May I suggest adding ‘temperature control’ as a major criteria for determining “the better” furnace? Temperature (of the furnace) depends on “air flow”, fuel (wood or …), raw material (to become charcoal), size and shape of the furnace (how well it can distribute heat) and ability of the furnace’s wall (material) to retain heat. Once the temperature can be controlled, quality of charcoal can be better determined for particular raw materials. Designs of furnace should be based on Physics and workflow/processflow –not Culture.
Research on “jet stove” (for cooking) offers useful information on “efficient heat source” ;-)