Theoretical rate coefficients for allyl+HO2 and allyloxy decomposition

305

189

Isobutene is an important intermediate in the pyrolysis and oxidation of higher-order branched alkanes, and it is also a component of commercial gasolines. To better understand its combustion characteristics, a series of ignition delay time (IDT) and laminar flame speed (LFS) measurements have been performed. In addition, flow reactor speciation data recorded for the pyrolysis and oxidation of isobutene is also reported. Predictions of an updated kinetic model described herein are compared with each of these data sets, as well as with existing jet-stirred reactor (JSR) species measurements.IDTs of isobutene oxidation were measured in four different shock tubes and in two rapid compression machines (RCMs) under conditions of relevance to practical combustors. The combination of shock tube and RCM data greatly expands the range of available validation data for isobutene oxidation models to pressures of 50 atm and temperatures in the range 666-1715 K. Isobutene flame speeds were measured experimentally at 1 atm and at unburned gas temperatures of 298-398 K over a wide range of equivalence ratios. For the flame speed results, there was good agreement between different facilities and the current model in the fuel-rich region.Ab initio chemical kinetics calculations were carried out to calculate rate constants for important reactions such as H-atom abstraction by hydroxyl and hydroperoxyl radicals and the decomposition of 2-methylallyl radicals.A comprehensive chemical kinetic mechanism has been developed to describe the combustion of isobutene and is validated by comparison to the presently considered experimental measurements. Important reactions, highlighted via flux and sensitivity analyses, include: (a) hydrogen atom abstraction from isobutene by hydroxyl and hydroperoxyl radicals, and molecular oxygen; (b) radical-radical recombination reactions, including 2-methylallyl radical self-recombination, the recombination of 2-methylallyl radicals with hydroperoxyl radicals; and the recombination of 2-methylallyl radicals with methyl radicals; (c) addition reactions, including hydrogen atom and 2 hydroxyl radical addition to isobutene; and (d) 2-methylallyl radical decomposition reactions. The current mechanism accurately predicts the IDT and LFS measurements presented in this study, as well as the JSR and flow reactor speciation data already available in the literature.The differences in low-temperature chemistry between alkanes and alkenes are also highlighted in this work. In normal alkanes, the fuel radical Ṙ adds to molecular oxygen forming alkylperoxyl (RȮ 2 ) radicals followed by isomerization and chain branching reactions which promote low-temperature fuel reactivity. However, in alkenes, because of the relatively shallow well (~20 kcal mol -1 ) for RȮ 2 formation compared to ~35 kcal mol -1 in alkanes, the Ṙ + O 2 ⇌ RȮ 2 equilibrium lies more to the left favoring Ṙ + O 2 rather than RȮ 2 radical stabilization. Based on this work, and related studies of allylic systems, it is apparent that reactivity fo...

Section: Ic 4 H 8 + O 2 ↔ Productsmentioning

confidence: 99%

Section: Iċ 4 H 7 + Hȯ 2 ↔ Productsmentioning

confidence: 99%

A comprehensive experimental and modeling study of isobutene oxidation

Zhou

O'Connor

et al. 2016

305

189

“…The resulting hydroperoxide has a weak O-O bond and scission results in a hydroxyl radical and an oxy radical. Rate coefficients for recombination reactions of resonantly stabilized radicals with hydroperoxy and associated reactions were taken from the theoretical study by Goldsmith et al on the reactions of allyl with hydroperoxy [70].…”

Section: Reaction Pathways Characteristic For Alkene and Alcohol Oxidmentioning

confidence: 99%

“…The aforementioned competing decomposition channels for isobutenyloxy radical are part of the base mechanism developed by Zhou et al [49]. Associated reaction rate coefficients are estimated from the analogous allyloxy system [70] and rate rules. This may lead to erroneous branching ratio of the isobutenyloxy decomposition channels which can be the cause of the overprediction regarding 2-methyl-2-propenal and discrepancies regarding allene.…”

Section: Reaction Path Analysismentioning

confidence: 99%

Understanding the reactivity of unsaturated alcohols: Experimental and kinetic modeling study of the pyrolysis and oxidation of 3-methyl-2-butenol and 3-methyl-3-butenol

Bruycker

Herbinet

Carstensen

et al. 2016

. Understanding the reactivity of unsaturated alcohols: Experimental and kinetic modeling study of the pyrolysis and oxidation of 3-methyl-2-butenol and 3-methyl-3-butenol. Combustion and Flame, Elsevier, 2016, 171, pp.237-251. 1 Understanding the reactivity of unsaturated alcohols: Experimental and kinetic modeling study of the pyrolysis and oxidation of 3-methyl-2-butenol and 3-methyl-3-butenol AbstractThe reactivity of unsaturated alcohols with a C=C double bond in the β-and γ-positions to the hydroxyl group is not well established. The pyrolysis and oxidation of two such unsaturated alcohols have been studied, i.e. 3-methyl-2-butenol (prenol) and 3-methyl-3-butenol (isoprenol). Experiments at three equivalence ratios, i.e. φ = 0.5, φ = 1.0 and φ = ∞ (pyrolysis), were performed using an isothermal jet-stirred quartz reactor at temperatures ranging from 500 to 1100 K, a pressure of 0.107 MPa and a residence time of 2 s. The reactant and product concentrations were quantified using gas chromatography. A kinetic model has been developed using the automatic network generation tool "Genesys". Several important rate coefficients are obtained from new quantum chemical calculations. Overall, there is a good agreement between model calculated mole fraction profiles and experimental data. Reaction path analysis reveals that isoprenol consumption is dominated by a unimolecular reaction to formaldehyde and isobutene. At the applied operating conditions, the equivalence ratio has no effect on the isoprenol conversion profile. Pyrolysis and oxidation of prenol is dominated by radical chemistry, with hydrogen abstractions from prenol forming resonantly stabilized radicals as dominating conversion path. Oxidation and decomposition of the resulting radicals are predicted to form 3-methyl-2-butenal and 2-methyl-1,3-butadiene, which have been detected as important products in the reactor effluent.

“…This rate constant modification should not be adopted universally. The authors acknowledge that high temperature propene oxidation chemistry is an area of research that warrants further investigation, and are aware of separate efforts to improve the AramcoMech propene sub-mechanism based on more comprehensive propene oxidation and pyrolysis studies [29].…”

Section: Classes Of Reactions Rate Constant Rules and Thermochemicamentioning

confidence: 99%

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...