Pyrolysis study of 1,2,4-trimethylcyclohexane with SVUV-photoionization molecular-beam mass spectrometry

Abstract

Pyrolysis of 1,2,4-trimethylcyclohexane (T124MCH) at 30 and 760 Torr has been investigated by using VUV-synchrotron photoionization molecular-beam mass spectrometry. In general, the ambient pressure leads to peak-shaped profiles of light hydrocarbons, while the monotonically increasing curves are observed for light hydrocarbons and aromatics in pyrolysis of T124MCH at low pressure. A detailed mechanism involving 530 species and 3160 reactions developed by the authors previously was used for the simulation of T124MCH with reasonable predictions. The consumption of T124MCH followed similar patterns at both pressures, namely the H-abstractions on the first, secondary, and tertiary carbon atoms are the main consumption reactions of T124MCH. The isomerization reactions transform the C₉H₁₇ radicals to branch-chain hydrocarbons, which are the precursors of light hydrocarbons such as ethane and propene. Sensitivity analysis indicates that the CH₃ consumption reactions are the most significant promoting reactions at both pressures. Isomerization reactions of T124MCH tend to play significant inhibiting effect on T124MCH pyrolysis. Compared to T124MCH oxidation, T124MCH pyrolysis tends to produce large quantities of benzene and toluene. These results will improve the understanding of the combustion and soot formation of T124MCH as a potential aviation surrogate fuel.

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.