Biochar: from kiln to pit

Those of you following the biochar-hawaii list know that i’ve stopped using my kiln, and am now focused on making biochar in a pit. This is both for reasons of scalability and wear; my 55-gal steel drum kiln/retort could only make ~23-gal of char, and the surrounding kiln blocks cracked from repeated heating.

Hence, a pit. Mine is lined with blocks for clean char and easy unloading. Continuously fed wood, pyrolysis occurs at the air-starved bottom of the pile, gradually the pit fills up, then i cover and let it cool for a day, before opening and scooping out the finished char:

That first small pit worked well, so i made it bigger and sure enough, it scales well:

Width Length Depth Gallons Cubic feet # of blocks Gallons of Char
24 32 16 53.2 7.11 33 16.5
32 48 16 106.4 14.22 48 34
32 48 24 159.6 21.33 60 68
32 48 24 On second burn: 82

That 82-gallon operation took 2.5 hours to do the burn, then 2.25 hours the next day to unload, crush, sort, sift, and load into buckets. That’s 82/4.75 = 17.25 gallons of char per hour of work. That’s not bad, given that i’m working with some cheap concrete blocks, a piece of old corrugated roofing, and a shovel. With more money and technology, like a continuous pyrolysis machine, you could certainly get vastly more char per hour of labor, but those machines start at $100,000. I’m feeling quite happy about my pit. The Biochar2010 album has all the pictures.

I gave a biochar talk to the Kona Coffee Grower’s Association on June 2. 10 minutes of that talk got uploaded to YouTube. I then addressed the Hawaii Tropical Fruit Growers on July 19, that time with a fancy presentation with charts and pictures. Next will probably be an evening talk in Waimea on August 8, and then a 1-day workshop on making and using biochar here at our farm, date TBA.

7 thoughts on “Biochar: from kiln to pit

  1. Is there a quality relation between the different charcoal peices, powder etc?
    Can you just grind everything all together, and then mix it with compost?


  2. Research is still ongoing about what charcoal size (pieces, power, etc.) give the best results with different coils, climates, and plants. In general, the answer is yes: mix together with compost. Beware ashes though! Ashes are a different subject, with its own pros and cons.

  3. Thanks Ben,

    Have you done any analysis of char from the concrete blocks pit?

    How do you use the biochar? 50/50 with compost?

    Do you know where I can learn more about it?

  4. Nitya,
    No, i have not done analysis. Like all open pits, i would expect a whole spectrum of properties, with the range of low to high-temperature pyrolization, and diverse input materials. I use the biochar in many ways, mostly in soil improvement of our acidic clay soil, and as a potting medium, and also as a compost ingredient (although much less than 50%, because we make a large amount of compost, and don’t have that much char).
    To learn more about it, there are countless website and now, two books. I recommend as a starting point.

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    This book will help !

    Here practice and theory merge under a single cover of “The Biochar Revolution” and reveals hidden secrets of science called Biochar

  6. I love the simplicity and affordability of this pit kiln design. I’m worried, however, about the climate change implications of not flaring the gasses produced by the pyrolisis happening in the pit. Are you flaring the gasses coming out? If not, have you heard of any way to produce biochar with a pit kiln while flaring the exhaust gasses? That’s exactly what I’m looking for. Low tech + low cost + high return for labor + low/no release of gasses into atmosphere.

  7. I also love how simple this pit kiln is as really anyone could do it. How have things been going with it? Can you update?

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