Metallurgical coal, also known as coking coal, is used to produce coke, the primary source of carbon used in steelmaking. Metallurgical coal differs from thermal coal, which is used for energy and heating, by its carbon content and its coking ability. Coking refers to the coal's ability to be converted into coke, a pure form of carbon that can be used in basic oxygen furnaces. Bituminous coal—generally classified as metallurgical grade—is harder and blacker and contains more carbon and less moisture and ash than low-rank coals.
The grade of coal and its coking ability are determined by the coal's rank—a measure of volatile matter and degree of metamorphism—as well as mineral impurities and the ability of the coal to melt, swell and resolidify when heated.
Coke making is effectively the carbonization of coal at high temperatures. Production normally takes place in a coke battery located near an integrated steel mill. In the battery, coke ovens are stacked in rows. Coal is loaded into the ovens and then heated in the absence of oxygen up to temperatures around 1,100 degrees Celsius (2,000 degrees Fahrenheit).
Without oxygen, the coal does not burn; it begins to melt. The high temperatures volatize unwanted impurities, such as hydrogen, oxygen, nitrogen, and sulfur. These off gasses can either be collected and recovered as by-products or burned off as a source of heat.
After cooling, the coke solidifies as lumps of porous, crystalline carbon large enough to be used by blast furnaces.
Properties inherent in the initial input coal heavily influence the ultimate quality of the coke produced. A lack of a reliable supply of individual coal grades means that coke-makers today often use blends of up to 20 different coals in order to offer steelmakers a consistent product.
Approximately 1.5 metric tons of metallurgical coal are required to produce 1 metric ton (1,000 kilograms) of coke.
A blast furnace is fed with coke, iron ore and fluxes, and hot air is blown into the mixture. Air causes the coke to burn, raising temperatures to 1,700 degrees Celsius, which oxidizes impurities. The process reduces the carbon content by 90 percent and results in a molten iron known as hot metal.
The hot metal is then drained from the blast furnace and sent to the basic oxygen furnace (“BOF”) where scrap steel and limestone are added to make new steel. Other elements, such as molybdenum, chromium, or vanadium can be added to produce different grades of steel.
On average, about 630 kilograms of coke are required to produce 1 metric ton of steel.
Production efficiency in the blast furnace process is highly dependent upon the quality of raw materials used. A blast furnace fed with high-quality coke will require less coke and flux, lowering production costs and resulting in a better hot metal.