No description available.

<p> Demand ever increases for clean-burning, high-efficiency, and power-dense jet engines. This demand raises the thermal requirements and stresses on fuel systems for every new generation of gas turbine engine. Fuel is used to cool subsystems such as engine oil, pumps, electronics, valves, etc. resulting in elevated fuel temperatures upstream of combustor nozzles. Carbonaceous deposits or fouling occurs if the wetted wall temperature is elevated sufficiently, especially at fuel nozzle tips where temperatures are maximized. Fouling within fuel nozzles diminish atomization performance producing incomplete combustion, instability, and polluting byproducts. Therefore, the industry seeks strategies to mitigate carbon deposition without reducing the thermal requirements placed on the fuel. Existing carbon mitigation techniques rely on coating the fuel-wetted surfaces in an inert layer via anodic oxidation, chemical vapor deposition, etc. In this proposal, we aim to investigate a novel approach: inducing the lotus effect (heterogenous wetting) along the walls of fuel passageways. The lotus effect minimizes wetting area along a liquid-solid interface using a highly ordered set of micro or nano features with weak interfacial energy resulting in the liquid only wetting the peaks of said features. We hypothesized that the combination of a chemically inert surface with reduced wetting area diminishes the opportunity for deposit to form. The mitigating effect can be enhanced by the thermal insulation provided by the vapor or gas pockets trapped between the liquid-solid interface, passively reducing the thermal loading of the fuel. As a preliminary step, we produced the lotus effect on multiple aerospace alloys such as Inconel 718, stainless steel 304, and pure titanium via electrochemical etching and surface modification. We then exposed treated tubes to fuel under fouling-favorable conditions to compare their relative deposition rates. Our results indicate that the lotus effect loses stability at pressures well below those used in practical applications. However, the electrochemical etch we developed consistently produced negligible deposit where it would typically be maximized. Depending on if the surface is etched, FAS17 (a perfluoroalkyl silane used to generate superphobicity) can act to encourage or discourage carbon deposition. We determined that the electrochemical etch or FAS17 alone may be a method to mitigate carbon deposition regardless of the wetting behavior </p>

The objective of the thesis was to analyze the ageing processes and the modifications of an Ir/CeO2catalyst for steam reforming of ethanol. Over a model Ir/CeO2 catalyst, the initial and fast deactivationwas ascribed to ceria surface restructuring and the build-up of intermediates monolayer (acetate,carbonate and hydroxyl groups). In parallel, a progressive and slow deactivation was found to come fromthe structural changes at the ceria/Ir interface linked to Ir sintering and ceria restructuring. Theencapsulating carbon, coming from C2 intermediates polymerization, did not seem too detrimental to theactivity in the investigated operating conditions. By doping ceria with PrOx, the oxygen storage capacityand thermal stability were greatly promoted, resulting in the enhanced activity and stability. The Ir/CeO2catalyst was then modified by changing the shape of ceria. It was found that the shape and therefore thestructure of ceria influenced the activity and stability significantly. A simplified modeling of theseprocesses has contributed to support the new proposals of this work.

No description available.

The surface of a dense Ni-YSZ anode was modified with a thin-film coating of niobium oxide (Nb2O5) in order to understand the mechanism of sulfur tolerance and the behavior of carbon deposition. Results suggest that the niobium oxide was reduced to NbO2 under operating conditions, which has high electrical conductivity. The NbOx coated dense Ni-YSZ showed sulfur tolerance when exposed to 50 ppm H2S at 700°C over 12 h. Raman spectroscopy and XRD analysis suggest that different phases of NbSx formed on the surface. Further, the DOS (density of state) analysis of NbO2, NbS, and NbS2 indicates that niobium sulfides can be considered as active surface phases in the H2S containing fuels. It was demonstrated that carbon formation was also suppressed with niobium oxide coating on dense Ni-YSZ in humidified CH4 (3% H2O) at 850ºC. In particular, under active operating conditions, there was no observable surface carbon as revealed using Raman spectroscopy due probably to electrochemical oxidation of carbon. Stable performances of functional cells consisting of Pt/YSZ/Nb2O5 coated dense Ni-YSZ in the fuel were achieved; there was no observable degradation in performance due to carbon formation. The results suggest that a niobium oxide coating has prevented carbon from formation on the surface probably by electrochemically oxidation of carbon on niobium oxide coated Ni-YSZ. On the other hand, computational results suggest that, among the metals studied, Mo seems to be a good candidate for Ni surface modification. Ni-based anodes were modified with Mo using wet-impregnation techniques, and tested in 50 ppm H2S-contaminated fuels. It was found that the Ni-Mo/CeO2 anodes have better sulfur tolerance than Ni, showing a current transient with slow recovery rather than slow degradation in 50 ppm H2S balanced with H2 at 700°C.

Le reformage catalytique du méthane en hydrogène, vecteur d'énergie pour les piles à combustibles de type Solid Oxide Fuel Cell (SOFC), a été étudié sur des matériaux d'anode à base de métaux supportés (NiO/CeO2, NiO-Y2O3-ZrO2) et également sur des catalyseurs de structure définie (La0,8Sr0,2TiO3+δ). La première famille de catalyseurs a été synthétisée par deux méthodes de préparation : la technique d'imprégnation en milieu aqueux et en milieu organique sur des supports du commerce CeO2 et Y2O3-ZrO2 ou préparés au laboratoire, et par le procédé sol-gel. Le titanate de lanthane dopé au strontium a été préparé par la méthode de co-précipitation et également par la méthode sol-gel. La méthode de préparation a un effet important sur les propriétés physico-chimiques des catalyseurs synthétisés et par conséquent affecte à la fois leur activité catalytique en reformage du méthane et leur résistance à l'empoisonnement par le dépôt de carbone. Les catalyseurs à base de nickel supporté sur cérine ont été par la suite dopés avec l'oxyde de magnésium (formation d'une solution solide MgO-NiO) ainsi qu'avec l'oxyde de lanthane (La2O3-NiO) en vue de limiter la formation de carbone sur la surface catalytique et augmenter ainsi la durée de vie des catalyseurs lors du reformage du méthane. Les résultats obtenus montrent que l'effet de promotion de la phase active NiO par MgO ou La2O3 diminue à la fois le dépôt de carbone mais également les performances catalytiques. Les propriétés physico-chimiques et les performances catalytiques de NiO-Y2O3-ZrO2 préparé par le procédé sol-gel ont été comparées à celles de matériaux commerciaux (Aldrich & Jülich) de même composition. Les résultats expérimentaux montrent que les matériaux synthétisés par la méthode sol-gel sont plus actifs en vaporeformage du méthane que ceux du commerce (dans le domaine de fonctionnement d'une pile SOFC) alors qu'ils présentent une activité similaire à ces derniers en reformage à sec du méthane. La quantité de carbone graphitique formée, quoique supérieure à celle observée dans le cas des catalyseurs commerciaux, demeure faible (< 2%). Ce dépôt de carbone ne provoque qu'une légère diminution des performances catalytiques en reformage à sec du méthane. Ceci est probablement lié à la diminution des sites actifs / The catalytic reforming of methane into hydrogen, for direct operation of Solid Oxide Fuel Cells (SOFCs) on methane, was studied on anode materials such as NiO/CeO2, NiO-Y2O3-ZrO2 and La0.8Sr0.2TiO3+δ. The first group of catalysts was synthesized by two methods: the impregnation technique both in aqueous and organic media (commercial and laboratory made CeO2 and Y2O3-ZrO2), and also using sol-gel process. Lanthanumtitanium oxide host structure doped with strontium was prepared both by co-precipitation and sol-gel process. The method of preparation has an important effect on the physico-chemical properties of the synthesized catalysts and affects consequently both their catalytic performances in methane reforming and their resistance to poisoning by carbon deposition. In order to limit carbon formation on the catalytic surface and to increase the lifetime of catalysts during the catalytic reforming of methane, ceria supported nickel based-catalysts were doped with magnesium oxide (forming MgO-NiO solid solution) as well as with lanthanum oxide (La2O3-NiO). The obtained results show that the effect of promotion of NiO active phase by MgO and La2O3 decreases carbon deposition but also the catalytic performances. Physico-chemical properties and catalytic performances of NiO-Y2O3-ZrO2 (Ni-YSZ) prepared by the sol-gel process were compared with those of commercial (Aldrich and Jülich) materials having the same composition. The experimental results showed that materials synthesized by the sol gel method are more active in methane steam reforming than commercial catalysts while sol gel and commercial samples show similar performances in methane dry reforming. Amounts of graphitic carbon, although being higher for sol gel samples compared to commercial ones, remain low (< 2%). This carbon deposit provokes only a slight decrease of catalytic performances of sol gel prepared materials in methane dry reforming, probably by decreasing the number of active sites

Ce travail rapporte l’étude des processus de désactivation et des modifications d’un catalyseur Ir supportésur cérine en vaporeformage de l’éthanol. Différentes causes de désactivation ont été identifiées selon lesconditions opératoires : température, temps de contact et temps de réaction. La désactivation initiale,rapide mais limitée a été attribuée à la restructuration de surface de la cérine et à la formation d’unemonocouche d’intermédiaires de type acetate, carbonate et hydroxyls. En parallèle, une désactivationlente et progressive a été mise en évidence, ayant pour origine les changements structurels de l’interfaceentre la cérine et l’iridium, liés au frittage des particules d’iridium et à la restructuration profonde de lacérine. Par contre, la formation continue, à température modérée, d’une couche de carbone encapsulantissu de la polymérisation d’intermédiaires C2 n’a pas semblé contribuer significativement à ladésactivation du catalyseur dans nos conditions opératoires. Pour limiter ce phénomène de désactivation,des modifications ont été apportées au catalyseur. Le dopage du catalyseur par PrOx a permis defortement améliorer la capacité de stockage de l’oxygène et la stabilité thermique du catalyseur,entraînant une augmentation de son activité et de sa stabilité en vaporeformage de l’éthanol. Lecatalyseur Ir/CeO2 a ensuite subi une mise en forme de la cérine (nano-tubes), avec une influencesignificative sur l'activité et la stabilité en vaporeformage de l’éthanol, liée à des effets structuraux. Unemodélisation simplifiée de ces divers phénomènes a également contribué à soutenir les propositionsoriginales de ce travail. / The objective of the thesis was to analyze the ageing processes and the modifications of an Ir/CeO2catalyst for steam reforming of ethanol. Over a model Ir/CeO2 catalyst, the initial and fast deactivationwas ascribed to ceria surface restructuring and the build-up of intermediates monolayer (acetate,carbonate and hydroxyl groups). In parallel, a progressive and slow deactivation was found to come fromthe structural changes at the ceria/Ir interface linked to Ir sintering and ceria restructuring. Theencapsulating carbon, coming from C2 intermediates polymerization, did not seem too detrimental to theactivity in the investigated operating conditions. By doping ceria with PrOx, the oxygen storage capacityand thermal stability were greatly promoted, resulting in the enhanced activity and stability. The Ir/CeO2catalyst was then modified by changing the shape of ceria. It was found that the shape and therefore thestructure of ceria influenced the activity and stability significantly. A simplified modeling of theseprocesses has contributed to support the new proposals of this work.

No description available.

Liquid methane is becoming an increasingly attractive rocket propellant due to its high performance characteristics and potential to support in-situ resource utilisation. Methane, however, when heated, can thermally decompose in a process known as pyrolysis. In regeneratively cooled rocket engines, the solid carbon products from the pyrolysis reactions are deposited on the walls of the cooling channels. This increases the thermal resistance of the channel walls, resulting in higher wall temperatures. In turn, this can facilitate cracking and crack propagation, presenting a potential problem in rockets, especially for future reusable designs. It will therefore be necessary to inspect the state of the cooling channels between flights. The carbon layer also changes the catalytic properties of the surface, affecting the onset temperature of methane pyrolysis, and thus impacting the pyrolysis behaviour during subsequent flights. It is possible to clean the channel using a mixture of gaseous oxygen and ozone, however, preliminary testing has indicated that not all the carbon is removed within a reasonable time frame. An experimental facility exists which can control the thermal and flow conditions in straight test channels to replicate the conditions seen in methane rocket nozzle cooling channels. The purpose of this project is to develop a system to quantitatively assess the amount of carbon deposition in these test channels after methane pyrolysis has occurred within them, and following ozone cleaning. The developed system is an optical method which uses a borescope to capture images within the coked channel. These images are then run through bespoke image processing software to determine the proportion of the inner channel wall that is coked. The software has been developed and a provisional mechanical setup has been designed. Initial validation tests have been conducted to assess the accuracy of the software used in conjunction with the borescope and camera. The results indicate that the system is capable of quantifying coke in a metal channel with an error of 1.489%±0.232% or less. / Flytande metan är på väg att bli ett mera attraktivt raketbränsle på grund av sina högprestanda-egenskaper samt potential för att stödja resursanvändning, in situ. Hursomhelst så kan metan, då uppvärmt, termiskt brytas ned i en process kallad pyrolys. I regenerativt kylda raketmotorer så utfälls de solida kolprodukterna från pyrolysen på väggarna av kylkanalerna. Detta höjer den termiska resistansen hos kanalens väggar vilket resulterar i högre väggtemperaturer. Detta kan, i sin tur, leda till spricktillväxt som väcker ett potentiellt problem med raketer, speciellt för framtida återanvändningsbara designer.Det kommer därför vara nödvändigt att inspektera skicket av kylkanalerna mellan flygningar. Kollagret förändrar också de katalyserande egenskaperna av ytan, vilket har en inverkan på begynnelsetemperaturen av metanpyrolys, som påverkar hur pyrolysen beter sig för följande flygningar.Däremot är möjligt att rena kanalerna genom att använda en blandning av syre i gasform, och ozon. Preliminära tester indikerar på att inte allt kol är borttaget inom en rimlig tidsram. En experimentell anläggning finns, som kan kontrollera tillstånd för värme och flöde i raka testkanaler för att replikera tillstånden som setts i kylkanaler i dysor för metanraketer. Syftet med detta projekt är att utveckla ett system för att kvatitativt bedöma mängden koldeposition i dessa testkanaler efter att pyrolys av metan har skett i dem, följt av ozon-rening. Det utvecklade systemet är en optisk metod som använder ett boroskop för att fånga bilder inuti den kanalen med koks. Dessa bilder körs genom ett skräddarsytt bildprocesseringsprogram för att bestämma proportionerna av den inre kanalväggen med koks. Mjukvaran har utvecklats och en provisorisk mekanisk anordning har utformats. Initiella valideringstester har genomförts för att bedöma noggrannheten av mjukvaran som använts i samband med boroskopet och kameran. Resultaten indikerar på att systemet är kapabelt att kvatifiera koks in en metallkanal med ett fel på 1,489%±0,232% eller mindre.

Στην παρούσα διδακτορική διατριβή έγινε η μελέτη της φύσης των επιφανειακών ειδών άνθρακα που δημιουργούνται στην επιφάνεια κεραμομεταλλικών υλικών βασισμένα στο Ni-YSZ κατά την θερμοδυναμική ισορροπία της διασπαστικής ρόφησης του μεθανίου. Κύριος στόχος της μελέτης ήταν να διερευνηθεί η επίδραση της παρουσίας ενός δεύτερου μετάλλου στο Ni-YSZ, όσον αφορά τον σχηματισμό και/ή την αναστολή του ανενεργού άνθρακα. Η επικάλυψη της επιφάνειας του Ni με μικρή ποσότητα Au (≤1%at σε σχέση με το Ni) επηρέασε σημαντικά την κινητική της διασπαστικής ρόφησης του CH4 και την ενέργεια δεσμού των ροφημένων ειδών CHx στην επιφάνεια του Ni. Ο σχηματισμός γραφιτικού άνθρακα παρεμποδίστηκε, ενώ η επιφανειακή δραστικότητα για την υδρογόνωση των CHx προς CH4 ήταν χαμηλότερη από ότι στον Ni-YSZ. Αυτό έχει ως αποτέλεσμα την υψηλότερη σταθερότητα και την επιμήκυνση του χρόνου ζωής των CHx πάνω στην επιφάνεια του NiAu/YSZ πριν την διάσπαση τους προς επιφανειακά καρβιδικά είδη. Με βάση τις πειραματικές μετρήσεις, προτάθηκε κινητικό μοντέλο για την περιγραφή της μερικής οξείδωσης του μεθανίου για την παραγωγή αερίου σύνθεσης. Οι αντιδράσεις οξείδωσης περιλαμβάνουν την οξείδωση των ειδών CHx προς CHxO και την επακόλουθη διασπασή τους σε υψηλές θερμοκρασίες (700 Κ) προς CO και H2, ενώ τα καρβιδικά είδη οξειδώνονται εκλεκτικά προς CO2 από χαμηλότερες θερμοκρασίες (500 Κ). Από αυτή την άποψη, η εκλεκτικότητα ως προς το αέριο σύνθεσης εξαρτάται από δύο παράγοντες: (i) τον χρόνο ζωής και την επιφανειακή συγκέντρωση των ειδών CHx, και (ii) τη θερμοκρασία διάσπασης των ειδών CHxO. Ως γενικό συμπέρασμα, ο ηλεκτροκαταλύτης Ni(1%at Au)/YSZ αναμένεται να έχει υψηλή αντοχή στην εναπόθεση άνθρακα όπως και εκλεκτικότητα για την μερική οξείδωση του CH4 προς αέριο σύνθεσης. / In order to investigate the possibility of the direct introduction of CH4 into a SOFC, the dissociative adsorption of CH4 and the nature of the various carbon adspecies on Ni-based cermets were studied in the present thesis by means of thermodynamic equilibrium measurements and temperature programming methods. The aim of this study was mainly to elucidate the effect of a second metal addition on Ni-YSZ, regarding the formation and/or inhibition of inactive carbon. The decoration of the Ni surface with very small quantity of Au (≤1at% with respect to Ni) affected significantly both the kinetics of CH4 dissociative adsorption and the binding strength of the adsorbed CHx species on the Ni surface. The formation of graphitic carbon was significantly inhibited, while the surface reactivity towards the hydrogenation of CHx species into CH4 was lower than on the unmodified Ni/YSZ surface. This resulted in the higher stability and elongation of the CHx species life time on the NiAu/YSZ surface prior to their decomposition into surface carbidic species. Based on the experimental results and discussion, a kinetic model is proposed for the description of the partial oxidation of methane towards the production of synthesis gas. The oxidation reactions involve both the oxidation of CHx species into CHxO and its subsequent decomposition at elevated temperatures (700 K) into CO and H2, while carbidic species are selectively oxidized into CO2 at temperatures as low as 500 K. In this respect, the selectivity towards synthesis gas depends on two factors: (i) the lifetime and surface concentration of CHx species, and (ii) the decomposition temperature of the CHxO species. As a conclusion, the Ni(1%at Au with respect to Ni)/YSZ catalyst is expected to be highly carbon tolerant and selective catalyst/electrode for the CH4 partial oxidation reaction for the production of synthesis gas.