High-temperature oxidation mechanisms of m- and p-xylene

Deep neural networks for simultaneous BTEX sensing at high temperatures

In the study of chemical reactions, it is desirable to have a diagnostic strategy that can detect multiple species simultaneously with high sensitivity, selectivity, and fast time response. Laser-based selective detection of benzene, toluene, ethylbenzene, and xylenes (BTEX) has been challenging due to the similarly broad absorbance spectra of these species. Here, a mid-infrared laser sensor is presented for selective and simultaneous BTEX detection in high-temperature shock tube experiments using deep neural networks (DNN). A shock tube was coupled with a non-intrusive mid-infrared laser source, scanned over 3038.6-3039.8 cm^-1, and an off-axis cavity enhanced absorption spectroscopy (OA-CEAS) setup of ∼ 100 gain to enable trace detection. Absorption cross-sections of BTEX species were measured at temperatures of 1000-1250 K and pressures near 1 atm. A DNN model with five hidden layers of 256, 128, 64, 32, and 16 nodes was implemented to split the composite measured spectra into the contributing spectra of each species. Several BTEX mixtures with varying mole fractions (0-600 ppm) of each species were prepared manometrically and shock-heated to 1000-1250 K and 1 atm, and the composite measured absorbance were split into contributions from each BTEX species using the developed DNN model, and thus make selective determinations of BTEX species. Predicted and manometric mole fractions were in good agreement with an absolute relative error of ∼ 11%. We obtained a minimum detection limit of 0.73-1.38 ppm of the target species at 1180 K. To the best of our knowledge, this work reports the first successful implementation of multispecies detection with a single narrow wavelength-tuning laser in a shock tube with laser absorption spectroscopy.

Insights into the Decomposition and Oxidation Chemistry of p-Xylene in Laminar Premixed Flames

This work reports an experimental and kinetic modeling investigation on laminar premixed flame of p-xylene at 0.04 atm and equivalence ratios of 0.75, 1.0, and 1.79. Intermediates such as the p-xylyl radical, p-xylylene, and styrene, as well as polycyclic aromatic hydrocarbons (PAHs), were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry. Based on our previous aromatic kinetic model, a detailed kinetic model of p-xylene combustion was developed, and the model was validated against the present flame structure data. Model analysis work was also performed in order to reveal the important reactions in p-xylene decomposition and oxidation. The H-abstraction reactions leading to the p-xylyl radical are found to control the consumption of p-xylene in all the three flames. In the rich flame, p-xylyl mainly suffers the H-elimination and isomerization reactions, which produce p-xylylene and the o-xylyl radical, respectively. The further decomposition reactions of the o-xylyl radical contribute to the production of styrene, which is another important C₈ intermediate observed in the rich flame. In the stoichiometric and lean flames, p-xylyl mainly suffers the oxidation reactions by O, which give p-methylbenzaldehyde as major product. The growth pathways of PAHs in the rich flame were also investigated in this work. Indenyl, indene, naphthalene, and phenanthrene were observed as the abundantly produced bicyclic and tricyclic PAHs due to the existence of direct formation pathways from the decomposition of p-xylyl radical.

Review of Oxidation of Gasoline Surrogates and Its Components

There has been considerable progress in the area of fuel surrogate development to emulate gasoline fuels’ oxidation properties. The current paper aims to review the relevant hydrocarbon group components used for the formulation of gasoline surrogates, review specific gasoline surrogates reported in the literature, outlining their utility and deficiencies, and identify the future research needs in the area of gasoline surrogates and kinetics model.

Oxidation of the para-Tolyl Radical by Molecular Oxygen under Single-Collison Conditions: Formation of the para-Toloxy Radical

Crossed molecular beam experiments were performed to elucidate the chemical dynamics of the para-tolyl (CH₃C₆H₄) radical reaction with molecular oxygen (O₂) at an average collision energy of 35.3 ± 1.4 kJ mol^-1. Combined with theoretical calculations, the results show that para-tolyl is efficiently oxidized by molecular oxygen to para-toloxy (CH₃C₆H₄O) plus ground-state atomic oxygen via a complex forming, overall exoergic reaction (experimental, -33 ± 16 kJ mol^-1; computational, -42 ± 8 kJ mol^-1). The reaction dynamics are analogous to those observed for the phenyl (C₆H₅) plus molecular oxygen system which suggests the methyl group is a spectator during para-tolyl oxidation and that application of phenyl thermochemistry and reaction rates to para-substituted aryls is likely a suitable approximation.