[Stoves] Pyrolytic gasifiers - gasifying the carbon

[Kevin Chisholm]

Dear Crispin

Carbon has a boiling point of 4,027 C. You can't "boil off" carbon. You must 
oxidize it.

Oxidation must inherently involve diffusion of "oxidizer gases" into the 
clay, so that they can react with carbon.  For this mechanism to proceed, 
you must have an "open cell structure", so that there are continuous 
passages for the "oxidizer" to diffuse into the clay. Then, the CO produced 
must diffuse out of the clay, to allow the entry of additional oxidizer.

I would suggest for your consideration the use of wood pulp, fine sawdust, 
or perhaps coarse sawdust, rather than charcoal as your "removable spacer 
material.".

I would guess that wood would be better than charcoal, in that for a given 
volume of wood, compared to charcoal, simple heating will destroy the wood, 
and conceptually, there would remain a little grain of charcoal rattling 
around in the hole where the wood used to be.

I would guess also that there is a critical volume of sawdust to add to the 
clay to permit rapid burnout. Above a certain volume percentage of wood, 
there would be a significant "interconnection of passages" to allow 
relatively easy diffusion of oxidizer in, and CO out.

Best wishes,

Kevin
----- Original Message ----- 
From: "Crispin Pemberton-Pigott" crispin at newdawn.sz

...del...
> +++++++++
>
> I have been making a few insulative rings for Rock and other stoves in the 
> workshop, made from a blend of two black clays and charcoal powder.  Here 
> is an example:
>
> Moulded weight after drying for a week = 1889 gm
> Heated to 105 degrees for several hours to dry it, heated to 600 degrees 
> for 3 hours to get out the crystal water, heated briefly to 1000 to fire 
> the clay and burn out the charcoal.
>
> After firing the mass dropped to 1543 gm.  1543 is simply too heavy, based 
> on the inputs.  It was 1/3 charcoal powder.  Clearly the charcoal (carbon) 
> was not removed from the centre of the 60mm thick ring.
>
> After a second firing holding at 1000 for 2-3 hours the mass is now 1397 
> gm for a density of 0.95.  My explanation is that 146 more gm more 
> charcoal has been removed.
>
> As the sides are porus, one could say it has 'burned' but I really doubt 
> that air is getting 30mm into the material to combust the carbon.
>
> My feeling is that is has to be evaporating rather than burning to be 
> removed.
>
> John is right about the 400 C and 'burning' but I think we should look at 
> the toolmaker's approach when there is no air.  Case hardening of steel is 
> done in a sealed stainless steel box filled with charcoal in one form or 
> another.  It has to be 900 C to create the gas and to open the steel 
> structure to accept the carbon.  I am not sure which dominates - probably 
> the steel's need for heat.
>
> To get out the carbon gas I will have to hold the (insulative) ring at 
> 900+ for long enough for the heat to get to the centre of and evaporate 
> the charcoal.  This is a serious issue for rural kilns where these things 
> are supposed to be made.  At the moment people believe their insulative 
> rings have no carbon left in them after kiln firing.
>
> Alternatively, the ring should be held at a temperature at which carbon 
> evaporates because there is no 'air in there'.
>
> While it may be common cause that the charcoal 'burns out', it is not 
> burning out 30mm inside the ring.  It is sitting there, hot, and later, 
> heat-conductive unless it has been gasified and given a chance to move 
> through the remaining clay sponge.
>
> Comments anyone?
>
> Regards
> Crispin