Measurements of Intermediate Species in Fuel-Rich Oxidation of Ethylene, Toluene, and n-Decane

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are important precursors to the formation of soot and are also pollutant emissions from combustion devices, including internal combustion engines. To understand the formation of PAHs, the low-molecular-weight hydrocarbon chemistry leading to their formation needs to be understood. Toward this goal, fuel-rich oxidation of three different hydrocarbons, i.e., ethylene, toluene, and n-decane, has been investigated in an atmospheric pressure flow reactor at varying temperatures (1000–1350 K), equivalence ratios (ϕ = 3.0–12.0), and residence times of 0.25–1.5 s. The major C1 to C7 intermediates, such as methane, acetylene, ethylene, allene, propyne, propylene, vinylacetylene, 1,3-butadiene, cyclopentadiene, benzene, and toluene, were quantified using a gas chromatograph equipped with a flame ionization detector. The experimental tendency of intermediate species formation, e.g., the dependence of temperature, equivalence ratio, and residence time, was similar in ethylene oxidation and n-decane oxidation. The concentrations of intermediate species up to C4 were higher in ethylene/n-decane oxidation than toluene oxidation, while a nearly equal or larger amount of C5–C7 species was produced in toluene oxidation. The experimental data were compared with modeling results using a detailed chemical kinetic mechanism. The calculated data using the kinetic model were in agreement with the experimental results. A comprehensive kinetic analysis on the reaction pathways of each species was conducted to assess the differences in the oxidation chemistry with the change in the structure of hydrocarbons. In particular, in-depth analysis of benzene formation was performed, elucidating that two benzene formation pathways were important in ethylene and n-decane oxidation: (1) H elimination of the 2,4-cyclohexadienyl radical produced from the isomerization of the 2,4-cyclopentadienylmethyl radical, and (2) dehydrogenation of cyclohexadiene produced from reactions of vinyl radical with 1,3-butadiene. In toluene oxidation, it was found that benzene was primarily produced from toluene through the replacement of the methyl group with hydrogen.

Ring Growth Mechanism in the Reaction between Fulvenallenyl and Cyclopentadienyl Radicals

“…To examine potential contributions of the studied reactions to the acenaphthylene formation during combustion, preliminary kinetic simulations using a kinetic model incorporating the proposed reaction mechanism were performed for the fuel-rich oxidation of toluene in a flow reactor. − The simulation method is the same as that used in ref except that the acenaphthylene formation mechanisms from the fulvenallenyl + cyclopentadienyl and propargyl + indenyl reactions were newly added to the kinetic model. The fulvenallenyl + cyclopentadienyl reaction was implemented by using the mechanism shown in Table or using the overall reaction described above.…”

“…For the propargyl + indenyl reaction, the mechanism proposed in ref was used. The simulation was performed for the inlet gas of a mixture of 0.6 vol % toluene and 0.6 vol % oxygen diluted with nitrogen at an atmospheric pressure and with a residence time of 1.2 s. Experimentally, the formation of species with a molecular weight of 152, most of which was assumed to be acenaphthylene, peaked at the mean gas temperature of 1250 K with the concentration of ≈170 ppm. The present simulation gives a peak acenaphthylene concentration of ≈200 ppm at 1300 K (see Figure S16 for comparison).…”