I finally got to fire up the tube kiln - it is 4ft diameter and 4 ft tall. I made a heat shield out of some thin, finely corrugated steel from an above ground swimming pool - got it at a yard sale for $5!
Here is a slide show video:
The heat shield makes a big difference. It's all about holding as much heat in the flame zone as you can. You can see from the pictures that there is not much smoke emissions. The cap of flame on top burns the smoke for clean operation.
We actually tried 3 different methods at once - Tube Kiln, Pyramid Kiln and TLUD - a Three Ring Circus of Pyrolysis!