An experimental and kinetic modeling study of NH₃/n-decane laminar diffusion flames

Combustion-generated soot harms health and the climate; adding ammonia (NH₃) is being explored as a low-carbon countermeasure. This study explores the effects of NH₃ addition on soot and polycyclic aromatic hydrocarbons (PAHs) formation in n-decane laminar co-flow diffusion flames. Laser induced incandescence and laser induced fluorescence are used to measure soot, PAHs and OH. A detailed kinetic mechanism is developed to simulate soot formation and the chemical role of NH₃. Increasing the NH₃ blending ratio from 0 % to 40 % leads to a marked reduction in soot and PAH signals. The soot peak moves upward and becomes more radially diffuse. The LIF intensities of A1, A2-A3 and A4 decrease by about 45 %, 20 % and 15 %, while the OH-PLIF peak also decreases with little change in shape. A cross fuel comparison with methane, ethylene and n-heptane data shows that, at similar NH₃ levels, soot and large PAHs are suppressed much less in n-decane flames than in small fuel flames. Numerical simulations based on rate-of-production and sensitivity analyses demonstrate that NH₃ changes the routes that control aromatic formation. The dominant A1 formation reaction via propargyl recombination is notably weakened in NH₃-containing flames. At the same time, the oxidation of C₃H₃ by OH and the decomposition of C₃H₅ to C₂H₂ and CH₃ are enhanced, promoting the consumption of PAH intermediates. Sensitivity analysis reveals that these inhibitory reactions exhibit increased negative sensitivity under NH₃-rich conditions, indicating that NH₃ strengthens the influence of reactions unfavorable to aromatic growth. Additionally, NH₃ consumes reactive radicals such as H, OH, and CH₃, reducing their availability for hydrocarbon chain propagation and ring formation. This means that the main control of A1 changes from C₃ and C₄ growth reactions to C₃ consumption reactions. Correspondingly, the concentrations of key precursors C₂H₂ and C₃H₃ are significantly reduced with increasing NH₃ blending ratio. Model results further show that moderate NH₃ fractions already give most of the soot reduction, whereas higher fractions mainly increase NO, NO₂ and N₂O, revealing a soot-NO_X trade off. The present work clarifies how ammonia influences soot formation and offers insights for controlling soot in zero-carbon combustion. Novelty and significance statement: This study presents the first combined OH-PLIF and LII diagnostics applied to n-decane/ ammonia laminar diffusion flames, systematically investigating how ammonia addition affects soot and polycyclic aromatic hydrocarbons formation. Additionally, a kinetic model consisting of 423 substances was developed, and it was proven that this model can accurately capture the experimental trends and elucidate the chemical pathways by which ammonia inhibits the growth of polycyclic aromatic hydrocarbons. Its significance lies in demonstrating that ammonia acts as an effective soot suppressant, providing molecular-level insights for designing low-soot amino fuels and advancing carbon-neutral combustion technologies.