[Stoves] Gross verses net calorific value for dry wood

Sun, 06 Oct 2013 17:30:59 -0700

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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