Formation and emission of large furans and oxygenated hydrocarbons from flames

Abstract

Many oxygenated hydrocarbon species formed during combustion, such as furans, are highly toxic and detrimental to human health and the environment. These species may also increase the hygroscopicity of soot and strongly influence the effects of soot on regional and global climate. However, large furans and associated oxygenated species have not previously been observed in flames, and their formation mechanism and interplay with polycyclic aromatic hydrocarbons (PAHs) are poorly understood. We report on a synergistic computational and experimental effort that elucidates the formation of oxygen-embedded compounds, such as furans and other oxygenated hydrocarbons, during the combustion of hydrocarbon fuels. We used ab initio and probabilistic computational techniques to identify low-barrier reaction mechanisms for the formation of large furans and other oxygenated hydrocarbons. We used vacuum-UV photoionization aerosol mass spectrometry and X-ray photoelectron spectroscopy to confirm these predictions. We show that furans are produced in the high-temperature regions of hydrocarbon flames, where they remarkably survive and become the main functional group of oxygenates that incorporate into incipient soot. In controlled flame studies, we discovered ∼100 oxygenated species previously unaccounted for. We found that large alcohols and enols act as precursors to furans, leading to incorporation of oxygen into the carbon skeletons of PAHs. Our results depart dramatically from the crude chemistry of carbonand oxygen-containing molecules previously considered in hydrocarbon formation and oxidation models and spearhead the emerging understanding of the oxidation chemistry that is critical, for example, to control emissions of toxic and carcinogenic combustion by-products, which also greatly affect global warming.furans | oxygenated hydrocarbons | soot | organic carbon | black carbon

The Molecular Composition of Soot

“…The potential incorporation of oxygen within PAH molecules is of significant interest, particularly with regard to their environmental consequences. [14] In this study, while a small number of low-abundance ions in some flame areas could potentially be assigned as oxygenated PAH, they were very weakly detected (< 5 % relative abundance) and did not show coherent spatial distributions corresponding to the physical flame shape and thus may be background-or noise-related. Flames where oxygenated-aromatic species have been identified have been burned in a premixed configuration, where the oxygen availability during molecular growth is typically much greater, or in oxygen-rich (and high temperature) regions of counterflow diffusion flames where phenols, ethers, and furanembedded species have been observed.…”

Radical–radical reactions, pyrene nucleation, and incipient soot formation in combustion

“…SNAPS models the formation and growth/decomposition of PAHs starting from an initial "seed" molecule, reconstructing the probabilities of forming an indefinite number of species throughout the flame. [33,34] , and all reactions are fully reversible. SNAPS has been validated against experimental data from premixed and diffusioncontrolled combustion [32,33,35] .…”

“…[33,34] , and all reactions are fully reversible. SNAPS has been validated against experimental data from premixed and diffusioncontrolled combustion [32,33,35] . The gas-phase environment (needed as input for SNAPS) was obtained using the PREMIX program in CHEMKIN [35] , using the deterministic kinetic mechanism of Appel-Bockhorn-Frenklach [23] .…”

Critical Assessment of Photoionization Efficiency Measurements for Characterization of Soot-Precursor Species

“…[45][46] Thus, gas-phase PAHs have to condense onto existing particles that are either larger than, or that grow to, ~50 nm in the sampling line in order to be efficiently detected by the aerosol mass spectrometer, and the strong suppression of light gas-phase species using this instrument has been revealed in previous studies. [30][31][32]47 Because detected species are in a condensed phase at ambient conditions and are vaporized from soot particles in the ionization region, we refer to the mass spectra as aerosol mass spectra.…”