John M. Simmie

Detailed chemical kinetic models for the combustion of hydrocarbon fuels

Abstract The status of detailed chemical kinetic models for the intermediate to high-temperature oxidation, ignition, combustion of hydrocarbons is reviewed in conjunction with the experiments that validate them. All classes of hydrocarbons are covered including linear and cyclic alkanes, alkenes, alkynes as well as aromatics.

Burning velocities of dimethyl ether and air

Flame speeds of dimethyl ether and air mixtures have been measured in a spherical bomb by using two experimental methods, for equivalence ratios ranging from 0.7 to 1.7 at an initial pressure of 1 bar and a temperature of 295 K. The corrected funda-mental velocities were deduced from the measured flame speeds and thermodynamic data. The maximum burning velocity of 47.5 cm/s was observed at an equivalence ratio of 1.13. Partial agreement with a model for dimethyl ether oxidation was obtained. Chemical kinetic modeling of the flame speeds was performed with a detailed mechanism for the oxidation of DME comprising 77 species and 351 reactions. In addition, sensitivity analyses were used to identify the most important reactions and showed methyl and formyl radicals are key species influencing the flame speed.

The oxidation and ignition of dimethylether from low to high temperature (500–1600 K): Experiments and kinetic modeling

The oxidation of dimethylether (DME) has been studied in a fused silica jet-stirred reactor (JSR) at 10 atm, 0.2≤φ≤1, 550–1100 K. Concentration profiles of reactants, intermediates, and products of the oxidation were measured by low-pressure sonic probe sampling and off-line gas chromatography analyses. The results obtained in the cool flame regime are the first to be reported. The ignition delays of DME/O2/Ar mixtures have been measured in a shock tube at 1200 to 1600 K, at 3.5 atm and 0.5≤φ≤2. A numerical model consisting of a detailed kinetic reaction mechanism with 331 reactions (most of them reversible) among 55 species is proposed to describe both the low and high-temperature oxidation of DME in the JSR (550–1275 K, 1–10 atm) and the ignition of DME in shock tubes from low to high temperature (650–1600 K, 3.5–40 bar). A general good agreement between the data and the model was observed. A kinetic analysis involving sensitivity and reaction path analysis is used to interpret the results.

Barrier heights for H-atom abstraction by HȮ2 from n-butanol—A simple yet exacting test for model chemistries?

The barrier heights involved in the abstraction of a hydrogen atom from n‐butanol by the hydroperoxyl radical have been computed with both compound (CBS‐QB3, CBS‐APNO, G3) and coupled cluster methods. In particular, the benchmark computations CCSD(T)/cc‐pVTZ//MP2/6‐311G(d,p) were used to determine that the barrier heights increase in the order α

Shock tube ignition of ethanol, isobutene and MTBE: Experiments and modeling

The ignition of ethanol, isobutene and methyl tert-butyl ether (MTBE) has been studied experimentally in a shock tube and computationally with a detailed chemical kinetic model. Experimental results, consisting of ignition delay measurements, were obtained for a range of fuel/oxygen mixtures diluted in Argon, with temperatures varying over a range of 1100–1900 K. Mixtures ranged from very lean to very rich, including equivalence ratios of 0.1–4.0 for isobutene, 0.25–1.5 for ethanol, and 0.15–2.4 for MTBE. The numerical model consisted of a detailed kinetic reaction mechanism with more than 400 elementary reactions, chosen to describe reactions of each fuel and the smaller hydrocarbon and other species produced during their oxidation. The overall agreement between experimental and computed results was excellent, particularly for mixtures with greater than 0.3% fuel. The greatest sensitivity in the computed results was found to falloff parameters in the dissociation reactions of isobutene, ethane, methane, and ethyl and vinyl radicals, to the C 3 H 4 and C 3 H 5 reaction submechanisms in the model, and to the reactions in the H 2 −O 2 −CO submechanism.

Accurate Benchmark Calculation of the Reaction Barrier Height for Hydrogen Abstraction by the Hydroperoxyl Radical from Methane. Implications for CnH2n+2 where n = 2 → 4

The CH4 + HO2(*) reaction is studied by using explicitly correlated coupled-cluster theory with singles and doubles (CCSD-R12) in a large 19s14p8d6f4g3h basis (9s6p4d3f for H) to approach the basis-set limit at the coupled-cluster singles-doubles level. A correction for connected triple excitations is obtained from the conventional CCSD(T) coupled-cluster approach in the correlation-consistent quintuple-zeta basis (cc-pV5Z). The highly accurate results for the methane reaction are used to calibrate the calculations of the hydroperoxyl-radical hydrogen abstraction from other alkanes. For the alkanes C(n)H(2n+2) with n = 2 --> 4, the reactions are investigated at the CCSD(T) level in the correlation-consistent triple-zeta (cc-pVTZ) basis. The results are adjusted to the benchmark methane reaction and compared with those from other approaches that are commonly used in the field such as CBS-QB3, CBS-APNO, and density functional theory. Rate constants are computed in the framework of transition state theory, and the results are compared with previous values available.

A Multiple Shock Tube and Chemical Kinetic Modeling Study of Diethyl Ether Pyrolysis and Oxidation

The pyrolysis and oxidation of diethyl ether (DEE) has been studied at pressures from 1 to 4 atm and temperatures of 900-1900 K behind reflected shock waves. A variety of spectroscopic diagnostics have been used, including time-resolved infrared absorption at 3.39 mum and time-resolved ultraviolet emission at 431 nm and absorption at 306.7 nm. In addition, a single-pulse shock tube was used to measure reactant, intermediate, and product species profiles by GC samplings at different reaction times varying from 1.2 to 1.8 ms. A detailed chemical kinetic model comprising 751 reactions involving 148 species was assembled and tested against the experiments with generally good agreement. In the early stages of reaction the unimolecular decomposition and hydrogen atom abstraction of DEE and the decomposition of the ethoxy radical have the largest influence. In separate experiments at 1.9 atm and 1340 K, it is shown that DEE inhibits the reactivity of an equimolar mixture of hydrogen and oxygen (1% of each).

Formation Enthalpies and Bond Dissociation Energies of Alkylfurans. The Strongest C—X Bonds Known?

Enthalpies of formation, DeltaH(f)(298.15 K), of 2-methyl-, 3-methyl-, 2-ethyl-, 2-vinyl-, 2,3-dimethyl-, 2,4-dimethyl-, and 3,4-dimethylfurans are computed with three compound quantum chemical methods, CBS-QB3, CBS-APNO, and G3, via a number of isodesmic reactions. We show that previously experimentally determined enthalpies of formation of furan itself, 2,5-dimethyl-, 2-tert-butyl-, and 2,5-di-tert-butylfurans are self-consistent but that for 2-vinylfuran is most probably in error. The formation enthalpies of over 20 furyl and furfuryl radicals have also been determined and consequently the bond dissociation energies of a number of C-H, C-CH(3), C-F, C-Cl, and C-OH bonds. The ring-carbon-H bonds in alkylfurans are much stronger than previously thought and are among the strongest ever C-H bonds recorded exceeding 500 kJ mol(-1). The relative thermodynamic instability of the various furyl radicals means that bonds to methyl, fluorine, and chlorine are also unusually strong. This is as a consequence of the inability of the radical to effectively delocalize the unpaired electron and the geometrical inflexibility of the five-membered heterocyclic ring. By way of contrast the furfuryl radicals are more stable than similar benzyl radicals which results in weaker side-chain C-H bonds than the corresponding toluene derivatives (although stronger than the corresponding cyclopentadiene analogue). These results have implications for the construction of detailed chemical kinetic models to account for the thermal decomposition and oxidation of alkylfurans either in engines or in the atmosphere.

Enthalpies of Formation and Bond Dissociation Energies of Lower Alkyl Hydroperoxides and Related Hydroperoxy and Alkoxy Radicals

The enthalpies of formation and bond dissociation energies, D(ROO-H), D(RO-OH), D(RO-O), D(R-O 2) and D(R-OOH) of alkyl hydroperoxides, ROOH, alkyl peroxy, RO, and alkoxide radicals, RO, have been computed at CBS-QB3 and APNO levels of theory via isodesmic and atomization procedures for R = methyl, ethyl, n-propyl and isopropyl and n-butyl, tert-butyl, isobutyl and sec-butyl. We show that D(ROO-H) approximately 357, D(RO-OH) approximately 190 and D(RO-O) approximately 263 kJ mol (-1) for all R, whereas both D(R-OO) and D(R-OOH) strengthen with increasing methyl substitution at the alpha-carbon but remain constant with increasing carbon chain length. We recommend a new set of group additivity contributions for the estimation of enthalpies of formation and bond energies.

Ab Initio Study of the Decomposition of 2,5-Dimethylfuran

The initial steps in the thermal decomposition of 2,5-dimethylfuran are identified as scission of the C-H bond in the methyl side chain and formation of β- and α-carbenes via 3,2-H and 2,3-methyl shifts, respectively. A variety of channels are explored which prise the aromatic ring open and lead to a number of intermediates whose basic properties are essentially unknown. Once the furan ring is opened demethylation to yield highly unsaturated species such as allenylketenes appears to be a feature of this chemistry. The energetics of H abstraction by the hydroxyl radical (and other abstracting species) from a number of mono- and disubstituted methyl furans has been studied. H-atom addition to 2,5-dimethylfuran followed by methyl elimination is shown to be the most important route to formation of the less reactive 2-methylfuran. Identification of 2-ethenylfuran as an C(6)H(6)O intermediate in 2,5-dimethylfuran flames is probably not correct and is more likely the isomeric 2,5-dimethylene-2,5-dihydrofuran for which credible formation channels exist.