I tried natural barbecue "hardwood characoal"--not briquettes--that I bought at a hardware store. It worked great. It's so light (fluffy) I had to make the pile a little deep. It boils down to dollars per btu. Someday I'll have to burn $50 of characoal then $50 of coal and see which heats more metal.
Neighbors complain when you make charcoal in a retort in town. The fumes STINK even if you burn them. And there's a lot of smoke. It's funny, the finished product, characoal, burns so cleanly in a forge--after the initial smoke is gone.