Pyrolysis study of 1,2,4-trimethylcyclohexane with SVUV-photoionization molecular-beam mass spectrometry
Introduction
In recent years, the combustion studies of aviation fuels, as the energy source of aircrafts, have attracted more and more attention with respect to high efficiency and low pollutant emissions. Zheng et al. [1] identified the compositions of RP-3 aviation kerosene with alkanes (53.0%), cycloalkanes (37.7%) and minor quantity of aromatics as well as olefins with gas chromatography-mass spectrometer (GC–MS). Widegren and Bruno. [2] presented the compositions of Jet-A POSF 4658: alkanes (68.2%), aromatics (25.5%), naphthalenes (3.0%) and cycloalkanes (3.3%) with the same technology. Surrogate fuels, which make the modeling the behavior of real aviation fuels feasible, have been widely used in the combustion field. Recently, cyclohexane (CH) and alkyl cyclohexanes have attracted more attention as the important components of surrogate model fuels, as they can generate both alkanes and olefins in the combustion process [3], [4], [5], [6].
Most of the cyclohexane combustion studies were focused on CH and alkyl cyclohexanes, especially methylcyclohexane (MCH). Several studies of MCH were reported with respect to its pyrolysis [7], [8], [9], flame [10,11], oxidation [12], and ignition [13,14]. Long side-chains alkyl cyclohexane (e.g., ethylcyclohexane, propylcyclohexane, butylcyclohexane and etc.) were also reported regarding their combustion experiments and kinetic modeling. However, the combustion studies of alkyl CH with multiple side-chains are quite limited. Gillespie et al. studied the pyrolysis of 1,4-MCH and identified a number of products by using a flow reactor in 1979 [15]. Kang et al. measured the ignition delay time of 1,2-MCH and 1,3-MCH in a motored engine in 2015 [16]. Eldeeb et al. studied the ignition and pyrolysis of 1,3-MCH under high-temperature and presented a detailed kinetic model [17]. Up to 2018, there were neither experimental nor modeling studies on trimethylcyclohexane (tri-MCH) available in the literature. Tri-MCH can play an important role in kerosene surrogate, which is a typical kind of cycloalkanes and could isomerize to more kinds of linear alkenes than ethyl-CH or n-propylcyclohexane (NPCH) which are widely used in surrogate fuels. More recently, our group reported an oxidation study on 1,2,4-trimethylcyclohexane (T124MCH) with a new kinetic model [18]. However, due to the lack of experimental data, the model still needs to be further validated against other kinds of experimental results such as pyrolysis data.
This work aims to identify and quantify intermediates and products in the pyrolysis of T124MCH. Based on the experimental data, the previous kinetic model which can reproduce the experimental results in the oxidation work would be validated. Rate-of-production (ROP) and sensitivity analyses were performed to identify the consumption and sensitive pathways of T124MCH pyrolysis. The validated mechanism could provide a detailed study on tri-MCH as a potential surrogate compound, and improve the understanding of its combustion characteristics of diesel and jet fuels for future applications.
Section snippets
Experimental
The experiments were carried out at the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China, which has been described elsewhere [19], and only a brief description is given here. A tubular flow reactor made of corundum (α-Al2O3) whose outer diameter, inner diameter, and length is 10, 7 and 220 mm respectively, was used. T124MCH was vaporized by a high-pressure syringe pump, and carried by helium. The total flow rate was 1000 sccm, consisting of 985 sccm of He, 10 sccm of Kr for
Modeling
The PFR code in the CHEMKIN-II software was used to simulate the pyrolysis experiments. The details of kinetic model were presented in our previous work [19]. This is a high-temperature oxidation mechanism coupled with an acetylene oxidation mechanism [21] The newly developed T124MCH sub-mechanism was estimated based on the previous mechanism of MCH proposed by Wang et al. [22]. According to Wang's mechanism, the initiation of MCH gives rise to five C7H13 radicals, which then would decompose to
Mole fraction profiles
More than 44 species were detected in the pyrolysis of T124MCH, as shown in Table S3 (see SM). Generally, ethylene, propylene, allene, propyne, butadiene and butyne are main light hydrocarbons formed in pyrolysis at both pressures, among which ethylene plays a dominant role. Benzene, toluene and ethylbenzene were observed at both pressure, and benzene is the most abundant among aromatics. At atmospheric pressure, T124MCH begins to decompose at a lower temperature with more ethylene, allene,
Conclusions
This work reports the pyrolysis study of T124MCH in a flow reactor equipped with an SVUV-photoionization molecular-beam mass spectrometry at 30 and 760 Torr. The results indicate that the pyrolysis of T124MCH is significantly influenced by the pressure. The onset temperature of decomposition decreases from 1050 to 875 K as the pressure increases from 30 to 760 Torr. The main light hydrocarbons and aromatics increase monotonically with temperature at 30 Torr, and have a peak value at 760 Torr.
Declaration of Competing Interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
Acknowledgement
ZYT is thankful for the financial support from the NSFC (No. 51888103/51976216), MOST (2017YFA0402800) and Recruitment Program of Global Youth Experts.
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2022, Combustion and FlameCitation Excerpt :To the best of our knowledge, none of both experimental and kinetic-model studies aiming at low-temperature oxidation behaviors of multi-alkylated cycloalkanes has been reported until now. Thus, a representative fuel with a highly symmetric molecular structure is chosen as our research target in the present work: 1,3,5-trimethylcyclohexane (T135MCH), which has been considered as a typical component of surrogate fuels in previous studies [45,46]. In this work, a combined experimental and kinetic modeling effort concerning low-temperature oxidation process of T135MCH at 1 atm was made in a JSR under lean (ϕ = 0.5), stoichiometric (ϕ = 1.0) and rich (ϕ = 1.5) conditions, respectively.
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Both contributed equally.