Experimental And Kinetic Modeling Study Of Cyclohexane And Its Mono-alkylated Derivatives Combustion

The combustion of fossil fuels provides most of the energy used worldwide, promoting the rapid growth of society and economy. However, large amount of pollutants were emitted, which are harmful to human health and the sustainability of human society. The design of high efficiency and low emission engines, as well as the development of detailed kinetic model of fossil fuels are crucial to alleviate the energy shortage and reduce the pollution. Cycloalkanes are an important component family of fossil fuel and their surrogates. Moreover, the new discovery of oil sands may contain larger fractions of cycloalkanes. The China NO.3Kerosene also has large mass fraction of cycloalkanes. The combustion of cyclohexane and its derivatives produces large amount of dienes, such as carcinogenic1,3-butadiene. They also have relatively high sooting tendency via the dehydrogenation process to generate aromatics. Thus, considering the importance of cycloalkane fuels and pollutants emission controlling, it is required to get a deep understanding of their combustion mechanism and develop their detailed kinetic models.In this work, three of the classic cycloalkanes, including cyclohexane, methylcyclohexane (MCH) and ethylcyclohexane (ECH) were chosen for a systemic study. In experiment, the pyrolysis of the three fuels at30-760Torr was investigated in a flow reactor by the state-of-the-art synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The pyrolysis species were identified by the energy scan to achieve the photoionization efficiency spectra. Their mole fractions with temperatures were quantified by near threshold ionization. The flow reactor pyrolysis apparatus was coupled with the GC/GC-MS analysis to investigate ethylcyclohexane pyrolysis. More isomers were separated and low concentration intermediates such as3-octene was quantified. The data sets by the two analytical methods are in good agreement and complementary. The laminar premixed flames of MCH and ECH were also studied by SVUV-PIMS with equivalence ratio of1.75at30Torr. The hydrocarbons measured in flames are in accord with the pyrolysis intermediates. The mole fractions of reactants, diluent Ar and intermediates were quantified along the axial of the flame. The temperature profiles along the flow reactor were measured by an S-type thermocouple, while the flame temperature was measured by B-type thermocouple coated with Y2O3-BeO anti-catalytic ceramic. In pyrolysis and flames, lots of chain and branched dienes were measured. The tendency for long chain dienes formation increases as the side chain increases of the cyclohexanes. A set of cyclic C6-C8alkenes, dienes and trienes were measured, which provide experimental evidence of novel aromatic formation pathways.There is scarce theoretical study on cyclohexanes, especially for cyclohexanes with side chains. For methylcyclohexane, there exists dispute for the initial unimolecular channels. In this work, the reaction pathways as well as the rate constants were studied by high level quantum chemistry calculation. Firstly, the ring-opening isomerization and dissociation channels of MCH were investigated. From the viewpoint of energy barriers, the ring opening via the C-C bond fission adjacent to the side chain has the lowest barrier among all the isomerization pathways; the dissociation channel via methyl loss is competitive with the ring-opening pathways. The temperature-and pressure-dependent rate constants were calculated by RRKM/ME theory based on the potential energy surface. Secondly, the H-abstraction reactions of MCH via H atom attack were calculated. The obtained rate constants agree well with the literature values. In combustion process, the MCH and ECH radicals connect the consumption of fuels and the formation of intermediates. The branching ratios of these radicals have significant effect on the distribution of the combustion intermediates. To get a more reasonable prediction of the branching ratios, the energy barriers of these radicals’isomerization and dissociation were investigated. The pressure-and temperature-dependent rate constants were calculated for the MCH radicals. The calculation on isomerization and dissociation of MCH, and reactions of MCH and ECH radicals provides new insight into the reaction mechanism of mono-alkylated cyclohexanes. They are beneficial for the rate constants and branching ratio estimation in kinetic model development.A detailed kinetic model of cyclohexane, methylcyclohexane and ethylcyclohexane combustion was developed. The rate constants come from literature review and calculation in this work. In the submechanism development, the reaction classes include the isomerization and dissociation of fuels; the H-abstraction of fuels; the dissociation and H-abstraction of C6-C8alkenes; the dissociation and isomerization of fuel radicals; the dissociation and isomerization of C6-C8alkenyl radicals; and the dissociation and stepwise dehydrogenation of C6-C8cyclic alkenes. Detailed description was given in the text for the source of the rate constants, including the latest experimental measurement and theoretical calculation. The detailed kinetic model of the three fuels was built through the combination of the newly developed submechanism with the C0-C4base model. The C0-C4mechanism mainly comes from USC Mech II and the pyrolysis model of three butene isomers. The simulation was carried out with the CHEMKIN-PRO software. The pyrolysis and flame data in this work were used as validation target. Through the comparison between experiment and simulation, the model was optimized. Furthermore, the model was validated by species mole fraction in premixed flames and jet-stirred reactor (JSR) oxidation, and global combustion properties of ignition delay times and laminar flame speeds reported in the literature. The covered experimental conditions compose of equivalence ratio0.25-∞, pressure30-76000Torr, and temperature700-2100K. The simulation predicts satisfactorily the species speciation in pyrolysis, oxidation and flames, and also global combustion properties.Under the pyrolysis circumstance in this work, the three fuels were consumed by both unimolecular decomposition and H-abstraction reactions. The contribution of these two types of reactions has pressure dependence. In the flame and JSR oxidation, the fuel was mainly consumed by H-abstraction reactions. In the fuel-rich flames, the H-abstraction by H atom attack is dominant, while in JSR oxidation, OH radical attack on the fuels is much more prevalent. For radicals, its direct β-C-C scission and isomerization are prevalent in pyrolysis and rich flames, while in JSR oxidation, the oxidation reactions of radicals play important roles. The formation of aromatics was discussed based on the experimental observation and kinetic modeling of the three fuels combustion.The chemical kinetics of the three fuels combustion were discussed in detail, such as the reactivity of the three fuels and the mole fraction distribution of intermediates in flow reactor pyrolysis; the flame structure and intermediates distribution of the three fuel-rich low pressure laminar premixed flames; and the disparity of laminar flame speeds between cyclohexane and the C1-C2mono-alkylated cyclohexanes from1to10atm.In the conclusion and perspective chapter, a brief summary of this work and perspective for cycloalkanes combustion chemistry study were given. In the future, it is needed to carry out experimental and kinetic modeling study for cycloalkanes with long and complex side chains; also for the important intermediates formed in cyclohexane and mono-alkylated cyclohexanes combustion process. It is required to develop the low-temperature oxidation mechanism of cyclohexane and mono-alkylated cyclohexanes.