The effect of CO2/H2O on the formation of soot particles in the homogeneous environment of a rapid compression facility

Previous investigations show that soot particle volume fraction and number density were significantly reduced by exhaust gas recirculation (EGR) diluents CO2 and H2O. 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 CO2 (up to 9.5% volume fraction) for both C2H2 (1.00% volume fraction) and CH4 (1.85% volume fraction) fuels in homogeneous mixtures. Computed effect of H2O 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 CO2 on polycyclic aromatic hydrocarbons (PAH) and formation of nascent soot particles with negligible oxygen influence. Experimental results show that CO2 enhanced the soot inception rate when added to C2H2 but had an undetectable affect on CH4. Gas chromatography confirmed that CO2 increases CO mole fraction and reduces C2H2 fuel concentration. Chemical kinetic simulations showed that the C2H2 was being converted to soot precursors. CO2 enhanced the soot inception rate for C2H2 by producing OH radicals. Images of nascent soot particles produced in the presence of CO2 were used to determine the size of PAH molecules in the particles and particle morphology. Both attributes were similar to particles formed without CO2. CO2 had little impact on the long reaction pathway from CH4 to PAH molecules because H and CH3 radicals propagated these reactions more readily than OH radicals.