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. _______________________________________________ Stoves mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org for more Biomass Cooking Stoves, News and Information see our web site: http://www.bioenergylists.org/
