PAH formation characteristics in hydrogen-enriched non-premixed hydrocarbon flames

The utilisation of hydrogen with conventional hydrocarbons offers an excellent opportunity to decarbonise current energy systems without significant hardware upgrades. However, this presents fresh scientific challenges, one of which is the difficulty in effective control of pollutant soot emissions due to complex reaction kinetics of hydrogen enriched flames. This paper focuses on polycyclic aromatic hydrocarbons (PAHs), which are the building blocks of soot and responsible for its carcinogenicity. Detailed understanding of the effect of H-2 on the underlying processes of PAH formation and growth is important for the development of effective strategies to curtail PAH formation and hence, reduce soot emissions from combustion systems. In this study, an experimental methodology was employed to analyse PAH formation and growth characteristics of laminar inverse diffusion flames of various hydrocarbon fuels (alkanes and alkenes) enriched with H-2 using simultaneous planar laser induced fluorescence (PLIF) imaging of PAHs and hydroxyl radicals (OH). OH PLIF was used to indicate peak temperature locations in the flame (flame front), while PAH PLIF was used to determine PAH formation characteristics. Methane (CH4) was also separately added to the same hydrocarbon fuels to study effects of carbon-bound hydrogen addition, in comparison to H-2 addition. It was observed that only the addition of H-2 to CH4 showed significant variation in the magnitude of PAH reduction levels as the length along the flame front, L-f increased. The results also showed that while the addition of H2 was more effective in reducing the rate of PAH fluorescence signal increase (indicative of concentration growth) when compared to CH4 addition, both fuels showed two distinct regions in the PAH growth curve; a steep growth region followed by a slower growth region. This is potentially indicative of the self-limiting nature of PAH formation and growth. The study concluded that the growth rate of PAHs lies within a narrow band irrespective of the fuel bonding, molecular structure and the H:C ratio of the fuel mixtures tested.