Biochar kiln progress

I recently did a second and third burn in my biochar kiln, tweaking each time. The story is best told in pictures:

Upon detailed inspection, the April test burn actually gave good results. Four white buckets are completely charred material, two orange buckets incomplete, one mixed and one of material from the surrounding fire.

Completely charred wood from retort, and the incompletely charred – only a small amount, and generally from the bottom of the barrel, perhaps due to a lower temperature there.

Preparing for burn #2, using smaller wood and some changes to the kiln.

Added a layer of firebrick at the base. Ideally, it should enclose the whole chamber, but that would take a lot of actual masonry.

More air inlets, allowing air into all four corners.

The ‘chimney’ is formed by the blocks themselves.

Smaller wood scraps for burn #2.

Opening the kiln after burn #2.

As before, the material at the bottom of the barrel (top, when inverted like this) is less charred, but everything above (below) it is completely charred

Much of the sticks that look brownish on the outside are actually completely charred black on the inside

Burn #3

Got the fire real hot this time, you can clearly hear the “whoosh” of the pyrolysis gasses from the barrel joining the fire

Sifting/crushing/sorting the result. Some 1/2″-minus has direct uses. The rest will soak in nutrients to charge it, then goes through the chipper-shredder to make “charged fines” – biochar fertilizer.

9 thoughts on “Biochar kiln progress

  1. So the charcoal has to be soaked in a nutrient solution still? What kind of solution? Should i just stay tuned ; )

    Could purchased charcoal be used the same way?

  2. Hi Julie:. Yes, you can buy “all natural lump hardwood charcoal” Cowboy Charcoal. The trick is to wet it down with nutrient solution and smash it to tiny, tiny bits (use mask!) by hand with rubber mallet or use chipper/shredder.

  3. Yes Julie, there are advantages to “pre-charging” the char with nutrients. Plain char will have long-term benefits of retaining nutrients and boosting microbes, but if you want immediate results, charged char is better. Popular methods are:
    1. Mixing the char into warm, live compost and letting it sit. Char will absorb nutrients.
    2. Soaking the char in a nutrient solution, for example fish emulsion fertilizer, or urine. Urine actually has astonishing levels of N-P-K, if the yuck factor doesn’t bother you, it’s an excellent free source.
    You could pour water into a bucket of that fresh horse manure i see on your blog, and use that aqueous solution to charge char.

  4. Hi Ben! I’m looking for some design schematics for a double barrel oven design for a community demonstration project in Hampton VA. Have you run across anything besides some pictures and instruction like “Put some holes in the bottom”? This is a bunch of engineers so winging it means everyone will write and run complicated thermodynamic modeling program so we can argue about it in a meeting. I’m really trying to avoid that whole thing.



  5. Melissa, I don’t recommend the double metal barrel approach. 1. It requires finding or manufacturing barrels of different sizes, 2. It limits you to output size of the smaller barrel and 3. you want the outer layer to insulate, to hold in heat, and a metal barrel is very poor for that. I recommend instead an approach like you see here in my blog, with a single barrel (or other metal chamber open at the bottom), surrounded by any kind of insulated chamber you can build – earthen, concrete blocks, bricks, it does not matter, anything is better than a thin metal barrel. Consult my notes at and feel free to give a phone call (for contact info click ‘Directions’ in the right.)

  6. Thanks for making this info. available. I also have a lot where I’m living on the Big Island and this inspires me to want to try it. I remember seeing charcoal pits before, now I regret not have studied harder how they were doing it, if I recall it took about 3-5 days to do it in a 10’x20′ pit.

  7. You must read this
    “The Biochar Revolution” with “The Biochar Solution”
    The Biochar Revolution collects the results and best practical advice that these entrepreneurs have to offer to the biochar community. When practice and theory advance to the point where they meet in the middle, then we will truly see a biochar revolution.

Leave a Reply

Your email address will not be published. Required fields are marked *