Numerical investigation of the hydrogen addition effect on NO formation in ammonia/air premixed flames at elevated pressure using an improved reaction mechanism

Ammonia (NH3) is emerging as a promising carbon-free fuel for power generation systems; however, its limitations in ammonia/air flames have led to the proposal of hydrogen as an additive to improve combustion performance. Understanding the impact of hydrogen addition on NO emissions in gas turbines is critical for optimizing combustion strategies. This study presents a revised chemical kinetic model, developed through sensitivity analyses, that accurately predicts ignition delay time, laminar burning velocity, and NO emissions. Numerical investigations were conducted on ammonia/air premixed flames at various equivalence ratios with hydrogen addition up to 60 % at 20 atm. Significant differences were observed between fuel-lean and fuel-rich flames. In fuel-lean flames (phi = 0.8-0.9), NO concentrations increased substantially, reaching approximately 2300 ppm, while in fuel-rich flames, NO concentrations remained stable, with variations of less than 100 ppm. To isolate temperature effects on NO emissions, an argon dilution approach was employed to maintain consistent maximum flame temperatures in both lean and rich ammonia/air flames with hydrogen addition. Key NO formation pathways, particularly involving HNO and NH intermediates, were identified. In fuel-lean flames (phi = 0.8), NO formation was dominated by HNO-related reactions, with hydrogen addition expanding the radical pool and amplifying these pathways, especially through the reaction of HNO with H radicals. In fuel-rich flames (phi = 1.2), net NO production remained stable up to 76 % hydrogen content, where NO formation and consumption were balanced. Beyond this point, NO production surged due to an increasing imbalance between NO formation and consumption.