impacts. However, no such measures are yet commercially deployed or have been demonstrated that can substantially reduce emissions of carbon dioxide from large coal-fired power plants, to ameliorate their contribution to global warming. Accordingly, the greatest difficulty in projecting future coal usage arises from uncertainty about the nature of future government action to limit CO2 emissions from power plants and the viability of large-scale applications of technology to capture and sequester CO2 emissions from power plants and other coal-based energy conversion facilities. Consequently, any projections of a substantial future expansion of coal use in the context of a meaningful CO2 reduction mandate depend on the development and widespread deployment of technology to reduce CO2 emissions from coal-based power plants.

Impacts on Air Quality

Atmospheric emissions of SO2, NOx, and particulate matter (PM) from coal combustion are significant sources of air pollution that have been linked to respiratory and other human health problems (e.g., NRC, 2005a; EPA, 2006a). However, as a result of a series of actions following the 1990 Clean Air Act Amendments, emissions of NOx and SO2 from coal-fired power plants in the United States are being substantially reduced even as U.S. coal consumption is increasing. As other requirements of the 1990 amendments are implemented, emissions of hazardous air pollutants such as mercury (EPA, 1997, 1998; NRC, 2002c) from coal-fired power plants are also expected to decline.

Projections of future coal use already reflect current government regulations aimed at reducing emissions of NOx, SO2, particulate matter, and mercury from coal-fired power plants. Both the technology needed to satisfy the regulatory requirements for NOx and SO2 emissions and the technology to eliminate more than 99 percent of PM emissions are currently in widespread commercial use (NESCAUM, 2005). The technology to significantly reduce mercury emissions in compliance with federal regulations is also expected to be commercially available by 2010 (STAPPA/ALAPCO, 2005; Feeley, 2006).

The present availability and continuing evolution of technology to respond to emissions of these pollutants make it unlikely within the 25-year time horizon of this study that current or future governmental requirements to control such emissions will materially affect future coal usage overall, although air quality considerations will continue to affect the siting of new coal-based facilities. Requirements to control air pollution from power plants are generally structured in one of two ways—as specific standards imposed on each individual electric generating unit in a regulated class; or as a “cap-and-trade” program (Box 6.2), which limits the aggregate emissions from all units in the regulated class but does not impose limits on individual units. Each approach can shift demand—either intentionally or unintentionally—to coals with differing characteristics (e.g., from bituminous to subbituminous coal or vice versa).

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