Andrew, list, Paul, Crispin I can agree with the various explanations offered in this thread vis-a-vis fuel moisture. But I think there is another fundamental chemical 'WATER GAS" explanation. One can find this chemistry explanation many places, for example: http://www.webelements.com/carbon/chemistry.html which says:
"Reaction of carbon with water Carbon, either as graphite or diamond does not react with water under normal conditions. Under more forsing conditions, the reaction becomes important. In industry, water is blown through hot coke. The resulting gas is called water gas and is a mixture of hydrogen (H 2 , 50%), carbon monoxide (CO, 40%), carbon dioxide (CO 2 , 5%), nitrogen and methane (N 2 + CH 4 , 5%). It is an important feedstock gas for the chemical industry. C + H 2 O → CO + H 2 This reaction is endothermic (ΔH° = +131.3 kJ mol -1 ; ΔS° = +133.7 J K -1 mol -1 ) which means that the coke cools down during the reaction. To counteract this, the steam flow is replaced by air to reheat the coke allowing further reaction." (Emphasis added) Ron ----- Original Message ----- From: [email protected] To: "Discussion of biomass cooking stoves" <[email protected]> Sent: Sunday, November 11, 2012 3:17:24 AM Subject: Re: [Stoves] Smoke-free biomass pellet fueled stove [Default] On Thu, 8 Nov 2012 23:01:44 +0000,"Crispin Pemberton-Pigott" <[email protected]> wrote: >Dear Andrew > >I an becoming convinced that the char disappears for other reasons as well. >The heat needed to get rid of the moisture is far less than the charcoal. That >leads me to think there is some reaction involving water that breaks the >carbon out of the char. > >Any ideas what that would be? Crispin I think Jaakko has nailed that one and I completely agree, it's the heat required to dry the particle below the pyrolysis front before it reaches pyrolysis temperature, all the while the bulk of the heat is rising through the already charred layers as sensible heat of the offgas. The more of the char burns at the pyrolysis front because the downward movement of the front has slowed and it remains in contact with air for longer. When the wood is dry the front moves downward from the fresh char and the offgas, being devoid of oxygen, shields the hot char from further oxidation. Jaakko that's an interesting cite with regard to the autogasification, I haven't found the original yet, I'm not sure it is relevant to tlud as generally the burn is complete before larger particles are pyrolysed, also the watergas reaction needs a higher temperature at equilibrium that we normally see in the pyrolysis front ( which is about 600C I think). I have also in the past pointed out that large logs of dry wood burn differently from the same log when green as pyrolysis moves quickly through the log evolving offgas which again forms a shield as it burns preventing oxygen reaching the outer parts of the log, once the offgas slows down the char then burns. With the green log because the heat required to dry successive layers of the log is large and offgas evolution is slow air does reach fresh char on the surface, so the log gradually disappears. Also as the water vapour and CO2 given off dilute the offgas often this does not burn and the log smoulders away with little or no flame but lots of acrid smoke. Sorry to be a bit slow on the response but I'm glad Jaakko chipped in. AJH _______________________________________________ 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/
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