Air Emissions and Electricity Generation at U.S. Power Plants (GAO-12-545R, Apr 18, 2012) found that "Older electricity generating units—those that began operating in or before 1978—provided 45 percent of electricity from fossil fuel units in 2010 but produced a disproportionate share of emissions, both in aggregate and per unit of electricity generated. Overall, in 2010 older units contributed 75 percent of sulfur dioxide emissions, 64 percent of nitrogen oxides emissions, and 54 percent of carbon dioxide emissions from fossil fuel units. For each unit of electricity generated, older units collectively emitted about 3.6 times as much sulfur dioxide, 2.1 times as much nitrogen oxides, and 1.3 times as much carbon dioxide as newer units.
difference in emissions between older units and their newer counterparts
may be attributed to a number of factors. First, 93 percent of the
electricity produced by older fossil fuel units in 2010 was generated by
coal-fired units. Compared with natural gas units, coal-fired units
produced over 90 times as much sulfur dioxide, twice as much carbon
dioxide and over five times as much nitrogen oxides per unit of
electricity, largely because coal contains more sulfur and carbon than
Second, fewer older units have
installed emissions controls, which reduce emissions by limiting their
formation or capturing them after they are formed. Among
coal-fired units—which produce nearly all sulfur dioxide emissions from
electric power generation—approximately 26 percent of older units used
controls for sulfur dioxide, compared with 63 percent of newer units.
Controls for nitrogen oxide emissions were more common among all types
of fossil fuel units, but these controls vary widely in their
effectiveness. Among older units, 14 percent had installed selective
catalytic reduction (SCR) equipment, the type of control capable of
reducing the greatest amount of nitrogen oxides emissions, compared with
33 percent of newer units. In addition, approximately 38 percent of
older units did not have any controls for nitrogen oxides, compared with
6 percent of newer units.
Third, lower emissions
among newer units may be attributable in part to improvements in the
efficiency with which newer units convert fuel into electricity.
Nonetheless, older units remain an important part of the electricity
generating sector, particularly in certain regions of the United