Influence of ammonia blending on soot formation and nanostructure in laminar n-decane diffusion flames

Ammonia blending with hydrocarbon fuels has drawn increasing attention as a strategy for reducing soot emissions and mitigate combustion related particulate pollution. In this study, the influence of ammonia addition on soot formation and structural evolution was investigated in laminar coflow diffusion flames of n- decane. Ammonia was blended at 0–40% volume fraction, with the carbon mass flow rate maintained constant to minimize thermal variations and emphasize chemical effects. Flame imaging and axial thermocouple measurements showed that ammonia increased the visible flame height and enlarged central dark zone, while peak flame temperatures decreased by less than 5%, indicating a limited thermal influence. Quantitative soot volume fraction measurements further confirmed that ammonia addition significantly reduced soot concentrations throughout the flame. Soot samples collected at 2, 3, and 4 cm above the burner were characterized by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Ammonia addition suppressed primary particle growth, with the mean particle diameter at 3 cm decreasing from 23 nm (N00) to 19 nm (N40). HRTEM analysis revealed that ammonia blending reduced fringe length by up to 40%, while increasing fringe tortuosity, and widened inter-fringe spacing. The increased occurrence of multicore structures and reduced internal layering indicated less ordered soot, has typically associated with higher oxidative reactivity. Numerical simulations indicate that NH₃ suppresses benzene formation primarily by weakening the propargyl recombination pathway, while the increased formation of CN and HNCO reflects enhanced C–N chemistry and the redistribution of reactive carbon, thereby reducing the formation of A1 and subsequent PAH growth. These findings highlight the dual role of ammonia in suppressing soot formation and modifying soot nanostructure in ammonia-hydrocarbon combustion systems, providing insight into the development of cleaner low-emission combustion technologies.