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.