Effects of oxygen-enrichment and fuel unsaturation on soot and NOx emissions in ethylene, propane, and propene flames

We have performed a computational study on the effects of oxygen enrichment1 and fuel unsaturation on the flame structure, PAHs, soot, and NOx emissions. Counterflow flames burning ethylene, propane, and propene are simulated with CHEMKIN-Pro, using a validated mechanism with 197 species and around 5000 reactions. The stoichiometric mixture fraction (ζst) is varied by simultaneously using O2-enriched airstream and N2-diluted fuel stream such that the adiabatic flame temperature is nearly constant. Dominant reaction paths are analyzed to examine the relative roles of hydrodynamics and changes in flame structure on PAHs and soot emissions. As ζst is increased, results indicate a significant reduction in acetylene and PAHs formation, and with additional soot oxidation in the post flame region, it leads to a nearly non-sooting flame. The drastic reduction in PAHs and soot formation can be attributed to both the hydrodynamic and the flame structure effects. At moderate oxygenation levels, changes in flame structure seems to play a more prominent role, while at higher oxygenation levels, the hydrodynamic effect seems to be more important. With the increase in ζst, the O, OH, and H radical pool is enhanced, and, consequently, the intermediate species (propargyl, allene, and propyne) are reduced to smaller hydrocarbons, decreasing the formation of PAHs and soot. With further increase in ζst, the flame location shifts from oxidizer to fuel side, and, consequently, PAH species and soot get oxidized in the oxygen rich region, leading to nearly soot free flames. However, as ζst is increased, NO emissions increase monotonically. At low ζst values, the prompt route contributes more to NO formation, while at high ζst values, the thermal route contributes more. The rate of production analysis indicates that the presence of double bond promotes reactions which produce higher amounts of allyl and propargyl species, and thus higher amounts of soot precursors; benzene and pyrene. Consequently, propene and ethylene flames produce significantly higher amount of soot compared to propane flames.