Electron Ionization Molecular-beam Mass Spectrometry Research Articles

Interactions in Ammonia and Hydrogen Oxidation Examined in a Flow Reactor and a Shock Tube

Ammonia (NH3) is a promising fuel, because it is carbon-free and easier to store and transport than hydrogen (H2). However, an ignition enhancer such as H2 might be needed for technical applications, because of the rather poor ignition properties of NH3. The combustion of pure NH3 and H2 has been explored widely. However, for mixtures of both gases, mostly only global parameters such as ignition delay times or flame speeds were reported. Studies with extensive experimental species profiles are scarce. Therefore, we experimentally investigated the interactions in the oxidation of different NH3/H2 mixtures in the temperature range of 750-1173 K at 0.97 bar in a plug-flow reactor (PFR), as well as in the temperature range of 1615-2358 K with an average pressure of 3.16 bar in a shock tube. In the PFR, temperature-dependent mole fraction profiles of the main species were obtained via electron ionization molecular-beam mass spectrometry (EI-MBMS). Additionally, for the first time, tunable diode laser absorption spectroscopy (TDLAS) with a scanned-wavelength method was adapted to the PFR for the quantification of nitric oxide (NO). In the shock tube, time-resolved NO profiles were also measured by TDLAS using a fixed-wavelength approach. The experimental results both in PFR and shock tube reveal the reactivity enhancement by H2 on ammonia oxidation. The extensive sets of results were compared with predictions by four NH3-related reaction mechanisms. None of the mechanisms can well predict all experimental results, but the Stagni et al. [React. Chem. Eng. 2020, 5, 696-711] and Zhu et al. [Combust. Flame 2022, 246, 115389] mechanisms perform best for the PFR and shock tube conditions, respectively. Exploratory kinetic analysis was conducted to identify the effect of H2 addition on ammonia oxidation and NO formation, as well as sensitive reactions in different temperature regimes. The results presented in this study can provide valuable information for further model development and highlight relevant properties of H2-assisted NH3 combustion.

Plasma-Assisted Chemical-Looping Combustion: Low-Temperature Methane and Ethylene Oxidation with Nickel Oxide

The chemical reaction network of low-temperature plasma-assisted oxidation of methane (CH4) and ethylene (C2H4) with nickel oxide (NiO) was investigated in a heated plasma reactor through time-dependent species measurements by electron-ionization molecular beam mass spectrometry (EI-MBMS). Methane (ethylene) oxidation by NiO was explored in temperature ranges from 300-700 °C (300-500 °C) and 300-800 °C (300-600 °C) for the plasma and nonplasma conditions. Significant enhancement of methane oxidation was observed with plasma between 400 and 500 °C, where no oxidation was observed under nonplasma conditions. For the oxidation of methane at higher temperatures, three different oxidation stages were observed: (I) a period of complete oxidation, (II) a period of incomplete CO oxidation, and (III) a period of carbon buildup. For the C2H4 experiments, and unlike the CH4 experiments, the plasma resulted in a significant amount of new intermediate oxygenated species, such as CH2O, CH3OH, C2H4O, and C2H6O. Carbon deposits were observed under both methane and ethylene conditions and verified by X-ray photoelectron spectroscopy (XPS). ReaxFF (reactive force field) simulations were performed for the oxidation of CH4 and C2H4 in a nonplasma environment. The simulated intermediates and products largely agree with the species measured in the experiments, though the predicted intermediate oxygenated species such as CH2O and C2H6O were not observed in experiments under nonplasma conditions. A reaction pathway analysis for CH4 and C2H4 reacting with NiO was created based on the observed species from the MBMS spectra along with ReaxFF simulations.

A detailed uncertainty analysis of EI-MBMS data from combustion experiments

Electron ionization molecular-beam mass spectrometry (EI-MBMS) is a widely used analytical method for studying the chemistry of combustion processes and for providing quantitative validation targets for kinetic model development often in form of species mole fraction profiles. The present study focuses on the determination of the uncertainties of such data, which result from the mass spectrometric measurements as well as from the data evaluation procedure, in particular through the use of different calibration methods, i.e., through a direct calibration of the instrument, a convolution procedure, and through the method of Relative Ionization Cross Sections (RICS). A series of calibration experiments were carried out using two independent time-of-flight mass spectrometers in order to check the influence of experimental and instrumental parameters on the uncertainties. The data obtained reveal several key contributors to the uncertainty, which are, among others, the signal intensity of a species in the mass spectrum, the instability of instrumental parameters, the species itself, and the calibration method.The propagation of these uncertainties from the different calibration methods as well as from the instrumental inconstancies into the final uncertainties of the mole fractions is further demonstrated using a gaussian error propagation and provision of sensitivities for different uncertainty sources is given. It was found that most of the uncertainties of the mole fractions reported in the literature are smaller than those reported here and presumably only consider the reproducibility of the signal.The use of two independent mass spectrometers for the experiments also revealed that the uncertainties determined for one instrument cannot be directly transferred to another especially for the method of direct calibration due to differences in the signal reproducibility, but that the uncertainties only change marginal for the semi-theoretical approaches.

Dimethyl ether (DME) and dimethoxymethane (DMM) as reaction enhancers for methane: Combining flame experiments with model-assisted exploration of a polygeneration process

The potential of dimethyl ether (DME) and dimethoxymethane (DMM), representatives of the attractive oxymethylene ether (OME) alternative fuel family, are explored here as reactivity enhancers for methane-fueled polygeneration processes. Typically, such processes that can flexibly generate power, heat, or chemicals, operate under fuel-rich conditions in gas turbines or internal combustion engines. To provide a consistent basis for the underlying reaction mechanisms, it is recognized that speciation data for the DME/CH4 fuel combination are available for such conditions while such information for the DMM/CH4 system is largely lacking. In addition, it should be noted that a detailed speciation study in flames, i.e., combustion systems involving chemistry and transport processes over a large temperature range, is still missing in spite of the potential of such systems to provide extended species information. In a systematic approach using speciation with electron ionization molecular-beam mass spectrometry (EI-MBMS), we thus report, as a first step, investigation of six fuel-rich premixed flames of DME and DMM and their blends with methane with special attention on interesting chemicals. Secondly, a comprehensive but compact DME/DMM/CH4 model (PolyMech2.1) is developed based on these data. This model is then examined against available experimental data under conditions from various facilities, focusing preferentially on elevated pressure and fuel-rich conditions. Comparison with existing literature models is also included in this evaluation. Thirdly, an analysis is given on this basis, via the extensively tested PolyMech2.1 model, for assumed polygeneration conditions in a homogeneous charge compression ignition (HCCI) engine environment. The main interest of this model-assisted exploration is to evaluate whether addition of DME or DMM in a polygeneration process can lead to potentially useful conditions for the production of syngas or other chemicals, along with work and heat. The flame results show that high syngas yields, i.e., up to ∼78% for CO and ∼35% for H2, can be obtained in their burnt gases. From the large number of intermediates detected, predominantly acetylene, ethylene, ethane, and formaldehyde show yields of 2.1−4.4% (C2 hydrocarbons) and 3.4−8.7% (CH2O), respectively. Also, methanol and methyl formate show comparably high yields of up to 0.6−6.7% in the flames with DMM, which is 1–2 orders of magnitude higher than in those with DME as the additive. In the modeling-assisted exploration of the engine process, the PolyMech2.1 model is seen to perform at significantly reduced computational costs compared to a recently validated model without sacrificing the prediction performance. Promising conditions for the assumed polygeneration process using fuel combinations in the DME/DMM/CH4 system are identified with attractive syngas yields of up to 77% together with work and heat output at exergetic efficiencies of up to 89% with DME.

MBMS study on plasma‐assisted low‐temperature oxidation of n‐heptane and iso‐octane in a flow reactor

AbstractNonequilibrium plasma has a great potential in the lean combustion of gasoline engines. However, fundamental studies on plasma‐assisted combustion on gasoline components are limited. In the present study, Low‐temperature oxidation behaviors for n‐heptane and iso‐octane in a plasma flow reactor were investigated. Species measurements were performed at room temperature and low pressure of 40 mbar using the electron ionization molecular‐beam mass spectrometry (EI‐MBMS). A total of 72 and 57 species were quantified in n‐heptane and iso‐octane oxidation, respectively. Based on measured species data, reaction networks of n‐heptane and iso‐octane in the plasma system were discussed. Particularly, some typical low‐temperature intermediates, such as cyclic ethers, diones, and ketohydroperoxides, were observed, suggesting the applicability of the classical low‐temperature reaction scheme in the plasma system of large hydrocarbons. The results indicate that the major role of plasma discharge on the low‐temperature oxidation of large hydrocarbons is either to breakdown large fuel molecules or to accelerate the chain initiation reactions. We also investigated the parametric effects of voltage, frequency, and residence time on the species pools, and found that the increase of these parameters leads to higher production of intermediates. The whole system reactivity was most sensitive to the plasma voltage. The difference between plasma frequency and residence time on the species formation was found. Moreover, some highly temperature‐sensitive reactions, such as the RO2 → QOOH, proceed at very low temperatures (around 340 K) in the plasma system, which was potentially attributed to the additional energy from plasma‐generated species.

Laminar premixed and non-premixed flame investigation on the influence of dimethyl ether addition on n-heptane combustion

Several different ether components have been proposed as renewable replacements for diesel fuel in compression ignition engines. From a fundamental point of view, blends of n-heptane and dimethyl ether (DME) have been considered in this study, where n-heptane is a single-component diesel surrogate and DME acts as a representative for oxymethylene ethers. n-Heptane and n-heptane / DME blends have been investigated experimentally under laminar premixed low-pressure and non-premixed atmospheric-pressure counterflow flame conditions. For the premixed case, a series of three fuel-rich flames was studied with oxygen as the oxidizer, at a constant C/O ratio of 0.47, pressure of 4 kPa, with 50% argon dilution, and an equivalence ratio ϕ near 1.5, burning pure n-heptane and mixtures of n-heptane with DME in the ratios 1:1 and 1:4. In the non-premixed counterflow configuration, two fuel/oxygen/argon flames were studied with pure n-heptane and a 1:1 DME–n-heptane mixture as the fuel. Electron ionization molecular-beam mass spectrometry was used for both flame configurations to investigate the flame structure and determine quantitative mole fraction profiles of stable and reactive species formed in the combustion process. Particular attention was given to the changes caused by partial replacement of n-heptane by DME.Notwithstanding the experimental focus of this work, the experimental results were compared with predictions from a model suitable for both n-heptane and DME that was combined for this case from recent mechanisms available in the literature for these fuels. While the overall flame structure was not significantly altered upon DME addition, with some smaller differences mainly due to temperature effects, more prominent changes and interactive effects were found for a number of primary decomposition products, oxygenated species, and higher-molecular hydrocarbon compounds. In most cases, experimentally observed trends, but not always quantitative changes, could be satisfactorily reproduced and explained by the model.

Probing the low-temperature chemistry of di-n-butyl ether: Detection of previously unobserved intermediates

Di-n-butyl ether (DBE, C8H18O) has been proposed as a promising biofuel for diesel engines, but details of its low-temperature (LT) oxidation chemistry are not well understood. This paper reports new speciation data obtained in the temperature range of 400–1100 K at ϕ=1 and nearly-atmospheric pressure, using a plug flow reactor combined with electron ionization molecular-beam mass spectrometry (MBMS) and two different jet-stirred reactors coupled with either online gas chromatography or tunable synchrotron vacuum ultraviolet photoionization-MBMS.The experimental results confirm that DBE is very reactive and exhibits two negative-temperature coefficient (NTC) zones around 500–550 K and 650–750 K. Speciation data with about 40 C0-C8 species are presented, including about 20 LT species not reported previously. Among those, fuel-specific C8H16O2 cyclic ethers were quantified. Also, butanoic acid, which is present in highest amounts among the detected LT intermediates, and C8H14O3 diones, were found to peak already near 500 K, suggesting their importance in the LT chemistry of DBE. Signals of several highly oxygenated peroxides (e.g., C8H14O5 and C8H16O6) were detected, indicating third O2 addition steps. Respective reaction pathways are suggested and discussed based on these experimental results.To better understand the LT chemistry of DBE, the present data were compared to two recent DBE models (Cai et al., 2014; Thion et al., 2017). Significant discrepancies between the experimental data and both models were found for important LT intermediates, of which many were not included in the respective mechanisms. The results reported in the present study thus provide new opportunities for refining DBE kinetic models.

An experimental laminar flame investigation of dual-fuel mixtures of C4 methyl esters with C2–C4 hydrocarbon base fuels

Blending petroleum-based fuels with biofuel components is deemed attractive to reduce soot and CO2 emissions, but fundamental studies of the combustion behavior of such fuel blends suited for model development and validation remain rather scarce. To contribute to the understanding of the combustion chemistry effects of such blending strategies, we have investigated laminar premixed low-pressure flames of three hydrocarbon base fuels, namely 1-butene (1-C4H8), isobutene (i-C4H8), and ethene (C2H4), blended each with two different ester fuels, namely methyl crotonate (C5H8O2, MC) and methyl butanoate (C5H10O2, MB). A series of 13 flames with different argon dilution was investigated to study effects of the specific fuel structure on the combustion chemistry. Full speciation analyses were performed for fuel-rich (ϕ = 1.6) conditions by electron ionization molecular-beam mass spectrometry (EI-MBMS). More than 35 species in the range of C0–C7 were identified and quantified in these flames, resulting in ∼450 mol fraction profiles. The experimental data were compared to simulations by the kinetic model reported by Yang et al. [Proc. Combust. Inst. 2011, Phys. Chem. Chem. Phys. 2013] that was chosen because it includes basic mechanisms of all studied fuels. Overall, the agreement of experiment and this model seems satisfactory but calls for further improvements regarding ester as well as hydrocarbon sub-mechanisms. It was noted that the unsaturation degree in the methyl esters affects the formation of hydrocarbons, that depend mainly on the structure of the respective base fuel, and of oxygenated intermediates. The methyl esters have different decomposition pathways leading to some specific oxygenated species. Both methyl esters promote the formation of formaldehyde and methanol, while acetic acid is significantly increased by the presence of MB. The effect of the ester addition is also influenced by the species pool of the respective hydrocarbon base fuel.

Chemical interaction of dual-fuel mixtures in low-temperature oxidation, comparing n-pentane/dimethyl ether and n-pentane/ethanol

With the aim to study potential cooperative effects in the low-temperature oxidation of dual-fuel combinations, we have investigated prototypical hydrocarbon (C5H12) / oxygenated (C2H6O) fuel mixtures by doping n-pentane with either dimethyl ether (DME) or ethanol (EtOH). Species measurements were performed in a flow reactor at an equivalence ratio of ϕ = 0.7, at a pressure of p = 970 mbar, and in the temperature range of 450–930 K using electron ionization molecular-beam mass spectrometry (EI-MBMS). Series of different blending ratios were studied including the three pure fuels and mixtures of n-pentane containing 25% and 50% of C2H6O. Mole fractions and signals of a significant number of species with elemental composition CnH2n+xOy (n = 1–5, x = 0–(n + 2), y = 0–3) were analyzed to characterize the behavior of the mixtures in comparison to that of the individual components. Not unexpectedly, the overall reactivity of n-pentane is decreased when doping with ethanol, while it is promoted by the addition of DME. Interestingly, the present experiments reveal synergistic interactions between n-pentane and DME, showing a stronger effect on the negative temperature coefficient (NTC) for the mixture than for each of the individual components. Reasons for this behavior were investigated and show several oxygenated intermediates to be involved in enhanced OH radical production. Conversely, ethanol is activated by the addition of n-pentane, again involving key OH radical reactions. Although the main focus here is on the experimental results, we have attempted, in a first approximation, to complement the experimental observations by simulations with recent kinetic models. Interesting differences were observed in this comparison for both, fuel consumption and intermediate species production. The inhibition effect of ethanol is not predicted fully, and the synergistic effect of DME is not captured satisfactorily. The exploratory analysis of the experimental results with current models suggests that deeper knowledge of the reaction chemistry in the low-temperature regime would be useful and might contribute to improved prediction of the low-temperature oxidation behavior for such fuel mixtures.

Influence of dimethyl ether and diethyl ether addition on the flame structure and pollutant formation in premixed iso-octane flames

While the combustion reactions of many individual fuels in flames have been studied in detail, often providing numerous experimental species profiles in combination with the development and critical testing of kinetic models, similar work on fuel mixtures has remained scarcer, especially for blends of conventional and biogenic fuels. In the present study, we have thus chosen iso-octane, one of the primary reference fuels that determine octane number and knocking tendency of gasoline, as the base fuel, and dimethyl ether (DME) and diethyl ether (DEE) as potentially bio-derived fuel components. Premixed low-pressure flames of iso-octane without and with different amounts of DME or DEE, of up to 50% of the fuel in the mixture, have been investigated systematically to analyze the change of the flame structure and species composition upon blending. All flames were studied at a pressure of 40 mbar (4kPa) with 50% argon dilution and a cold gas velocity of 60 cms−1. The carbon-to-oxygen ratio was fixed at 0.47 while the equivalence ratio changed from 1.47 to 1.84. Up to 70 species in the range of C0–C9 were identified and quantified in each flame by electron ionization molecular-beam mass spectrometry (EI-MBMS). Moreover, about 300 experimental species profiles were provided.The aim of this study is thus threefold. First, a new species dataset for a pure iso-octane flame is reported, since speciation data under premixed flame conditions for this fuel is surprisingly scarce. Second, the effects of DME and DEE addition in increasing amounts to the iso-octane base flame are investigated with special attention to the flame structure and pollutant species formation. Third, a joint combustion model for iso-octane, DME, and DEE mixtures consisting of 379 species and 1931 reactions is proposed that was examined against the present data and reference experiments for a larger range of conditions. Key pathways of interesting species, including aldehydes, alkenes, and soot precursors were revealed with the present mixture model. DME and DEE addition were found to have a similar influence on the flame structure of iso-octane, except for some C2 and C3 intermediates. The addition of DME as well as of DEE decreases the formation of soot precursors while it enhances the production of formaldehyde.

1-, 2- and 3-Pentanol combustion in laminar hydrogen flames – A comparative experimental and modeling study

Oxidation pathways of three different pentanol isomers 1-, 2- and 3-pentanol are investigated using the electron–ionization molecular-beam mass spectrometry (EI-MBMS) technique. New experimental speciation data was obtained from laminar, flat, low-pressure H2/O2/Ar base flames seeded with equal amounts of pre-vaporized 1-, 2- or 3-pentanol. The experimental investigation is supported by kinetic modeling. Here, one single detailed reaction mechanism for the three linear pentanol isomers has been constructed and compared against new and existing experimental data. The model itself was obtained by an open-source reaction model generation software (RMG) and tested against existing ignition delay times, flame speeds and new quantitative experimental results for mole fraction profiles of major and intermediate species.The overall discussion of individual species profiles is guided by the model-based reaction flow analysis focusing on the initial steps of the fuel destruction paths for the three pentanols down to C3-hydrocarbon species. Therefore, the reaction pathways for the initial fuel destruction steps are shown and analyzed using combined experimental and predicted results. Analysis is performed by means of secondary decay products for each of the three pentanols. The kinetic reaction model was successfully tested against available ignition delay time and laminar flame speed data. Comparisons against 23 new quantitative species profiles are presented for each of the pentanol doped flames. In general, a good predictive capability of the detailed model can be noted for all three investigated straight-chain pentanols regarding the mole fraction profiles of major and intermediate species.

Influence of substituted furans on the formation of Polycyclic Aromatic Hydrocarbons in flames

The structure of low-pressure (40kPa) rich laminar premixed flames of two potential new biofuels, 2,5-dimethylfuran (DMF) and 2-methylfuran (MF), has been investigated using gas chromatography after sampling by a sonic probe. This work follows a study of the same flames made using electron-ionization molecular-beam mass spectrometry (EI-MBMS). Temperature profiles have been measured using a thermocouple with and without the probe and compared with those obtained during EI-MBMS experiments, showing the important influence of the probe geometry. The mole fraction profiles above the burner of numerous products and intermediates (31 for MF, 40 for DMF) have been quantified, including all the isomers which were globalized during EI-MBMS experiments, as well as some heavier aromatic products such as ethylbenzene and styrene. The profiles of these aromatics confirmed their significantly enhanced formation in the case of DMF compared to MF, which was already shown in the previous study. The model of the oxidation of these two substituted furans have been up-dated to include new reactions for the formation of C8 aromatics, as well as reactions proposed in literature for large Polycyclic Aromatic Hydrocarbons (PAH) formation. Simulations show a significant enhancement effect on the formation of PAH, such as pyrene or phenanthrene: their maximum mole fractions are increased by a factor of more than 400 in the DMF flame compared to that of MF, while it is only a factor 3 in the case of benzene. Simulations show that PAH mole fractions are also slightly larger in the DMF flame than in a flame of a gasoline surrogate containing toluene as an octane improver under the same conditions. However the PAH mole fractions in the DMF flame are significantly lower than in a flame of pure toluene, showing that this biofuel could be an interesting octane improver in replacement of aromatics.

Flame structure and kinetic studies of carbon dioxide-diluted dimethyl ether flames at reduced and elevated pressures

The flame structure and kinetics of dimethyl ether (DME) flames with and without CO2 dilution at reduced and elevated pressures were studied experimentally and computationally. The species distributions of DME oxidation in low-pressure premixed flat flames were measured by using electron-ionization molecular-beam mass spectrometry (EI-MBMS) at an equivalence ratio of 1.63 and 50mbar. High-pressure flame speeds of lean and rich DME flames with and without CO2 dilution were measured in a nearly-constant-pressure vessel between about 1 and 20bar. The experimental results were compared with predictions from four kinetic models: the first was published by Zhao et al. (2008) [9], the second developed by the Lawrence Livermore National Laboratory (LLNL) (Kaiser et al., 2000) [13], and the third has been made available to us as the Aramco mechanism (Metcalfe et al., 2013) [14]; as the fourth, we have used an updated model developed in this study. Good agreement was found between measurements and predictions from all four models for all major and most typical intermediate species with and without CO2 addition in low-pressure flat flame experiments. However, none of the models was able to reliably predict high-pressure flame speeds. Although the updated model improved the prediction of flame speeds for lean mixtures, errors remained for rich conditions at elevated pressure, likely due to uncertainty in the rates of CH3+H(+M)=CH4(+M) and the branching and termination reaction pair of CH3+HO2=CH3O+OH and CH3+HO2=CH4+O2. CO2 addition considerably decreased the flame speed. Kinetic comparisons between inert and chemically active CO2 in DME flames showed that CO2 addition affects rich and lean DME flame kinetics differently. For lean flames, both the inert third-body effect and the kinetic effect of CO2 reduce H-atom production. However, for rich flames, the inert third-body effect increases H-atom production via HCO(+M)=H+CO(+M) and suppression of the kinetic effect of CO2 by shifting the equilibrium of CO+OH=CO2+H.

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 40mbar) 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. [Combust. 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.

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 40mbar) 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 40mbar) 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.

Identification of Tetrahydrofuran Reaction Pathways in Premixed Flames

Abstract Premixed low-pressure tetrahydrofuran/oxygen/argon flames are investigated by photoionization molecular-beam mass spectrometry using vacuum-ultraviolet synchrotron radiation. For two equivalence ratios (φ = 1.00 and 1.75), mole fractions are measured as a function of distance from the burner for almost 60 intermediates with molar masses ranging from 2 (H2) to 88 (C4H6O2), providing a broad database for flame modeling studies. The isomeric composition is resolved by comparisons between experimental photoionization efficiency data and theoretical simulations, based on calculated ionization energies and Franck-Condon factors. Special emphasis is put on the resolution of the first reaction steps in the fuel destruction. The photoionization experiments are complemented by electron-ionization molecular-beam mass-spectrometry measurements that provide data with high mass resolution. For three additional flames with intermediate equivalence ratios (φ = 1.20, 1.40 and 1.60), mole fractions of major species and photoionization efficiency spectra of intermediate species are reported, extending the database for the development of chemical kinetic models.

Structure of a stoichiometric propanal flame at low pressure

In view of the steady increase in the use of oxygenated fuels, aldehydes play an increasingly important role as intermediates in combustion. The combustion chemistry of propanal (CH3CH2CHO) was studied in a stoichiometric low-pressure, premixed flat flame. Species mole fraction profiles were measured by electron-ionization molecular beam mass spectrometry (MBMS) for the combustion products, the majority of stable reaction intermediates and several radical species. Based on the mole fraction profiles reactions of the aldehyde fuel are discussed. A number of oxygenated species with a molecular mass exceeding that of the fuel are formed during the early stages of the combustion process. Their presence suggests that addition reactions of small radicals to fuel derived molecules with aldehyde functions play an important role in this flame.