Impact of Ammonia Cracking on NOx Emissions in Turbulent Ammonia Combustion

This paper investigates the impact of ammonia cracking at varying equivalence ratios on NOx emissions during combustion. A series of CFD simulations in Ansys Fluent were conducted across cases representing different equivalence ratios and levels of ammonia cracking. Four parametric sweeps were performed: (1) varying the equivalence ratio for pure ammonia, (2) varying the ammonia cracking fraction at the equivalence ratio corresponding to peak NOx from the first sweep, (3) varying the equivalence ratio for fully cracked ammonia, and (4) varying the ammonia cracking fraction at the equivalence ratio corresponding to peak NOx from the first sweep while adjusting the fuel flow rate to maintain a constant heat release rate. Combustor design and operating conditions were adapted from reference articles that conducted lab-scale experiments with ammonia combustors to accommodate two-dimensional axisymmetric simulations for this investigation. NOx emissions increased with ammonia cracking, reaching a maximum near 60%, then declined sharply, with fully cracked ammonia producing at least an order of magnitude less NOx than the peak. These results suggest that fully cracking ammonia prior to combustion can substantially mitigate the high NOx emissions typically associated with ammonia fuel.