Initially I needed charcoal to run a forge. On the journey to find out how to make charcoal for that purpose I discovered biochar and gasification to fuel an internal combustion engine. At the time, it seemed that the 2-barrel method with 30 gallon retort inside a 55 gallon barrel with a 6 inch chimney was the best choice. I couldn’t find a 30 gallon barrel with a removable lid so I had to fill the 30 gallon barrel with wood, place the 55 over it on blocks, and then insert a rod under the bottom of the 30 to hold the barrels in place while I turned the barrels so the 30 was upside down. In the videos it looked easy to flip the barrels without the rod but I didn’t have much luck doing so without getting wood under the bottom of the 30. After a burn or two I became unsatisfied with that method. It’s a royal pain trying to stuff fuel between the barrels, and I had some charcoal that wasn’t completely converted because there wasn’t room for enough fuel between the barrels to complete the process.
At that point I cut a 1/3 section of a 55 gallon barrel, with an expanded metal grate set on 4 bricks and an access hole in the side for addition of wood, as a firebox beneath the original 55 gallon barrel. I cut the firebox barrel just above a rib so I simply stacked the 55 gallon barrel on top of the firebox. The original 55 gallon barrel had holes cut around it’s bottom perimeter and sides at the bottom to allow air flow, yet the middle of the bottom was solid so that no air could enter the 30 gallon retort. With that tweak, I no longer had problems with an incomplete conversion and it was easy to add wood.
When to bottom of the original 55 gallon barrel burned/rusted out I had to find another way to support the 30 gallon retort inside the 55. I used ½ inch rebar welded together in a triangle set in notches in the top of the firebox. Four legged tables can rock, three legged tables do not. However, the ½ inch rebar would get soft and bend under weight so I had to weld angle iron on them for more strength. I eventially make 2 rebar triangles to make a star shaped support. Since I needed a bottom for the 30, I bolted a disk blade slightly larger than it’s diameter for a removable bottom/top. I bolted the center hole in the disk blade closed because the gasses exit between the disk and the barrel and I have a few holes drilled in the top now bottom of the 30. The disk blade was a lucky coincidence. It’s heavy enough that I doubt it will burn out any time soon. It’s also curved, so it settles nicely onto the rebar/angle under heat and directs the flames from the firebox and gasses from the 30 up.
That worked pretty good but it still seemed that I had more smoke out the top than was politically correct so I stole an idea from Gary Gillmore and used the remaining 2/3 barrel set on a rebar spacer as chimney. It’s not completely smokeless, but it smokes less…………less than cooking a hamburger on a hibache.
As assembled and ready to fire: 1/3 of 55 gallon barrel as firebox with grate on bricks and support for 30 gallon barrel, 55 gallon barrel with no top or bottom with air holes around bottom sides, 30 gallon barrel filled with wood upside down with disk blade as bottom, rebar spacer for air on the 55, 2/3 55 gallon barrel as chimney.
After lighting the wood in the firebox, the 30 will off-gas in 20-30 minutes and burn is complete in 2 ½ -3 1/2 hours. Off-gassing is not strictly linear and regular, small and dry to larger and damper, means that once off-gassing begins to burn on its own flame, at least a small fire must be maintained in the firebox. The off-gassing process will start and at times diminish to the point that the burn will not continue to completion without a source of heat and flame in the firebox to maintain off-gassing. It’s possible for the off-gas burn to take over and burn very vigorously for such a duration that the fire in the firebox will nearly go out before off-gassing is complete. Once the burn is complete I just walk away until the charcoal has cooled completely. I have fired twice a day by dumping the nearly cooled charcoal from the first burn into an airtight 15 gallon barrel. Depending on the size and moisture content of the wood in the retort, approximately 30 gallons of wood is converted to 15 gallons of really good charcoal while burning 15 gallons of wood in the firebox. . I suspect that if you want to start a fire and leave it completely unattended until complete, a larger firebox (2/3 barrel) with more wood would get the job done.
This retort isn’t for the faint of heart. Although I’ve never seen red metal other than the 30 gallon barrel supports, it gets HOT. Laser thermometer readings on the outside barrels were around 250 degrees, but internal temperatures were higher. The thermometer I used goes to 900 degrees and then simply says “higher”. Pointing the laser through cracks and holes, in some places read “higher”. With good dry oak it sounds like a jet engine when it off-gasses and the flames may be 8 foot high. Considering that flammable wood and wood gas from the retort is at roughly a foot off the ground, it does get warm.