Coke is generally produced after the volatile constituents of the coal are driven off by baking bituminous coal in the airless furnaces at around 2000°C. These volatile constituents are water, coal gas and coal tar. This is artificial Coke. Natural Coke is formed when the coal seam is intersected by a volcanic intrusion. These intrusions heat the surrounding coal in an anoxic atmosphere producing coke in a zone (usually several meters) around the intrusion.
The production of metallurgical coke is an important use of coal. As mentioned above, Coke is nearly pure carbon that is produced after heating bituminous coal in the absence of oxygen. This process drives off water and other volatile compounds, making the resulting coke virtually smokeless. One ton of coal (2000 pounds) produces about 1400 pounds of coke. Coke has a higher heat value than coal itself, and can therefore be used to generate the extremely high temperatures (up to 4000°F) required for metal refining (particularly steelmaking). In addition to providing the heat, coke acts as a chemical reducing agent to produce pure iron from molten iron ore in a blast furnace.