[Stoves] oven-dried vs sun-dried biomass and TLUD stoves

ajheggie at gmail.com ajheggie at gmail.com
Sun Feb 26 04:45:53 CST 2012


On Sat, 25 Feb 2012 10:51:19 -0800, Frank Shields wrote:

>Back on the farm we would pile up very wet green wood and burn. A tractor
>tire buried in the pile and a little gasoline would help get it started but
>once going (and the tire gone) no amount of wet wood seemed to stop it. It
>seems if calculating the energy needed to evaporate the water vs energy
>produced from the biomass that it should not burn - or very well. So I
>wonder if the process of water evaporating and then turning to H and CO
>requiring a lot of energy then back to water at the outside releasing its
>energy, all being energy neutral, make this all happen?

Frank even wet wood burns, it's just that it's difficult to burn it
cleanly and top lighting will not work because the propagation of a
top lit fire depends on the heat necessary to dry, pyrolyse and ignite
the layer below being conducted down from above.

With a traditional bottom lit bonfire all the heat rises through the
wet layers above, drying them as it does so. Now while this may make
it smoky because the offgas from the fire at the bottom has become
cooled and diluted with waster vapour the char being formed will
continue to burn and pyrolyse layers above. Eventually the offgas will
get hot enough to sustain a flame  and as you know once the fire is
big enough the radiant heat from the flame is high enough to dry
surrounding wood.

Even if a flame never develops say 50% of the original dry weight of
the wood on the fire is charred and then only this burns in a
smouldering fire. The charcoal burning has only to supply enough heat
to dry the wood beside and above it and then raise it through another
200 degrees C to propagate the fire and the only losses are the heat
going up in the smoke and convection/conduction to the surrounds. As
the fire gets bigger these losses become proportionately smaller
because the losses depend on the surface area which is related to the
dimension squared whilst the heat production is related to volume, or
dimension cubed, as long as enough air can get in. Cover the lot with
soil and you have a charcoal clamp.

Say 1000kg wood at 50% mc and no secondary combustion produces 250 kg
of 400C char that burns yielding 24MJ/kg at 400C. That's releasing
6000MJ, the water can be evaporated by 1350MJ and discharged at over
150C, drying material above as it cools from 400 to 150C. So even a
lossy smouldering bonfire can sustain burning. In a high tech device
it should be possible to cleanly burn material up to 80% moisture
content.

Burning tyres other than in an approved incinerator is unlawful in UK
but much practiced by the traveling community and some farmers who
view it as Dunlop coal in large ( non exempt) outside boilers.




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