as an energy source for industrial processes, transportation, and residential or commercial buildings. In 2005, the dominant use of coal in the United States (92 percent of the total) was for electric power generation (Table 6.1). Coke plants (2 percent) and other industrial uses (5 percent) account for most of the remainder, with a small amount of coal still used in residential and commercial buildings (EIA, 2006c).

Extensive discussions of the technologies used in each of these sectors, as well as the status and needs of ongoing R&D, can be found in other reports (e.g., NRC, 1994, 1995, 2000, 2002a, 2002b). Accordingly, this chapter presents only a brief overview of the major coal utilization technologies that influence the projections for future coal use in Chapter 2.

Electric Power Sector Technologies

About 90 percent of the 313,000 megawatts (MW) of coal-fired generating capacity in the United States today is based on combustion of pulverized coal (PC). This process involves reducing the coal to a powder that is burned in a boiler to generate high-pressure, superheated steam that drives a turbine connected to an electric generator (Figure 6.1). The steam is then condensed back to a liquid and returned to the boiler to repeat the cycle. Although this process can use a variety of coal types, boilers capable of burning a more uniform quality of coal are generally less expensive than those designed for a broader range of coals. Consequently, it is common for many run-of-mine coals to be cleaned to reduce coal ash and (to a lesser extent) sulfur content, thereby providing a more uniform fuel supply with a higher heating value (see Chapter 4). Several of the newer coal-fired units constructed in recent years have employed atmospheric fluidized bed combustion (AFBC) technology offering greater flexibility in fuel quality.

The overall efficiency of PC power generation is affected by many factors, including the thermodynamic cycle design, steam temperature and pressure, coal particle size (coal grind), combustion air-to-fuel ratio, fuel mixing, air leakage into the system, cooling (condenser) water temperature, and the “parasitic” energy loads required to power auxiliary equipment such as grinding mills, pumps, fans, and environmental control systems. Since 1960, the average thermal

TABLE 6.1 Coal Use in the United States by End-Use Sector in 2005

End-Use Sector

Coal Use (thousand short tons)

Electric power sector

1,037,485

Coke plants

23,434

Other industrial processes

60,340

Residential and commercial

4,217

Total

1,125,476

SOURCE: EIA (2006c).



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