UV absorption, laser excited fluorescence and direct sampling in the study of the formation of polycyclic aromatic hydrocarbons in rich CH4/O2 flames

Abstract

UV absorption and laser excited visible fluorescence techniques have been used to analyze systematically the early regions of atmospheric pressure, premixed flat CH4/O2 flames. A range of CH4/O2 ratios (R) from 0.1 to 1.27 was explored and different flow rates and inert addition were also considered for the sooting R=1.27 flame. Soot-forming flames show fluorescence and visible absorption profiles, which increase higher in the flame and with increasing CH4/O2 ratio; the UV absorption spectra evidence broad peaks in the 220–250 nm range, which are shifted toward the red for richer mixtures. A new sampling and collecting method allowed us to follow the formation of individual polycyclic aromatic hydrocarbons (PAH) immediately downstream of the reaction zone. Almost down to the reaction zone, the relative concentration of two and three ring PAH is quite high and then undergoes a rapid decline downstream, where four ring PAH are predominate; the concentrations of 5–7 ring PAH are almost negligible. The influence of the probe on the PAH concentration levels was also investigated using fluorescence emission. These experimental results and the spectral evidence suggest that the fluorescence is due to four ring PAH, whereas the UV absorption peaks, downstream of the reaction zone, may be due to two- and three ring PAH. The fluorescence profiles evidence a very early initial formation of PAH which also takes place in the preheating region near the reaction zone.