Coals for Coke and Steel Making

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.

Generalized diagram showing how steel is made.

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).

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