[Stoves] Fuel Testing

[AJH]

On Mon, 2 Oct 2006 19:18:26 -0400, Crispin Pemberton-Pigott wrote:

>The effect of moisture in the wood seems to be to moderate the production of 
>volatiles and perhaps acting as a catalyst (?) to clean up the combustion, 
>but sucking heat out as well.

Crispin I think you are right, I cannot remember what the definition
of a catalyst is but the water does have the ability to change the
reaction without itself altering. I think it acts opposite to how we
normally see a catalyst (something that speeds up a reaction by
reducing a threshold to the reaction) in that it slows things down.

An easy demonstration is to take a piece of wood freshly cut and a
bone dry piece of wood. place them separately on an open fire and you
will see the green wood burns away gradually from outside to nothing.
The bone dry piece of wood  immediately breaks into a sooty flame and
rapidly turns to a piece of charcoal before that itself burns. My take
on this is that the heat necessary to vapourise the water in the green
wood is so high that the temperature of the inside of the stick never
gets high enough to pyrolyse the bits away from the edge, so the char
depth stays constant as air burns away the surface. The dry log can
rapidly conduct heat to the interior and only a small amount of this
is necessary to get the wood to pyrolysis temperature so the volatiles
quickly evolve and leave the wood. As the temperature near the surface
quickly rises these offgases meet any free oxygen and combine in a
flame, so no oxygen gets to react with the char until the evolution of
volatiles drops at the end.

We can lead on from this and see how tlud is so constant in its power
output during the pyrolysis phase, even with bone dry wood, and why
increasing the moisture content of the wood actually leads to a lower
production of char. Similarly we can see how a batch loaded updraught
fire can have its power characteristic modified by changing the size
and moisture content of the wood.

I have mentioned before how closed wood burners left to smoulder can
reach a stage where eventually the whole batch becomes dry such that
once enough heat has been supplied to drive off the moisture the whole
batch quickly rises to 400C and above and suddenly the fire bursts
into a thermal runaway.

There seems to be another effect that water has in moderating the
temperature of the combustion. With bone dry wood the flame can be
sooty, I take this to be the partial burning of volatiles in an air
starved condition. These sooty PICs then don't burn out subsequently.
I wonder if by lowering the temperature in this air starved zone the
water allows different species of offgas products to reach the
secondary air and burn out more cleanly. If I remember correctly Tom
Reed posted a long time back that wood at 12% mc burned more cleanly
than bone dry wood.

AJH