| Soot particles generated by fossil fuel burning are harmful to the global climate, atmospheric visibility, and human health. So in order to reduce particulate emissions, the study on soot formation mechanism has become research focuses. Because of high temperature, high pressure and unstable heterogeneous combustion process in the engine cylinder, the formation mechanism of soot needs to be further studied and analyzed.A methane/air diffusion flame combustion system was established in this study. Soot was collected from flame at different heights above the burner(HAB) by using thermophoretic sampling particle diagnostic system(TSPD) and in-situ capillary sampling system(ICCS). Besides, the microstructure, surface functional groups and oxidation reactivity of soot in the flame were studied systematically using TEM, Raman, FT-IR, XPS and TGA to analyze the formation, evolution of soot in the diffusion flame.Results show that in the diffusion flame, soot formation process can be divided into: the growth stage of single particles, coagulation stage, aggregation stage and oxidation stage. With the increase in HAB, liquid-like particles gradually grow to carbonized mature soot aggregates. The fractal dimensions of agglomerates range from 1.42 to 1.69 and increase with the increase in HAB. All primary particles present unimodal size distributions, their mean diameters of primary particles are in a range of 7.84-22.57 nm and show a decrease after an increase with the increase in HAB. The mean fringe separation distance, the fringe length and tortuosity of soot are in ranges of 0.36-0.40 nm, 0.85-1.53 nm and 1.41-1.51 respectively. With the increase in HAB, the fringe separation distance and tortuosity decrease, and fringe length gradually increases. The ratio of AD1/AG in soot Raman experiments ranges from 2.00 to 2.45 and presents an increase after a decrease with the increase in HAB. This phenomenon suggests that the graphitization degree of soot firstly decreases and then increases, that is, the soot varied from a disordered microstructure to an ordered one in the diffusion flame.The equivalent ratios of aliphatic C–H surface functional groups(A2920/A1595) ranges within 0.46-2.18. The content of oxygenated C-OH and C=O surface functional groups are in ranges of 5.37-10.57% and 3.39-5.30% respectively, and they all show a decrease after an increase with the increase in HAB. These results indicate that the content of soot surface functional groups firstly increases and then decreases in the diffusion flame.The onset, the maximum combustion rate and the burn-out temperatures of soot are in ranges of 530.75-573.83 °C, 564.67-630.93 °C and 654.50-686.58 °C respectively. The apparent activation energies are in a range of 106.51-140.40 KJ/mol. All these parameters firstly decrease and then increase with the increase in HAB, showing that the oxidation reactivity of soot varies from an increase to a decrease. The microstructure and surface functional groups of soot have direct correlation with its oxidation activity, that is, more disorder and more surface functional groups could lead to smaller apparent activation energies and higher oxidation activity. |
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