The easiest way I've found to make bio char is by using a cut-down 300 gallon fuel barrel along with a 55 gallon oil drum. I cut the 300 gallon barrel down so it's about 12 inches taller than the 55 gallon barrel. This end piece is made into a lid. Add a 6-inch stovepipe to the lid to help the air draft up through the fire, thus getting the fire super-hot. Some draft holes are placed around the bottom of the 300 gallon barrel to provide air for the fire.
The 55 gallon barrel goes inside the 300 gallon barrel. Place some small holes at the first ring up from the bottom, about a third-of-the-way up from the bottom of the 55 gallon barrel. These holes will assist in directing the heated combustible gases from the char material into the fire to get the super-heating action (positive feedback reaction). Cut the top out of the 55 gallon barrel, leave a lip so a piece of stove gasket can be placed between the barrel and the lid to make a seal. The plant material you want to make into bio char goes into the smaller, 55 gallon barrel.
I use Eastern Red Cedar pieces to make bio char. Let the cedar dry for a year, cut them up into ~3 inch chunks. Place the chunked material into the 55 gallon barrel. After a year of drying most of the water moisture is gone from the wood (in our environment), but still has residual cedar oil that enhances the burn. Put the stove gasket in place and put on the lid. With the 55 gallon barrel inside the 300 gallon barrel start adding fire material around the 55 gallon barrel.
In the first layer of burn material I place a few feed bags to assist with getting the burn started. Pack the burn material around and over the 55 gallon barrel, put on the lid and start the fire. I prefer to place most of the burn material vertically in the burn barrel. The fire drafts from the holes along the bottom of the 300 gallon barrel, up the vertically aligned seams in the burn material, then out the central smoke stack. As the fire gets hot, the inner char material will start to emit wood alcohols and oil vapor. The wood alcohols and oil vapor are combustible and will pass out the small, knee-high holes in the 55 gallon barrel. The knee-high level is at the level of the fire so the oil feeds the fire and gets the burn even hotter. After about 12 hours the burn is complete. I usually start a burn in the evening, it's done by morning. After the burn, the char material only occupies two-thirds of the space (a 1/3 reduction).
If you expose the hot char to the air it will continue to burn and you will be left with nearly nothing for your effort. Once you take the lid off, wet the char down to stop the burn. I like to place the char into a 6-foot stock tank. I add water to stop the combustion. When I keep the char wet, a few winters of thawing and freezing pulverizes the char into small shards. I've also placed char on a tarp and hammered it into smaller pieces. We use the char directly as a soil amendment, though I think it works best when mixed into a compost pile. The organisms in the compost pile fill the pores in the bio char with nitrogenous material which enhances soil life.
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