Flame structure of laminar premixed anisole flames investigated by photoionization mass spectrometry and photoelectron spectroscopy

Abstract

Two laminar, premixed, fuel-rich flames fueled by anisole-oxygen-argon mixtures with the same cold gas velocity and pressure were investigated by molecular-beam mass spectrometry at two synchrotron sources where tunable vacuum-ultraviolet radiation enables isomer-resolved photoionization. Decomposition of the very weak O–CH3 bond in anisole (C6H5OCH3) by unimolecular decomposition yields the resonantly-stabilized phenoxy radical (C6H5O). This key intermediate species opens reaction routes to five-membered ring species, such as cyclopentadiene (C5H6) and cyclopentadienyl radicals (C5H5). Anisole is often discussed as model compound for lignin to study the phenolic-carbon structure in this natural polymer. Measured temperature profiles and mole fractions of many combustion intermediates give detailed information on the flame structure. A very comprehensive reaction mechanism from the literature which includes a sub-scheme for anisole combustion is used for species modeling. Species with the highest measured mole fractions (on the order of 10−3–10−2) are CH3, CH4, C2H2, C2H4, C2H6, CH2O, C5H5 (cyclopentadienyl radical), C5H6 (cyclopentadiene), C6H6 (benzene), C6H5OH (phenol), and C6H5CHO (benzaldehyde). Some are formed in the first destruction steps of anisole, e.g., phenol and benzaldehyde, and their formation will be discussed and with regard to the modeling results. There are three major routes for the fuel destruction: (1) formation of benzaldehyde (C6H5CHO), (2) formation of phenol (C6H5OH), and (3) unimolecular decomposition of anisole to phenoxy (C6H5O) and CH3 radicals. In the experiment, the phenoxy radical could be measured directly. The phenoxy radical decomposes via a bicyclic structure into the soot precursor C5H5 and CO. Formation of larger oxygenated species was observed in both flames. One of them is guaiacol (2-methoxyphenol), which decomposes into fulvenone. The presented speciation data, which contain more than 60 species mole fraction profiles of each flame, give insights into the combustion kinetics of anisole.