Biochar in Hawaii

SUBHEAD: Small kiln/retort experiment for producing biochar on farm in Ahualoa, Hawaii.

I built a small kiln/retort on my farm in Ahualoa, Hawaii. Description, photos and observations from my first trial run at on my blog here:
Summary: it's a hybrid of the two-barrel and twin-oaks approaches, aiming for higher volume than the two-barrel and lower cost/complexity than the twin-oaks. My approach needs tuning, but it seems promising.

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.


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