Rate constant of the elementary reaction of carbon monoxide with hydroxyl radical

Insight into the Ozone-Assisted Low-Temperature Combustion of Dimethyl Ether by Means of Stabilized Cool Flames

The low-temperature combustion kinetics of dimethyl ether (DME) were studied by means of stabilized cool flames in a heated stagnation plate burner configuration using ozone-seeded premixed flows of DME/O₂. Direct imaging of CH₂O* chemiluminescence and laser-induced fluorescence of CH₂O were used to determine the flame front positions in a wide range of lean and ultra-lean equivalence ratios and ozone concentrations for two strain rates. The temperature and species mole fraction profiles along the flame were measured by coupling thermocouples, gas chromatography, micro-chromatography, and quadrupole mass spectrometry analysis. A new kinetic model was built on the basis of the Aramco 1.3 model, coupled with a validated submechanism of O₃ chemistry, and was updated to improve the agreement with the obtained experimental results and experimental data available in the literature. The main results show the efficiency of the tested model to predict the flame front position and temperature in every tested condition, as well as the importance of reactions typical of atmospheric chemistry in the prediction of cool flame occurrence. The agreement on the fuel and major products is overall good, except for methanol, highlighting some missing kinetic pathways for the DME/O₂/O₃ system, possibly linked to the direct addition of atomic oxygen on the fuel radical, modifying the product distribution after the cool flame.

Dual Fuel Reaction Mechanism 2.0 including NOx Formation and Laminar Flame Speed Calculations Using Methane/Propane/n-Heptane Fuel Blends

This study presents the further development of the TU Wien dual fuel mechanism, which was optimized for simulating ignition and combustion in a rapid compression expansion machine (RCEM) in dual fuel mode using diesel and natural gas at pressures higher than 60 bar at the start of injection. The mechanism is based on the Complete San Diego mechanism with n-heptane extension and was attuned to the RCEM measurements to achieve high agreement between experiments and simulation. This resulted in a specific application area. To obtain a mechanism for a wider parameter range, the Arrhenius parameter changes performed were analyzed and updated. Furthermore, the San Diego nitrogen sub-mechanism was added to consider NOx formation. The ignition delay time-reducing effect of propane addition to methane was closely examined and improved. To investigate the propagation of the flame front, the laminar flame speed of methane–air mixtures was simulated and compared with measured values from literature. Deviations at stoichiometric and fuel-rich conditions were found and by further mechanism optimization reduced significantly. To be able to justify the parameter changes performed, the resulting reaction rate coefficients were compared with data from the National Institute of Standards and Technology chemical kinetics database.

A Shock-Tube Study of the CO + OH Reaction Near the Low-Pressure Limit

Rate coefficients for the reaction between carbon monoxide and hydroxyl radical were measured behind reflected shock waves over 700-1230 K and 1.2-9.8 bar. The temperature/pressure conditions correspond to the predicted low-pressure limit of this reaction, where the channel leading to carbon dioxide formation is dominant. The reaction rate coefficients were inferred by measuring the formation of carbon dioxide using quantum cascade laser absorption near 4.2 μm. Experiments were performed under pseudo-first-order conditions with tert-butyl hydroperoxide (TBHP) as the OH precursor. Using ultraviolet laser absorption by OH radicals, the TBHP decomposition rate was measured to quantify potential facility effects under extremely dilute conditions used here. The measured CO + OH rate coefficients are provided in Arrhenius form for three different pressure ranges: kCO+OH(1.2-1.6 bar) = (9.14 ± 2.17) × 10(-13) exp(-(1265 ± 190)/T) cm(3) molecule(-1) s(-1); kCO+OH(4.3-5.1 bar) = (8.70 ± 0.84) × 10(-13) exp(-(1156 ± 83)/T) cm(3) molecule(-1) s(-1); and kCO+OH(9.6-9.8 bar) = (7.48 ± 1.92) × 10(-13) exp(-(929 ± 192)/T) cm(3) molecule(-1) s(-1). The measured rate coefficients are found to be lower than the master equation modeling results by Weston et al. [J. Phys. Chem. A, 2013, 117, 821] at 819 K and in closer agreement with the expression provided by Joshi and Wang [Int. J. Chem. Kinet., 2006, 38, 57].

Structure, chemical mechanism and properties of premixed flames in mixtures of carbon monoxide, nitrogen and oxygen with hydrogen and water vapour

Implicit solutions of the time-dependent flame equations have been used to calculate, for assumed reaction mechanisms, the expected structures and properties of a series of hydrogen-carbon monoxide-oxygen-nitrogen flames, some containing traces of added water vapour, at atmospheric and reduced pressures. Predicted burning velocities at atmospheric pressure have been compared with: ( a ) recent measurements, reported here, of the effect of addition of up to 10 % carbon monoxide on the burning velocity of a low temperature hydrogen-oxygennitrogen flame; ( b ) previous measurements by Scholte & Vaags (1959c) on dry hydrogen-carbon monoxide-air mixtures over the whole composition range on the fuel-rich side of stoichiometric; and ( c ) previously reported measurements by Jahn (1934), Badami & Egerton (1955), Scholte & Vaags (1959 b )and Wires et al . (1959) for moist carbon monoxide-air or carbon monoxide-oxygen mixtures, with or without traces of added hydrogen. Additionally, the following comparisons are made: ( d )The mole fraction profile for the decay of a trace of carbon dioxide added to the low temperature hydrogen-oxygen-nitrogen flame has been recalculated with the aid of the full reaction mechanism, for comparison with the previously reported measurements of Dixon-Lewis et al. (1965). ( e ) Computed structures of two hydrogen-carbon monoxide-oxygen-argon flames burning at reduced pressure have been compared with previous measurements by Fenimore & Jones (1959) and Vandooren et al . (1975). ( f ) The mole fraction ratio X co /X CO 2 in the burnt gas from a low temperature, fuel-rich hydrogen-carbon monoxide-oxygen-argon flame at atmospheric pressure was measured by using a mass spectrometer. The measured ratio agreed to within 1 % with that predicted by computation of the complete flame properties. Both the calculated and measured ratios were higher than would correspond with the establishment of the water gas equilibrium in the flame. The major part of the observed changes in burning velocity from those of hydrogen-air mixtures can be satisfactorily explained by the addition of the single reaction (xxi) ,

OH + CO ⇌ C O 2 + H , ( xxi )

to the mechanism already established for the hydrogen-oxygen-nitrogen flame system (Dixon-Lewis 1979). This applies particularly to fuel-lean flames and to fuel-rich mixtures not too far from stoichiometric. For fuel-rich flames further from stoichiometric, and particularly for the measurements in §(a), agreement between predicted and measured burning velocities is improved by adding to the mechanism a series of chain terminating steps involving the formation and subsequent reactions of the formyl radical. For reasonable values of its rate coefficient, reaction (xxii),

O + CO + M ⇌ C O 2 + M , ( xxii )

never exerts more than a minor influence on the burning velocity. The major features of the structure of the flames are: ( a ) a preferential oxidation of hydrogen in the early stages of the reaction zones, leading to overshoot in the water concentration followed by a slow approach to the water gas equilibrium from the carbon monoxide-water side; and ( b ) marked enrichment of the oxygen atom concentration in the radical pool as the hydrogen content of the flames is decreased. In the flames containing only traces of hydrogen, the degree of enrichment is markedly influenced by reaction (xxii).