RE: Inexpensive biochar retort, experimental design
- I have now done 4 burns in my kiln/retort, with some small design
modifications, and have some results to report. As they say, "the more you
burn, the more you learn".
For pictures, see http://washedashore.com/eggsntea/category/biochar/
1. It works well. The outer walls provide decent insulation, the outer fire
burns well without large amounts of wood, the contents of the barrel
pyrolyze thoroughly, and the pyrolysis gasses combust completely by passing
through the burn chamber. Once hot, it cooks itself very cleanly.
2. I could continue to improve the design a little. I could mortar the
blocks together, put a real chimney pipe on the back, and bury it deeper in
the ground (like David Yarrow's hole + air pipes). That could improve
efficiency and emissions a little more, but it's already quite good. I
could also improve the durability, so the barrel lasts longer and the blocks
don't crack so often from the heat.
3. A 55-gallon barrel just isn't very large. A full barrel of scrap wood
shrinks to roughly 1/2 in volume as it chars. I weighed the output, and
it's 23.2 lbs per burn. It take more than 2 hours to gather wood, load the
barrel, close the kiln, build and start the outside fire, then (next day)
open the kiln, unload the barrel. That's around 10 lbs of char per hour of
work. At $.50/pound, that's only $11.60 worth of char. Even assuming the
kiln materials and wood are free, it's not even $5/hour of labor - and the
fact it need to cool overnight means i can only produce one batch per day.
So, batch size needs to go up. It seems my options are:
A. Find a larger metal container, to build a kiln around. I may not be able
to tip it into place like a barrel, so it would need to have an airtight
door for loading and unloading. I've heard of people using a metal
dumpster? Not sure what kind of metal chamber would be easily found or
affordable. Perhaps thick metal barrels are available in sizes larger than
B. Switch to a in-place burn approach. Options are an open fire, a covered
pit, or a traditional charcoal kiln where the wood is lit directly and the
burn is controlled by limiting air. Efficiency would go down, and emissions
would go up - but it's very easy to scale up to a large fire/large pit/large
kiln. I'm not wild about this approach, and it might even run into trouble
with State fire/emission laws. But, i'll try it out and see what i get for
wood consumption and (more importantly) finished char per hours of labor.