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coke, substance obtained by the destructive distillation of bituminous coal. Coke bears the same relation to coal as does charcoal to wood. A hard, gray, massive, porous fuel, coke is the solid residue remaining after bituminous coal is heated to a high temperature out of contact with air until substantially all components that easily vaporize have been driven off. The residue is chiefly carbon, with minor amounts of hydrogen, nitrogen, sulfur, and oxygen. Also present in coke is the mineral matter in the original coal, chemically altered and decomposed.
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.
a man-made solid fuel of high strength; it is produced by heating natural fuels or products of their processing to high temperatures (950°–1050°C) in the absence of air. A distinction is made among coal, pitch, and petroleum coke, depending on the type of the raw material. Most coke is produced from coal.
Coal coke is used mainly in the blast furnace process for smelting iron (metallurgical coke). Here the coke is used simultaneously as a fuel and as an agent for reducing the iron ore. Coke is used in significantly smaller quantities in the foundry process (foundry coke) for the sintering of ores and in the chemical industry and nonferrous metallurgy.
The production of coal coke began in the 18th century, when it became necessary to replace charcoal, which was becoming increasingly scarce, for use in blast furnaces. The first industrial melting with coke was carried out in Great Britain in 1735. By 1970, world coke production was more than 300 million tons per year. In the USSR, which holds first place in coke production, 79.75 million tons were produced in 1972.
Coal coke has the form of elongated gray clumps. True relative density, 1.80–1.95 g/cm3; apparent density, taking the pores into account, 0.8–1.0 g/cm3; average porosity, about 50 percent; bulk weight, 400–500 kg/m3; heat of combustion, about 29 megajoules per kg (MJ/kg, or 7,000 kcal/kg); heat of combustion of combustible mass, about 33 MJ/kg (about 8,000 kcal/kg).
The carbon content in the combustible mass of coke is about 96 percent, and the volatiles yield is 0.8–1.0 percent. The moisture content in coke with dry quenching does not exceed 0.5 percent; with wet quenching, usually 2–4 percent. The sulfur content in metallurgical coke from Donets coals is 1.5–1.9 percent, and from Kuznetsk coals it is 0.4–0.5 percent; for foundry coke, it should not exceed 1.2 percent. The phosphorus content in coke in smelting Bessemer pig iron should not exceed 0.015 percent. The ash content of metallurgical coke should not exceed 9–10.5 percent. As the quantity of these components increases, the quality of the metal deteriorates, the consumption of coke and the charge increases, and the output of the blast furnace is sharply reduced.
Pitch coke and petroleum coke, in comparison with coal coke, have a very low ash content—usually not more than 0.3 percent (up to 0.8 percent for petroleum coke). Pitch coke is produced by coking; high-melting coal-tar pitch in compartment-type Dinas brick ovens. Petroleum coke is produced by coking liquid petroleum residues and pitches in metal coke vats or special furnaces. Petroleum coke is also formed during cracking and pyrolysis of products of petroleum distillation. Pitch coke and petroleum coke are the main raw material for producing electrodes.
REFERENCESSpravochnik koksokhimika, vol. 2. Moscow, 1965.
Goftman, M. V. Prikladnaia khimiia tverdogo topliva. Moscow, 1963.
D. D. ZYKOV