11.4. The most important contaminants from fossil-fueled power plant operation are carbon dioxide, sulfur dioxide, nitrogen oxides, and partic­ulate matter. Combustion processes contribute about 10 percent of the carbon dioxide emitted to the atmosphere, the remainder being primarily the result of natural decay. The carbon dioxide concentration in the at­mosphere is the result of the balance between generation and removal processes such as photosynthesis. However, as will be discussed in the next section, increasing carbon dioxide concentration levels could affect the earth’s climate as a result of the greenhouse effect.

11.5. Power plants contribute the major proportion of sulfur dioxide emitted to the atmosphere, but industrial sources can be very significant in some areas. Local concentrations vary widely and tend to be highest in urban industrial regions. High concentrations on the order of 10 parts per million (ppm) can cause breathing problems.

11.6. Only about 4 percent of the total emission of nitrogen oxides is caused by power plants. However, when carried along by wind currents, sulfur compounds and nitrogen oxides combine with the water vapor in the atmosphere to form sulfuric and nitric acids as aerosols which precip­itate back to earth with rain or snow long distances from the point of origin. This phenomenon, known as acid rain, has become a serious environmental problem. Therefore, flue-gas desulfurization systems are now required for new fossil-fueled power plants to minimize sulfur emission. These systems are expensive, generate a great deal of solid waste requiring disposal, and require significant maintenance.