The effect of CO 2/H 2O on the formation of soot particles in the homogeneous environment of a rapid compression facility

Abstract

Previous investigations show that soot particle volume fraction and number density were significantly reduced by exhaust gas recirculation (EGR) diluents CO 2 and H 2O. However, these investigations were often convoluted by their experimental flame configurations and primarily focused on soot volume fraction rather than soot inception. To isolate the effects on soot inception and the corresponding chemistry, the current study measured the reactivity of CO 2 (up to 9.5% volume fraction) for both C 2H 2 (1.00% volume fraction) and CH 4 (1.85% volume fraction) fuels in homogeneous mixtures. Computed effect of H 2O on these and other fuels are also presented. Experiments were performed at high temperature (1640 K and 1770 K) and high equivalence ratios ( Φ = 55 and 75) to understand the effect of CO 2 on polycyclic aromatic hydrocarbons (PAH) and formation of nascent soot particles with negligible oxygen influence. Experimental results show that CO 2 enhanced the soot inception rate when added to C 2H 2 but had an undetectable affect on CH 4. Gas chromatography confirmed that CO 2 increases CO mole fraction and reduces C 2H 2 fuel concentration. Chemical kinetic simulations showed that the C 2H 2 was being converted to soot precursors. CO 2 enhanced the soot inception rate for C 2H 2 by producing OH radicals. Images of nascent soot particles produced in the presence of CO 2 were used to determine the size of PAH molecules in the particles and particle morphology. Both attributes were similar to particles formed without CO 2. CO 2 had little impact on the long reaction pathway from CH 4 to PAH molecules because H and CH 3 radicals propagated these reactions more readily than OH radicals.