Guy-Marie Côme

Computer-Aided Derivation of Gas-Phase Oxidation Mechanisms: Application to the Modeling of the Oxidation of n-Butane

This paper describes a system that permits the computer-aided formulation of comprehensive primary mechanisms and simplified secondary mechanisms, coupled with the relevant thermochemical and kinetic data in the case of the gas-phase oxidation of alkanes and ethers. This system has been demonstrated by modeling the oxidation of n-butane at temperatures between 554 and 737 K, i.e., in the negative temperature coefficient regime, and at a higher temperature of 937 K. The system yields satisfactory agreement between the computed and the experimental values for the rates, the induction period and conversion, and also for the distribution of the products formed.

Computer based generation of reaction mechanisms for gas-phase oxidation

This paper describes EXGAS, an advanced software for the automatic generation of reaction mechanisms. It has been developed to model the gas-phase oxidation of some components of gasoline, alkanes and ethers. The chemistry involved in these validated mechanisms relies both on a reaction base for some particular species and for the largest part on generic elementary reactions, which are well known for the oxidation of hydrocarbons. The programming of this system is mainly based on a referenced canonical treelike description of molecules and free radicals and can handle both acyclic and cyclic compounds. Mechanisms are generated in a way to ensure their comprehensiveness. Chemical models, which can directly be used by codes of simulations, are obtained as a result.

Construction and simplification of a model for the oxidation of alkanes

This paper analyzes some of the chemical and kinetic principles that rule the automatic generation of reaction mechanisms by the system EXGAS for the oxidation of alkanes. The systematic inclusion in the mechanism of all possible reaction pathways, in comparison with other models published, permits one to discuss the role of the different classes of reactions and to deduce some methods for simplifying a priori these mechanisms from a kinetic basis. The first technique is based on an analysis of the reactivity of free radicals and of the relative rates of generic reactions versus temperature. This analysis of reaction rates allows us to discuss the temperature ranges where it is necessary to consider one or two successive additions of an oxygen molecule to a hydrocarbon radical when modeling the oxidation of alkanes. A method to decrease the number of species and reactions deriving from the second addition of oxygen is also reported. Substantial reductions of the number of reactions and species included in the primary mechanism can be attained by these techniques. The validity of the simplified mechanisms obtained by these methods is illustrated with some examples derived from the modeling of the oxidation of n-heptane and n-decane.

Computer-aided design of gas-phase oxidation mechanisms—Application to the modeling of n-heptane and iso-octane oxidation

This paper first describes a computer package that permits the automatic generation of detailed oxidation and combustion kinetic models in the case of paraffins and isoparaffins. The system, which provides kinetic models in a CHEMKIN II format, includes the following: u ⊙ A reaction base for small free radicals and molecules having fewer than three carbon atoms. ⊙ A generator of detailed and comprehensive primary mechanisms. ⊙ A generator of lumped secondary reactions of the lumped primary products. ⊙ Computerized thermochemical and kinetic databases that provide data by means of the thermochemical kinetics techniques, as well as by using quantitative structure-reactivity relationships. The system was then applied to the generation of kinetic models of the oxidation of n-heptane and isooctane. The predictions of the models were compared with experimental results obtained by means of perfectly stirred reactors both in the high temperature range (950–1150 K, 1 atm) and in the low temperature range (600–850 K, 10 atm), which includes the negative temperature coefficient area. The agreement between the computed and the experimental values is correct both for conversions and for the distribution of the products formed, considering that no fitting of any kinetic parameter was done.

Experimental and modeling study of the gas-phase oxidation of methyl and ethyl tertiary butyl ethers

The gas-phase oxidation of methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) has been experimentally studied using a jet-stirred reactor between 750 and 1150 K (pressure of 10 atm, equivalence ratios from 0.5 to 2 with an important dilution in nitrogen). These experiments have been modeled using a kinetic mechanism automatically generated by EXGAS, the system developed in Nancy. The modeling of the oxidation of several mixtures of these ethers with n-heptane has also been performed in an extended temperature range, between 580 and 1100 K, covering the regions of cool flames and a negative temperature coefficient. The agreement between the computed and the experimental values is mostly good, both for conversions and for the distribution of major products, except for the lowest temperatures, where catalytic effects should be taken into account. The decrease of reactivity due to the addition of MTBE or ETBE to n-heptane at low temperatures is well predicted by the model.

Modeling of the gas-phase oxidation of n-decane from 550 to 1600 K

To improve the performances of diesel engines and to reduce the emission of pollutants at their outlet, it is necessary to be able to model the combustion and the oxidation of higher alkanes. Up to now, only a few detailed kinetic mechanisms were written for modeling the combustion of alkanes higher than n -heptane and iso -octane and even fewer for modeling their oxidation at low temperature in the cool flame region or in the negative temperature coefficient (NTC) regime. This paper presents a modeling study of the oxidation and combustion of n -decane in a range of temperatures, from 550 to 1600 K, aiming at reproducing experiments performed in a jet-stirred reactor and in a premixed laminar flame. The study covered an important part of the wide range of temperatures that is observed in engines. It is worth noting that n -decane is actually present in diesel fuel. Detailed kinetic mechanisms have been automatically generated by using the computer package EXGAS developed in our laboratory. The predictions of the mechanisms were compared to the experimental results without any adjustment of kinetic data. The mechanism used for simulation at low temperature included 7920 reactions. A satisfactory agreement was obtained for the two kinds of experimental apparatus, both for the consumption of reactants and for the formation of most products. In the flame, the formation of pollutants, such as unsaturated compounds, was well reproduced. In the perfectly stirred reactor, a flow rate (flux) analysis at 650 K in the cool flame region showed a scheme of reaction close to that of n -heptane. Nevertheless, the higher reactivity of n -decane compared with that of lower linear alkanes such as n -heptane seems to be due not only to faster metathesis reactions favored by additional secondary abstractable atoms of hydrogen, but also to a lower relative flow rate of oxidations giving alkenes and the very unreactive HO 2 radicals. The long linear chain favors internal isomerizations and then reduces the relative flow rates of reactions competing with the addition of oxygen.

A linear chemical notation

Abstract A linear notation for chemical compounds is proposed which allows input of chemical formulae into the computer from standard devices. The notation is unambiguous, but not canonical. This leads to a simple grammar which is easy to learn to read or to write.

A topological method for determining the external symmetry number of molecules

Abstract A method for determining the external symmetry number of molecules and free radicals is described. The method of calculation is based on a topological approach, mainly by means of a factorized representation of molecules and free radicals. The input of formulae into the computer is achieved by a linear notation. The corresponding program is a part of a more important software dealing with the computation of thermochemical properties of molecules and free radicals in the gas phase.

Detailed mechanism of the oxidative coupling of methane

To determine the relative importance of gas-phase and surface reactions in the oxidative coupling of methane (OCM), experimental investigations were performed. Our experimental results were compared to simulated values derived from a kinetic model taking into account heterogeneous and gas-phase reactions. We propose an original approach derived from Bensons techniques to estimate the kinetic parameters of surface reactions.

Kinetic study of the Chddative Coupling of Methane in a Catalytic Jet Stirred Reactor

It is now well-established that the oxidative coupling of methane is a homogeneous-heterogeneous reaction. Lunsford [1] has been a pioneer in this field, showing that methyl radicals are produced on the surface of the catalyst, released and coupled in the gas phase. However, the question of interaction between the homogeneous and heterogeneous processes is not completely understood. In order to obtain more information on this reaction, a new type of reactor was developed, which contains a large well-stirred gas phase volume in contact with catalyst pellets laid on the bottom of the reactor. The chosen catalyst is lanthanum oxide because it has a high stability and activity. This reactor permits an investigation of the influence of the usual parameters: space time, temperature, ratio of reactants, dilution, …; it also makes possible the modification of the relative importance of the gas phase and surface reactions by varying the number of catalyst pellets (i. e. the active catalyst surface) with a fixed gas phase volume. It is also possible to choose different operating temperatures for the catalyst and for the gas phase. The effect of these new parameters has been investigated at low conversion for the major part of this study, in order to limit as far as possible any change in the reaction due to the consumption of reactants.