The formation of NOx has been well understood by burner manufacturers, who are able to offer several methods to reduce the formation of NOx in steam generators. Gas turbine manufacturers also have come up with design improve­ments to lower NOx emissions.

During the combustion process, several complex reactions occur within the flame, and NOx formation is a function of temperature, oxygen, and time of residence in the high temperature zones. Figure 4.1 Shows the effect of temperature on NOx formation. As the combustion temperature is reduced from 2700°F to 2300°F, NOx is reduced by a factor of 10.

As the excess air increases, the NOx increases and drops off as shown in Fig. 4.10 Because CO is another pollutant, its emissions should also be limited. As the excess air increases, CO decreases. Hence there is a band of excess air in which one can operate the burner to minimize both NOx and CO.

Gas turbine manufacturers have come up with dry low-NOx (DLN) combustors, which limit the NOx to single-digit levels. Most of the NOx emitted by a gas turbine firing natural gas is generated by the fixation of atmospheric nitrogen in the flame, and the amount of this ‘‘thermal NOx’’ is an exponential function of flame temperature. The DLN combustor lowers the flame temperature by burning a leaner mixture of fuel and air in premixed mode. To reduce NOx emissions in traditional combustors, steam or water is injected to reduce the flame temperature; benefits include additional power output. However, there is a loss in engine life and shortening of combustor life. CO formation also increases as the amount of water or steam increases, as shown iN Fig. 4.11.