In the U.S. the 1980 NOx emissions are contributed by the transportation sector (45%), power plants (35%), and industrial sources (25%) (EPA 1986). In the transportation sector, the NOx emissions result from internal combustion engines. In power plants and industrial sources, NOx is produced in boilers. The overwhelming fraction of nitrogen oxide emissions arises from the high temperature combustion of fossil fuels; emissions from metal-processing plants and open-air burning of biomass are insignificant.
In internal combustion engines the main parameter that determines the NOx production is the combustion temperature, which in turn depends on the air-fuel ratio. In industrial boilers the combustion temperature is also the main factor. In addition, fuel-bound nitrogen in coal and residual oil also contributes about 20% to the NOx emissions (Darmstadter et al. 1987). For this reason, combustion technology plays a significant role in the quantity of NOx emissions. In this sense, NOx emission estimates, as well as the suitable control strategies are significantly different from those of sulfur.
Estimating historical emission trends of nitrogen oxides is difficult because most of the nitrogen oxide is formed by the fixation of atmospheric nitrogen at high temperatures of combustion rather than by oxidation of the nitrogen contained in the fuel. Thus, nitrogen oxide emissions depend primarily on the combustion temperature and to a lesser degree on the fuel properties. Since combustion processes in internal-combustion engines and boilers have undoubtedly changed since the turn of the century, it is likely that nitrogen oxide emission factors have also changed historically. Because combustion parameters can vary over a wide range, and because information on historical combustion processes is generally lacking, assumptions concerning changes in emission factors over time constitute the major source of uncertainty in developing trends in nitrogen oxide emissions.
NOx emissions are obtained from fuel consumption data weighted by an appropriate emission factor. For a given source of combustion, this factor is the quantity of nitrogen oxide emitted per unit of fuel consumed. The emission factors used here were derived from extensive inventories that list nitrogen oxide emission factors according to source type of combustion (U.S. Environmental Protection Agency 1977, 1978). The numerous emission factors listed in these compilations were aggregated into four weighted-average emission factors by fuel type: coal, gasoline, natural gas, and other petroleum products. The emission factors before 1970 were estimated to reflect the fact that the average combustion temperature, and hence the production of nitrogen oxides per unit of fuel consumed, was lower, especially for coal combustion, over the past 100 years (Figure 12). A simple linear trend was assumed for all emission factors. For coal combustion the emission factor was assumed to increase fivefold from 1880 to 1970. For combustion of gasoline and natural gas the emission factors were assumed to increase by 50 and 100 percent, respectively. The emission factor for other petroleum products was assumed to be constant over time.
Figure 12. Trends in emission factors of nitrogen oxide by fuel type. Emission factors from the
period of 1970 to 1980 were derived from data given from the U.S. Environmental Protection Agency (1977,
1978). For the period from 1880 to 1970, trends of historical emission factors were assumed to be linear, with
slopes varying by fuel type.
Figure 13 (a) Trends in emission of nitrogen oxides in the eastern United States (the aggregate
of Regions B, C,D, and E); (b) Region B; (c) Region C; (d) Region D; (e) Region E; (f) trends in emission densities of regions north
(Regions B, D, and E) and south (Region C) of the Ohio River.
Based on these estimates of emission factors and data on fuel consumption, national emission trends were calculated as shown in Figure 13. It is evident that there was a monotonic increase of U.S. NOx emissions from the turn of the century to about 1970. Since then the emissions have remained roughly constant.
A comparison of the sulfur nitrogen emissions reveals significant differences: 1.) National sulfur emissions have fluctuated between 8 and 16 million tons a year, since the turn of the century. Nitrogen oxides, on the other hand, have monotonically increased until the 1970s. 2.) Since the 1970s the main source of sulfur oxides is coal combustion in power plants, while nitrogen oxides are contributed primarily by internal combustion engines. 3.) Sulfur emissions result from oxidation of the sulfur impurity contained in fossil fuels and metal ores. Nitrogen oxides are formed primarily by fixation of atmospheric nitrogen at high temperatures and to a lesser degree by oxidation of fuel-bound nitrogen. 4.) The control of sulfur oxides will have to relay on the removal of sulfur from the fuel or flue gases. Nitrogen oxides controls can be accomplished by technological changes in the combustion itself.