Natural gas used to fuel gas turbines is one of the cleanest types of fuels used for power production. It produces little sulfur dioxide (SO 2) or carbon monoxide (CO). The high overall efficiency of modern gas turbines and combined cycles contributes to lower carbon dioxide (CO 2) emissions. However, since power and thermal efficiency increase with increasing firing temperature, modern gas turbines are emitting higher nitrogen oxides (NO, NO 2, termed NO X). Figure 20.1 illustrates the increase in NO X with combustion temperature.
Volatile organic compounds and sunlight combine with this pollutant to form ground-level ozone, or smog. Respiratory systems and vegetation are seriously affected by elevated ozone concentrations.
NO X is also a contributor to acid rain, and it is implicated as a greenhouse gas. Its emissions are transported over long distances, causing harmful effects in other geographic areas. It is produced by high-temperature (2000 F) oxidation of nitrogen.
NO X production is promoted by higher pressures and by long residence time of the very hot mixture, which ensures complete combustion of fuel to minimize CO and smoke. In general, the CO emissions increase as the NO X emissions decline. Liquid fuels that have higher local-flame temperature and some nitrogen compounds also form NO X. SO 2 emissions are also produced by liquid-fueled units, depending on the fuel sulfur content.
NO X emissions are normally measured as a fraction of NO 2-equivalent...
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