Intriguing approach, I'm not entirely sure I understand the approach.
As I understand it, biochar is created by burning carbaceous materials in a low oxygen environment. I'm not seeing how you keep the oxygen levels low in this setup.
I do, however, think you've inspired an alternate approach. Digging a burn pit that is slightly smaller than the tub would allow a good fire to be built, and then the tub inverted on top. That (if my brain is sufficiently caffinated and active) should inhibit oxidization of the burn, while letting the drain act as the exhaust/chimney. Conversely, two tubs of similar dimensions could be placed with drain spouts at polar and planar opposite ends (yeah, it's gonna be heavy.) if you build a slight platform beneath the entire setup, it SHOULD allow air to draw in the bottom, through the firebox, and out the opposite end of the kiln via the top tub's drain.
Having never played with biochar or similar approaches, however, I'm confident in saying that while it might look good on paper, I have no idea what I'm talking about.
I guess the proof is in the burning; I obtained a decent amount of charcoal using this method.
As for retort style kilns I read that retort yield is much the same as open kilns due to the extra material needed to get the fire going. And of course there's a lot more work involved in constructing such kilns, whereas mine is a one off burn that I probably won't repeat.
If you send it by car it's a shipment, but if by ship it's cargo. This tiny ad told me:
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