In a blast furnace a mixture of iron oxide (Fe2O3), coke (carbon derived from coal), and limestone are heated to produce elemental iron (Fe) plus residual slag.
External heat is used to preheat the air introduced into the bottom of the blast furnace but the main source of heat in the furnace is the burning of the coke with oxygen.
C + O2 --> CO2 + heat
In addition, the CO2 reacts with some of the coke to produce carbon monoxide (CO). The carbon monoxide then reacts with the iron ore to reduce it to iron.
C + CO2 --> 2 CO
Fe2O3 + 3CO --> 2 Fe + 3 CO2