I love my Japanese Cone Kiln. I had a welder make it for me 2 years ago, but I did not understand how to use it. I finally figured it out about a month ago and it is so easy! 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.
It takes about 2 hours of very easy work. You can do other chores (like splitting wood) while you watch the fire. And the yield is very impressive. I start with two 6 cu ft wheelbarrows full of 2" kindling wood. I end up with about 5 cu ft of nice, crumbly biochar (water quenching really helps make your biochar hydrophilic).
Compare this with my JRO (Jolly Roger Oven) TLUD and my retort kiln, both using 55 gallon drums:
My JRO TLUD takes about 45 minutes when loaded with big wood chips. The yield is about 2 cu ft. Finding, screening and drying wood chips is a pain. And the tall stack of afterburner and chimney is hard to handle for me (at 5' 2" I need a step ladder).
My retort kiln takes about 2 hours to complete. The yield is about 1.5 cu ft and I usually use about one full wheelbarrow load of wood to fire it, sometimes more. The feedstock prep is similar to the cone kiln, although I can use any size wood to fire it.
See why I love my Cone Kiln? Here is a video clip and some pictures:
Here is a report on the Japanese "Cool Veggie" program that uses these cone kilns to make biochar for small farms in Japan: A Rural Revitalization Scheme in Japan Utilizing Biochar and Eco-Branding: The Carbon Minus Project, Kameoka City.