Coal Processing

“Raw” or “run-of-mine” coal can be processed2 using physical separation methods to remove unwanted mineral matter to produce a “clean” coal. Processing adds value in several ways:

  • Removal of the mineral matter (or “ash”),3 which is largely noncombustible and may constitute up to 65 percent of the raw coal, increases the heating value of the coal on a mass basis.4 Although some combustible material is lost as part of the cleaning process, the removal of unwanted material reduces the mass and volume of coal for a given heating value thereby reducing shipping costs as well as minimizing coal handling and ash management costs for the end user.

  • Processing allows greater control over the “quality” of the coal—principally ash and moisture—which improves its consistency for end users, such as electricity generators or coke manufacturers. Improved and consistent quality increases the efficiency and availability of steam boilers and is particularly important for the quality of metallurgical coke.

  • Physical processing (see Appendix E) can, to some extent, reduce sulfur and trace element contents, particularly on a heating value basis. However, generally coal cleaning is not practiced primarily for this purpose except for the metallurgical coal market.

The decision whether or not to process a particular raw coal depends on the coal and its intended market. The subbituminous coal of the Powder River Basin is almost always shipped to market raw because it has inherently low ash content and poor “washability,”5 and the region has low water availability—a critical requirement for conventional coal beneficiation.

Most coal preparation plants are in the eastern states, with more than 80 percent of the plants and almost 80 percent of capacity located in West Virginia, Virginia, Pennsylvania, Ohio, and Kentucky (Fiscor, 2005; see Appendix E). The 11 coal preparation plants in the western states are located at bituminous

2

The terms “coal preparation,” “cleaning,” “washing,” “processing,” and “beneficiation” are used synonymously to refer to physical enrichment of the combustible portion of the coal by selective removal of the noncombustible components, principally mineral matter and water.

3

It is customary to refer to the mineral content of a coal as “ash,” and it is usually reported as such in coal quality descriptions. Ash content is determined by combusting the coal in air and converting the inorganic elements to their oxides.

4

Commercial coal characteristics, such as heating value, ash, moisture, sulfur, etc., are determined in the United States according to standards established by the American Society for Testing and Materials (ASTM), and are usually denominated in English units (e.g., Btu/lb for heating value on a mass basis).

5

The term “washability” is used to describe the ease with which mineral matter can be separated from the coal, and depends on the degree of incorporation of the mineral matter in the coal’s organic matrix and its specific gravity relative to the coal.



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