Anthony M. Dean

Combustion Chemistry of Nitrogen

The purpose of this chapter is to examine reactions of nitrogen-containing species that are important in high-temperature gas-phase systems so as to provide the best set of rate coefficients presently available for use in combustion chemistry modeling. Since the 1984 review of N/H/O rate coefficients by Hanson and Salimian 1984 there has been a major review of nitrogen chemistry under combustion conditions by Miller and Bowman (1989). Several compilations of evaluated rate coefficients have also appeared. We update these discussions where appropriate and then analyze a number of chemically activated reactions that are relevant to understanding nitrogen chemistry.

High-pressure rate rules for alkyl + O2 reactions. 1. The dissociation, concerted elimination, and isomerization channels of the alkyl peroxy radical.

The reactions of alkyl peroxy radicals (RO(2)) play a central role in the low-temperature oxidation of hydrocarbons. In this work, we present high-pressure rate estimation rules for the dissociation, concerted elimination, and isomerization reactions of RO(2). These rate rules are derived from a systematic investigation of sets of reactions within a given reaction class using electronic structure calculations performed at the CBS-QB3 level of theory. The rate constants for the dissociation reactions are obtained from calculated equilibrium constants and a literature review of experimental rate constants for the reverse association reactions. For the concerted elimination and isomerization channels, rate constants are calculated using canonical transition state theory. To determine if the high-pressure rate expressions from this work can directly be used in ignition models, we use the QRRK/MSC method to calculate apparent pressure and temperature dependent rate constants for representative reactions of small, medium, and large alkyl radicals with O(2). A comparison of concentration versus time profiles obtained using either the pressure dependent rate constants or the corresponding high-pressure values reveals that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2).

Direct Detection of Products from the Pyrolysis of 2-Phenethyl Phenyl Ether

The pyrolysis of 2-phenethyl phenyl ether (PPE, C(6)H(5)C(2)H(4)OC(6)H(5)) in a hyperthermal nozzle (300-1350 °C) was studied to determine the importance of concerted and homolytic unimolecular decomposition pathways. Short residence times (<100 μs) and low concentrations in this reactor allowed the direct detection of the initial reaction products from thermolysis. Reactants, radicals, and most products were detected with photoionization (10.5 eV) time-of-flight mass spectrometry (PIMS). Detection of phenoxy radical, cyclopentadienyl radical, benzyl radical, and benzene suggest the formation of product by the homolytic scission of the C(6)H(5)C(2)H(4)-OC(6)H(5) and C(6)H(5)CH(2)-CH(2)OC(6)H(5) bonds. The detection of phenol and styrene suggests decomposition by a concerted reaction mechanism. Phenyl ethyl ether (PEE, C(6)H(5)OC(2)H(5)) pyrolysis was also studied using PIMS and using cryogenic matrix-isolated infrared spectroscopy (matrix-IR). The results for PEE also indicate the presence of both homolytic bond breaking and concerted decomposition reactions. Quantum mechanical calculations using CBS-QB3 were conducted, and the results were used with transition state theory (TST) to estimate the rate constants for the different reaction pathways. The results are consistent with the experimental measurements and suggest that the concerted retro-ene and Maccoll reactions are dominant at low temperatures (below 1000 °C), whereas the contribution of the C(6)H(5)C(2)H(4)-OC(6)H(5) homolytic bond scission reaction increases at higher temperatures (above 1000 °C).

High-pressure rate rules for alkyl + O2 reactions. 2. The isomerization, cyclic ether formation, and β-scission reactions of hydroperoxy alkyl radicals.

The unimolecular reactions of hydroperoxy alkyl radicals (QOOH) play a central role in the low-temperature oxidation of hydrocarbons as they compete with the addition of a second O(2) molecule, which is known to provide chain-branching. In this work we present high-pressure rate estimation rules for the most important unimolecular reactions of the β-, γ-, and δ-QOOH radicals: isomerization to RO(2), cyclic ether formation, and selected β-scission reactions. These rate rules are derived from high-pressure rate constants for a series of reactions of a given reaction class. The individual rate expressions are determined from CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. Next we use the rate rules, along with previously published rate estimation rules for the reactions of alkyl peroxy radicals (RO(2)), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of n-butyl radical with O(2) by comparison of concentration versus time profiles that were obtained using two mechanisms at 10 atm: one that contains pressure-dependent rate constants that are obtained from a QRRK/MSC analysis and another that only contains high-pressure rate expressions. These simulations reveal that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2) and QOOH. For the same conditions, we also address whether the various isomers equilibrate during reaction. These results indicate that equilibrium is established between the alkyl, RO(2), and γ- and δ-QOOH radicals.

CHEMACT: A Computer Code to Estimate Rate Constants for Chemically-Activated Reactions

Abstract CHEMACT is a computer code that uses the QRRK treatment of chemical activation reactions to estimate apparent rate constants for the various channels that can result in addition, recombination and insertion reactions. The working equations are developed, the necessary input file is described, and sources of required input data are discussed. Three applications of the code to representative examples of reactions relevant to combustion are presented. These include C2H5 + O2, H + C6H5Cl, and CH3 + OH. These reactions illustrate some of the expected behavior with different types of chemical activation and demonstrate the necessity of explicitly accounting for the effects of chemical activation in high temperature gas-phase reactions. Methods by which these results can be included in detailed kinetic mechanisms are discussed.

Rate constant rules for the automated generation of gas-phase reaction mechanisms.

The capability of kinetic models to predict complex chemical systems has enormously improved in the last decades, making them an increasingly important tool for process development and optimization. Extension of these approaches to even more complex systems is hindered not only by the geometrically increasing number of reactions and species to be considered but also by the necessity of assigning accurate rate constants to all of the reactions. The recent developments in automated mechanism generators can address the tedious bookkeeping issues. The requirement for development of accurate rate constant estimates remains the job of the kineticist. This task has been aided immeasurably by the combined advances in electronic structure methods and computer performance. This article describes two areas of rate estimation. First, we discuss the development of H abstraction rate estimates from C-H bonds in alkanes, cycloalkanes, and allylic systems by H atoms and point to a surprising result found for cyclopentane. Second, we briefly review our investigation of the ethyl + O(2) reaction and demonstrate the suitability of the QRRK/MSC approach for automated mechanism generation. We conclude with some suggestions for future work in this area.

Laser induced fluorescence and absorption measurements of NO in NH3/O2 and CH4/air flames

Laser diagnostics have been used to probe NO in atmospheric pressure flames. Laser induced fluorescence techniques (LIF) were used to measure relative concentration profiles of NO at fuel equivalence ratios φ=1.28, 1.50, and 1.81 in NH3/O2/N2 flames and φ=1.7 and 1.8 in CH4/air/O2 flames. Laser absorption measurements were made to derive an absolute concentration of NO in a lean NH3/O2/N2 flame. This measured NO concentration agreed well with the calculated equilibrium concentration. The fluorescence signals from rich flames were then calibrated by comparing the fluorescence signals to that of the lean flame where absolute concentrations were derived. In rich NH3/O2/N2 flames NO concentrations decay more rapidly throughout the burnt gases than one would expect from the conventional mechanism of ammonia oxidation. This suggests that new reactions such as NH2+NH2 and NH+NH2 to ultimately yield N2 are important in these rich flames. LIF measurements on the CH4/air/O2 flames were able to resolve the growth an...

Detailed Modeling of Low-Temperature Propane Oxidation: 1. The Role of the Propyl + O2 Reaction

Accurate description of reactions between propyl radicals and molecular oxygen is an essential prerequisite for modeling of low-temperature propane oxidation because their multiple reaction pathways either accelerate the oxidation process via chain branching or inhibit it by forming relatively stable products. The CBS-QB3 level of theory was used to construct potential energy surfaces for n-C(3)H(7) + O(2) and i-C(3)H(7) + O(2). High-pressure rate constants were calculated using transition state theory with corrections for tunneling and hindered rotations. These results were used to derive pressure- and temperature-dependent rate constants for the various channels of these reactions under the framework of the Quantum Rice-Ramsperger-Kassel (QRRK) and the modified strong collision (MSC) theories. This procedure resulted in a thermodynamically consistent C(3)H(7) + O(2) submechanism, which was either used directly or as part of a larger extended detailed kinetic mechanism to predict the loss of propyl and the product yields of propylene and HO(2) over a wide range of temperatures, pressures, and residence times. The overall good agreement between predicted and experimental data suggests that this reaction subset is reliable and should be able to properly account for the reactions of propyl radicals with O(2) in propane oxidation. It is also demonstrated that for most conditions of practical interest only a small subset of reactions (e.g., isomerization, concerted elimination of HO(2), and stabilization) controls the oxidation kinetics, which makes it possible to considerably simplify the mechanism. Moreover, we observed strong similarities in the rate coefficients within each reaction class, suggesting the potential for development of relatively simple rate constant estimation rules that could be applied to analogous reactions involving hydrocarbon radicals that are too large to allow accurate detailed electronic structure calculations.

Laser absorption measurements of OH, NH, and NH2 in NH3/O2 flames: Determination of an oscillator strength for NH2

We have used laser diagnostics to probe atmospheric pressure ammonia–oxygen flames. Absorption from a tunable dye laser was used to measure concentration profiles of OH, NH, and NH2 radicals at fuel equivalence ratios φ of 1.28, 1.50, and 1.81. The absolute concentrations of OH and NH and the product of the NH2 concentration and the NH2 oscillator strength were derived as a function of height above the flat flame burner. These measurements indicate that the reaction NH2+OH?NH+H2O is equilibrated near the flame front, as well as in the post flame region. It was possible to use these measurements to derive an oscillator strength fi = (2.04±0.44)×10−4 for the PQ1,7 line in the (0,9,0)←(0,0,0) vibrational band of the A 2A1←X 2B1 transition of NH2. Rotational temperatures of OH were obtained from absorption measurements on a variety of rotational lines. These temperatures suggest the possibility of excess rotational energy in OH in the flame front regions of the φ = 1.28 and φ = 1.50 flames.

The numerical solution of some kinetics models with VODE and CHEMKIN II

Abstract We describe some numerical experiments we have performed in solving chemical kinetics models with CHEMKIN II and two high quality ordinary differential equations (ODE) packages—VODE and LSODE. For this mode of kinetics modeling, we give performance figures for these codes and for three chemical systems of varying size and behavior. In all cases, the Jacobian matrix is internally computed by the ODE solver, owing to the complexity of the models. Although these results are somewhat more conservative than others we have obtained, they do show that significant performance improvement can be obtained by using VODE instead of LSODE for these problems. We believe that the problems are typical of those that one of us (AMD) has been working with for several years. The main objective of this report is to make kineticists and other practitioners aware of VODE and its potential advantages in such applications. Performance results are given. For example, a small mechanism (17 species and 31 reactions) required 0.48 s CPU time with VODE vs 0.83 s with LSODE. A larger mechanism (135 species and 497 reactions) showed a more dramatic difference: 60 s CPU time for VODE vs 213 s with LSODE.