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This work presents a comprehensive experimental and kinetic modeling study on the combustion of n -propylbenzene. Flow reactor pyrolysis of n -propylbenzene at 0.04, 0.2 and 1 atm and laminar premixed flames of n -propylbenzene at 0.04 atm with equivalence ratios of 0.75 and 1.00 were investigated with synchrotron vacuum ultraviolet photoionization mass spectrometry. Jet stirred reactor (JSR) oxidation of n -propylbenzene at 10 atm with equivalence ratios of 0.5, 1.0, 1.5 and 2.0 was investigated with gas chromatography. A detailed kinetic model for n -propylbenzene combustion with 340 species and 2069 reactions was developed and validated against the data measured in this work. Model analyses such as rate of production analysis and sensitivity analysis were also performed to reveal the key pathways in the consumption of fuel and formation of polycyclic aromatic hydrocarbons (PAHs). The analysis results demonstrate that the benzylic C C bond dissociation reaction is crucial for the decomposition of n -propylbenzene in the pyrolysis and rich flame. Low temperature oxidation reactions play important roles in the high pressure JSR oxidation of n -propylbenzene. In addition, the formation pathways of PAHs are strongly related to the fuel structure, especially for the formation of bicyclic PAHs such as indene and naphthalene. Furthermore, the present model was also validated against previous experimental data of n -propylbenzene combustion under a wide range of conditions, including ignition delay times, laminar flame speeds, extinction strain rates, speciation profiles in atmospheric pressure JSR oxidation, flow reactor oxidation and high pressure shock tube pyrolysis and oxidation. [ABSTRACT FROM AUTHOR]

The flow reactor pyrolysis of n -pentanol at 30, 150, and 760 Torr and laminar premixed flames of n -pentanol with equivalence ratios of 0.7 and 1.8 at 30 Torr are investigated using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The pyrolysis products and flame species are identified and the mole fraction profiles are measured. A detailed kinetic model of n -pentanol is developed and validated on the new experimental results. The C–C bond dissociation reactions play a significant role in the pyrolysis of n -pentanol. The contribution of the water elimination reaction becomes less important in the pyrolysis of n -pentanol than in the pyrolysis of n -butanol. Olefins and C n H 2 n O species are found to be the two major product families in the pyrolysis and combustion of n -pentanol, and specific products are also observed for most of the H-atom abstraction reactions of n -pentanol. The model is further validated on the experimental data of n -pentanol combustion in literature, including the species profiles in jet-stirred reactor oxidation, the laminar flame speeds and ignition delay times. [ABSTRACT FROM AUTHOR]

The flow reactor pyrolysis of toluene from low to atmospheric pressures and jet stirred reactor oxidation of toluene at high pressure have been studied in Part I of this serial work. The targets in this part are to collect and review the experimental data of toluene pyrolysis and oxidation reported in recent years, and validate the kinetic model of toluene combustion developed in Part I. The vast amount of data used in this work mainly include species profiles in shock tube pyrolysis and oxidation, flow reactor oxidation and low pressure premixed laminar flames, and global combustion parameters such as ignition delay times and laminar flame speeds. Along with the validation of new pyrolysis and oxidation data in Part I, a wide range of conditions from low to high temperatures, subatmospheric to high pressures, very lean to pyrolysis conditions were validated in this work. The satisfactory reproduction of these data demonstrates that the present model can be used in combustion simulation of surrogate fuels containing toluene and as a reliable basis to develop kinetic models of large alkylbenzenes and bicyclic aromatics. [ABSTRACT FROM AUTHOR]

Abstract: A rich premixed flame of n-propylbenzene (ϕ =1.79) was investigated at low pressure. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to detect flame species including a lot of radicals, isomers and polycyclic aromatic hydrocarbons (PAHs) and measure their mole fraction profiles. A preliminary kinetic model of n-propylbenzene combustion was developed from recently reported toluene and ethylbenzene models [Y.Y. Li et al., Proc. Combust. Inst. 33 (2011) 593–600, 617–624] and validated by the experimental results. Rate constants of the important pathways of n-propylbenzene consumption, i.e. the H-abstraction reactions by H atom and the benzylic C–C bond dissociation, were calculated theoretically and included in this model. Based on the rate of production analysis and experimental observations, styrene, benzyl and benzene are confirmed as significant intermediates in the n-propylbenzene flame. For the formation pathways of PAHs, n-propylbenzene flame has a fuel specific pathway to form indene compared to smaller alkylbenzene, which increases the concentrations of indene and indenyl radical. It is concluded that high concentrations of important PAH precursors such as benzyl and indenyl radicals result in the enhanced PAHs formation in this flame compared with smaller alkylbenzenes. [Copyright &y& Elsevier]

The pyrolysis of decalin was studied in a flow reactor using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at the pressures of 4, 20, and 101 kPa and the temperatures from 920 to 1500 K. Dozens of pyrolysis species were identified and their mole fraction profiles were measured as a function of temperature, particularly several free radicals and aromatic species. Besides, a detailed kinetic model consisting of 484 species and 1774 reactions was proposed and used to simulate the present experimental data. The rate of production analysis and sensitivity analysis was performed in order to provide deep insight into the pyrolysis kinetics of decalin. The decomposition of decalin is initiated by the C–C bond dissociation reactions of both decalin and its C 10 H 18 isomers, while the H-atom abstraction reactions consume most of decalin. Monocyclic aromatic hydrocarbons (MAHs) are mainly produced in the fuel decomposition process instead of the mass growth process from small molecules, indicating that the two fused six-membered ring structure offers a solid infrastructure to form MAHs. Polycyclic aromatic hydrocarbons (PAHs) are mainly produced in the mass growth process from PAH precursors such as benzyl radical (A1CH 2 ), phenyl radical (A1-), and 1,3-cyclopentadienyl radical (CYC 5 H 5 ). The relatively high degree of saturation of decalin facilitates ring-opening pathways in its decomposition. This enhances the formation of MAHs but limits the formation of PAHs, leading to the lower sooting tendency of decalin than tetralin. Furthermore, the model was also validated on the shock tube pyrolysis data of decalin in the literature. [ABSTRACT FROM AUTHOR]

Abstract: Methylcyclohexane is the simplest alkylated cyclohexane, and has been broadly used as the representative cycloalkane component in fuel surrogates. Understanding its combustion chemistry is crucial for developing kinetic models of larger cycloalkanes and practical fuels. In this work, the synchrotron vacuum ultraviolet photoionization mass spectrometry combined with molecular-beam sampling was used to investigate the species formed during the pyrolysis of methylcyclohexane and in premixed flame of methylcyclohexane. A number of pyrolysis and flame intermediates were identified and quantified, especially including radicals (e.g. CH3, C3H3, C3H5 and C5H5) and cyclic C6- and C7-intermediates (benzene, 1,3-cyclohexadiene, cyclohexene, toluene, C7H10 and C7H12, etc.). In particular, the observation of cyclic C6- and C7-intermediates provides important experimental evidence to clarify the special formation channels of toluene and benzene which were observed with high concentrations in both pyrolysis and flame of methylcyclohexane. Furthermore, the rate constants of H-abstraction of methylcyclohexane via H attack, and the isomerization and decomposition of the formed cyclic C7H13 radicals were calculated in this work. A kinetic model of methylcyclohexane combustion with 249 species and 1570 reactions was developed including a new sub-mechanism of MCH. The rate of production and sensitivity analysis were carried out to elucidate methylcyclohexane consumption, and toluene and benzene formation under various pyrolytic and flame conditions. Furthermore, the present kinetic model was also validated by experimental data from literatures on speciation in premixed flames, ignition delays and laminar flame speeds. [Copyright &y& Elsevier]

Abstract: The pyrolysis of cyclohexane at low pressure (40mbar) was studied in a plug flow reactor from 950 to 1520K by synchrotron VUV photoionization mass spectrometry. More than 30 species were identified by measurement of photoionization efficiency (PIE) spectra, including some radicals like methyl, propargyl, allyl and cyclopentadienyl radicals, and stable products (e.g., 1-hexene, benzene and some aromatics). Among all the products, 1-hexene is formed at the lowest temperature, indicating that the isomerization of cyclohexane to 1-hexene is the dominant initial decomposition channel under the condition of our experiment. We built a kinetic model including 148 species and 557 reactions to simulate the experimental results. The model satisfactorily reproduced the mole fraction profiles of most pyrolysis products. The rate of production (ROP) analysis at 1360 and 1520K shows that cyclohexane is consumed mainly through two reaction sequences: cyclohexane→1-hexene→allyl radical+ n-propyl radical, and cyclohexane→cyclohexyl radical→hex-5-en-1-yl radical that further decomposes to 1,3-butadiene via hex-1-en-3-yl and but-3-en-1-yl radicals. Besides the stepwise dehydrogenation of cyclohexane, C3+C3 channels, i.e. C3H3 +C3H3 and C3H3 +aC3H5 also have important contribution to benzene formation. The simulation reveals that C3H3 +C3H3 =phenyl+H reaction is the key step for other aromatics formation, i.e. toluene, phenylacetylene, styrene, ethylbenzene and indene in this work. [Copyright &y& Elsevier]

The oxidation of 1-butene and i -butene with and without addition of 1000 ppm NO was experimentally and numerically studied primarily at fuel-rich (ϕ = 2.0) conditions under high dilution (96% Ar) in a flow reactor operated at atmospheric pressure in the low temperature range of approximately 600-1200 K. Numerous intermediate species were detected and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). An elementary-step reaction mechanism consisting of 3996 reactions among 682 species, based on literature and this work, was established to describe the reactions and interaction kinetics of the butene isomers with oxygen and nitrogenous components. Model predictions were compared with the experimental results to gain insight into the low- and high-temperature fuel consumption without and with NO addition and thus the respective interaction chemistry. This investigation firstly contributes a consistent set of temperature-dependent concentration profiles for these two butene isomers under conditions relevant for engine exhaust gases. Secondly, the observed oxidation kinetics is significantly altered with the addition of NO. Specifically, NO promotes fuel consumption and introduces for i -butene a low-temperature behavior featuring a negative temperature coefficient (NTC) region. The present model shows reasonable agreement with the experimental results for major products and intermediate species, and it is capable to explain the promoting effect of NO that is initiated by its contribution to the radical pool. Further, it can describe the observed NTC region for the i -butene/NO mixture as a result of the competition of chain propagation and chain terminating reactions that were identified by reaction flow and sensitivity analyses. [ABSTRACT FROM AUTHOR]

Bio-butanol is an important alternative fuel and has many advantages in engine combustion. In the present work, laminar flame propagation of n- butanol/air and n- butanol/(14% O 2 /86% He) mixtures was investigated using a constant-volume combustion vessel at the unburnt temperature of 423 K, initial pressures of 1–20 atm and a series of equivalence ratios. Flame instabilities of n -butanol, including cellular instability and pulsating instability, were studied based on flame morphology. The unsteady transition from weak flame to normal flame was investigated based on the flame trajectories and this phenomenon became less perceptible in the flame with stronger combustion intensity and smaller Lewis number. The accelerated laminar burning velocity (LBV), thickened flame thickness, larger Markstein length and Markstein number indicate that the thermal diffusivity of n- butanol/O 2 /He flames are greatly enhanced by helium addition. The smaller densities and lower reactant concentrations result in the smaller laminar burning fluxes of n- butanol/O 2 /He flames than those of n -butanol/air flames. The high temperature kinetic model of n -butanol reported by Cai et al. was further developed with recent theoretical progresses on n -butanol reactions. The updated high temperature model was validated on the present experimental data and a vast amount of previous n -butanol combustion data. Sensitivity analysis was performed using the updated model under wide pressure and equivalence ratio ranges. The similarity in the sensitivities indicates that the sensitive reactions in n- butanol/air flames can be validated by using n- butanol/O 2 /He flames, which greatly compensates the problem that the n- butanol/air flames are susceptible to flame instabilities at high pressures. The weaker pressure dependence of the LBVs of n -butanol/O 2 /He flames indicates that their flame chemistry changes less to resist the pressure variation. [ABSTRACT FROM AUTHOR]

In order to examine in detail the effect of C C bond on the combustion chemistry of biodiesel fuels, two C4 fatty acid methyl esters (FAMEs) were investigated, namely methyl butanoate (MB) and methyl crotonate (MC). Pyrolysis experiments of these two FAMEs at 30, 150 and 780 Torr were conducted in a flow reactor over the temperature range of 773–1323 K, using gas chromatography-mass spectrometry. A number of pyrolysis species including C1 to C4 hydrocarbons, oxygenated products, esters and aromatics were observed and identified. A comprehensive kinetic model for MB and MC combustion was developed, and applied to validate against the new experimental data. In this work, peak mole fraction of benzene, as well as other unsaturated hydrocarbons in MC pyrolysis, was found to be in slightly higher amounts in comparison with that of MB. Kinetic modeling analysis revealed that the dominant formation pathway of benzene was the self-combination reaction of propargyl radical. Furthermore, the model was also validated against the previous experimental data on MB and MC combustion, including oxidation in jet stirred reactor, pyrolysis in shock tube and laminar premixed flame. This study suggests that the effect of C   C double bond in FAMEs might give rise to a growing tendency of initial PAH and soot precursors in the whole thermal decomposition process. [ABSTRACT FROM AUTHOR]