Detailed mass spectrometric and modeling study of isomeric butene flames (original) (raw)
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Combustion Chemistry of the Butane Isomers in Premixed Low-Pressure Flames
Zeitschrift für Physikalische Chemie, 2011
The combustion chemistry of the two butane isomers represents a subset in a comprehensive description of C1–C4 hydrocarbon and oxygenated fuels. A critical examination of combustion models and their capability to predict emissions from this class of fuels must rely on high-quality experimental data that address the respective chemical decomposition and oxidation pathways, including quantitative intermediate species mole fractions. Premixed flat low-pressure (40 mbar) flames of the two butane isomers were thus studied under identical, fuel-rich (φ=1.71) conditions. Two independent molecular-beam mass spectrometer (MBMS) set-ups were used to provide quantitative species profiles. Both data sets, one from electron ionization (EI)-MBMS with high mass resolution and one from photoionization (PI)-MBMS with high energy resolution, are in overall good agreement. Simulations with a flame model were used to analyze the respective reaction pathways, and differences in the combustion behavior o...
An experimental and kinetic modeling study of combustion of isomers of butanol
Combustion and Flame, 2010
A kinetic model is developed to describe combustion of isomers of butanol-n-butanol (n-C 4 H 9 OH), secbutanol (sec-C 4 H 9 OH), iso-butanol (iso-C 4 H 9 OH), and tert-butanol (tert-C 4 H 9 OH). A hierarchical approach is employed here. This approach was previously found to be useful for developing detailed and semidetailed mechanism of oxidation of various hydrocarbon fuels. This method starts from lower molecular weight compounds of a family of species and proceeds to higher molecular weight compounds. The pyrolysis and oxidation mechanisms of butanol isomers are similar to those for hydrocarbon fuels. Here, the development of the complete set of the primary propagation reactions for butanol isomers proceeds from the extension of the kinetic parameters for similar reactions already studied and recently revised for ethanol, n-propanol and iso-propanol. A detailed description leading to evaluation of rate constants for initiation reactions, metathesis reactions, decomposition reactions of alkoxy radicals, isomerization reactions, and four-center molecular dehydration reactions are given. Decomposition and oxidation of primary intermediate products are described using a previously developed semi-detailed kinetic model for hydrocarbon fuels. The kinetic mechanism is made up of more than 7000 reactions among 300 species. The model is validated by comparing predictions made using this kinetic model with previous and new experimental data on counterflow non-premixed flames of n-butanol and iso-butanol. The structures of these flames were measured by removing gas samples from the flame and analyzing them using a gas chromatograph. Temperature profiles were measured using coated thermocouples. The flame structures were measured under similar conditions for both fuels to elucidate the similarities and differences in combustion characteristics of the two isomers. The profiles measured include those of butanol, oxygen, carbon dioxide, water vapor, carbon monoxide, hydrogen, formaldehyde, acetaldehyde, and a number of C 1 -C 4 hydrocarbon compounds. The predictions of the kinetic model of flame structures of the two isomers were satisfactory. Validation of the kinetic model was also performed by comparing predictions with experimental data reported in the literature. These data were obtained in batch reactors, flow reactors, jet-stirred reactors, and shock tubes. In these configurations, combustion is not influenced by molecular transport. The agreement between the kinetic model and experimental data was satisfactory.
Chemical Structures of Premixed iso-Butanol Flames
The combustion chemistry of iso-butanol was investigated by determining the chemical composition of premixed, laminar low-pressure flames and by an automatically generated combustion chemistry model. The flames, which were stabilized on a flatflame burner under a reduced pressure of 15 to 30 Torr, were quantitatively analyzed by flame-sampling molecular-beam mass spectrometry. The new set of data consists of isomer-resolved mole fraction profiles for more than 40 species in each of the four flames and provides a comprehensive benchmark for testing of high-temperature oxidation chemistry model for iso-butanol. Temperature profiles were measured using OH laser-induced fluorescence. The kinetic model, which has been extensively tested against other experimental data (ignition delay times, profiles in jet-stirred reactor and flow reactor) shows impressive capabilities for predicting the new flame data presented here. Predictions of the C 2 H 4 O, C 3 H 6 O, and C 4 H 8 O enol-aldehyde-ketone isomers were significantly improved compared to the earlier work by Hansen et al. [Phys. Chem. Chem. Phys. 13 (2011) 20262-20274] on similar nbutanol flames. A reaction path analysis identified prominent fuel-consumption oxidation sequences, and some significant differences with previous published models are highlighted.
Chemical insights into the larger sooting tendency of 2-methyl-2-butene compared to n-pentane
Combustion and Flame, 2019
A comprehensive, chemically detailed mechanism for the combustion of 2-methyl-2-butene and npentane is presented to provide insights into the different sooting tendencies of these two structurally different C 5 hydrocarbons. A hierarchically assembled mechanism has been developed to specifically target speciation data from low-pressure premixed flames of 2-methyl-2-butene [Ruwe et al., Combust. Flame, 175, 34-46, 2017 ] and newly measured mole fraction data for a fuel-rich (ɸ = 1.8) n-pentane flame, in which species profiles up to phenol were quantified. The partially isomer-resolved chemical composition of this flame was determined using flame-sampling molecular-beam mass spectrometry with single-photon ionization by tunable, synchrotron-generated vacuum-ultraviolet radiation. The presented model, which includes a newly determined, consistent set of the thermochemistry data for the C 5 species, presents overall satisfactory capabilities to predict the mole fraction profiles of common combustion intermediates. The analysis of the model predictions revealed the fuel-structure dependencies (i.e. saturated vs. unsaturated and linear vs. branched) of the formation of small aromatic species that are considered as soot precursors. The propensity of the 2-methyl-2-butene flame to form larger concentrations of aromatic species was traced back to the readily available formation routes of several small precursor molecules and the efficient formation of "first aromatic rings" beyond benzene.
Combustion and pyrolysis of iso-butanol: Experimental and chemical kinetic modeling study
Combustion and Flame, 2013
The first reaction mechanism for iso-butanol (372 species and 8723 reversible elementary reactions) pyrolysis and combustion that includes pressure dependent kinetics and proposes reaction pathways to soot precursors has been automatically generated using the open-source software package RMG. High-pressure reaction rate coefficients for important hydrogen abstraction reactions from iso-butanol by hydrogen, methyl and HO 2 were calculated using quantum chemistry at the CBS-QB3 level. The mechanism was validated with recently published iso-butanol combustion experiments as well as new pyrolysis speciation data under diluted and undiluted conditions from 900 to 1100 K at 1.72 atm representative of fuel rich combustion conditions. Sensitivity and rate of production analysis revealed that the overall good agreement for the pyrolysis species, and in particular for the soot precursors like benzene, toluene and 1,3-cyclopentadiene, depends strongly on pressure dependent reactions involving the resonantly stabilized iso-butenyl radical. Laminar flame speed, opposed flow diffusion flame speciation profiles, and autoignition are also well-captured by the model. The agreement with speciation profiles for the jet-stirred reactor could be improved, in particular for temperatures lower than 850 K. Flux and sensitivity analysis for iso-butanol consumption revealed that this is primarily caused by uncertainty in iso-butanol + OH, iso-butanol + HO 2 and the low temperature peroxy chemistry rates. Further theoretical and quantum chemical studies are needed in understanding these rates to completely predict the combustion behavior of iso-butanol using detailed chemistry.
Practical hydrocarbon fuels are complex mixtures of several hundreds of individual species. Their models are mostly established to contain the species from 4 main families of hydrocarbons (n/i-paraffins, naphthenes, aromatics) and to predict various combustion characteristics and pollution formation. As the mixture of simplest substituted aromatic (C 7 H 8 , toluene) and "smallest" large n-paraffin (n-C 7 H 16 , nheptane) used in fuel blends, this combination can be effectively used for an investigation of specifics of the poly-aromatic hydrocarbon (PAH) formation and growth in engines.
Modeling of Rich Premixed C2H4/O2/Ar and C2H4/ Dimethoxymethane/O2/Ar Flames
Two rich premixed ethylene/oxygen/argon and ethylene/dimethoxymethane/oxygen/argon flat flames burning at 50 mbar were investigated experimentally by using molecular beam mass spectrometry to study the effect of methylal (dimethoxymethane) addition on species concentration profiles (Renard C, Van Tiggelen P.J. and Vandooren J., Proc. Combust. Inst., 29, 1277-1284, 2002. The replacement of 5.7% C 2 H 4 by 4.3% C 3 H 8 O 2 , keeping the equivalence ratio equal to 2.50, is responsible for a decrease of the maximum mole fractions of most of the detected intermediate species. If this phenomenon is barely noticeable for C 2 to C 4 intermediates, it becomes more efficient for C 5 to C 10 species. Previously, a reaction mechanism has been validated against a premixed rich C 2 H 4 /O 2 /Ar flame (φ = 2.50) which describes in detail the formation of soot precursors and more precisely the main pathways involving benzene (Dias, V., Renard, C., Van Tiggelen, P.J. and Vandooren, J., European Combustion Meeting, Orléans -France, p.221, 2003). The aim of this work is to extend this original model by building a sub-mechanism taking into account the formation and the consumption of oxygenated species involved in dimethoxymethane combustion. The new mechanism contains 474 elementary reactions and involves 90 chemical species in order to simulate both ethylene flames with and without methylal addition. The model leads to a good simulation for all species detected in these flames, and underlines the effect of methylal addition on species concentration profiles. According to this mechanism, the two main degradation pathways of methylal (CH 3 OCH 2 OCH 3 ) in C 2 H 4 /methylal/oxygen/argon flame are: 1)
Combustion and Flame, 2010
Fuel-rich premixed flames of seven monocyclic aromatic hydrocarbons (MAHs) including benzene, toluene, styrene, ethylbenzene, ortho-xylene, meta-xylene, and para-xylene were studied at the pressure of 30 Torr and comparable flame conditions (C/O = 0.68). The measurement of photoionization efficiency (PIE) spectra facilitated the comprehensive identification of combustion intermediates from m/z = 15 to 240, while mass spectrometric analysis was performed to gain insight into the flame chemistry. Features of the sidechain structure in fuel molecule affect the primary decomposition and aromatics growth processes, resulting in different isomeric structures or compositions of some primary products. This effect becomes weaker and weaker as both processes proceed. The results indicate that most intermediates are identical in all flames, leading to similar intermediate pools of these fuels. Consequently the chemical structures of flames fueled by different MAHs are almost identical, subsequent to the initial fuel-specific decomposition and oxidation that produce the primary intermediates. On the other hand, special features of the sidechain structure can affect the concentration levels of PAHs by increasing the concentrations of the key intermediates including the benzyl radical and phenylacetylene. Therefore, the total ion intensities of the PAH intermediates in the flames were observed to increase in the order of: benzene < toluene and styrene < four C 8 H 10 , which implies the same order of the sooting tendency. j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e 144 Y. Li et al. / Combustion and Flame 157 (2010) 143-154
A comprehensive iso-octane combustion model with improved thermochemistry and chemical kinetics
Combustion and Flame, 2017
iso-Octane (2,2,4-trimethylpentane) is a primary reference fuel and an important component of gasoline fuels. Moreover, it is a key component used in surrogates to study the ignition and burning characteristics of gasoline fuels. This paper presents an updated chemical kinetic model for iso-octane combustion. Specifically, the thermodynamic data and reaction kinetics of isooctane have been reassessed based on new thermodynamic group values and recently evaluated DRAFT FOR COMBUSTION AND FLAME 2 rate coefficients from the literature. The adopted rate coefficients were either experimentally measured or determined by analogy to theoretically calculated values. Furthermore, new alternative isomerization pathways for peroxy-alkyl hydroperoxide (ȮOQOOH) radicals were added to the reaction mechanism. The updated kinetic model was compared against new ignition delay data measured in rapid compression machines (RCM) and a high-pressure shock tube. These experiments were conducted at pressures of 20 and 40 atm, at equivalence ratios of 0.4 and 1.0, and at temperatures in the range of 632-1060 K. The updated model was further compared against shock tube ignition delay times, jet-stirred reactor oxidation speciation data, premixed laminar flame speeds, counterflow diffusion flame ignition , and shock tube pyrolysis speciation data available in the literature. Finally, the updated model was used to investigate the importance of alternative isomerization pathways in the low temperature oxidation of highly branched alkanes. When compared to available models in the literature, the present model represents the current state-of-the-art in fundamental thermochemistry and reaction kinetics of iso-octane; and thus provides the best prediction of wide ranging experimental data and fundamental insights into iso-octane combustion chemistry.