Characteristics of NO/NO₂/N₂O Formation and Distribution in Ammonia/<i>n</i>-Heptane Dual-Fuel Spray Combustion under Different Ammonia Concentrations

Ammonia, a zero-carbon fuel, shows promise for replacing fossil fuels in internal combustion engines to reduce the level of CO2 emissions. Using a dual-fuel model with a pilot diesel spray igniting the ammonia/air mixture is a practical approach for engine applications. However, adding ammonia leads to nitrogen pollutants like NO, NO2, and N2O. This study uses a validated large-eddy simulation model in OpenFOAM to examine ignition and pollutant formation in ammonia/n-heptane dual-fuel spray combustion at various ammonia concentrations (ϕNH3). First, the effects of ϕNH3 on the ignition of the n-heptane spray are investigated. Competition between ammonia and n-heptane for the OH radical leads to a nonlinear delay in n-heptane spray ignition with rising ϕNH3. This prolonged delay increases the NH3 mixing time during n-heptane spray combustion, affecting pollutant formation. The total masses of NO, NO2, and N2O increase with the increase of ϕNH3. Furthermore, pollutants presented notable characteristics in spatiotemporal distribution with the effects of turbulence and multistage ignition in the n-heptane spray. NO2 is concentrated in the n-heptane spray zone of ϕ > 1 and 900–1150 K, followed by N2O in the zone of ϕ > 1 and 1150–1500 K. NO distributes on the tip of the n-heptane spray, where the temperature and ϕ are around 2500 K and 1, respectively. Finally, chemical kinetic analyses for NH3 consumption present intricate interconversion pathways among NO, NO2, and N2O. The generation pathways of pollutants indicate that above 80% NO2 and N2O were generated via reactions related to NO in ammonia/n-heptane dual-fuel spray combustion. NO2 is mainly generated by the reaction that occurred below 1100 K, and N2O is dominated by the reaction that occurred below 1400 K. The findings of this study provide a fundamental understanding of pollutant formation in low-carbon ammonia/diesel dual-fuel spray combustion.