the sorbent utilization for sulfur removal in advanced fluidized-bed combustors and gasifiers. Current PFBC systems produce the largest volume of solid waste per unit of sulfur removed. The presence of unreacted lime (as well as sulfides in the case of gasifiers) adds to the difficulty and cost of waste disposal. Pilot plant data for circulating PFBC designs show improved sorbent utilization relative to bubbling bed designs, but more work is needed to achieve commercially acceptable systems. More intensive research on reuse of spent sorbent also is needed if DOE's goal for solid waste reduction is to be achieved.

Control technologies for noncriteria pollutants also need to be addressed. To deal with the emerging issue of air toxics (see Chapter 3), trace substance emissions and fate must be characterized for current and advanced technologies. It is anticipated that existing high-efficiency particulate control technologies will be adequate to deal with most heavy metal emissions from coal combustion, but specific regulations have yet to be established. Similarly, the extent to which vapor-phase emissions such as mercury, chlorides, and selenium will have to be controlled is not yet clear; technologies to control these emissions may well be needed in the near future. Should that be the case, an additional risk of hot gas cleanup systems is their uncertain capability to control emissions of air toxics, since they presently do not remove vapor-phase species. Additional controls for air toxics may impose additional economic costs.

The ability of control technology to reduce or eliminate emissions of potential air toxics is currently under study by DOE, EPRI, and others. The most prevalent data are for conventional cold-side ESPs, which show high removal efficiencies for most heavy metals but much lower removal rates for volatile species such as mercury (Rubin et al., 1993). Wet FGD systems in conjunction with an upstream particulate collector appear to offer the greatest removal rates of volatile species and other potential air toxics such as chlorides. However, there is large uncertainty in the data, with relatively little information currently available for wet scrubbers operating in the United States. Experiments with carbon-based additives show an enhanced ability to remove mercury in some cases, particularly with high chloride coals. Research on novel control methods for air toxics is being pursued by EPRI, DOE, and others.

As noted in Chapter 3, a major concern for all coal-based technologies is the potential requirement to control carbon dioxide (CO2) emissions. The most economical means is to improve the efficiency of energy conversion and utilization so that less CO2 is emitted per unit of useful energy delivered. For coal-fired power plants, average U.S. energy losses are about 2 percent in coal preparation, 67 percent in power generation, and 8 percent in transmission and distribution (EIA, 1993), yielding an overall efficiency of about 30 percent for fuel to delivered electricity. Within the limits of thermodynamic cycles, the greatest opportunity for energy efficiency improvements thus lies in the power generation process. As noted previously, the most efficient PC-fired plants commercially available today have efficiencies in the range of 38 to 42 percent. Thus, advanced

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