I have just finished putting all my plans and photos online of my latest Biochar making system. I have called the unit FatBoy because it sort of looks like one
The system was built for about $20 or so and with some mild engineering know how, you should be able to knock one up over a weekend.
I have put some focus into areas of common gasifier design which can be a real pain to deal with. it's dead easy to light and get going without using toxic flammable fuel, it does not produce smoke after less than 30 seconds of lighting. It's a fast process making a batch in about 15 minutes. It has a simple indicator for when the unit is ready to quench. The size of the flue and a secondary air inlet have been designed to greatly reduce emissions. It's still a work in progress and I'm starting to design a larger 150 litre unit.
If you have any questions or ideas please feel free to ask