I've been smelting aluminium, making ingots for future experiments in sand-casting. I found that the cost of barbecue charcoal problematic and my yard is full of twigs so I decided to make my own smelting fuel.
The direct method involves making a fire and smothering it to the point where it burns off the volatiles and contaminates. This method was very popular but I had problems with quality and if you screw up it can explode. As I like my hair,eyebrows, fingers and dangly bits I looked into the indirect method also known as the "retort" method.
This method involves placing the material to be made into charcoal into a airflow restricted container and applying heat bringing the material above combustion temperature without allowing any combustion.
I did this with various food cans. Notably soup. I made the retorts by placing a progresso soup can over a cambels can as an insert. Fill the cambels can with twigs cover with the progresso can and you have a retort with restricted airflow that also allows for pressure release of the pyrolytic gas around the bottom which then burns and adds energy to the process.
For a furnace I used a bunch of old bricks, a small 12 volt muffin fan, a grill from a dead toaster oven and a random piece of metal cut from the back of an old computer case as a spark catcher. (the part with all the holes in it.)
The assembled furnace.
The muffin fan as an air mover.
Next morning. I save the ash for making wood lye.
cost of materials: 0
labour hours : about 1
time for process : approx 3 hours.
Yield : Variable based on density of feed stock. harder/denser woods= better yield.