R. Minetti

A rapid compression machine investigation of oxidation and auto-ignition of n-heptane: measurements and modeling

n-Heptane oxidation and auto-ignition in a rapid compression machine is studied in the low and intermediate temperature regimes at high pressures. Experimental ignition delay times and some phenomenological aspects related to knock in engines are presented, providing additional information at lower temperatures on previously published delays from shock tube experiments. The products of oxidation are identified and time profiles are measured during a two-stage ignition process. Eight C7 heterocycles, heptenes, lower 1-alkenes, aldehydes, and carbon monoxide are the main species. Their origin is discussed in relation to the isomerization and decomposition of heptylperoxy radicals. The high selectivity observed in the formation of lower 1-alkenes is explained by the scission of the β CC bond of the β-hydroperoxyheptyl radicals weakened by the presence of oxygen atoms. Numerical simulation of the experiments with Warnatzs comprehensive chemical mechanism gives satisfactory results for cool flame and total ignition delays, but fails to reproduce the detailed chemistry before auto-ignition.

Oxidation and combustion of low alkylbenzenes at high pressure: comparative reactivity and auto-ignition

Abstract The auto-ignition features of 11 alkylbenzenes in a rapid compression machine have been compared for stoichiometric mixtures in the lower temperature region (600–900 K), and at compressed pressures up to 25 bar, by following pressure traces and light emission. They are classified in two groups. Toluene, m -xylene, p -xylene, and 1,3,5-trimethylbenzene ignite only above 900 K and 16 bar. o -Xylene, ethylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, n -propylbenzene, 2-ethyltoluene, and n -butylbenzene ignite at much lower temperature and pressure. The second group shows a complex phenomenology similar to alkanes and alkenes when submitted to adapted conditions of reactant concentrations. Ignition in two steps and negative temperature dependence of ignition delays are observed in favorable cases. Some of them show a low-temperature luminescence. Ignition features of o -xylene and n -butylbenzene are similar, in spite of their dissimilar molecular structure. The higher degree of reactivity of the second group is ascribed to the close proximity and/or length of their alkyl chains.

Experimental and Modeling Study of Oxidation and Autoignition of Butane at High Pressure

Oxidation and autoignition of stoichiometric, lean (ϕ = 0.8), and rich (ϕ = 1.2) butane-“air” mixtures are studied in a rapid compression machine between 700–900 K and 9–11 bar. Information is obtained concerning cool flames and ignition delays. Product profiles for selected major and minor species are measured during a two-stage ignition process. The presence of C4 heterocycles may be connected to isomerization and decomposition of butylperoxy radicals. The experimental results are compared with numerical predictions of an homogeneous adiabatic model based on the Pitz-Westbrook comprehensive chemical mechanism of 1990. The experimental and predicted delays are in the same order of magnitude. A relatively good agreement is found for the major species profiles. Improvement of the mechanism is needed to account for the minor products. The different paths of OH formation are discussed.

The production of benzene in the low-temperature oxidation of cyclohexane, cyclohexene, and cyclohexa-1,3-diene

Abstract The oxidation and auto-ignition of cyclohexane, cyclohexene, and cyclohexa-1,3-diene have been studied by rapid compression between 600 K to 900 K and 0.7 MPa to 1.4 MPa to identify the low-temperature pathways leading to benzene from cyclohexane. Auto-ignition delay times were measured and concentration-time profiles of the C 6 intermediate products of oxidation were measured during the auto-ignition delays. Cyclohexane showed two-stage ignition at low temperatures, but single-stage ignition at higher temperatures, and a well-marked negative-temperature coefficient. By contrast there was neither a cool flame, nor a negative-temperature coefficient for cyclohexa-1,3-diene. Cyclohexene behaved in an intermediate way without a cool flame, but with a narrow, not very marked negative-temperature coefficient. The identified C 6 products belong to three families: the bicyclic epoxides and cyclic ketones, the unsaturated aliphatic aldehydes, and the conjugated alkenes, which are always the major products. The formation of C 6 products from cyclohexane is explained by the classical scheme for low-temperature oxidation, taking into account addition of O 2 to cyclohexyl radicals and the various isomerizations of the resulting peroxy radicals. Most of the C 6 products from cyclohexene are predicted by the same scheme, beginning with the formation of the allylic cyclohexenyl radical. However, addition of HO 2 to the double bond has to be included to predict the formation of 1,2-epoxycyclohexane. For cyclohexa-1,3-diene, the classical scheme is not valid: the C 6 oxygenated products are only formed by addition of HO 2 to the double bond. For all three hydrocarbons, the pathways to benzene are those leading to conjugated alkenes, and they are always more efficient than those producing oxygenated products, either by adding HO 2 to double bonds, or by addition of O 2 to the initial cyclic radical.

High pressure auto-ignition and oxidation mechanisms of o-xylene, o-ethyltoluene, and n-butylbenzene between 600 and 900 K

Abstract A complex phenomenology of auto-ignition, similar to n -alkanes and n -alkenes, has been revealed between 600–900 K and at pressures above 14 bar by studying in a rapid compression machine stoichiometric mixtures of o -xylene ( o -methyltoluene), o -ethyltoluene, or n -butylbenzene in oxygen with lower concentrations than in air. Extensive chemical analyses of the reacting mixtures before ignition were performed to elucidate the mechanisms of reaction. The classical low temperature scheme, modified for the reactivities of benzylic-type hydrogen atoms and radicals, is valid. It appears that the addition of molecular oxygen to benzylic-type radicals leads to a double peroxidation and low temperature branching only when the transfer of hydrogen in the isomerization step occurs either from an ortho-alkyl group, or from another carbon atom of the same alkyl chain. The products observed are shown to be consistent with the proposed mechanism. The same complex pattern of auto-ignition is found, not only for o -xylene, o -ethyltoluene, and n -butylbenzene, but also for 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, n -propylbenzene, and ethylbenzene. When easily transferable hydrogen atoms are not available for selective radicals such as peroxy radicals, branching occurs through completely different pathways, which require higher temperatures and pressures. Then, the pattern of auto-ignition is much simpler, as already observed for toluene, m -xylene, p -xylene, and 1,3,5-trimethylbenzene [1] .

Autoignition Delays of a Series of Linear and Branched Chain Alkanes in the Intermediate Range of Temperature

A set of ignition data of linear and branched chain alkanes (n-butane, n-pentane, neopentane, n-heptane, and isooctane) measured in an original rapid compression machine is provided. It allows a comparison of the ignition conditions of pressure, temperature and equivalence ratio for these hydrocarbons. Detailed mechanisms from different research groups based on a similar generic scheme of hydrocarbon oxidation are tested against the experimental ignition delays. The differences between experimental and modeling results are discussed.

The low-temperature autoignition of alkylaromatics: Experimental study and modeling of the oxidation of n-butylbenzene

The low-temperature oxidation of n -butylbenzene, an intermediate structure between alkanes and short-chain alkylaromatics, was studied between 640 and 840 K by rapid compression and by modeling. Delay times of one- and two-stage autoignitions were measured, and intermediate species after the cool flame were analyzed. First, a detailed mechanism for n -butane was developed with existing material. Then, an n -butylbenzene mechanism was built by taking into account the change of reactivity due to the introduction of the aromatic nucleus. Both mechanisms have been validated by simulations of the delays and the product concentrations. Finally, the n -butylbenzene mechanism was used to analyze the main low-temperature reaction pathways. The comparative calculation of the concentrations of alkyl, alkylperoxy, and hydroperoxyalkyl radicals in the cool flame of n -butane and n -butylbenzene illustrates the effects of the aromatic nucleus on the first steps of oxidation. A study of the competitive channels to the main molecular intermediate species shows that the internal transfer of a benzylic hydrogen to the peroxy sites is a major event in the development of reactions leading to branching and ignition. This can explain a previous observation that alkylaromatics with two oitho -alkyl groups or a long single lateral chain have the possibility of an internal transfer of a benzylic hydrogen and manifest a greater low-temperature reactivity than aromatics that have neither oitho -alkyl groups nor a long lateral chain.

A theoretical study of the kinetics of the benzylperoxy radical isomerization.

The rate constant of the benzylperoxy isomerization reaction has been computed using 54 different levels of theory and has been compared to the experimental value reported at 773 K. The aim of this methodology work is to demonstrate that standard theoretical methods are not adequate to obtain quantitative rate constants for the reaction under study. The use of the elaborated CASPT2 method is essential to estimate a quantitative rate constant. Geometry optimizations and vibrational frequency calculations are performed using three different methods (B3LYP, MPW1K, and MP2) and six different basis sets (6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), and cc-pVDZ). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the 6-31G(d,p) basis set, and with the CASPT2 level of theory with the ANO-L-VDZP basis set. Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. We recommend the use of the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory to compute the temperature dependence of the rate constant of the four-center isomerization of the benzylperoxy radical. It is given by the following relation: k(600-2000 K) (in s (-1)) = (1.29 x 10 (10)) T (0.79) exp[(-133.1 in kJ mol (-1))/ RT]. These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. This computational procedure can be extended to predict rate constants for other similar reactions where no available experimental data exist.

On the reactivity of hydroperoxy radicals and hydrogen peroxide in a two-stage butane-air flame☆

Abstract n -Butane autoignition chemistry was studied in a two-stage butane-air flame stabilized on a special flat flame burner at 1.8 bar and an inlet temperature of 670 K. HO 2 , H 2 O 2 , major molecular products, and temperature profiles have been obtained. The rate constants of HO 2 reactions that are reviewed enable us to calculate the reaction rates of HO 2 and H 2 O 2 in the flame. The main reactions for producing H 2 O 2 have been identified as the bimolecular recombination of HO 2 and hydrogen abstraction from butane. The destruction of H 2 O 2 proceeds principally via radical abstraction except in the high-temperature region where the homogeneous decomposition becomes an important step to convert the HO 2 radical into the more reactive OH radical.

Influence of N-methylaniline on a two-stage butane-air flame

Abstract The influence of N-methylaniline (NMA) on flame stability and structure of a two-stage butane-air flame is studied. The stability of the flame system is reduced as increasing amount of the inhibitor are added, particularly at lower pressures. At 1.8 bar and a low level of NMA (0.3% of butane) the structure of the two-stage butane-air flame, which was described in a previous communication with a particular emphasis on the H2O2/HO2 chemistry, is slightly modified. Among the 18 chemical species profiles studied, the hydrogen peroxide and peroxy radicals profiles are most sensitive to inhibition. The second stage ignition is particularly affected as it results from balanced concentration of the branching agent H2O2 controlled by the inhibitor. At higher concentration of NMA, the inhibition is more important, as revealed by the lifted positions of the flames.