Kelpie Wilson has been playing with the Japanese Cone Kiln for Biochar see the latest on her site: http://www.greenyourhead.com/

She's finding that the Cone Kiln is easier to use than the alternatives and it produces more char too.

in her words:

I love my Japanese Cone Kiln. ... It is basically just a cone-shaped fire ring - a truncated cone. All you do is start a small fire in the bottom, and once that is all burned to glowing coals, you add small stick wood or branches on in layers. Each time the wood gets black and starts to ash, you add another layer. The layers underneath continue to cook out tar and gas, but they don't burn because air is excluded. When the cone is full you quench it with water. If you like, you can throw a grill on it and cook your dinner before you put it out.