Low-cost, Low-emission Charcoal Kiln Design Elements

Paul Taylor, Greg Hall, John Seed, Geoff Moxham, November 2008

A useful URL:
http://www.fpl.fs.fed.us/documnts/fplr/fplr2213.pdf
For now I want to talk about basic Design Brief Essentials, then design particulars.
The design essentials identified so far :

1. Good conversion quality of feedstock to char.
Requires:- 500C core temp of char vessel (need: probe), therefore ~800C outer firebox
temperature, and enough thermal momentum to hold the firebox temps for an hour (soak);
therefore efficient insulation R values, and thermal mass, so: firebricks and cladding/mudding
are favoured. Additional small bourry box for soak may be necessary.

2. Clean burning in terms of GHG and visual and environmental pollution.
A suburbia essential. In the first 10 -15 minutes there is a tricky juggle of airflow, start-up fire
fuel mass and dryness. This is the nutshell of the task. Big airflows imply a good height of sealed
chimney/stack/flue or fan forced air. The idea of a small "jockey" firebox, like the bourry box, at
the flue end of the kiln, may be the best for consuming any smoke from the startup of the main
firebox, and any unburned post-pyrolysing gases. This would be done by including the unburned
gases in the air feed to this start-up fire, for ~15 minutes. Cladding and ducting this hot envelope
of air and smoke on start-up to the air inlet venturi will help emissions. A cyclone added to the
flue outlet will remove particulates.
Hot operation should be clean and easy to adjust air/fuel ratios, once draught and pyrolysis
begin.

3. Efficiency in terms of utilization of waste heat.
At operating temperatures the kiln will provide excess heat. Attached to the cyclone and flue
could be heat exchange coils in 3/4" copper, or water-jackets in stainless steel. At the low-tech
end, just any metal tank, an old copper, 44