[Stoves] Smoke-free biomass pellet fueled stove

[Crispin Pemberton-Pigott]

Is this not exactly the reaction taking place as the moisture comes out of the fuel particles and passes through the charred surface?

The point of discussion is that it consumes the char in doing so. 

Regards
Crispin
-----Original Message-----
From: rongretlarson at comcast.net
Date: Sun, 11 Nov 2012 20:56:14 
To: Discussion of biomass cooking stoves<stoves at lists.bioenergylists.org>; Andrew Heggie<ajheggie at gmail.com>
Cc: paul anderson<psanders at ilstu.edu>; Crispin Pemberton-Pigott<crispinpigott at gmail.com>
Subject: Re: [Stoves] Smoke-free biomass pellet fueled stove


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: ajheggie at gmail.com 
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org> 
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" 
<crispinpigott at gmail.com> 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 

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