Blacksmithing: Differences Between Coal, Charcoal and Coke As Fuel
The type of fuel you use to forge steel depends upon the wind, what type of setup you have, and the amount of air you can get into and control your forge. The gas forge, which is probably the one most professionals will use, has a good many advantages over coal/coke or charcoal fires, but seriously, with practice you can get good results and adequate heats just using lump charcoal, which is not the briquettes, but the natural charcoal that still looks like wood, the briquettes burn away and leave a mess of residue and binder. Charcoal burns cleaner, hotter and more consistently than coal, but the primary disadvantages are the sparks, which can hurt and burn, and the fact that it requires a lot of charcoal to get a good heat.
A bit about forging fire, if coke is used, coke is the coal equivalent of charcoal, meaning that once coal has burned down in a lower oxygen environment then coke is the result, which is mostly carbon, and like charcoal, has a lot of surface area which retains the oxygen to produce greater heat, that is why charcoal, when superheated steam is passed through it, creates "activated" charcoal, which is an exceptional filter - it contains thousands of miles of surface area.
Coke also has so much internal surface area that the heat is increased as more air is fed into the surfaces, and trapped in the bowl area of the forge. It has to pass through the coke, and as it does so, the heat increases. Do realize that if enough air is added, then even a small forge can melt or combust steel, it is difficult, but not impossible.
What I have found, especially for small projects, and those made in a wheel-drum forge or with smaller projects or forges, is that if you mix a combination of coal, charcoal and coke, you get the high heat, the steadiness of burning and the durability of fire to be able to work for a while on a project. The best way to do this is in layers, charcoal on the bottom, then coke, then coal on top, as needed, to feed the fire. As the coal burns down into coke, the expent material tends to either fall into then dust grate and so the coal replaces the expent coke.
The advantage of having a bed of charcoal is that if the fire gets too low, you can renew the fire by pumping air into the thing and adding more charcoal, relighting the already warm coke. A coke fire creates less flame, but tends to go out because most of the fuel is gone, with charcoal the fire and sparks are more apparent, but it is an quiet as it gets when working with traditional forges. the biggest advantage you have with this fuel mix is you can stop forcing air into it when filing or doing other work.