Search

The low-temperature oxidation of a stoichiometric 2-methylhexane/O2/Ar mixture was examined in a jet-stirred reactor coupled with synchrotron vacuum UV photoionization molecular-beam MS. Low-temperature oxidation intermediates with two to five oxygen atoms were observed. The detection of C7H14O5 and C7H14O4 species implies that a third O2 addition process occurred in 2-methylhexane low-temperature oxidation. A detailed kinetic model was developed to describes the third O2 addition and subsequent reactions leading to C7H14O5 (keto-dihydroperoxide and dihydroperoxy cyclic ether) and C7H14O4 (diketo-hydroperoxide and keto-hydroperoxy cyclic ether) species. The kinetics of the third O2 addition reactions revealed that the addition reactions promoted 2-methylhexane auto-ignition at low temperatures.

This work provides new temperature-dependent mole fractions of elusive intermediates relevant to the low-temperature oxidation of dimethyl ether (DME). It extends the previous study of Moshammer et al. [ J. Phys. Chem. A 2015 , 119 , 7361 - 7374 ] in which a combination of a jet-stirred reactor and molecular beam mass spectrometry with single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation was used to identify (but not quantify) several highly oxygenated species. Here, temperature-dependent concentration profiles of 17 components were determined in the range of 450-1000 K and compared to up-to-date kinetic modeling results. Special emphasis is paid toward the validation and application of a theoretical method for predicting photoionization cross sections that are hard to obtain experimentally but essential to turn mass spectral data into mole fraction profiles. The presented approach enabled the quantification of the hydroperoxymethyl formate (HOOCH2OCH2O), which is a key intermediate in the low-temperature oxidation of DME. The quantification of this keto-hydroperoxide together with the temperature-dependent concentration profiles of other intermediates including H2O2, HCOOH, CH3OCHO, and CH3OOH reveals new opportunities for the development of a next-generation DME combustion chemistry mechanism.

Understanding gas‐phase reactions in model gas mixtures approximating pre‐turbine heavy‐duty natural gas engine exhaust compositions containing NO, NH3, NO2, CH4, CO, and C2H4 is extremely relevant for aftertreatment procedure and catalyst design and is thus addressed in this work. In a plug‐flow reactor at atmospheric pressure, five different model gas mixtures were investigated in the temperature range of 700‐1 200 K, using species analysis with electron ionization molecular‐beam mass spectrometry. The mixtures were designed to reveal influences of individual components by adding NO2, CH4, CO, and C2H4 sequentially to a highly argon‐diluted NO/NH3 base mixture. Effects of all components on the reactivity for NOx conversion were investigated both experimentally as well as by comparison with three selected kinetic models. Main results show a significantly increased reactivity upon NO2 and hydrocarbon addition with lowered NO conversion temperatures by up to 200 K. Methane was seen to be of dominant influence in the carbon‐containing mixtures regarding interactions between the carbon and nitrogen chemistry as well as formaldehyde formation. The three tested mechanisms were capable to overall represent the reaction behavior satisfactorily. On this basis, it can be stated that significant gas‐phase reactivity was observed among typical constituents of pre‐turbine natural gas engine exhaust at moderate temperature. [ABSTRACT FROM AUTHOR]

This work aims to provide insight into the interaction of propene with NO x from both experimental and kinetic modeling perspectives. The oxidation of propene at fuel-lean (ϕ =0.23) condition and the oxidation of propene doped with NO x at fuel-lean (ϕ =0.23) and fuel-rich (ϕ =1.35) conditions have been investigated in a laminar flow reactor at atmospheric pressure in the temperature range of 725-1250 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used to achieve comprehensive, isomer-resolved identification of major products and critical nitrogenous, carbonyl and hydrocarbon intermediates. To complement the experiments, a detailed kinetic model, starting from widely used core mechanisms, was developed. Rate of production analyses and sensitivity analyses were performed to interpret the experimental observations. The results show that the promoting effects of NO x on the oxidation reactivity of propene are initiated by the reactions of allyl radical with NO 2 at low temperature, i.e. C 3 H 5 A+NO 2 C 3 H 5 O+NO. For the oxidation of the fuel-rich propene/NO x mixture, temperature-dependent mole fraction profiles of propene, O 2 and products show several distinct regions reflecting a competition between chain propagation via C 3 H 5 A+NO 2 C 3 H 5 O+NO and chain termination via C 3 H 5 A+NO C 3 H 5 NO. The formation and consumption chemistry of carbonyl and hydrocarbon intermediates in the presence of NO x was also analyzed and discussed. [ABSTRACT FROM AUTHOR]

n -Heptane has been used extensively in various fundamental combustion experiments as a prototypical hydrocarbon fuel. While the formation of polycyclic aromatic hydrocarbon (PAH) in n -heptane combustion has been studied preferably in premixed flames, this study aims to investigate the combustion chemistry of n -heptane in less-studied diffusion flame and highly rich high-temperature homogeneous oxidation configurations by using a counterflow burner and a flow reactor, respectively. This work addresses the formation of higher-molecular species in the mass range up to about 160 u in both configurations. Samples are analyzed by time-of-flight (TOF) molecular beam mass spectrometry (MBMS) using electron-impact (EI) and single-photon ionization (PI). Highly resolved speciation data are reported. Laminar flow reactor experiments cover a wide temperature range. Especially the measurements at low temperatures provide speciation data of large oxygenates produced in the low-temperature oxidation of n -heptane, which are scarce in the literature. Important precursor molecules for PAH and soot formation, such as C 9 H 8 , C 10 H 8 , C 11 H 10, and C 12 H 8 , are formed during the high-temperature combustion process in the counterflow flame, while oxygenated growth species are observed under low-temperature conditions, even at the fuel-rich equivalence ratio of ϕ =4.00. Numerical modeling for both conditions is performed by using a newly developed kinetic model of n -heptane, which includes the n -heptane and PAH formation chemistry with state-of-the-art kinetic knowledge. Good agreement between model predictions and experimental data of counterflow flame and flow reactor is observed for the major species and some intermediates of n -heptane oxidation. While the concentrations of benzene and toluene measured in the counterflow burner are well-reproduced, the numerical results for flow reactor data are not satisfactory. Differences are found between the formation pathways of fulvene, from whose isomerization benzene is produced in diffusion flame and flow reactor. [ABSTRACT FROM AUTHOR]

With the aim to study potential cooperative effects in the low-temperature oxidation of dual-fuel combinations, we have investigated prototypical hydrocarbon (C 5 H 12 ) / oxygenated (C 2 H 6 O) 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 C 2 H 6 O. Mole fractions and signals of a significant number of species with elemental composition C n H 2n+x O y ( 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. [ABSTRACT FROM AUTHOR]

This work provides new temperature-dependent mole fractions of elusive intermediates relevant to the low-temperature oxidation of dimethyl ether (DME). It extends the previous study of Moshammer et al. [ J. Phys. Chem. A 2015 , 119 , 7361 - 7374 ] in which a combination of a jet-stirred reactor and molecular beam mass spectrometry with single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation was used to identify (but not quantify) several highly oxygenated species. Here, temperature-dependent concentration profiles of 17 components were determined in the range of 450-1000 K and compared to up-to-date kinetic modeling results. Special emphasis is paid toward the validation and application of a theoretical method for predicting photoionization cross sections that are hard to obtain experimentally but essential to turn mass spectral data into mole fraction profiles. The presented approach enabled the quantification of the hydroperoxymethyl formate (HOOCH 2 OCH 2 O), which is a key intermediate in the low-temperature oxidation of DME. The quantification of this keto-hydroperoxide together with the temperature-dependent concentration profiles of other intermediates including H 2 O 2 , HCOOH, CH 3 OCHO, and CH 3 OOH reveals new opportunities for the development of a next-generation DME combustion chemistry mechanism.