Effect of pressure and fuel–air unmixedness on NOx emissions from industrial gas turbine burners

The effect of fuel–air unmixedness on NOx emissions from industrial lean premixed gas turbine burners fueled with natural gas is analyzed in the pressure range from 1 to 30 bar. The analysis is based on a model where NOx production is split, according to a Damköhler number criterion, into a “prompt” (fast) contribution generated within the very narrow instantaneous heat release region (flamelet) and a “postflame” (slow) one, generated in the combustion products. Using GRI3.0 chemical kinetics, it is found that (a) the prompt NOx contribution is approximately a factor of 3 less sensitive to adiabatic flame temperature variations than postflame NOx and (b) prompt and postflame NOx change with pressure respectively according to an exponent αPR≃−0.45αPR≃−0.45 and αPF≃0.67αPF≃0.67. It is shown that total NOx emissions change from being mostly of prompt type at 1 bar to being mostly of postflame type at 30 bar, so that the effect of fuel–air unmixedness on NOx emissions significantly increases with increasing pressure. The combination of these findings yields a negative NOx pressure exponent under fully premixed conditions across a rather large range of equivalence ratios but a positive one for levels of fuel–air unmixedness typical of industrial burners. This result is confirmed by the application of the NOx model in the large eddy simulation of the ALSTOM EV double cone burner, which gives, in line with experimental data, an NOx pressure exponent growing, with equivalence ratio, from ≃0.1 to ≃0.67.