Modeling Study of the Low-Temperature Oxidation of Large Methyl Esters

This study focuses on the automatic generation by the software EXGAS of kinetic models for the oxidation of large methyl esters using a single set of kinetic parameters. The obtained models allow to well reproduce the oxidation of n-decane / methyl palmitate mixture in a jet-stirred reactor. This paper also presents the construction and a comparison of models for methyl esters from C7 up to C17 in terms f conversion in a jet-stirred reactor and of ignition delay time in a shock tube. This comparison study showed that methyl esters larger than methyl octanoate behave similarly and have very close reactivities.Comment: European Combustion Meeting 2009, Vienne : Autriche (2009

First Study of the Pyrolysis of a Halogenated Ester: Methyl Chloroacetate

International audienceThe pyrolysis of a halogenated ester, methyl chloroacetate (MC), under dilute atmosphere and quasi-atmospheric pressure was studied at temperatures from 473 to 1048 K using an alumina tubular reactor. MC was chosen as a surrogate to model the thermal decomposition of ethyl bromoacetate, a chemical warfare agent. A maximum MC conversion of 99.8% was observed at a residence time of 2 s, a temperature of 1048 K, and an inlet mole fraction of 0.01. The following products were quantified: CO, CO2, HCl, methane, ethylene, ethane, propene, chloromethane, dichloromethane, vinyl chloride, chloroethane, and dichloroethane. For the first time, a detailed kinetic model of MC pyrolysis was developed and gave a good prediction of the global reactivity and the formation of most of the major products. Flow rate and sensitivity analyses were made to highlight the different pathways of decomposition during the MC pyrolysis. In a first attempt to extrapolate the results obtained with methyl chloroacetate to ethyl bromoacetate, simulations were run with a modified version of the model developed in this study taking into account the differences in bond dissociation energies induced by the change of the chlorine atom by a bromine one

The role of chemistry in the oscillating combustion of hydrocarbons : an experimental and theoretical study

The stable operation of low-temperature combustion processes is an open challenge, due to the presence of undesired deviations from steady-state conditions: among them, oscillatory behaviors have been raising significant interest. In this work, the establishment of limit cycles during the combustion of hydrocarbons in a wellstirred reactor was analyzed to investigate the role of chemistry in such phenomena. An experimental investigation of methane oxidation in dilute conditions was carried out, thus creating quasi-isothermal conditions and decoupling kinetic effects from thermal ones. The transient evolution of the mole fractions of the major species was obtained for different dilution levels (0.0025 <= X-CH4 <= 0.025), inlet temperatures (1080K <= T <= 1190K) and equivalence ratios (0.75 <= Phi <= 1). Rate of production analysis and sensitivity analysis on a fundamental kinetic model allowed to identify the role of the dominating recombination reactions, first driving ignition, then causing extinction. A bifurcation analysis provided further insight in the major role of these reactions for the reactor stability. One-parameter continuation allowed to identify a temperature range where a single, unstable solution exists, and where oscillations were actually observed. Multiple unstable states were identified below the upper branch, where the stable (cold) solution is preferred. The role of recombination reactions in determining the width of the unstable region could be captured, and bifurcation analysis showed that, by decreasing their strength, the unstable range was progressively reduced, up to the full disappearance of oscillations. This affected also the oxidation of heavier hydrocarbons, like ethylene. Finally, less dilute conditions were analyzed using propane as fuel: the coupling with heat exchange resulted in multiple Hopf Bifurcations, with the consequent formation of intermediate, stable regions within the instability range in agreement with the experimental observations

Thermal stability of n-dodecane : experiments and kinetic modelling

The thermal decomposition of n-dodecane, a component of some jet fuels, has been studied in a jet-stirred reactor at temperatures from 793 to 1093 K, for residence times between 1 and 5 s and at atmospheric pressure. Thermal decomposition of hydrocarbon fuel prior the entrance in the combustion chamber is an envisaged way to cool the wall of hypersonic vehicles. The products of the reaction are mainly hydrogen, methane, ethane, 1,3-butadiene and 1-alkenes from ethylene to 1-undecene. For higher temperatures and residence times acetylene, allene, propyne, cyclopentene, 1,3-cyclopentadiene and aromatic compounds from benzene to pyrene through naphthalene have also been observed. A previous detailed kinetic model of the thermal decomposition of n-dodecane generated using EXGAS software has been improved and completed by a sub-mechanism explaining the formation and the consumption of aromatic compounds

Study of oscillations during methane oxidation with species probing

Biogas has been considered as a renewable energy source with respect to fossil fuels due to its sustainability, security supply, and environmental friendly potential [1-4]. Methane occupies a large part in biogas. It is of great value to review the methane oxidation for a primary understanding of the features associated with biogas combustion. It was found that dynamic behavior in terms of methane oxidation occurred under specific conditions. The first methane oxidation oscillation experiments were conducted by [5] in a jet-stirred reactor (JSR) and were extended to a higher inlet temperature [6]. The map of dynamic behavior was drawn in terms of various C/O ratios and temperatures ranging from 1025-1275 K at a fixed 90% nitrogen bath gas. Recently, Lubrano Lavadera et al. [7] investigated the main parameters, such as, equivalence ratios (0.5-1.5), residence time (1.5-2 s), various bath gases (N2, CO2, He, H2O), on the oscillatory behavior of methane oxidation. However, to our best knowledge, studies of dynamic phenomenology with species probing have never been reported. Because of the heat release in terms of the exothermic or endothermic reactions, the temperature and species oscillations are strongly coupled during fuel oxidation. In order to put emphasis on species dynamic behavior, very diluted conditions are needed to decouple as much as possible temperature and species oscillations. The purpose of this work is to investigate the effects of various parameters: inlet mole fraction of methane (0.1-0.5%), stoichiometric condition (=1) and reactor temperatures (950-1200 K), on the species oscillations during methane oxidation. A detailed kinetic mechanism (POLIMI) [8] is selected to interpret the experimental data

An experimental and kinetic modeling study of NH3 oxidation in a Jet Stirred Reactor

The increasing interest towards renewable, and more sustainable energy sources imposes a widerange analysis of the underlying chemistry, in order to maximize the efficiency of combustion devices and reduce pollutant emissions. In this context, ammonia chemistry has recently gained major attention: it is present in biogas and bio-oil, in trace amounts. Investigating ammonia chemistry can benefit from several studies carried out in the past decades on its pyrolysis and oxidation behavior. However, scarce literature is available on the conditions of interest previously mentioned, since the presence of ammonia in trace amounts results in superoxidative conditions. The available kinetic models of ammonia have been built up by mostly relying on hightemperature data, obtained in ideal reactors. On the other side, few work has been carried out to investigate its oxidation at lower temperatures. In order to further investigate this topic, and to provide a stronger support for kinetic model validation, in this study the oxidation of ammonia in diluted conditions, at relatively low temperatures (T < 1200 K) and a pressure close to atmospheric, is investigated by using a Jet Stirred Reactor. In addition to ammonia conversion, the formation of Nitrogen Oxides (NOx) is also analyzed. At the same time, a detailed kinetic mechanism for ammonia oxidation is developed by leveraging the most recently available kinetic data on experimental and theoretical reaction rates, and is used to analyze the obtained data, after being validated against the literature data in similar conditions