[Stoves] Pyrolytic gasifiers - gasifying the carbon

[Crispin Pemberton-Pigott]

Dear Paul and John (nice to hear from you!)

John notes:

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All oxygen present will react with carbon above about  400c to form  CO2.-
 exothermic reaction.- heat evolved  ~2X

When CO2 without oxygen comes in contact with carbon at
about 800c it will react and form CO -
endothermic reaction.- Heat absorbed ~X

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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