Vuthisa Technologies in South Africa have been working on improving the Portable Metal Kiln Charcoal Making Method and using a retort design to reduce emissions and improve efficiency making charcoal.

They have a great description with lots of detail on their web site: http://vuthisa.com/biochar/
as well as a Google Group:
https://groups.google.com/forum/?hl=en-GB&fromgroups#!forum/portable-kiln

In short the system is composted of an outer drum, often fabricated of sheet steel enclosing an inner set of 30 gallon drums.

David Yarrow, Four Oaks Farm

sunday we conducted our 18th test burn with our TLUD biochar burn barrel.
this time brad mostly loaded wood chips in the barrel.
this burn ran over 95 minutes, quietly, smoothly
and produced over 15 gallons of solid, dense biochar,

for more details see: http://www.dyarrow.org/18thBurn/

monday brad did two more wood chip burns.
the first ran 90 minutes.
the second ran over 2 hours.
we like burning wood chips!!

REMINDER:
this weekend is our first biochar workshop at four oaks farm.
saturday at 10am, sunday at 1pm, and monday at 6pm
http://www.dyarrow.org/biochar/

Today Chris fired up his own creation, which took an oil drum (with Texaco logo still intact) and a butane tank with the bottom cut off. Total cost, including welding & machining labor, was US$40. He stuffs the inner chamber (inverted gas tank -- the top handles serve as a stand) with guadua (bambusa vulgaris) -- great for the cellular structure -- and the outer chamber with woody scraps from the farm as he limbs trees, opens trails, etc. It takes about 20 min from the time he fires the barrel to get up to pyrolysis temperature, at which point the smoke coming from the barrels ignite and the burn is pretty clean. He puts food for his pigs -- breadfruit, chocho, bananas -- on top, to use some of the heat (it comes to a boil in a minute or two), but I have to say there is still a lot of waste heat that would be good to find some uses for.

The nice thing about this rig is that it is so simple and easy to operate that it can be used every day by either the farmer or his wife or son. Chris throws the bamboo-char into his pig pen for the pigs to pulverize. They ate it the first time, which was even better, but haven't eaten it again since.