offers significant design, performance, environmental compliance, and cost advantages over AFBC technologies. As noted earlier, a second-generation of PFBC technology offering additional performance (efficiency) benefits is entering the pilot and demonstration phase. These systems employ a coal pyrolyzer to produce a fuel gas that is burned in the turbine topping cycle. Since only a portion of the coal is gasified, this design has the potential for higher efficiencies than IGCC systems, where all of the coal is gasified. Maude (1993) estimates that the efficiency advantage may be approximately four percentage points. Because PFBC operates at a higher pressure and increased efficiency compared with AFBC, the same power output can be achieved with a unit that requires less land area (i.e., smaller "footprint" of equipment). The steam flows for PFBC units also are compatible with steam turbines at existing power plants. Thus, the technology is especially attractive for repowering existing units at existing power plant sites, avoiding the need and difficulty of developing new sites. The higher cost of equipment operating at higher pressures and temperatures is partially offset by the reduced equipment size and higher efficiency. Efficiencies on the order of 39 to 42 percent can be achieved with newer PFBC designs, compared with 34 percent efficiency for AFBC. EPRI estimates the capital cost of a 340-MW bubbling bed supercritical PFBC boiler (42 percent efficiency) at $1,318/kW (in 1992 dollars), with a total levelized cost of 37 mills/kWh (80 percent capacity factor, eastern bituminous coal) (EPRI, 1993a).

Substantially higher efficiencies (45 to greater than 48 percent) are expected from second-generation PFBC systems. It is questionable whether the advanced PFBC systems can achieve DOE's goal of 20 to 25 percent reduction in electricity cost as well as capital cost reductions relative to current PC plants. In general, the higher degree of complexity of advanced systems makes it likely that capital costs will tend to increase rather than decrease, although the resultant efficiency gains will have a positive effect in lowering the cost of electricity. At present, however, there remains considerable uncertainty as to the future costs of advanced power systems.

One of the key performance and cost uncertainties for advanced PFBC systems is the development of hot gas cleanup technology. Reliable hot gas particulate cleanup plus advanced (1370 °C [2500 °F] or higher) turbine systems will be required for PFBC technology to achieve DOE's projected performance potential of more than 50 percent efficiency while meeting environmental compliance requirements. At the present time these technologies are under development. The status of hot gas cleanup technology and advanced turbine systems (ATS) is discussed later in this chapter.

Related issues concern the development of adequate SO2 and NOx controls and their associated costs. Current DOE flowsheets for advanced PFBC systems are beginning to incorporate the possible need for selective or nonselective catalytic reduction systems for NOx control in addition to the combustion controls inherent in FBC systems. Added NOX controls would increase the base cost of the



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