The Application of Argon-Oxygen Atmosphere to Investigate the Formation and Evolution of Fuel Nitrogen Oxides During the Oxidation of Hydrogen-Ammonia Fuel Mixtures Under Representative Engine In-Cylinder Conditions

Abstract

Abstract In the face of climate change, reducing reliance on fossil fuels and transitioning to renewable energy sources is crucial. Ammonia, as a carbon-free and renewable fuel, exhibits significant potential as an alternative energy source. By incorporating hydrogen as an additive, its flammability can be improved to meet the requirements of existing spark ignition engines. However, comprehending the characteristics of nitrogen oxides (NOx) emissions resulting from the combustion of ammonia-hydrogen fuel blends poses challenges due to the contribution of both fuel NOx and thermal NOx. Therefore, gaining a comprehensive understanding of the fuel NOx behavior during ammonia-hydrogen combustion is of great importance. This paper focuses on investigating fuel NOx exclusively by utilizing an argon-oxygen atmosphere, which eliminates nitrogen from the oxidizer and its role in thermal NOx formation. The study conducts a fundamental investigation into the formation and evolution characteristics of fuel NOx during ammonia-hydrogen combustion under engine-like conditions. The findings indicate that fuel NOx acts as intermediate products, potentially originating from chemical equilibrium calculations. While fuel NO predominantly forms in the burning zone, it undergoes reduction in the burned zone, despite being the primary NOx species. N2O, absent in thermal NOx mechanisms, exhibits significant concentrations within the burning zone and is mostly converted to N2, resulting in limited N2O concentration in the ultimate NOx concentration. Lean burn conditions, hydrogen addition, and oxyfuel combustion all promote fuel NOx formation. Furthermore, the equivalence ratio influences the structure of the ammonia-hydrogen premixed flame due to the de-NOx effect of ammonia. Collectively, these findings highlight that fuel NOx mechanisms differ from thermal NOx mechanisms, necessitating distinct NOx reduction technologies to achieve low-NOx-emission spark ignition engines.