Coal is needed to produce molten pig iron in the blast furnace, but it cannot be used directly in the blast furnace. It contains too many harmful or unusable byproducts for the blast furnace process, and it is not strong enough to support the load in the blast furnace. Therefore, the coal is first converted into coke.

A coal mixture is heated in coke ovens to approximately 1,250°C. Since there is no oxygen in these sealed ovens, the coal does not burn. This process is called "dry distillation." It takes about 18 hours to convert 35 tons of coal into 25 tons of coke. During the distillation process, a large amount of gas and smoke is released, which, after purification, yields coke oven gas and other valuable byproducts, such as tar, sulfur, ammonia, naphthalene, and benzol. The coke produced is used as fuel in the blast furnace and plays a role in the chemical processes that take place there. The coke oven gas is used entirely internally as fuel.