[Stoves] Fuel Testing

[AJH]

On Sun, 1 Oct 2006 12:05:06 -0700, frank at compostlab.com wrote:

>My questions are: If you use dry wood to boil a pot of water and determine the
>energy produced. Then take the same type of wood and soak into it, say, 100
>mls water and repeat the test is some of the energy in the wood reduced
>because of the water or, because the wood will dry during the fire and the
>carbon later used, it doesn't make a difference? 

I see Crispin has answered this but I'll put a slightly different
slant on it. The products of combustion all exit past the pot and they
are nitrogen from the air supply, which passes through the system un
reacted and the products of combustion. If the fire is clean we can
assume these products are CO2 and water. So the flue gas will be
nitrogen CO2 and water and will have a mass equal to the combustion
air supply plus the mass of fuel. All this massflow will leave the pot
at a high temperature. The only energy that goes into the pot is the
difference between the energy in the wood and the energy in the flue
gas past the pot.

So essentially the energy in the wood is distributed over the whole
mass flow that goes past the pot. As Crispin has said in the early
stages the coolness in the pot will cause the water vapour to condense
on the pot, giving up it's latent heat to the pot, this is actually a
good mechanism for transferring heat but it's irrelevant to the energy
equation because it ether drips back or is subsequently re vapourised.

As water/steam have high specific heats and the latent heat of water
is very high these are significant amounts of heat leaving the stove
that cannot be recovered.

The latent heat of water is about 2.3 MJ/kg and this is totally lost
from the system and cannot contribute to heating the pot one boiling. 

It's actually worse than that because the reject temperature is much
higher with a simple heat exchange surface like the bottom and sides
of a pot. So not only do you lose the heat of vapourisation of the
water in the wood but also you throw away the steam and all its
sensible heat at a high temperature.

At a minimum I reckon you can say that 2.7MJ of energy is lost from
the stove for every unnecessary kg of water in the wood. It's likely
much worse than this because wetter wood means a cooler fire and
necessitates higher excess air, so despite the massflow going up the
temperature the pot sees is lower and there is worse heat transfer.


> If some of the energy
>(carbon) in the fuel is needed to first remove the water (cooled to make
>steam) is there an estimate of the amount of carbon 'wasted' to remove a gram
>of water? Am I thinking about this in the right way?

If you model wood as carbon plus associated water then the energy from
burning the wood  cleanly to carbon dioxide and water is just the
oxidation energy of the carbon mass in the wood. If you use my 2.7 MJ
figure for losses due to excess water content in the wood then it's a
simple calculation to see how much carbon you would need to vapourise
and reject this water. In practice as a number of others have pointed
out a simple formula is mass of fuel minus mass of residual ash  gives
the organic content. Mass of oven dry fuel minus ash times 18.6 gives
the energy content for most biomass and subtract from this 2.7MJ/kg
for every kg of water in the wood.

AJH