Étude expérimentale et modélisation cinétique de la combustion de composés modèles de biocarburants : Détermination d'intermédiaires par spectrométrie de masse à photo-ionisation par rayonnement synchrotron

With the depletion of fossil fuel resources, the search for new alternatives such as biofuels produced from biomass has intensified. A wide variety of compounds can be produced by treating this biomass. Ethers are one of them and can be used as additives to conventional fuels or as fuel. To this end, it is necessary to have a better knowledge of the kinetics of the oxidation of these compounds which differ from conventional fuels by the presence of oxygen atoms in their structure, making the chemistry more complex. This thesis aims to develop kinetic mechanisms of the oxidation of simple ethers based on experimental data obtained in jet stirred reactor. The oxidation of four ethers at atmospheric pressure and high pressure (10 atm) was studied over a wide range of temperatures (450 - 1250 K) under different equivalence ratios (0.5 to 4). The mole fraction profiles of the reactants, products of combustion and main stable intermediates were measured by gas chromatography, infra-red spectrometry or photoionization mass spectrometry using synchrotron radiation. The results thus obtained then served as a basis for validating kinetic mechanisms modeling the oxidation of these species whose agreement with the measurements is globally satisfactory for compounds for which few studies are available.Avec l’épuisement des ressources en combustible fossile, la recherche de nouveaux carburants alternatifs tels que les biocarburants produits à partir de biomasse s’est intensifiée. Une grande variété de composés peut être produite par traitement de cette biomasse. Les éthers en font partie et peuvent être utilisés comme additifs aux carburants classiques ou comme carburant. À cette fin, il faut disposer d’une meilleure connaissance de la cinétique chimique d’oxydation de ces composés qui diffèrent des carburants conventionnels par la présence d’atomes d’oxygène dans leur structure, rendant la chimie plus complexe. Cette thèse a pour objectif de développer des mécanismes cinétiques d’oxydation d’éthers simples à partir de données expérimentales obtenues en réacteur auto-agité par jets gazeux. L’oxydation de quatre éthers à pression atmosphérique et haute pression (10 atm) a été étudiée sur un large domaine de température (450 - 1250 K) dans différentes conditions de richesse de mélange (0,5 à 4). Les profils de fraction molaire des réactifs, produits de combustion et intermédiaires réactionnels stables ont été mesurés par chromatographie en phase gazeuse, spectrométrie infra-rouge ou spectrométrie de masse à photo-ionisation par rayonnement synchrotron. Les résultats ainsi obtenus ont ensuite servi de base pour valider des mécanismes cinétique modélisant l’oxydation de ces espèces dont l’accord avec les mesures est globalement satisfaisant pour des composés pour lesquels peu d’études sont disponibles

Experimental study and kinetic modeling of the combustion of biofuel : Determination of intermediates by photoionization mass spectrometry using synchrotron radiation

Avec l’épuisement des ressources en combustible fossile, la recherche de nouveaux carburants alternatifs tels que les biocarburants produits à partir de biomasse s’est intensifiée. Une grande variété de composés peut être produite par traitement de cette biomasse. Les éthers en font partie et peuvent être utilisés comme additifs aux carburants classiques ou comme carburant. À cette fin, il faut disposer d’une meilleure connaissance de la cinétique chimique d’oxydation de ces composés qui diffèrent des carburants conventionnels par la présence d’atomes d’oxygène dans leur structure, rendant la chimie plus complexe. Cette thèse a pour objectif de développer des mécanismes cinétiques d’oxydation d’éthers simples à partir de données expérimentales obtenues en réacteur auto-agité par jets gazeux. L’oxydation de quatre éthers à pression atmosphérique et haute pression (10 atm) a été étudiée sur un large domaine de température (450 - 1250 K) dans différentes conditions de richesse de mélange (0,5 à 4). Les profils de fraction molaire des réactifs, produits de combustion et intermédiaires réactionnels stables ont été mesurés par chromatographie en phase gazeuse, spectrométrie infra-rouge ou spectrométrie de masse à photo-ionisation par rayonnement synchrotron. Les résultats ainsi obtenus ont ensuite servi de base pour valider des mécanismes cinétique modélisant l’oxydation de ces espèces dont l’accord avec les mesures est globalement satisfaisant pour des composés pour lesquels peu d’études sont disponibles.With the depletion of fossil fuel resources, the search for new alternatives such as biofuels produced from biomass has intensified. A wide variety of compounds can be produced by treating this biomass. Ethers are one of them and can be used as additives to conventional fuels or as fuel. To this end, it is necessary to have a better knowledge of the kinetics of the oxidation of these compounds which differ from conventional fuels by the presence of oxygen atoms in their structure, making the chemistry more complex. This thesis aims to develop kinetic mechanisms of the oxidation of simple ethers based on experimental data obtained in jet stirred reactor. The oxidation of four ethers at atmospheric pressure and high pressure (10 atm) was studied over a wide range of temperatures (450 - 1250 K) under different equivalence ratios (0.5 to 4). The mole fraction profiles of the reactants, products of combustion and main stable intermediates were measured by gas chromatography, infra-red spectrometry or photoionization mass spectrometry using synchrotron radiation. The results thus obtained then served as a basis for validating kinetic mechanisms modeling the oxidation of these species whose agreement with the measurements is globally satisfactory for compounds for which few studies are available

On the similarities and differences between the products of oxidation of hydrocarbons under simulated atmospheric conditions and cool flames

International audienceAbstract. Atmospheric oxidation chemistry and, more specifically, photooxidation show that the long-term oxidation of organic aerosol (OA) progressively erases the initial signature of the chemical compounds and can lead to a relatively uniform character of oxygenated organic aerosol (OOA). This uniformity character observed after a long reaction time seems to contrast with the great diversity of reaction mechanisms observed in the early stages of oxidation. The numerous studies carried out on the oxidation of terpenes, and more particularly on limonene for its diversity of reaction sites (endo- and oxocyclic), allow this evolution to be studied. We have selected, for their diversity of experimental conditions, nine studies of limonene oxidation at room temperature over long reaction times to be compared to the present data set obtained at elevated temperature and short reaction time in order to investigate the similarities in terms of reaction mechanisms and chemical species formed. Here, the oxidation of limonene–oxygen–nitrogen mixtures was studied using a jet-stirred reactor at elevated temperature and atmospheric pressure. Samples of the reacting mixtures were collected and analyzed by high-resolution mass spectrometry (Orbitrap) after direct injection or after separation by reverse-phase ultra-high-pressure liquid chromatography and soft ionization, i.e., (+/-) HESI and (+/-) APCI. Unexpectedly, because of the diversity of experimental conditions in terms of continuous-flow tank reactor, concentration of reactants, temperature, reaction time, mass spectrometry techniques, and analysis conditions, the results indicate that among the 1138 presently detected molecular formulae, many oxygenates found in earlier studies of limonene oxidation by OH and/or ozone are also produced under the present conditions. Among these molecular formulae, highly oxygenated molecules and oligomers were detected in the present work. The results are discussed in terms of reaction pathways involving the initial formation of peroxy radicals (RO2), isomerization reactions yielding keto-hydroperoxides, and other oxygenated intermediates and products up to C25H32O17, products which could derive from RO2 autoxidation via sequential H shift and O2 addition (C10H14O3,5,7,9,11) and products deriving from the oxidation of alkoxy radicals (produced by RO2 self-reaction or reaction with HO2) through multiple H shifts and O2 additions (C10H14O2,4,6,8,10). The oxidation of RO2, with possible occurrence of the Waddington mechanism and of the Korcek mechanism, involving H shifts is also discussed. The present work demonstrates similitude between the oxidation products and oxidation pathways of limonene under simulated atmospheric conditions and in those encountered during the self-ignition of hydrocarbons at elevated temperatures. These results complement those recently reported by Vereecken and Nozière and confirm for limonene the existence of an oxidative chemistry of the alkylperoxy radical beyond 450 K based on the H shift (Nozière and Vereecken, 2019; Vereecken and Nozière, 2020)

Towards a Comprehensive Characterization of the Low-Temperature Autoxidation of Di-n-Butyl Ether

International audienceIn the present study, we investigated the oxidation of 2500 ppm of di-n-butyl ether under fuel-rich conditions (φ = 2) at low temperatures (460–780 K), a residence time of 1 s, and 10 atm. The experiments were carried out in a fused silica jet-stirred reactor. Oxidation products were identified and quantified in gas samples by gas chromatography and Fourier transform infrared spectrometry. Samples were also trapped through bubbling in cool acetonitrile for high-pressure liquid chromatography (HPLC) analyses. 2,4-dinitro-phenylhydrazine was used to derivatize carbonyl products and distinguish them from other isomers. HPLC coupled to high resolution mass spectrometry (Orbitrap Q-Exactive®) allowed for the detection of oxygenated species never observed before, i.e., low-temperature oxidation products (C8H12O4,6, C8H16O3,5,7, and C8H18O2,5) and species that are more specific products of atmospheric oxidation, i.e., C16H34O4, C11H24O3, C11H22O3, and C10H22O3. Flow injection analyses indicated the presence of high molecular weight oxygenated products (m/z > 550). These results highlight the strong similitude in terms of classes of oxidation products of combustion and atmospheric oxidation, and through autoxidation processes. A kinetic modeling of the present experiments indicated some discrepancies with the present data

A JET-STIRRED REACTOR STUDY OF THE OXIDATION AND PYROLYSIS OF DI-N-PROPYL-ETHER

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Experimental and numerical kinetic study of the oxidation of C5H10O2 esters isomers

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On the autoxidation of terpenes: Detection of oxygenated and aromatic products

International audienceLimonene-O 2-N 2 and α-pinene-O 2-N 2 mixtures were oxidized in a jet-stirred reactor at atmospheric pressure, in the cool flame regime, and fuel-lean conditions. Samples of the reacting mixtures were analyzed by on-line Fourier transform infrared (FTIR) and collected, dissolved in acetonitrile for analysis by flow injection or chromatographic separation by ultra-high performance liquid chromatography and Orbitrap mass spectrometry. OH/OD exchange using D 2 O and reaction with 2,4dinitrophenylhydrazine were carried out for probing the existence of hydroxyl or hydroperoxyl, and carbonyl functions in the products, respectively. A large number of oxidation products, including highly oxygenated organic products with more than ten oxygen atoms, were observed. Unexpectedly, aromatic and polyunsaturated products with a contribution of 8-10% were detected for both terpenes over the range of temperatures studied. Van Krevelen plots, computed oxidation state of carbon, aromaticity index, and aromaticity equivalent index in products were used to rationalize the results

On the formation of highly oxidized pollutants by autoxidation of terpenes under low-temperature-combustion conditions: the case of limonene and α -pinene

International audienceThe oxidation of monoterpenes under atmospheric conditions has been the subject of numerous studies. They were motivated by the formation of oxidized organic molecules (OOMs), which, due to their low vapor pressure, contribute to the formation of secondary organic aerosols (SOA). Among the different reaction mechanisms proposed for the formation of these oxidized chemical compounds, it appears that the autoxidation mechanism, involving successive events of O2 addition and H migration, common to both low-temperature-combustion and atmospheric conditions, leads to the formation of highly oxidized products (HOPs). However, cool-flame oxidation (∼500–800 K) of terpenes has not received much attention even if it can contribute to atmospheric pollution through biomass burning and wildfires. Under such conditions, terpenes can be oxidized via autoxidation. In the present work, we performed oxidation experiments with limonene–oxygen–nitrogen and α-pinene–oxygen–nitrogen mixtures in a jet-stirred reactor (JSR) at 590 K, a residence time of 2 s, and atmospheric pressure. Oxidation products were analyzed by liquid chromatography, flow injection, and soft-ionization–high resolution mass spectrometry. H–D exchange and 2,4-dinitrophenyl hydrazine derivatization were used to assess the presence of OOH and C=O groups in oxidation products, respectively. We probed the effects of the type of ionization used in mass spectrometry analyses on the detection of oxidation products. Heated electrospray ionization (HESI) and atmospheric-pressure chemical ionization (APCI) in positive and negative modes were used. We built an experimental database consisting of literature data for atmospheric oxidation and presently obtained combustion data for the oxidation of the two selected terpenes. This work showed a surprisingly similar set of oxidation products' chemical formulas, including oligomers, formed under the two rather different conditions, i.e., cool-flame and simulated atmospheric oxidation. Data analysis (in HESI mode) indicated that a subset of chemical formulas is common to all experiments, independently of experimental conditions. Finally, this study indicates that more than 45 % of the detected chemical formulas in this full dataset can be ascribed to an autoxidation reaction

Characterization of the Autoxidation of Terpenes at Elevated Temperature Using High-Resolution Mass Spectrometry: Formation of Ketohydroperoxides and Highly Oxidized Products from Limonene

International audienceLow-temperature experiments on the oxidation of limonene-O2-N2 mixtures were conducted in a jet-stirred reactor (JSR) over a range of temperatures (520 to 800 K), fuel-lean conditions (equivalence ratio φ = 0.5), short residence time (1.5 s) and a pressure of 1 bar. Collected samples of the reacting mixtures were analyzed by (i) on-line Fourier transform infrared spectroscopy (FTIR) and (ii) Orbitrap Q-Exactive® high resolution mass spectrometry after direct injection or chromatographic separation using reversed-phase ultra-high-performance liquid chromatography (RP-UHPLC) and soft ionization (+/‒ heated electrospray ionization and +/‒ atmospheric pressure chemical ionization). H/D exchange using deuterated water (D2O) and reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH) were performed to probe the presence of OH, OOH and C=O groups in the oxidized products, respectively. A broad range of oxidation products ranging from water to highly oxygenated products, containing five and more O-atoms, was detected (C7H10O4,5, C8H12O2,4, C8H14O2,4, C9H12O, C9H14O1,3-5, C10H12O2, C10H14O1-9, C10H16O2-5, and C10H18O6). Mass spectrometry analyzes were only qualitative whereas quantification was performed with FTIR. The results are discussed in terms of reaction routes involving the initial formation of peroxy radicals, H-atom transfer and O2 addition sequences producing a large set of chemical products among which ketohydroperoxides and more oxygenated products. Carbonyl compounds deriving from the Waddington oxidation mechanism on exo- and endo-double bonds (C=C) were observed as well as their products of further oxidation. Products of the Korcek mechanism (carboxylic acids and carbonyls) were also detected

Exploration of the oxidation chemistry of dimethoxymethane: Jet-stirred reactor experiments and kinetic modeling

Cinétique chimique et Aérothermodynamique pour des Propulsions et des Systèmes Energétiques Propre