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RE: Inexpensive biochar retort, experimental design

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  • Ben Discoe
    I have now done 4 burns in my kiln/retort, with some small design modifications, and have some results to report. As they say, the more you burn, the more
    Message 1 of 12 , Jun 7, 2010
      I have now done 4 burns in my kiln/retort, with some small design
      modifications, and have some results to report. As they say, "the more you
      burn, the more you learn".

      For pictures, see http://washedashore.com/eggsntea/category/biochar/

      1. It works well. The outer walls provide decent insulation, the outer fire
      burns well without large amounts of wood, the contents of the barrel
      pyrolyze thoroughly, and the pyrolysis gasses combust completely by passing
      through the burn chamber. Once hot, it cooks itself very cleanly.

      2. I could continue to improve the design a little. I could mortar the
      blocks together, put a real chimney pipe on the back, and bury it deeper in
      the ground (like David Yarrow's hole + air pipes). That could improve
      efficiency and emissions a little more, but it's already quite good. I
      could also improve the durability, so the barrel lasts longer and the blocks
      don't crack so often from the heat.


      3. A 55-gallon barrel just isn't very large. A full barrel of scrap wood
      shrinks to roughly 1/2 in volume as it chars. I weighed the output, and
      it's 23.2 lbs per burn. It take more than 2 hours to gather wood, load the
      barrel, close the kiln, build and start the outside fire, then (next day)
      open the kiln, unload the barrel. That's around 10 lbs of char per hour of
      work. At $.50/pound, that's only $11.60 worth of char. Even assuming the
      kiln materials and wood are free, it's not even $5/hour of labor - and the
      fact it need to cool overnight means i can only produce one batch per day.

      So, batch size needs to go up. It seems my options are:

      A. Find a larger metal container, to build a kiln around. I may not be able
      to tip it into place like a barrel, so it would need to have an airtight
      door for loading and unloading. I've heard of people using a metal
      dumpster? Not sure what kind of metal chamber would be easily found or
      affordable. Perhaps thick metal barrels are available in sizes larger than

      B. Switch to a in-place burn approach. Options are an open fire, a covered
      pit, or a traditional charcoal kiln where the wood is lit directly and the
      burn is controlled by limiting air. Efficiency would go down, and emissions
      would go up - but it's very easy to scale up to a large fire/large pit/large
      kiln. I'm not wild about this approach, and it might even run into trouble
      with State fire/emission laws. But, i'll try it out and see what i get for
      wood consumption and (more importantly) finished char per hours of labor.

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