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Now, i'm not an expert, but I believe if you have a fuzzy export bus on a container, place the crafting pattern in the bus, and set it to craft mode, it will constantly keep crafting that recipe until it runs out of materials.
Stick a precision export bus on a chest and point a level emitter on the bus. Then set the bus to always craft and redstone mode to high. Place a coal essence in the level emitter and set it so that it gives a redstone signal as soon as you have enough essence to craft coal, afaik 8 essence. Then you can hook an import or storage bus to your chest. Done.
But imo the better way is to use BC autocrafting tables. Just set the recipe in it and attach a normal export and import bus to it. Uses less energie and your MAC is free for other crafting jobs, that might only be important if you have more different items or a very fast essence farm.