1-Hexene Ozonolysis across Atmospheric and Combustion Temperatures via Synchrotron-Based Photoelectron Spectroscopy and Chemical Ionization Mass Spectrometry.

This study investigates the complex interaction between ozone and the autoxidation of 1-hexene over a wide temperature range (300-800 K), overlapping atmospheric and combustion regimes. It is found that atmospheric molecular mechanisms initiate the oxidation of 1-hexene from room temperature up to combustion temperatures, leading to the formation of highly oxygenated organic molecules. As temperature rises, the highly oxygenated organic molecules contribute to radical-branching decomposition pathways inducing a high reactivity in the low-temperature combustion region, i.e., from 550 K. Above 650 K, the thermal decomposition of ozone into oxygen atoms becomes the dominant process, and a remarkable enhancement of the conversion is observed due to their diradical nature, counteracting the significant negative temperature coefficient behavior usually observed for 1-hexene. In order to better characterize the formation of heavy oxygenated organic molecules at the lowest temperatures, two analytical performance methods have been combined for the first time: synchrotron-based mass-selected photoelectron spectroscopy and orbitrap chemical ionization mass spectrometry. At the lowest studied temperatures (below 400 K), this analytical work has demonstrated the formation of the ketohydroperoxides usually found during the LTC oxidation of 1-hexene, as well as of molecules containing up to nine O atoms.

A Combined Experimental and Modeling Study on Isopropyl Nitrate Pyrolysis.

Alkyl nitrates thermally decompose by homolytic cleavage of the weak nitrate bond at very low temperatures (e.g., around 500 K at reaction times of a few seconds). This provides the opportunity to study the subsequent chemistry of the initially formed radical (or its subsequent pyrolysis products, if unstable) and nitrogen dioxide at such mild conditions. In this work this idea is applied to isopropyl nitrate (iPN) pyrolysis, which is studied in a tubular reactor at atmospheric pressure, temperatures ranging from 373 to 773 K, and residence times of around 2 s. At the experimental conditions, iPN decomposition starts at 473 K with O-N bond fission producing isopropoxy radical (i-C3H7O) and NO2. i-C3H7O is rapidly converted to acetaldehyde (CH3CHO), which is the most abundant product detected, and methyl radicals. Other major products detected are formaldehyde (CH2O), methanol (CH3OH), nitromethane (CH3NO2), NO, methane, formamide (CHONH2), and methyl nitrite (CH3ONO). Four literature nitrogen chemistry models─three of those augmented with iPN specific reactions─have been tested for their ability to predict the iPN decomposition and product profiles. The mechanism by the Curran group performs best, but it still underpredicts the observed high formaldehyde and methanol yields. A rate analysis indicates that the branching ratio of the reaction between methyl radicals and nitrogen dioxide is of significant importance. Based on recent theoretical and experimental data, new rate expressions for the two reactions CH3 + NO2 → CH3O + NO and CH3 + NO2 + He → CH3ONO2 + He are calculated and incorporated in the kinetic models. It is shown that this change clearly improves the predictions, although additional work is needed to achieve good agreement between calculated and measured species profiles.

Experimental and Updated Kinetic Modeling Study of Neopentane Low Temperature Oxidation.

Neopentane is an ideal fuel model to study low-temperature oxidation chemistry. The significant discrepancies between experimental data and simulations using the existing neopentane models indicate that an updated study of neopentane oxidation is needed. In this work, neopentane oxidation experiments are carried out using two jet-stirred reactors (JSRs) at 1 atm, at a residence time of 3 s, and at three different equivalence ratios of 0.5, 0.9, and 1.62. Two different analytical methods (synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography) were used to investigate the species distributions. Numerous oxidation intermediates were detected and quantified, including acetone, 3,3-dimethyloxetane, methacrolein, isobutene, 2-methylpropanal, isobutyric acid, and peroxides, which are valuable for validating the kinetic model describing neopentane oxidation. In the model development, the pressure dependencies of the rate constants for the reaction classes Q̇OOH + O2 and Q̇OOH decompositions are considered. This addition improves the prediction of the low-temperature oxidation reactivity of neopentane. Another focus of model development is to improve the prediction of carboxylic acids formed during the low-temperature oxidation of neopentane. The detection and identification of isobutyric acid indicates the existence of the Korcek mechanism during neopentane oxidation. Regarding the formation of acetic acid, the reaction channels are considered to be initiated from the reactions of ȮH radical addition to acetaldehyde/acetone. This updated kinetic model is validated extensively against the experimental data in this work and various experimental data available in the literature, including ignition delay times (IDTs) from both shock tubes (STs) and rapid compression machines (RCMs) and JSR speciation data at high temperatures.

Experimental evidence for the elusive ketohydroperoxide pathway and the formation of glyoxal in ethylene ozonolysis.

Despite decades of research on alkene ozonolysis, the kinetic network of the archetypal case of ethylene (CH2CH2) with ozone (O3) still lacks consensus. In this work, experimental evidence of an elusive diradical pathway is provided through the detection of the 2-hydroperoxyacetaldehyde ketohydroperoxide and its decomposition product, glyoxal.

Product Identification in the Low-Temperature Oxidation of Cyclohexane Using a Jet-Stirred Reactor in Combination with SVUV-PEPICO Analysis and Theoretical Quantum Calculations.

Cyclohexane oxidation chemistry was investigated using a near-atmospheric pressure jet-stirred reactor at T = 570 K and equivalence ratio ϕ = 0.8. Numerous intermediates including hydroperoxides and highly oxygenated molecules were detected using synchrotron vacuum ultraviolet photoelectron photoion coincidence spectroscopy. Supported by high-level quantum calculations, the analysis of photoelectron spectra allowed the firm identification of molecular species formed during the oxidation of cyclohexane. Besides, this work validates recently published gas chromatography and synchrotron vacuum ultraviolet photoionization mass spectrometry data. Unambiguous detection of characteristic hydroperoxides (e.g., γ-ketohydroperoxides) and their respective decomposition products provides support for the conventional O2 addition channels up to the third addition and their relative contribution to the cyclohexane oxidation. The results were also compared with the predictions of a recently proposed new detailed kinetic model of cyclohexane oxidation. Most of the predictions are in line with the current experimental findings, highlighting the robustness of the kinetic model. However, the analysis of the recorded slow photoelectron spectra indicating the possible presence of C5 species in the kinetic model provides hints that the substituted cyclopentyl radicals from cyclohexyl ring opening might play a minor role in cyclohexane oxidation. Potentially important missing reactions are also discussed.

Accounting for molecular flexibility in photoionization: case of tert-butyl hydroperoxide.

tert-Butyl hydroperoxide (tBuOOH) is a common intermediate in the oxidation of organic compounds that needs to be accurately quantified in complex gas mixtures for the development of chemical kinetic models of low temperature combustion. This work presents a combined theoretical and experimental investigation on the synchrotron-based VUV single photon ionization of gas-phase tBuOOH in the 9.0 - 11.0 eV energy range, including dissociative ionization processes. Computations consist of the determination of the structures, vibrational frequencies and the energetics of neutral and ionic tBuOOH. The Franck-Condon spectrum for the tBuOOH+ (X+) + e- ← tBuOOH (X) + hν transition is computed, where special treatment is undertaken because of the flexibility of tBuOOH, in particular regarding the OOH group. Through comparison of the experimental mass-selected threshold photoelectron spectra with explicitly correlated coupled cluster calculations and Franck-Condon simulations that account for the flexibility of the molecule, an estimation of the ionization energy is given. The appearance energy of the only fragment observed within the above-mentioned energy range, identified as the tert-butyl C4H9+, is also reported. Finally, the signal branching ratio between the parent and the fragment ions is provided as a function of photon energy, essential to quantify tBuOOH in gas-phase oxidation/combustion experiments via advanced mass spectrometry techniques.

Pyrolysis and Combustion Chemistry of Pyrrole, a Reference Component for Bio-oil Surrogates: Jet-Stirred Reactor Experiments and Kinetic Modeling.

Fast-pyrolysis bio-oils (FPBOs) obtained from lignocellulosic biomass are gaining attention as sustainable fuels for various applications, including the transport sector and power production. A significant fraction of bio-oils is constituted by nitrogen-containing compounds (N fuels) that should be considered when developing surrogate models for FPBOs. Moreover, the content of N fuels in FPBOs is expected to strongly contribute to the production of nitrogen oxides (NO x ) directly from fuel-bound nitrogen (fuel NO x ), in addition to the thermal NO x formation pathways typical of high-temperature combustion conditions. This work investigates the pyrolysis and combustion chemistry of pyrrole (C4H5N), a candidate reference fuel component for FPBO surrogate models. Speciation measurements in an atmospheric pressure jet-stirred reactor have been performed for both pyrolysis and oxidation conditions. Pyrolysis experiments have been performed for 1% pyrrole/helium mixtures over the temperature range T = 925-1200 K. Oxidation experiments were carried out for 1% pyrrole/oxygen/helium mixtures at three equivalence ratios (φ = 0.5, 1.0, and 2.0) over the temperature range T = 700-1200 K. These new data significantly extend the number of experimental targets for kinetic model validation available at present for pyrrole combustion. After a thorough revision of previous theoretical and kinetic modeling studies, a preliminary kinetic model is developed and validated by means of comparison to new experimental data and those previously reported in the literature. The rate of production and sensitivity analyses highlight important pathways deserving further investigations for a better understanding of pyrrole and, more in general, N fuel combustion chemistry. A critical discussion on experimental challenges to be faced when dealing with pyrrole is also reported, encouraging further experimental investigation with advanced diagnostics.

Isomer-sensitive characterization of low temperature oxidation reaction products by coupling a jet-stirred reactor to an electron/ion coincidence spectrometer: case of n-pentane.

Through the use of tunable vacuum ultraviolet light generated by the DESIRS VUV synchrotron beamline, a jet-stirred reactor was coupled for the first time to an advanced photoionization mass spectrometer based upon a double imaging PhotoElectron PhotoIon COincidence (i2PEPICO) scheme. This new coupling was used to investigate the low-temperature oxidation of n-pentane, a prototype molecule for gasoline or diesel fuels. Experiments were performed under quasi-atmospheric pressure (1.1 bar) with a residence time of 3 s for two equivalence ratios (1/3 and 0.5) with a fuel initial mole fraction of 0.01. The measured time-of-flight mass spectra are in good agreement with those previously obtained with other photoionization mass spectrometers and, like those previous ones, display several m/z peaks for which the related species assignation is ambiguous. This paper shows how the analysis of the coincident mass-tagged Threshold PhotoElectron Spectra (TPES) together with first principle computations, consisting of the determination of the adiabatic ionization energies and the spectra of some products, may assist products' identification. The results mostly confirm those previously obtained by photoionization mass spectrometry and gas chromatography, but also allow a more accurate estimation of the 1-pentene/2-pentene mole fraction ratio. Our data also indicate a higher formation of acetone and methyl ethyl ketone than what is predicted by current models, as well as the presence of products that were not previously taken into account, such as methoxyacetylene, methyl vinyl ketone or furanone. The formation of three, four and five membered ring cyclic ethers is confirmed along with linear ketones: 2- and 3-pentanone. A significant general trend in indicating higher amounts of ketones than are indicated by gas chromatography is noted. Finally, TPES of alkenylhydroperoxides are also provided for the first time and constrains on the isomers identification are provided.

Hydroperoxide Measurements During Low-Temperature Gas-Phase Oxidation of n-Heptane and n-Decane.

A wide range of hydroperoxides (C1-C3 alkyl hydroperoxides, C3-C7 alkenyl hydroperoxides, C7 ketohydroperoxides, and hydrogen peroxide (H2O2)), as well as ketene and diones, have been quantified during the gas-phase oxidation of n-heptane. Some of these species, as well as C10 alkenyl hydroperoxides and ketohydroperoxides, were also measured during the oxidation of n-decane. These experiments were performed using an atmospheric-pressure jet-stirred reactor at temperatures from 500 to 1100 K and one of three analytical methods, time-of-flight mass spectrometry combined with tunable synchrotron photoionization with a molecular beam sampling: time-of-flight mass spectrometry combined with laser photoionization with a capillary tube sampling, continuous wave cavity ring-down spectroscopy with sonic probe sampling. The experimental temperature at which the maximum mole fraction is observed increases significantly for alkyl hydroperoxides, alkenyl hydroperoxides, and then more so again for hydrogen peroxide, compared to ketohydroperoxides. The influence of the equivalence ratio from 0.25 to 4 on the formation of these peroxides has been studied during n-heptane oxidation. The up-to-date detailed kinetic oxidation models for n-heptane and for n-decane found in the literature have been used to discuss the possible pathways by which these peroxides, ketene, and diones are formed. In general, the model predicts well the reactivity of the two fuels, as well as the formation of major intermediates.

Study of the Formation of the First Aromatic Rings in the Pyrolysis of Cyclopentene.

The thermal decomposition of cyclopentene was studied in a jet-stirred reactor operated at constant pressure and temperature to provide new experimental information about the formation of the first aromatic rings from cyclic C5 species. Experiments were carried out at a residence time of 1 s, a pressure of 106.7 kPa, temperatures ranging from 773 to 1073 K and under diluted conditions (cyclopentene inlet mole fraction of 0.04). Species were quantified using three analytical methods: gas chromatography, synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS), and single photon laser ionization mass spectrometry (SPI-MS). Several species could be quantified using both methods allowing comparison of experimental data obtained with the three apparatuses. Discrepancies observed in mole fraction profiles of some large aromatics suggest that the direct sampling in the gas phase (with a molecular beam or a capillary tube) provide more reliable results. The main reaction products are 1,3-cyclopentadiene and hydrogen. The formation of many unsaturated C2-C6 olefins, diolefins and alkynes was also observed but in smaller amounts. Benzene, toluene, styrene, indene, and naphthalene were detected from 923 K. SVUV-PIMS data allowed the identification of another C6H6 isomer which is 1,5-hexadien-3-yne rather than fulvene. The quantification of the cyclopentadienyl radical was obtained from SVUV-PIMS and SPI-MS data with some uncertainty induced by the possible contribution to the signal for m/z 65 of a fragment from the decomposition of a larger ion. This is the first time that a radical is quantified in a jet-stirred reactor using non-optical techniques. SPI-MS analyses allowed the detection of species likely being combination products of allyl and cyclopentadienyl radicals. A model was developed for the pyrolysis of cyclopentene. This model includes routes of formation of aromatics from the cyclopentadienyl radical. The comparison of experimental and computed data is overall satisfactory for primary reaction products whereas discrepancies are still observed for aromatics.

Experimental and Modeling Investigation of the Low-Temperature Oxidation of Dimethyl Ether.

The oxidation of dimethyl ether (DME) was studied using a jet-stirred reactor over a wide range of conditions: temperatures from 500 to 1100 K; equivalence ratios of 0.25, 1, and 2; residence time of 2 s; pressure of 106.7 kPa (close to the atmospheric pressure); and an inlet fuel mole fraction of 0.02 (with high dilution in helium). Reaction products were quantified using two analysis methods: gas chromatography and continuous wave cavity ring-down spectroscopy (cw-CRDS). cw-CRDS enabled the quantification of formaldehyde, which is one of the major products from DME oxidation, as well as that of hydrogen peroxide, which is an important branching agent in low-temperature oxidation chemistry. Experimental data were compared with data computed using models from the literature with important deviations being observed for the reactivity at low-temperature. A new detailed kinetic model for the oxidation of DME was developed in this study. Kinetic parameters used in this model were taken from literature or calculated in the present work using quantum calculations. This new model enables a better prediction of the reactivity in the low-temperature region. Under the present JSR conditions, error bars on predictions were given. Simulations were also successfully compared with experimental flow reactor, jet-stirred reactor, shock tube, rapid compression machine, and flame data from literature. The kinetic analysis of the model enabled the highlighting of some specificities of the oxidation chemistry of DME: (1) the early reactivity which is observed at very low-temperature (e.g., compared to propane) is explained by the absence of inhibiting reaction of the radical directly obtained from the fuel (by H atom abstraction) with oxygen yielding an olefin + HO2·; (2) the low-temperature reactivity is driven by the relative importance of the second addition to O2 (promoting the reactivity through branching chain) and the competitive decomposition reactions with an inhibiting effect.

Experimental and Kinetic Modeling Study of 2-Methyl-2-Butene: Allylic Hydrocarbon Kinetics.

Two experimental studies have been carried out on the oxidation of 2-methyl-2-butene, one measuring ignition delay times behind reflected shock waves in a stainless steel shock tube, and the other measuring fuel, intermediate, and product species mole fractions in a jet-stirred reactor (JSR). The shock tube ignition experiments were carried out at three different pressures, approximately 1.7, 11.2, and 31 atm, and at each pressure, fuel-lean (ϕ = 0.5), stoichiometric (ϕ = 1.0), and fuel-rich (ϕ = 2.0) mixtures were examined, with each fuel/oxygen mixture diluted in 99% Ar, for initial postshock temperatures between 1330 and 1730 K. The JSR experiments were performed at nearly atmospheric pressure (800 Torr), with stoichiometric fuel/oxygen mixtures with 0.01 mole fraction of 2M2B fuel, a residence time in the reactor of 1.5 s, and mole fractions of 36 different chemical species were measured over a temperature range from 600 to 1150 K. These JSR experiments represent the first such study reporting detailed species measurements for an unsaturated, branched hydrocarbon fuel larger than iso-butene. A detailed chemical kinetic reaction mechanism was developed to study the important reaction pathways in these experiments, with particular attention on the role played by allylic C-H bonds and allylic pentenyl radicals. The results show that, at high temperatures, this olefinic fuel reacts rapidly, similar to related alkane fuels, but the pronounced thermal stability of the allylic pentenyl species inhibits low temperature reactivity, so 2M2B does not produce "cool flames" or negative temperature coefficient behavior. The connections between olefin hydrocarbon fuels, resulting allylic fuel radicals, the resulting lack of low-temperature reactivity, and the gasoline engine concept of octane sensitivity are discussed.

Experimental investigation of the low temperature oxidation of the five isomers of hexane.

The low-temperature oxidation of the five hexane isomers (n-hexane, 2-methyl-pentane, 3-methyl-pentane, 2,2-dimethylbutane, and 2,3-dimethylbutane) was studied in a jet-stirred reactor (JSR) at atmospheric pressure under stoichiometric conditions between 550 and 1000 K. The evolution of reactant and product mole fraction profiles were recorded as a function of the temperature using two analytical methods: gas chromatography and synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Experimental data obtained with both methods were in good agreement for the five fuels. These data were used to compare the reactivity and the nature of the reaction products and their distribution. At low temperature (below 800 K), n-hexane was the most reactive isomer. The two methyl-pentane isomers have about the same reactivity, which was lower than that of n-hexane. 2,2-Dimethylbutane was less reactive than the two methyl-pentane isomers, and 2,3-dimethylbutane was the least reactive isomer. These observations are in good agreement with research octane numbers given in the literature. Cyclic ethers with rings including 3, 4, 5, and 6 atoms have been identified and quantified for the five fuels. While the cyclic ether distribution was notably more detailed than in other literature of JSR studies of branched alkane oxidation, some oxiranes were missing among the cyclic ethers expected from methyl-pentanes. Using SVUV-PIMS, the formation of C2-C3 monocarboxylic acids, ketohydroperoxides, and species with two carbonyl groups have also been observed, supporting their possible formation from branched reactants. This is in line with what was previously experimentally demonstrated from linear fuels. Possible structures and ways of decomposition of the most probable ketohydroperoxides were discussed. Above 800 K, all five isomers have about the same reactivity, with a larger formation from branched alkanes of some unsaturated species, such as allene and propyne, which are known to be soot precursors.

Combustion chemistry and flame structure of furan group biofuels using molecular-beam mass spectrometry and gas chromatography - Part III: 2,5-Dimethylfuran.

This work is the third part of a study focusing on the combustion chemistry and flame structure of furan and selected alkylated derivatives, i.e. furan in Part I, 2-methylfuran (MF) in Part II, and 2,5-dimethylfuran (DMF) in the present work. Two premixed low-pressure (20 and 40 mbar) flat argon-diluted (50%) flames of DMF were studied with electron-ionization molecular-beam mass spectrometry (EI-MBMS) and gas chromatography (GC) under two equivalence ratios (φ=1.0 and 1.7). Mole fractions of reactants, products, and stable and radical intermediates were measured as a function of the distance to the burner. Kinetic modeling was performed using a reaction mechanism that was further developed in the present series, including Part I and Part II. A reasonable agreement between the present experimental results and the simulation is observed. The main reaction pathways of DMF consumption were derived from a reaction flow analysis. Also, a comparison of the key features for the three flames is presented, as well as a comparison between these flames of furanic compounds and those of other fuels. An a priori surprising ability of DMF to form soot precursors (e.g. 1,3-cyclopentadiene or benzene) compared to less substituted furans and to other fuels has been experimentally observed and is well explained in the model.

Combustion chemistry and flame structure of furan group biofuels using molecular-beam mass spectrometry and gas chromatography - Part II: 2-Methylfuran.

This is Part II of a series of three papers which jointly address the combustion chemistry of furan and its alkylated derivatives 2-methylfuran (MF) and 2,5-dimethylfuran (DMF) under premixed low-pressure flame conditions. Some of them are considered to be promising biofuels. With furan as a common basis studied in Part I of this series, the present paper addresses two laminar premixed low-pressure (20 and 40 mbar) flat argon-diluted (50%) flames of MF which were studied with electron-ionization molecular-beam mass spectrometry (EI-MBMS) and gas chromatography (GC) for equivalence ratios φ=1.0 and 1.7, identical conditions to those for the previously reported furan flames. Mole fractions of reactants, products as well as stable and reactive intermediates were measured as a function of the distance above the burner. Kinetic modeling was performed using a comprehensive reaction mechanism for all three fuels given in Part I and described in the three parts of this series. A comparison of the experimental results and the simulation shows reasonable agreement, as also seen for the furan flames in Part I before. This set of experiments is thus considered to be a valuable additional basis for the validation of the model. The main reaction pathways of MF consumption have been derived from reaction flow analyses, and differences to furan combustion chemistry under the same conditions are discussed.

Combustion chemistry and flame structure of furan group biofuels using molecular-beam mass spectrometry and gas chromatography - Part I: Furan.

Fuels of the furan family, i.e. furan itself, 2-methylfuran (MF), and 2,5-dimethylfuran (DMF) are being proposed as alternatives to hydrocarbon fuels and are potentially accessible from cellulosic biomass. While some experiments and modeling results are becoming available for each of these fuels, a comprehensive experimental and modeling analysis of the three fuels under the same conditions, simulated using the same chemical reaction model, has - to the best of our knowledge - not been attempted before. The present series of three papers, detailing the results obtained in flat flames for each of the three fuels separately, reports experimental data and explores their combustion chemistry using kinetic modeling. The first part of this series focuses on the chemistry of low-pressure furan flames. Two laminar premixed low-pressure (20 and 40 mbar) flat argon-diluted (50%) flames of furan were studied at two equivalence ratios (φ=1.0 and 1.7) using an analytical combination of high-resolution electron-ionization molecular-beam mass spectrometry (EI-MBMS) in Bielefeld and gas chromatography (GC) in Nancy. The time-of-flight MBMS with its high mass resolution enables the detection of both stable and reactive species, while the gas chromatograph permits the separation of isomers. Mole fractions of reactants, products, and stable and radical intermediates were measured as a function of the distance to the burner. A single kinetic model was used to predict the flame structure of the three fuels: furan (in this paper), 2-methylfuran (in Part II), and 2,5-dimethylfuran (in Part III). A refined sub-mechanism for furan combustion, based on the work of Tian et al. [Combustion and Flame 158 (2011) 756-773] was developed which was then compared to the present experimental results. Overall, the agreement is encouraging. The main reaction pathways involved in furan combustion were delineated computing the rates of formation and consumption of all species. It is seen that the predominant furan consumption pathway is initiated by H-addition on the carbon atom neighboring the O-atom with acetylene as one of the dominant products.

Products from the oxidation of linear isomers of hexene.

The experimental study of the oxidation of the three linear isomers of hexene was performed in a quartz isothermal jet-stirred reactor (JSR) at temperatures ranging from 500 to 1100 K including the negative temperature coefficient (NTC) zone, at quasi-atmospheric pressure (1.07 bar), at a residence time of 2 s and with dilute stoichiometric mixtures. The fuel and reaction product mole fractions were measured using online gas chromatography. In the case of 1-hexene, the JSR has also been coupled through a molecular-beam sampling system to a reflectron time-of-flight mass spectrometer combined with tunable synchrotron vacuum ultraviolet photoionization. A difference of reactivity between the three fuels, which varies with the temperature range has been observed and is discussed according to the changes in the possible reaction pathways when the double bond is displaced. An enhanced importance of the reactions via the Waddington mechanism and of those of allylic radicals with HO2 radicals can be noted for 2- and 3-hexenes compared to 1-hexene.

Quantification of OH and HO2 radicals during the low-temperature oxidation of hydrocarbons by Fluorescence Assay by Gas Expansion technique.

•OH and •HO2 radicals are known to be the key species in the development of ignition. A direct measurement of these radicals under low-temperature oxidation conditions (T = 550-1,000 K) has been achieved by coupling a technique named fluorescence assay by gas expansion, an experimental technique designed for the quantification of these radicals in the free atmosphere, to a jet-stirred reactor, an experimental device designed for the study of low-temperature combustion chemistry. Calibration allows conversion of relative fluorescence signals to absolute mole fractions. Such radical mole fraction profiles will serve as a benchmark for testing chemical models developed to improve the understanding of combustion processes.

A comprehensive experimental and detailed chemical kinetic modelling study of 2,5-dimethylfuran pyrolysis and oxidation.

The pyrolytic and oxidative behaviour of the biofuel 2,5-dimethylfuran (25DMF) has been studied in a range of experimental facilities in order to investigate the relatively unexplored combustion chemistry of the title species and to provide combustor relevant experimental data. The pyrolysis of 25DMF has been re-investigated in a shock tube using the single-pulse method for mixtures of 3% 25DMF in argon, at temperatures from 1200-1350 K, pressures from 2-2.5 atm and residence times of approximately 2 ms. Ignition delay times for mixtures of 0.75% 25DMF in argon have been measured at atmospheric pressure, temperatures of 1350-1800 K at equivalence ratios (ϕ) of 0.5, 1.0 and 2.0 along with auto-ignition measurements for stoichiometric fuel in air mixtures of 25DMF at 20 and 80 bar, from 820-1210 K. This is supplemented with an oxidative speciation study of 25DMF in a jet-stirred reactor (JSR) from 770-1220 K, at 10.0 atm, residence times of 0.7 s and at ϕ = 0.5, 1.0 and 2.0. Laminar burning velocities for 25DMF-air mixtures have been measured using the heat-flux method at unburnt gas temperatures of 298 and 358 K, at atmospheric pressure from ϕ = 0.6-1.6. These laminar burning velocity measurements highlight inconsistencies in the current literature data and provide a validation target for kinetic mechanisms. A detailed chemical kinetic mechanism containing 2768 reactions and 545 species has been simultaneously developed to describe the combustion of 25DMF under the experimental conditions described above. Numerical modelling results based on the mechanism can accurately reproduce the majority of experimental data. At high temperatures, a hydrogen atom transfer reaction is found to be the dominant unimolecular decomposition pathway of 25DMF. The reactions of hydrogen atom with the fuel are also found to be important in predicting pyrolysis and ignition delay time experiments. Numerous proposals are made on the mechanism and kinetics of the previously unexplored intermediate temperature combustion pathways of 25DMF. Hydroxyl radical addition to the furan ring is highlighted as an important fuel consuming reaction, leading to the formation of methyl vinyl ketone and acetyl radical. The chemically activated recombination of HȮ2 or CH3Ȯ2 with the 5-methyl-2-furanylmethyl radical, forming a 5-methyl-2-furylmethanoxy radical and ȮH or CH3Ȯ radical is also found to exhibit significant control over ignition delay times, as well as being important reactions in the prediction of species profiles in a JSR. Kinetics for the abstraction of a hydrogen atom from the alkyl side-chain of the fuel by molecular oxygen and HȮ2 radical are found to be sensitive in the estimation of ignition delay times for fuel-air mixtures from temperatures of 820-1200 K. At intermediate temperatures, the resonantly stabilised 5-methyl-2-furanylmethyl radical is found to predominantly undergo bimolecular reactions, and as a result sub-mechanisms for 5-methyl-2-formylfuran and 5-methyl-2-ethylfuran, and their derivatives, have also been developed with consumption pathways proposed. This study is the first to attempt to simulate the combustion of these species in any detail, although future refinements are likely necessary. The current study illustrates both quantitatively and qualitatively the complex chemical behavior of what is a high potential biofuel. Whilst the current work is the most comprehensive study on the oxidation of 25DMF in the literature to date, the mechanism cannot accurately reproduce laminar burning velocity measurements over a suitable range of unburnt gas temperatures, pressures and equivalence ratios, although discrepancies in the experimental literature data are highlighted. Resolving this issue should remain a focus of future work.

An experimental and modeling study of the low- and high-temperature oxidation of cyclohexane.

The experimental study of the oxidation of cyclohexane has been performed in a jet-stirred reactor at temperatures ranging from 500 to 1100 K (low- and intermediate temperature zones including the negative temperature-coefficient area), at a residence time of 2 s and for dilute mixtures with equivalence ratios of 0.5, 1, and 2. Experiments were carried out at quasi-atmospheric pressure (1.07 bar). The fuel and reaction product mole fractions were measured using online gas chromatography. A total of 34 reaction products have been detected and quantified in this study. Typical reaction products formed in the low-temperature oxidation of cyclohexane include cyclic ethers (1,2-epoxycyclohexane and 1,4-epoxycyclohexane), 5-hexenal (formed from the rapid decomposition of 1,3-epoxycyclohexane), cyclohexanone, and cyclohexene, as well as benzene and phenol. Cyclohexane displays high low-temperature reactivity with well-marked negative temperature-coefficient (NTC) behavior at equivalence ratios 0.5 and 1. The fuel-rich system (ϕ = 2) is much less reactive in the same region and exhibits no NTC. To the best of our knowledge, this is the first jet-stirred reactor study to report NTC in cyclohexane oxidation. Laminar burning velocities were also measured by the heated burner method at initial gas temperatures of 298, 358, and 398 K and at 1 atm. The laminar burning velocity values peak at ϕ = 1.1 and are measured as 40 and 63.1 cm/s for Ti = 298 and 398 K, respectively. An updated detailed chemical kinetic model including low-temperature pathways was used to simulate the present (jet-stirred reactor and laminar burning velocity) and literature experimental (laminar burning velocity, rapid compression machine, and shock tube ignition delay times) data. Reasonable agreement is observed with most of the products observed in our reactor, as well as the literature experimental data considered in this paper.