Effects of ammonia addition on the soot nanostructure and oxidation reactivity in n-heptane/toluene diffusion flames

Abstract

Co-firing NH₃ with conventional hydrocarbon fuels is an important approach for reducing CO₂ emissions in existing combustion systems. Besides CO₂, the blending of NH₃ would also notably affect soot formation and its oxidation behaviors. In the present study, we focus on the effects of NH₃ on the nanostructure and oxidation characteristics of soot produced in diffusion flames of n-heptane/toluene mixtures. Two configurations of laminar co-flow diffusion flame, including both normal and inverse diffusion flames (NDF and IDFs), were used for investigation. High-resolution transmission electron microscopy (HRTEM), Raman spectroscopy (Raman), and Thermogravimetric analysis (TGA) were employed for soot characterization. The HRTEM and Raman spectra showed that with the increase of NH₃ blending ratio, the fringe length (L_a) and the degree of graphitization decreased while the microcrystal tortuosity (T_f) increased. The results are in consistent with TGA analysis which suggests the promoting effects of NH₃ on the soot oxidation reactivity. Difference between NDF and IDF with respect to the soot nanostructure and oxidation activity were discussed. It is our hope that the present results could deepen our understanding on the effects of NH₃ on soot nanostructure and oxidation behavior and benefit the design of particulate filters for combustion devices fueled with hydrocarbon/NH₃ mixtures.