I have just bought an old forge - its made of Brick, set in a steel case and has a good strong electric blower with a controlable fan speed.

It seems to be full of old (spent) coke

I have plenty of coal around as I have steam engines, will this do for fuel

I intent to work outside with this forge

sure,it will work great,some coal is better than others but give er a go, have fun,jimmy

What you want is a coal with low ash, low clinker, high BTU, low sulfur and low volatiles. For best results it should be sized between peas and walnuts. Many (but not all) coals and cokes available in the UK will work adequately but if you get much into it you'll probably want to buy a few sacks of smithing coke. This is what almost all professionals (who burn solid fuel) use in the UK, as it's somewhat consistent and the Clean Air Act gets in the way of using a good bituminous smithing coal, which some would prefer.

Try a few scoops of whatever you have on hand, keep notes and perhaps visit a more experienced smith to compare his/her fuel.

Steam engine coal should be ideal, some of the best forging coal I have used came from steam engined ships, once you have used it you can decide the best way to work it, you may need to dampen it, see how it cokes, does it make a nice covered fire and so on I am sure you should get on well with it.