Coal Tar
. Coal Tar, viscous black liquid produced in the destructive distillation of coal to make coke and gas. Coal tar is a complex mixture of organic compounds, mostly hydrocarbons . Its composition varies with the coal, the temperature at which it is formed, and the equipment used. When formed at temperatures below 600° C (1112° F), coal tar consists of hydrocarbons of the paraffin and olefin types, together with alcohols , phenols , and water. Coke is usually produced at about 1000° C (about 1832° F), and coal tar formed in that temperature range consists mostly of aromatic hydrocarbons plus phenols and some compounds containing nitrogen, sulfur, and oxygen. The variation in composition means that most of the compounds in coal tar are formed during the coking process and are not present as such in the original coal. Some 300 distinct compounds have been identified in coal tar, of which about 50 are separated and used commercially. Some of the constituents are carcinogens, that is, they i