[Stoves] Burning coal in cookstoves

[IPC]

Dear Dean

I was intrigued that you were thinking of trying coal.  It is not an easy
fuel.  We burn about 1 million tons annually in homes in South Africa, and
it causes 40% of our total particulate air pollution load.  

Part of the problem is that the stove design is basically an 1870's wood
burner, cast iron with a ceramic lining.  To get a proper air flow, you need
a sized coal - the wood stove needs -30 +12mm.  Anything coarser packs
badly; anything finer cuts down air flow and you don't get a proper fire.

It is bottom lit, and that is the soure of the problem, because the coal
gives off smoke while it is being heated from below.  You lose a lot of
energy as "smoke" - the "smoke" is actually high-grade combustible material.
We captured some in an electrostatic precipitator, and it had a heating
value of over 35MJ/kg, whereas the dry coal was about 25MJ/kg.

The smoke levels increase soon after the fire is lit, until temperatures
reach the point where the smoke will burn.  While there are visible flames,
it is relatively clean.  Then, when most of the volatiles have been driven
off, there is not enough volatile fuel to support flaming and the flames go
out - but the volatiles continue to be emitted, i.e. the thing starts
smoking again. The smoke levels are usually at a lower level than the first
smoke, but smoking goes on for longer. 

Top-lit designs smoke much less, but stoves with a top-lit feature are not
commercially available. There is a huge "market inertia" - the old
wood-burning design has a certain social cachet, and people have "arrived"
socially when they can invite their friends to sit round the stove of a
winter's night.  Spare parts are readily available, and there is an
extensive distribution network.  So, inefficient as it may seem, it is not
going to go away any day soon.

Because of the air pollution, Government has been seeking an alternative.
The best seems to be devolatilised coal.  It is possible to come up with a
satisfactory fuel if you reduce the volatiles to about 8% by heating in the
absence of air. It would add about 20% to the pithead price, which would
increase the retail price by about 5%, but so far no-one has had the courage
to do this.

Hope this experience helps.

(Dr)Philip Lloyd
Energy Research Centre
University of Cape Town
Private Bag Rondebosch 7701
South Africa
Tel +27 (0)21 650 3896
Fax +27 (0)21 650 2830
e-mail philip.lloyd at uct.ac.za