cient wet scrubbers reduce SO2 emissions to about one-fourth to one-sixth of NSPS requirements (98 percent control). The most efficient commercial systems yield particulate emissions of about one-half to one-quarter NSPS levels (99.9 percent control). U.S. technology for power plant NOx control has focused on combustion modification methods that currently reduce emissions to about one-half to two-thirds of NSPS levels (50 to 60 percent control). In Japan and Germany, postcombustion controls achieving up to 80 percent NOx reduction (about one-third to one-sixth NSPS levels) are in widespread use on low-sulfur coal plants. These controls have not yet been deployed in the United States, but such systems are now being demonstrated at U.S. plants as part of DOE's CCT program, and several are offered commercially. Post combustion NOx controls that employ selective catalytic reduction have been installed on several gas-fired power plants, including combustion turbines, to meet state and local air quality requirements. Over the next 10 years, new requirements for NOx reductions at existing and new coal-based power plants are likely to achieve national ambient air quality standards for tropospheric ozone. Also possible are new standards for fine particulates. Future NOx controls would likely exceed the modest reductions (10 percent of 1980 levels) already required for acid deposition control.

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 per year from designated industrial and other sources. Emissions of these hazardous air pollutants from fossil-fueled power plants were exempted from the CAAAs provisions pending further study by the U.S. Environmental Protection Agency (EPA). Air toxics of primary concern to utilities are the 10 to 20 trace substances commonly found in coal, including arsenic, mercury, selenium, nickel, cadmium, and other heavy metals. The basis for regulating emissions of these species from electric utilities would be an EPA finding of an unacceptable health risk or an ecological risk to one or more regions of the country named in the 1990 CAAAs (Zeugin, 1992). Independent of EPA action, however, individual states may impose regulations or guidelines on emissions of hazardous air pollutants.

Current worldwide concern over potential global warming may pose the greatest long-term threat to expanded coal use, primarily because of the emissions of the "greenhouse gas" carbon dioxide (CO2) from coal combustion. Over the mid to long-term, CO2 emission reductions may be critical to address these concerns, although policy measures could force such reductions sooner. At the present time there is significant scientific uncertainty regarding timing, magnitude, and consequences of increased greenhouse gas emissions. Inevitably, such uncertainty is reflected in varying views about the need for CO2 emissions controls. However, the preponderance of scientific opinion—as reflected, for example, by a recent NRC study (NRC, 1992b)—suggests that the threats are of sufficient concern to warrant some initial actions. Together with some 150 other nations, the United States is already committed to a program of CO2 reduction by virtue of being a signatory to international agreements stemming from the 1992



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