[Stoves] Burning coal in cookstoves

[andrew]

On Wednesday 02 January 2008 08:06, Crispin Pemberton-Pigott wrote:
> Dear AJ
>
> >Bone, seashell, limestone or chalk (not gypsum) may serve the
> > same purpose.
>
> Do you have any idea about quantities? Surely it must be related
> to the amount of sulphur involved.

Yes, from Tom Miles recent post he's suggesting a fluidized bed 
burner would use 4 times the molar weights of reagents so if the 
sulphur is 2% of the coal and it burns out to SO2 then the reagent 
you add will have to be in a concentration of 4 times what the 
simple reaction suggests. So 1kg of coal will contain 20 grammes of 
sulphur which has an atomic weight of 16 but I don't know what 
molecular form it exists as, if solid sulphur it would be 
64gram-mol. If it oxidises to SO2 (48 gram-mol) then that 20 grammes 
of sulphur makes about 60 grammes of sulphur dioxide if I remember 
my chemistry from 40 years ago correctly.

I'm guessing that what happens is CaCO3 (100gram-mol) is split to CaO 
and CO2 by heat and the CaO then reacts with SO2 in an oxidising 
atmosphere which adds an O to yield CaSO4 , the solid gypsum which 
stays with the ash. So the initial 20 grammes of sulphur would need  
120 grammes of limestone if it were a 1:1 relationship or 480 
grammes to to give the 4 times which Tom suggests is necessary, it 
begins to look like a big proportion of 1kg!

Now from Tom's further points it looks like this reagent needs to be 
in the oxidising atmosphere of the primary air and wouldn't work on 
top of a TLUD burn because that is a reducing atmosphere.

> It appears that downdraft is the way to go for ultra low emissions
> with a small fire. 

Peter Verhaart will be pleased with that conclusion!

> So...perhaps the best answer is a particular shape of briquette
> with a limestone content sufficient to clean up the SO2 somewhat. 

It looks a bit like the briquette with limestone is the way to go.

AJH