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This option includes the transition to solar energy, nuclear energy, and other methods of obtaining energy. Of the pollutants generated by burning fuels, the emission of sulfur oxides and ash are directly attributable to fuel composi tion. Using the right fuel can cut sulfur dioxide and ash emissions. This option means switching to coal with lower sulfur content or replacing coal with gaseous fuel or desul-furized fuel oils. The sulfur in heavy fuel oils can be removed in a high-pressure catalytic reactor, in which hydrogen combines with the sulfur to form hydrogen sulfide which is then recovered.

An alternative to using lower-sulfur coal is to remove some of the sulfur in the coal (see Figure 5.19.1). Sulfur in the form of discrete iron pyrite crystals can be removed because they have different properties from the organic coal matrix in which they are embedded. This difference makes separation by physical processing possible. The key economic factor is the cost of the losses of combustible material which occur during the cleaning process. To date, none of the coal cleaning processes have been proven commercially successful. Their attractiveness is also diminished by their inability to recover, on average, more than half the sulfur content (e.g., the pyritic fraction) of the fuel (Bradshaw, Southward, and Warner 1992).

Effluent

FIG. 5.19.1 Schematic diagram of a limestone-gypsum FGD plant.

Effluent

FIG. 5.19.1 Schematic diagram of a limestone-gypsum FGD plant.

Nitrogen oxides form at firebox temperatures by the reaction of the oxygen and nitrogen in the air and fuel. The thermal fixation of atmospheric nitrogen and oxygen in the combustion air produces thermal NOX, while the conversion of chemically bound nitrogen in the fuel produces fuel NOx.

For natural gas and light distillate oil firing, nearly all NO emissions result from thermal fixation. With residual fuel oil, the contribution of fuel-bound nitrogen can be significant and in some cases predominant. This contribution is because the nitrogen content of most U.S. coals ranges from 0.5 to 2% whereas that of fuel oil ranges from 0.1 to 0.5%. The conversion efficiencies of fuel mixture to NOx for coals and residual oils have been observed between 10 and 60% (U.S. EPA 1983). Figure 5.19.2 shows the possible fates of fuel nitrogen. One option of reducing NOx is to use low-nitrogen fuel.

FIG. 5.19.2 Possible fates of nitrogen contained in coal. (Reprinted, with permission, from M.P. Heap et al., 1976, The optimization of burner design parameters to control NO formation in pulverized coal and heavy oil flames, Proceedings of the Stationary Source Combustion Symposium—Vol. II: Fuels and Process Research Development, EPA-600/2-76-152b [Washington, D.C.: U.S. EPA].

FIG. 5.19.2 Possible fates of nitrogen contained in coal. (Reprinted, with permission, from M.P. Heap et al., 1976, The optimization of burner design parameters to control NO formation in pulverized coal and heavy oil flames, Proceedings of the Stationary Source Combustion Symposium—Vol. II: Fuels and Process Research Development, EPA-600/2-76-152b [Washington, D.C.: U.S. EPA].

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