The composition of emissions from electric power plants

The composition of emissions from electric power plants is, in part, a function of the completeness of the combustion process. The primary fuels burned in electric power plants (coal, natural gas, and distillate or residual oils) are carbon-hydrogen compounds that produce CO2 and water vapor byproducts when completely combusted (oxidized).

However, combustion is seldom complete, and incomplete combustion yields unburned fuel molecules, smoke particles (primarily carbon), and partially oxidized carbon as CO. Nitrogen oxides result from the combustion of hydrocarbons in the presence of air, which is 21 percent oxygen and 78 percent nitrogen. During combustion, portions of both the atmospheric nitrogen and the fuel-bound nitrogen react with oxygen to form NO and NO2. These compounds are referred to collectively as nitrogen oxides.(6)

Fossil fuels also contain varying amounts of sulfur, which is oxidized to sulfur dioxide (SO2) during combustion. The level of SO2 emitted is a function of the type of fuel burned and the control equipment used rather than the combustion process. Sulfur is present in virtually all coals and fuel oils at levels ranging from trace amounts to 6 percent by weight.(7)

Electric utility power plants currently account for only a small percentage of U.S. total particulate emissions (Figure FE1) because control devices, such as baghouse filters and electrostatic precipitators, remove most of the particulates from power plant waste gases. Similarly, electric utility power plants contribute only small percentages of total emissions of VOCs, CO, N2O, and CH4.(8) On the other hand, 72 percent, 35 percent, and 33 percent of total emissions of SO2,(9) CO2, (10) and NOx,(11) respectively, come from utility power plants.

DOE has increasingly recognized that the lack of accurate and consistent (across fuel types) information on external costs distorts Federal energy research decisions and PUC decisions about emission control technologies. In 1991, DOE and the Commission of the European Communities committed to a joint study to develop comparative analytical methodologies to determine the external costs of the major fuels. Preliminary emissions data from the application of these methodologies by Oak Ridge National Laboratory indicate that substituting any major fuel for coal (or using clean-coal technology) reduces emissions of the key pollutants SO2, NOx, and CO2 (Table FE1).(12)


Environmental Problems Related to Electric Power Plant Emissions
Electric power plant emissions are factors in three major environmental issues: acid rain, urban air quality, and global climate change. These issues are discussed below.

Acid rain. Acid rain refers to rain, fog, mist, or snow that is more acidic than normal. The acidity of precipitation is stated in terms of its pH level, which describes the concentration of hydrogen ions along a scale (from 0 to 14) that defines the continuum from acid to base. The pH scale is logarithmic; pH levels of 4.0 and 3.0, for example, are 10 and 100 times more acidic, respectively, than a pH level of 5.0. Although a pH level of 7.0 is neutral, unpolluted rainfall is normally slightly acidic (pH=5.6). Acid rain is defined as any precipitation with a pH of 5.5 or less.

Chemical analysis of data collected by means of cloud sampling and experimentation reveals the presence of sulfuric acid and nitric acid in precipitation in the United States (Figure FE2).(13) Sulfur dioxide and nitrogen oxides in the air, partly the result of emissions from electric power plants, gradually react with water vapor and become acids. Precipitation becomes acidic by mixing with these acids. The acidity of the precipitation depends upon the amount of acid in the atmosphere and the amount of water in which it is dissolved. Undissolved acids may also fall to Earth by themselves or in combination with dust particles.

The most severely acidic conditions are found in the eastern United States. EPA believes that acid rain has been the primary cause of the acidification of hundreds of streams in the mid-Atlantic highlands and the New Jersey Pine Barrens and of many lakes in the Adirondack Mountains of New York.(14) The National Acid Precipitation Assessment Program (NAPAP) identified acid rain as one of several possible causes of increased nitrate leaching and acidification of surface waters in several northeastern watersheds. Episodes of acidification are believed to harm populations of fish and invertebrates in small streams and lakes.(15)

Field studies have implicated acid rain in observed damage to high-elevation red spruce forests in the northeastern United States. Nutrient leaching and changes in soil chemistry due to acid deposition have also been detected in forests south of the Great Lakes. In general, NAPAP concluded that acid deposition, among other stressors, threatens the long-term structure, function, and productivity of many sensitive ecosystems.(16)