the turbine from particle-induced damage exceed the current requirements for environmental protection. The most promising systems to date have employed barrier filters designed to achieve emissions of less than 2 ppm by weight of particles greater than 5 microns in diameter. The major problem, however, has been longevity. Current candle filter designs have operated no more than several hundred hours at the required temperatures (760 °C to 870 °C [1400 °F to 1600 °F]) before breaking, whereas lifetimes on the order of 16,000 hours are needed for economical PFBC systems (DOE, 1994c). Improved designs, as well as testing in the reducing gas environment of IGCC systems, are planned as part of the CCT demonstration projects.

With respect to solid waste emissions, many state-of-the-art air pollution control systems offer improved prospects for waste reduction through the production of salable by-products, especially with regard to sulfur emissions control. Modern FGD systems produce gypsum, which can be upgraded to commercial quality and sold (which is common practice in Europe and Japan). Several advanced flue gas cleanup systems being demonstrated in the CCT program produce by-product sulfur or sulfuric acid, as do the hot and cold gas cleanup systems employed with coal gasifiers. Only advanced PFBC systems increase rather than decrease the total solid wastes generated from coal use. In all cases the economic viability of by-product recovery systems depends on site-specific factors and markets. In the United States today, waste disposal in landfills is still more attractive for many electric utilities.

Technical Issues, Risks, and Opportunities

Existing control technologies for the criteria air pollutants (SO2 , NOx, and particulates) associated with PC-fired power plants are capable of meeting current or anticipated emission reduction requirements in the near-term (i.e., prior to 2005). The same is true of cold gas cleanup control technologies for gasification-based systems. Cost reduction and minimization of solid waste remain important goals to improve the viability of these coal-based systems. For the medium term (post-2005), additional performance improvements also may be required, especially for NOx controls.

Control technologies applicable to advanced combustion and gasification technologies need further development. In particular, hot gas cleanup systems for SO2 and particulate removal, which are critical to several of the advanced high-efficiency technologies-especially PFBC-have yet to achieve the performance, reliability, or durability needed for commercial applications. In IGCC systems, hot gas cleanup does not presently control nitrogen emissions (in the form of gaseous ammonia), which increases downstream costs and complexity for NOx controls in the gas turbine/heat recovery system. Research to address these issues is in progress.

With respect to solid waste minimization, one of the key needs is to improve



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