LX: in general lump charcoal isn't as dense as coal, hence you need 5-10X charcaol compared to regular coal. While you can use anthracite in a forges, its more of a PITA. I was using anthracite until i got my hands on bituminous. Alot of heat, easier to control and doesn't need nearly as much air to keep it going.
The bituminous is small 3/4 to 1/2 " in size. It will leave larger lumps when done and then you can reuse the leftovers. I am still new to all this but there is a lot of info out there. Google rotor or brake drum forges and have some fun.
What would you say the temperature of the forge is, ball park? I have no clue on forging metals but have been looking for creating something to melt metals and do some very small scale casting. The idea of not having to create some crazy propane set up is appealing.