Esposito, Vincenzo (2026) Investigation of synthetic alternative fuels through advanced diagnostics in controlled flames. [Tesi di dottorato]

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Abstract

The increasing demand for sustainable energy carriers has renewed interest in alternative fuels capable of reducing greenhouse gas (GHG) and pollutant emissions while remaining compatible with existing combustion technologies. Among these, oxygenated bio- and synthetic fuels show strong potential to suppress soot formation, although their emission characteristics and underlying mechanisms are not yet fully understood. This thesis presents an experimental and numerical investigation of gas-phase chemistry, nanoparticle formation, and soot evolution in canonical laminar flames fueled with sustainable oxygenated alternatives. Premixed laminar flames (PLFs) and counterflow diffusion flames (CDFs) were used as controlled configurations to isolate chemical and transport effects. The study focused primarily on ethanol (EtOH) and oxymethylene ether-3 (OME3), with additional investigations on higher alcohols (n-butanols), to assess the influence of molecular structure on pollutant formation. Overall, results show that oxygenated fuels strongly suppress key soot precursors, delay nanoparticle inception, and significantly reduce soot volume fractions (SVF), especially under nucleation-dominated conditions. Numerical simulations using a discrete sectional model (DSM) for soot formation, extended to include oxygenated-fuel chemistry, reproduced the experimental trends without empirical tuning. Three specific findings are particularly noteworthy. First, oxygen incorporation into soot is governed by reactive gas-phase intermediates, particularly acetaldehyde (CH3CHO), rather than by condensation of oxygenated polycyclic aromatic hydrocarbons (PAHs). Second, fuel molecular structure strongly influences pollutant formation and oxidation mechanisms, resulting in different wt. % O embedded in soot particles. Third, strain-rate (K) effects in CDFs further highlighted the role of residence time, fuel molecular structure, and gas-phase kinetics in controlling soot growth and oxidation. Finally, this thesis provides a consistent mechanistic framework for soot formation and functionalization in oxygenated-fuel flames, offering valuable insights for the design of low-sooting, sustainable fuel blends and for the development of predictive combustion and emission models.

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