Characteristics of Fuel-Borne Nitrogen Pollutants in Hydrogen–Ammonia Mixtures Using Argon–Oxygen Atmosphere Under Engine Conditions

Amid climate change, reducing reliance on fossil fuels and transitioning to renewable energy sources is crucial. Ammonia, a carbon-free and renewable fuel, shows significant potential as an alternative energy source. By incorporating hydrogen as an additive, its flammability can be enhanced to suit existing spark-ignition engines. However, understanding the characteristics of nitrogen pollutant emissions (i.e., NO_x, which includes NO and NO₂, and N₂O) from ammonia–hydrogen combustion is challenging due to contributions from both fuel-borne and air-borne nitrogen. However, understanding the characteristics of nitrogen pollutant emissions from ammonia–hydrogen combustion is challenging due to contributions from both fuel-borne and air-borne nitrogen. Therefore, a comprehensive understanding of fuel-borne nitrogen pollutants during ammonia–hydrogen combustion is essential. This study focuses on investigating fuel-borne nitrogen pollutants in argon–oxygen atmosphere, thereby eliminating nitrogen from the oxidizer and its role in thermal NO_x formation. The research examines the formation and evolution of fuel-borne nitrogen pollutants during ammonia–hydrogen combustion under engine-like conditions. Results indicate that fuel-borne nitrogen pollutants act as intermediates, potentially originating from chemical equilibrium. While fuel NO predominantly forms in the burning zone, it undergoes a reduction in the burned zone. N₂O, absent in thermal NO_x mechanisms, shows significant concentrations in the burning zone and is mostly converted to N₂, leading to limited N₂O in the final fuel-borne nitrogen pollutant concentration. Lean-burn conditions, hydrogen addition, and oxyfuel combustion promote fuel NO_x formation. Additionally, the equivalence ratio affects the ammonia–hydrogen premixed flame structure due to the de-NO_x effect of ammonia. Overall, these findings highlight that fuel-borne nitrogen pollutant mechanisms differ from thermal NO_x mechanisms, necessitating specially designed reduction technologies for clean spark-ignition engines.