Effect of dialkyl carbonates on soot precursor formation in ethylene laminar opposed-flow flames

Abstract

Diethyl carbonate (DEC) and dimethyl carbonate (DMC) are environmentally friendly oxygenated fuel additives due to their low toxicity, engine compatibility, miscibility with conventional fuels, and sustainable synthesis from alcohols and CO₂. While DEC has demonstrated strong potential for particulate matter reduction, the fundamental chemical mechanisms governing soot suppression remain insufficiently understood. In this work, the effects of linear dialkyl carbonates on soot precursor formation are investigated in ethylene laminar non-premixed counterflow diffusion flames using a combination of non-intrusive optical diagnostics and detailed kinetic modeling. Soot volume fractions and polycyclic aromatic hydrocarbons (PAHs) were measured by light extinction/scattering and laser-induced fluorescence, respectively. DMC addition leads to an initial increase in soot volume fraction and PAH formation, followed by a decrease at higher doping levels, whereas DEC consistently suppresses soot formation over the entire range of investigated concentrations while moderately enhancing lighter PAHs. Quantum chemical calculations suggest temperature-dependent conformational effects in DEC that favor alternative unimolecular decomposition pathways associated with soot suppression. Ab initio calculations at the CCSDT/6-311++G**//MP2/6-311++G** level were used to refine key reaction pathways and incorporated into detailed kinetic mechanisms accounting for alkene–carbonate interactions, PAH growth, and resonance-stabilized radical chemistry. Kinetic simulations suggest that DMC enhances methyl and propargyl radical formation, promoting aromatic growth, whereas DEC is associated with lower concentrations of radicals and intermediates linked to PAH growth pathways represented in the mechanism, supporting the interpretation that soot inhibition is primarily chemical rather than thermal in origin. These results provide evidence for a molecular-level connection between fuel structure and soot chemistry and identify DEC as a particularly effective soot-mitigating additive under diffusion flame conditions. Novelty and significance statement: This work provides novel mechanistic insight into the role of linear dialkyl carbonates in soot precursor chemistry under non-premixed counterflow diffusion flame conditions. By combining detailed experimental measurements, quantum chemistry calculations, and kinetic modeling, the study provides evidence linking fuel molecular structure to resonance-stabilized radical chemistry and polycyclic aromatic hydrocarbon growth. The results challenge the common assumption that oxygenated additives suppress soot primarily through dilution or thermal effects and instead highlight the importance of fuel-specific chemical pathways. The integration of high-fidelity experimental data with updated kinetic mechanisms that incorporate alkene–carbonate interaction pathways provide new insight into soot formation and inhibition. These findings underscore the importance of fuel-specific chemistry in predictive soot modeling and demonstrate the potential of linear carbonates, particularly diethyl carbonate, as structurally driven soot-mitigating additives with implications for cleaner combustion strategies.