(Clean Air Act amendments), NOx emissions from power plants will be twice as great as automobiles and the largest source of NOx emissions nationally around the turn of the century (Bachman, 1994). Therefore, it can be expected that ozone reduction strategies in the Northeast and other parts of the United States will focus increasingly on NO x emissions from new and existing fossil-fueled power plants. A tightening of the ambient ozone standard also is under consideration by EPA based on recent health studies (Bachman, 1994). Though the timing and magnitude of NOx reduction requirements to achieve ambient ozone standards is highly uncertain, the implication of current trends in regulation and technology development is that stringent NOx controls of coal-based technologies could well emerge within the next decade.

Acid Deposition

The acid deposition provisions of the 1990 CAAAs established for the first time an absolute cap on total U.S. SO2 emissions. In contrast to ambient air quality standards, which primarily protect human health, acid deposition regulations primarily guard against a host of cultural and ecological concerns, including damage to aquatic systems, forests, visibility, and materials. The regional nature of acid deposition and the role of long-range transport of pollutants require reductions in SO2 and NOx emissions over a broad geographical area, primarily the eastern half of the United States. The SO2 cap of 10 million tons per year established by the 1990 amendments will require a reduction of roughly 40 percent in current SO2 emissions from electric power plants, to be phased in by the turn of the century. A smaller reduction of 2 million tons per year in NOx emissions, about 10 percent of 1980 levels, also is mandated for acid deposition control. There is no cap on total NOx emissions, however.

The anticipation of acid rain controls was the prime factor motivating SO2 and NOx control technology development during the 1980s. The longer-term implications of acid rain regulations for coal technology development are somewhat speculative. The absolute cap on SO2 emissions could provide incentives to seek high levels of SO2 control in order to accommodate long-term growth. Some scenarios, however, suggest that future SO2 emissions will continue to decline using power generation technologies that are currently available or will become available commercially in the near-term period, especially integrated gasification combined-cycle systems (NAPAP, 1991). Other factors also could affect future developments. For example, a new air quality standard for fine particulates, discussed earlier, could require additional SO 2 controls to reduce particulate sulfate emissions.

Air Toxics

Title III of the 1990 CAAAs lists 189 substances as "air toxics" subject to "maximum-achievable control technology" when emitted at rates of 10 to 25 tons



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