Biochar retort, experimental design, first trial run

My friends Josiah and Jay down in Puna are producing biochar using a classic pit method, which seems to work well. I may end up making char that way as well, but there is some criticism on the biochar list of open burns, saying that emissions aren’t fully combusted and carbon yield is low, recommending a kiln or even better, a retort (closed “cooking vessel”). So, i looked at plans online and found two approaches, the two-drum and the Twin Oaks, particularly as built by Kelpie in Oregon. The first approach is too small a batch and requires multiple drum sizes, the second requires expensive metalwork including pipes and welding. I came up with a hybrid of the two approaches which should be cheap, simple and high yield.

I did my first trial fire-up yesterday. The trial results are from this picture onwards.

Results were promising, but need tuning. I learned a lot from this trial run. Some indications:

  1. The kiln fire needs to be strong and hot and heat up fast.  My kiln burned moderately, for a long time, so it didn’t fully cook the retort.
  2. More air inputs.  I was hoping to limit openings to focus the heat inside.  I put vents on the left and right and front, but the fire seemed to want more air.
  3. A round barrel in a square box isn’t great geometry for a fire, which tends to burn separately in the four corner “zones”.  I could try stacking the blocks in a more circular arrangement, like a hexagon/octagon.  If i stay with this arrangement, i’ll need air vents specifically pointing into each corner.
  4. Chimney.  I figured a simple rectangular hole at the back should suffice, since it worked for Kelpie.  But mine didn’t seem to draw well. Charcoal kilns for a thousand years have had proper chimneys.  I’ll probably need one too.
  5. Insulation.  I used regular CMUs because they’re cheap and available.  No doubt better insulation would result from using firebrick, thereby focusing more heat inside.  I could also fill/bury the hollow tile walls, even if they’re dry-stacked.

The half-charred results of this trial aren’t useless; they could still be used for a less-smoky cook fire, or dropped through my shredder to make mulch with a more stable carbon content.  However, the goal remains easy, cheap, reliable full pyrolysis.  If it doesn’t pan out with this design, i could always switch to a pit, or hybrid brick-lined pit, or other ideas.

2 thoughts on “Biochar retort, experimental design, first trial run

  1. GREAT WORK! Thanks for sharing. Looks like a great concept for an economical design. It appears to me that you have it down if you add a few tweaks. Maybe some holes in barrel at bottom to let wood gas to escape, burn and add to heat of fire. Maybe if wood was smaller dry chucks and you had a block with a vertical hole in middle of roof with a tall chimney pipe to increase draft effect. Please share your progress. I would like to try your design as time permits. Keep on Keepin’ On! Be the Change!

  2. I’d recommend moving up to a hot water heater tank as a core element as well. You’re going to find once you get it hot enough to do the job you’ll burn those barrels up quite quickly. The steel isn’t up to the task.

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