Next generation nuclear power plants (GEN-IV) will be capable of not only producing energy in a reliable, safe and sustainable way, but they will also be capable of reducing the amount of nuclear waste, which has been accumulated over the lifetime of current-generation nuclear power plants, through transmutation. Due to the use of new and different coolants, existing computational tools need to be tested, further developed and improved in order to thermal-hydraulically design these power plants.This work covers two different non-unity Prandtl number fluids which are considered as coolants in GEN-IV reactors, liquid lead/lead-bismuth-eutectic and supercritical water. The study investigates different turbulence modeling strategies, such as Large Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) modeling, and their applicability to these proposed coolants. It is shown that RANS turbulence models are partly capable of predicting wall heat transfer in annular flow configurations. However, improvements in these prediction should be possible through the use of advanced turbulence modeling strategies, such as the use of separate thermal turbulence models. A large blind benchmark study of heat transfer in supercritical water showed that the available turbulence modeling strategies are not capable of predicting deteriorated heat transfer in a 7-rod bundle at supercritical pressures. New models which take into account the strong buoyancy forces and the rapid change of the molecular Prandtl number near the wall occurring during the transition of the fluid through the pseudocritical point need to be developed. One of these strategies to take into account near-wall buoyancy forces is the use of advanced wall functions, which cannot only help in modeling these kind of flows, but also decrease computational time by 1 to 2 orders of magnitude. Different advanced wall function models were implemented in the open-source CFD toolbox OpenFOAM and their performance for different flows in sub- and supercritical conditions were evaluated. Based on those results, the wall function model UMIST-A by Gerasimov is recommended for further investigation and specific modeling tactics are proposed.Near-wall temperature and velocity behavior is important to and influenced by the wall itself. The thermal inertia of the wall influences the temperature in the fluid. However, a more important issue is how temperature fluctuations at the wall can induce thermal fatigue. With the help of LES thermal mixing in a simplified model of a control rod guide tube was investigated, including the temperature field inside the control rod and guide tube walls. The WALE sub-grid turbulence model made it possible to perform LES computations in this complex geometry, because it automatically adapts to near-wall behavior close to the wall, without the use of ad-hoc functions. The results for critical values, such as the amplitude and frequency of the temperature fluctuations at the wall, obtained from the LES computations are in good agreement with experimental results.The knowledge gained from the aforementioned investigations is used to optimize the flow path in a small, passively liquid-metal-cooled pool-type GEN IV reactor, which was designed for training and education purposes, with the help of 3D CFD. The computations were carried out on 1/4 of the full geometry, where the small-detail regions of the heat exchangers and the core were modeled using a porous media approach. It was shown that in order to achieve optimal cooling of the core without changing the global geometry a ratio of close to unity of the pressure drop over the core and the heat exchanger needs to be achieved. This is done by designing a bottom plate which channels enough flow through the core without choking the flow in the core. Improved cooling is also achieved by reducing heat losses from the hot leg through the flow shroud to the cold leg by applying thermal barrier coating similar to methods used in gas turbine design. / Nästa generations kärnkraftverk (GEN-IV) kan inte bara producera el på ett pålitligt, säkert och hållbart sätt, utan det kan också reducera mängden kärnavfall, som har producerats under tiden som man använt nuvarande generationen kärnkraftverk, genom att transmutera avfallen. Framtidens kärnkraftverk använder andra kylmedel än nuvarande kraftverk som t.ex. flytande bly, gas eller superkritiskt vatten. Det betyder att många beräkningsverktyg måste testas, utvecklas och förbättras så att man kan genomföra termohydrauliska designberäkningar. Den här avhandlingen omfattar två olika kylmedel, flytande bly och superkritiskt vatten, som har ett Prandtl-tal som skiljer sig från 1 och kommer att användas i GEN-IV reaktorer. Studien undersöker olika strategier för att modellera turbulens som Large Eddy Simulation (LES) och Reynolds-Averaged Navier-Stokes (RANS) och hur man kan använda dessa strategierna i beräkningar av strömning och värmetransfer i den nya kylvätskan. Undersökningen visar att RANS turbulensmodeller delvis kan förutsäga värmeöverföringen vid en vägg i en ringformad strömningsgeometri. Förbättringar av förutsägelsen ska vara möjlig genom användning av avancerade strategier för turbulensmodellering, t.ex. termiska turbulensmodeller. En stor prestandajämförelse för värmeöverföring i superkritiskt vatten visade att ingen av nuvarande strategier för turbulensmodellering kan förutsäga försämrad värmeöverföring i en 7-stavknippet under superkritiskt tryck. Nya modeller, som omfattar de starka flytkrafterna och den snabba förändringen av den molekulära Prandtl-tal vid väggen som uppstår när vätskan går genom pseudokritiska punkten, måste utvecklas. Avancerade väggfunktioner är en av strategierna som kan ta hänsyn till dessa fenomen. Väggfunktioner kan inte bara hjälpa till att modellera de typer av flöden som behövs utan kan också hjälpa till att sänka beräkningstiden med en eller två tiopotenser. Olika avancerade väggfunktioner i open-source beräkningsverktyget OpenFOAM implementerades och deras prestation i sub- och superkritiska vattenflödar värderades. Baserat på detta rekommenderas Gerasimovs modell för ytterligare utredning. Dessutom läggs olika strategier fram för att utöka modellens validitet till flöde med superkritiskt vatten i sammanband med försämrad och förbättrad värmeöverföring. Kunskap om beteendet av temperatur och hastighet i väggens närhet är viktigt för väggens integritet, detta då väggen även påverkar beteendet. Väggens termiska tröghet påverkar flödets temperatur och hastighet. Dock är ett ännu viktigare problem, som kan uppträda, är att temperaturfluktuationer kan framkalla termisk utmattning i en vägg. Med användning av LES utreds termisk blandning av varmt och kallt vatten i en simplifierad modell av ett styrstavsledrör, inklusive temperaturfältet i styrstaven och ledrörsväggen. Användningen av WALE LES-turbulensmodellen gör det möjligt att utföra beräkningar i den komplexa geometrin, detta eftersom modellen anpassar sig automatiskt till fenomenen nära väggen utan användning av ad-hoc funktioner. LES resultaten för alla värden som är viktiga för att bestämma utmattningsbeteende, som amplitud och frekvens av temperaturfluktuationer i väggens närhet och i väggen själv, är i god överensstämmelse med resultaten från experiment från KTH i samma geometri.Kunskapen som vunnits genom ovannämnda utredningar användes för att optimera den termohydrauliska designen av en liten, pool-typ GEN-IV reaktor som är passivt kyld med flytande bly. Reaktorn är designad som en utbildnings- och träningsreaktor och optimeringen genomfördes med hjälp av 3D CFD. Beräkningarna genomfördes på en fjärdedel av reaktorns hela geometrin. Regioner med små detaljer, som de åtta värmeväxlarna och reaktorns kärna, modellerades genom porösa material. Det visar sig att för att ha en optimal kylning av kärnan, utan att förändra reaktorns globala geometri, måste förhållandet mellan tryckförlust i reaktorkärnan och värmeväxlarna vara nära 1. Detta uppnås genom att designa plattan vid ingången till kärnan så att tillräckligt med bly flödar genom kärnan utan att kväva flödet i denna. Ytterligare en förbättring i reaktorkylningen uppnås genom att reducera värmeförlusten genom väggen som skiljer varm och kall vätska. Detta görs med en strategi som förekommer i gasturbinteknologin, genom att man lägger till ett tunt skikt av termiskt isolerande material på väggen, som reducerar värmeöverföring med ungefär 50%. / <p>QC 20151123</p> / THEMFA / GENIUS / THINS

L’Oxydation HydroThermale (OHT) est l’une des technologies mises en œuvre pour le traitement des déchets liquides organiques radio-contaminés. Grâce aux propriétés de l’eau supercritique, l’OHT permet d’obtenir une minéralisation complète des composés organiques, avec des temps de séjour très courts, dans des réacteurs continus et compacts. Un modèle thermohydraulique couplé à un modèle de cinétique de combustion a été précédemment développé par le CEA et implémenté sur la plateforme de mécanique des fluides numérique ANSYS Fluent. Dans l’objectif d’améliorer la description des écoulements dans ces réacteurs, des mesures de masse volumique de mélanges ont permis d’ajuster puis d’implémenter un modèle en accord avec les données expérimentales. La problématique de la précipitation des sels minéraux a également été traitée au cours de cette étude, en mettant en œuvre une méthodologie expérimentale permettant d’acquérir des données de solubilité de composés modèles, et aussi de collecter et de caractériser les dépôts obtenus lors de la précipitation de ces composés dans un dispositif continu ad hoc. Ce travail expérimental a été poursuivi par l’implémentation de ces phénomènes de précipitation et de dépôt au sein du modèle numérique / HydroThermal Oxidation (HTO) is one of the technologies applied for radio-contaminated organic liquid waste treatment. HTO uses the peculiar properties of supercritical water to achieve a complete destruction of organic compounds within very short residence times in continuous compact reactors. A thermohydraulic model, coupled with combustion kinetics, was previously developed by the CEA, and implemented using CFD software ANSYS Fluent. In order to improve the description of the flow in continuous HTO reactors, mixtures density measurements allowed to find a model fitting these experimental data. This model was consequently incorporated into the previous numerical simulation. The issue of mineral salts precipitation was also taken into account, by an experimental methodology allowing to acquire solubility data of model compounds, and also to collect and characterise the deposit obtained after precipitation of these compounds in an ad hoc continuous device. This experimental work was followed by the implementation of these precipitation and deposition phenomena in the numerical model

A gaseificação utiliza o conteúdo intrínseco de carbonos e hidrogênios das matérias primas sólidas ou líquidas na geração de uma mistura de hidrogênio (H2), monóxido de carbono (CO), dióxido de carbono (CO2) e metano (CH4). Tal mistura pode ser utilizada como matéria prima na síntese de novos produtos ou como combustível. A gaseificação pode ser utilizada no processamento de uma gama variada de produtos, independentemente de suas características ou estado físico. A utilização de biomassa como insumo da gaseificação vem sendo cada vez mais explorada e estudada, já que apresenta benefícios não somente na esfera ambiental, mas também em âmbitos econômicos e sociais. A vinhaça é um subproduto do processo de produção de álcool, que contém grandes concentrações de nutrientes e matéria orgânica em sua composição. A sua utilização hoje está limitada a fertirrigação e a aplicações isoladas em biodigestão e outros, que não são suficientes para o consumo da produção anual crescente do resíduo. Seu uso na gaseificação permitiria o aproveitamento do conteúdo orgânico da mesma e a produção de gases de alto valor agregado. Como a umidade do insumo interfere negativamente na eficiência da gaseificação clássica, a aplicação da mesma para matérias primas com alto teor de líquidos não é recomendada. Uma alternativa viável seria a utilização do meio gaseificante supercrítico, que resulta em rendimentos constantes, independentemente da umidade da corrente de entrada do reator. O presente trabalho consiste no projeto de um módulo de gaseificação de vinhaça em água supercrítica, a ser instalado como uma unidade anexa a usinas de açúcar e álcool. Ele compreende o projeto conceitual e análise de viabilidade deste módulo, incluindo estimativas de CAPEX (Capital Expenditure) e OPEX (Operation Expenditure) e uma análise de sensibilidade dos mesmos. O estudo apresenta ainda o estado da arte do conhecimento e da tecnologia de gaseificação com água supercrítica (SCWG), relacionando os gargalos a serem resolvidos, assim como os ganhos intrínsecos da definição conceitual do projeto. / The gasification process uses the carbon and hydrogen content in a given solid or liquid feedstock to produce a gaseous mixture of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2) and methane (CH4). This mixture can be used as a precursor in the synthesis of new products or directly as a fuel. The gasification can be used in the processing of a wide range of materials, regardless of its characteristics or physical state. The use of vinasse as a gasification feedstock has been increasingly explored and studied, since its appeal lies not only on the environmental sphere, but as well on economic and social scope. Vinasse is a byproduct of the ethanol/ sugar producing process and contains large concentrations of nutrients and carbon organic matter in its protein-rich composition. The use of this fluid is limited today to fertirrigation and isolated applications, that are not enough for the consumption of its growing production. The possibility of gasifying the vinasse would allow a more profitable use of the fluid. In the classical gasification, the moisture content of the product being gasified impairs the yield of the reaction. So, for liquid feedstock its use is not recommended. One viable alternative for this case would be the use of the supercritical water as a reaction medium, which results in constant yields regardless of the moisture content of the raw material. This work consists on the design of module for vinasse gasification in supercritical water, to be installed as a unit, attached to an alcohol/ sugar plant. It comprises the conceptual design and feasibility study of the module, including CAPEX and OPEX estimates, plus a sensitivity analysis. The study also presents the state of the art of the knowledge associated to SCWG technology, relating bottlenecks to be solved, as well as the intrinsic gains from conceptual design definition.

<p>Supercritical Water-cooled Reactor (SCWR) is a Generation-IV nuclear reactor design that operates on a direct energy conversion cycle above the thermodynamic critical point of water (374⁰C and 22.1 MPa), and offers higher thermal efficiency and considerable design simplification. As an essential step in the design of SCWR safety systems, the accident behaviour of the reactor is evaluated to ensure that the safety systems can achieve safe shutdown for all the design basis accidents. Unfortunately, the computational tools and computer codes that are currently employed for safety analysis have little application in the supercritical region, and faces significant challenges in simulating the transitions from subcritical to supercritical conditions.</p> <p>This thesis examines the predictive capabilities of Computational Fluid Dynamics (CFD) code STAR-CCM+ by evaluating critical flow (or choked flow) due to accidental release of coolant from supercritical fluid systems. The biggest challenge of this research is that the current version of STAR-CCM+ does not support supercritical simulations because the steam tables included in the package are only limited to the subcritical subset of the thermodynamic fluid properties.</p> <p>The research was carried out in two stages. In the first stage, the CFD code STAR-CCM+ was customized to simulate supercritical conditions by, (i) Generating updated steam tables to include subcritical and supercritical fluid properties and using more pressure and temperature points in the pseudo critical region (22 – 25 MPa, 645 -660 K) to handle the rapid changes in the fluid properties, and (ii) Implementing a multi-dimensional steam table interpolation scheme to access the fluid property data at any thermodynamic state during the simulation. In the second stage, the customized CFD code was extensively evaluated by simulating several accidental release scenarios from supercritical conditions using rounded-edge and sharp-edge nozzles and the model results were validated with experimental data. To overcome the solution stability (or convergence) issues encountered during the supercritical simulations, a fine tuning procedure was proposed that guaranteed convergence for all the case studies considered in this thesis.</p> <p>The simulation results revealed that the CFD model produced results that were in good agreement with experimental data and only about 10% prediction error was noticed for most cases considered in the thesis. Considering the sensitivity of the CFD model for upstream temperatures and pressures, these results appear to be quite reasonable. From the computational experience gained in this research , we believe that the CFD code STAR-CCM+ is a very useful tool to perform thermal hydraulic simulations for supercritical systems. However, an appropriate customization and extensive validation of the code is required before it can be exclusively used for safety analysis.</p> / Master of Applied Science (MASc)

Des nouveaux procédés éco-efficients basés sur une meilleure utilisation des ressources renouvelables sont nécessaires pour assurer la continuité du développement énergétique. La thèse étudie le procédé de gazéification en eau supercritique (T>374°C et P>22,1 MPa) de la biomasse très humide pour l’obtention de l’hydrogène, molécule ayant un potentiel énergétique très intéressant à valoriser avec un impact environnemental très favorable. L’étude porte sur l’application du procédé à la biomasse modèle (solutions de glucose, glycérol et leur mélange) ainsi qu’au bioglycérol, résidu de la fabrication du biodiesel. Les propriétés du solvant et les mécanismes prépondérants développés par l’eau en phase souset supercritique peuvent être contrôlés par les paramètres opératoires imposés au processus : température, pression, concentration en molécules organiques et catalyseur alcalin, temps de réaction... Les études paramétriques des systèmes réactionnels ont été menées dans des réacteurs batch à deux échelles différentes, les phases résultantes étant caractérisées par des protocoles analytiques élaborés et validés dans le cadre de l’étude. Le suivi du milieu réactionnel en batch lors de son déplacement vers l’état supercritique a mis en évidence une conversion avancée des molécules organiques et une identification de certains intermédiaires générés. Parmi les paramètres étudiés, la température et le temps de réaction influent le plus le rendement à l’obtention d’hydrogène en présence de catalyseur (K2CO3) dans les réacteurs batch, rendements de 1,5 et 2 mol d’H2 respectivement par mol de glycérol et de glucose introduites. Les gaz obtenus contiennent des proportions variables d’hydrocarbures légers et du CO2. Environ 75% du carbone est converti en phase gaz et liquide (sous forme de carbone organique et inorganique), le restant étant déposé sous forme solide ou huileuse. L’analyse du solide généré (plus de 90% de C) laisse apparaître différentes phases, y compris la formation de nanoparticules sphériques. Enfin, la gazéification en réacteur continu du glycérol préchauffé a montré de meilleurs rendements en hydrogène que le procédé batch, pendant que celle du bioglycérol demande une évolution du procédé à cause de la précipitation en phase supercritique des sels contenus dans le réactant. En conclusion, la gazéification en eau supercritique de la biomasse peut être considérée comme une alternative intéressante à d’autres procédés physico-chimiques de production de l’hydrogène. L’amélioration du procédé sera possible par son intensification menée en parallèle avec l’utilisation de matériaux plus performants et le contrôle de la salinité de la phase réactante. / Supercritical water (T > 374 ° C and P > 22.1 MPa) gasification of wet biomass for hydrogen production is investigated. This process converts a renewable resource into a gas, which is mainly composed of hydrogen and hydrocarbons with interesting energy potential, and which can be separated at high pressure. In addition, the greenhouse gas effect of the process is zero or negative. Model biomasses (glucose, glycerol and their mixture) and bio-glycerol, residue from bio-diesel production, have been gasified by different processes: two-scale batch reactors (5 mL and 500 mL) and a continuous gasification system. Supercritical water acts as a reactive solvent, its properties can be adjusted by the choice of the experimental (P, T) couple. The operating parameters, e.g. temperature, pressure, concentration of biomass and alkaline catalysts, reaction time… allow favoring certain reaction mechanisms. In order to characterize the processes, specific analytical protocols have been developed and validated. The intermediates, formed during the heating time in the batch reactors, have been identified. Among the investigated operating parameters, temperature and reaction time have the greatest influence on the hydrogen production in batch reactors. In the presence of catalyst (K2CO3), H2 yields of 1.5 mol/mol glucose and 2 mol/mol glycerol have been respectively observed. The obtained gas contains different proportions of light hydrocarbons and CO2. About 75% of the carbon is converted into gas and liquid (in form of organic and inorganic carbon). The conversion leads also to a solid or oily residue. In the generated solid phase (composed over 90% of C), spherical nanoparticles are observed via electronic microscopy. The hydrogen production from glycerol is improved in the continuous process compared to batch reactors, however, bio-glycerol supercritical water gasification requests process improvement due to the precipitation of the salt contained in the reactant. In conclusion, supercritical water gasification of biomass can be considered as an promising alternative process for hydrogen production. The process should be improved by more performing equipments and by the control of the salinity content of the crude biomass.

Cette thèse s'inscrit dans le cadre d'une collaboration étroite entre la société Innoveox (http://www.innoveox.com) et le groupe Fluides Supercritiques de l'Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB). La société Innoveox a mis en œuvre une technologie révolutionnaire non polluante qui détruit des déchets qu'ils soient dangereux ou non, urbains, industriels ou même militaires, avec une efficacité de 99.99%. Cette technologie s'appelle l'oxydation hydrothermale supercritique. Cette technologie est étudiée à l'ICMCB depuis le début des années 1990 au sein du groupe Fluides Supercritiques. Ce travail de recherche a pour objet l'étude des phénomènes de précipitation des sels en eau supercritique. La motivation de ce projet repose sur la compréhension des processus mis en jeu lors de la précipitation afin d'améliorer le fonctionnement du procédé industriel. Ce projet de thèse repose sur 3 principaux objectifs. Le premier concerne la détermination expérimentale de la solubilité des sels en conditions supercritiques, via la mise en place d'un système de mesure en continu. Le second objectif porte sur la détermination expérimentale des tailles de particule de sel, aussi bien au niveau de la formation de la cristallite que de l'agrégat. Pour ce faire, des analyses in situ sont notamment réalisées. Enfin, le dernier objectif est la réalisation d'une simulation numérique de la précipitation des sels en conditions supercritiques, utilisant les résultats expérimentaux des deux précédents objectifs. Les résultats de cette modélisation sont en accord avec les résultats expérimentaux et apportent également de nouveaux éléments de compréhension du mécanisme de précipitation. / This thesis is part of a close collaboration between the Innoveox Company (http ://www.innoveox.com) and the Supercritical Fluids group in the Institute of Chemistry and Condensed Matter of Bordeaux (ICMCB). The Innoveox Company has developed a revolutionary nonpolluting technology, degrading toxic and dangerous waste from industrial, urban or military use, with a 99.99% efficiency. This technology is called Supercritical Water Oxidation and is studied at the ICMCB since the beginning of the 90's in the Supercritical Fluids Group. The present research work objective is the study of the salt precipitation phenomenon in supercritical water, with the aim of understanding the processes and mechanisms involvedduring precipitation, to then improve the industrial process. The project is built on 3 main objectives. The first one is dedicated to the experimental determination of solubility data of salts, in supercritical conditions, with the development of a continuous experimental set-up. The second objective is related to the experimental determination of particle sizes, from crystallite to aggregates formation, using in situ analyses. Last objective concerns the numerical modeling of salt precipitation in supercritical water conditions, using previous experimental data obtained. The results of the modeling are in agreement with the experimental ones, and bring new understanding elements for the precipitation mechanism.technology is called Supercritical Water Oxidation and is studied at the ICMCB since the beginning of the 90's in the Supercritical Fluids Group.

L’eau supercritique peut être utilisée comme caloporteur à haute pression pour améliorer le rendement des centrales électriques. Pour un concept de réacteur nucléaire, la durée de vie des matériaux est un paramètre important en termes de sécurité. Par conséquent, les critères de sélection des matériaux pour un concept de réacteur à l’eau supercritique concernent les propriétés mécaniques à haute température pour une bonne tenue au fluage et à l’irradiation mais également une résistance à la corrosion généralisée et à la corrosion sous contrainte. Ce travail à pour objectif d’améliorer la compréhension des mécanismes de corrosion en eau supercritique à 600°C et 25 MPa pour des alliages c.f.c contenant du fer, du nickel et du chrome. Des essais de corrosion ont été réalisés sur des autoclaves échantillons d’alliages 316L et 690 en prenant en compte l’état de surface. Les couches d’oxydes formées ont été décrites en termes de morphologie, de composition et de structure, après caractérisations par microscopie électronique à balayage, par spectroscopie à décharge luminescente et par diffraction des rayons X. Si un comportement de type gazeux de l’eau supercritique est attendu dans les conditions d’essai, les résultats montrent une dissolution significative de certains éléments de l’alliage. Par conséquent, la corrosion en eau supercritique peut être considérée comme similaire à la corrosion aqueuse avec un effet de la température qui peut influencer la diffusion en phase solide par exemple. Pour l’alliage 690, la couche d’oxyde protectrice formée sur une surface polie est composée de chromine et surmontée d’un chromite ou d’un spinelle mixte de nickel et de fer. La double couche d’oxyde formée sur une surface de même finition pour l’alliage 316L semble moins protectrice. La couche externe de magnétite est poreuse et la couche interne riche en chrome est non homogène. Pour chaque alliage, l’étude des mécanismes de diffusion, grâce à des expériences utilisant des marqueurs ou des traceurs, révèle une croissance de la couche d’oxyde contrôlée par un processus anionique. Cependant, l’état de surface influence fortement les mécanismes de formation des couches d’oxyde. La comparaison des résultats sur l’acier suggère qu’il y a une concurrence entre l’oxydation du fer et celle du chrome. Une quantité suffisante de chrome est nécessaire pour former une fine couche d’oxyde protectrice. Les surfaces très déformées ou à microstructure à grains très fins conduisent à des fines couches d’oxyde de chrome, grâce à une forte densité de site de germination ou grâce aux courts-circuits de diffusion du chrome. L’état de surface est donc déterminant pour l’acier mais le même paramètre engendre des effets différents pour l’alliage à base de nickel. Les surfaces usinées deviennent sensibles à une oxydation interne du chrome, même si une fine couche d’oxyde continue et riche en chrome et manganèse se forme. Ce phénomène suggère une diffusion accélérée concurrentielle entre l’oxygène et le chrome. Pour conclure, ce travail propose un mécanisme de croissance de la couche d’oxyde dans chaque cas et discute des conditions favorables à la formation d’une couche d’oxyde protectrice riche en chrome dans l’optique d’une application au réacteur à eau supercritique. / Supercritical water can be use as a high pressure coolant in order to improve the thermodynamic efficiency of power plants. For nuclear concept, lifetime is an important safety parameter for materials. Thus materials selection criteria concern high temperature yield stress, creep resistance, resistance to irradiation embrittlement and also to both uniform corrosion and stress corrosion cracking.This study aims for supplying a new insight on uniform corrosion mechanism of Fe-Ni-Cr f.c.c. alloys in deaerated supercritical water at 600°C and 25MPa. Corrosion tests were performed on 316L and 690 alloys as sample autoclaves taking into account the effect of surface finishes. Morphologies, compositions and crystallographic structure of the oxides were determined using FEG scanning electron microscopy, glow discharge spectroscopy and X-ray diffraction. If supercritical water is expected to have a gas-like behaviour in the test conditions, the results show a significant dissolution of the alloy species. Thus the corrosion in supercritical water can be considered similar to corrosion in under-critical water assuming the higher temperature and its effect on the solid state diffusion. For alloy 690, the protective oxide layer formed on polished surface consists of a chromia film topped with an iron and nickel mixed chromite or spinel. The double oxide layer formed on 316L steel seems less protective with an outer porous layer of magnetite and an inhomogeneous Cr-rich inner layer. For each alloy, the study of the inner protective scale growth mechanisms by marker or tracer experiments reveals that diffusion in the oxide scale is governed by an anionic process. However, surface finishes impact deeply the growth mechanisms. Comparisons between the results for the steel suggest that there is a competition between the oxidation of iron and chromium in supercritical water. Sufficient available chromium is required in order to form a thin oxide layer. Highly deformed or ultra fine microstructure surfaces lead to thin chromium rich oxide layers thanks to either diffusion short circuiting or increasing Cr oxide nucleation site. The nature of the surface is a determining factor in the steel instance. The same parameter breeds different effects for the Ni-based alloy. Machined surfaces lead to internal oxidation on alloy 690 even if a thin Cr and Mn rich oxide scale is formed. Competitive diffusion of oxygen and Cr species through the diffusion short circuit paths of the alloy is suggested. This work proposes oxide growth mechanisms for each case. Finally the conditions leading to the formation of chromium-rich protective oxide films in supercritical water are discussed.

Electronic coarse graining is a technique improving the predictive power of molecular dynamics simulations by representing electrons via a quantum harmonic oscillator. This construction, known as a Quantum Drude Oscillator, provides all molecular long-range responses by uniting many-body dispersion, polarisation and cross interactions to all orders. To demonstrate the predictive power of electronic coarse graining and provide insights into the physics of water, a molecular model of water based on Quantum Drude Oscillators is developed. The model is parametrised to the properties of an isolated molecule and a single cut through the dimer energy surface. Such a parametrisation makes the condensed phase properties of the model a prediction rather than a fitting target. These properties are studied in four environments via two-temperature adiabatic path integral molecular dynamics: a proton ordered ice, the liquid{vapour interface, supercritical and supercooled water. In all these environments, the model predicts a condensed phase in excellent agreement with experiment, showing impressive transferability. It predicts correct densities and pressures in liquid water from 220 K to 647 K, and a correct temperature of maximum density. Furthermore, it predicts the surface tension, the liquid-vapour critical point, density of ice II, and radial distribution functions across all conditions studied. The model also provides insight into the relationship between the molecular structure of water and its condensed phase properties. An asymmetry between donor and acceptor hydrogen bonds is identified as the molecular scale mechanism responsible for the surface orientation of water molecules. The dipole moment is identified as a molecular scale signature of liquid-like and gas-like regions in supercritical water. Finally, a link between the coordination number and the anomalous thermal expansion of the second coordination shell is also presented.

L'objectif de cette thèse est d'étudier la valorisation de la liqueur noire non soufrée par deux procédés hydrothermaux : la gazéification en eau supercritique et la liquéfaction hydrothermale. Ceux-ci seront comparés au procédé actuel de valorisation (évaporation puis combustion dans chaudière Tomlinson) selon 3 critères : le rendement énergétique, la récupération du sodium et la production de molécules aromatiques biosourcées.Lors de la gazéification, il a été montré que la formation de gaz est compétitive à celle de char. Une chauffe rapide et des températures élevées vont favoriser le rendement gaz et donc le rendement énergétique. Cependant les rendements énergétiques sont plus faibles que le procédé actuel car la conversion des composés aromatiques provenant de la lignine est faible dans la gamme de température étudiée. Lors d'un procédé en continu, à plus haute température (700°C) avec une chauffe rapide, le rendement énergétique peut être le double au procédé actuel (simulé à l'équilibre thermodynamique). La préhydrolyse du bois et l'utilisation de bois de résineux vont défavoriser la conversion de la liqueur noire en gaz.La liquéfaction quant-à-elle permet la formation composés phénoliques et d'un biocrude dont la combustion permettant de meilleur rendements énergétique que le procédé actuel. En effet, la lignine de la liqueur noire est hydrolysée en fragments réactifs, pouvant être soit dégradés soit se recombiner pour former le biocrude. Cette dernière est favorisée par la présence des carbohydrates. L'utilisation de bois de feuillus et la préhydrolyse vont améliorer le rendement énergétique.La récupération du sodium est satisfaisante pour les deux procédés, validant la faisabilité de la substitution de la chaudière par ces procédés hydrothermaux. / This thesis aims to study sulfur free black liquor valorization through two hydrothermal processes: supercritical water gasification and hydrothermal liquefaction. These processes will be compared to the industrial process (evaporation and Tomlinson boiler) with 3 mains criteria: energetic yield, sodium recovery and phenolic molecules production.In supercritical conditions, gas formation is competitive with char formation. Fast heating and high temperature permit to increase gas yield, thus energetic yield. However, conversion of phenolic compounds from lignin is low below 500°C, leading to a lower energetic yield than reference. In a continuous process, at high temperatures (700°C) and fast heating, energetic yield should be 2 times higher than industrial process (simulation at thermodynamic equilibrium). Wood prehydrolysis and softwood lead to a lower conversion of black liquor.Hydrothermal liquefaction produces a biocrude which can be burnt and phenolic platform compounds. Indeed, lignin is depolymerized into reactive fragments which can be degraded into platform phenolic molecules. Moreover, the recombination of these fragments, leading to biocrude formation, is favored by the carbohydrates derivatives in black liquor. Wood prehydrolysis and hardwood lead to better energetic and phenolic molecules yields.Sodium recovery is satisfactory for both processes. Substitution of Tomlinson recovery by a hydrothermal process is then possible.

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No. of bitstreams: 2 Dissertação - Lucas Clementino Mourão - 2018.pdf: 2137358 bytes, checksum: fb0f843e55d7839ce63dfaae63046a2b (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2018-07-19 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The great environmental concern, coupled with the risk of depletion of non-renewable raw material, has driven the search for new sustainable technologies with major concern to the reduction of pollutant emissions. Hydrogen, a chemical of enormous importance to industrial plants, stands out as a clean and renewable energy source. This chemical is commonly produced from non-renewable sources, such as natural gas reforming. Due to specific reaction conditions, the supercritical water gasification of wet biomass is a promising way for the production of hydrogen and others high added value fuel gases. Ethanol is an attractive material because it is renewable, has low toxicity compared to other resources and has high hydrogen content in its molecule. In order to become this technology viable, a decisive point is the development of a catalyst aiming at cost reductions and high selectivity to the products of interest. In this work, ethanol gasification was carried out in supercritical water with heterogeneous catalysts. The tests were performed on an Inconel Alloy 625 tubular reactor under the following operating conditions: temperatures of 400, 450, 500, 550, 600 and 650 ºC, pressure of 250 bar, 5 g loading of heterogeneous catalyst, reactor feed: ethanol/water molar ratio of 1:10 and mass flow rate of 5 g/min. The catalysts were synthesized by wet impregnation method using aqueous solutions of Cu, Co and Zn nitrates as precursors for the active phase and spherical pellets of Al 2 O 3 as catalytic support. The catalysts and the catalytic support were characterized by Thermogravimetry and Differential Thermal Analysis (TG/DTA), X-Ray Fluorescence (XRF), Scanning Electron Microscopy (SEM), textural analysis by Adsorption/Desorption Isotherms of N 2 at 77 K and X-Ray Diffraction (XRD). The gasification results indicated that H 2 production was mainly due to ethanol dehydrogenation. The catalysts showed higher conversions than observed for catalytic support only. The CuAl catalyst showed higher H 2 selectivity as well as higher H 2 molar flow at 650 °C. The CoZnAl catalyst showed a high tendency for C 2 H 4 formation at any reaction temperature, especially at 650 °C. / A grande preocupação ambiental, junto da possibilidade de insuficiência de matéria prima não renovável, tem estimulado a busca de novas tecnologias sustentáveis com maior atenção à emissão de poluentes. O hidrogênio, substância química de enorme importância nas industriais, destaca-se como uma fonte de energia limpa e renovável. Hidrogênio é comumente produzido a partir de fontes não renováveis, como na reforma à vapor do gás natural. Devido a características reacionais específicas, a gaseificação de biomassas úmidas em meio contendo água em condições supercrítica é um caminho promissor para a produção de hidrogênio e outros gases combustíveis com alto valor agregado. O etanol se mostra um material atraente pois é renovável, apresenta baixa toxicidade em comparação com outros recursos e possui alto teor de hidrogênio em sua molécula. Em busca de viabilizar tal tecnologia um ponto determinante é o desenvolvimento de catalisadores visando reduções de custo e aumento de seletividade aos produtos de interesse. Neste trabalho foram realizados testes de gaseificação de etanol em água supercrítica com catalisadores heterogêneos. Os testes foram executados em reator tubular feito de Inconel 625 sob as seguintes condições operacionais: temperaturas de 400, 450, 500, 550, 600 e 650 ºC, pressão de 250 bar, carga de 5 g de catalisador heterogêneo, alimentação do reator com razão molar de Etanol:Água de 1:10 e vazão mássica de alimentação de 5 g/min. Os catalisadores foram sintetizados a partir do método de impregnação de soluções aquosas dos nitratos precursores de Cu, Co e Zn com excesso de solvente e usando como suporte catalítico pellets esféricos de Al 2 O 3 . Os catalisadores e o suporte catalítico foram caracterizados por Termogravimetria e Análise Térmica Diferencial (TG/ATD), Fluorescência de Raios X (FRX), Microscopia Eletrônica de Varredura (MEV), análise textural por Isotermas de Adsorção/Dessorção de N 2 a 77 K e Difração de Raios X (DRX). Os resultados de gaseificação indicaram que a produção de H2 se deu principalmente a partir da desidrogenação de etanol. Os catalisadores demonstraram conversões maiores ao observado apenas para o suporte catalítico. O catalisador CuAl apresentou maior seletividade a H 2 bem como maior vazão molar de H 2 à temperatura de 650 ºC. O catalisador CoZnAl apresentou elevada tendência a formação de C 2 H 4 em qualquer temperatura de reação, especialmente à temperatura de 650 ºC.