[Stoves] Fuel Testing

[frank]

Stovers,

I would think a wet piece of wood would conduct heat through the wood 
faster. Water transfers the heat. Then when it turns to steam the wood 
cells explode permitting more air movement through the wood. As the 
steam-water dries the wood is already hot, there is a lot of moving air 
from the steam next to the now dried wood that acts like a fans and the 
wood burns. It would be fun to embed thermistors along a wet stick and 
have a blow touch directed at one end to watch the heat transfer along 
the wood. Then compare that to a dry stick.

Frank


>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
>
>
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-- 
Frank Shields
Soil Control Lab
42 Hangar way
Watsonville, CA  95076
(831) 724-5422 tel
(831) 724-3188 fax
frank at compostlab.com
www.compostlab.com