I have a 55 gallon drum that I converted into a kiln; it's basically just a 55 gallon drum elevated with air intake holes in the bottom. Basically you get the wood burning really well then put the lid on with a small gap; you then build up sand around the base and let it smolder. I have been doing this for a while and find this design doesn't produce a substantial amount of charcoal, below half after a full barrel of wood. I believe that some of the reason could be that the heat is constantly leaving the kiln therefore it needs more fuel to draw off the water in the wood. If there was a way to keep in the heat so it takes less time to draw off the water then it would take less fuel to transform into energy; therefore leaving you with more charcoal. Has anybody ever heard of putting a layer of fiberglass insulation around the kiln along with another layer of sheet metal? Would this concept increase efficiency? Is there a possibility of the insulation catching fire?

Making Charcoal this method works really well, and is smokeless, along with being energy efficient (using the energy in the smoke being cooked off to cook the charcoal) :D

Blueprint BP0338 Making your own Charcoal

how to make your own charcoal

If you're getting less charcoal than you think you should, it is probably getting too much air. It isn't so much about driving mosture off as the other volatile componets of the wood if it is burning most of the water is gone and other stuff is burning. Depending on the wood you start with expect anything between 50-75% charcoal from a kiln.
As for the fiberglass insulation not the best idea, the heat would break the insulation down pretty fast.

ron