The federal NSPS for particulate emissions from coal-fired power plants has been tightened by a factor of three since standards were first promulgated in 1971 (see Figure 3-2b). Modern electrostatic precipitators and fabric filters routinely achieve emission levels significantly below the federal standard, with commercial designs today achieving one-third the NSPS level (Sloat et al., 1993). Some baghouses now in use achieve particulate emission levels of 0.005 lb/million Btu, or one-sixth the NSPS requirement (EPRI, 1992). With the likelihood of a new air quality standard for fine particles, the potential exists for future emission limits based on particle size as well as total mass.

Nitrogen Dioxide

The health-based ambient air quality standard for NO2 has not been a major forcing function for power plant control technology development. However, the federal NSPS for nitrogen oxides (NOx, a mix of NO and NO2) has brought about a number of changes in the design of new coal-fired boilers. The 1971 NSPS of 0.7 lb NOx/million Btu led to the extinction of new cyclone-fired boilers, which have high NOx emissions, and stimulated a variety of low-NOx burner designs. In 1979 the NSPS was tightened slightly, reflecting improvements in combustion-based controls (see Figure 3-2c). The overall level of NOx reduction now being achieved at new coal-fired plants is roughly 50 percent of uncontrolled pre-NSPS levels. As noted earlier, the environmental issues of acid deposition, fine particles, and urban ozone are likely to push requirements for greater NOx controls in the near future.

Ozone

Attainment of the health-related national air quality standard for tropospheric ozone, the major constituent of photochemical smog, poses some of the most difficult environmental challenges in the United States. Though this problem traditionally has been associated with Los Angeles and the automobile, evidence shows the problem to be far more widespread, with many metropolitan areas throughout the country exceeding the national standard (EPA, 1990). Photochemical ozone is formed from emissions of volatile organic compounds and NOx via a complex series of chemical reactions fueled by sunlight. To date, reductions in ozone have been sought primarily by reducing emissions of volatile organic compounds. Improved understanding of photochemical smog formation, however, now indicates that NOx controls must be a more significant component of ozone reduction strategies (NRC, 1991).

Federal standards for new automobiles already have reduced mobile source NOx emissions significantly in the past two decades. As a result, power plants today account for about half the total U.S. NOx emissions. After the further reductions in automotive and power plant NOx mandated by the 1990 CAAAs



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