Traitement de l'acétaldéhyde par décharges électriques impulsionnelles dans les mélanges de gaz atmosphériques (cinétique et efficacité énergétique)

Cette thèse a pour objet l analyse de la cinétique de la conversion de l acétaldéhyde, CH CHO, à des concentrations initiales inférieures ou égale à 5000 ppm dans un mélange de gaz à base d azote et contenant jusqu à 20% d oxygène, à température ambiante. L étude a été réalisée en utilisant trois réacteurs mettant en œuvre des décharges de qualités spatiales différentes. Il s agit d un réacteur (UV510) à décharge pré-ionisée (photo-déclenchée) par rayonnement UV produisant un plasma homogène, et de deux réacteurs à décharge à barrière diélectrique (DBD), de géométrie plane (plan-plan) et de géométrie cylindrique (tige-tube) alimentés par impulsion HT et produisant des plasmas non homogènes à faible (plan) ou forte (cylindre) filamentation ; un diagnostic d imagerie rapide (ns) de la DBD de géométrie plane montre que le plasma peut être considéré quasi-homogène. En s appuyant sur une modélisation 0D auto-cohérente de la décharge photo-déclenchée, l étude de la cinétique du mélange N /CH CHO montre l importance des états métastables de la molécule d azote, triplet A u , et singlets (groupe a' u, a g, et w u) dans la dissociation de l acétaldéhyde. Un coefficient minimum de 6.5.10 cm .s est estimé pour le quenching des singlets par l acétaldéhyde. Le coefficient du triplet est estimé entre 4.2.10 cm .s et 6.5.10 cm .s . Cette dissociation produit des radicaux (CH , CH CO, HCO, H, O) et des molécules (CH , CH CO, C H , C H , H , CO). Ainsi les sous-produits majoritaires mesurés à la fin de la post décharge temporelle sont le méthane, le dihydrogène, le monoxyde de carbone et l éthane. Les minoritaires sont l acétylène, l éthène, l acétone et l acétonitrile. Dans les mélanges contenant de l oxygène, l importance de la dissociation de CH CHO par quenching des états métastables de N diminue au profit des processus d oxydation par le radical hydroxyle, OH, et l oxygène atomique, O ( P). La mesure résolue en temps du radical OH dans la post-décharge du réacteur UV510 montre une très forte réactivité de ce radical avec les sous-produits de conversion de l acétaldéhyde. Une densité maximum de OH égale à 3.5.10 cm a été mesurée pour 10 % d oxygène et 5000 ppm d acétaldéhyde. Le schéma cinétique adopté pour ces mélanges donne, par la modélisation auto-cohérente, une valeur de densité plus élevée. Toutefois, la conversion de l acétaldéhyde dans N /O /CH CHO est bien expliquée par le modèle, de même que les concentrations produites de méthane et d éthane. Enfin, la comparaison de l efficacité énergétique des trois réacteurs étudiés montre que l homogénéité de la décharge favorise, pour des milieux pauvres en oxygène (moins de 2 %), la conversion de l acétaldéhyde.The present study deals with the kinetics analysis of acetaldehyde (CH CHO) conversion in electrical discharges with different spatial qualities et at room temperature. Acetaldehyde concentrations up to 5000 ppm in nitrogen-based gas mixture containing up to 20% of oxygen have been investigated. Three different plasma reactors were used: an UV510 reactor producing a homogeneous plasma thanks to a pre-ionization by UV radiation (photo-triggered), a plane-to-plane and a rod-tube dielectric barrier discharges (DBDs) reactors, In both DBDs reactors discharges were driven by high voltage pulses allowing the production of weakly inhomogeneous plasma in the plane geometry and highly filamentary discharges in the cylindrical one. A high speed imaging diagnostic (ns range) of the plane-to-plane DBD shows that the plasma can be considered quasi-homogeneous. Based on a self-consistent 0D model, the kinetics study of the N /CH CHO mixture conversion in the photo-triggered discharge shows the importance of nitrogen molecule metastable states , i.e. the triplet A u and the singlets group a' u, a g, et w u, in the acetaldehyde dissociation process. A minimum coefficient of 6.5.10 cm .s has been estimated for the quenching of N singlets state by acetaldehyde. For the triplet states quenching the coefficient of has been evaluated between 4.2.10 cm .s and 6.5.10 cm .s . This dissociation process produces radicals as CH , CH CO, HCO, H, O, and molecules like CH , CH CO, C H , C H , H , CO. Thus, the major by-products detected at the end of the post-discharge time are methane, hydrogen, carbon monoxide and ethane; smaller amounts of acetylene, ethene, acetone and acetonitrile were also detected. In containing oxygen mixtures, the importance of the CH3CHO dissociation processes due to N metastable states quenching of decreases in favor of oxidation processes promoted by the hydroxyl radical, OH, and atomic oxygen, O ( P). Time-resolved measurements of the OH radical in the photo-triggered post-discharge show a very high reactivity of this radical with the by-products of acetaldehyde conversion. A maximum density of OH radical equal to 3.5.10 cm was measured for 10% oxygen and 5000 ppm of acetaldehyde. The kinetic scheme adopted in the self-consistent model for the same gas mixture gives a higher density value; by the way the model is in good agreement with the acetaldehyde conversion in N /O /CH CHO mixtures, as well as with the methane and ethane produced concentrations. Finally, the comparison of the three studied reactors energy efficiency shows that, for low oxygen content (less than 2%), the homogeneity of the discharge promotes the acetaldehyde conversion.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Direct and real-time analysis in a plasma reactor using a compact FTICR/MS: degradation of acetone in nitrogen and by-products formation

Methods for reduction of Volatile Organic Compounds (VOCs) content in air depend on the 10 application considered. For low concentration and low flux, non-thermal plasma methods are often considered as efficient. However, the complex chemistry involved is still not well understood as there is a lack of datasets of byproducts formation. So as to overcome this issue, rapid analytical methods are needed. We present the coupling of a rapid chemical ionization mass spectrometer (CIMS) for the real-time analysis of the VOCs formed during a degradation experiment. The high resolution instrument used 15 allows for chemical ionization and direct quantification of non-targeted compounds. We present degradation experiments of acetone in a photo-triggered nitrogen plasma discharge. Two regimes were highlighted: efficient conversion at low concentrations (100ppm). Those two regimes were clearly delimited as the sum of two exponential curves occuring at respectively low and high concentrations. Many by-products were detected, in 20 particular HCN presented a significantly high yield. Nitrile compounds (acetonitrile, propionitrile,...) are formed as well. To a lower extent, ketene, acetaldehyde and formaldehyde are observed. The association of the high resolution mass spectrometer to the plasma reactor will allow further insights into the plasma chemistry and comparison to modelisation

Influence of water vapour on the propagation speed and mean energy of an atmospheric non-equilibrium diffuse discharge in air

We report results on the influence of humidity on the propagation and the energy of a pin-to-plane nanosecond pulse discharge at atmospheric pressure. Water vapour only impacts discharges in saturated gas mixtures, for which propagation is first slowed down, but accelerates faster than usual close to the plane. Energy is unchange

Simulation of the discharge propagation in a capillary tube in air at atmospheric pressure

International audienceThis paper presents simulations of an air plasma discharge at atmospheric pressure initiated by a needle anode set inside a dielectric capillary tube. We have studied the influence of the tube inner radius and its relative permittivity ε r on the discharge structure and dynamics. As a reference, we have used a relative permittivity ε r = 1 to study only the influence of the cylindrical constraint of the tube on the discharge. For a tube radius of 100 µm and ε r = 1, we have shown that the discharge fills the tube during its propagation and is rather homogeneous behind the discharge front. When the radius of the tube is in the range 300 to 600 µm, the discharge structure is tubular with peak values of electric field and electron density close to the dielectric surface. When the radius of the tube is larger than 700 µm, the tube has no influence on the discharge which propagates axially. For a tube radius of 100 µm, when ε r increases from 1 to 10, the discharge structure becomes tubular. We have noted that the velocity of propagation of the discharge in the tube increases when the front is more homogeneous and then, the discharge velocity increases with the decrease of the tube radius and ε r. Then, we have compared the relative influence of the value of tube radius and ε r on the discharge characteristics. Our simulations indicate that the geometrical constraint of the cylindrical tube has more influence than the value of ε r on the discharge structure and dynamics. Finally, we have studied the influence of photoemission processes on the discharge structure by varying the photoemission coefficient. As expected, we have shown that photoemission, as it increases the number of secondary electrons close to the dielectric surface, promotes the tubular structure of the discharge

Decharge creee par une onde progressive en presence d'un champ magnetique statique

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc