Coke oven gas, also called coal gas, is the gas obtained when coal is heated in an airtight place. It consists mainly of hydrogen and methane. Coke oven gas is burned to produce heat in industrial plants and in homes. It was formerly used for illumination. Coke oven gas is made by heating coal in a by-product coke oven that heats to about 2000 °F (1100 °C). As the coal heats, coke oven gas and other by-products are given off. The gas leaves the oven through pipes and is stored in large tanks called gas holders. See Gas (How gas is manufactured) .