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 (Φoverall=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 (Φoverall=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.