If the reaction takes place in a gaseous state, why does the laden heat of
evaporation apply?
----- Original Message -----
From: [email protected]
To: [email protected]
Sent: Sunday, October 06, 2013 8:29 PM
Subject: [Stoves] Gross verses net calorific value for dry wood
Dear Lanny
It is a lot more arbitrary that you might think. That is why there are
different values used in different industries.
Some fuels contain hydrogen. Some don't. Hydrogen burns to water. Water has a
phase change at 100C. Because the main application of heat in cooking is to
bring things to the boiling point it is obvious that condensed water vapour
just below the boiling point adds nothing to a boiling pot.
So the energy released by condensing water vapour to liquid is not useful for
cooking unless the pot is cold, and even then it would have to be evaporated
again later unless it dripped off.
When testing the whole fuel in a Bomb Calorimeter the latent heat of
condensation is counted. So while the heat is there it is not useful for
cooking, mostly.
So the Latent Heat is discounted when making efficiency calculations. Work
done divided by heat available = efficiency.
The value given to heat available is the HHV (total energy) minus the latent
heat of condensation of the water vapour created by burning the hydrogen in the
fuel. Obviously this varies with fuel type.
In biomass the H2 content doesn't vary much. The H2 mass burned to H2O gives
a new mass.
That mass (of water vapour) x 2257 J/g = 1,320,000 Joules for normal biomass.
It is never exactly that but it is close enough for government work.
HHV - 1.32 = LHV in MJ
Approximately.
Regards Crispin boarding in Yogyakarta' little airport
Can you explain that in a way...
Lanny
J
Crispin
What is the difference between
Gross calorific value (dry fuel)
and
Net calorific value (dry fuel)
Used in the WBT spreadsheet
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