Investigation of the formation mechanisms of nitrogen-based pollutants in ammonia-diesel dual-fuel engines by decoupling dilution, thermal, and kinetic effects

Abstract

This paper investigates the dynamic formation and conversion mechanisms of nitrogen-based pollutants in ammonia-diesel dual-fuel engines, where ammonia (NH₃) is introduced via port fuel injection. The dynamic mechanisms behind pollutant formation in this context are not well understood, primarily due to the focus in existing literature on decoupling thermal-type and fuel-type nitrogen-based pollutants in three-dimensional (3D) computational fluid dynamics (CFD) simulations of ammonia-diesel operation, which distorts the natural spatiotemporal distribution and kinetic interactions of nitrogen-based pollutants, thus falling short of fully revealing the real dynamic mechanisms. Rather than distinguishing the nitrogen sources from ammonia or atmospheric nitrogen, this study proposes an innovative approach by decoupling the dilution, thermal, and kinetic effects of ammonia in the 3D CFD model, enabling a detailed analysis of the dynamic formation and conversion mechanisms of nitrogen-based pollutants. Given the significant impact of ammonia kinetic effects, the study also introduces a novel method of partial closure of ammonia kinetics to examine the influence of gas movement. The results indicate that ammonia kinetics do not significantly alter the regions where nitrogen-based pollutants form (i.e., nitrogen oxides (NOx) in pure diesel engines), as ammonia oxidation occurs co-combustively with diesel in regions reached by the diesel plume in ammonia-diesel dual-fuel operation. However, ammonia kinetics affect the types and concentrations of nitrogen-based pollutants in these regions, such as increasing the concentration of nitrogen monoxide (NO) in high-temperature zones. The momentum of the bulk gas generated by the spray jet, piston downward movement, and combustion pushes the hot combustion products toward the unburned ammonia-air mixture, allowing NO formed during the main combustion stage and NH₃ remaining from the main combustion stage to meet, facilitating the de-NOx effect of ammonia, which results in nitrogen (N₂) formation when the process is locally efficient and nitrous oxide (N₂O) formation when it is not. Within the combustion chamber, N₂O is found in cooler regions with limited NO concentration. Some of the N₂O comes from the flow of N₂O surviving from the main combustion stage, while the remainder is generated by the inefficiency of the ammonia de-NOx effect during the late combustion stage. In the exhaust gases, the concentration of N₂O is comparable to NO, making it a major nitrogen-based pollutant. Although nitrogen dioxide (NO₂) is not a major component in the exhaust, it plays a crucial role in N₂O formation, as evidenced by its spatiotemporal distribution, which mirrors that of N₂O. Overall, the turbulent gas flow and ammonia de-NOx kinetics together influence the spatiotemporal distribution of nitrogen-based pollutants after the main heat release stage inside the combustion chamber, resulting in a complex composition and concentration of nitrogen-based pollutants that need to be considered for aftertreatment control.