What Are Coal Grades?

Coal grade is an economic or technological classification of the relative quality of a coal for a particular use. A variety of grades of coal are defined for different uses or markets in different industries and countries, and for the needs of a particular process or by regulations concerning the process or end-use product. The terms are process- or product-specific. For example, a coal which can be used to generate steam for electricity, may not be a high-grade coal for metallurgical uses. Terms such as low-ash or low-sulfur are also examples of grade terms. Different quality grades are used in different coal markets.

Steam coal (sometimes called thermal coal) is a grade of coal used in electric power plants to generate steam to create electricity. Most of the coals mined in the United States are steam coals. Steam coals for power plants must meet quality and heating characteristics of the boiler design and for the design of pollution-control equipment at a power plant.

Grades of steam coal are generally related to sulfur content and ash yield. Typically, the term low-sulfur coal is used for coals with less than 1 percent sulfur. In the United States, the Clean Air Act Amendments (1990) set a limit for sulfur emissions at power plants of 1.2 pounds of sulfur per 10,000 Btu, which amounts to approximately 1 to 1.2 percent sulfur in a bituminous coal (Cobb and Eble, 1992; U.S. Energy Information Administration, 1997). Hence, sulfur is used to define grades of coals that meet emissions standards. Low-ash coals are grades of coal with ash yields below 10 percent. High-ash coals are generally coals above 10 percent ash yield.

Some coals can be used to produce coke, an important raw material used in steel making. These coals are referred to as metallurgical coal, met coal, or coking coal.These can be considered grade terms.Coke (not the soft drink) is a hard, porous, carbon-rich compound. Only coals with specific quality characteristics can be used to make coke. Metallurgical grade coals for steel production must be very low in ash (generally less than 10 percent) and sulfur (less than 1 percent), have volatile matter contents from 20 to 30 percent (medium- to high-volatile bituminous rank), and have a favorable balance of reactive and inert components. Grade terms in the coking industry include low-ash, low-sulfur, and low-volatile, based on the requirements to make coke. Element concentrations are important as well, because certain elements (for example, phosphorous) can cause problems in the resulting steel (Zimmerman, 1979; Stach and others, 1982).

Are any Kentucky coals metallurgical grade coals?

Because the ideal coal for making coke rarely occurs in a single bed, it is common practice to blend several coals together to achieve the quality parameters needed to make coke for steel. Coals from western Kentucky are typically not used for steel production (e.g., met coal), because of their high sulfur content. Several coal beds in eastern Kentucky meet the low-ash and sulfur requirements of metallurgical-grade coal feedstock, but unfortunately they typically have volatile matter contents (in the mid to high 30’s) that are too high for use in steel production. As such, most eastern Kentucky coals are not used by themselves to produce coke. Rather, they can be blended with low-volatile bituminous coals to form very high-quality coke. Low-volatile bituminous coals are mined in adjacent West Virginia and Virginia and have volatile matter contents below 20 percent.

Certain coals can be used to produce chemicals and specialty products. This is a small percentage of the overall coal market. Chemical and specialty coals must meet very specific requirements for the product or chemical process in which they will be used. Requirements (and relative grade terms) may be related to ash and sulfur contents, as with steam coals and metallurgical-grade coals, but also may be related to mechanical properties (e.g., Hardgrove grindability, free-swelling index), or to chemical composition (e.g., trace elements, amounts of reactive macerals).

The Blue Gem Coal and Silicon Metal

The Blue Gem coal is an exceptionally high-quality coal found in several areas of southeastern Kentucky. The coal can have sulfur contents below 1 percent and ash yields below 1.5 percent, which is less than the ash content of the living plants from which the coal formed! Because of its low sulfur and extremely low ash content (among other properties), this coal is one of only a few coals in the world that can be used to make silicon metal.Silicon metal is metallurgical-grade silicon, which is used to make aluminum-silicon alloy materials, in the chemical industry, and in the manufacture of electric semi-conductors.

Other properties

Low ash-fusion temperature: The mineral fraction of the Blue Gem is dominated by siderite, rather than silicates. This gives the coal a low ash-fusion temperature that is favorable in the production of silicon metal (Gardner and others, 2007; Hower and others, 2007).

High silicon reactivity: The carbon in the Blue Gem exhibits high silicon reactivity, which is favorable in the production of silicon metal (Myrvagnes and Lindstad, 2007).

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