Flame-sampling Molecular-beam Mass Research Articles

Inhibition of Laminar Premixed Flames of Dimethyl Ether by a Phosphorus-Containing Compound

ABSTRACT The present paper reports experimental and modeling results on the laminar burning velocity and structure of stoichiometric and fuel-rich premixed laminar flames of DME with and without TMP additives. The spatial variations of the mole fractions of H, OH, PO, PO2, HOPO, HOPO2 and some intermediate hydrocarbons in the one-dimensional burner-stabilized flames with various equivalence ratios are measured by the flame-sampling molecular beam mass spectrometry. The novel measurement data for the DME flames are used to validate a kinetic mechanism available in the literature for flame inhibition by organophosphorus compounds. The TMP inhibition effectiveness of DME flames has been determined and revealed to be lower than that of CH4/air flame. The speeds of CH4/air and DME/air flames were found to be sensitive to the same reactions involving phosphorus compounds, while the sensitivity of the methane flame speed to inhibition reactions is higher than that of DME/air flame. The observed differences were explained by the higher concentration of the chain carriers H, OH, and O in the DME flame. Based on the results obtained, we conclude that the phosphorus-containing inhibitors are promising candidates for reducing the flammability and ensuring the fire safety of DME mixtures with air.

Flame Structure at Elevated Pressure Values and Reduced Reaction Mechanisms for the Combustion of CH4/H2 Mixtures

Understanding and controlling the combustion of clean and efficient fuel blends, like methane + hydrogen, is essential for optimizing energy production processes and minimizing environmental impacts. To extend the available experimental database on CH4 + H2 flame speciation, this paper reports novel measurement data on the chemical structure of laminar premixed burner-stabilized CH4/H2/O2/Ar flames. The experiments cover various equivalence ratios (φ = 0.8 and φ = 1.2), hydrogen content amounts in the CH4/H2 blend (XH2 = 25%, 50% and 75%), and different pressures (1, 3 and 5 atm). The flame-sampling molecular-beam mass spectrometry (MBMS) technique was used to detect reactants, major products, and several combustion intermediates, including major flame radicals. Starting with the detailed model AramcoMech 2.0, two reduced kinetic mechanisms with different levels of detail for the combustion of CH4/H2 blends are reported: RMech1 (30 species and 70 reactions) and RMech2 (21 species and 31 reactions). Validated against the literature data for laminar burning velocity and ignition delays, these mechanisms were demonstrated to reasonably predict the effect of pressure and hydrogen content in the mixture on the peak mole fractions of intermediates and adequately describe the new data for the structure of fuel-lean flames, which are relevant to gas turbine conditions.

Revisit flame chemistry of propene at elevated pressures: Insight into pressure effects on chemical structure and laminar flame propagation

This work reports an experimental and kinetic modeling investigation on flame chemistry of propene at elevated pressures. The chemical structures of premixed propene/O2/Ar flames at 2–5 atm and the equivalence ratio of 1.5 including reactants, major products and a series of intermediates were measured using the flame sampling molecular beam mass spectrometry. The maximum mole fractions of H, CH3 and C2H6 decrease with increasing pressure, while that of CH4 follows the opposite trend. Laminar burning velocities (LBVs) of propene/air mixtures at equivalence ratios of 0.7–1.5 and initial pressures up to 10 atm were measured using the spherically propagating flame method. Remarkable pressure effects can be observed, especially under the lean and rich conditions. A kinetic model for propene combustion was developed based on our recent model for ethylene combustion and recently reported chemically termolecular reactions and HCO prompt dissociation reactions. Key reactions in the propene flames were revealed using the modeling analysis, especially for those playing significant roles in the pressure effects. Two chain termination reactions including the self-combination of CH3 and the recombination between CH3 and H are found to be crucial for the observed pressure effects in maximum mole fractions of H, CH3, CH4 and C2H6. The HCO prompt dissociation can significantly promote the laminar flame propagation of propene/air mixtures due to the substantial formation of CH2O through CH2+O2 and CH3+O pathways, while the chemically termolecular reactions exhibit weak inhibition effects at equivalence ratios smaller than 1.2. The reason for the stronger pressure effects of LBVs under the lean and rich conditions is that the excess O2 and abundantly produced CH3 enhances the roles of H+O2(+M)=HO2(+M) and CH4(+M)=CH3+H(+M) in chain termination, respectively.

Propylene Oxide Addition Effect on the Chemical Speciation of a Fuel-Rich Premixed n-Heptane/Toluene Flame.

1,2-Propylene oxide (PO, C3H6O) is considered as a promising agent for improving fuel. In this work, the effect of PO additives on the species pool in a premixed burner-stabilized fuel-rich (ϕ = 1.6) flame fueled by n-heptane/toluene mixture (7/3 by volume of liquids) at atmospheric pressure is studied by the flame-sampling molecular beam mass spectrometry and numerical modeling in order to get insight into the chemical aspects of the influence of oxygenates with an epoxy group on the formation of abundant intermediates (including PAH precursors) during combustion of fossil fuels. The flames with various loadings of PO in the fuel blend (from 0 to 16.3% in mole basis) are examined, and detailed kinetic mechanisms available in the literature are validated against the measurements of mole fraction profiles of reactants, major products, and many intermediate species. A higher reactivity of the fresh mixture and a reduction in the peak mole fractions of intermediates playing an important role in PAH formation (benzene, styrene, ethylbenzene, phenol, acetylene, diacetylene, etc.) are observed when PO is added. This was found to be due to simultaneously two factors: the partial replacement of "sooting" fuel (toluene, which is the main precursor of these species) with oxygenated additive, and the changes in the flame radical pool caused by PO addition. Propylene oxide additive was found to change the ratio between H, OH, O, and CH3 toward an increase in the proportion of O and CH3. The detailed kinetic mechanisms considered in the work are found to overpredict the peak mole fraction of acetylene, a key species playing a crucial role in PAH growth. Its chemistry is revisited in order to provide a better prediction of C2H2 and, as a result, PAHs.

Improving the Predictive Accuracy for Ketene in Diacetyl Laminar Premixed Flames: Experiment and Model Analysis.

Ketene is an important species in core mechanisms for the combustion of hydrocarbon and oxygenated fuels, but direct experiments with ketene are challenging to conduct due to its high reactivity. Diacetyl can be used as a precursor of ketene, and abundant ketene is present in premixed flames of diacetyl. However, predictions of ketene in diacetyl flames with previous models have significant uncertainties. The study of Sun et al. [Sun, W.; Wang, J.; Huang, C.; Hansen, N.; Yang, B. Combust. Flame, 2019, 205, 11-21, DOI: 10.1016/j.combustflame.2019.03.037] shows that the flame structure measurements should be performed under certain conditions to improve the predictive accuracy of ketene in diacetyl flames. In this work, the structures of three laminar premixed flames of diacetyl under atmospheric pressure in a range of equivalence ratios are examined with flame-sampling molecular-beam mass spectrometry (MBMS). With the new experimental data and the data available in literature, Bayesian analysis is performed to optimize the kinetic model. The obtained optimized model is compared with the original one, and the results show that the optimized model agrees better with the experimental data than the original one. The uncertainties of the rate coefficients of some key reactions are constrained with these experimental data, which eventually leads to smaller modeling uncertainties for ketene concentrations under studied conditions.

Molecular-growth pathways in premixed flames of benzene and toluene doped with propyne

In a combined experimental and modeling effort, we investigated the molecular-growth pathways in propyne-doped low-pressure premixed flames of benzene and toluene. We determined the chemical structures of these two flames with flame-sampling molecular-beam mass spectrometry. The mole fraction profiles of the aromatic intermediates served as validation targets for two chemically detailed mechanisms that were independently developed at the German Aerospace Center (DLR) and at the Lawrence Livermore National Laboratory (LLNL). Reaction path analyses reveal the important pathways for indene, naphthalene, and phenanthrene. There is no appreciable fuel-structure effect and molecular growth was observed to be driven by radical-radical recombination reactions, and ring-closure and ring-enlargement reactions with little contribution from the classical HACA mechanism. Indene is formed in both flames through the reactions of the phenyl and propargyl radicals. The benzyl radical plays only a very minor role in the formation of indene through the reaction with acetylene. Reactions of the propargyl radical with fulvenallenyl, a C7H5 isomer, contribute significantly to naphthalene formation in both flames investigated here. Benzyl radicals contribute to naphthalene formation via reactions with propargyl radical through formation of phenyl-substituted butadienyl and vinylacetylene isomers. Ring-enlargement reactions converting indene's five-membered ring into naphthalene's six-membered ring also contribute in small amounts to naphthalene. Fulvenallenyl radicals also contribute substantially to phenanthrene formation.

Structure of premixed flames of propylene oxide: Molecular beam mass spectrometric study and numerical simulation

The knowledge of the combustion chemistry of oxygenated fuels is essential for the development of detailed kinetic mechanisms suitable for the combustion processes involving biofuels. Moreover, epoxidized olefins, are increasingly used as chemical intermediates or as bulk chemicals. Nevertheless, a dearth of data for their reactivity in the oxidative environment can be observed in the current literature. This study reports the experimental and the model characterization of the flame structure of propylene oxide at stoichiometric and fuel-rich conditions at atmospheric pressure. To this aim, the species mole fractions in three premixed flames stabilized on a flat-flame burner have been quantitatively measured by using the flame sampling molecular beam mass spectrometry. Three chemical kinetic mechanisms retrieved from the current literature involving propylene oxide chemistry have been validated against the novel experimental data. In general, the predictions appeared to be in satisfactory agreement with measurements except for acetaldehyde and ketene. The rate of production analysis in the flame has shown that the discrepancies observed for these species are related basically to the incorrect ratio between the rates of primary reaction pathways of propylene oxide destruction.

Entanglement of n-heptane and iso-butanol chemistries in flames fueled by their mixtures

This paper concerns itself with the entanglement of the high-temperature oxidation chemistry of n-heptane and iso-butanol in flames fueled by their mixtures. While in many cases the chemistries of the individual fuel components do not interact in mixture flames, in this work, we revealed interactions between the individual species pools originating from n-heptane and iso-butanol oxidation. In a coordinated experimental and modeling effort, chemical structures of three low-pressure premixed flames fueled by different blends of n-heptane and iso-butanol were determined using flame-sampling molecular-beam mass spectrometry with synchrotron-based single-photon ionization and chemical kinetic modeling. The chemical kinetic model, which is based on the reaction set that was used previously [Braun-Unkhoff et al., Proc. Combust. Inst.,2017, 36, 1311–1319], was now extended by an n-heptane sub-mechanism. The overall good performance of the model allows for an extraction of chemically relevant information that highlights the entanglement between the individual fuel-specific species pools. For example, it was shown that methyl radicals, in part from iso-butanol oxidation (i.e., from the decomposition of α-iso-butanol radicals) can participate in n-heptane consumption processes through H-abstraction reactions. Further interactions are related to the formation of the methylallyl radical and aromatics formation. The relevance of such interactions is also discussed regarding the formation of oxygenated byproducts.

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.

Inhibition of premixed flames of methyl methacrylate by trimethylphosphate

In the present paper, the laminar burning velocity and structure of near-stoichiometric premixed laminar flames of methyl methacrylate (MMA) with and without trimethylphosphate (TMP) additives have been studied experimentally and by numerical modeling. The MMA+TMP combustion system is considered as a model system, which simulates gas-phase combustion of polymethyl methacrylate (PMMA) with additive of phosphorus-containing fire retardants (PFRs). The motivation of the present research is to provide a basis for development of a predictive gas-phase chemical kinetic model for inhibition of PMMA by PFRs. The flame sampling molecular beam mass spectrometry was used to determine the spatial variation of the mole fractions of H, OH, PO, PO2, HOPO, HOPO2 and some intermediate hydrocarbons in the one-dimensional burner-stabilized flames. The effect of TMP on the hydrocarbon intermediates in the flames is investigated. The reaction mechanism for combustion of MMA+TMP system has been validated against the novel laminar burning velocity and chemical speciation data. Performances and deficiencies of the kinetic mechanism for MMA flame inhibition are discussed. The sensitivity analysis showed that the insufficiently accurate prediction of mole fraction of H, O and OH results in disagreement for mole fraction profiles of hydrocarbon intermediates in the inhibited flame. Inhibition effectiveness of MMA flame by TMP is compared with that derived from experimental data for other fuels and the observed tendencies are discussed.

Experimental and numerical study of the structure of premixed H2/CO/O2/Ar flames at atmospheric pressure

Elaboration of a predictive and well validated kinetic mechanism for combustion of H2/CO mixtures is the key to development of advanced technologies for effective syngas burning in power generation systems. Moreover, understanding the combustion chemistry of H2/CO mixtures is a prerequisite for a comprehensive description of hydrocarbons chemistry and development of predictive chemical kinetic models for combustion of transportation fuels. In this work we report new experimental data for chemical speciation in the atmospheric pressure burner-stabilized premixed flames diluted with Ar and fuelled with H2/CO (1:1) mixture at equivalence ratios of 1 and 2. Flame sampling molecular beam mass spectrometry with soft electron ionization was used to examine the flames. Recent detailed chemical kinetic mechanisms for syngas combustion available from literature were used to simulate the experimental profiles with the CHEMKIN code. A compact kinetic mechanism including 11 species and 16 reactions was elaborated to reproduce the experimental data.

Flame structure and a compact reaction mechanism for combustion of dimethyl ether at atmospheric pressure

To extend the available experimental database on DME flame speciation, this paper reports new measurement data on the chemical structure of premixed stoichiometric and fuel-rich (ϕ = 2.2) DME/O2/Ar flames stabilized on a Botha-Spalding burner at atmospheric pressure. Flame sampling molecular beam mass spectrometry with soft electron ionization was used to detect many DME combustion intermediates, including labile species such as H, CH3, and HO2. The new experimental data were used to develop and validate a compact mechanism of DME combustion which is able to reproduce not only laminar burning velocity and ignition delay times, but also peak mole fractions of important species in DME combustion at a minimum number of species and reactions included. The compact chemical kinetic mechanism consisting of 41 species and 110 elementary reactions is proposed based on the literature kinetic mechanism of DME combustion. Comparison of predictions made using the compact mechanism and detailed literature models with three sets of measurement data (chemical speciation profiles reported in this paper, laminar burning velocity and ignition delay time data obtained by other authors) shows that they are in satisfactory agreement, which generally proves the good predictive capability of the mechanism. Further investigations, however, are needed to better understand the kinetics of H2O2 and C2H2 included in the mechanism, because under fuel-rich conditions, the compact mechanism as well as other detailed models do not adequately predict the peak mole fractions of these important intermediates.

Chemical insights into the larger sooting tendency of 2-methyl-2-butene compared to n-pentane

A comprehensive, chemically detailed mechanism for the combustion of 2-methyl-2-butene and n-pentane is presented to provide insights into the different sooting tendencies of these two structurally different C5 hydrocarbons. A hierarchically assembled mechanism has been developed to specifically target speciation data from low-pressure premixed flames of 2-methyl-2-butene [Ruwe et al., Combust. Flame, 175, 34-46, 2017] and newly measured mole fraction data for a fuel-rich (ɸ=1.8) n-pentane flame, in which species profiles up to phenol were quantified. The partially isomer-resolved chemical composition of this flame was determined using flame-sampling molecular-beam mass spectrometry with single-photon ionization by tunable, synchrotron-generated vacuum-ultraviolet radiation. The presented model, which includes a newly determined, consistent set of the thermochemistry data for the C5 species, presents overall satisfactory capabilities to predict the mole fraction profiles of common combustion intermediates. The analysis of the model predictions revealed the fuel-structure dependencies (i.e. saturated vs. unsaturated and linear vs. branched) of the formation of small aromatic species that are considered as soot precursors. The propensity of the 2-methyl-2-butene flame to form larger concentrations of aromatic species was traced back to the readily available formation routes of several small precursor molecules and the efficient formation of “first aromatic rings” beyond benzene.

Flame structure of dimethyl ether/O2/Ar mixture at 1 atm

For diesel engines, dimethyl ether (DME) is widely used in the world, it is a perspective candidate to replace traditional diesel in the future. The consumption of DME as a motor fuel is constantly increasing. DME has a very low propensity to soot formation during combustion, which allows not only to reduce harmful emissions into the atmosphere, but also to improve the completeness of combustion. To validate available chemical kinetic models for DME combustion, in this work we report our experimental data on the chemical structure of a laminar premixed stoichiometric DME/O2/Ar flame stabilized at 1 atm on a flat burner. Flame sampling molecular beam-mass spectrometry is used to obtain spatial distribution of mole fractions of many species in the flame. The experimental data are compared with the calculation results obtained using PREMIX code and two detailed chemical kinetic mechanisms available from literature for DME combustion.

Providing effective constraints for developing ketene combustion mechanisms: A detailed kinetic investigation of diacetyl flames

Ketene is an important combustion intermediate, but due to its high reactivity, it has been difficult to validate the corresponding kinetic sub-mechanisms with direct experimental measurements. It is shown here the possibility to refine the ketene combustion model through relevant experimental and modeling analyses of diacetyl [2,3-butadione, (CH3CO)2] combustion. The consumption of diacetyl leads to abundant ketene under high-temperature oxidation conditions. From the aspect of the hierarchical structure of a kinetic mechanism, the ketene sub-mechanism acts as a part of a secondary mechanism in the kinetic scheme exploring diacetyl combustion. Therefore, this strategy to understand the kinetics of ketene through studying the combustion chemistry of diacetyl should be based on a good understanding of the diacetyl combustion kinetics. To this end, we investigated the flame chemistry of diacetyl through experimental, modeling, and theoretical approaches. Flame-sampling molecular-beam mass spectrometry with single-photon ionization was employed to measure the detailed chemical structure of a premixed diacetyl flame stabilized at the pressure of 18.0 Torr and the equivalence ratio of 1.2. The quantitative speciation measurements were essential in identifying important reactions in the fuel sub-mechanism, and the corresponding rate coefficients were obtained through high-level theoretical calculations. A kinetic model was proposed, which showed satisfactory performances in predicting the measured flame structure. With the diacetyl flame chemistry better understood, global sensitivity analyses were carried out to suggest conditions for future experiments potentially facilitating the development of ketene sub-mechanisms.

Experimental Study and a Short Kinetic Model for High-Temperature Oxidation of Methyl Methacrylate

ABSTRACTSynthetic and natural polymeric esters find applications in transport and construction sectors, where fire safety is an important concern. One polymer that is widely used is poly (methyl methacrylate) (PMMA), which almost completely undergoes thermal decomposition into methyl methacrylate (its monomer) (MMA) at –C. In order to analyze the high-temperature gas-phase oxidation of PMMA, and thereby predict its fire behavior (such as burning rate, temperature of the material, and heat fluxes) with less computational effort, a compact kinetic model for the oxidation of its primary decomposition product, MMA, is most essential. This is accomplished in the present work by obtaining a reduced mechanism for MMA oxidation from a detailed mechanism from the Lawrence Livermore National Laboratories group.To extend the available data base for model validation and present validation data at atmospheric pressure conditions, for the first time, (i) detailed measurements of species profiles have been performed in stoichiometric laminar flat flames using flame sampling molecular beam mass spectrometry (MBMS) technique and (ii) laminar burning velocities have been obtained using the heat flux method for various unburnt mixture temperatures. Evaluating the model against these data sets point to the need to revise the kinetic model, which is achieved by adopting rate constants of key reactions among analogous molecules from recent literature. The updated compact kinetic model is able to predict the major species in the flat flame as well as the burning velocity of MMA satisfactorily. The final “short MMA mechanism” consists of 88 species and 1084 reactions.

The Effect of Methyl Pentanoate Addition on the Structure of a Non-Premixed Counterflow n-Heptane/O2 Flame

The influence of methyl pentanoate (MP) addition to n-heptane on the species pool in a nonpremixed counterflow flame fueled with n-heptane at atmospheric pressure has been investigated experimentally and numerically. Two non-premixed flames in counterflow configuration have been examined: (1) n-heptane/Ar (5.3%/94.7%) vs O2/Ar (24.1%/75.9%) and (2) n-heptane/MP/Ar (2.5%/2.5%/95%) vs O2/Ar (19.6%/80.4%). Both flames had similar strain rates and stoichiometric mixture fractions to allow an adequate comparison of their structures. The mole fraction profiles of the reactants, major products, and intermediates in both flames were measured using flame sampling molecular beam mass spectrometry. These experimental data were used for validation of a detailed chemical kinetic mechanism, which was proposed earlier for prediction of combustion characteristics of n-heptane/iso-octane/toluene/MP mixtures. The addition of MP to n-heptane reduced the flame temperature and the peak mole fractions of many flame intermediat...

Influences of the molecular fuel structure on combustion reactions towards soot precursors in selected alkane and alkene flames.

In this study, we experimentally investigate the high-temperature oxidation kinetics of n-pentane, 1-pentene and 2-methyl-2-butene (2M2B) in a combustion environment using flame-sampling molecular beam mass spectrometry. The selected C5 fuels are prototypes for linear and branched, saturated and unsaturated fuel components, featuring different C-C and C-H bond structures. It is shown that the formation tendency of species, such as polycyclic aromatic hydrocarbons (PAHs), yielded through mass growth reactions increases drastically in the sequence n-pentane < 1-pentene < 2M2B. This comparative study enables valuable insights into fuel-dependent reaction sequences of the gas-phase combustion mechanism that provide explanations for the observed difference in the PAH formation tendency. First, we investigate the fuel-structure-dependent formation of small hydrocarbon species that are yielded as intermediate species during the fuel decomposition, because these species are at the origin of the subsequent mass growth reaction pathways. Second, we review typical PAH formation reactions inspecting repetitive growth sequences in dependence of the molecular fuel structure. Third, we discuss how differences in the intermediate species pool influence the formation reactions of key aromatic ring species that are important for the PAH growth process underlying soot formation. As a main result it was found that for the fuels featuring a C[double bond, length as m-dash]C double bond, the chemistry of their allylic fuel radicals and their decomposition products strongly influences the combination reactions to the initially formed aromatic ring species and as a consequence, the PAH formation tendency.

Aromatic ring formation in opposed-flow diffusive 1,3-butadiene flames

This paper is concerned with the formation of one- and two-ring aromatic species in near atmospheric-pressure opposed-flow diffusion flames of 1,3-butadiene (1,3-C4H6). The chemical structures of two different 1,3-C4H6/Ar–O2/Ar flames were explored using flame-sampling molecular-beam mass spectrometry with both electron and single-photon ionization. We provide mole fraction profiles of 47 components as function of distance from the fuel outlet and compare them to chemically detailed modeling results. To this end, the hierarchically developed model described by Seidel et al. [16] has been updated to accurately comprise the chemistry of 1,3-butadiene. Generally a very good agreement is observed between the experimental and modeling data, allowing for a meaningful reaction path analysis. With regard to the formation of aromatic species up to naphthalene, it was essential to improve the fulvene and the C5 chemistry description in the mechanism. In particular, benzene is found to be formed mainly via fulvene through the reactions of the C4H5 isomers with C2H2. The n-C4H5 radical reacts with CH3 forming 1,3-pentadiene (C5H8), which is subsequently oxidized to form the naphthalene precursor cyclopentadienyl (C5H5). Oxidation of naphthalene is predicted to be a contributor to the formation of phenylacetylene (C8H6), indicating that consumption reactions can be of similar importance as molecular growth reactions.

Consumption and hydrocarbon growth processes in a 2-methyl-2-butene flame

This paper is concerned with the investigation of the chemical structure of a low-pressure, fuel-rich (ɸ = 1.8) premixed laminar flame fueled with 2-methyl-2-butene employing flame-sampling molecular-beam mass spectrometry with vacuum-ultraviolet single-photon ionization. Partially isomer-resolved mole fraction profiles can be explained by a decomposition scheme based on hydrogen abstraction and addition reactions. The presence of 9 allylic CH bonds compared to only one vinylic CH bond is the key feature that governs the fuel consumption and subsequent hydrocarbon growth reactions. Compared to other alkenes, including e.g., 1-butene, 2-butene, and iso-butene (Schenk et al., 2013), 2-methyl-2-butene shows a remarkable tendency to form soot precursor molecules such as toluene. In particular, experimental evidence is provided here that toluene, o-xylene, and styrene can be a starting point for PAH formation, thus serving as first aromatic rings besides benzene. The formation of toluene, o-xylene, and styrene can be traced back to the reactions of the resonantly stabilized C4H5 [∙CH2CCCH3 and CH2CH∙CCH2] radicals and the C5H7 [CH2C(CH3)∙CCH2] radicals that are readily formed through fuel-specific decomposition channels. Our experimental data in form of mole fraction profiles as a function of height above the burner for a mass range from 2 to 210 u can serve as reliable validation targets for model development. A preliminary comparison to the model of Westbrook et al. [1] that was optimized to capture ignition delay times and the low-temperature oxidation regime, shows promising elements already for the initial fuel consumption.