Valérie Warth

An experimental and kinetic modeling study of the oxidation of the four isomers of butanol.

Butanol, an alcohol which can be produced from biomass sources, has received recent interest as an alternative to gasoline for use in spark ignition engines and as a possible blending compound with fossil diesel or biodiesel. Therefore, the autoignition of the four isomers of butanol (1-butanol, 2-butanol, iso-butanol, and tert-butanol) has been experimentally studied at high temperatures in a shock tube, and a kinetic mechanism for description of their high-temperature oxidation has been developed. Ignition delay times for butanol/oxygen/argon mixtures have been measured behind reflected shock waves at temperatures and pressures ranging from approximately 1200 to 1800 K and 1 to 4 bar. Electronically excited OH emission and pressure measurements were used to determine ignition-delay times. The influence of temperature, pressure, and mixture composition on ignition delay has been characterized. A detailed kinetic mechanism has been developed to describe the oxidation of the butanol isomers and validated by comparison to the shock-tube measurements. Reaction flux and sensitivity analysis illustrates the relative importance of the three competing classes of consumption reactions during the oxidation of the four butanol isomers: dehydration, unimolecular decomposition, and H-atom abstraction. Kinetic modeling indicates that the consumption of 1-butanol and iso-butanol, the most reactive isomers, takes place primarily by H-atom abstraction resulting in the formation of radicals, the decomposition of which yields highly reactive branching agents, H atoms and OH radicals. Conversely, the consumption of tert-butanol and 2-butanol, the least reactive isomers, takes place primarily via dehydration, resulting in the formation of alkenes, which lead to resonance stabilized radicals with very low reactivity. To our knowledge, the ignition-delay measurements and oxidation mechanism presented here for 2-butanol, iso-butanol, and tert-butanol are the first of their kind.

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.

Towards cleaner combustion engines through groundbreaking detailed chemical kinetic models

In the context of limiting the environmental impact of transportation, this critical review discusses new directions which are being followed in the development of more predictive and more accurate detailed chemical kinetic models for the combustion of fuels. In the first part, the performance of current models, especially in terms of the prediction of pollutant formation, is evaluated. In the next parts, recent methods and ways to improve these models are described. An emphasis is given on the development of detailed models based on elementary reactions, on the production of the related thermochemical and kinetic parameters, and on the experimental techniques available to produce the data necessary to evaluate model predictions under well defined conditions (212 references).

MODELING OF THE OXIDATION OF N-OCTANE AND N-DECANE USING AN AUTOMATIC GENERATION OF MECHANISMS

Detailed modeling of the oxidation of n-octane and n-decane in the gas phase was performed by using mechanisms written by means of a software recently developed in our laboratory. This computer-aided design of mechanisms permits the automatic generation of detailed oxidation and combustion kinetic models in the case of paraffins and isoparaffins [1]. For n-octane, the predictions of the model were compared with experimental results obtained by Dryer and Brezinsky by means of a turbulent plug flow reactor (1080 K, 1 atm) [2]. The experimental study of Bales–Gueret et al., performed in a perfectly stirred reactor (922–1033 K, 1 atm) [3], was used as a basis of comparison for the modeling of the oxidation of n-decane. Considering that no fitting of any kinetic parameter was done, the agreement between the computed and the experimental values is satisfactory both for conversions and for the distribution of the products formed. This modeling has required improvement in the generation of the secondary reactions of alkenes, which are the main primary products obtained during the oxidation of these two alkanes in the range of temperature studied and for which reaction paths are detailed.

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.

Comprehensive mechanism for the gas-phase oxidation of propene

Abstract This paper presents a first test of the extension of our computer code EXGAS to the generation of detailed mechanisms for the oxidation and combustion of alkenes. In the first part, an analysis of the elementary reactions from the literature allowed us to define new specific generic reactions involving alkenes and their free radicals, as well as correlations to estimate the related rate constants. The corresponding generic rules were then implemented in the EXGAS code. The second part, a mechanism for the oxidation of propene involving 262 species and including 1295 reactions was generated by EXGAS. The predictions of this mechanism were compared, without any change of the best available kinetic data, with two sets of experimental measurements: the first obtained in a static vessel between 580 K and 740 K; the second used a jet-stirred reactor between 900 K and 1200 K. If one takes into account that no fitting of individual rate constants was done, the mechanism reproduces correctly both the negative temperature coefficient (NTC) observed at ≈630 K and the variations of the concentrations with residence time of C 3 H 6 , CO, CO 2 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 2 , C 3 H 4 , HCHO, CH 3 CHO, C 2 H 3 CHO, and cyclic ethers (C 3 H 6 O), especially the general shape of these curves and their minima, maxima, and inflection points. Flux and sensitivity analyses were performed to get insight into the kinetic structure of the mechanism explaining the observed characteristics, such as the NTC or the autocatalytic behavior of the reaction. At low temperatures, these analyses showed that the NTC is mainly due to the reversibility of the addition to oxygen of the adducts, ·C 3 H 6 OH, which via a mechanism similar to that of alkyl radicals and involving two additions to oxygen, yields degenerate branching agents. At high temperatures, in both kind of reactor, the determining role of termination reactions involving the very abundant allyl radicals has been emphasized, especially the recombination of allyl and hydroperoxyalkyl radicals, which is the main source of acrolein.