of sufficient magnitude to justify development of appropriate technology for their control.

Emissions of nitrous oxide (N20), another greenhouse gas, also arise from coal combustion. Because N20 is formed primarily at relatively low temperature and pressure, the largest emissions rates are associated with atmospheric fluidized-bed combustion systems. Overall, N20 emissions from coal combustion worldwide are estimated to contribute less than 1 percent of total global warming emissions. The primary sources of N20 worldwide are fertilizers and agricultural wastes (NRC, 1992b).

Coal-fired electric power plants and fuel conversion processes are subject to state and federal regulations to protect the quality of surface waters, ground water, and drinking water. The principal environmental concerns are thermal discharges to waterways (discharges prohibited for new plants) and various chemical emissions, including heavy metals, organics, suspended solids, and other aqueous constituents found in power plant waste streams. In recent years there has been increasing attention to control of hazardous or toxic trace chemical species and a general tightening of effluent emission standards at existing and new facilities (Rubin, 1989). High-volume wastes, such as flyash from coal-fired power plants, have been declared "nonhazardous," with only some low-volume wastes such as boiler cleaning sludges falling under the "hazardous" category. The latter require more rigorous treatment and involve much higher disposal costs to avoid surface or ground water contamination. Nonetheless, to control the release of suspended solids and other chemical constituents of high- and low-volume wastes, water treatment systems similar to those found in other industrial processes are an integral requirement for modern power plants.

The large volumes of solid waste that must be disposed of, particularly ash from coal, represent a growing problem because of concern over contamination of ground water and surface waters and the decreased availability of landfill sites for waste disposal. Ash solubility and its effects on ground water can be greatly reduced by processes that fuse ash, resulting in products that can be used as construction materials, such as gravel substitutes. While research on the conversion of solid wastes to higher-value products has shown that by-product and reuse options are technically feasible, such conversion methods currently are not able to absorb the large quantities of material produced and often are not economical in today's markets. Another disposal option, especially applicable to western open-face mines where coal is transported by rail, is returning waste to the coal mine.

To an increasing extent, federal NSPS levels for power plants no longer set the benchmark for environmental control performance. Rather, state and local determinations of "lowest-achievable emission rates" now set the critical requirements in many cases. A related trend is the adoption by some state public utility commissions of "externality adders," economic costs added to the nominal cost of power generation that reflect the environmental damages due to emissions that



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