You're best bet is to build a kiln using corrugated iron to both reflect the heat back in & restrict the oxygen. You still need to quench at the end but it produces a lot more biochar. Simply cut lengths of corrugated iron to 1.2m (4ft) & bolt/screw them together to form a cube that's open at both ends. Star pickets wired to the top hold the general shape & support it when you throw long branches in. Feed the fire when the wood burning starts to turn to charcoal. Only the top layer of wood will be combusting because of the restricted oxygen.

Nice biochar kiln. I may make one at some point, but the pit method works pretty well too. I'm not too worried about getting less biochar, wood is abundant here. This was a quick and easy approach. I still have a large amount of char. If I wasn't burning wood that was 7-10 feet in length, I would have tried a dryer drum set on the ground to plug the holes. It works as a great burn barrel when off the ground. I like to build things out of pallets and bedframes, so when I have a pile of scraps this burns it up fast.

I have a lot of green & brown waste too. The simple kiln is incredible at burning through it all. Very minimal smoke even when burning green leaves. The size allows you to add whole branches and logs without spending hours chopping into smaller peices. It's a brilliant design (also not mine).

I imagine with a good bed of coals in that thing, it would burn almost anything. Do you have an infrared thermometer? I would be curious how hot you could get it.

No thermometer but it's very hot work - it's like being in a sauna working around it. You can feel the heat radiating from the iron quite a few metres away. When you put a branch with leaves in it, they seem to spontaneously combust in a ball of hot air. It wasn't until I did my own burn that I fully grasped how clever the design was.