On Soot Reduction Using Oxygenated Combustion in High Pressure Counterflow Flames

Reduction of NOx and soot emissions from combustion systems has been a major driver for combustion research in recent years. A promising approach for reducing soot in non-premixed flames is based on simultaneously using an O2-enriched oxidizer stream and a N2-diluted fuel stream. The effectiveness of this approach is due to the fact that it modifies the stoichiometric mixture fraction (st) without significantly altering the adiabatic flame temperature. The present work involves a computational study on oxygenated non-premixed counterflow flames burning different small hydrocarbon fuels (ethylene, propene and propane) at atmospheric and high pressures (ethylene). The computational model employs a validated reaction mechanism with 197 species and about 5000 reactions for gas-phase chemistry, and a fairly detailed soot model. Results focus on the effect of oxygenation, fuel unsaturation, and pressure on the flame structure, soot precursors and soot emissions. At a given pressure, as st is increased, there is 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 presence of double bond (C=C) leads to higher soot emissions, with the amount of soot formed being the highest in propene flames, followed by ethylene flames, and then propane flames. Dominant reaction paths are analyzed to examine the relative roles of double bond, hydrodynamics and changes in flame structure on PAHs and soot emissions. The drastic reduction in PAHs and soot formation can be attributed to both the hydrodynamic and flame structure effects. While the oxygenated combustion in reducing soot is also effective at higher pressures (1-8 atm), the effect of increasing pressure at a fixed st is to increase the PAHs and soot emissions.