Photoionization Molecular-beam Mass Spectrometry Research Articles

Oxidation of ethyl methyl ether: Jet-stirred reactor experiments and kinetic modeling

Larger ethers such as diethyl ether (DEE) and di-n-propyl ether (DPE) have different oxidation behavior (double-NTC behavior) compared to the simplest dimethyl ether (DME). Such phenomena are interpreted with different reactions and processes in different ether kinetic models, which also predict different formation pathways of oxidation intermediates such as acids. To gain further insights into the oxidation kinetics of linear ethers, ethyl methyl ether (EME), which has a nonsymmetrical structure, was studied in this work. Oxidation experiments of 1% of EME were performed in a jet-stirred reactor at 1 atm, a residence time of 2 s, an equivalence ratio of 1, and over a temperature range of 375–850 K. The intermediates were analyzed with photoionization molecular-beam mass spectrometry. To explain the oxidation behavior of EME, a detailed kinetic model was also constructed. The oxidation of EME spans a wider temperature range than DME, but no obvious double-NTC behavior was observed as DEE. Based on the model analysis and profiles of critical intermediates such as ketohydroperoxides (KHPs) and CH3O2H, the low-temperature oxidation behavior of EME was explained by the chain-branching reactions of the fuel itself and the oxidation intermediates. Abundant species such as aldehydes, acids, esters, and fuel-specific dione species were detected and could be well reproduced by the current model. In particular, acids are produced by the decomposition of KHPs and subsequent reactions of the intermediate CH3CHO. Esters and dione species are mainly formed via fuel-related pathways.

Exploring the pyrolysis chemistry of 1,3,5-trimethylcyclohexane with insight into fuel isomeric and multiple substitution effects

To reveal insights into the combustion mechanism of multiple alkyl substituent cycloparaffins, this work reports an experimental and modeling study of 1,3,5-trimethylcyclohexane (T135MCH) pyrolysis in an extended flow reactor at low and atmospheric pressures. More than 30 species were detected and quantified employing synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry, and a detailed kinetic model developed based on reaction classes and update kinetic data was validated against the measured species profiles with a reasonable agreement. The reaction flux analyses were performed to reveal the key pathways of the fuel decomposition, intermediates production and aromatics formation. For the primary decomposition, the branching ratios of reaction types show strong dependence on changes of pressures and temperatures, including unimolecular methyl elimination, unimolecular ring-opening isomerization and H-abstraction. Besides the direct dissociation channels, major intermediate hydrocarbons are formed via stepwise dehydrogenation, recombination with ĊH3 radical or “formally direct” chemically activated reactions triggered by Ḣ atom addition. Monocyclic aromatic hydrocarbons such as benzene and toluene can be produced by traditional H-abstraction/β-C-H scission sequence, cyclopentadiene-related pathways, or recombination mechanism from small linear products. The formations of indene and naphthalene are controlled by C5+C5 and C5+C4 mechanism respectively. The comparison work of species profiles combined with theoretical calculations of bond dissociation enthalpies (BDEs) was performed to reveal the multiple CH3-group substituent and isomeric effects of methylcyclohexane (MCH), 1,2,4-trimethylcyclohexane (T124MCH) and T135MCH on pyrolysis activity and ethylene/benzene formation. Besides the increased reaction active sites, the added CH3-group and ortho-substitution can both weaken the strength of CC and CH bonds, leading to the promoting decomposition activity. The different formation tendencies of products are caused by different BDEs, length of carbon skeleton, as well as complex fuel-specific pathways.

A combustion chemistry study of tetramethylethylene in a laminar premixed low-pressure hydrogen flame

The combustion chemistry of tetramethylethylene (TME) was studied in a premixed laminar low-pressure hydrogen flame by combined photoionization molecular-beam mass spectrometry (PI-MBMS) and photoelectron photoion coincidence (PEPICO) spectroscopy at the Swiss Light Source (SLS) of the Paul Scherrer Institute in Villigen, Switzerland. This hexene isomer with the chemical formula C6H12 has a special structure with only allylic CH bonds. Several combustion intermediate species were identified by their photoionization and threshold photoelectron spectra, respectively. The experimental mole fraction profiles were compared to modeling results from a recently published kinetic reaction mechanism that includes a TME sub-mechanism to describe the TME/H2 flame structure. The first stable intermediate species formed early in the flame front during the combustion of TME are 2-methyl-2-butene (C5H10) at a mass-to-charge ratio (m/z) of 70, 2,3-dimethylbutane (C6H14) at m/z 86, and 3-methyl-1,2-butadiene (C5H8) at m/z 68. Isobutene (C4H8) is also a dominant intermediate in the combustion of TME and results from consumption of 2-methyl-2-butene. In addition to these hydrocarbons, some oxygenated species are formed due to low-temperature combustion chemistry in the consumption pathway of TME under the investigated flame conditions.

Towards a predictive kinetic model of 3-ethyltoluene: Evidence concerning fuel-specific intermediates in the flow reactor pyrolysis with insights into model implications

To reveal insights into high temperature kinetics of dialkylaromatics, a pyrolysis investigation of 3-ethyltoluene in a flow reactor together with its reaction kinetics are presented in this work. Concentrations and chemical structures of specific species covering temperature range from 796 to 1383K at the pressure of 30 and 760 Torr were recorded and quantified by using synchrotron vacuum ultraviolet photoionization molecular-beam mass spectrometry (VUV-PI-MBMS). Important C8 and C9 fuel-specific intermediates relevant to primary decomposition of 3-ethyltoluene and isomerization of methylbenzyl and ethylbenzyl radicals were detected and identified. The kinetic model interpreting high temperature pyrolysis chemistry of 3-ethyltoluene was developed and reasonably predicted the measurements in this work. The model analyses reveal that the methyl-dissociated reaction from the ethyl group of 3-ethyltoluene is dominant in the fuel decomposition at low pressure, while the fuel is mainly consumed by hydrogen abstraction reactions at atmospheric pressure. The experimental observations of three methylbenzyl isomers, o-xylylene, p-xylylene, styrene and benzocyclobutene provide evidence for the relationships between products involving isomerization of methylbenzyl radicals, formation of xylylenes and decomposition of o-xylylene. The fuel structure effects of 3-ethyltoluene and m-xylene are revealed by comparing the pyrolysis behaviors in both cases. It has been found that the m-methylbenzyl-generating channel in the 3-ethyltoluene pyrolysis improved the reaction reactivity initially. Furthermore, the fuel with longer substituent ethyl group facilitates the formation of cycloalkenes and aromatics.

Pyrolysis of norbornadiene: An experimental and kinetic modeling study

Pyrolysis of norbornadiene (NBD) was investigated in a jet-stirred reactor at 760 torr and within the temperature range of 560 to 1100 K. 37 products and intermediates ranging from m/z = 26 to 178 were identified and quantified by using synchrotron radiation photoionization molecular-beam mass spectrometry accompanied by gas chromatography. The C7H8 isomers, including NBD, cycloheptatriene (CHT) and toluene (A1CH3), were differentiated. Acetylene (C2H2), cyclopentadiene (C5H6), C7H8 isomers and other aromatics are abundant during the pyrolysis process. A detailed kinetic model was developed with reasonable predictions against the measured results, involving 264 species and 1259 reactions, which is helpful to understand the kinetic and heat features of initial consumption as well as the synergistic effect of C2H2, C5H6 and A1CH3 to the formation of aromatics. Rate-of-production analysis shows that the pyrolysis of NBD experiences two stages. The first stage involves the isomerization and decomposition of NBD, yielding A1CH3, CHT, C5H6 and C2H2. Branching ratio analysis shows that C5H6 and C2H2 are dominant products in the first stage. Heat release analysis indicates that the first stage is endothermic. The second stage involves the pyrolysis and interactions of first-stage products to produce light hydrocarbons and polycyclic aromatic hydrocarbons like indene, naphthalene, acenaphthylene, fluorene and phenanthrene. Three routes dominate the formation of indene, including C5H6 + C5H5, benzyl radical + C2H2 and cycloheptatrienyl radical + C2H2, the last one of which has been scarcely considered in the previous investigation. Sensitivity analysis indicates the reactions in the first stage exhibit strong promoting effects for the consumption of NBD, while the second-stage reactions play minor roles in NBD consumption. This work would enrich the understanding of NBD consumption, PAH formation and heat release.

Experimental and kinetic study of pyridine oxidation under the fuel-lean condition in a jet-stirred reactor

Pyridine is an important model compound of fuel-nitrogen during the coal combustion process. The lean oxidation of pyridine was studied over 800–1100 K at atmospheric pressure with the equivalence ratio of 0.4 in a jet-stirred reactor. Pyridine and its oxidation products were measured by tunable synchrotron vacuum ultraviolet photoionization molecular-beam mass spectrometry and gas chromatography analyzer. Three-stage consumption phenomenon of pyridine and O2 was observed including a fast consumption stage at 910–920 K, a moderate consumption stage at 920–1000 K, and a slow consumption stage at 1000–1100 K. HCN, N2, N2O, and HNCO are the major nitrogenous oxidation products of pyridine. CO, CO2, HCN, and HNCO are the dominant carbonaceous products. HNCO is uniquely detected in the lean oxidation of pyridine compared with rich oxidation. A new pyridine LTO 2.0 model comprising 210 species and 1349 reactions was proposed, providing a good prediction of pyridine and its major oxidation products against experimental data. N2O emission is larger than NO at 1100 K. HCN is the dominant nitrogenous intermediate under both fuel-lean and fuel-rich conditions. Results of this work provide a detailed analysis of the nitrogen evolution mechanism of pyridine oxidation under the fuel-lean condition, which is close to the real industrial low-temperature coal combustion processes including the MILD combustion, fluidized bed combustion, etc.

Probing combustion and catalysis intermediates by synchrotron vacuum ultraviolet photoionization molecular-beam mass spectrometry: recent progress and future opportunities

Soft photoionization molecular-beam mass spectrometry (PI-MBMS) using synchrotron vacuum ultraviolet (SVUV) light has been significantly developed and applied in various fields in recent decades. Particularly, the tunability of SVUV light enables two-dimensional measurements, i.e. mass spectrum and photoionization efficiency spectrum measurements, affording isomer distinguishment in complex reaction processes. Many key intermediates have been successfully detected in combustion and catalysis reactions with the help of the state-of-the-art SVUV-PI-MBMS, promoting the understanding of the chemical mechanisms. Herein, we present a brief review of the instrumentation of beamline and PI-MBMS machines at the current synchrotron user facility Hefei Light Source II and exemplify the advantages of the SVUV-PI-MBMS method with recent applications in combustion and catalysis research, especially in probing key reaction intermediates. Future opportunities with the next generation synchrotron light source and bench-top light source have also been discussed.

Experimental and kinetic modeling study of α-methyl-naphthalene pyrolysis: Part I. Formation of monocyclic aromatics and small species

α-Methyl-naphthalene is a very important intermediate in fuel oxidation and a raw material in industrial and scientific fields. Unraveling its detailed chemistry is important for its practical applications. Pyrolysis of α-methyl-naphthalene (C11H10) is studied in a flow reactor at low and atmospheric pressures (30 and 760 Torr) using synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). A kinetic model is developed to predict the decomposition of α-methyl-naphthalene under pyrolytic conditions. A general map of α-methyl-naphthalene thermal decomposition is presented according to the experimental observations and model analysis. Benzo[b]benzyl radical is the dominant primary product in α-methyl-naphthalene pyrolysis. Benzo[b]benzyl radical dissociates to monocyclic aromatics and small intermediates mainly via benzo-fulvenallene and ethynyl-indenyl. The yielded o-benzyne and cyclopent-1-en-3‑yne then contributes to the formation of C5 – C8 species. Phenylacetylene has a direct formation route from α-methyl-naphthalene by H-addition allyl-elimination. In addition to naphthalene (C10H8) and indene (C9H8) sub-mechanisms, C11 sub-mechanism is shown to be critical to unveil the degradation from bicyclic to monocyclic aromatics.

Experimental and kinetic modeling study of α-methyl-naphthalene pyrolysis: Part II. PAH formation

α-Methyl-naphthalene plays an important role as a functional material in petrochemical industries and as a precursor of soot particles. The formation chemistry of polycyclic aromatic hydrocarbons (PAHs) from α-methyl-naphthalene, therefore, warrants detailed investigations. In this work, we studied PAH formation from its pyrolysis using experiments and kinetic models. Flow reactor pyrolytic experiments at low and atmospheric pressures (30 and 760 Torr) were performed using synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). A kinetic model was then developed to predict PAH formation from α-methyl-naphthalene. According to the kinetic analysis of the proposed model, naphth-1-yl-methyl, benzo-fulvenallene, and benzo-fulvenallenyl are three critical intermediates in the formation of large PAHs. Other than the traditional H-abstraction acetylene-/vinylacetylene-addition mechanisms, three prototypical PAH formation pathways are identified in α-methyl-naphthalene pyrolysis: 1) addition and cyclization reactions of naphth-1-yl-methyl and naphth-1-yl radicals; 2) recombination of resonance stabilized radicals (indenyl, benzo-fulvenallenyl, phenalenyl, etc.) and the subsequent ring expansion reactions; 3) sequential propargyl addition reactions.

First aromatic ring formation by the radical-chain reaction of vinylacetylene and propargyl

Abstract Recent investigations illustrated that clustering of hydrocarbons by radical-chain reaction (CHRCR) mechanism provides key mechanistic steps for the rapid synthesis of polycyclic aromatic hydrocarbons (PAHs) and soot. Resonance-stabilized radicals (RSRs) play critical roles in this mechanism, and non-benzene first-ring species have attracted considerable attention as precursors of larger aromatic hydrocarbons. C7H7 RSRs, such as benzyl, tropyl, vinyl-cyclopentadienyl, are particularly stable and are thus quite important in the growth of PAHs. The addition of vinylacetylene to propargyl radical, a prototypical CHRCR reaction, provides a facile route to C7H7 RSRs. We have directly investigated the reaction of propargyl and vinylacetylene in isomer-resolved elementary experiments by synchrotron vacuum ultra-violet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). In good agreement with theoretical predictions, vinyl-cyclopentadienyl is found to be the major product of vinylacetylene and propargyl reaction while benzyl is minor. This work demonstrates a feasible CHRCR pathway, not proceeding through benzene, for PAH formation.

Exploring chemical kinetics of plasma assisted oxidation of dimethyl ether (DME)

Abstract Chemical kinetics of plasma assisted oxidation of dimethyl ether was investigated by photoionization molecular beam mass spectrometry (PI-MBMS) and kinetic modeling. A series of hydrocarbon and oxygenated intermediates, especially some fuel-specific oxygenated intermediates including methyl formate, ethyl methyl ether and dimethoxymethane, were identified by the measured mass spectra and photoionization efficiency (PIE) curves. The quantification results displayed two main change patterns of species mole fractions with increasing oxygen, indicating the different dominating formation pathways. In addition to the oxygenates, C1 and C2 hydrocarbon intermediates were observed at low temperatures in the DME/O2/Ar and DME/Ar plasma systems, while the formation of these intermediates usually occurs at temperatures higher than 800 K in the thermal oxidation of DME. A kinetic model containing gas-phase reactions and plasma reactions was developed. The model analysis suggests that both hydrogen abstractions and plasma reactions involving electron/Ar*/O(1D)/Ar+ contribute to the fuel consumption. The subsequent reactions of the resulting CH3OCH2, CH3O and CH3 lead to the yield of the observed hydrocarbons and oxygenates. The experimental results together with kinetic modeling indicate that the fuel and fuel radical may decompose into methyl radical, oxygen atom and formaldehyde directly, which account for the under prediction of formaldehyde and the observation of oxygenates with moderate concentrations in the DME/Ar plasma.

Molecular-Weight Growth in Ozone-Initiated Low-Temperature Oxidation of Methyl Crotonate.

We report experiments of ozone-initiated low-temperature oxidation of methyl crotonate (MC, CH3-CH═CH-C(O)OCH3) from 420 to 660 K in a near-atmospheric-pressure jet-stirred reactor using photoionization molecular-beam mass spectrometry as a sampling technique. In this temperature regime, no typical low-temperature combustion (LTC) reactions have been observed for MC when oxygen (O2) is used as the oxidizer. Upon ozone addition, significant oxidation of methyl crotonate is found. On the basis of experimentally observed energy-dependent mass peaks in combination with temperature-dependent mole fraction profiles and photoionization efficiency curves, we provide new insights into the methyl crotonate ozonolysis reaction network. The observed MC + O3 products, C5H8O5, are found to be related to the keto-hydroperoxides resulting from the isomerization of the primary ozonide. Evidence is also provided that molecular growth mainly results from cycloaddition reactions of the Criegee intermediate into aldehydes and alkenes as well as addition reactions of the Criegee intermediates to the double bond of methyl crotonate and sequential decomposition into ketones. Furthermore, species that contribute in large amounts to the low-temperature oxidation of methyl crotonate, like H2O2, CH3OOH, CH3OH, and HC(O)OH, are identified, and their mole fractions are reported. Additionally, preliminary modeling is performed which qualitatively captures the observed NTC behavior and reveals future research opportunities.

Experimental flat flame study of monoterpenes: Insights into the combustion kinetics of α-pinene, β-pinene, and myrcene

Pinenes and pinene dimers have similar energy densities to petroleum-based fuels and are regarded as alternative fuels. The pyrolysis of the pinenes is well studied, but information on their combustion kinetics is limited. Three stoichiometric, flat premixed flames of the C10H16 monoterpenes α-pinene, β-pinene, and myrcene were investigated by synchrotron-based photoionization molecular-beam mass spectrometry (PI-MBMS) at the Advanced Light Source (ALS). This technique allows isomer-resolved identification and quantification of the flame species formed during the combustion process. Flame-sampling molecular-beam mass spectrometry even enables the detection of very reactive radical species. Myrcene was chosen because of its known formation during β-pinene pyrolysis. The quantitative speciation data and the discussed decomposition steps of the fuels provide important information for the development of future chemical kinetic reaction mechanisms concerning pinene combustion. The main decomposition of myrcene starts with the unimolecular cleavage of the carbon-carbon single bond between the two allylic carbon atoms. Further decompositions by β-scission to stable combustion intermediates such as isoprene (C5H8), 1,2,3-butatriene (C4H4) or allene (aC3H4) are consistent with the observed species pool. Concentrations of all detected hydrocarbons in the β-pinene flame are closer to the myrcene flame than to the α-pinene flame. These similarities and the direct identification of myrcene by its photoionization efficiency spectrum during β-pinene combustion indicate that β-pinene undergoes isomerization to myrcene under the studied flame conditions. Aromatic species such as phenylacetylene (C8H6), styrene (C8H8), xylenes (C8H10), and indene (C9H8) could be clearly identified and have higher concentrations in the α-pinene flame. Consequently, a higher sooting tendency can generally be expected for this monoterpene. The presented quantitative speciation data of flat premixed flames of the three monoterpenes α-pinene, β-pinene, and myrcene give insights into their combustion kinetics.

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

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

Cool flame chemistry of diesel surrogate compounds: n-Decane, 2-methylnonane, 2,7-dimethyloctane, and n-butylcyclohexane

Elucidating the formation of combustion intermediates is crucial to validate reaction pathways, develop reaction mechanisms and examine kinetic modeling predictions. While high-temperature pyrolysis and oxidation intermediates of alkanes have been thoroughly studied, comprehensive analysis of cool flame intermediates from alkane autoxidation is lacking and challenging due to the complexity of intermediate species produced. In this work, jet-stirred reactor autoxidation of four C10 alkanes: n-decane, 2-methylnonane, 2,7-dimethyloctane, and n-butylcyclohexane, as model compounds of diesel fuel, was investigated from 500 to 630 K using synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PIMS). Around 100 intermediates were detected for each fuel. The classes of molecular structures present during the autoxidation of the representative paraffinic functional groups in transport fuels, i.e., n-alkanes, branched alkanes, and cycloalkanes were established and were found to be similar from the oxidation of various alkanes. A theoretical approach was applied to estimate the photoionization cross sections of the intermediates with the same carbon skeleton as the reactants, e.g., alkene, alkenyl keto, cyclic ether, dione, keto-hydroperoxide, diketo-hydroperoxide, and keto-dihydroperoxide. These species are indicators of the first, second, and third O2 addition reactions for the four C10 hydrocarbons, as well as bimolecular reactions involving keto-hydroperoxides. Chemical kinetic models for the oxidation of these four fuels were examined by comparison against mole fraction of the reactants and final products obtained in additional experiments using gas chromatography analysis, as well as the detailed species pool and mole fractions of aforementioned seven types of intermediates measured by SVUV-PIMS. This works reveals that the models in the literature need to be improved, not only the prediction of the fuel reactivity and final products, but also the reaction network to predict the formation of many previous undetected intermediates.

The C5 chemistry preceding the formation of polycyclic aromatic hydrocarbons in a premixed 1-pentene flame

The formation of small polycyclic aromatic hydrocarbons (PAHs) and their precursors can be strongly affected by reactions of C5 species. For improving existing combustion mechanisms for small PAH formation, it is therefore valuable to understand the fuel-specific chemistry of C5 fuels. To this end, we provide quantitative isomer-resolved species profiles measured in a laminar premixed (ϕ = 1.8) low-pressure (4 kPa) flame of 1-pentene with photoionization molecular-beam mass spectrometry (PI-MBMS) using tunable synchrotron vacuum-ultraviolet (VUV) radiation. These experimental results are accompanied with numerical simulations, starting from models from the literature by Wang et al. [JetSurF version 2.0 (2010)] and Healy et al. [Energy Fuels 24 (2010) 1521–1528] that were developed for different fuels, but which include 1-pentene as an intermediate, and by Narayanaswamy et al. [Combust. Flame 157 (2010) 1879–1898] focusing on the small PAH chemistry. Taking observed discrepancies between experimental results and simulations into consideration, a mechanism for C5 chemistry was newly developed including PAH formation pathways, and its performance analyzed in detail. Special emphasis was placed on the initial fuel consumption of 1-pentene as well as on formation pathways of small aromatics. The mechanisms show differences regarding fuel decomposition and hydrocarbon growth reactions. These contribute to noticeable differences between the simulations with different models on one hand, and deviations between model predictions and experimental results on the other. While the new model presents overall satisfactory capabilities to predict the mole fraction profiles of common combustion intermediates, the predictive capability of the literature models was not fully satisfying for some intermediate species, including C4H6, C7H8, and C10H8. The results indicate that the fuel-specific C5 reaction routes as well as the mechanism for small PAH formation need further investigation.

Polycyclic aromatic hydrocarbons in pyrolysis of gasoline surrogates (n-heptane/iso-octane/toluene)

Toluene primary reference fuels (TPRFs), i.e., a ternary mixture of toluene, n-heptane and iso-octane, better match the combustion properties of real gasoline fuels compared to simpler binary n-heptane/iso-octane mixtures. While there has been significant research on combustion of n-heptane/iso-octane mixtures, fundamental data characterizing polycyclic aromatic hydrocarbons (PAHs) formation in TPRFs combustion is lacking, especially under pyrolysis conditions. In this work, the pyrolysis of two TPRF mixtures (TPRF70 and TPRF97.5), representing low octane (research octane number 70) and high octane (research octane number 97.5) gasolines, respectively, was studied in a jet-stirred reactor coupled with gas chromatography (GC) analysis and a flow reactor coupled with synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). The experiments indicate that pyrolysis of TPRF70 produced slightly higher benzene and naphthalene than TPRF97.5. In contrast, TPRF97.5 pyrolysis produced slightly higher phenanthrene and pyrene than TPRF70. The mole fraction profiles of aromatics from benzene to pyrene were used to validate TPRF kinetic models from the literature. Specifically, the KAUST-Aramco PAH Mech 1-GS kinetic model was updated to match and elucidate the experimental observations. The kinetic analysis reveals that propargyl radical is a crucial intermediate forming benzene and naphthalene, while benzyl radical, generated from the dehydrogenation of toluene, plays an important role in formation of larger PAHs.

A further experimental and modeling study of acetaldehyde combustion kinetics

Acetaldehyde is an important intermediate and a toxic emission in the combustion of fuels, especially for biofuels. To better understand its combustion characteristics, a detailed chemical kinetic model describing the oxidation of acetaldehyde has been developed and comprehensively validated against various types of literature data including laminar flame speeds, oxidation and pyrolysis in shock tubes, chemical structure of premixed flames, and low-temperature oxidation in jet-stirred reactors. To extend the validation range, the chemical structure of a counterflow flame fueled by acetaldehyde at 600 Torr has been measured using vacuum ultra-violet photoionization molecular-beam mass spectrometry. In addition, ignition delay times at 10 atm and 700-1100 K were measured in a rapid compression machine, and a negative temperature coefficient (NTC) behavior was observed. The present kinetic model well reproduces the results of various acetaldehyde combustion experiments covering wide ranges of temperatures (300–2300 K) and pressures (0.02–10 atm), and explains well the observed NTC behavior based on the competition between multiple oxidation pathways for the methyl radicals and their self-recombination forming ethane, a relatively stable species at temperatures below 1000 K.

Insights into the oxidation kinetics of a cetane improver – 1,2-dimethoxyethane (1,2-DME) with experimental and modeling methods

A kinetic investigation was carried out in this work for an oxygenated fuel additive, 1,2-dimethoxyethane (1,2-DME) to reveal the oxidation chemistry responsible for its practical function as a cetane improver. Experiments were conducted for 1,2-DME/O2/N2 mixtures with different equivalence ratios (0.5, 1.0 and 2.0) in a high-pressure (10 atm) jet-stirred reactor (JSR) facility over the temperature range of 450–1100 K. Species concentration evolutions with the temperature were monitored with on-line Fourier transform infrared (FTIR) spectrometry and off-line gas chromatography (GC). The technique of photoionization molecular-beam mass spectrometry (PI-MBMS) was combined with another JSR setup operated at near-atmospheric pressure (700 Torr), for the purpose of probing reactive intermediates from 1,2-DME oxidation. A kinetic model was constructed based on the “reaction classes” strategy, which could satisfactorily predict speciation measurements in the current work. Pronounced low-temperature reactivity of 1,2-DME was observed under all investigated conditions, and crucial intermediates like ketohydroperoxides were detected with the PI-MBMS. Some specificities of 1,2-DME oxidation were elucidated through further model interpretations. The double ether groups imbedded in the fuel molecule make hydrogen abstractions easier and meanwhile hinder the chain-terminating concerted eliminations, so the low-temperature reactivity starts at relatively low temperatures (compared to n-hexane for example). Chain-branching reaction sequences following the second O2 additions proceed in the oxidizing mixtures, leading to the remarkable low-temperature reactivity of 1,2-DME which can be utilized as a cetane-improver.

A high-temperature study of 2-pentanone oxidation: experiment and kinetic modeling

Abstract Small methyl ketones are known to have high octane numbers, impressive knock resistance, and show low emissions of soot, NOx, and unburnt hydrocarbons. However, previous studies have focused on the analysis of smaller ketones and 3-pentanone, while the asymmetric 2-pentanone (methyl propyl ketone) has not gained much attention before. Considering ketones as possible fuels or additives, it is of particular importance to fully understand the combustion kinetics and the effect of the functional carbonyl group. Due to the higher energy density in a C5-ketone compared to the potential biofuel 2-butanone, the flame structure and the mole fraction profiles of species formed in 2-pentanone combustion are of high interest, especially to evaluate harmful species formations. In this study, a laminar premixed low-pressure (p = 40 mbar) fuel-rich (ϕ = 1.6) flat flame of 2-pentanone has been analyzed by vacuum-ultraviolet photoionization molecular-beam mass-spectrometry (VUV-PI-MBMS) enabling isomer separation. Quantitative mole fraction profiles of 47 species were obtained and compared to a model consisting of an existing base mechanism and a newly developed high-temperature sub-mechanism for 2-pentanone. High-temperature reactions for 2-pentanone were adapted in analogy to 2-butanone and n-pentane, and the thermochemistry for 2-pentanone and the respective fuel radicals was derived by ab initio calculations. Good agreement was found between experiment and simulation for the first decomposition products, supporting the initial branching reactions of the 2-pentanone sub-mechanism. Also, species indicating low-temperature chemistry in the preheating zone of the flame have been observed. The present measurements of a 2-pentanone flame provide useful validation targets for further kinetic model development.