[Stoves] Fuel Testing

[Jeff Davis]

Dear All,

Would the below link be of any help??

http://www.biomassenergyfoundation.org/LEARN/papers/high_heat_of_fast_pyrolysis.doc



Jeff


> 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
>>
>>
>>_______________________________________________
>>Stoves mailing list
>>Stoves at listserv.repp.org
>>http://listserv.repp.org/mailman/listinfo/stoves_listserv.repp.org
>>
>>
>>
>>
>>
>
> --
> 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
>
>
>
>
> _______________________________________________
> Stoves mailing list
> Stoves at listserv.repp.org
> http://listserv.repp.org/mailman/listinfo/stoves_listserv.repp.org
>
>


-- 
Jeff Davis

Some where 20 miles south of Lake Erie, USA