Crispin and all,
I think that the char being created in a TLUD is not hot enough to
cause the water gas reaction.
Also, there is no net increase in energy. The water gas reaction just
takes energy from the reacting carbon plus water and puts it (saves it)
into the desirable gases (H2, and CO mainly) that permits the energy to
be moved to a different place where it is needed when the gases are
combusted. That makes sense in the larger gasifiers where the gases
can be moved many meters, cooled, and put into internal combustion
engines. But in a TLUD, the gases would be moved only 5 to 20 cm
upward before being combusted under the pot.
Paul
Paul S. Anderson, PhD aka "Dr TLUD"
Email: [email protected] Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com
On 11/11/2012 6:46 PM, Crispin Pemberton-Pigott wrote:
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
------------------------------------------------------------------------
*From: * [email protected]
*Date: *Sun, 11 Nov 2012 20:56:14 +0000 (UTC)
*To: *Discussion of biomass cooking
stoves<[email protected]>; Andrew Heggie<[email protected]>
*Cc: *paul anderson<[email protected]>; Crispin
Pemberton-Pigott<[email protected]>
*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 (?H° = +131.3 kJ mol^-1 ; ?S° = +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
------------------------------------------------------------------------
*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
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