Each of these technical factors can set limiting conditions when considering the economic recoverability for a given coal, mining method, and market. The important economic parameters are the relative costs of mining coal by surface and underground methods including costs associated with any site-specific land use constraints, the cost of removing the material above the coal seam in the surface method, and the price of coal. The price for any particular coal is related directly to coal quality (Box 4.1 and Table 4.2). Because more than 90 percent of the coal mined in the United States is used in power plants to generate electricity using steam turbines, the price for steam coal is dependent primarily on its heating value and sulfur content.

Relatively shallow coal deposits are generally extracted by surface mining, and deeper deposits are extracted by underground mining (more detailed descriptions of surface and underground mining processes can be found in Appendix E). There are also situations in which a seam is mined by surface methods first, and then if adequate reserves are still available, the mine is developed for underground extraction. Where remaining reserves are limited, other methods of mining—such as auger mining or highwall mining—may be used.

Surface mining has many advantages compared to underground mining. In general, coal recovery is very high (85 to 90+ percent), compared to 40 to 70 percent in underground mines. The productivity of surface mines is generally higher than that of underground mines (Figure 4.4), and health and safety statistics for surface mining are also generally better than those of underground mining. Surface-mined coal from the Powder River Basin is usually simply sized and screened in preparation for market, whereas underground-mined coal and surface-mined coal from the Interior and Appalachian basins often requires a greater amount of processing (see below) to improve its marketability. The cost per ton of mining coal by surface methods is generally lower than that by underground methods.

In the United States, in addition to a continuous growth in coal production since the 1960s, there has been a dramatic shift in production from underground mining to surface mining (Figure 1.6). In the Powder River Basin (PRB), where deposits of coal more than 100 feet thick occur close to the surface, individual surface mines can produce more than 90 million tons each year. Underground coal mining is more common east of the Mississippi River, particularly in Appalachia. Some of the largest underground coal mines, each producing around 10 million tons annually, are located in Pennsylvania and West Virginia. The largest underground mining complex in the United States produces about 20 million tons per year.

Bituminous coals in the eastern and central United States are mined by both surface and underground mining methods. Anthracite coal is mined exclusively in northeastern Pennsylvania, also by both underground and surface mining methods. Lignite and subbituminous coal production is centered in a small number of large mines (Table 4.3). Subbituminous coal and lignite comprise about 50

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