Wednesday, September 3, 2008

New charcoal retort

I made a new charcoal retort, as the first one burned up after a dozen batches. This one is closer in design to what Folke Gunther describes, in that the outer barrel has a bottom and the inner barrel sits upside down in it. (In the first one my outer barrel was open top and bottom, and the inner barrel had a lid.) Also per Gunther, the outer barrel has triangular notches in the side near the bottom to let in combustion air, and it is not insulated.

The new inner barrel is ten times the weight of the previous one (35 vs. 3.5 kg.) It's the inner pressure vessel from a carbon dioxide tank. My idea was that it would last longer, and might do a better job of cooking the charcoal to completion by retaining heat longer. Unfortunately it's slightly larger and taller than the old trashcan and there is at most a two inch gap between the inner and outer barrel.

So far this thing is a disappointment. I've done three trials and had to refuel it during the burn each time. It tends to start burning only on one side. I suspect the narrow gap is to blame and possibly the lack of insulation. It did cook to completion but the yielded a low volume of ashy charcoal, indicating overcooking or afterburning of the completed charcoal. In one case, the inner vessel was still hot after sitting overnight, and had live coals inside. The heavy inner tank is retaining heat but this appears to be a mixed blessing, and it is slower to heat up at the beginning. Also I may have cut too many or too large notches in the edge of the inner tank to let the gases escape, this may be allowing air to get into the inner tank and feed combustion of the charcoal.

I realize now that I made three changes which tend to slow the thing down: there is less area at the bottom for combustion air to enter, the inner tank is much heavier, and more heat can radiate out the sides with no insulation. Also there is less room for fuel outside. Probably the next thing to do is insulate the outer barrel again.

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