[Stoves] Smoke-free biomass pellet fueled stove

[Saastamoinen Jaakko]

Dear Crispin

I think this is due to the fact that the velocity of the ignition front in the fuel bed depends on the moisture. Moist wood particles require longer time to ignite. For higher moisture increasing amount of energy from char is required to heat and dry the fuel below the ignition front to the ignition temperature. Only a part of the energy from char is radiated below and much of it is radiated and carried by convection above the ignition front.    

Autogasification could take place for larger wood particles, if there is still moisture in the centre of the particle when the outer layer is already charred . Then steam from drying in the inner part  of the particles reacts with the outer char layer (see M.P. Järvinen, R. Zevenhoven, E.K. Vakkilainen, Auto-gasification of a biofuel, Combustion and Flame, Volume 131, 2002, 357-370). 

Jaakko Saastamoinen

-----Original Message-----
From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Crispin Pemberton-Pigott
Sent: 9. marraskuuta 2012 1:02
To: Stoves
Subject: Re: [Stoves] Smoke-free biomass pellet fueled stove

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?

Regards
Crispin
------Original Message------
From: ajheggie at gmail.com
Sender: Stoves
To: Stoves
ReplyTo: Stoves
Subject: Re: [Stoves] Smoke-free biomass pellet fueled stove
Sent: Nov 8, 2012 17:13

[Default] On Sun, 4 Nov 2012 11:54:00 -0800,"Tom Miles"
<tmiles at trmiles.com> wrote:

>I have often wondered how consistent the char is from a TLUD. While the 
>maximum heat treat temperature (HTT) is probably consistent, the time 
>that the char is exposed to these temperatures probably varies.

Tom Reed used to say that "Denver dry" wood (10%mc wwb) yielded 25% char from a tlud burn and 25% none, so as the initial mc goes up the char becomes higher in ash and lower in charcoal. This is because the enthalpy of water vapour being produced has to be provided by heat from burning char.

AJH

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