Coke, iron ore and limestone are the principal inputs for blast furnace (BF) ironmaking. Coke provides thermal energy, combines with the oxygen in the ore to release iron, and ensures a permeable physical structure within the furnace to allow hot gases to move upwards, heating the incoming materials, and molten iron and slag to move downwards for tapping.
The quality of coke has a significant influence on furnace productivity and iron production costs.
Coke is produced by heating coal to about 1,100oC in a reducing (oxygen deficient) atmosphere. This is done in coke ovens, and volatile compounds like tars are released along with hydrogen and methane to leave a carbon-rich product.
Desirable qualities for coke are high carbon content; low sulphur, phosphorus and moisture content; low ash residue; and physical strength.
Coke consumption per tonne of liquid iron produced has fallen significantly over time and currently is around 400kg. Consumption, and costs, can be further reduced by the injection of pulverised (non-coking) coal. And for more detail click and see wikipedia entry.