Coke used as blast furnace fuel is mainly produced from bituminous coal worldwide. There is coke from petroleum (usually called petcoke) where one of the main usages is material for anodes in smelters (so speaking in-game, petcoke should be used for what graphite is now). Some petcoke is also used in blast furnaces, but far less common than coal coke.
Proper graphics for the coking unit will come whenever I manage to produce something (very much not a 3D artists) or someone contributes.
The ratio of coking units to blast furnace II is exactly 1:1, and that is fully intentional. The way I'm trying to balance the various possible pathways to make steel includes workers, space, electricity, maintenance etc of the various supply buildings. I could of course double the output, but to keep balance, size, workers etc would also have to be doubled.