[Stoves] Crispin´s kiln. (was Re: Traditonal Charcola Making Process / retort

[AJH]

On Fri, 17 Aug 2007 00:02:58 +0200, Crispin Pemberton-Pigott wrote:

>[1] Crispin says some combustion air is allowed in to burn char which
>leaves pores in the stove body, so there is a massflow and energy
>contribution from this.
>
>I had not given this enough thought.  The dry mass of charcoal in the PK11
>mix is 10%. At a firing mass of about 4 Kg the stove has 400 gm of charcoal
>in it.  This has quite a lot of heat, were it to be burned properly.

Yes it is probably a large portion of the total, 400gm of typical
charcoal has the potential to contribute 2.8kWhr(t).
>
>So I agree there is heat released by the burning of the charcoal powder, but
>it won't by at full heat because it is removed in the form of CO and the
>temperature in the kiln is too low to ignite it.  I expect it is all
>released as CO from the top of the furnace.

I suspect you have been inferring too much from contributions of our
more erudite gasification contributors. How do you think you can turn
carbon powder to CO under these conditions? Tom Reed has suggested the
conditions necessary will be:

Temperatures over 700C
No free Oxygen
Bed depth of 20 particle diameters

None of these conditions exist in your electrically heated kiln.

I cannot see any other mechanism, than oxidation, for the char to
leave the stove body ( Carbon doesn't change state till about 3000C
IIRC)

> If I could measure CO2 I guess I
>could prove it.

Yes
>
>I was looking for a heat content for a kilo of charcoal burned to CO but I
>have not found a consistent figure. Perhaps you can offer something.

We can calculate it but it's not necessary if the char burns out to
CO2.
>
>Suppose all the carbon burned out and the temperature did not rise at all?
>It means there is a balance in heat put in from elements and heat from
>burning carbon (to CO) minus loses from air flow and through or into the
>walls.

I don't follow this, we know that as the carbon burns it leaves the
kiln at the exit temperature, we can calculate the mass flow of the
CO2, Nitrogen and excess oxygen, from this we can see what the flue
losses are. As the air has been heated by the electric elements and
the combustion takes place in the stove body I cannot see how it
cannot contribute to increasing the temperature of the stove body.
>
>It will be an interesting modelling calculation to see if there is any
>meaningful benefit from the burning of the carbon. 

Crispin There's a saying here that "you cannot make bricks without
straw" this comes from a time that bricks were hand made and fired in
simple trenches, the straw contributed both char and oxygen to heating
the brick internally. 

> All the carbon burns out
>of the clay at or before 600 C.  The air hole is open at the time, wasting a
>certain amount of the heat

Yes it does contribute to flue losses.

> and venting the CO. 

OK I give up!

Incidentally I've been ploughing through recent posts on
[gasification] which is too verbose and very high noise to sound ratio
to read when mobile with slow download speeds, I'm grateful that
message on this list tend to be more succinct. I noted two threads of
interest, one from Tom Reed acknowledging a charcoal gasifier was
worthwhile *IF* the charcoal were made in a manner that energy
released was utilised, a TLUD does this. The second was a post by
Philip Raufast (sp?) suggesting running a TLUD continuously by
refilling it with fresh wood when the pyrolysis zone reached the
bottom and started back up in updraught mode, then inverting the whole
fuel canister and letting it run TLUD again. Has anybody tried it? I
wonder at what stage the ash would start interfering.

AJH