[Stoves] Smoke-free biomass pellet fueled stove

[ajheggie at gmail.com]

[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