Unlike thermal or steaming coal used in power stations or for industrial or domestic heating, coking (or metallurgical) coal has unique properties that suit it to iron making.
A key characteristic is that when heated in the absence of air, this coal, unlike steaming coal, will form the hard, sponge-like material known as coke. As steelmakers are looking to produce high iron purity, the coal needs to have high carbon, but relatively low ash, sulphur and phosphorus content.
The terms “hard”, “semi-hard” and “semi-soft” actually refer to the coal’s readiness to form coke (“hard” is best), and not its physical hardness. However, “hard” coal does happen to produce physically hard coke. About 10% of world mined coal is coking coal, with the major exporters being Australia, Canada and the USA, followed by China and Russia. And for more detail click and see wikipedia entry.