Coke is a hard, grayish substance obtained when soft coal is heated in an airtight coke oven. It is hard and porous (full of tiny holes), and, in most cases, contains 87 to 89 percent carbon. Coke produces intense, smokeless heat when it burns.
Coke is made by heating pulverized coal (coal ground to a powder) in an airtight oven. As the coal heats, it decomposes (decays). It cannot burn completely without air. Coal tar and coke oven gas evaporate from the decomposed coal and are drawn out of the oven. The escaping tar and gas form the pores in the coke. The hot coke is taken out of the oven and cooled with water at a quenching tower to keep it from burning in the air.
Coke is valuable in smelting (melting) iron ore. Coke used in smelting is called metallurgical coke. In the United States, over 95 percent of such coke is made in giant coking plants with by-product ovens equipped to save coal tar and coke oven gas. These ovens hold from 4 to 20 tons (3.6 to 18 metric tons) or more of coal. If not properly sealed, by-product ovens can leak coal dust and harmful gases. Environmental concerns have led the coke industry to reduce leaks and to improve the operation and design of ovens.
