I have a friend back home that makes his own charcoal for use in his foundry.
There are a few of us in the area who have one man saw mill operations and he would utilize our slabs of scrap. branches and anything else he could get his hands on.
His set up was fairly simple
A 55gal drum with a removable lid. The lid had the 2 bung holes in it. I believe they are a 2.5 and a 1 in plug.
With the lid attached, the 1 in bung on the up side of the drum, a 1n pipe. it would come out of the bung, down and then under the drum. The section under the drum traveled the full length of the drum. This section of pipe had small holes drilled in it aprox every 3 to 4 in ant a off center angle. The drub would be filled with the chunks of green wood, then closed up and set on a brick work. He then built a enclosure around the barrel also with bricks. leaving space in the front to tend the fire that was to be built under the barrel. As the barrel was heated up, the gas would be cooked off and escape out the 1 in pipe and would then be ignited by the fire. It sounded like a jet on afterburner. when the fire coming out of the pipe subsided, the external fire was extinguished and the barrel was left to cool off. Opening the next day revealed about a 30 % loss of mass but none the less a barrel full of usable charcoal.
When I am home next, I will try to get some photos