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...
Rate constants of hydrogen‐atom abstraction from n‐butanol by the HȮ2 radical have been calculated. Conventional transition state theory employing rigid‐rotor harmonic‐oscillator approximations for all but the torsional degrees of freedom is used with tight transition states. The Pitzer–Gwinn‐like approximation using Fourier fits to internal rotations was applied to determine the one‐dimensional hindered potentials. Asymmetric Eckart barriers were used to model tunneling in one‐dimensional through saddle points. Activation entropies for all of the reaction channels have been determined. Hydrogen bonds formed in the transition states lead to ring structures, which lower the energy barrier and thus an increase in the rate constant for abstraction. Conversely, entropy is lost when the ring structure is formed and this decreases the frequency factor for abstraction; therefore, both of these effect influence the rate constants in opposite ways. Abstraction of an α hydrogen atom is dominant throughout the whole temperature range, and the branching ratio decreases from 96.1% at 500 K to 46.6% at 2000 K. As the carbon chain lengthens, the influence from the OH group lessens and hence δ hydrogens behave in a similar fashion to primary H‐atoms in n‐butane. The estimated uncertainty for the individual rate constants is a factor of 2.5. Computed total, kt, and individual rate constants, based on the CCSD(T)/cc‐pVTZ//MP2/6‐311G(d,p) potential energy surface, in the temperature range of 500–2000 K for n‐butanol + HȮ2 are reported as follows (cm3 mol−1 s−1): © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 155–164, 2012
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