Fuel Flexibility of an Existing Natural-Gas Burner for NH3/H2/NG Co-Combustion: Flame Stability, Flame Characteristics, and NOx Emissions

Ammonia is a carbon-free energy carrier, but its low reactivity, limited flame stability, and potential for high NOx emissions hinder its use in existing natural-gas combustion systems. This study assessed the fuel flexibility of a pilot-scale natural-gas burner operated in a semi-open chamber with natural gas/hydrogen/ammonia blends and a highly N2-diluted H2/N2/NH3 test mixture matching a composition reported in the literature. Measurements included fuel flow rates, axial temperatures, visual assessment of flame stability, flue-gas composition, and image-based flame-color indicators. Stable combustion was maintained with mixtures containing up to 32 vol.% H2 and 21 vol.% NH3, including operation with 25 vol.% NH3 without hydrogen. Within the blow-off tests, ammonia reduced resistance to extinction, whereas hydrogen increased it. Increasing the ammonia fraction produced a more uniform axial temperature distribution and shifted the visible flame emission from blue towards yellow–orange. The H2/N2/NH3 test mixture was unstable without natural-gas support at the investigated operating point. NOx emissions increased markedly with ammonia addition. The results demonstrate the feasibility of using ammonia-containing fuels in existing natural-gas burners while highlighting the need for burner-specific stabilization and NOx-control strategies.