Quantitative investigation of the formation of oxygenated aromatics in an anisole-doped flame

Recent studies have demonstrated that several oxygenated aromatics, including oxygenated polycyclic aromatic hydrocarbons (OPAHs) possessing different functional groups, are formed during anisole combustion. However, a quantitative analysis for these species is still very limited. This limitation inhibits the development and validation of formation kinetic mechanisms for these toxic air pollutants. This study addresses this gap by investigating quantitatively, a fuel-rich anisole-doped laminar premixed flame stabilized on a Holthuis burner at atmospheric pressure with an equivalence ratio of 1.90. Gas samples were extracted from the flame using a quartz nozzle and analyzed by gas chromatography (GC) preceded by a special online pre-concentration trap system, which decreases the detection limit by a factor of over 1000 compared to a conventional GC. Major species (reactants, CO, etc.), 32 small intermediates (C₁-C₅ like formaldehyde, acetaldehyde, acetylene, cyclopentadiene, etc.), 12 non-oxygenated aromatics (benzene, naphthalene, phenanthrene, etc.), and especially 24 oxygenated aromatics (phenol, 2,2-biphenol, dibenzofuran, 9H-xanthene…) including several OPAHs at ppb concentration levels were quantified. Interestingly, flame structure analysis shows that oxygenated aromatics peak closer to the burner surface as compared to non-oxygenated aromatics. The number of rings for non-oxygenated aromatics was observed to increase with the height above the burner, indicating that one-ring aromatics form before two- or three-ring aromatics. However, this is not always the case for oxygenated aromatics. Three-ring OPAHs are almost as abundant as the three-ring PAHs in terms of quantity, some three-ring OPAHs are even twice as abundant as their analogous PAHs (e.g., benzofuran vs fluorene, 9H-xanthene vs anthracene, etc.), which emphasizes their importance and certainly implies that these species need to be considered in kinetic studies. However, unlike PAHs, only less than half of the quantified OPAHs are currently present in literature models for anisole combustion.