[Stoves] Burning coal in cookstoves

[Tom Miles]

Philip,

We found that by burning 25% wood with 75% bituminous coal in an
institutional boiler we created a volatile flame above the coal bed that
burned out the combustible gases and particulate. Since it was a continuous
feed stoker it might not work the same way in a batch stove or furnace. 

The coal was 40% volatile, 10% MC and 20% ash 32 mm stoker coal (25 MJ/kg
LHV). The wood was wet sawdust, 50% MC, 1% ash 6-20 mm (8 KMJ/kg LHV).
Moisture from the wood cooled the coal flame and reduced agglomeration. Wood
ash captured the sulfur in the ash. High volatiles from the wood (80%) of
the fire and cleaned up the hydrocarbons which reduced emissions, cleaning
up the stack and burning out the CO. (We burned up to 2/3 wet wood with coal
at which point we were "burning" 1/3 wood, 1/3 coal and 1/3 water.) We
engineered the combustion air for good burnout at and above the grate. Now
if we could use a little wood to clean up a coal stove . . .

Tom Miles         




> -----Original Message-----
> From: stoves-bounces at listserv.repp.org [mailto:stoves-
> bounces at listserv.repp.org] On Behalf Of IPC
> Sent: Friday, December 28, 2007 12:07 PM
> To: 'Discussion of biomass cooking stoves'
> Subject: Re: [Stoves] Burning coal in cookstoves
> 
> 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
> 
> 
> 
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