The blast furnace in 1709

Up to 1709, furnaces could only use charcoal to produce iron. However, wood (which is what charcoal is made from) was becoming more expensive, as forests were being cleared for farmland and timber.
Coal was a possible alternative to wood, but although it was cheap and plentiful, it wasn't a feasible fuel for making iron, because it contained sulphur, and this made the iron too brittle to be of any use.
However, in 1709, a man called Abraham Darby finally succeeded in smelting iron using coke  as fuel.
In this animation you can realize the process of obtaining steel.