Control of nitrogen oxide emissions from power plant boilers

The control of nitrogen oxide emissions from power plant boilers is discussed. Pollutant NOx is essentially all in the form of nitric oxide and nitrogen dioxide. In the atmosphere of urban centers the concentration of NOx can be 50 to 100 times that of the natural background, which is only about 1 to 5 ppb. Frequently, control strategies are contradictory to good combustion practice. Basic information on the chemical and physical factors affecting NOx formation in combustion was developed and applied to combustion modification techniques to control NOx emissions. Lowering the level of excess air to limits set by achieving good combustion and avoiding the emission of unburned combustibles lowers NOx emissions. Reducing the load and the firing rate in boilers results in lower heat release rates which lower combustion zone temperatures and NOx emissions. Increasing the heat absorption rate quenches the combustion gases more rapidly resulting in reduced levels of NOx emissions. Decreasing the turbulence in the flame zone results in less intense combustion conditions, and reduced NOx emissions. The tangential firing design utilizes this principle to good advantage.