Nitrogen Sources and Formation Pathways of NO in Ammonia Combustion: The Effect of Hydrogen Blending Ratio

Hydrogen (H 2 ) addition can enhance the reactivity and stability of ammonia (NH 3 ) combustion but could increase both combustion temperature and NO x emissions. In particular, it strengthens interactions between NH 3 -derived NO and air’s N 2 -derived NO pathways, making NO source attribution and evolution unclear. This study investigates the NO formation in premixed NH 3 /H 2 combustion using a novel nitrogen source-differentiated kinetic model, combined with perfectly stirred reactor (PSR) calculations and computational fluid dynamics (CFD) simulations of a premixed furnace. It is demonstrated that NO is dominated by the NH 3 -derived pathway at low temperatures, while the N 2 -derived contribution grows with temperature. Although hydrogen addition generally promotes NO formation, a specific temperature range exists in PSR where the total-NO changes slightly or even decreases with increasing H 2 blending ratio, due to enhanced NO consumption offsetting part of the NO increment. When the H 2 fraction increases from 0% to 40% under equivalence ratio of 1.0, total-NO decreases by up to approximately 6.5% within this regime. Kinetic analysis shows that this behavior results from two coupled effects: enhanced NO consumption through N–N* coupling reactions, especially N* + NO ↔ N*N + O and N + N*O ↔ N*N + O, and suppressed NH 3 -derived NO formation through the HNO route (NH 2 /NH → HNO → NO) due to the reduced NH 3 concentration in the fuel mixture. CFD results further confirm this trend under practical flow and mixing conditions. At a furnace wall temperature of approximately 2100 K, increasing the H 2 fraction from 10 to 30% reduces the outlet total-NO by approximately 12.6%. The N 2 -derived pathway also promotes NH 3 oxidation and burnout at high temperatures. This study provides new mechanistic insights and theoretical support for low-emission ammonia-hydrogen combustion.