25 to 100-gallon burners
A 55-gallon barrel is a cheap and easy choice to build your first significant scale backyard biochar burner. Barrels are easy to obtain, easy to modify, cheap if not free, re-usable, standardized. With simple tools and minimal skills, most of us can convert a 55-gallon drum into a barrel burner that can produce up to 30 pounds of biochar from local feedstock—enough in one batch to cover a decent size garden test plot.
Without any modification, a 55-gallon metal drum easily becomes a very simple charcoal-making burner. Tip a barrel on its side and start a fire in the bottom. Split kindling that ignites and burns quickly works best. Slowly add more wood until a hot fire is burning in the barrel bottom, then tip the barrel up.
At this point, the fire will start choking for oxygen, since air can't easily get down into the barrel bottom with hot flames rushing up and out. Add one more layer of fuel and close the barrel with a lid to starve the fire of oxygen. This forces it to smolder to char instead a full-scale, complete combustion to ash.
One modification is to add a few air slot at the bottom of the barrel. These can be open for full combustion, or have dirt kicked over them to restrict air flow and choke the fire for char-making.
This method is very inefficient and smoky, but makes a test batch for soil use. The resulting char will also include significant ash, torrified (scorched) wood, and tar, all of which are also useful to soil organisms.
But with a small bit of work and money, a 55-gallon barrel can become a rather efficient “barrel burner.” Outfitted with simple valves to restrict air intake to the combustion chamber, and chimney exhaust flue and damper, even some insulation, a 55-gallon drum makes a nice size charcoal cooker. Add a smaller size drum, this becomes a 2-barrel burner with a nested retort.
A hardware store can equip a few modifications to turn a barrel into an easy-to-operate, controlled-burn retort. An hour or two of metal fabrication will produce a simple barrel burner, with air intake and gas exhaust controls. I’ve seen over a dozen designs in less than a year.
The next sections feature basic barrel burner designs—super-simple, easy to build, vertical and horizontal. In the beginning, simple is best—focus on mastering the simple burn process and understand the gasification phase before you develop complicated burners.