Rate constants for the H abstraction from alkanes (R–H) by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si22.gif" overflow="scroll"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:msup><mml:mrow><mml:mtext>R</mml:mtext></mml:mrow><mml:mrow><mml:mo>′</mml:mo></mml:mrow></mml:msup><mml:mtext>O</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mtext/></mml:mrow><mml:none/><mml:none/></mml:mmultiscripts></mml:mrow></mml:math> radicals: A systematic study on the impact

Diethyl carbonate (DEC) is an attractive biofuel that can be used to displace petroleum-derived diesel fuel, thereby reducing CO 2 and particulate emissions from diesel engines. A better understanding of DEC combustion characteristics is needed to facilitate its use in internal combustion engines. Towards this goal, ignition delay times for DEC were measured at conditions relevant to internal combustion engines using a rapid compression machine (RCM) and a shock tube. The experimental conditions investigated covered a wide range of temperatures (660-1300 K), a pressure of 30 bar, and equivalence ratios of 0.5, 1.0 and 2.0 in air.To provide further understanding of the intermediates formed in DEC oxidation, species concentrations were measured in a jet-stirred reactor at 10 atm over a temperature range of 500-1200 K and at equivalence ratios of 0.5, 1.0 and 2.0. These experimental measurements were used to aid the development and validation of a chemical kinetic model for DEC.The experimental results for ignition in the RCM showed near negative temperature coefficient (NTC) behavior. Six-membered alkylperoxy radical (R 2 ) isomerizations are conventionally thought to initiate low-temperature branching reactions responsible for NTC behavior, but DEC has no such possible 6-and 7-membered ring isomerizations. However, its molecular structure allows for 5-, 8-and 9-membered ring R 2 isomerizations. To provide accurate rate constants for these ring structures, ab initio computations for R 2 QOOH isomerization reactions were performed. These new R 2 isomerization rate constants have been implemented in a chemical kinetic model for DEC oxidation. The model simulations have been compared with ignition delay times measured in the RCM near the NTC region. Results of the 3 simulation were also compared with experimental results for ignition in the high-temperature region and for species concentrations in the jet-stirred reactor. Chemical kinetic insights into the oxidation of DEC were made using these experimental and modeling results.

Section: R 2 Qooh Isomerization and Related Reactionscontrasting

confidence: 58%

An experimental and modeling study of diethyl carbonate oxidation

Nakamura

Curran

Córdoba

et al. 2015

“…Aguilera-Iparraguirre et al [25] found that energy barriers for H abstractions by HO 2 radicals from C 1 -C 4 alkanes vary significantly with the adopted theoretical approach. Their reaction rate constants, estimated based on computed activation energies at the CCSD(T)/cc-pVTZ level, were in accord with corresponding data calculated by Carstensen et al [26] using the CBS-QB3 composite method.…”

Section: Computational Detailssupporting

confidence: 77%

“…A faster overall rate constant for the reaction ethylbenzene + HO 2 , in reference to that of toluene + HO 2 , stems not only from a larger number of abstractable H sites in ethyl benzene but also from a lower A factor in the toluene system. Aguilera-Iparraguirre et al [25] calculated theoretically reaction rate constants for H abstractions by HO 2 radicals from the two distinct positions in propane, and n-butane, and contrasted their estimations with analogous theoretically derived values of Carstensen et al [26] and recommended values of Orme et al [41], Baldwin et al [42], and Scott and Walker [20]. Aguilera-Iparraguirre et al [25] found that their calculated values reasonably agree with these four sets of data.…”

Section: Abstraction Of Propyl H Atom By Ho 2 Radicalsmentioning

confidence: 96%

Reactions of HO2 with n-propylbenzene and its phenylpropyl radicals

Altarawneh

Dlugogorski

2015

Abstraction of an H atom from the propyl side chain by hydroperoxyl radicals (HO 2 ) constitutes a central reaction in the low-temperature oxidation of n-propylbenzene (nPB).Herein, we calculate reaction rate constants for H abstraction from primary, secondary and benzylic sites in nPB. Rate of abstraction of a benzylic H atom dominates that of a secondary H atom with negligible abstraction of the primary H atom at T ≥ 600 K. We present the reaction enthalpies for 1-phenyl-1-propyl (R1), 1-phenyl-2-propyl (R2) and 1-phenyl-3-propyl (R3) radicals, and compare the computed reaction rate constants and bond dissociation enthalpies with analogous scarce literature values. Addition of HO 2 radicals to radical sites in R1, R2 and R3 proceeds in a highly exothermic process and results in the formation of HO 2 -phenylpropyl adducts. Mapped potential energy surfaces illustrate all plausible exit channels of the three HO 2 -phenylpropyl adducts. Master equation calculations for the three phenylpropyl + HO 2 reactions indicate that direct O-OH bond fission and water elimination control the fate of the adducts leading to the formation of ketonic-type structures. Results from this study should be useful to update kinetic models for the low-temperature oxidation of alkylbenzenes in general.

“…This was done only for peroxy radicals derived from 2,5-dimethylhexane and not for small alkylperoxy radicals. The A-factors for H-atom abstractions by HO 2 radical were increased by a factor of 3 to provide better agreement with measured ignition delay times measured at 900 to 1200 K. The previously used rate constants [20] were from the quantum-level calculations of Aguilera-Iparraguirre et al [27], whereas the new rate constants are in closer agreement with the calculations of Carstensen and Dean [28].…”

Section: Classes Of Reactions Rate Constant Rules and Thermochemicamentioning

confidence: 68%

A comprehensive combustion chemistry study of 2,5-dimethylhexane

Sarathy

Javed

Karsenty

et al. 2014

Iso-paraffinic molecular structures larger than seven carbon atoms in chain length are commonly found in conventional petroleum, Fischer-Tropsch (FT), and other alternative hydrocarbon fuels, but little research has been done on their combustion behavior. Recent studies have focused on either mono-methylated alkanes and/or highly branched compounds (e.g., 2,2,4-trimethylpentane). In order to better understand the combustion characteristics of real fuels, this study presents new experimental data for the oxidation of 2,5-dimethylhexane under a wide variety of temperature, pressure, and equivalence ratio conditions. This new dataset includes jet stirred reactor speciation, shock tube ignition delay, and rapid compression machine ignition delay, which builds upon recently published data for counterflow flame ignition, extinction, and speciation profiles. The low and high temperature oxidation of 2,5-dimethylhexane has been modeled using a comprehensive chemical kinetic model developed using established reaction rate rules. The agreement between the model and data is presented, along with suggestions for improving model predictions. The importance of propene chemistry is highlighted as critical for correct prediction of high temperature ignition delay. The oxidation behavior of 2,5-dimethylhexane is also compared with oxidation behavior of other linear and branched octane isomers, in order to determine the effects of the number of methyl branches on combustion properties. Both experimental data and model predictions indicate that increasing the level of branching decreases fuel reactivity at low and intermediate temperatures. The model is used to elucidate the structural features and reaction pathways responsible for inhibiting the reactivity of 2,5-dimethylhexane. FUEL or CNF, Sarathy et al. submitted July 2013Introduction Detailed chemical kinetic models for transportation fuels have reached a level of fidelity where accurate predictions can be made of combustion phenomenon relevant to the operation of practical devices. Schofield [1] states that these large scale models are adequate as engineering tools for studying the combustion of new fuel molecules. A recent review paper by Pitz and Mueller [2] describing the development of diesel surrogate fuel models concluded that major research gaps remain in modeling high molecular weight (i.e., C 8 and greater) aromatics, alkyl aromatics, cyclo-alkanes, and lightly branched iso-alkanes. The present study is concerned with the combustion of branched alkanes, specifically 2,5-dimethylhexane, which has been reported as a component of petroleum combustion exhaust, smog, and tobacco smoke [3]. Branched alkanes are important components of conventional diesel and jet fuels derived from petroleum [2,4]; synthetic Fischer-Tropsch diesel and jet fuels derived from coal, natural gas, and/or biomass [5,6]; and renewable diesel and jet fuels derived from thermochemical treatment of bio-derived fats and oils (e.g., hydrotreated renewable jet (HRJ) fuels) [7,8]. Detailed com...