Numerical study on the temperature dependence of soot formation in acetylene pyrolysis blended with methane, formaldehyde, methanol, and dimethyl ether

This paper addresses the combined effects of varying C/H and C/O ratios as well as of the molecular structure of the fuels selected on the normalized soot volume fraction f V. For the simulations, an already existing and validated reaction mechanism for the pyrolysis of C2H2 in argon, Aghsaee et al. (Combust. Flame 2014, 161, 2263–2269), was used in the current work. It was extended with PAH reactions from coronene (C24H12) up to ovalene (C32H14), whereas general principles for the rapid build-up of large PAHs were presented. Soot formation was modeled according to Appel et al. (Combust. Flame 2000, 121, 122–136) by applying the method of moments. A validation of the extended reaction model was carried out for shock-wave-induced O2/C2H2 mixtures from literature. In the following, the influence of blends of methane (CH4), formaldehyde (CH2O), methanol (CH3OH), and dimethyl ether (CH3)2O on soot formation during C2H2 pyrolysis diluted in Ar was studied. Special emphasis was laid on the inception chemistry of soot formation. The role of intermediates, such as the propargyl radical (C3H3), leading towards benzene and polyaromatic hydrocarbon (PAH) formation and their interplay with hydrogen molecules (H2) to H atoms (H) ratio was examined. All blends increased the ratio of the concentrations of H2 and H leading thus to reduced soot inception and soot formation. However, soot suppressing effects were overrun by supporting ones when the additives provided suitable molecular groups, such as methyl radicals (CH3), in sufficient high concentrations for early aromatic ring formation. Thus, a prominent synergistic effect on soot formation was found for the CH4/C2H2 mixture only. Besides, species able to mirror characteristics of the soot formation process, such as the peak value of the normalized soot volume fraction, are presented. The findings of this work indicate the synergistic effect of H2/H and C/O ratios as well as of methyl radicals on the PAHs’ production of appropriate size able to initiate soot inception process in an aliphatic fuel.