Coal carbonization is the process for producing metallurgical coke for use in iron-making blast furnaces and other metal smelting processes. Carbonization of coal (qv) entails heating coal to temperatures as high as 1100 • C in the absence of oxygen in order to distill out tars and light oils (see Tar and pitch). A gaseous by-product referred to as coke oven gas (COG) along with ammonia, water, and sulfur compounds are also thermally removed from the coal. The coke that remains after this distillation largely consists of carbon (qv), in various crystallographic forms, but also contains the thermally modified remains of various minerals that were in the original coal. These mineral remains, commonly referred to as coke ash, do not combust and are left as a residue after the coke is burned. Coke also contains a portion of the sulfur from the coal. Coke is principally used as a fuel, reductant, and support for other raw materials in ironmaking blast furnaces (see Furnaces, fuel-fired; Iron). A much smaller tonnage of coke is similarly used in cupola furnaces in the foundry industry. The carbonization by-products are usually refined, within the coke plant, into commodity chemicals such as elemental sulfur (qv), ammonium sulfate, benzene, toluene, xylene, and naphthalene (qv) (see also Ammonium compounds; BTX processing). Subsequent processing of these chemicals produces a host of other chemicals and materials. The COG is a valuable heating fuel used mainly within steel (qv) plants for such purposes as firing blast furnace stoves, soaking furnaces for semifinished steel, annealing furnaces, and lime kilns as well as heating the coke ovens themselves.Coke making dates to seventeenth century England where it was discovered that interrupting the burning of coal heaps produced solid blocks of carbon from the botton of the heap (1). This carbon quickly supplanted wood charcoal as the main blast furnace fuel. The first commercially successful coal carbonization plant was developed in 1709 (2). Subsequent generations of cokemaking facilities proceeded to ever more effectively exclude air (oxygen) from contact with the carbonizing coal. These facilities evolved from the initial coal heaps first to pits, then to masonry-walled nonroofed ovens. Dome-shaped mud-walled ovens, and then domed refractory brick ovens, commonly called beehive ovens, appeared by 1840 (1). At about this same time, rectangular-shaped ovens having arched roofs and removable doors on one or both ends of the oven appeared. This latter type allowed for pushing the coke out of the oven so that the coke could be quenched with water. Earlier ovens were designed for quenching the coke within the oven, necessitating subsequent reheating of the oven as well as repair of damage caused by thermal shock to the oven structure.Within 15 years, the enclosing of the coal during carbonization allowed for the first attempts at profitably recovering the off-gases from the coking process. Commercial success in this endeavor is generally credited to Germany's by-product coke ...
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