blast-furnace gas[′blast ‚fər·nəs ‚gas]
(top gas), the waste gas from blast furnaces; mainly the products of the incomplete combustion of carbon. Its chemical composition during the smelting of cast iron on coal coke is 12–20 percent carbon dioxide, 20–30 percent carbon monoxide, up to 0.5 percent methane, 1–4 percent hydrogen, and 55–58 percent nitrogen. Blast-furnace gas is utilized as a fuel in metallurgical works. The heat of combustion of blast-furnace gas is approximately 3.6–4.6 megajoules per cu m (850–1,100 kcal per cu m). When the blast is enriched with oxygen, the nitrogen content of the gas is lowered and the amount of other gases (including carbon monoxide and hydrogen), as well as the heat of combustion, increases correspondingly.