Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents (including water, coal-gas and coal-tar) are driven off by baking in an airless oven at temperatures as high as 1,000 degrees Celsius so that the fixed carbon and residual ash are fused together. The carbon content of coke is partially converted to graphite.
Coke typically has a specific gravity in the range 1.85 - 1.9. It is highly porous, and a mass of coke has 40% greater volume than the equivalent mass of coal.
Since the smoke-producing constituents are driven off during the coking of the coal, coke forms a desirable fuel for stoves and furnaces in which conditions are not suitable for the complete burning of bituminous coal itself. Coke may be burned with little or no smoke under combustion conditions which would result in a large amount of smoke were bituminous coal the fuel. Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Coke from coal is grey, hard, and porous and has a heating value of 28 million joules/kilogram.
The solid residue remaining from the refinement of petroleum by the "cracking" process is also a form of coke. Petroleum coke has many uses besides being a fuel, such as the manufacture of dry cells, electrodes, etc. Gas works that manufacture syngas also produce coke as an end product, called gas house coke.