Study of polycyclic aromatic hydrocarbons (PAHs) in hydrogen-enriched methane diffusion flames

Polycyclic aromatic hydrocarbons (PAHs) are the carcinogenic components of soot. Detailed understanding of PAH formation characteristics is required for development of effective strategies to curtail PAH formation and reduce soot in combustion devices. This study presents an experimental methodology to analyse PAH formation characteristics of a non-premixed methane-air flame with and without hydrogen (H₂) addition, using simultaneous planar laser induced fluorescence (PLIF) imaging of PAH and hydroxyl radical (OH). OH PLIF was used to represent peak temperature regions in the flame front. One-dimensional, opposed-jet laminar non-premixed flame simulations were also carried out for the same fuel mixture conditions. This work describes comparison of trends from both sets of studies. PAH fluorescence intensity values were observed to increase with increasing height above burner, however this rate of increase reduced with H₂ addition. This observed rate of change in PAH fluorescence (that is, PAH growth characteristics) is indicative of the sooting potential of the fuel mixture. PAH fluorescence from experiments and PAH concentration from simulation show strong reduction with increase in H₂ addition. The percentage reduction in PAH fluorescence signal with H₂ addition closer to the burner tip was of a similar magnitude to that observed with flame simulations. The reduction in PAH with H₂ addition could be attributed to the reduction in acetylene and propargyl concentrations, and reduced H-abstraction rates, which reduced the availability of active sites for PAH growth. The proposed experimental methodology for PAH measurements can be readily applied to any fuel mixtures.