Tried out my new charcoal furnace this last weekend, and it seems to work okay.
I've been disappointed with the old technique for making charcoal for a long time. Seems like I went thru tons of fuel and made too much nasty, billowing clouds of smoke for the couple of cubic feet of charcoal I got. I felt I could do better than that.
I started with a 24" by 40" tank that had already been converted into a wood stove. I punched some more holes in that to accommodate a 48" by 14" propane cylinder. I cut the bottom off the propane cylinder and fitted it up with a mate collar and a gasket (made from welder's drop cloth). Effectively gasketing that joint is the only tricky part of this assembly. I also replaced the valve with some ¾" black iron piping that I routed back around underneath the smaller tank within the fire box, and drilled some holes in that. The idea was that the gasses evolving off the wood in the smaller tank would contribute to the fuel heating it.
Here is a CAD sketch of the basic structure.
Separating the wood to be converted into charcoal from the heating fire also would mean that I could burn household trash, yard waste and other wood unsuitable for charcoal and still end up with clean charcoal.
As I said, it worked okay. It still took a handful of hours and it still smoked rather a lot, though now it was more from the dried resinous scotch broom and rhododendron bush trimmings I was using as fuel. I wasn't sure how well the feedback line was working during the bulk of the run but as you can see in this picture taken late in the process, it seemed to be working fine.
As an unexpected side benefit I noticed black goo dripping from the seal on the inner tank. I collected about ½ cup of wood tar from that. I haven't decided what to do with it yet, but I've got it, and I know it should be useful for some stuff. I happened to choose to make this batch of charcoal from splintered Indonesian hardwood pallet boards. It will be interesting to see if I get different tars from different feed stocks. Or different performance from the charcoal, for that matter.
Here is one more picture taken at the same time as the last, but showing more of the furnace.