International Journal of Chemical Kinetics

A comprehensively tested H₂/O₂ chemical kinetic mechanism based on the work of Mueller et al. 1 and recently published kinetic and thermodynamic information is presented. The revised mechanism is validated against a wide range of experimental conditions, including those found in shock tubes, flow reactors, and laminar premixed flame. Excellent agreement of the model predictions with the experimental observations demonstrates that the mechanism is comprehensive and has good predictive capabilities for different experimental systems, including new results published subsequent to the work of Mueller et al. 1, particularly high‐pressure laminar flame speed and shock tube ignition results. The reaction H + OH + M is found to be primarily significant only to laminar flame speed propagation predictions at high pressure. All experimental hydrogen flame speed observations can be adequately fit using any of the several transport coefficient estimates presently available in the literature for the hydrogen/oxygen system simply by adjusting the rate parameters for this reaction within their present uncertainties. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 566–575, 2004

An updated H₂/O₂ kinetic model based on that of Li et al. (Int J Chem Kinet 36, 2004, 566–575) is presented and tested against a wide range of combustion targets. The primary motivations of the model revision are to incorporate recent improvements in rate constant treatment and resolve discrepancies between experimental data and predictions using recently published kinetic models in dilute, high‐pressure flames.

Attempts are made to identify major remaining sources of uncertainties, in both the reaction rate parameters and the assumptions of the kinetic model, affecting predictions of relevant combustion behavior. With regard to model parameters, present uncertainties in the temperature and pressure dependence of rate constants for HO₂ formation and consumption reactions are demonstrated to substantially affect predictive capabilities at high‐pressure, low‐temperature conditions. With regard to model assumptions, calculations are performed to investigate several reactions/processes that have not received much attention previously. Results from ab initio calculations and modeling studies imply that inclusion of H + HO₂ = H₂O + O in the kinetic model might be warranted, though further studies are necessary to ascertain its role in combustion modeling. In addition, it appears that characterization of nonlinear bath‐gas mixture rule behavior for H + O₂(+ M) = HO₂(+ M) in multicomponent bath gases might be necessary to predict high‐pressure flame speeds within ∼15%.

The updated model is tested against all of the previous validation targets considered by Li et al. as well as new targets from a number of recent studies. Special attention is devoted to establishing a context for evaluating model performance against experimental data by careful consideration of uncertainties in measurements, initial conditions, and physical model assumptions. For example, ignition delay times in shock tubes are shown to be sensitive to potential impurity effects, which have been suggested to accelerate early radical pool growth in shock tube speciation studies. In addition, speciation predictions in burner‐stabilized flames are found to be more sensitive to uncertainties in experimental boundary conditions than to uncertainties in kinetics and transport. Predictions using the present model adequately reproduce previous validation targets and show substantially improved agreement against recent high‐pressure flame speed and shock tube speciation measurements. Comparisons of predictions of several other kinetic models with the experimental data for nearly the entire validation set used here are also provided in the Supporting Information. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 444–474, 2012

A previous technique for the calculation of rate constants for the gas‐phase reactions of the OH radical with organic compounds has been updated and extended to include sulfur‐ and nitrogen‐containing compounds. The overall OH radical reaction rate constants are separated into individual processes involving (a) H‐atom abstraction from CH and OH bonds in saturated organics, (b) OH radical addition to >CC< and CC unsaturated bonds, (c) OH radical addition to aromatic rings, and (d) OH radical interaction with NH₂, >NH, >N, SH, and S groups. During its development, this estimation technique has been tested against the available database, and only for 18 out of a total of ca. 300 organic compounds do the calculated and experimental room temperature rate constants disagree by more than a factor of 2. This suggests that this technique has utility in estimating OH radical reaction rate constants at room temperature and atmospheric pressure of air, and hence atmospheric lifetimes due to OH radical reaction, for organic compounds for which experimental data are not available. In addition, OH radical reaction rate constants can be estimated over the temperature range ca. 250–1000 K for those organic compounds which react via H‐atom abstraction from CH and OH bonds, and over the temperature range ca. 250–500 K for compounds containing >CC< bond systems.

We have developed a computer code for an IBM PC/XT/AT or compatible which can be used to estimate, edit, or enter thermodynamic property data for gas phase radicals and molecules using Benson's group additivity method. The computer code is called THERM (THermo Estimation for Radicals and Molecules). All group contributions considered for a species are recorded and thermodynamic properties are generated in old NASA polynomial format for compatibility with the CHEMKIN reaction modeling code. In addition, listings are created in a format more convenient for thermodynamic, kinetic, and equilibrium calculations. Polynomial coefficients are valid from 300–5000 K using extrapolation methods based upon the harmonic oscillator model, an exponential function, or the Wilhoit polynomials. Properties for radical and biradical species are calculated by applying bond dissociation increments to a stable parent molecule to reflect loss of H atom. THERM contains a chemical reaction interpreter to calculate thermodynamic property changes for chemical reactions as functions of temperature. These include equilibrium constant, heat release (required heat, ΔH_r), entropy change (ΔS_r), Gibbs free energy change (ΔG_r), and the ratio of forward to reverse Arrhenius A‐factors (for elementary reactions). This interpreter can also process CHEMKIN input files. A recalculation procedure is incorporated for rapid updating of a database of chemical species to reflect changes in estimated bond dissociation energies, heats of formation, or other group values. All input and output files are in ASCII so that they can be easily edited, expanded, or updated.

The unimolecular decomposition reaction of dimethyl ether (DME) was studied theoretically using RRKM/master equation calculations. The calculated decomposition rate is significantly different from that utilized in prior work (Fischer et al., Int J Chem Kinet 2000, 32, 713–740; Curran et al., Int J Chem Kinet 2000, 32, 741–759). DME pyrolysis experiments were performed at 980 K in a variable‐pressure flow reactor at a pressure of 10 atm, a considerably higher pressure than previous validation data. Both unimolecular decomposition and radical abstraction are significant in describing DME pyrolysis, and hierarchical methodology was applied to produce a comprehensive high‐temperature model for pyrolysis and oxidation that includes the new decomposition parameters and more recent small molecule/radical kinetic and thermochemical data. The high‐temperature model shows improved agreement against the new pyrolysis data and the wide range of high‐temperature oxidation data modeled in prior work, as well as new low‐pressure burner‐stabilized species profiles (Cool et al., Proc Combust Inst 2007, 31, 285–294) and laminar flame data for DME/methane mixtures (Chen et al., Proc Combust Inst 2007, 31, 1215–1222). The high‐temperature model was combined with low‐temperature oxidation chemistry (adopted from Fischer et al., Int J Chem Kinet 2000, 32, 713–740), with some modifications to several important reactions. The revised construct shows good agreement against high‐ as well as low‐temperature flow reactor and jet‐stirred reactor data, shock tube ignition delays, and laminar flame species as well as flame speed measurements. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 40: 1–18, 2008

Atmospheric photodissociation rate coefficients and photodissociation lifetimes for nitromethane, methyl nitrite, and methyl nitrate were calculated as a function of altitude from their measured visible and near ultraviolet photoabsorption cross sections at 298 K. The lifetime of methyl nitrite is nearly independent of altitude and is approximately 2 min. From 0 to 50 km the lifetime of nitromethane varies from 10 to 0.5 hr, while that of methyl nitrate changes from 5.3 to 0.09 days, respectively.

We propose a new high temperature pathway for NO formation that involves the reaction of NNH with oxygen atoms. This reaction forms the HNNO* energized adduct via a rapid combination reaction; HNNO* then rapidly dissociates to NH + NO. The rate constant for O + NNH  NH + NO is calculated via a QRRK chemical activation analysis to be 3.3 × 10¹⁴ T^−0.23exp(+510/T) cm³ mol⁻¹ s⁻¹. This reaction sequence can be an important or even major route to NO formation under certain combustion conditions. The presence of significant quantities of NNH results from the reaction of H with N₂. The H + N₂  NNH reaction is only ca. 6 kcal/mol endothermic with a relatively low barrier. The reverse reaction, NNH dissociation, has been reported in the literature to be enhanced by tunneling. Our analysis of NNH dissociation indicates that tunneling dominates. We report a two‐term rate constant for NNH dissociation: 3.0 × 10⁸ + [M] {1.0 × 10¹³T^0.5exp(−1540/T)} s⁻¹. The first term accounts for pressure‐independent tunneling from the ground vibrational state, while the second term accounts for collisional activation to higher vibration states from which tunneling can also occur. ([M] is the total concentration in units of mol cm⁻³.) Use of this dissociation rate constant and microscopic reversibility results in a large rate constant for the H + N₂ reaction. As a result, we find that NNH  H + N₂ can be partially equilibrated under typical combustion conditions, resulting in NNH concentrations large enough for it to be important in bimolecular reactions. Our analysis of such reactions suggests that the reaction with oxygen atoms is especially important. © 1995 John Wiley & Sons, Inc.

Rate constants for the gas‐phase reactions of O₃ with a series of monoterpenes and related compounds have been determined at 296 ± 2 K and 740 torr total pressure of air or O₂ using a combination of absolute and relative rate techniques. Good agreement between the absolute and relative rate data was observed, and the rate constants obtained (in units of 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹) were: α‐pinene, 8.7; β‐pinene, 1.5; Δ³‐carene, 3.8; 2‐carene, 24; sabinene, 8.8; d‐limonene, 21; γ‐terpinene, 14; terpinolene, 140; α‐phellandrene, 190; α‐terpinene, 870; myrcene, 49; trans‐ocimene, 56; p‐cymene, <0.005; and 1,8‐cineole, <0.015. While these rate constants for α‐ and β‐pinene and sabinene are in good agreement with recent absolute and relative rate determinations, those for the other monoterpenes are generally lower than the literature data by factors of ca. 2–10. The measured rate constants for the monoterpenes are reasonably consistent with predictions based upon the number and positions of the substituent groups around the 〉CC〈 bond(s).

The mechanism of NH₃ pyrolysis was investigated over a wide range of conditions behind reflected shock waves. Quantitative time‐history measurements of the species NH and NH₂ were made using narrow‐linewidth laser absorption. These records were used to establish an improved model mechanism for ammonia pyrolysis. The risetime and peak concentrations of NH and NH₂ in this experimental database have also been summarized graphically.

Rate coefficients for several reactions which influence the NH and NH₂ profiles were fitted in the temperature range 2200 K to 2800 K. The reaction and the corresponding best fit rate coefficients are as follows: with a rate coefficient of 4.0 × 10¹³ exp(−3650/RT) cm³ mol⁻¹ s⁻¹, with a rate coefficient of 1.5 × 10¹⁵T^−0.5 cm³ mol⁻¹ s⁻¹ and with a rate coefficient of 5.0 × 10¹³ exp(−10000/RT) cm³ mol⁻¹ s⁻¹. The uncertainty in rate coefficient magnitude in each case is estimated to be ±50%. The temperature dependences of these rate coefficients are based on previous estimates.

The experimental data from four earlier measurements of the dissociation reaction were reanalyzed in light of recent data for the rate of NH₃ + H → NH₂1 + H₂, and an improved rate coefficient of 2.2 × 10¹⁶ exp(−93470/RT) cm³ mol⁻¹ s⁻¹ in the temperature range 1740 to 3300 K was obtained. The uncertainty in the rate coefficient magnitude is estimated to be ± 15%.

Rate constants for the gas‐phase reactions of O₃ with the sesquiterpenes α‐cedrene, α‐copaene, β‐caryophyllene, α‐humulene, and longifolene, and with the monoterpenes limonene, terpinolene, α‐phellandrene, and α‐terpinene, have been measured using a relative rate technique at 296 ± 2 K and atmospheric pressure of air. The rate constants obtained (in units of 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹) are: limonene, 20.1 ± 5.1; terpinolene, 188 ± 67; α‐phellandrene, 298 ± 105; α‐terpinene, 2110 ± 770; α‐cedrene, 2.78 ± 0.71; α‐copaene, 15.8 ± 5.6; β‐caryophyllene, 1160 ± 430; α‐humulene, 1170 ± 450; and longifolene, <0.07, where the indicated errors include the estimated overall uncertainties in the rate constants for the reference organics. Hydroxyl radical formation yields were also determined for the O₃ reactions with the sesquiterpenes, of 0.67 for α‐cedrene, 0.35 for α‐copaene, 0.06 for β‐caryophyllene, and 0.22 for α‐humulene, all with estimated overall uncertainties of a factor of ca. 1.5. The tropospheric lifetimes of the sesquiterpenes due to reaction with O₃ are calculated. © 1994 John Wiley & Sons, Inc.