The effect of ozone on soot formation in partially premixed laminar methane/air flames

The effect of ozone addition on soot formation in partially premixed laminar methane flames at two equivalence ratios, Φ = 11.9 and Φ = 7.6 (with 500 ppm and 570 ppm of ozone, respectively), has been investigated. Soot particles collected in the centerline of the flames at several heights above the burner have been examined in terms of size, morphology, and chemical/structural characteristics, by differential mobility analysis, atomic force microscopy, and Raman spectroscopy respectively. The results show that adding ozone to the flame reduces the number of soot particles, as well as their average size. The typical height profiles of the collected particles on substrates are shown to be also flatter as compared to the pristine flames, possibly indicating a lower level of cross-linking in the aromatic network forming the particles. Also, the Raman spectroscopy analysis indicates that ozone addition promotes the formation of larger aromatic soot constituents. Moreover, within the selected experimental conditions and ozone concentration of several hundred parts per million, the flame temperature is unaffected regardless of the presence or absence of ozone. Consequently, it can be inferred that the observed modifications in soot characteristics are predominantly attributed to chemical factors. The observed effects are all consistent with a possible chemical interaction of atomic oxygen, resulting from the decomposition of ozone in the post-flame zone, with aromatic π-radicals, precursors of the soot particles.