Synergistic effect on PAH and soot formation in ethylene counterflow diffusion flames by the addition of 1,3-dioxolane - a bio-hybrid fuel

Abstract

Bio-hybrid fuels combine the utilization of bio-based feedstocks and carbon dioxide with renewable electricity to achieve a carbon-neutral and low-emissions future for the transportation sector. A potential candidate is the heterocyclic acetal 1,3-dioxolane (C3H6O2). However, little is known about the mechanism behind the impact of 1,3-dioxolane on soot formation. In this study, we investigated ethylene counterflow diffusion flames with 1,3-dioxolane addition in different amounts. The equilibrium temperature, the stoichiometric mixture fraction, and the strain rate were kept nearly constant to highlight the chemical effect of 1,3-dioxolane addition. Measurements of soot volume fraction with laser-induced incandescence showed that the addition of 1,3-dioxolane up to 30 % leads to a synergistic effect on soot formation with a maximum at 10 %. To reveal the chemistry behind the synergistic effect and its attenuation, speciation measurements were performed on three representative flames (0 %, 10 %, and 30 % fuel mole fraction of 1,3-dioxolane) with gas chromatography-mass spectrometry. The experimental data were compared to kinetic model simulations for interpretation. It was found that the addition of 1,3-dioxolane enhances methyl radical formation, which promotes C3 species formation via C1+C2 pathways. As a consequence, C3 species increase monotonically, whereas some C4 species show a non-monotonic behavior. Pathway analyses revealed that benzene dominantly forms via the C3 route. The main soot precursor naphthalene primarily forms via C4+2C3 pathways through C7 species, and benzene-based pathways, i.e., C6+C4 and H-abstraction-C2H2-addition (HACA), play a minor role. The observed synergistic effect on naphthalene formation is promoted by C3 species for 1,3-dioxolane addition in small amounts. At higher fuel mole fractions of 1,3-dioxolane, C4 species become the bottleneck in naphthalene formation and the synergistic effect subsides. Naphthalene passes this behavior mainly via the HACA pathway to larger polycyclic aromatic hydrocarbons and subsequently to soot.