Coal to Make Coke and Steel
Metallurgical coal (also called "met" coal) is an important raw material used in the steel-making process, although very small amounts of coal (relative to the amount used for electricity) are needed. The coal used to make steel is heated without air in an oven at temperatures of as much as 2,060°F (1,125°F), until most of its volatile matter is released. During this process, it softens, then liquefies, and re-solidifies into a hard porous material called "coke". This is not the cola soft drink. Coke is a porous, carbon-rich material used to make steel. The coke is mixed with iron ore and limestone to make molten iron, which is then further treated and heated to make steel.
In the steel-making process, coke is used in the blast furnace as a (1) fuel to produce added heat; (2) chemical-reducing agent for the reduction of iron oxides; and (3) as a permeable support in the molten material in the furnace. By-products from coke ovens are used in the chemical industry (see Coal for chemicals and specialty products).
Not all coals can be used as metallurgical coals to make coke. Met coals must have low sulfur content (<1%), low ash yield (<7%), low volatile matter (15 to 40%) and low phosphorous-, low chlorine-, and low alkali content. Ideally, they are medium-volatile bituminous in rank, although other coal ranks can be used and blended to make a suitable met coal. Because of the relative rarity of met coals, they are more expensive than coals used for heating or electricity.
Both the coals and subsequent cokes are tested for their quality. Another important quality of the coke, which is tested, is its strength. Coals and cokes are also microscopically analyzed (called petrography) to determine the relative proportions of "reactive" components (vitrinite and liptinite macerals) vs. "inert" components (inertinite macerals and mineral matter), which influence coking properties.
The production of steel in the United States has declined in recent years because of the increased use of imported and recycled steel. Not surprisingly, the amount of coal mined in Kentucky to make coke has also declined. In 2014, in the United States, approximately 22,000 tons of coal was sold in the met market, which was less than three percent of the total mined coal that year (EIA, 2016 data).
Other Uses of Coal:
- Coal for Electricity Generation
- Coal for Industry and Residential Markets
- Coal for Specialty Products and Chemicals
- Rare Earth Elements from Coal
References for Coal for Coke and Steel
- Ammosov, I.I., Eremin, I.V., Sukhenko, S.F., Oshurkova, L.S., 1957, Calculation of coking changes on basis of petrographic characteristics of coals: Coke Chem. U.S.S.R., v.2, p. 9– 12.
- Benedict, L.G., Thompson, R.R., Wenger, R.O., 1968, Relationship between coal petrographic composition and coke stability: Blast Furnace and Steel Plant, v.56 (3), p.217–224.
- Dıez, M.A., Alvarez, R., Barriocanal, C., 2002, Coal for metallurgical coke production: predictions of coke quality and future requirements for coke making: International Journal of Coal Geology, v. 50, p. 389– 412.
- Dutcher, L.A.F., Crelling, J.C., 2000, History of applied coal petrology in the United States: I. Early history of the application of coal petrology in the steel industry: International Journal of Coal Geology, v.42, p. 93– 101.
- Gray, R.J., 1978, Selection of coals for coke making, in Meissner, C.R., Jr., Cecil, C.B., and Stricker, G.D. (eds.), Coal Geology and the Future Symposium Abstracts and Selected References: U.S. Geological Survey, Circular 757, p. 15-16.
- Karr, C. (ed.), 2013, Analytical methods for coal and coal products: Academic Press, v.2, 669 p.
- Nakamura, N., Togino, Y., Tateoka, T., 1977, Behaviour of coke in large blast furnace. Coal, Coke and Blast Furnace: London, The Metals Society, p. 1-18.
- Schapiro, N., and Gray, R.J., 1964, The use of coal petrography in coke making: Journal of the Institute of Fuel, v. 43, p. 234–242.
- Thompson, R.R., 2000, History of applied coal petrology in the United States: III. Contributions to applied coal and coke petrology at the Bethlehem Steel Corporation: International Journal of Coal Geology, v. 42, p. 115–128.
- Thompson, R.R., and Benedict, L.G., 1976, Coal composition and its influence on coke making. Ironmaking Proceedings, v.3 (2), p. 21– 31.
- U.S. Environmental Protection Agency (EPA), 1995, Coke production: U.S. EPA Emissions Factors and AP 42, Compilation of Air Pollutant Emission Factors, Chapter 12, Metallurgical Industry, varied pagination.
- Zimmerman, R.E., 1979, Evaluating and testing the coking properties of coal: San Francisco, Miller Freeman Publications, San Francisco, California, 144 p.