Prof. Dr. Tina Kasper
University of Duisburg-Essen
- Investigation of the combustion of iron pentacarbonyl and the formation of key intermediates in iron oxide synthesis flames
Karakaya, Y. and Kluge, S. and Wiggers, H. and Schulz, C. and Kasper, T.
Chemical Engineering Science 230 (2021)The information of the gas phase kinetics are relevant for the development of detailed reaction mechanisms as well as for process design and control in flame synthesis. In this study, the decomposition of iron pentacarbonyl and the reaction pathways towards iron oxide clusters and particles in laminar H2/O2/Ar low-pressure synthesis flames are investigated. Gas-phase species are analyzed by photoionization and electron ionization mass spectrometry. The extraction of a representative sample from the particle-laden flow of a synthesis flame by an intrusive sampling technique for the analysis is challenging, because iron-intermediate species can condense easily. Cations can be extracted from the flame with a high efficient ion sampling technique that results in high sensitivity. Iron-containing cations provide evidence of the presence of key intermediates, e.g., Fe(OH)2, Fe(OH)3, Fe2O3, and larger Fe-O-clusters which are the dominant intermediates with respect to particle formation and need to be considered in future gas-phase reaction mechanisms. © 2020 Elsevier Ltd
view abstract 10.1016/j.ces.2020.116169
- Plug-flow reactor and shock-tube study of the oxidation of very fuel-rich natural gas/DME/O2 mixtures
Kaczmarek, D. and Herzler, J. and Porras, S. and Shaqiri, S. and Fikri, M. and Schulz, C. and Atakan, B. and Maas, U. and Kasper, T.
Combustion and Flame 225 (2021)A polygeneration process with the ability to provide work, heat, and useful chemicals according to the specific demand is a promising alternative to traditional energy conversion systems. By implementing such a process in an internal combustion engine, products like synthesis gas or unsaturated hydrocarbons and very high exergetic efficiencies can be obtained through partial oxidation of natural gas, in addition to the already high flexibility with respect to the required type of energy. To enable compression ignition with natural gas as input, additives such as dimethyl ether are needed to increase the reactivity at low temperatures. In this study, the reaction of fuel-rich natural gas/dimethyl ether (DME) mixtures is investigated to support the further development of reaction mechanisms for these little studied reaction conditions. Temperature-resolved species concentration profiles are obtained by mass spectrometry in a plug-flow reactor at equivalence ratios ϕ = 2, 10, and 20, at temperatures between 473 and 973 K and at a pressure of 6 bar. Ignition delay times and product-gas analyses are obtained from shock-tube experiments, for ϕ = 2 and 10, at 710 – 1639 K and 30 bar. The experimental results are compared to kinetic simulations using two literature reaction mechanisms. Good agreement is found for most species. Reaction pathways are analyzed to investigate the interaction of alkanes and DME. It is found that DME forms radicals at comparatively low temperatures and initiates the conversion of the alkanes. Additionally, according to the reaction pathways, the interaction of the alkanes and DME promotes the formation of useful products such as synthesis gas, unsaturated hydrocarbons and oxygenated species. © 2020
view abstract 10.1016/j.combustflame.2020.10.004
- Threshold photoionization shows no sign of nitryl hydride in methane oxidation with nitric oxide
Hoener, M. and Bodi, A. and Hemberger, P. and Endres, T. and Kasper, T.
Physical Chemistry Chemical Physics 23 (2021)Methane was doped with nitric oxide and oxidized in a high-pressure flow reactor. The nitrogen chemistry during partial oxidation was studied using photoelectron photoion coincidence spectroscopy with vacuum ultraviolet synchrotron radiation. The adiabatic ionization energy of nitrous acid, HONO, has been determined as 10.95 ± 0.03 eV. The HONO breakdown diagram was plotted based solely on the measured parent signal and the computed Franck-Condon envelope of trans-HONO, confirming the trans-HONO dissociative photoionization threshold to NO+ + OH at 11.34 eV. The spectra show strong indication for the presence of cis-HONO. We expected the m/z 47 photoion mass selected threshold photoelectron signal to rebound near 12 eV, i.e., at the ionization energy of nitryl hydride, the third HNO2 isomer. Recent computational studies suggest nitryl hydride is formed at a rate similar to trans-HONO, is more thermally stable than nitrous acid, its cation is bound, and its photoelectron spectrum is predicted to exhibit a strong origin band near 12 eV. The absence of its mass selected threshold photoelectron signal shows that nitryl hydride is either not formed in measurable amounts or is consumed faster than nitrous acid, for instance by isomerization to trans-HONO. This journal is © the Owner Societies.
view abstract 10.1039/d0cp04924g
- A pressurized flow reactor combustion experiment interfaced with synchrotron double imaging photoelectron photoion coincidence spectroscopy
Hoener, M. and Kaczmarek, D. and Bierkandt, T. and Bodi, A. and Hemberger, P. and Kasper, T.
Review of Scientific Instruments 91 (2020)A new pressurized low-temperature combustion experiment has been commissioned at the Swiss Light Source, Paul Scherrer Institute. The experiment uses photoionization with tunable synchrotron radiation and double imaging photoelectron photoion coincidence (i2PEPICO) detection at the vacuum ultraviolet beamline. The experimental setup is described, including the high-pressure reactor experiment, sampling interface, and reactant delivery system. The CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence) endstation and VUV beamline are briefly elaborated. The novel aspects of the apparatus and the new components are elucidated in detail, such as the fluid supply system to the reactor and the reactor integration into the endstation. We also present a system overview of the experimental setup. The technical details are followed by a description of the experimental procedure used to operate the pressurized flow reactor setup. Finally, first experimental results demonstrating the capability of the setup are provided and analyzed. A major advantage of this new experiment is that the excellent isomer resolution capabilities of the i2PEPICO technique can be transferred to the investigation of reactions at elevated pressures of several bars. This enables the investigation of pressure effects on the reactivity of fuel mixtures and covers more realistic conditions found in technical combustors. The capability to obtain quantitative oxidation data is confirmed, and the main and certain intermediate species are quantified for a selected condition. The results show excellent agreement with a chemical kinetics model and previously published reference measurements performed with a gas chromatography setup. © 2020 Author(s).
view abstract 10.1063/1.5141168
- An experimental and modeling study on the reactivity of extremely fuel-rich methane/dimethyl ether mixtures
Porras, S. and Kaczmarek, D. and Herzler, J. and Drost, S. and Werler, M. and Kasper, T. and Fikri, M. and Schießl, R. and Atakan, B. and Schulz, C. and Maas, U.
Combustion and Flame 212 (2020)Chemical reactions in stoichiometric to fuel-rich methane/dimethyl ether/air mixtures (fuel air equivalence ratio ϕ = 1–20) were investigated by experiment and simulation with the focus on the conversion of methane to chemically more valuable species through partial oxidation. Experimental data from different facilities were measured and collected to provide a large database for developing and validating a reaction mechanism for extended equivalence ratio ranges. Rapid Compression Machine ignition delay times and species profiles were collected in the temperature range between 660 and 1052 K at 10 bar and equivalence ratios of ϕ = 1–15. Ignition delay times and product compositions were measured in a shock tube at temperatures of 630–1500 K, pressures of 20–30 bar and equivalence ratios of ϕ = 2 and 10. Additionally, species concentration profiles were measured in a flow reactor at temperatures between 473 and 973 K, a pressure of 6 bar and equivalence ratios of ϕ = 2, 10, and 20. The extended equivalence ratio range towards extremely fuel-rich mixtures as well as the reaction-enhancing effect of dimethyl ether were studied because of their usefulness for the conversion of methane into chemically valuable species through partial oxidation at these conditions. Since existing reaction models focus only on equivalence ratios in the range of ϕ = 0.3–2.5, an extended chemical kinetics mechanism was developed that also covers extremely fuel-rich conditions of methane/dimethyl ether mixtures. The measured ignition delay times and species concentration profiles were compared with the predictions of the new mechanism, which is shown to predict well the ignition delay time and species concentration evolution measurements presented in this work. Sensitivity and reaction pathway analyses were used to identify the key reactions governing the ignition and oxidation kinetics at extremely fuel-rich conditions. © 2019 The Authors
view abstract 10.1016/j.combustflame.2019.09.036
- Decomposition Reactions of Fe(CO)5, Fe(C5H5)2, and TTIP as Precursors for the Spray-Flame Synthesis of Nanoparticles in Partial Spray Evaporation at Low Temperatures
Gonchikzhapov, M. and Kasper, T.
Industrial and Engineering Chemistry Research 59 (2020)Flame spray pyrolysis is an important method of nanoparticle manufacturing. Reactions of the precursor and the solvent determine which intermediates can contribute to particle formation. To investigate the chemical interaction between the solvent and the precursor during the partial evaporation of the spray preceding ignition, precursor solutions were sprayed into an externally heated flow reactor. The thermal decomposition of the precursors Fe(CO)5, Fe(C5H5)2, and Ti(i-OC3H7)4 in solutions of xylene and ethanol was investigated. Decomposition products were analyzed by mass spectrometry. The relevance of reactions at these low temperatures for the spray-flame process is substantiated by measurements of the spatial temperature distribution of the spray flame. Depending on the relative thermal stabilities of the precursor and the solvent, the less stable component can initiate decomposition of the more stable component, resulting in different reaction patterns of the solutions. The results are discussed with regard to their potential influence on particle formation pathways. © 2020 American Chemical Society.
view abstract 10.1021/acs.iecr.9b06667
- Experimental and numerical study on the influence of equivalence ratio on key intermediates and silica nanoparticles in flame synthesis
Karakaya, Y. and Janbazi, H. and Wlokas, I. and Levish, A. and Winterer, M. and Kasper, T.
Proceedings of the Combustion Institute (2020)Tetramethylsilane is a precursor often used for the production of flame-synthesized silica nanoparticles or coatings. This study investigates the chemical reaction mechanism of tetramethylsilane in a series of H 2 /O 2 /Ar low-pressure (p = 30 mbar) flames from fuel-lean to slightly fuel-rich flame conditions (ϕ= 0.8, 1.0 and 1.2). Mole fraction profiles are obtained by molecular-beam mass spectrometry. The experimental data are compared to simulations using a recently published reaction mechanism. The present study reveals the influence of the flame composition on the depletion of the precursor TMS, the formation of its main carbon-containing products (e.g. CO 2 and CO) and the main silicon-containing intermediates (e.g. Si(CH 3) 3 (CH 2)OO), Si(OH) 4, SiO 2, Si 4 O 10 H 4) appearing along the routes of particle formation. TEM images of synthesized particles reveal that the nanoparticles obtained from the gas-phase synthesis are spheres with a low degree of agglomeration. The particle size distribution appears to be dependent on the equivalence ratio of the synthesis flames and the changes can tentatively be traced to different particle formation pathways. The data set provided in this work can serve a basis for improvements to the reaction mechanisms of the Si/C/H/O system that are urgently needed to improve particle synthesis processes. © 2020 The Combustion Institute. Published by Elsevier Inc.
view abstract 10.1016/j.proci.2020.06.096
- Experimental flat flame study of monoterpenes: Insights into the combustion kinetics of α-pinene, β-pinene, and myrcene
Bierkandt, T. and Hoener, M. and Gaiser, N. and Hansen, N. and Köhler, M. and Kasper, T.
Proceedings of the Combustion Institute (2020)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. © 2020 The Combustion Institute.
view abstract 10.1016/j.proci.2020.06.204
- Flexible energy conversion and storage via high-temperature gas-phase reactions: The piston engine as a polygeneration reactor
Atakan, B. and Kaiser, S.A. and Herzler, J. and Porras, S. and Banke, K. and Deutschmann, O. and Kasper, T. and Fikri, M. and Schießl, R. and Schröder, D. and Rudolph, C. and Kaczmarek, D. and Gossler, H. and Drost, S. and Bykov, V. and Maas, U. and Schulz, C.
Renewable and Sustainable Energy Reviews 133 (2020)Piston engines are typically considered devices converting chemical energy into mechanical power via internal combustion. But more generally, their ability to provide high-pressure and high-temperature conditions for a limited time means they can be used as chemical reactors where reactions are initiated by compression heating and subsequently quenched by gas expansion. Thus, piston engines could be “polygeneration” reactors that can flexibly change from power generation to chemical synthesis, and even to chemical-energy storage. This may help mitigating one of the main challenges of future energy systems – accommodating fluctuations in electricity supply and demand. Investments in devices for grid stabilization could be more economical if they have a second use. This paper presents a systematic approach to polygeneration in piston engines, combining thermodynamics, kinetics, numerical optimization, engineering, and thermo-economics. A focus is on the fuel-rich conversion of methane as a fuel that is considered important for the foreseeable future. Starting from thermodynamic theory and kinetic modeling, promising systems are selected. Mathematical optimization and an array of experimental kinetic investigations are used for model improvement and development. To evaluate technical feasibility, experiments are then performed in both a single-stroke rapid compression machine and a reciprocating engine. In both cases, chemical conversion is initiated by homogeneous-charge compression-ignition. A thermodynamic and thermo-economic assessment of the results is positive. Examples that illustrate how the piston engine can be used in polygeneration processes to convert methane to higher-value chemicals or to take up carbon dioxide are presented. Open issues for future research are addressed. © 2020 The Authors
view abstract 10.1016/j.rser.2020.110264
- Probing the low-temperature chemistry of methyl hexanoate: Insights from oxygenate intermediates
Rogers, C.O. and Kaczmarek, D. and Kasper, T. and Labbe, N.J.
Proceedings of the Combustion Institute (2020)Understanding the combustion of methyl esters is crucial to elucidate kinetic pathways and predict combustion parameters, soot yields, and fuel performance of biodiesel, however most kinetic studies of methyl esters have focused on smaller, surrogate model esters. Methyl hexanoate is a larger methyl ester approaching the chain length of methyl esters found in biodiesel and has not received as much research attention as other smaller esters. The purpose of this work is to present the first atmospheric pressure combustion data of methyl hexanoate, CH3CH2CH2CH2CH2COOCH3. Mixtures of 2% methyl hexanoate in O2 and N2 are studied using a plug flow reactor at atmospheric pressure, wall temperatures from 573 to 973 K, residence times from roughly 1-2 s., and fuel equivalence ratios of 1, 1.5, and 2. Exhaust gases are analyzed by a gas chromatograph-mass spectrometer system and species mole fractions are presented. The literature model shows satisfactory agreement with the experimental species profiles and improvements for future mechanistic studies are suggested. In particular, this work proposes new unimolecular decomposition pathways of methyl hexanoate to form methanol or methyl acetate. Furthermore, the experiment detected three unsaturated esters that are direct products of the low temperature oxidation chemistry and it provides more insight into branching ratios for the formation of methyl hexanoate radicals and for the decomposition of hydroperoxyalkyl radicals. © 2020 The Combustion Institute.
view abstract 10.1016/j.proci.2020.07.056
- The influence of pressure and equivalence ratio on the NTC behavior of methane
Kaczmarek, D. and Shaqiri, S. and Atakan, B. and Kasper, T.
Proceedings of the Combustion Institute (2020)Methane based polygeneration processes in piston engines offer the possibility of a controllable and flexible conversion of energy, to up-convert low value chemicals and to store energy. These processes preferably take place under fuel-rich conditions and at high pressures. Under fuel-rich conditions, there was one experimental report that a distinctive negative temperature coefficient (NTC) behavior occurs in methane oxidation (Petersen et al., 1999). To design a polygeneration process, reliable kinetic models are required to capture the impact of pressure and equivalence ratio variations on reactivity of the gas mixtures. Here, the experimental basis for methane oxidation is expanded to high pressures and very fuel-rich conditions and compared to literature models, both with special emphasis on the NTC behavior. The oxidation of methane/oxygen mixtures at 2 ≤ Φ≤ 20 and pressures ranging from 1 to 20 bar is investigated. The literature reaction mechanisms are assessed with respect to their ability to predict this phenomenon and used to identify reaction pathways. It is found that NTC behavior occurs in a temperature range between 700 and 1000 K and at pressures higher than 5 bar. The lower temperature limit is slightly shifted towards higher temperatures with decreasing equivalence ratio. In addition, the higher the equivalence ratio, the broader the pressure range, in which the NTC behavior is observed. In general, predictions of some models are in good agreement with the experimental data. Reaction path analyses reveal that the competition between oxidation and recombination pathways are responsible for the NTC region in methane oxidation. © 2020 Published by Elsevier Inc.
view abstract 10.1016/j.proci.2020.06.112
- Chemistry of iron nitrate-based precursor solutions for spray-flame synthesis
Stodt, M.F.B. and Gonchikzhapov, M. and Kasper, T. and Fritsching, U. and Kiefer, J.
Physical Chemistry Chemical Physics 21 (2019)Understanding the chemistry of iron-based metal-organic precursor solutions for spray-flame synthesis is a key step to developing inexpensive and large scale applications for gas-phase synthesized, nano-sized iron oxide particles. Owing to the large variety of available organic solvents and iron compounds, the choice of a suitable precursor-solvent pair is challenging. Systematic investigations of the precursor chemistry of iron-based systems are currently not available. This work aims at filling this gap by providing a detailed spectroscopic analysis of mixtures containing iron(iii) nitrate nonahydrate and alkyl alcohols (C2-4). Moreover, the impact of adding 2-ethylhexanoic acid is explored. The FTIR spectra reveal the formation of carboxylates and allow deriving information about the coordination of the metal-carboxylate complexes. The stability of the precursor solutions is investigated by monitoring precipitation phenomena and turbidity. Furthermore, gas chromatography is employed to provide additional information on oxidation products and esters as well as to aid the interpretation of the FTIR data. It is found that the formation of esters has an enhancing effect on iron sorption and, thus, it promotes precursor stability. © 2019 the Owner Societies.
view abstract 10.1039/c9cp05007h
- Development and evaluation of a chemical kinetics reaction mechanism for tetramethylsilane-doped flames
Janbazi, H. and Karakaya, Y. and Kasper, T. and Schulz, C. and Wlokas, I. and Peukert, S.
Chemical Engineering Science 209 (2019)Tetramethysilane (TMS) is a precursor for flame synthesis of silica (SiO2) nanoparticles. A chemical reaction mechanism was developed for the oxidation of TMS in a lean low-pressure (p ≈ 30 mbar) H2/O2/Ar flame using species mole fractions, obtained from molecular-beam mass spectrometry (MBMS) measurements in a matrix-supported flat flame doped with 600 ppm TMS. The thermodynamic data of Si-containing species were determined from quantum-chemical calculations at the G4 level of theory. The formation and subsequent consumption of Si(OH)4, one of the main products of TMS oxidation, and the formation of Si4O10H4 clusters are hypothesized to be the primary pathway in the synthesis of silica nanoparticles. The reaction rate coefficients are either estimated via an algorithmic optimization procedure or are assumed based on analogies to similar reactions in the literature. The mechanism was further validated based on MBMS measurements with the same base flame doped with 400 and 800 ppm TMS. © 2019 Elsevier Ltd
view abstract 10.1016/j.ces.2019.115209
- Flame structure of laminar premixed anisole flames investigated by photoionization mass spectrometry and photoelectron spectroscopy
Bierkandt, T. and Hemberger, P. and Oßwald, P. and Krüger, D. and Köhler, M. and Kasper, T.
Proceedings of the Combustion Institute 37 (2019)Two laminar, premixed, fuel-rich flames fueled by anisole-oxygen-argon mixtures with the same cold gas velocity and pressure were investigated by molecular-beam mass spectrometry at two synchrotron sources where tunable vacuum-ultraviolet radiation enables isomer-resolved photoionization. Decomposition of the very weak O-CH3 bond in anisole (C6H5OCH3) by unimolecular decomposition yields the resonantly-stabilized phenoxy radical (C6H5O). This key intermediate species opens reaction routes to five-membered ring species, such as cyclopentadiene (C5H6) and cyclopentadienyl radicals (C5H5). Anisole is often discussed as model compound for lignin to study the phenolic-carbon structure in this natural polymer. Measured temperature profiles and mole fractions of many combustion intermediates give detailed information on the flame structure. A very comprehensive reaction mechanism from the literature which includes a sub-scheme for anisole combustion is used for species modeling. Species with the highest measured mole fractions (on the order of 10?3-10?2) are CH3, CH4, C2H2, C2H4, C2H6, CH2O, C5H5 (cyclopentadienyl radical), C5H6 (cyclopentadiene), C6H6 (benzene), C6H5OH (phenol), and C6H5CHO (benzaldehyde). Some are formed in the first destruction steps of anisole, e.g., phenol and benzaldehyde, and their formation will be discussed and with regard to the modeling results. There are three major routes for the fuel destruction: (1) formation of benzaldehyde (C6H5CHO), (2) formation of phenol (C6H5OH), and (3) unimolecular decomposition of anisole to phenoxy (C6H5O) and CH3 radicals. In the experiment, the phenoxy radical could be measured directly. The phenoxy radical decomposes via a bicyclic structure into the soot precursor C5H5 and CO. Formation of larger oxygenated species was observed in both flames. One of them is guaiacol (2-methoxyphenol), which decomposes into fulvenone. The presented speciation data, which contain more than 60 species mole fraction profiles of each flame, give insights into the combustion kinetics of anisole.
view abstract 10.1016/j.proci.2018.07.037
- Investigation of the partial oxidation of methane/n-heptane-mixtures and the interaction of methane and n-heptane under ultra-rich conditions
Kaczmarek, D. and Atakan, B. and Kasper, T.
Combustion and Flame (2019)The homogeneous partial oxidation of methane is an interesting approach to obtain useful chemicals like synthesis gas, higher hydrocarbons, aldehydes or alcohols. Because of the low reactivity of methane, the homogeneous conversion needs high temperatures to proceed at reasonable reaction rates. Additives like n-heptane form reactive intermediates at comparatively low temperatures and initiate the conversion. To study the kinetics of doped conversion reactions, fuel-rich diluted methane/n-heptane/oxygen/argon-mixtures (2 ≤ Φ ≤ 20) were investigated in a plug-flow reactor at a pressure of 6 bar, at intermediate temperatures between 423 and 973 K and at relatively long residence times (7 ≤ τ ≤ 14 s). The product composition at the reactor outlet is analyzed by gas chromatography and mass spectrometry. Species profiles as a function of equivalence ratio and temperature are compared with simulations, and serve as validation data for different reaction mechanisms. Rates of production and reaction paths are analyzed to investigate the interaction of methane and n-heptane during the oxidation process. They show that the chemical interaction of the oxidation products of both fuels has a promoting effect on the formation of different useful products like carbon monoxide, methanol or ethane. To prove this observation, mole fraction profiles as a function of temperature were compared between experiments with an equivalence ratio of Φ = 8 using neat methane, neat n-heptane and methane/n-heptane mixtures as fuels. The results show that the yields of these species are much higher in case of the methane/n-heptane mixture compared to the yields obtained in the neat methane and neat n-heptane conversions or the sum of both. © 2019 The Combustion Institute
view abstract 10.1016/j.combustflame.2019.04.005
- Plug-Flow Reactor Study of the Partial Oxidation of Methane and Natural Gas at Ultra-Rich Conditions
Kaczmarek, D. and Atakan, B. and Kasper, T.
Combustion Science and Technology 191 (2019)The homogeneous partial oxidation of fuel-rich CH4/O2, CH4/C2H6/C3H8/O2 as well as CH4/C2H6/C3H8/H2/O2 mixtures is investigated in a plug-flow reactor at intermediate temperatures (473 ≤ T ≤ 973 K) and a pressure of 6 bar. Experiments are carried out at equivalence ratios (Φ) of 2, 10, and 20. Product species are analyzed using time-of-flight molecular-beam mass spectrometry. The experimental results are further compared with kinetic simulations. It was found that under the investigated conditions, the onset temperature for CH4 oxidation is above 773 K. The highest methane conversion at equivalence ratios of 10 and 20 was between 0–3% for neat methane as fuel and 10–13% for natural gas as fuel. The conversions yield useful chemicals like synthesis gas (H2/CO), C2H4, C2H6, or C3H6. Higher CH4 conversion in the natural gas mixtures results in much higher yields of all products. The natural gas components ethane and propane do not influence the reaction onset temperature. © 2019, © 2019 Taylor & Francis Group, LLC.
view abstract 10.1080/00102202.2019.1577829
- The fate of the OH radical in molecular beam sampling experiments
Krüger, D. and Oßwald, P. and Köhler, M. and Hemberger, P. and Bierkandt, T. and Kasper, T.
Proceedings of the Combustion Institute 37 (2019)The collisional history of ionized molecules in a molecular beam mass spectrometric flame experiment is target of our present investigation. Measurements in a double imaging photoelectron photoion coincidence spectroscopy (i2PEPICO) were performed at the Swiss Light Source (SLS) of the Paul Scherrer Institute to use the ion imaging device for separating the molecular beam ions from rethermalized ions. This enables the precise composition study of the individual types of ions. Results show clearly for the OH radical that the complete signal is obtained from the molecular beam, while the signal from other combustion compounds features additional rethermalized molecules. As for OH radicals, the mole fraction is reduced by sampling effects and contact with the ionization vessel walls significantly. Consequently, this leads to signal loss and lower mole fractions, when using ionization cross sections for the quantification. To improve on this, a beam fraction (BF) factor is presented. The factor describes the ratio of the separated beam signal without rethermalized ions with the total ion signal, consisting of the mass to charge ratio from the molecular beam and additional rethermalized ions. Since the detected OH radicals are solely from the molecular beam, a new method of comparing two molecular beam alignments using the OH to H2O signal ratio is presented. This method has a decent potential for the optimization of the quality of molecular beams. Finally, the separated beam signal (without the rethermalized ions) was used to determine mole fraction profiles for the OH radical using ionization cross sections. These profiles are in good agreement with model predictions of the USC-II and the Aramco Mech 2.0 mechanisms, while the total signal leads to factor of 12 smaller OH mole fractions. © 2018 Elsevier Ltd.
view abstract 10.1016/j.proci.2018.05.041
- Hydrogen abstraction ratios: A systematic iPEPICO spectroscopic investigation in laminar flames
Krüger, D. and Oßwald, P. and Köhler, M. and Hemberger, P. and Bierkandt, T. and Karakaya, Y. and Kasper, T.
Combustion and Flame 191 (2018)The radicals produced by hydrogen abstraction in the initial fuel decomposition step are essential in combustion kinetics, but their experimental detection is very challenging. Imaging photoelectron photoion coincidence spectroscopy enables the detection and identification of even these isomeric radicals. Laminar low-pressure (40 mbar) hydrogen flames doped with different alkanes and alkenes are investigated systematically with the goal to identify the formation pathways and the fate of fuel radicals formed in hydrogen abstraction reactions. The abstraction reactions of primary, secondary, tertiary, and vinylic H atoms were never target of a systematic, direct semiquantitative investigation in a flame environment and this paper describes such a study for the first time. Performing the measurements at the vacuum ultraviolet beamline located at the Swiss Light Source enables isomer-selective detection of reactive radical species by imaging photoelectron photoion coincidence spectroscopy. For unambiguous identification of several isomeric radicals, threshold photoelectron spectra were compared with reference photoelectron spectra. H-abstraction ratios of isomeric radicals were determined and compared to literature reaction barriers and rate coefficients. In addition to the quantitative information, the peak positions of the profiles of radicals formed by hydrogen abstraction or addition to the fuel molecules as function of distance from the burner show faster H-abstraction for unbranched alkanes and alkenes than for branched fuels and faster H-addition than H-abstraction, respectively. © 2018 The Combustion Institute
view abstract 10.1016/j.combustflame.2017.12.025
- Synchrotron-based VUV photoionization mass spectrometry in combustion chemistry research
Hansen, N. and Yang, B. and Kasper, T.
Synchrotron Radiation Applications (2018)Since more than a decade, synchrotron-based methods play a major role in combustion chemistry research. This chapter focuses on the use of synchrotron-generated vacuum-ultraviolet (VUV) radiation as an ionization source in mass spectrometric applications. The use of tunable VUV radiation allows for the detection, identification, and quantification of different isomeric structures, thus adding a new dimension to mass spec- trometric experiments which has not been previously accessible. In particular, this chapter highlights flame-sampling and kinetic experiments that provided new detailed insights into combustion chemistry processes. Examples are provided that are concerned with soot precursor formation and the combustion of prototypical biofuels. © 2018 by World Scientific Publishing Co. Pte. Ltd.
view abstract 10.1142/9789813227675_0002
- Insights in m-xylene decomposition under fuel-rich conditions by imaging photoelectron photoion coincidence spectroscopy
Bierkandt, T. and Hemberger, P. and Oßwald, P. and Köhler, M. and Kasper, T.
Proceedings of the Combustion Institute 36 (2017)A fuel-rich (Φ=1.79) m-xylene flame (7.3% m-C8H10, 42.7% O2, 50.0% Ar) at low-pressure (40mbar) was investigated with focus on the reactive fuel radicals (C8H9) and the first decomposition steps leading to C8H8 isomers. The results show that an isomerization of the m-xylyl radical to o- and p-xylyl must take place to explain the observed intermediates in agreement with pyrolysis experiments. Important higher polycyclic aromatic hydrocarbons (PAHs) relevant to soot formation were also identified. All Measurements were performed with a molecular-beam mass spectrometry (MBMS) setup at the Swiss Light Source (SLS), where single-photon ionization with VUV radiation offers soft ionization of the sampled species. Isomer-selective detection with unprecedented resolution is achieved by a combination of time-of-flight mass spectrometry and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. In principle, species can be identified by comparison of measured ionization efficiency (PIE) curves with known or calculated ionization energies of expected species. For convoluted signals of several species, this procedure works well for the isomer with the lowest ionization energy. Changes in the slopes of the ionization efficiency curve do not necessarily correlate with ionization thresholds of other isomers and the assignment of higher thresholds can become difficult. PEPICO spectrometry, which detects the electrons that are produced in the ionization process in coincidence with the ions, enables the measurement of mass-selected threshold photoelectron spectra (ms-TPES). These spectra improve the detection capability of isomers because vibrational transitions from the neutral into ionic states can be observed and used as a fingerprint of a specific molecule. The obtained ms-TPES are compared with reference spectra from the literature or Franck-Condon simulations. Quantification of the major species as well as several intermediate species for this fuel-rich m-xylene flame yields a data set for model validation and experimental results are compared with five kinetic reaction models from the literature. © 2016.
view abstract 10.1016/j.proci.2016.06.143
- New insights into methane-oxygen ion chemistry
Alquaity, A.B.S. and Chen, B. and Han, J. and Selim, H. and Belhi, M. and Karakaya, Y. and Kasper, T. and Sarathy, S.M. and Bisetti, F. and Farooq, A.
Proceedings of the Combustion Institute 36 (2017)External electric fields may reduce emissions and improve combustion efficiency by active control of combustion processes. In-depth, quantitative understanding of ion chemistry in flames enables predictive models to describe the effect of external electric fields on combustion plasma. This study presents detailed cation profile measurements in low-pressure, burner-stabilized, methane/oxygen/argon flames. A quadrupole molecular beam mass spectrometer (MBMS) coupled to a low-pressure (P =30Torr) combustion chamber was utilized to measure ion signals as a function of height above the burner. Lean, stoichiometric and rich flames were examined to evaluate the dependence of ion chemistry on flame stoichiometry. Additionally, for the first time, cataloging of flame cations is performed using a high mass resolution time-of-flight mass spectrometer (TOF-MS) to distinguish ions with the same nominal mass. In the lean and stoichiometric flames, the dominant ions were H3O+, CH3O2 +, C2H7O+, C2H3O+ and CH5O+, whereas large signals were measured for H3O+, C3H3 + and C2H3O+ in the rich flame. The spatial distribution of cations was compared with results from numerical simulations constrained by thermocouple-measured flame temperatures. Across all flames, the predicted H3O+ decay rate was noticeably faster than observed experimentally. Sensitivity analysis showed that the mole fraction of H3O+ is most sensitive to the rate of chemi-ionization CH+O↔CHO+ +E-. To our knowledge, this work represents the first detailed measurements of positive ions in canonical low-pressure methane flames. © 2016.
view abstract 10.1016/j.proci.2016.05.053
- Shock-tube and plug-flow reactor study of the oxidation of fuel-rich CH4/O2 mixtures enhanced with additives
Sen, F. and Shu, B. and Kasper, T. and Herzler, J. and Welz, O. and Fikri, M. and Atakan, B. and Schulz, C.
Combustion and Flame 169 (2016)Partial oxidation of hydrocarbons under well-controlled conditions opens a path to higher-value chemicals from natural gas with small exergy losses if the chemical conversion proceeds in an internal combustion engine as a polygeneration process (Gossler et al., 2015). For the relevant reaction conditions, kinetics models are not sufficiently validated due to the atypical reaction conditions, e.g., high equivalence ratios and pressures. The purpose of this study is to obtain experimental validation data for chemical reaction mechanisms that can be used to predict polygeneration processes in practical applications. In case of methane these processes proceed under fuel-rich conditions and yield primarily syngas (CO/H2). In this study, the partial oxidation of methane was investigated for an equivalence ratio of φ=2 in a shock-tube and a plug-flow reactor (PFR) in order to cover a wide temperature range. Time-resolved CO mole fractions were measured in shock-heated mixtures between 1600 and 2100K at ~1bar. Good agreement was found between the experiment and the models (Yasunaga et al., 2010; Burke et al., 2015; Zhao et al., 2008). Stable reaction products were monitored by time-of-flight mass spectrometry between 532 and 992K at 6bar in a tubular flow reactor at reaction times >4s. The influence of dimethyl ether (DME) and n-heptane addition on methane reactivity and conversion was investigated. The additives significantly lower the initial reaction temperature by producing significant amounts of OH. The results were compared to simulations and serve as validation data for the development of reaction mechanisms for these atypical reaction conditions. Good agreement was found between the experiment and the models for most of species. © 2016 The Combustion Institute.
view abstract 10.1016/j.combustflame.2016.03.030
- Electron ionization, photoionization and photoelectron/photoion coincidence spectroscopy in mass-spectrometric investigations of a low-pressure ethylene/oxygen flame
Felsmann, D. and Moshammer, K. and Krüger, J. and Lackner, A. and Brockhinke, A. and Kasper, T. and Bierkandt, T. and Akyildiz, E. and Hansen, N. and Lucassen, A. and Oßwald, P. and Köhler, M. and Garcia, G.A. and Nahon, L. and Hemberger, P. and Bodi, A. and Gerber, T. and Kohse-Höinghaus, K.
Proceedings of the Combustion Institute 35 (2015)Quantitative species data for the development and critical examination of combustion mechanisms are in high demand regarding the need for predictive combustion models that may assess the emission potential of current and emerging fuels. Mass spectrometric investigation is one of the often-used techniques to provide mole fractions of stable and reactive intermediates including radicals from specifically designed laboratory experiments. Molecular-beam mass spectrometry (MBMS) has been coupled with electron ionization (EI) and photoionization (PI) to determine the species compositions, and combinations of these techniques have been successful in the investigation of the combustion pathways in flames of numerous hydrocarbon, oxygenated and nitrogenated fuels. Photoelectron/photoion coincidence spectroscopy (PEPICO) has recently emerged as a novel diagnostics to be combined with flame-sampling mass spectrometry, and its potential as a complement of existing techniques is just about being explored. In a multi-laboratory investigation, the present study has thus combined four different MBMS spectrometers (in Bielefeld, Germany, the Advanced Light Source in Berkeley, USA, the Swiss Light Source in Villigen, Switzerland, and the SOLEIL synchrotron in St. Aubin, France) to study a rich premixed argon-diluted low-pressure (40 mbar) ethylene-oxygen flame under comparable conditions. This was done with the aim of illustrating the respective properties and capabilities of the methods under these conditions, with an emphasis on the power offered by the synchrotron-based techniques, including PEPICO, for combustion chemistry studies. Examples include comparisons of selected species quantification as well as PEPICO spectra measured at different instruments. © 2014 The Combustion Institute.
view abstract 10.1016/j.proci.2014.05.151
- Flame structure of a low-pressure laminar premixed and lightly sooting acetylene flame and the effect of ethanol addition
Bierkandt, T. and Kasper, T. and Akyildiz, E. and Lucassen, A. and Oßwald, P. and Köhler, M. and Hemberger, P.
Proceedings of the Combustion Institute 35 (2015)The flame structure of a fuel-rich (φ = 2.4), laminar premixed, and lightly sooting acetylene flame at 40 mbar and the influence of ethanol addition on the species pool was investigated. Special emphasis was put on the analysis of important soot precursors like propargyl, benzene, and the polyynes. The mole fractions of more than 50 stable and radical species up to m/z = 170 are obtained experimentally in the flames by molecular-beam mass spectrometry (MBMS) in combination with single-photon ionization (SPI) by vacuum ultraviolet (VUV) radiation from the Advanced Light Source (ALS) in Berkeley, CA, USA. For the neat acetylene flame, successful measurements were performed with a combination of MBMS and imaging photoelectron photoion coincidence spectrometry (iPEPICO) at the VUV beamline at the Swiss Light Source (SLS) in Villigen, Switzerland and adding additional species information to the data set. Some interesting isomers (C3H2, C4H5, C4H2O) can be clearly identified by comparison of measured photoionization efficiency (PIE) curves or threshold photoelectron (TPE) spectra with Franck-Condon simulations or literature spectra, respectively. Because of apparatus improvements, the chemical resolution in this study goes beyond prior work and provides a high-quality data set for the development of reaction mechanisms at fuel-rich, low-pressure conditions.
view abstract 10.1016/j.proci.2014.05.094
- Partial oxidation of methane at elevated pressures and effects of propene and ethane as additive: Experiment and simulation
Sen, F. and Kasper, T. and Bergmann, U. and Hegner, R. and Atakan, B.
Zeitschrift fur Physikalische Chemie 229 (2015)Partial homogeneous oxidation of methane (CH<inf>4</inf>) within stationary engines may be one concept for conversion of available energy to alternatively mechanical energy, heat, and additional useful chemicals like syngas (CO/H<inf>2</inf>), formaldehyde (CH<inf>2</inf>O), methanol (CH<inf>3</inf>OH) or hydrocarbons (e.g. C<inf>2</inf>H<inf>4</inf>). The present study investigates the formation reactions of chemicals experimentally and theoretically. Methane oxidation is studied under fuel-rich conditions (σ=17.50-22.25) at high pressures (6bar) and high temperatures (T <inf>max</inf>=1030K) for long residence times in a tubular reactor. The gas composition is determined experimentally by time-of-flight mass spectrometry for different reactor temperatures. Through variation of reactor temperature an overview of the maximum mole fractions of target chemicals, the temperature of observed reaction onset, and the optimal temperature to increase target yields can be determined. The experimental results are compared to kinetic simulations of the methane conversion using literature mechanisms to assess how well the data are reproduced for these uncommon reaction conditions. The potential of activating the conversion reactions with ethane (C<inf>2</inf>H<inf>6</inf>) and propene (C<inf>3</inf>H<inf>6</inf>) as additives is investigated. Methanol is chosen as one target compound. Its yield is increased by both additives. In addition, propene as additive reduces the temperature of reaction onset in the experiments and in the simulation. CH<inf>2</inf>O and C<inf>2</inf>H<inf>4</inf> can be identified as other useful chemicals produced in the experiments and the influence of the additives on the yields is discussed. © 2015 Walter de Gruyter Berlin/Boston.
view abstract 10.1515/zpch-2014-0576
- In situ flame chemistry tracing by imaging photoelectron photoion coincidence spectroscopy
Oßwald, P. and Hemberger, P. and Bierkandt, T. and Akyildiz, E. and Köhler, M. and Bodi, A. and Gerber, T. and Kasper, T.
Review of Scientific Instruments 85 (2014)Adaptation of a low-pressure flat flame burner with a flame-sampling interface to the imaging photoelectron photoion coincidence spectrometer (iPEPICO) of the VUV beamline at the Swiss Light Source is presented. The combination of molecular-beam mass spectrometry and iPEPICO provides a new powerful analytical tool for the detailed investigation of reaction networks in flames. First results demonstrate the applicability of the new instrument to comprehensive flame diagnostics and the potentially high impact for reaction mechanism development for conventional and alternative fuels. Isomer specific identification of stable and radical flame species is demonstrated with unrivaled precision. Radical detection and identification is achieved for the initial H-abstraction products of fuel molecules as well as for the reaction controlling H, O, and OH radicals. Furthermore, quantitative evaluation of changing species concentrations during the combustion process and the applicability of respective results for kinetic model validation are demonstrated. Utilization of mass-selected threshold photoelectron spectra is shown to ensure precise signal assignment and highly reliable spatial profiles. © 2014 AIP Publishing LLC.
view abstract 10.1063/1.4861175
- Buoyancy induced limits for nanoparticle synthesis experiments in horizontal premixed low-pressure flat-flame reactors
Weise, C. and Faccinetto, A. and Kluge, S. and Kasper, T. and Wiggers, H. and Schulz, C. and Wlokas, I. and Kempf, A.
Combustion Theory and Modelling 17 (2013)Premixed low-pressure flat-flame reactors can be used to investigate the synthesis of nanoparticles. The present work examines the flow field inside such a reactor during the formation of carbon (soot) and iron oxide (from Fe(CO)5) nanoparticles, and how it affects the measurements of nanoparticle size distribution. The symmetry of the flow and the impact of buoyancy were analysed by three-dimensional simulations and the nanoparticle size distribution was obtained by particle mass spectrometry (PMS) via molecular beam sampling at different distances from the burner. The PMS measurements showed a striking, sudden increase in particle size at a critical distance from the burner, which could be explained by the flow field predicted in the simulations. The simulation results illustrate different fluid mechanical phenomena which have caused this sudden rise in the measured particle growth. Up to the critical distance, buoyancy does not affect the flow, and an (almost) linear growth is observed in the PMS experiments. Downstream of this critical distance, buoyancy deflects the hot gas stream and leads to an asymmetric flow field with strong recirculation. These recirculation zones increase the particle residence time, inducing very large particle sizes as measured by PMS. This deviation from the assumed symmetric, one-dimensional flow field prevents the correct interpretation of the PMS results. To overcome this problem, modifications to the reactor were investigated; their suitability to reduce the flow asymmetry was analysed. Furthermore, 'safe' operating conditions were identified for which accurate measurements are feasible in premixed low-pressure flat-flame reactors that are transferrable to other experiments in this type of reactor. The present work supports experimentalists to find the best setup and operating conditions for their purpose. © 2013 Copyright Taylor and Francis Group, LLC.
view abstract 10.1080/13647830.2013.781224
- Flame chemistry of tetrahydropyran as a model heteroatomic biofuel
Labbe, N.J. and Seshadri, V. and Kasper, T. and Hansen, N. and Oßwald, P. and Westmoreland, P.R.
Proceedings of the Combustion Institute 34 (2013)The flame chemistry of tetrahydropyran (THP), a cyclic ether, has been examined using vacuum-ultraviolet (VUV)-photoionization molecular-beam mass spectrometry (PI-MBMS) and flame modeling, motivated by the need to understand and predict the combustion of oxygen-containing, biomass-derived fuels. Species identifications and mole-fraction profiles are presented for a fuel-rich (U = 1.75), laminar premixed THP-oxygen-argon flame at 2.66 kPa (20.0 Torr). Flame species with up to six heavy atoms have been detected. A detailed reaction set was developed for THP combustion that captures relevant features of the THP flame quite well, allowing analysis of the dominant kinetic pathways for THP combustion. Necessary rate coefficients and transport parameters were calculated or were estimated by analogies with a recent reaction set [Li et al., Combust. Flame 158 (2011) 2077-2089], and necessary thermochemical properties were computed using the CBS-QB3 method. Our results show that under the low-pressure conditions, THP destruction is dominated by H-abstraction, and the three resulting THP-yl radicals decompose primarily by b-scissions to two- and four-heavy-atom species that are generally destroyed by b-scission, abstraction, or oxidation. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
view abstract 10.1016/j.proci.2012.07.027
- An experimental and kinetic modeling study of methyl formate low-pressure flames
Dooley, S. and Dryer, F.L. and Yang, B. and Wang, J. and Cool, T.A. and Kasper, T. and Hansen, N.
Combustion and Flame 158 (2011)The oxidation of methyl formate (CH3OCHO), the simplest methyl ester, is studied in a series of burner-stabilized laminar flames at pressures of 22-30Torr and equivalence ratios (Φ) from 1.0 to 1.8 for flame conditions of 25-35% fuel. Flame structures are determined by quantitative measurements of species mole fractions with flame-sampling molecular-beam synchrotron photoionization mass spectrometry (PIMS). Methyl formate is observed to be converted to methanol, formaldehyde and methane as major intermediate species of mechanistic relevance. Smaller amounts of ethylene and acetylene are also formed from methyl formate oxidation. Reactant, product and major intermediate species profiles are in good agreement with the computations of a recently developed kinetic model for methyl formate oxidation [S. Dooley, M.P. Burke, M. Chaos, Y. Stein, F.L. Dryer, V.P. Zhukov, O. Finch, J.M. Simmie, H.J. Curran, Int. J. Chem. Kinet. 42 (2010) 527-529] which shows that hydrogen abstraction reactions dominate fuel consumption under the tested flame conditions. Radical-radical reactions are shown to be significant in the formation of a number of small concentration intermediates, including the production of ethyl formate (C2H5OCHO), the subsequent decomposition of which is the major source of observed ethylene concentrations. The good agreement of model computations with this set of experimental data provides a further test of the predictive capabilities of the proposed mechanism of methyl formate oxidation. Other salient issues in the development of this model are discussed, including recent controversy regarding the methyl formate decomposition mechanism, and uncertainties in the experimental measurement and modeling of low-pressure flame-sampling experiments. Kinetic model computations show that worst-case disturbances to the measured temperature field, which may be caused by the insertion of the sampling cone into the flame, do not alter mechanistic conclusions provided by the kinetic model. However, such perturbations are shown to be responsible for disparities in species location between measurement and computation. © 2010 The Combustion Institute.
view abstract 10.1016/j.combustflame.2010.11.003
- Fuel-structure dependence of benzene formation processes in premixed flames fueled by C6H12 isomers
Hansen, N. and Kasper, T. and Yang, B. and Cool, T.A. and Li, W. and Westmoreland, P.R. and Oßwald, P. and Kohse-Höinghaus, K.
Proceedings of the Combustion Institute 33 (2011)The fuel-structure-dependent significance of various benzene formation pathways is analyzed using data from rich (φ = 1.7) flames fueled by four C6H12 isomers: 1-hexene, cyclohexane, methylcyclopentane, and 3,3-dimethyl-1-butene. The isomer-resolved chemical compositions of the four premixed, laminar low-pressure flat flames are determined by flame-sampling molecular-beam mass spectrometry employing single-photon ionization by synchrotron generated vacuum-ultraviolet photons. Isomer-resolving photoionization efficiency curves and quantitative mole fraction profiles reveal the dominant fuel destruction pathways, the influence of different fuel consumption processes on the formation of commonly considered benzene precursors, and the contributions of several routes towards benzene formation. While propargyl and allyl radicals dominate benzene formation in the combustion of 1-hexene, contributions from reactions involving i-C4H5 and C5H5 radicals are revealed in the flames of 3,3-dimethyl-1-butene and methylcyclopentane, respectively. Close to the burner surface, successive dehydrogenation of the fuel is found to be important for the cyclohexane flame and to some smaller extent for the methylcyclopentane flame. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.
view abstract 10.1016/j.proci.2010.05.056
- Identification of tetrahydrofuran reaction pathways in premixed flames
Kasper, T. and Lucassen, A. and Jasper, A.W. and Li, W. and Westmoreland, P.R. and Kohse-Höinghaus, K. and Yang, B. and Wang, J. and Cool, T.A. and Hansen, N.
Zeitschrift fur Physikalische Chemie 225 (2011)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 (C 4H6O2), 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. © by Oldenbourg Wissenschaftsverlag, München.
view abstract 10.1524/zpch.2011.0163
- Multiple benzene-formation paths in a fuel-rich cyclohexane flame
Li, W. and Law, M.E. and Westmoreland, P.R. and Kasper, T. and Hansen, N. and Kohse-Höinghaus, K.
Combustion and Flame 158 (2011)Detailed data and modeling of cyclohexane flames establish that a mixture of pathways contributes to benzene formation and that this mixture changes with stoichiometry. Mole-fraction profiles are mapped for more than 40 species in a fuel-rich, premixed flat flame (φ=2.0, cyclohexane/O2/30% Ar, 30Torr, 50.0cm/s) using molecular-beam mass spectrometry with VUV-photoionization at the Advanced Light Source of the Lawrence Berkeley National Laboratory. The use of a newly constructed set of reactions leads to an excellent simulation of this flame and an earlier stoichiometric flame (M.E. Law et al., Proc. Combust. Inst. 31 (2007) 565-573), permitting analysis of the contributing mechanistic pathways. Under stoichiometric conditions, benzene formation is found to be dominated by stepwise dehydrogenation of the six-membered ring with cyclohexadienyl⇄benzene+H being the final step. This finding is in accordance with recent literature. Dehydrogenation of the six-membered ring is also found to be a dominant benzene-formation route under fuel-rich conditions, at which H2 elimination from 1,3-cyclohexadiene contributes even more than cyclohexadienyl decomposition. Furthermore, at the fuel-rich condition, additional reactions make contributions, including the direct route via 2C3H3⇄benzene and more importantly the H-assisted isomerization of fulvene formed from i-/n-C4H5+C2H2, C3H3+allyl, and C3H3+C3H3. Smaller contributions towards benzene formation arise from C4H3+C2H3, 1,3-C4H6+C2H3, and potentially via n-C4H5+C2H2. This diversity of pathways is shown to result nominally from the temperature and the concentrations of benzene precursors present in the benzene-formation zone, which are ultimately due to the feed stoichiometry. © 2011 The Combustion Institute.
view abstract 10.1016/j.combustflame.2011.03.014
- Biofuel combustion chemistry: From ethanol to biodiesel
Kohse-Höinghaus, K. and Oßwald, P. and Cool, T.A. and Kasper, T. and Hansen, N. and Qi, F. and Westbrook, C.K. and Westmoreland, P.R.
Angewandte Chemie - International Edition 49 (2010)Biofuels, such as bio-ethanol, bio-butanol, and biodiesel, are of increasing interest as alternatives to petroleum-based transportation fuels because they offer the long-term promise of fuel-source regenerability and reduced climatic impact. Current discussions emphasize the processes to make such alternative fuels and fuel additives, the compatibility of these substances with current fuel-delivery infrastructure and engine performance, and the competition between biofuel and food production. However, the combustion chemistry of the compounds that constitute typical biofuels, including alcohols, ethers, and esters, has not received similar public attention. Herein we highlight some characteristic aspects of the chemical pathways in the combustion of prototypical representatives of potential biofuels. The discussion focuses on the decomposition and oxidation mechanisms and the formation of undesired, harmful, or toxic emissions, with an emphasis on transportation fuels. New insights into the vastly diverse and complex chemical reaction networks of biofuel combustion are enabled by recent experimental investigations and complementary combustion modeling. Understanding key elements of this chemistry is an important step towards the intelligent selection of next-generation alternative fuels. © 2010 Wiley-VCH Verlag GmbH & Co. KCaA.
view abstract 10.1002/anie.200905335
- The importance of fuel dissociation and propargyl + allyl association for the formation of benzene in a fuel-rich 1-hexene flame
Hansen, N. and Li, W. and Law, M.E. and Kasper, T. and Westmoreland, P.R. and Yang, B. and Cool, T.A. and Lucassen, A.
Physical Chemistry Chemical Physics 12 (2010)Fuel decomposition and benzene formation processes in a premixed, laminar, low-pressure, fuel-rich flame of 1-hexene (C6H12, CH 2CH-CH2-CH2-CH2-CH3) are investigated by comparing quantitative mole fraction profiles of flame species with kinetic modeling results. The premixed flame, which is stabilized on a flat-flame burner under a reduced pressure of 30 Torr (= 40 mbar), is analyzed by flame-sampling molecular-beam time-of-flight mass spectrometry which uses photoionization by tunable vacuum-ultraviolet synchrotron radiation. The temperature profile of the flame is measured by OH laser-induced fluorescence. The model calculations include the latest rate coefficients for 1-hexene decomposition (J. H. Kiefer et al., J. Phys. Chem. A, 2009, 113, 13570) and for the propargyl (C3H3) + allyl (a-C3H 5) reaction (J. A. Miller et al., J. Phys. Chem. A, 2010, 114, 4881). The predicted mole fractions as a function of distance from the burner are acceptable and often even in very good agreement with the experimentally observed profiles, thus allowing an assessment of the importance of various fuel decomposition reactions and benzene formation routes. The results clearly indicate that in contrast to the normal reactions of fuel destruction by radical attack, 1-hexene is destroyed mainly by decomposition via unimolecular dissociation forming allyl (a-C3H5) and n-propyl (n-C 3H7). Minor fuel-consumption pathways include H-abstraction reactions producing various isomeric C6H11 radicals with subsequent β-scissions into C2, C3, and C4 intermediates. The reaction path analysis also highlights a significant contribution through the propargyl (C3H3) + allyl (a-C3H5) reaction to the formation of benzene. In this flame, benzene is dominantly formed through H-assisted isomerization of fulvene, which itself is almost exclusively produced by the C3H 3 + a-C3H5 reaction. © 2010 the Owner Societies.
view abstract 10.1039/c0cp00241k
time of flight mass spectrometry