Effects of ammonia addition on the physicochemical characteristics and nitrogen speciation of particulate matter emitted from diesel engine

As a carbon-free fuel, ammonia is considered a promising low-carbon alternative for reducing CO 2 emissions from diesel engines. However, its direct application in diesel engines is limited by poor auto-ignition characteristics and low laminar burning velocity. Therefore, ammonia/diesel dual-fuel combustion is a feasible approach for using ammonia in diesel engines. Ammonia contains approximately 82.4 wt% nitrogen. The introduction of ammonia into diesel combustion may change the physicochemical characteristics of particulate matter (PM) and affect the chemical states of nitrogen retained in the PM. In this study, PM emitted from diesel and ammonia/diesel dual-fuel combustion in a single-cylinder diesel engine was collected at 2700 r/min and 100% load. SEM/EDS, GC–MS, and XPS were used to analyze the microscopic morphology, elemental composition, soluble organic fraction (SOF), PAH composition, surface elemental composition, and chemical states of nitrogen in PM. The results showed that, compared with diesel PM, ammonia/diesel PM exhibited a looser agglomerated structure, a smaller primary particle size, and a lower total mass concentration of PAHs. In contrast, the relative proportion of nitrogen-containing organic compounds in the SOF changed only slightly. The average primary particle diameter decreased from 42.7 nm for diesel PM to 38.3 nm for ammonia/diesel PM. The total mass concentration of PAHs decreased from 562.8 mg/kg to 9.4 mg/kg. Among the detected PAHs, high-ring PAHs showed a more pronounced decrease. This indicated that ammonia addition suppressed PAH formation and PAH ring growth. XPS analysis further revealed the nitrogen retention characteristics of PM after ammonia addition. The surface N content increased from 0.55 at.% in diesel PM to 1.20 at.% in ammonia/diesel PM. After SOF removal, the surface N content of the ammonia/diesel soot fraction reached 2.26 at.%, approximately 2.57 times that of the diesel soot fraction at 0.88 at.%. These results indicated that a larger proportion of the increased nitrogen after ammonia addition was retained in the soot fraction after SOF removal. The N 1 s deconvolution results showed that pyrrolic N was the dominant nitrogen species, accompanied by small amounts of pyridinic N, graphitic N, and N-Ox species. These results reveal the inhibitory effects of ammonia addition on particle morphology evolution and PAH growth and clarify the retention characteristics and chemical forms of ammonia-derived nitrogen in PM. The findings provide an experimental basis for understanding the participation of nitrogen-containing intermediates in particle formation and surface chemical evolution in practical ammonia/diesel engines.