On Soot Reduction Using Oxygenated Combustion in Counterflow Diffusion Flames

Abstract

Reduction in NO_x and soot emissions from combustion systems has been a major driver for combustion research in recent years. A promising approach for substantially reducing soot in nonpremixed flames is based on simultaneously using an oxygen-enriched oxidizer stream and nitrogen-diluted fuel stream. The effectiveness of this approach is due to the fact that it modifies (increases) the stoichiometric mixture fraction ($\zeta_{\text{st}}$) without significantly altering the adiabatic flame temperature. In this chapter, we discuss computational results examining this strategy for small hydrocarbon fuels in flames at atmospheric and high pressures. The computational model employs a 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 and pressure on the flame structure, soot precursors, and soot emissions. At a given pressure, as $\zeta_{\text{st}}$ is increased, it leads to a noticeable reduction in acetylene and PAHs formation, and due to increased soot oxidation in the post-flame region, a nearly non-sooting flame can be achieved. Such drastic reduction in PAHs and soot is attributed to both the hydrodynamic effect and the change in flame structure. While the oxygenated combustion in reducing soot is also effective at higher pressures (1–8 atm), the effect of increasing pressure at a fixed $\zeta_{\text{st}}$ is to increase the PAHs and soot emissions. The presence of double bond (C=C) also leads to higher soot emissions, and consequently, the soot formation is the highest in propene flames and then in ethylene flames followed by propane flames.