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Real biodiesel fuels are mixtures comprising many high molecular weight components, making it a challenge to predict their combustion chemistry with detailed kinetic models. Our group previously proposed a functional-group approach (FGMech) to model the combustion chemistry of real gasoline and jet fuels; this approach has now been extended to model real biodiesel combustion and mixtures with petroleum fuels. As in our previous work, a decoupling philosophy is adopted for construction of the model. A lumped reaction mechanism describes the (oxidative) pyrolysis of fuels, while a detailed base chemistry model represents the oxidation of key pyrolysis intermediates. However, due to the presence of the ester group, several oxygenated species are identified as additional primary products and incorporated into the lumped reaction steps. In addition to the lumped reactions initiated by unimolecular decomposition and H-atom abstraction reactions, a lumped H-atom addition-elimination reaction is also incorporated as a new reaction class to account for the presence of double bonds. Stoichiometric parameters are obtained based on a multiple linear regression (MLR) model, which establishes relationships between the fuel's functional group distributions and the stoichiometric parameters of the lumped reactions. Global rate constants are developed from consistent rate rules obtained from pure fuels. New pyrolysis experimental data for methyl pentanoate/methyl nonanoate and methyl heptanoate/ n -heptane mixtures (50%/50% in mol) are obtained in a jet-stirred reactor at atmospheric pressure. In general, kinetic models developed using the FGMech approach can reasonably reproduce all the validation targets obtained in this work, as well as those in the literature, confirming that functional-group-modeling is a promising approach to simulate combustion behavior of diesel/biodiesel surrogate fuels and real biodiesels. [ABSTRACT FROM AUTHOR]

Construction of kinetic models to predict real-fuel combustion properties requires significant human and computational resources. In the first of this two-part study, a functional group correlation approach called FGMech was proposed for predicting the stoichiometric parameters in lumped pyrolysis reactions. The stoichiometric parameters were implemented in a recent real-fuel kinetic model, HyChem (Xu et al., 2018), and the validity of this approach was demonstrated for simulating real-fuel combustion. The present work extends the FGMech approach for developing surrogate and real-fuel kinetic models. Our approach is fundamentally different from the HyChem development approach in that no parameters are tuned to match actual real-fuel pyrolysis/oxidation data, and all model parameters are derived only from functional group data. Along with the stoichiometric parameters obtained in the first part of this study, the thermodynamic data, lumped reaction rate parameters and transport data were predicted in this work based on the functional group characterization of real fuels. The Benson group additivity method was adopted to estimate the thermodynamic data of real fuels, while rate rules developed for pure fuels were used to estimate the rate constants of lumped reactions in real-fuel models. For transport data, normal boiling point, critical temperature and pressure (estimated using the Joback group contribution method) were used to obtain Lennard-Jones parameters. The format of lumped reactions in FGMech followed the HyChem approach, and the base mechanism was adopted from the AramcoMech 2.0 and USC Mech II, respectively, to compare the model performance with different base mechanisms. Fourteen surrogate and twelve real-fuel models were developed based on this approach; they were validated against the experimental data in the literature. FGMech's performance was also compared with detailed and reduced models available in the literature. FGMech reasonably captures the experimental data in the literature, indicating that the present modeling approach is promising for modeling the combustion behavior of fuel, including surrogate mixtures and real fuels. [ABSTRACT FROM AUTHOR]

In this work, the oxidation of ethylcyclohexane was studied in a jet-stirred reactor at 780 Torr, 480–780 K and equivalence ratios of 0.5, 1.0 and 2.0. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for the detection of oxidation products, including a series of reactive intermediates such as cycloalkylhydroperoxides, keto-hydroperoxides, alkenal-hydroperoxides and highly oxygenated molecules. Quantum chemistry calculations were performed to obtain ionization energies for the identification of some important intermediates and energy barriers of several important pathways. On the other hand, this work presents the first efforts on developing a low-temperature oxidation model of ethylcyclohexane. The present model can reasonably capture the low-temperature oxidation reactivities and negative temperature coefficient behaviors observed in both present and previous experimental work of ethylcyclohexane oxidation. Modeling analyses were performed to provide insight into the low-temperature oxidation chemistry of ethylcyclohexane. The two-stage O 2 addition mechanism is concluded to dominate the chain-branching process in the low-temperature region. The concerted elimination reactions of cycloalkylperoxy radical and "formally direct" chemically activated reactions of cycloalkyl+O 2 result in cycloalkenes and HO 2 formation at the negative temperature coefficient region and serve as main chain-termination pathways. Compared with smaller cycloalkanes like cyclohexane and methylcyclohexane, the ethyl sidechain structure in ethylcyclohexane reduces the energy barriers of cycloalkylperoxy radical isomerization and facilitates the formation of keto-hydroperoxides which leads to more pronounced low-temperature oxidation reactivity of ethylcyclohexane. [ABSTRACT FROM AUTHOR]

With the growing attention on ammonia (NH 3) combustion, understanding NH 3 and nitric oxide (NO) interaction at temperatures higher than DeNOx temperature region or even flame temperature becomes a new research need. In this work, the outwardly propagation spherical flame method was used to investigate the laminar flame propagation of NH 3 /NO/N 2 mixtures and constrain the uncertainties of the specific kinetics. The present experiments were conducted at initial pressure of 1 atm, temperature of 298 K and equivalence ratios from 1.1 to 1.9. A kinetic model of NH 3 /NO combustion was updated from our previous work. Compared with several previous models, the present model can reasonably reproduce the laminar burning velocity data measured in this work and speciation data in literature. Based on model analyses, the interaction of NH 3 and NO was thoroughly investigated. As both the oxidizer and a carrier of nitrogen element, NO frequently reacts with different decomposition products of NH 3 including NH 2 , NH and NNH, and converts nitrogen element to the final product N 2. It is found that the laminar burning velocity experiment of NH 3 /NO/N 2 mixtures using the outwardly propagating spherical flame method can provide highly sensitive validation targets for the kinetics in NH 3 and NO interaction. [ABSTRACT FROM AUTHOR]

This work reports an experimental and kinetic modeling investigation on the laminar flame propagation of three butylbenzene isomers (n -butylbenzene, iso -butylbenzene and tert -butylbenzene)/air mixtures. The experiments were performed in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 423 K, initial pressures of 1–10 atm, and equivalence ratios (ϕ) of 0.7–1.5. The laminar burning velocities of butylbenzene/O 2 /He mixtures were also measured at 423 K, 10 atm and ϕ = 1.5 to provide additional experimental data under conditions that the butylbenzene/air experiments are susceptible of cellular instability. Comparison among the laminar burning velocities of butylbenzenes including both the three isomers investigated in this work and sec -butylbenzene investigated in our recent work [Combust. Flame 211 (2020) 18–31] shows remarkable fuel isomeric effects, that is, iso -butylbenzene has the slowest laminar burning velocities, followed by n -butylbenzene and tert -butylbenzene, while sec -butylbenzene has the fastest laminar burning velocities. A kinetic model for butylbenzene combustion was developed to simulate the laminar flame propagation of butylbenzenes. Sensitivity analysis was performed to reveal important reactions in laminar flame propagation of butylbenzenes, including both small species reactions and fuel-specific reactions. Kinetic effects are concluded to result in the different laminar burning velocities of four butylbenzene isomers. Small species reactions control the laminar flame propagation under lean conditions, which results in small differences of laminar burning velocities. Chain termination reactions, especially fuel-specific reactions, have important contributions to inhibit the laminar flame propagation under rich conditions. The structural features of butylbenzene isomers can significantly affect the formation of some crucial radicals such as methyl, cyclopentadienyl and benzyl radicals under rich conditions, which leads to remarkable fuel isomeric effects on their laminar burning velocities, especially at high pressures. [ABSTRACT FROM AUTHOR]

In order to better understand the low-temperature oxidation chemistry of alkenes, 1-butene and i -butene oxidation experiments triggered by dimethyl ether (DME) were conducted in a jet-stirred reactor at 790 Torr, 500–725 K and the equivalence ratio of 0.35. Low-temperature oxidation intermediates involved in alcoholic radical chemistry and allylic radical chemistry were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). To better interpret the experimental data, a kinetic model was proposed based on our low-temperature oxidation model of DME and comprehensive oxidation models of 1-butene and i -butene in literature. Based on present experimental results and modeling analysis, alcoholic radical chemistry initiated by OH addition is mainly responsible for the low-temperature chain propagation of butenes, since the Waddington mechanism plays a dominant role compared with the chain-branching pathways through the second O 2 addition. Allylic radical+HO 2 reactions producing alkenyl hydroperoxides and fuel+O 2 serve as the major chain-branching and chain-termination pathways, respectively, and they are competitive in the negative temperature coefficient (NTC) region. In contrast, chain-branching pathways originating from allylic radical+O 2 and alkyl-like radical+O 2 reactions have little contribution to the OH formation. Comparison with the simulation results of butane/DME mixtures demonstrates that butenes can largely inhibit the reactivity of DME at low temperatures due to its reduced low-temperature chain-branching process. However, in the NTC region, butenes may not be good OH absorbents since the allylic radicals can convert HO 2 to OH and consequently enhance the oxidation reactivity. [ABSTRACT FROM AUTHOR]

This work reports an experimental and kinetic modeling investigation on the flow reactor pyrolysis of iso -butylbenzene and tert -butylbenzene at 0.04 and 1 atm. Pyrolysis products were detected and identified using synchrotron vacuum ultraviolet photoionization mass spectrometry, and their mole fractions versus heating temperature were measured. High-pressure-limit and pressure-dependent rate constants of unimolecular decomposition reactions of iso -butylbenzene were calculated in this work using the same method as the theoretical calculation investigation on similar reactions of n -butylbenzene, sec -butylbenzene and tert -butylbenzene reported by Belisario-Lara et al. [ J. Phys. Chem. A , 122 (2018) 3980–4001]. Furthermore, a pyrolysis model of four butylbenzene isomers was developed from our previous models of n -butylbenzene and sec -butylbenzene and validated by the present experimental data. Modeling analysis was performed to reveal key pathways in fuel decomposition and polycyclic aromatic hydrocarbon (PAH) formation of iso -butylbenzene and tert -butylbenzene. The dominant decomposition reactions of iso -butylbenzene and tert -butylbenzene under pyrolysis conditions are benzylic C C bond dissociation reactions, while the major products in pyrolysis process are produced by β - scission reactions of primary radical products. Fuel-specific pathways are found to strongly affect the formation of PAHs, especially for indene, naphthalene and phenanthrene. Fuel isomeric effects on the pyrolysis of four butylbenzene isomers were also analyzed with special concerns on the fuel decomposition and PAH formation processes. Different sidechain structures result in different distributions of major products, while different formation pathways and concentrations of precursors lead to different formation tendency of typical PAHs. [ABSTRACT FROM AUTHOR]

In order to study the combustion chemistry of carboxyl functionality, the laminar burning velocity of acetic acid/air and propanoic acid/air mixtures was investigated in a high-pressure constant-volume cylindrical combustion vessel at 423 K, 1 atm and equivalence ratios of 0.7–1.4. Experimental results reveal that the flame propagation of propanoic acid flame is much faster than that of acetic acid flame, especially under rich conditions, and the laminar burning velocity of propanoic acid/air mixtures peaks at richer conditions than that of acetic acid. The present theoretical calculations for the isomerization and decomposition of propanoic acid radicals indicate that the primary radical products are HOCO, H and C 2 H 5 , while those in acetic acid flame are CH 3 and OH based on previous studies. A kinetic model of the two acids was developed mainly based on previous and the present theoretical calculation results. It could reasonably capture the measured laminar burning velocities of acetic acid/air and propanoic acid/air mixtures in this work, as well as the previous experimental data in literature. Based on the present model, CH 3 - and ketene-related pathways play an important role in acetic acid flames. Under rich conditions, ketene is mostly converted to CH 3 via CH 2 CO+H CH 3 +CO, and the chain-termination reaction of CH 3 +H(+M)=CH 4 (+M) is enhanced, which strongly inhibits the propagation of rich acetic acid flames. In contrast, C 2 H 5 and ethylene chemistry play an important role in propanoic acid flames. Rich conditions promote the decomposition of C 2 H 5 , yielding ethylene and H, which can facilitate the flame propagation. This can explain the shift of the peak laminar burning velocity of propanoic acid/air mixtures towards a slightly richer condition compared with that of acetic acid/air mixtures. [ABSTRACT FROM AUTHOR]

In this work, low temperature (low-T) oxidation of 1-heptene was investigated in a jet-stirred reactor (JSR) over the temperatures of 450–800 K, 770 Torr and equivalence ratios of 0.5–2.0. The intermediates were identified and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography combined with mass spectrometry (GC–MS). The SVUV-PIMS experiment combined with quantum chemistry calculation of ionization energy enables the identification of fuel-specific intermediates, including C 7 alkenylperoxy radical and hydroperoxides, such as diolefinic-hydroperoxide, alkenylhydroperoxide, alkenyl-ketohydroperoxide and cyclic ether hydroperoxide. Among them, alkenylperoxy radical, diolefinic-hydroperoxide and cyclic ether hydroperoxide have not been detected in alkene oxidation before. In order to accurately identify and quantify other fuel-specific intermediates such as aldehyde and cyclic ether isomers, the GC–MS experiment was conducted under the same conditions as the SVUV-PIMS experiment. On the other hand, a detailed low-T oxidation model of 1-heptene was developed, which can reasonably capture the fuel oxidation rate and negative temperature coefficient behaviors observed in this work. The present model can not only interpret the formation of different kinds of hydroperoxides and predict their temperature windows, but also capture the formation of 2-heptenal, hexanal and heptanal, and branched tetrahydrofurans, which are derived from the H-abstraction by OH, OH addition and H addition reactions of 1-heptene, respectively, revealing that the competition between these reactions can be well characterized. [ABSTRACT FROM AUTHOR]

Ammonia is attracting more and more attentions due to its role as both a carbon-free fuel for gas turbines and an effective H 2 carrier. Only a limit number of investigations on the laminar flame propagation and laminar burning velocity of ammonia have been performed on elevated pressures, which were focused on ammonia/air mixtures and suffered strong buoyancy effect. In this work, laminar flame propagation of ammonia/O 2 /N 2 mixtures covering wide ranges of equivalence ratios, oxygen contents and initial pressures was investigated in a high-pressure constant-volume cylindrical combustion vessel. The oxygen enrichment speeds up the spherically expanding flames and consequently reduces buoyancy effect on the laminar flame propagation of ammonia. The laminar burning velocity was observed to increase with the increasing oxygen content, but decrease with the increasing initial pressure. A kinetic model of ammonia combustion consisting 38 species and 265 reactions was constructed from previous models with updated rate constants of important reactions. The present model can reasonably reproduce the laminar burning velocity data in this work and literature, as well as the ignition delay time and speciation data in literature. Based on the model analysis, effects of oxygen enrichment, equivalence ratio and initial pressure on laminar burning velocities of ammonia were analyzed in detail. It is revealed that the enhanced flame propagation with oxygen enrichment is mainly due to the increase of adiabatic flame temperature which in turn leads to higher concentrations of key radicals like H, OH and NH 2. For NH 3 and its major decomposition products like NH 2 and NH, reactions with oxygenated species such as OH, O, O 2 and NO are generally more important in the lean flames, while the role of reactions with H, NH and NH 2 becomes crucial in the rich flames. The calculated pressure dependent coefficient indicates that NH 3 /O 2 /N 2 flames exhibit clear pressure dependence, while this pressure dependence is weaker than those of the hydrocarbon and biofuel flames. [ABSTRACT FROM AUTHOR]

In this work, laminar flame speeds of CH 4 /CO/air mixtures were measured at the unburnt temperature of 353 K and pressures from 1 to 10 atm in a high-pressure constant-volume cylindrical combustion vessel. Effects of pressure, equivalence ratio and CO content in CH 4 /CO mixtures on laminar flame speeds were investigated. A kinetic model for CH 4 /CO combustion was developed based on recent progress in elementary reactions and validated against previous and present experimental targets. It is found that both the thermal effect originating from different adiabatic flame temperatures and chemical effect originating from differences in the radical pool play important roles in the variation of laminar flame speed. The separate contribution of each effect varies at different pressures, equivalence ratios and CO contents. Besides, the transition from CH 4 -related chemistry to CO-related chemistry can be monitored by the increasing concentration of O atom in the radical pool under all the investigated conditions. Based on the modeling analysis, R18 (CH 3 + CH 3 (+M) = C 2 H 6 (+M)), R19 (HCO (+M) = H + CO (+M)), R6 (CH4 (+M) = CH 3 + H (+M)) and R7 (CH 4 + H = CH 3 + H 2) have major contributions to the transition chemistry, especially under the rich conditions. Compared with the atmospheric pressure conditions, R20 (CO + O (+M) = CO 2 (+M)) and R22 (H + O 2 (+M) = HO 2 (+M)) are enhanced at high pressures, which leads to the decrease of O atom in the radical pool at high pressures. [ABSTRACT FROM AUTHOR]

Abstract In order to investigate the pressure-dependent effects of carbon chain growth from methyl radical, pyrolysis experiments of butane-2,3‑dione (also known as diacetyl) were conducted both in a flow reactor over 780–1520 K at low and atmospheric pressures and in a jet-stirred reactor (JSR) over 650–1130 K at 10 bar. Identification and quantification of the intermediates were achieved by synchrotron vacuum ultraviolet photoionization mass spectrometry in a flow reactor and Fourier transform infrared spectrometry and gas chromatography in a JSR. A pyrolysis model of butane-2,3‑dione, incorporating the sub-mechanisms of butane-2,3‑dione, ketene, C1–C4 hydrocarbons and the formation of benzene, was developed from our recently reported C1 model in this work. It was validated against the present pyrolysis data of butane-2,3‑dione and the pyrolysis data of C2–C4 hydrocarbon fuels and ketene in the literature. Generally, the present model could reasonably predict all the experimental targets. Based on the rate of production analysis, the unimolecular decomposition reaction (CH 3 COCOCH 3 (+ M) = 2CH 3 CO (+ M)) is the dominant pathway to consume the fuel at low pressure. However, at atmospheric and high pressures, both the unimolecular decomposition reaction and the H abstraction reaction of butane-2,3‑dione by methyl radical are important. Ketene, acetone and acetaldehyde were measured in this work and their formation is also pressure dependent. For the methyl-related growth chemistry, addition-elimination reactions play an important role at low pressure. Their contribution becomes less at atmospheric pressure and can be neglected at high pressure. In contrast, the combination reactions are responsible for methyl-related growth at high pressure. For the formation of benzene, the recombination of propargyl radical is the most important pathway at low pressure, while the H-assisted isomerization reaction of fulvene becomes dominant at atmospheric and high pressures. [ABSTRACT FROM AUTHOR]

Abstract The kinetics of propanal oxidation was studied in a jet-stirred reactor (JSR) at atmospheric pressure. The investigated temperature range is from 450 to 800 K at two different equivalence ratios (0.35 and 4.0). Thanks to the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS), critical intermediates were identified, including methylperoxy, methyl hydroperoxide, ethylperoxy, ethyl hydroperoxide, α-lactone, β-propiolactone and ketohydroperoxides. A kinetic model for propanal oxidation was also developed and validated against the present experimental results, as well as those available from the literature. Main chain-branching pathways under the investigated conditions were analyzed based on present experimental data and kinetic model. The O 2 addition reaction to the propanoyl radical and subsequent oxidation reactions play an important role in determining the oxidation rate of propanal under fuel-lean conditions, especially in the low-temperature oxidation region. Under fuel-rich conditions, the decomposition of the propanoyl radical is predominant and C 2 H 5 relevant reactions consequently determine the kinetics of oxidation of propanal. [ABSTRACT FROM AUTHOR]

Acetaldehyde oxidation was investigated in a jet-stirred reactor at temperatures from 460 to 900 K, equivalence ratios from 0.5 to 4.0 and pressures of 710–720 Torr. Reactive intermediates under low-temperature conditions, such as methylperoxy, methylhydroperoxide and ketohydroperoxide were detected using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A kinetic model for acetaldehyde oxidation was constructed by incorporating recent theoretical and modeling progress of acetaldehyde kinetics, as well as the calculated results of H-atom abstraction reactions of acetaldehyde by acetylperoxy, methylperoxy and methoxy in this work. The present model was then comprehensively validated against the measurements in this work and the experimental data from literature. Modeling analysis reveals that the main chain-branching pathways of acetaldehyde under low-temperature oxidation conditions are the decomposition of acetylhydroperoxide and methylhydroperoxide. The second O 2 -addition process was verified to exist in the low-temperature oxidation of acetaldehyde as ketohydroperoxide was observed, while its contribution to the overall reactivity was found to be minor under present investigated conditions. On the other hand, the sub-mechanism of acetylperoxy is crucial in the low-temperature regime while the methyl oxidation mechanism dominates in the negative temperature coefficient (NTC) region at all equivalence ratios investigated in this work. [ABSTRACT FROM AUTHOR]

• A comprehensive kinetic model for NH 3 /NO x /syngas/CH 3 OH/CH 4 is proposed. • The kinetic model validation conditions cover temperatures of 473–2000 K, pressures of 0.04–100 atm, and equivalence ratios of 0.04–116. • C/N cross reactions are less competitive with C/H ones under flame conditions, while are possible to compete with C/H ones under ignition conditions. The application of ammonia (NH 3) as a fuel could contribute to mitigating global warming and achieving carbon neutrality by mid-century. To enhance the reactivity of NH 3 combustion, cofiring NH 3 with hydrogen or C 1 fuels like syngas, methanol and methane is proposed. In previous work, we studied the combustion chemistry of NH 3 /H 2 mixtures using a comprehensive kinetic model. In this work, we expanded our comprehensive kinetic model to cover NH 3 /syngas, NH 3 /methanol and NH 3 /methane mixtures. Rate constants of critical cross reactions between C- and N -containing species were evaluated based on previous experimental and theoretical studies for selection and incorporation in the present model. Experimental data for both NO x /C 1 and NH 3 /C 1 fuels were selected from literature to test the present model, including speciation data measured in shock tubes, flow reactors, jet-stirred reactors, burner-stabilized flames, and the global parameters like ignition delay times and laminar flame speeds. The kinetic model validation conditions cover temperatures of 473–2000 K, pressures of 0.04–100 atm, and equivalence ratios of 0.04–116. In general, the present model adequately captures the selected experimental data. The present model can not only be used to predict the combustion of NH 3 /C 1 fuel mixtures, but is also capable to predict the mutual interaction of NO x /C 1 fuels. It is found that the rate constants between C-containing species and H 2 -related radicals are generally faster than (or close to) those between C- and N -containing species. At high temperatures, H 2 -related radicals (i.e. H, O, OH) have higher concentrations than reactive N -related radicals (NH, NH 2 , NO 2). Therefore, under these conditions, C-containing species are more likely to react with H 2 -related radicals rather than N -related ones, making the C/N cross reactions less prominent. However, under low- and intermediate-temperature conditions, the concentrations of H and O radicals become lower, while those of NH 2 and NO 2 become higher, making the cross reactions between C- and N -containing species possible to compete with C/H cross reactions. The present model can be used as the base chemistry model for NH 3 /larger hydrocarbon fuels. [ABSTRACT FROM AUTHOR]

In this work, oxidation chemistry of methanol was investigated in laminar premixed flames. Laminar burning velocities of methanol/air mixtures at 423 K and 1–10 atm were measured in a spherical combustion vessel, extending the range of equivalence ratio up to 2.1. A stoichiometric premixed flat flame of methanol/O 2 /Ar at 0.04 atm was also conducted using synchrotron VUV photoionization mass spectrometry (SVUV-PIMS) to obtain more detailed kinetic information. Particularly, fuel-derived radicals including methoxy radical and hydroxymethyl radical were observed, while formaldehyde (CH 2 O) and formic acid (HOCHO) were identified as the abundantly produced intermediates in methanol flame. A methanol model was developed and validated against the experimental data obtained in this work, as well as in literature. In the predictions of laminar burning velocities, HO 2 radical plays an important role under very rich conditions. Besides, the measurement of formaldehyde and formic acid in methanol flame is helpful to constrain the rate constant of CH 2 OH + O 2 = CH 2 O + HO 2 , which presents large uncertainties at high temperatures. [ABSTRACT FROM AUTHOR]

In order to better understand the high temperature chemistry of large alcohol containing 5 carbon atoms, the pyrolysis of 2-methyl-1-butanol from 750 to 1400 K was investigated in a flow reactor at the pressures of 30 and 760 Torr. About 25 pyrolysis species including some radicals and reactive intermediates were identified and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry. A kinetic model with 177 species and 994 reactions was developed and tested by the measured mole fraction profiles of pyrolysis species. Rate of production (ROP) and sensitivity analysis were used to reveal the decomposition characters of 2-methyl-1-butanol and the effect of pressure. Compared with butanol isomers, the contributions of unimolecular reactions in 2-methyl-1-butanol are much lower than H abstraction reactions, which shows a similar decomposition rule to i -butanol rather than 1-butanol. [ABSTRACT FROM AUTHOR]

To investigate the oxidation of ammonia (NH 3)/hydrogen (H 2) mixtures at intermediate temperatures, this work has implemented jet-stirred reactor (JSR) oxidation experiments of NH 3 /H 2 mixtures at atmospheric pressure and over 800-1280 K. The H 2 content in the NH 3 /H 2 mixtures is varied from zero to 70 vol% at equivalence ratios of 0.25 and 1.0. Species identification and quantification are achieved by using Fourier-transform infrared (FTIR) spectroscopy. A kinetic model for pure NH 3 and NH 3 /H 2 mixtures is also developed for this research, and validated against the present experimental data for pure NH 3 and NH 3 /H 2 mixtures, as well as those for pure NH 3 , H 2 /NO, H 2 /N 2 O, NH 3 /NO, NH 3 /NO 2 and NH 3 /H 2 mixtures in literature. The model basically captures the experimental data obtained here, as well as in literature. Both measured and predicted results from this work show that H 2 blending enhances the oxidation reactivity of NH 3. Based on the model analysis, under the present experimental conditions, NH 3 + H = NH 2 + H 2 proceeds in its reverse direction with increasing H 2 content. The H atom produced is able to combine with O 2 to produce either O and OH via a chain-branching reaction, or to yield HO 2 through a chain-propagation reaction. HO 2 is an important radical under the present intermediate-temperature conditions, which can convert NH 2 to OH via NH 2 + HO 2 = H 2 NO + OH; H 2 NO is then able to convert H to NH 2 and OH. In this reaction sequence, NH 2 and H 2 NO are chain carriers, converting HO 2 and H to two OH radicals. Since the OH radical is the dominant radical to consume NH 3 under the present conditions, the enhanced OH yield via H + O 2 = O + OH, NH 2 + HO 2 = H 2 NO + OH and H 2 NO + H = NH 2 +OH, with increasing H 2 content, promotes the consumption of NH 3. For NO x formation, non-monotonous trends are observed by increasing the content of H 2 at the 99% conversion of NH 3. These trends are determined by the competition between the dilution effects and the chemical effects of H 2 addition. Nitrogen related radicals, such as NH 2 , NH and N, decrease as H 2 increases, and this dilution effect reduces NO x formation. For chemical effects, the yields of oxygenated radicals, such as O, OH and HO 2 , are enhanced with increasing H 2 content, which results in enhancing effects on NO formation. For N 2 O formation, the enhanced oxygenated radicals (O, OH and HO 2) suppress its formation, while the enhanced NO promotes its formation. [ABSTRACT FROM AUTHOR]

This paper explores the low-temperature (low-T) oxidation chemistry of 2,4,4-trimethyl-1-pentene (IC8D4, diisobutylene) by using jet-stirred reactor (JSR) experiments of both IC8D4/dimethyl ether (DME) mixture and pure IC8D4 at near atmospheric pressure and low temperatures. Oxidation species are measured using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS), gas chromatography (GC) and GC combined with mass spectrometry (GC/MS). It is found that the oxidation of pure IC8D4 at atmospheric pressure presents negligible low-T reactivity and negative temperature coefficient (NTC) behavior, and that the oxidation reactivity of IC8D4/DME mixture is lower than that of butene/DME mixtures previously studied. A kinetic model for low-T IC8D4/DME oxidation is developed from recent oxidation models of IC8D4 and DME. Thermodynamic data of IC8D4 and key species in its sub-mechanism are obtained from theoretical calculations in this work, while rate constants of critical reactions are updated from recent theoretical calculation studies in literature. Based on the modeling analysis, four main pathways are found to be responsible for the consumption of IC8D4 at low temperatures. Among them, the first three pathways initiated by H addition, OH addition and H abstraction on allylic carbon sites are similar to those in 1-butene/DME and isobutene/DME oxidation. These three pathways are mainly responsible for promoting or retaining OH formation. The fourth pathway initiated by H abstraction on the alkyl carbon site differs from the other three in that it is only important in IC8D4/DME oxidation while not important in butenes/DME oxidation. This fourth pathway incorporates stepwise O 2 addition and cycloaddition reaction sequences, which can promote and inhibit OH formation, respectively. The increasing contribution of this fourth pathway in IC8D4/DME oxidation reduces its reactivity compared to that of butenes/DME oxidation. [ABSTRACT FROM AUTHOR]

This work reports investigations on laminar premixed ethylene (C 2 H 4) flames with special interests on measurements and kinetic modeling at elevated pressures. Chemical structures of laminar premixed C 2 H 4 /O 2 /Ar flames at 3 and 5 atm with the equivalence ratio of 1.4 were measured with molecular beam mass spectrometry. Both the stable and reactive species were identified and quantified in this work. Among them, the mole fractions of H and OH are evidently dependent on pressures. Laminar flame propagation of C 2 H 4 /air mixtures at 1 and 2 atm and that of C 2 H 4 /O 2 /He mixtures at 2 and 5 atm were also investigated in a high pressure constant-volume cylindrical combustion vessel at an unburnt temperature (T u) of 298 K and equivalence ratios of 0.7–1.5. Besides, a kinetic model for C 2 H 4 combustion was developed based on the evaluations of critical reactions with special concerns on recent theoretical calculation progress and validated against both the new data in this work and previous data of ethylene combustion in literature. The updated reactions of C 2 H 4 + O from recent theoretical calculations in the present model can improve the predictions of both the flame speciation results and the laminar burning velocities (LBVs) under rich and elevated pressure conditions. Compared with previous models, C 2 H 4 + O = 3CH 2 + CH 2 O instead of C 2 H 4 + O = CH 3 + HCO becomes a major consumption pathway of C 2 H 4 in the present model. This results in the lower production of CH 3 which is responsible for the chain termination under rich and elevated pressure conditions. Therefore, the present model could lower down the predictions of the CH 3 -related speciation data and improve the LBV predictions under rich and elevated pressure conditions. [ABSTRACT FROM AUTHOR]

In this work, pyrolysis experiments of ethyl valerate were performed in a flow reactor over 705–1051 K at low and atmospheric pressures and in a jet-stirred reactor over 633–1013 K at near-atmospheric pressure. Products were measured with synchrotron vacuum ultraviolet photoionization mass spectrometry in the flow reactor pyrolysis and gas chromatography in the jet-stirred reactor pyrolysis. Valeric acid and ethylene were observed as the most abundant pyrolysis products in both experiments. Laminar burning velocities of ethyl valerate/air mixtures were also measured in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 443 K and initial pressures of 1–10 atm. A kinetic model of ethyl valerate combustion incorporated with recent theoretical progress was developed to predict the new experimental data in this work, as well as the speciation data under flame conditions and laminar burning velocities at different initial temperatures and pressures in literature. Experimental observations and modeling analyses both confirm the significant role of the intramolecular elimination reaction of ethyl valerate producing valeric acid and ethylene. In particular, this reaction has exclusive significance in decomposition of ethyl valerate under pyrolysis conditions, indicating pyrolysis experiments can provide crucial constraints for its rate constant. Subsequent decomposition reactions of valeric acid at higher temperatures enrich the intermediate pool, especially radicals, and can continue producing ethylene to make its mole fraction keep growing under the investigated temperature ranges in the jet-stirred reactor pyrolysis. Under the flame propagation conditions, C 0 C 1 reactions have the highest sensitivity coefficients to the flame propagation, while ethylene- and vinyl-involved reactions also play important roles due to the abundant production of ethylene. [ABSTRACT FROM AUTHOR]

• Pyrolysis of iso -pentanol was conducted at 30 and 760 Torr using SVUV-PIMS. • Olefins and oxygenated products are the two major product families. • A kinetic model of iso -pentanol pyrolysis was developed and validated. • Fuel isomeric effects on the pyrolysis chemistry of pentanol were explored. • Different reactivities and product distributions were found for pentanol isomers. Flow reactor pyrolysis of iso -pentanol was conducted at 30 and 760 torr using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Pyrolysis products, especially radicals and enols, were identified and quantified. Olefins and oxygenated products (mainly aldehyde and enol) were found to be the two major product families in the pyrolysis of iso -pentanol. A kinetic model of iso -pentanol pyrolysis was developed and validated against the new experimental data. The rate of production (ROP) analysis and sensitivity analysis were conducted to understand the pyrolysis chemistry of iso -pentanol. The contribution of unimolecular decomposition reactions to the consumption of iso -pentanol is far less than that of H abstraction reactions at both pressures, which is similar to the situation in 2-methyl-1-butanol pyrolysis, but different from that in n -pentanol pyrolysis. Fuel isomeric effects on the pyrolysis chemistry of pentanol isomers were also investigated. Mole fraction profiles of fuels and major products were measured for other two typical pentanol isomers, i.e. n -pentanol and 2-methyl-1-butanol under the same pyrolysis conditions, while our previous models of n- pentanol and 2-methyl-1-butanol were used for the simulation. The initial decomposition temperatures of two branched pentanol isomers are slightly lower than that of n- pentanol at both pressures. The branched fuel structures strongly influence the molecular structures and formation pathways of fuel decomposition products such as C 4 and C 5 olefins, as well as the formation of benzene and fulvene. [ABSTRACT FROM AUTHOR]

Reactivity enhancement is crucial for the potential applications of ammonia (NH 3) as a gas turbine fuel. Doping more reactive fuels like H 2 , methane and syngas is a widely adopted strategy in this field, however fundamental combustion studies of NH 3 /reactive fuel mixtures under gas turbine-relevant high pressure conditions are still very limited. This work reports an effort to study laminar flame propagation of NH 3 /syngas mixtures up to 10 atm. Laminar burning velocities (LBVs) of NH 3 /syngas/air mixtures were measured at 298 K, various syngas contents in fuel mixtures (α) and H 2 contents in syngas (β), and equivalence ratios of 0.7–1.5 in a high-pressure constant-volume cylindrical combustion vessel. A kinetic model was developed for NH 3 /syngas combustion based on our recent NH 3 model and a recent syngas model in literature. It shows reasonable predictions on the present NH 3 /syngas LBVs at 1–10 atm, as well as previous data including NH 3 /syngas LBVs at 1 atm, NH 3 /syngas ignition delay times at various pressures and pure NH 3 LBVs at various pressures. Modeling analysis including the sensitivity analysis and rate of production analysis provides kinetic interpretation for the effects of fuel composition (α and β), equivalence ratio and pressure on NH 3 /syngas LBVs. It is found that the addition of syngas shifts the chemistry from NH 3 sub-mechanism to syngas sub-mechanism. A modified fictitious diluent gas method was proposed to separate the chemical and thermal effects of syngas addition, which shows that the chemical effect is more responsible for the enhanced laminar flame propagation. NH 3 /syngas/air flames have similar reaction networks but different preferred pathways under lean and rich conditions. Compared with pure NH 3 flames, the addition of syngas also improves the importance of H + O 2 (+ M) = HO 2 (+ M) and consequently leads to strong pressure dependency of NH 3 /syngas/air flames. [ABSTRACT FROM AUTHOR]

This work reports the investigation on pyrolysis and laminar flame propagation of sec- butylbenzene. The pyrolysis experiments were performed in a flow reactor using synchrotron vacuum ultraviolet photoionization mass spectrometry from 780 to 1166 K at 0.04 and 1 atm. The pyrolysis products were identified and their mole fraction profiles versus the heating temperature were evaluated. The laminar burning velocities of sec- butylbenzene/air mixtures were measured at the initial temperature of 423 K and initial pressures of 1–10 atm in a high-pressure constant-volume cylindrical combustion vessel with the equivalence ratio from 0.7 to 1.5. Furthermore, a kinetic model was developed to predict the pyrolysis and laminar flame propagation of sec ‑butylbenzene. Validation of the present model was performed against the new experimental data in this work. Rate of production analysis and sensitivity analysis were performed to provide insight into the chemistry in fuel decomposition and polycyclic aromatic hydrocarbons (PAHs) formation. In the flow reactor pyrolysis, the consumption of sec ‑butylbenzene is mainly controlled by the unimolecular decomposition reactions and H-atom abstraction reactions at both low and atmospheric pressures. Fuel specific pathways through propenylbenzene and α-methylstyrene become the dominant formation pathways of indene and naphthalene. In the laminar flame propagation, the laminar burning velocity of sec ‑butylbenzene is sensitive to the reactions of both small species and fuel-relevant intermediates under all investigated conditions. In particular, the pyrolysis reactions in the fuel sub-mechanism play inhibition effects on the laminar flame propagation of sec ‑butylbenzene under rich conditions. The laminar burning velocity of sec ‑butylbenzene is also compared with benzene, toluene and ethylbenzene under same initial conditions. Both the thermodynamic and kinetic effects are responsible for the difference in laminar burning velocities of these aromatic fuels. [ABSTRACT FROM AUTHOR]

Kinetic investigations into small ethyl esters lay the foundation for a good understanding of combustion properties, particularly those brought about by the ethyl ester functional group, for biodiesels in forms of fatty acid ethyl esters. A systematical study was conducted in this work, aimed at better interpreting the high-temperature reaction schemes for the simple C 3 C 5 ethyl esters. Quantitative information for crucial intermediates closely related to fuel consumptions was probed in the low-pressure premixed flame of each ester with a molecular beam mass spectrometer. Efforts based on theoretical calculations and extensive literature reviews were devoted to developing a kinetic model which can well predict the current experimental flame structures as well as speciation measurements under pyrolysis conditions reported in literature (Ren et al. (2014)). Model analyses in combination with experimental results helped in revealing the significant effects of specific reactions on the fates of key intermediates in ethyl esters combustion. The competitions between eliminations producing ethylene and acids and hydrogen abstractions by H atoms account for fuel consumptions under flame conditions, but the relative importance of the two reaction categories vary among the three flames. During the shock tube pyrolysis, acids are rapidly produced from fuel decompositions, but consumed in much longer periods, resulting in species formation and enriching the radical pools. [ABSTRACT FROM AUTHOR]

Diethyl ether (DEE) and its isomer n -butanol are both considered as promising fuel additives or neat biofuels. While some effects of their addition to hydrocarbon fuels under engine conditions have been reported, fundamental studies that aim at understanding the joint reaction pathways of such fuel mixtures remain quite scarce. Here, we have chosen n -butane as a well-studied hydrocarbon base fuel, and we have added these oxygenated isomers individually under identical conditions in premixed low-pressure flames. Different combustion behavior of the respective alkane-biofuel mixtures must then be related to the fuel structure. Analyses were performed in five fuel-rich flames including flames of n -butane, DEE, and n -butanol as well as two flames of n -butane doped with 50% DEE or 50% n -butanol. In this series, the carbon-to-oxygen ratio, argon dilution, pressure, and gas velocity were kept constant. More than 40 species in the range of C 0 –C 8 were identified and quantified in each flame by electron ionization (EI) molecular-beam mass spectrometry (MBMS) coupled with gas chromatography (GC). The experiments were partially complemented by tunable synchrotron vacuum ultraviolet (SVUV) photoionization (PI)-MBMS. To assist in the interpretation of the data, a kinetic model was established by combining different sub-mechanisms for these fuels available in the literature. As expected, the formation of toxic carbonyls, such as formaldehyde and acetaldehyde, increased significantly upon addition of both oxygenated fuels to n -butane. Blending n -butane with DEE noticeably reduces the formation of soot precursors, because primary reactions of DEE mainly release C 1 –C 2 hydrocarbon species to the system. n -Butanol addition, however, shows no significant reduction effects or even higher formation of soot precursors. These trends were observed both in the experiments and model predictions, and the higher ability of n -butanol to form soot precursors compared to DEE indeed results mainly from the differences in the fuel structure. [ABSTRACT FROM AUTHOR]

Low-temperature oxidation of cyclohexane is investigated in two jet-stirred reactors (JSRs) at 1.04 bar and the equivalence ratio of 0.25. Reactive hydroperoxides and highly oxygenated molecules are detected using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The isomers of C 6 H 10 O (5-hexenal, cyclic ethers and cyclohexanone) are separated using gas chromatography combined with mass spectrometry (GC–MS). Detection of characteristic hydroperoxides verifies that the conventional two-stage oxygen addition channels and recently reported third oxygen addition channels both contribute to the low-temperature oxidation of cyclohexane. Conformation-dependent channels theoretically investigated in Part I of this work are found correlated with the experimental observations of ketohydroperoxide (KHP) and alkenyl-hydroperoxide (AnHP) intermediates. A new detailed kinetic model of cyclohexane oxidation is constructed with consideration of the investigated conformation-dependent pathways in Part I and the experimental revisit of OH attack reactions over 889–1301 K and 1.22–1.84 bar. The model is validated against the newly measured oxidation data in this work and previous experimental data over a variety of pressure, temperature and equivalence ratio conditions. Modeling analysis reveals that the KHP channel and AnHP channel dominate the chain-branching process under the investigated conditions. The third oxygen addition channels and bimolecular reaction channels are found to play less important roles under the investigated conditions, while these reactions can provide more significant contributions to OH formation under high-pressure and lean conditions. [ABSTRACT FROM AUTHOR]