The origin of the oxygen bonded in soot particles in biofuel combustion: The ethanol case

This study investigates the origin of oxygen in soot particles formed during ethanol combustion. Carbon particulate matter (PM) and flame structure were analyzed in fuel-rich ethylene and ethanol-doped (20 %) flames. Gas-phase composition was assessed using probe sampling and gas chromatography, revealing that ethanol addition had minimal impact on CO, CO2, and C2H2 formation but reduced methane and C3-C6 hydrocarbons concentration. PM was sampled at different formation stages and the total sample was analyzed using laser desorption ionization-time of flight mass spectrometry (LDI-TOFMS). A Fast Fourier Transform (FFT) approach was applied to interpret complex mass spectra. Three PM fractions were also examined: species <20 nm (organic carbon and primary particles), dichloromethane-soluble fraction (organic carbon), and acetonitrile-soluble fraction (PAH-enriched fraction). FFT analysis showed that only ethanol-doped flames contained oxygen embedded in total PM and small particle fractions, while soluble fractions lacked oxygen atoms. Since oxy-PAHs were not detected during early soot formation, they cannot explain the observed oxygen incorporation. The data indicate that oxygen is primarily incorporated into soot via gas-phase oxidation of carbon particles. Of the gas-phase species quantified, acetaldehyde—unique in being markedly enriched in the ethanol-doped flame—emerges as the most plausible oxidant promoting oxygen addition to the particles. These findings challenge assumptions about oxy-PAHs being the primary source of oxygen in soot and highlight the role of gas-phase species in modifying soot composition. Understanding these mechanisms is crucial for optimizing oxygenated fuel combustion and mitigating harmful emissions.