Recent advances in soot formation mechanisms: Oxidation and oxidation-induced fragmentation

Abstract

Emissions of soot particles are solid products of incomplete combustion of hydrocarbons, which is generally considered to induce serious inverse effect to environment and climate. The main process of soot formation is essentially the result of the competition between the growth and oxidation of soot. Soot particles react with the oxidant during the oxidation process, resulting in substantial changes in the morphology, nano-structure, and particle size distribution of soot particles. Meanwhile, soot oxidation-induced fragmentation can be regarded as an accidentally but unavoidable secondary consequence of soot oxidation, which is of great significance in determining the particle burnout rate correctly. Quite a few of previous studies including numerical and experimental work are summarized in this paper, mainly on the oxidation process of soot produced by the incomplete combustion of hydrocarbon fuels. The external oxidation mode, internal oxidation mode, and oxidation-induced fragmentation are reviewed as well. Regarding the oxidation-induced fragmentation, it is summarized in this paper the mechanisms, characteristics, and its related phenomena. In addition, the numerical models and simulation approaches of oxidation-induced fragmentation are presented, along with the deficiencies in numerical simulations and experiments related to internal oxidation and the fragmentation of primary particles due to internal oxidation. Finally, the influence factors on the tendency towards external oxidation or internal oxidation of soot are summarized, and the double-peak structure of soot particle size distribution caused by soot oxidation and oxidation-induced fragmentation is discussed in detail.