Have you seen this video?
After several lousy attempts to make biochar, I decided to do just like this video, and it really works well. It takes anywhere from 4-6 hours to do a complete burn where all the material in the barrel is completely charred, but that is not a problem as it doesn't need to be constantly tended. The only difference I have is that my chimney is not a 30 gal. barrel, but is just two no.10 food cans fitted together.
And don't believe these people that say there should be no smoke. When water vapor released during combustion hits the ambient air outside, it condenses and the steam cloud continues until you run out of cellulose to burn. When it's all charcoal in the barrel, and that begins to burn, that's when the exhaust is pure CO2 and you will have no smoke.