Key Elementary Reactions Research Articles

The decomposition of nitrous oxide at 1.5 ⩽ P ⩽ 10.5 atm and 1103 ⩽ T ⩽ 1173 K

AbstractReaction experiments on mixtures of N2O/H2O/N2 were performed in a variable pressure flow reactor over temperature, pressure, and residence time ranges of 1103–1173 K, 1.5–10.5 atm, and 0.2–0.8 s, respectively. Mixtures of approximately 1% N2O in N2 were studied with the addition of varying amounts of water vapor, from background to 3580 ppm. Experimentally measured profiles of N2O, O2, NO, NO2, H2O, and temperature were compared with predictions from detailed kinetic modeling calculations to assess the validity of a reaction mechanism developed from currently available literature thermochemical and rate constant parameters. Sensitivity and reaction flux analyses were performed to determine key elementary reaction path processes and rates.Reaction rate constants for the uni‐molecular reaction, N2O → N2 + O, were determined at various pressures in order to match overall experimental and numerical decomposition rates of N2O. The numerical model included a newly determined rate constant for N2O + OH → HO2 + N2 with an upper limit of 5.66 × 108 cm3 mol−1 sec−1 at 1123 K. This is considerably smaller than presently reported in the literature. The experimentally observed rate of N2O decomposition was found to be slightly dependent on added water concentration. The rate constant determined for the elementary decomposition is strongly dependent on the choice of rate constants for the N2O + O ⇔ N2 + O2 and N2O + O ⇔ NO + NO reactions. In the absence of accurate data at the temperatures of this work, and based on these and other experiments in this laboratory, we presently recommend rate constants from the review of Baulch et al. The basis for this recommendation is discussed, including the impact on the rate constants derived for elementary nitrous oxide decomposition. The uncertainties in the rate constants as reported here are ±30%.The present mechanism was applied to previously reported high‐pressure shock tube data and yields a high‐pressure limit rate constant a factor of three larger than previously reported at these temperatures. The following expressions for the elementary decomposition reaction are recommended: k = 9.13 × 1014 exp (−57, 690/RT) cm3 mol−1 s−1 and k∞ = 7.91 × 1010 exp(−56020/RT) s−1. Simple Lindemann fits utilizing these parameters reproduce the pressure dependent rate constants measured here within ±25%. © 1995 John Wiley & Sons, Inc.

Oxidative pyrolysis of methane and monochloromethane to higher hydrocarbons with steam

Direct conversion of methane to acetylene, ethane, and ethylene via homogeneous chlorine-catalyzed oxidative pyrolysis using steam has been investigated. Experiments were conducted in an isothermal quartz flow reactor. Monochloromethane, methane, or mixtures of these were used as the carbon feed. The effects of methane to monochloromethane ratio in the feed, reactor temperature, and reactor space time on the product distribution in the reactor effluent were studied. Addition of steam in the feed was highly beneficial in obtaining higher yields and/or selectivities of the C2 species and suppressing the formation of coke. The key elementary reactions responsible for formation of the products detected are discussed.

Dynamics of vibrationally excited ozone formed by three-body recombination. II. Kinetics and mechanism

Spectrally resolved infrared chemiluminescence from vibrationally excited ozone, O3(v), has been used to study the reaction kinetics of O3(v) in discharged O2/Ar mixtures at ∼1 Torr and 80–150 K. Dependences of the excited state number densities on temperature and O2 mole fraction indicate O3(v) is formed primarily by three-body recombination of O with O2 and is destroyed by rapid chemical reaction with O. Several secondary excitation reactions involving vibrationally and electronically excited O2 are also indicated. The data are treated with a detailed steady-state analysis of the discharge kinetics, to extract estimates for rate coefficients of the key elementary reactions. The effective ‘‘quasinascent’’ state distribution in recombination is also inferred; this distribution shows selective recombination into the asymmetric stretching mode, but an apparently statistical (i.e., collisionally scrambled) behavior among the vibrational states within that mode. The results are discussed in terms of the detailed dynamics of three-body recombination.

Formation of free radicals in the RH + O 2 system: I. Tetralin, cyclohexanol, cyclohexanone

Gas explosion in coal mines caused a large number of deaths, injuries and property losses. Gas explosion still happens occasionally in recent years. It is known that most people died from CO poisoning in gas explosion. But the details of toxic gases produced and their impacts on people need further investigation. In order to study the products and species in gas explosion, an analysis of chemical kinetic characteristics was conducted using CHEMKIN III with detailed mechanism (GRI-Mech3.0) package. A closed homogeneous 0-D reactor was adopted to investigate the kinetic behavior of gas explosion. The initial conditions were set, similar to those in the real tunnel environment. The profiles of four reactants, toxic gases and free radicals were obtained. The threshold values of people exposed to different poisonous gases were analyzed to study their impacts on people. Sensitivity analysis of key elementary reaction to free radical was performed to find possible reactions which can suppress gas explosion. The results show that the ignition delay time is the longest at fuel-rich condition. The O2 concentration decreases from 19% to 2% at stoichiometric ratio, which could not support normal respiration after explosion. It is found that the concentration of CO ranks the highest among the four toxic gases. Toxicity analysis indicates that those exposed to such condition will soon die of asphyxia. This is consistent with the accident investigation findings that the majority of fatalities result from inhalation of CO. The concentration of CO at fuel-rich condition is higher than that at fuel-lean condition. The relationship between the final free radical concentration and initial CH4 concentration exhibits a reverse U shape. Sensitivity analysis reveals that free radicals are mostly sensitive to the intermediate reaction R156 (forward direction) and R158 (reverse direction). It provides a possible direction to research the corresponding chemicals for gas explosion suppression. This work would provide theoretical support for gas explosion suppression technology and decision-making of personal protection in rescuing trapped coal miners. It is of great importance for developing suppression chemicals of gas explosion.

Limits of inflammability

This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH3 in air at 295 K and 1 atm. The experiments were carried out using a Hartmann bomb apparatus. The kinetic modelling was performed using Ansys Chemkin-Pro with opposed-flow premixed flame model employing three detailed reaction mechanisms, namely, the Mathieu and Petersen, Otomo et al., and Okafor et al. mechanisms. The degree of NH3 dissociation was varied from 0 to 25% (0 to 20%v/v H2 in the fuel mixture with a fixed H2/N2 ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH3 in air were 15.0%v/v and 30.0%v/v, respectively, consistent with the literature data. The flammability limits of the mixture widened significantly with increasing the degree of NH3 dissociation. At 25% NH3 dissociation, LFL decreased to 10.1%v/v and UFL increased to 36.6%v/v. All tested mechanisms were able to predict the extinction characteristics exhibited by the lean and rich mixtures of partially dissociated NH3 in air with non-unity Lewis numbers. While all three mechanisms predict well LFL, the Otomo et al. mechanism showed the best agreement with the experimental data of UFL. The rate of production of radicals, sensitivity, and reaction path analyses were performed to identify the key elementary reactions and radicals during combustion of partially dissociated NH3. The production of key radicals including OH, H, O, and NH2 was enhanced in the presence of H2 and thus the conversion of NH to NO and then NO to N2 near LFL and the conversion of NH2 and NO to N2 near UFL leading to wider flammability limits.