Someone has defined creativity as combining two existing things in a new way. As I was thinking about this definition I thought about charcoal kilns and retorts. Kilns are direct heating and retorts are indirect heating. So I combined them.
I bolted the bottom of a 30gallon drum to the inside bottom of a 55 gallon drum. Then I cut several 6 inch criss-crossed slices through this double bottom and folded in the “pie slices” to accept a 6 inch flue pipe. I then drilled six 2 inch holes through the double bottom halfway between the flue and the perimeter of the 30 gallon drum. I insulated between the barrels and insulated the 30 gallon drum lid. The 30 gallon lid has a hole to allow the flue pipe to pass through snugly. The flue pipe is short enough so that the solid 55 gallon drum lid can be sealed. This hybrid kiln/retort works great!.
The 2 inch holes allow me to use a fan underneath to dry the feed stock and more importantly these holes allow a fire underneath to quickly start pyrolysis. The trick is to leave the 30 gallon lid off until the moisture (steam cloud) is gone and the wood gas starts burning. When the gas coming up out of the feedstock is ignited by the flue fire, I quickly put the 30 gallon donut-shaped lid on. I wear welding gloves for this critical operation. I then scoop ashes on top to seal around the flue pipe and lid.
If the wood is dry, this kiln becomes a self-sustaining retort in 20 minutes and roars with a 4 1/2 foot flame for an hour before starting to die down.
The 2 inch holes start out as passages for upward flowing heat from the starter fire to initiate pyrolysis and then become downdraft gas jets when the 30 gallon lid is put on top.
I use an 8 X 18 inch piece of insulated stove pipe to weight the retort lid. Beside the initial steam cloud from the wood moisture, there is very little smoke from this clean-burning setup. When the flames stop (about 2 hours total from first match) the 55 gallon lid is put on top and dirt is pushed around the base for cooling.