When you are running a coal fire once you get it started you keep it surrounded by wet, green coal. This coal heats and gives off the volatiles, expands and becomes very light compared to the original coal, this is breeze (commonly called coke). You are actually buring the breeze in your forge fire which is effectively a charcoal fire. You constantly make more breeze by slowly pushing the green coal in to the fire so more is being converted to breeze as you work. This stops the panic when you run out of breeze and are forced to pile on green coal which produces great clouds of heavy, smelly smoke. Good fire control ensures a virtually smoke free fire with a constantly refreshing supply of breeze.
Breeze is very easy to light and gives off very little smoke so you always keep the breeze from your last fire to get the next one going. Also, a coal fire will keep burning while the air is turned off, a great characteristic if you are using a small bellows or hand cranker.
Now, real coke is made from coal that is baked to force off the volatiles so these gases can be used in industry. The left over is a hard sort of glassy rock which is hard to light and needs a constant air supply (an electric blower is essential or a blower slave). After saying the bad bits it is virtually smoke free as the voaltiles are not there to form the smoke and you don't have to worry about making more, its already there.
Pick your poison :)