Since the vapor-producing constituents are driven off during coke production, coke is an ideal fuel for stoves and furnaces in which the environment is unsuitable for the complete burning of bituminous coal itself. In the form of oven coke it is primarily used when a porous fuel with few impurities and high carbon content is desired, as in the blast furnace to make iron. Coke is also used in other metallurgical processes, such as the manufacture of ferro-alloys, lead, and zinc, and in kilns to make lime and magnesium. Exceptionally large strong coke is known as foundry coke and is used in foundry cupolas to smelt iron ores. The smallest sizes of coke are used to heat buildings.
The majority of coke produced in the United States comes from byproduct coke ovens. The coke is prepared in retorts or furnaces of silica brick, and the byproducts (chiefly ammonia, coal tar, and gaseous compounds) are saved. These volatile gases are collected and sent to the byproduct plant where various byproducts are recovered. In nonrecovery coke plants, originally referred to as beehive ovens, the coal is carbonized in large oven chambers; the partially combusted gases collect in a common tunnel and exit via a stack. In recovery coke plants the waste gas exits into a waste heat recovery boiler which converts the excess heat into steam for power generation.
Petroleum coke is the solid residue left by the cracking process of oil refining. Natural coke, or carbonite, is formed by metamorphism from bituminous coal when intrusive igneous rock cuts across a vein of coal.
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