[Stoves] Burning coal in cookstoves

[Kevin Chisholm]

Dear John

John Davies wrote:
> Crispin,
>
> A little clarity on my thoughts.  I use the term wood ash very loosely. My
> thoughts go in the direction that any wood would burn to ash before the coal
> was giving off sulphur compounds. In my TLUD coal stove. the ignition is
> started by burning about 100g of wood kindling above a layer of fine coal.
> The ash from this wood forms a layer above the coal which can be clearly
> seen once the wood has been burned. after a while it cannot be clearly seen.
> Has it dropped into the coal bed below , or has it left the bed as particles
> in the flue gas ???. I do not know.!
>   

Wood ash captures sulphur because it reacts with it and forms a solid 
compound containing sulphur. In pyrometallurgy, lime is commonly added 
to combine with sulphur so that it can be "slagged off" and be removed 
from the metallic phase. Similarily, in the treatment of flue gases from 
a coal burning power plant, the sulphurous flue gases are reacted with 
lime to form CaS, or CaSO4. In such a "flue gas desulphurizing 
operation", the efficiency of lime utilization can be poor... instead on 
only one mole of Ca being required to capture one mole of S, the molar 
ratio may be excessive... perhaps the ratio of Ca/S could be 1.2, 1.5 or 
perhaps as high as 2, in very inefficient cases.

The potassium in the wood ash seems to act as a catalyst, giving an 
overall reaction along the lines of
     S + K + Ca ----> KS + Ca ----> CaS + K

The reaction can be thwarted by conditions which lead to the K being 
consumed by other species. For example, the K can react with SiO2 in the 
ash to yield a K20-SiO2 glassy phase. Along these lines, too much wood 
ash could be very bad, in that it could react with the high silica coal 
ash to make a liquid slag phase at operating temperatures, that could 
lead to serious clinkering.

The next time you run your TLUD, you might consider checking the 
resulting coal ash for evidence of glassiness or different appearance. 
My guess is that your wood ash disappeared by reacting with the coal ash.

Best wishes to all for a Happy and Prosperous 2008

Kevin


> With anything up to  5 Kg of coal to be burned, I doubt that there is enough
> ash to capture all the sulphur. I also doubt that a user would be prepared
> to add sufficient wood chips into the coal bed to achieve removal.
>
> It has to be borne in mind, that those using coal do so because it is much
> cheaper than firewood in their location. So while the idea is interesting to
> us, it is unlikely to be utilized by the end user, unless they are forced to
> do so.
>
> At this stage I believe that our aim should be to improve the combustion
> efficiency, make all the usable heat available, and remove the toxic gasses
> from the inside of the dwelling. This alone would reduce coal consumption by
> up to 50 %. And eliminate many deaths due to CO poisoning. That would be a 
> huge step forward.
>
> Wishing all a prosperous and happy 2008.
>
> John Davies.
>
>