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

Abstract

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.

Experimental study of the influence of hydrogen as a fuel additive on the formation of soot precursors and particles in atmospheric laminar premixed flames of methane

“…The impact of H 2 on the formation of soot precursors and soot particles have also been the subject of numerous fundamental studies but mostly for mixtures of H 2 with fuels other than CH 4 [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]. By contrast, only a few similar studies, to the best of our knowledge, were addressed to characterize the impact of H 2 on the combustion of CH 4 flames [38][39][40][41]. Liu et al [38] and Xu et al [39] studied the effects of H 2 as a fuel additive (up to 40% comparing to the base fuel) on soot formation in diffusion CH 4 flames and showed that the addition of H 2 strongly decreases the formation of soot.…”

“…Liu et al [38] and Xu et al [39] studied the effects of H 2 as a fuel additive (up to 40% comparing to the base fuel) on soot formation in diffusion CH 4 flames and showed that the addition of H 2 strongly decreases the formation of soot. Ezenwajiaku et al [41] also investigated the impact of H 2 (added up to 20% in CH 4 ) on the formation of PAHs also in a diffusion flame. The authors studied the formation of PAHs by recording their Laser Induced Fluorescence (LIF) signals collected in the wavelength range 420-480 nm after 283 nm excitation.…”

In situ observation of the evolution of polyaromatic tar precursors in packed-bed biomass pyrolysis

“…In this study we use 20 mJ energy (increased from 12 mJ used in ref. 21) to improve the signal to noise ratio, while the effect of the laser beam profile on the PAH LIF was found to be insignificant at ranges used in the present work. The beam was formed into a light sheet with height and thickness of 50 mm and 0.15 mm respectively using a combination of right-angle prism, plano-concave cylindrical lens ( f = −40 mm) and bi-convex lens ( f = 500 mm).…”

“…The PLIF setup to detect PAHs has been explained in detail in ref. 21 and briefly described here. PAHs were excited using the 532 nm output from an Nd:YAG laser (Litron Nano PIV) to pump a tunable dye laser containing rhodamine 6G dye.…”

“…The fundamental wavelength from the dye laser was frequency-doubled and tuned to near 283 nm to give approximately 20 mJ of pulse energy output. In our previous work 21 we reported that the PAH LIF signal is independent of laser intensity variation beyond 8 mJ. In this study we use 20 mJ energy (increased from 12 mJ used in ref.…”