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T > 1000 K)
Published in J. F. Griffiths, J. A. Barnard, Flame and Combustion, 2019
J. F. Griffiths, J. A. Barnard
The oxidation of aromatic hydrocarbons is confined essentially to the high temperature regime. Unlike most aliphatic hydrocarbons, the aromatics do not undergo vigorous oxidation or give rise to spontaneous ignition at temperatures below about 800 K. The benzyl and phenyl radicals, that are the primary propagating chain species, are relatively unreactive (Fig. 6.4 [92]). This arises in each case because the electron associated with the free radical centre is able to become delocalised across the electron-deficient, aromatic ring structure. There are consequences of this for the reactivity of fuel mixtures which include benzene, toluene or the xylenes (Chapter 13). The aromatic ring structure appears to be broken down via the formation of the unsaturated, cyclopentadienyl radical before fragmentation into smaller carbon-containing units.
Probing PAH Formation from Heptane Pyrolysis in a Single-Pulse Shock Tube
Published in Combustion Science and Technology, 2023
Alaa Hamadi, Leticia Piton Carneiro, Fabian-Esneider Cano Ardila, Said Abid, Nabiha Chaumeix, Andrea Comandini
Four different C14H10 species are identified and quantified in 2000 ppm heptane pyrolysis, including the dominant phenanthrene (PC14H10), and its isomers, diphenylacetylene (C6H5CCC6H5), 9-methylene-fluorene (C13H8CH2), and anthracene (AC14H10). Reaction pathways leading to C14H10 isomers formation at 1500 K are presented in Figure 6. C6H5CCC6H5 (Figure 3o) and C13H8CH2 (not shown in Figure 3) are the products of the C6H5C2H+C6H5 addition-elimination reaction (Sun et al. 2020). AC14H10 (Figure 3p) is totally formed from C7H5 self-recombination. PC14H10 (Figure 3h) mainly comes from the C7H7 self-recombination and AC14H10 isomerization. Other reaction channels which contribute to PC14H10 formation include: the C7H5 self-recombination, H-assisted isomerization of C6H5CCC6H5 and C13H8CH2, and the reaction of C9H7 with cyclopentadienyl radical (C5H5), which results from the consumption of C5H6 and the addition of C3H3 to C2H2.
Formation of Polycyclic Aromatic Hydrocarbons (PAHs) and Oxygenated PAHs in the Oxidation of Ethylene Using a Flow Reactor
Published in Combustion Science and Technology, 2022
Shunsuke Suzuki, Shota Kiuchi, Koichi Kinoshita, Yoshinaka Takeda, Kotaro Tanaka, Mitsuharu Oguma
The effect of mean gas temperature on the formation of MAHs, PAHs, and OPAHs was investigated. The equivalence ratio and residence time were fixed at 9.0 and 1.2 second, respectively, and the mean gas temperature was changed from 1050 K to 1350 K at 100 K intervals. Table 5 summarizes the experimental conditions. Figure 8 shows the experimental and simulated mole fractions of MAHs and PAHs. The analyzed mole fractions of PAHs that are not expressed in Figure 8 are shown in Figure S11. The behavior of some products was strongly affected by the gas temperature. In benzene, the measured concentrations increased monotonically with increasing gas temperature, which was reflected in the similar calculation by the LLNL model. However, the benzene concentrations in both KAUST models plateaued at temperatures above 1200 K. Regarding toluene and styrene, a local maximum value was observed in both experiments and calculations; however, the temperature at which the peak occurred differed with each other. For the small PAHs, each model predicted the tendencies in the measurements relatively well for naphthalene and acenaphthylene, though the LLNL model departed from the experimental trends of indene and phenanthrene. The deviation in indene concentration observed in the LLNL model at high temperatures may be due to the absence of reactions that form larger PAHs from indene or the indenyl radical. The reason why the phenanthrene trend exhibited in the LLNL model deviated from that of the experiments has been previously discussed in relation to Figure 4(g). In phenanthrene formation, as illustrated in Figure 8(g), the importance of hydrogenation of the phenanthrene radical (A3-4) formed by the reaction of acenaphthylene radical and acetylene grew at elevated temperatures in addition to indenyl radical (C9H7) + cyclopentadienyl radical (C5H5) reaction in the KAUST models. The main formation pathway of phenanthrene in the LLNL model was the recombination of fulvenallenyl radicals (C7H5), irrespective of temperature. Predictions of fluoranthene and chrysene by all models largely deviated from quantified results. For pyrene, the KAUST model (2019) predicted the experimental data well, while the maximum value predicted by the KAUST model (2017) was not observed in the experiments. For benzo(ghi)perylene, both KAUST models, especially the KAUST model (2017), agreed well with the experimental data.