Oxidation kinetic of soot generated from ammonia-acetylene laminar diffusion flame

Co-combustion of ammonia with hydrocarbon fuels and its effect on soot emission characteristics have garnered interest. In this study, the oxidation kinetics of soot generated in a laminar co-flow acetylene diffusion flame were investigated under 30 –800 °C temperature-programmed oxidation and isothermal oxidation at 500°C, 600°C, and 700°C using thermogravimetric analysis (TGA). The evolution of functional groups on soot surfaces and gaseous products were monitored by Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis coupled with infrared spectroscopy (TG-IR), respectively. Results indicate that the activation energy for soot oxidation increases with higher NH3 substitution ratios (XNH₃) and elevated temperatures. Isothermal oxidation tests also show that the oxidation rate constant increases with increasing XNH₃. FTIR results show that increasing XNH₃ reduces aliphatic C–H groups and increases oxygenated groups on soot surfaces. The detected C–N bonds are attributed to dehydrogenation of aliphatic carbon atoms on polycyclic aromatic hydrocarbons (PAHs) surfaces. TG-IR analysis revealed the C–N bonds in urethanes on soot surfaces may release as the gaseous C–N species during low-temperature (500 °C) oxidation of soot particles. Nevertheless, the higher-temperature facilitated the cleavage of C–N bonds, and then the generated NH₂ radicals react with oxygen radicals, leading to the formation of HNO on the soot surface.