Biochar Kiln
Well I finally got my biochar kiln built, cost me about 2000 pesos ($150 U.S.) for parts and labor, here in Colima, Mexico. I just showed the blacksmith pictures from the Internet, and he produced a very similar looking device.
Unfortunately, on the first burn I discovered it didn’t work. The door was not sealed good enough to keep oxygen out after the fire beneath it goes out, so the hot charcoal inside catches fire, and smolders. I thought I was leaving it to cool over-night, but by morning it was still smoking. I had to use water to put out the embers.
I was using the kiln to make charcoal on that first run, thinking that the charcoal would make a hotter fire for cooking the biochar. I ended up with about 1/2 charcoal and 1/2 un-burned wood, and the whole volume was less than half I started with — some reduction from the process, some just burned away from the night-long smoldering fire, due to too much air getting in.
So for my second try, I used the unfinished wood from the first try, plus some more wood, and tried again to make charcoal. This time I sealed the lid with a thin strip of clay. Our soil is over half clay, so I just extracted the clay from our yard. I also raised the fire up closer to the barrel on a layer of rocks. That worked pretty good, but the fire was still not hot enough to entirely ‘cook’ all the contents. There was very little smoke seeping out from the lid this time, at least the clay seemed to seal it good.
Most of the Internet photos of similar kilns show them surrounded with brick walls or cement-board, to hold the heat in. I guess that is an essential part of the system. I’ll report here again when I’ve built a containing system and get to try the thing out again. I have sawdust that I plan to use for the biochar, once the kiln is working properly.