Studies of soot oxidation and fragmentation in a two-stage burner under fuel-lean and fuel-rich conditions.

Abstract: In this work, soot oxidation and fragmentation studies were carried out experimentally in a two-stage burner, where soot was produced in a first-stage premixed burner, while in a second stage, the soot was oxidized under fuel-lean and slightly-rich conditions. Temperature, particle size distributions (PSD), and measurements of specific gas-phase compounds were performed to evaluate the effects of temperature, O₂, and OH on the evolution of soot size distributions during oxidation. Results for the leanest ethylene/air flame (Φ _overall =0.8), showed a decrease in particle mean diameter and an increase in particle number concentration for ultrafine-sized particles with increasing height above burner (HAB), which indicates fragmentation of the fine-sized particles. At higher HAB, the soot oxidation process was dominated by soot burnout as confirmed by the decrease in number and mass concentration. The fuel rich condition (Φ _overall =1.14) did not show particle fragmentation; in fact, the size distribution was governed by soot burnout starting at burner surface. Measurements of gas-phase compounds, O₂, CO, CO₂, H₂, validated a detailed kinetic model, which was used to predict OH concentration in the secondary flame. Using the concentrations for O₂ and OH , soot oxidation rates were calculated and compared with experimental data. Furthermore, the regions where O₂ and OH were dominating the process of soot oxidation were identified. Soot oxidation via O₂ was favored under fuel lean conditions close to the burner surface. Oxidation via OH appeared to be the major path for soot oxidation under near-stoichiometric and fuel-rich conditions after O₂ concentrations dropped, and faster soot oxidation rates were obtained. [ABSTRACT FROM AUTHOR]