COAL GOES IN, COKE COMES OUT
In case you've never heard of coke ovens, this is what they do. They turn coal into "coke", not the kind you drink.
Coal is placed in the "coke" ovens and burned at a very high but controlled temperature. At the end of the burning cycle, water is poured over what is now coke. This, of course, makes the coke cool and crack. The coke is removed from the ovens, and loaded onto rail cars. The oven is reloaded with a new batch of coal which self ignites because of the heat retained within the walls of the ovens.
Coke is mostly pure carbon and burns at a much higher temperature than coal.
The steel industry was the major market for the coke.