An experimental study of the rich premixed ethylbenzene flame at low pressure
Introduction
Aromatic hydrocarbons are major gasoline components with total concentration greater than 20% [1], [2]. Due to the high-energy rating and high-knock rating, they are desirable in gasoline for the improvement of fuel octane number, and in diesel oils and jet fuels as well. On the other hand, their toxicity and ability to form toxic species in oxidation are hazardous to human health. It has been aware that small aromatics are precursors of polycyclic aromatic hydrocarbons (PAHs), which are well-known to be mutagenic and carcinogenic. As one of the smallest aromatics, ethylbenzene has weight fractions of 0.2–4% in gasoline blends and presents with small quantities in diesel oils and jet fuels [3]. It is involved in PAHs formation, not only due to its role as actual fuel, but also because it serves as a common intermediate in combustion of other aromatic fuels, such as benzene [4] and toluene (Y.Y. Li et al., unpublished data).
Compared with the extensive concerns on small aliphatic fuels, laboratorial combustion researches on aromatic fuels are still insufficient, partly because of their high boiling points and heavy soot productions [5]. There are a limited number of experimental researches on ethylbenzene oxidation and combustion [3], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Among these valuable works, mole fraction profiles of combustion intermediates were measured using probe sampling method in non-premixed methane doped-ethylbenzene flames [11], [12] and in premixed atmospheric-pressure ethylbenzene flames [3], [14], [15]. Besides, kinetic studies were carried out basing on the results of the premixed atmospheric-pressure ethylbenzene flames [3], [14]. Due to the limitation of detection procedures in these works, identification of combustion intermediates in ethylbenzene flames remains ambiguous and incomplete, especially for radicals and isomeric species, preventing the further understanding of ethylbenzene involved flame chemistry.
Here, we report an experimental study of the slightly sooting premixed ethylbenzene flame (ϕ = 1.90) at low pressure using molecular-beam mass spectrometry (MBMS) and synchrotron vacuum ultraviolet (VUV) photoionization. Benefiting from the high-energy resolution of the ionization source, about ninety combustion intermediates, including a number of radicals and isomeric species, were identified from the ionization threshold measurements of photoionization efficiency (PIE) spectra. Mole fraction profiles of observed flame species have also been evaluated for further modeling purpose. Basing on the experimental results, the degradation of ethylbenzene and the formation of some interested PAHs are discussed in detail.
Section snippets
Experimental
The experimental work was performed at National Synchrotron Radiation Laboratory in Hefei, China. The detailed description of beamline setup is presented in Supplemental material #1. The experimental instrument has been reported elsewhere [16]. In brief, a low-pressure laminar premixed flame stabilized on a 6-cm-diameter McKenna burner is sampled by a quartz cone-shaped nozzle with a 40° included angle and a ∼500 μm orifice at the tip. The sampled flame species forms a molecular beam, which then
Mole fraction profiles of flame species
Combustion intermediates up to C19H12 were identified here, including many radicals and isomeric species. The identification is mainly based on the accordance of the measured thresholds with the literature IEs from the NIST online database [19]. The illustrations of intermediate identification can be found in our previous studies [4], [20] and will not be presented here. Mole fraction profiles of the observed species were evaluated from the measurements of burner scan. The evaluation method is
Conclusion
A slightly sooting premixed ethylbenzene flame has been studied using MBMS and tunable synchrotron VUV photoionization. About ninety combustion intermediates were observed, including many radicals and isomeric species. A number of newly detected intermediates can largely extend the known intermediate pool of ethylbenzene combustion, leading to a more comprehensive knowledge of flame chemistry of aromatic fuels. Basing on the experimental results, the degradation of ethylbenzene, as well as the
Acknowledgments
FQ is grateful for the funding supports from Chinese Academy of Sciences, Natural Science Foundation of China under Grant no. 20533040, National Basic Research Program of China (973) under Grant no. 2007CB815204, and Ministry of Science and Technology of China under Grant no. 2007DFA61310. We thank Dr. Jing Wang and Mr. Taichang Zhang for valuable help on experimental works.
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Current address: School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14850, USA.