Coke

Coke, hard, porous residue left after the destructive distillation of coal. Used as a reducing agent in the smelting of pig iron and as a fuel, coke is blackish-grey and has a metallic lustre. It is composed largely of carbon, usually about 92 per cent; most of the remainder is ash. When used as a fuel, it has a high heating value.

Coke was first produced as a by-product in the manufacture of illuminating gas. The growth of the steel industry, however, produced a rising demand for metallurgical coke, making it inevitable that coke should be manufactured as a chief product.

The earliest method of coking coal was simply to pile it in large heaps out of doors, leaving a number of horizontal and vertical flues through the piles. These flues were filled with wood, which was lighted and which, in turn, ignited the coal. When most of the volatile elements in the coal had been driven off, the flames would die down; the fire would then be partly smothered with coal dust, and the heap sprinkled with water.

A later development was the coking of coal in the beehive oven, so named because of its shape. As in open-air coking, no attempt was made to recover the valuable gas and tar that were by-products of the process. Beehive ovens have now been almost entirely supplanted by the modern by-product coke ovens. These ovens, usually arranged in batteries of about 60, are narrow vertical chambers with silica-brick walls, heated by burning gas flowing between adjoining ovens. Each oven is charged through an opening in the top with anywhere from 10 to about 20 tonnes of coal, which is heated to temperatures as high as 1,480° C (2,000° F) for about 17 hours. During this period the gases from the oven are collected through another opening in the top. The coal tar is condensed by contact with water in a hydraulic main, while the gas, after being scrubbed with water to remove ammonia and with oil to remove benzene, is used to heat the ovens. At the end of the coking period the red-hot coke is forced by a ram out of the oven directly into a car that carries it to the quenching hood, where it is sprinkled with water. The emptying process takes only about 3 minutes, so that the oven is ready for recharging with little loss of heat.