[Stoves] Why doesn't charcoal burn in the Champion TLUD?

[Anand Karve]

Dear List,
In most cooking processes, high heat is required in the first part of
the cooking process, after which one needs relatively low heat. We are
working on improving our Sampada stove, a natural draft gasifier
stove, made of sheet metal. Because it radiates a lot of heat outside,
the user found it uncomfortable to sit in front of the stove. We
therefore insulated the stove body with rock wool.  This increased the
overall efficiency of the stove because it now allows the heat of the
embers to rise up to the pot.
Yours
A.D.Karve

On Thu, Oct 16, 2014 at 5:24 AM, kgharris <kgharris at sonic.net> wrote:
> Huck,
>
> I was not planning to venture an answer to you question since it was
> addressed to Paul Anderson, but he answered and opened it to the list so
> here goes.
>
> Your question asks why the heat generated to drive off the gasses from the
> fuel doesn't burn the charcoal in the Champion stove. The reason is that
> there is not enough oxygen. It also doesn't burn most of the wood gas. Only
> a small amount of primary air is allowed into the stove providing enough
> oxygen to support the pyrolysis process to heat the fuel and continue
> producing more wood gas, but not enough oxygen to support burning either the
> char or most of the gas. Only a small amount of wood gas burns with the
> limited primary air to keep the process going. The three components needed
> for a fire are fuel, heat, and oxygen, and oxygen is limited here. Most of
> the gas, which contains flamable hydrogen and carbonmonoxide rises. It is
> hot but mostly unburned. Secondary air is injected into it above the char
> and that is where the gas begins burning. Once all the volitiles are driven
> out of the fuel, then the small amount of primary oxygen will start
> supporting the burning of the char, producing both CO and CO2. The charcoal
> fire is adjustable with the primary adjustment, so increasing the primary
> air increases the charcoal fire. If the char is hot enough and there is
> adiquate oxygen from the secondary, there will be a CO flame. It is bluish
> and very dim in a lighted area, but quite visible in darkness. I do not know
> if this flame burns all of the CO forming CO2, or leaves a lot of CO
> unburned. The charcoal fire can provide good heat for a time after the
> hydrocarbon flame goes out. I have even brought a small amount of water to
> boil using the charcoal fire. As the others have stated, the red hot
> charcoal is very hot and damaging to the metal of the stove.
>
> The pyrolysis process is interesting as well. The heated fuel produces wood
> gas. Most of this gas rises to the secondary burn area, but a small amount
> of it is burned at the pyrolysis front using the primary air. This small
> wood gas flame is what keeps heating the fuel and keeps the process going.
> The mechanism by which the pyrolysis front moves down into the fuel is
> mostly radiant heat. The small primary wood gas flame and hot char will
> radiate heat downward to a piece of fuel, heating it. At some point the
> temperature of this piece reaches the the flash point. At this point a very
> small flame moves around the surface of the piece and chars the surface.
> This tiny flame lasts maybe 2 seconds and then goes out and the piece is now
> producing wood gas, a small amount of which is burned using the primary air,
> thus continuing the process.
>
> The making of charcoal from wood is a pyrolysis process. The wood is put
> into a container which keeps oxygen out. Then some heat source heats the
> container to the point where the volatiles are driven off. The wood gas must
> have some way of exiting the container to avoid explosion. Without oxygen,
> the charcoal does not burn. The gas is often vented to the atmosphere where
> it cools and becomes smoke, a very wasteful process.
>
> I like the idea of having a replaceable sacraficial lining for the reactor
> so that the char can be burned in place if desired.
>
> Kirk
>
> ----- Original Message -----
> From: Huck Rorick
> To: psanders at ilstu.edu ; stoves at lists.bioenergylists.org
> Sent: Tuesday, October 14, 2014 9:36 AM
> Subject: [Stoves] Why doesn't charcoal burn in the Champion TLUD?
>
> Hi Paul,
>
>
>
> Can you explain why the charcoal in a Champion TLUD doesn't burn?  There is
> heat generated to drive off the gases from the wood.  Why doesn't that heat
> burn the carbon as well?
>
>
>
> Under what circumstances would it burn?
>
>
>
> If you leave the charcoal in the bottom of the TLUD and don't dump it out
> into an airtight container will it slowly burn away in the bottom of the
> TLUD?  If you shut off the primary air?  Will it burn away quickly if you
> give maximum primary air?
>
>
>
> Huck
>
> Huck Rorick
>
> Executive Director
> Groundwork Institute
> 2640 Silvercrest Street
> Pinole, CA 94564
> Phone: 510-222-4111
> email: huckrorick at groundwork.org
> Website: www.groundwork.org
>
>
>
>
>
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-- 
***
Dr. A.D. Karve
Trustee & Founder President, Appropriate Rural Technology Institute (ARTI)