NOx Reduction Through Swirl Injector for Hydrogen and Methane Combustion in Mild Conditions

Abstract The objective of this paper is to investigate the impact of low-swirl injectors (LSIs) on NOx and CO emissions on hydrogen and methane combustion under Moderate or Intense Low-Oxygen Dilution (MILD) conditions in stagnation point reverse-flow (SPRF) combustor for a micro gas turbine (mGT) application. Experimental and numerical analyses demonstrate that LSIs enhance fuel-air mixing, reduce localized peak temperature zones, and significantly lower NOx emissions compared to conventional straight injectors (SI). In hydrogen combustion, straight injectors create a highly intensified shear zone between fuel and air, resulting in high enthalpy release that extends still the end of flame tube. This leads to zones of significantly elevated temperatures due to localized combustion. Conversely, the LSI disrupts these shear layers, promoting more uniform burning in the flame tube. This controlled uniform burning reduces peak temperatures and leads to lower emissions. The optimized LSI for hydrogen combustion achieves stable flame behavior across start-up and nominal operating conditions, minimizing emissions while maintaining high combustion efficiency. For methane combustion, the sleeve enhances CO recombination by insulating low-temperature secondary flows, extending residence time. Strong recirculation in the Primary Mixing Zone (PMZ) significantly reduced peak temperatures inside the flame tube, where the position of the vortex in PMZ coincides with peak temperature zones for hydrogen and methane combustion. In the LSI, increased tangential velocity reduces PMZ recirculation strength (to maintain the same pressure drop), resulting in higher peak temperatures, decreased CO emissions, but elevated NOx levels. These findings highlight the potential of LSIs to support the transition toward sustainable hydrogen-based energy systems with reduced environmental impact.