Enhancing ammonia combustion: Study of laminar burning velocity, flame instability, and NOx emissions under oxygen-enriched conditions

Abstract

This study aims to characterize the premixed combustion of ammonia, a promising zero-carbon fuel, under oxygen-enriched conditions to guide its application in clean energy systems. Laminar burning velocity (LBV) was measured using the constant pressure method of the spherically expanding flame across a range of oxygen concentrations (25 %, 30 %), equivalence ratios (0.8–1.2), initial temperatures (313–373 K), and pressures (1–3 bar). The results indicate that LBV is significantly enhanced under oxygen enriched conditions, with the promoting effect of oxygen enrichment being even more pronounced at higher pressures. The oxygen enriched condition accelerates the dominant reaction pathway NH₃ → NH₂ → NH → N → N₂, leading to increased NO_X emissions. However, high pressure reduces NO_X emissions by promoting NO consumption, while elevated temperature slightly increases NO_X formation. Emissions reach a minimum at an equivalence ratio of ϕ = 1.2. The combined analysis of LBV and emission characteristics confirms the feasibility of employing oxygen-enriched combustion strategies for ammonia in practical high-temperature and high-pressure applications. An increase in oxygen content, initial temperature, and initial pressure all reduce flame thickness and raise the adiabatic flame temperature, thereby intensifying hydrodynamic instability. Nevertheless, thermal diffusion stabilizes the flame surface front. Buoyancy instability becomes significant under low flame speeds and can be suppressed by high oxygen concentration, high temperature, and lean-fuel mixture conditions, whereas high pressure and rich mixtures promote its development. These findings provide a theoretical foundation for designing and optimizing future engine technologies using ammonia combustion, demonstrating how to balance high performance with low emissions and advancing the development of zero-carbon power technology.