Making some charcoal

I've been doing some work on making charcoal.  Say why?  Well, I'm mainly interested in it as a soil amendment (see Terra Preta.)  Charcoal in the soil is supposed to increase the cation exchange capacity (CEC), that is, the ability of the soil to hold onto vital trace nutrients such as potassium and calcium until plants and their symbiotic root fungi are ready to take them up.  Charcoal is also potentially useful in greywater filtering, and of course, barbecue.

According to the internet, there are two basic approaches to charcoal making, the direct method and the retort method.  I decided to try a retort method, described very well by Gunther the big Swede.  

I started with a minature version.  The retort is a juice can upside down in a piece of six-inch class-A insulated chimney pipe.  I stuffed the inner can as tight as I could with flax straw.  The inner can is placed upside down in the outer pipe.

Fuel around the outside of the inner can is burned, producing heat that cooks the material in the inner vessel.  I decided to try top-down burning as in the wood-gas camp stove.  I placed a few cubes of wood at the bottom and flax straw in the rest of the gap.

The inner can rests on a grate, allowing the smoke and gases to escape at the bottom and be burned in the outer fire, adding to the heat.  The outer sleeve is spaced up on bolts to allow combustion air to enter.

Here you can see the gas flare as the volatiles from the straw in the inner can are driven out and burned.  This is a positive feedback or bootstrapping phenomenon.  On my first attempt the fire went out before this got going.  I had to refuel it and start again, then it took off and burned a lot longer.

It worked.  It converted all the straw inside to charcoal with very little ash.  The volume of charcoal was about half the original volume of straw.