PAH formation in the pyrolysis of benzene and dimethyl ether mixtures behind shock waves

Abstract

In this work the laser-induced fluorescence (LIF) method was used to study the formation of polyaromatic hydrocarbons (PAHs) in the pyrolysis of benzene C6H6 and dimethyl ether C2H6O (DME) mixtures diluted with argon behind reflected shock waves within a temperature range of 1300–2000 K and pressures of 3.5–5.7 bar. To monitor the volume fraction of the condensed phase, laser extinction at a wavelength of 633 nm at the temperature range 1600–2500 K was measured. The LIF spectra of PAHs formed at varying temperatures and at different times relative to the onset of the pyrolysis process were recorded. It was found that the LIF spectra shift toward longer wavelengths with increasing temperature and reaction time. It has been shown that PAHs with four or more aromatic rings are the precursors of the appearance of the condensed phase. In mixtures with DME, greater amounts of small aromatics (consisting of one-two rings and fewer larger PAHs) were detected relative to benzene pyrolysis. Besides, the DME presence significantly reduced the optical density of the mixture at 633 nm, indicating a decrease in the soot yield. Modeling of PAHs and soot growth for the conditions of experiments using the modern sectional kinetic mechanism was carried out. The simulation results have demonstrated a change in PAH formation pathways that depend on fuel composition and temperature. The effect of DME on the growth of PAHs in benzene pyrolysis is both a decrease in temperature due to heat consumption during the decomposition of DME and the appearance of additional pathways for small PAHs formation and, at the same time, a decrease in the yield of heavier compounds consisting of four or more aromatic rings.