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Author: Subject: Does wood pyrolysis generate heat ??
metalresearcher
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[*] posted on 7-2-2021 at 03:34
Does wood pyrolysis generate heat ??


I read on wiki that pyrolyzing wood releases energy. I always thought that that process is endothermic (i.e. costs energy) and this is one of the reasons, only dry wood burns well, as only the burning of the pyrolyzed gases release energy, which is partially used by the pyrolyzing of the logs in the wood fire.

If this is true, then burning wood (assumed dry wood and complete combustion of the gases, so no PAH release) is more efficient than I thought ?


[Edited on 2021-2-7 by metalresearcher]
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Fulmen
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[*] posted on 7-2-2021 at 10:40


The reason why wet wood burns (and heats) poorly is simple, all that water needs to be boiled away. If every pound of wood need to boil off a pound of water there is less energy left to produce heat.



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[*] posted on 14-2-2021 at 09:54


I don't think that part of the wiki article is correct. Doesn't make sense. As a chemical reaction I think it is endothermic. If viewed from a bigger perspective and if the gasses that are evolved are actually burned, then it could be considered exothermic.

Burning wood is always a two step process. First is pyrolysis, where volatiles are being boiled out of the wood and combusted, generating lots of flames. Once pyrolysis is complete, step two starts which is the combustion of rhe charcoal that's left. This step can get much hotter since it's not being cooled down from boiling volatiles.
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[*] posted on 18-2-2021 at 07:23


The pyrolysis process is exothermic. The reaction starts at about 200 degrees C. The heat of reaction is sufficient to increase the temperature to about 350 degrees. The heat of reaction is about 1600 J/gram which is 8-10% of the heat of combustion.

Source: Ullmanns Encyklop├Ądie der technischen Chemie, 4th edition.

If the heat needed to heat up the wood to 200 degrees is included the reaction is probably endothermic.
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