223 = 29.55 MJ/Kg

Following Baldwin's calculation of a further heat loss of 0.07 MJ for
rejecting the gases at 100 C not 25, the LHV would be 29.77 - 0.223 - 0.07 =
29.477 MJ/Kg for calculating the lower heating value of charcoal burned in a
stove.  This figure could be refined for the higher CO2 content in charcoal
fire emissions.

I propose the figure 29.477 be used for rating the heat content of charcoal
left over after a biomass stove test.

Comments and corrections are always welcome.  

I would be happy to use the figure 150 C as the final temperature of the
gases because that is close to reality in a stove and is used in some
sectors of industry.  I think that would make the final answer something
close to 29.43 MJ/Kg.

Regards
Crispin