Comparative Study of Combustion Characteristics of CH4, NH3–H2, and O2-Enriched NH3 Turbulent Flames

Abstract

Ammonia (NH3) is a promising carbon-free fuel, but its low flame speed, combustion instability, and high NOx emissions limit practical use. Hydrogen (H2) addition and oxygen enrichment enhance its reactivity and flame stability, making its combustion comparable to methane (CH4). This study presents a comparative investigation of turbulent nonpremixed flames of CH4, NH3–H2, and O2-enriched NH3 under matched laminar burning velocities. Experiments were conducted in a swirl-stabilized combustion chamber with radial fuel injection through eight ports. The swirl number was set to 1.4 and flame power maintained at 10 kW, with equivalence ratios ranging from 0.5 to 1.0. O2-enriched NH3 flames contained 42% O2, while NH3–H2 mixtures comprised 55.5% NH3 and 44.5% H2. Flame characteristics were analyzed using OH* and NH2* chemiluminescence imaging, while particle image velocimetry (PIV) captured the flow field. Exhaust emissions of NO, NO2, and H2 were quantified, alongside optical emission spectroscopy, and exhaust gas temperature was measured within the chamber. Results show that oxidizer composition strongly affects flame topology and emissions. O2-enriched NH3 flames exhibit compact, highly reactive structures with intense radical emissions and peak temperatures, promoting thermal NO formation. NH3–H2 flames reach CH4-like temperatures with better stability, demonstrating methanelike performance under carbon-free conditions. These findings highlight the potential of NH3–H2 blends as viable alternatives to conventional hydrocarbon fuels, offering stable, low-carbon, and efficient combustion.