A semiempirical technique for predicting the NOx emission index from the combustion of distillate type fuels with air was developed. This technique was devised to help evaluate combustion modification procedures for lowering NOx emissions. Equilibrium calculations, generally used to obtain directional estimates of pollutant concentration, can lead to errors. Some possible pitfalls in using equilibrium calculations are illustrated. The semiempirical technique is based on chemical kinetics and neglects fluid-dynamic effects. The kinetics are based on the modified Zeldovich chain mechanism for NO production from hot air and Fenimore’s data for “prompt NOx”. The resulting expression lends itself to hand calculation provided the nitric oxide equilibrium value is known at the temperature and pressure of interest. Excellent agreement was obtained with experimental results from gas turbines. The apparent time required to produce NOx was the only adjustable parameter used to fit the data. A large volume of data from aircraft gas turbines was correlated by assuming an apparent residence time of 0.5 millisec. The effectiveness of water addition in minimizing NOx emissions was predicted for a model of an industrial gas turbine using 2 millisec residence time. These residence times are somewhat short but physically reasonable. The calculations predict that the maximum NOx emission index shifts from stoichiometric combustion to lean combustion as air preheat temperature is lowered. This prediction has not been confirmed experimentally.

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