Consumption and hydrocarbon growth processes in a 2-methyl-2-butene flame

Abstract

This paper is concerned with the investigation of the chemical structure of a low-pressure, fuel-rich (ɸ = 1.8) premixed laminar flame fueled with 2-methyl-2-butene employing flame-sampling molecular-beam mass spectrometry with vacuum-ultraviolet single-photon ionization. Partially isomer-resolved mole fraction profiles can be explained by a decomposition scheme based on hydrogen abstraction and addition reactions. The presence of 9 allylic CH bonds compared to only one vinylic CH bond is the key feature that governs the fuel consumption and subsequent hydrocarbon growth reactions. Compared to other alkenes, including e.g., 1-butene, 2-butene, and iso-butene (Schenk et al., 2013), 2-methyl-2-butene shows a remarkable tendency to form soot precursor molecules such as toluene. In particular, experimental evidence is provided here that toluene, o-xylene, and styrene can be a starting point for PAH formation, thus serving as first aromatic rings besides benzene. The formation of toluene, o-xylene, and styrene can be traced back to the reactions of the resonantly stabilized C4H5 [∙CH2CCCH3 and CH2CH∙CCH2] radicals and the C5H7 [CH2C(CH3)∙CCH2] radicals that are readily formed through fuel-specific decomposition channels. Our experimental data in form of mole fraction profiles as a function of height above the burner for a mass range from 2 to 210 u can serve as reliable validation targets for model development. A preliminary comparison to the model of Westbrook et al. [1] that was optimized to capture ignition delay times and the low-temperature oxidation regime, shows promising elements already for the initial fuel consumption.