Numerical investigation of the NOX emissions of a perfectly premixed NH3-H2 flame at moderate pressure levels

Blends of green hydrogen-ammonia for Gas Turbines (GT) are gaining more and more interest, as their carbon neutrality can abate CO2 emissions. Such fuel mixture could be particularly effective since the low reactivity of ammonia can compensate the aggressive properties of hydrogen in terms of both flame speed and low heating value, reducing the re-design effort of the traditional dry-low emission combustors. From the environmental perspective, the main drawback is represented by the NOx emissions, mostly related to the fuel-bound pathway activated by the cracking of NH3: its mitigation requires the proper control of the equivalence ratio of the blend along the selected ratio of ammonia and hydrogen in the fuel mixture. Additionally, at relevant GT conditions, a moderate benefit for NOx reduction can be played by the pressure rise. In this work, the effect of the operating pressure onto the NOx emission is investigated numerically in the range 1.1–2 bar along a perfectly premixed mixture of 25 % NH3 – 75 % vol. H2 at constant equivalence ratio, leveraging the corresponding experimental data. The tests employ a radial swirler whose performances are measured not only in terms of NOx but also through detailed flame imaging. The latter is used to evaluate the accuracy of a Thickened Flame Model (TFM) in predicting the flame shape and position. The TFM is based on a skeletal mechanism consisting of 27 species that embeds the OH* to directly compare the numerical line of sight with the images from the experiment and the NOx chemistry as well. Regarding the NOx, the numerical results show a reasonable accuracy: not only can the overall flame length be captured but also a quantitative estimation can be retrieved from the numerical model. Lastly, the impact onto NOx of the hydrogen preferential diffusion related to the flame curvature is discussed.