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Interest in non-traditional fuel sources, carbon dioxide sequestration, and cleaner combustion has brought attention on gasification to supplement fossil fueled energy, particularly by a fluidized bed. Developing tools and methods to predict operation and performance of gasifiers will lead to more efficient gasifier designs. This research investigates bed fluidization and particle decomposition for fluidized materials. Experimental methods were developed to model gravimetric and energetic response of thermally decomposing materials. Gravimetric, heat flow, and specific heat data were obtained from a simultaneous thermogravimetric analyzer (DSC/TGA). A method was developed to combine data in an energy balance and determine an optimized heat of decomposition value. This method was effective for modeling simple reactions but not for complex decomposition. Advanced method was developed to model mass loss using kinetic reactions. Kinetic models were expanded to multiple reactions, and an approach was developed to identify suitable multiple reaction mechanisms. A refinement method for improving the fit of kinetic parameters was developed. Multiple reactions were combined with the energy balance, and heats of decomposition determined for each reaction. From this research, this methodology can be extended to describe more complex thermal decomposition. Effects of particle density and diameter on the minimum fluidization velocity were investigated, and results compared to empirical models. Effects of bed mass on pressure drop through fluidized beds were studied. A method was developed to predict hydrodynamic response of binary beds from the response of each particle type and mass. Resulting pressure drops of binary mixtures resembled behavior superposition for individual particles. / Master of Science

Although more than a hundred papers dealing with the agglomeration problem in combustion and gasification of biomass can be found in the literature, very few studies focusing on the bed particle layer formation process in fluidized bed combustion (FBC) and fluidized bed gasification (FBG) can be found. With increased knowledge of the bed particle layer formation process — i.e. the main route behind bed agglomeration and bed material deposition in wood combustion/gasification — suitable combinations of fuel/bed material and/or bed material management measures can be suggested. This would not only aim to reduce the risk of ash related operational problems but also to enhance the catalytic activity of the bed material (e.g. for tar removal in gasification). The present investigation was therefore undertaken to determine the layer formation process on and within typical bed materials (i.e. quartz and olivine) and for a potentially interesting new bed material, K-feldspar. Bed material samples were collected from four different combustion and two different gasification appliances: two bubbling fluidized beds (BFB) (5 kWth/30 MWth), two full-scale circulating fluidized beds (CFB) (90/122 MWth), and two dual fluidized bed gasifiers (DFB) (8/15 MWth). Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD) were used to explore layer morphology and elemental composition and to gain information about crystalline phases of the layers. Phase diagrams and thermodynamic equilibrium calculations (TECs) were used to interpret the melting behavior of the layers and the melt fragments in deposits. In addition, a diffusion model was used to interpret the layer growth process. For quartz bed particles taken from BFB, the younger particles (< around 1 day) had only one thin layer, but for particles older than 3 days, the layer consisted of inner and outer layers. In addition to the inner and outer layers, a K-rich inner-inner layer was found for bed particles taken from CFB and DFB. No outer layers were found for quartz bed particles taken from DFB. The thin/absence of an outer layer could have resulted from the more significant attrition between particles in CFB and DFB. Reduced availability of Ca and a risk of layer breakage from the particle lead to the formation of the inner-inner layer. Similar elemental compositions of the layers upon the quartz bed particles taken from different fluidized bed techniques were found. The inner-inner layers are dominated by Si, K and Ca (excluding O), and the outer layers are rich in Ca, Si and Mg, which seem to resemble more closely the fuel ash composition. The inner layers, mainly consisted of Si and Ca, were found to have higher concentrations of Ca for older particles. The layer thickness increases with particle age, but the growth rate decreases. Melt was estimated to exist in the inner layer for younger particles (< around 1 day) and in the inner-inner layer. The existence of partially melted inner-inner layers, in particles from CFB and DFB, points towards higher risk of bed agglomeration in these techniques compared to BFB. Based on the experimental results, thermodynamic equilibrium calculations, and diffusion model analyses, a layer formation process on quartz bed particle was suggested: the layer formation is initiated by reaction of gaseous K compounds with quartz to form K-rich silicate melt, which prompts the diffusion of Ca2+. The gradual incorporation of Ca into the melt followed by the precipitation of Ca-silicates, e.g. Ca2SiO4, will result in the continuous inner layer growth. However, because of increasing concentration of Ca and release of K from the inner layer, the melt disappears in the inner layer and the layer formation process gradually becomes Ca diffusion controlled. The diffusion resistance increases with increasing thickness of a more Ca-rich layer, resulting in a decreasing layer growth rate. Crack layers with similar compositions dominated by Si, K and Ca were observed in relatively old quartz bed particles. A melt was predicted to exist in the crack layer according to thermodynamic equilibrium calculations. The crack layers found in quartz particles from BFB and CFB connect with the cracks in the inner layer, whereas for bed samples collected from DFB, the crack layers were found along existing cracks in the quartz particle. The different morphologies may indicate different routes of formation for crack layers in bed particles from different fluidized bed technologies. For quartz particles from BFB and CFB, crack formation through the inner layer down to the interface between the inner layer and the core of quartz bed particle initiates the cracks in the quartz bed particle. This allows for diffusion of gaseous alkali compounds to react with quartz in the bed particle core, thereby forming crack layers. The reaction is accelerated with bridge formation between crack layers. This may later lead to the breakdown of the bed particle into smaller alkali-silicate-rich fragments. For K-feldspar bed particles from BFB and CFB, only one layer was found for particles with an age of 1 day. For bed particles with ages older than 3 days, two layers including a homogenous inner layer containing cracks and a more particle-rich outer layer can be distinguished. Compared to bed particles from BFB with similar ages, the outer layer is thinner for bed particles from CFB. The inner layer is dominated by Ca, Si and Al (excluding O), whereas the outer layer is dominated by Ca, Si and Mg. The average concentration of Ca in the inner layer increases with bed particle age. Increasing layer thickness with decreasing growth rate was found, similar to that on quartz particles. For particles from DFB, the inner layer is also mainly consisted of Ca and Si, but cracks in the inner layer were not found. For all the particles, the Ca/Si molar ratio in the layer decreases towards the bed particle core and the change of concentration is more significant at the bed particle core/layer interface. The overall inner layer growth is resultant from the gradual incorporation of Ca into the layer. For olivine bed particles from DFB, the younger bed particles (< around 24 h) have only one layer, but after 24 h, an inner layer and an outer layer appear. Furthermore, for bed particles older than 180 h, the inner layer is separated into a distinguishable Ca-rich and Mg-rich zone. Two kinds of cracks in the inner layer either perpendicular or parallel to the particle surface were observed. Compared to the younger bed particles, the Ca concentration in the layer of older particles is much higher. A detailed mechanism for layer formation on olivine particles in fluidized bed gasification (most likely also applicable to combustion) based on the interaction between woody biomass ash and olivine has been proposed. The proposed mechanism is based on a solid-solid substitution reaction. However, a possible enabling step in the form of a Ca2+ transport via melts may occur. Ca2+ is incorporated into the crystal structure of olivine by replacing either Fe2+ or Mg2+. This substitution occurs via intermediate states where Ca-Mg silicates, such as CaMgSiO4, are formed. Mg2+ released from the crystal structure most likely forms MgO, which can be found in a distinguishable zone between the main particle layers. Due to a difference in the bond lengths between Mg/Fe and incorporated Ca2+ with their respective neighboring oxygen atoms, the crystal structure shifts, resulting in formation of cracks. The dominating elements in the inner layers are similar for each kind of bed material from BFB, CFB, and DFB, indicating limited effects of atmosphere on the inner layer formation. The initiation of layer formation differs depending on the bed material, but increasing Ca concentration in the inner layer with time for all bed materials indicates that the layer growth resulted from the incorporation of Ca into the layer. Compared to quartz, K-feldspar and olivine are more promising bed materials in wood combustion/gasification, especially in CFB and DFB techniques, from the perspective of mitigating bed agglomeration and bed material deposit build-up.

Over the past few decades, a growing interest in the thermal conversion of alternative fuels such as biomass and waste-derived fuels has been observed among the energy-producing companies. Apart from meeting the increasing demand for sustainable heat and power production, other advantages such as reducing global warming and ameliorating landfilling issues have been identified. Among the available thermal conversion technologies, combustion in grate-fired furnaces is by far the most common mode of fuel conversion. In recent years, Fluidized-Bed (FB) technologies have grown to become one of the most suitable technologies for combustion and gasification of biomass and waste-derived fuels.In spite of the benefits, however, some difficulties are attributed to the thermal conversion of the alternative fuels. Ash-related issues could be a potential problem, as low-grade fuels may include considerable concentrations of ash-forming elements such as K, Na, S, Ca, Mg, P, Si and Cl. These elements undergo many undesirable chemical and physical transformations during the thermal conversion, and often cause operational problems such as deposition-related issues, slag formation in furnaces, corrosion of the heat transfer surfaces, and bed agglomeration of the fluidized-beds. Ash-related problems in the utility boilers are a major concern that may result in decreased efficiency, unscheduled outages, equipment failures, increased cleaning and high maintenance costs.This thesis investigated the ash behavior and ash-related problems in two different FB conversion systems: a Bubbling Fluidized-Bed (BFB) boiler combusting solid waste, and a Dual Fluidized-Bed (DFB) gasifier using biomass as feedstock. Full-scale measurements, chemical analysis of fuel and ash, as well as thermodynamic equilibrium modeling have been carried out for the BFB boiler (Papers I-IV), to investigate the impact of reduced-bed temperature (RBT) and also co-combustion of animal waste (AW) on the ash transformation behavior and the extent of ash-related issues in the boiler. For the DFB gasifier (Paper V), a thermodynamic equilibrium model was developed to assess the risk of bed agglomeration when forest residues are used as feedstock.The experimental results showed that the RBT and AW co-combustion could decrease or even resolve the ash-related issues in the BFB boiler, resulting in a lower deposit-growth rate in the superheater region, eliminating agglomerates, and a less corrosive deposit (in RBT case). Thermodynamic equilibrium modeling of the BFB boiler gave a better understanding of the ash transformation behavior, and also proved to be a reliable tool for predicting the risk of bed agglomeration and fouling. The modeling of the DFB gasifier indicated a low risk of bed agglomeration using the forest residues as feedstock and olivine as bed material, which was in good agreement following the observations in a full-scale DFB gasifier.

Le procédé par coprécipitation est l’un des plus utilisé dans l’industrie nucléaire pour le traitement des effluents liquides radioactifs car il peut être appliqué à tous les effluents quelque soit leur composition. Ce procédé consiste à former in situ des particules solides par précipitation dans le but de capter sélectivement un ou plusieurs radioéléments. L’objectif de ce travail de thèse est de mettre en évidence les phénomènes impliqués lors de la coprécipitation d’un élément présent en faible concentration. Pour cela, cette étude propose une nouvelle modélisation des phénomènes de coprécipitation dont l’originalité tient dans la possibilité de simuler le phénomène hors équilibre thermodynamique et à l’échelle d’un réacteur chimique. Ce modèle, couplé avec la résolution du bilan de population, permet d’identifier l’influence des paramètres de procédés (débits, agitation…) sur la décontamination. Afin d’éprouver ce nouveau modèle, celui-ci est appliqué au traitement, dans les conditions industrielles, du strontium par le sulfate de baryum en réacteur continu et semi-fermé. A partir de ces simulations, des lois d’évolution de l’efficacité du traitement en fonction de différents paramètres de procédé (Temps de passage ou d’injection, agitation, concentration de BaSO4) ont été dégagées puis vérifiées expérimentalement. Cette étude permet de définir les meilleures conditions de traitement. Trois dispositifs (à recyclage, à lit fluidisé et réacteur/décanteur) permettant d’approcher ces meilleures conditions ont été testés avec succès. Ceux-ci ouvrent d’importantes perspectives pour la réduction de la quantité de boue produite. Deux brevets ont été déposés suite à ce travail / To decontaminate liquid nuclear wastes, the coprecipitation process is the most commonly used in nuclear field because it can be applied to any type of aqueous effluents whatever their composition may be. This process deals with the in situ precipitation of solid particles to selectively remove one or more radioelements. The aim of this PhD work is to investigate phenomena which take place during the coprecipitation of a trace component. To reach this objective, we have proposed a new modelling of the coprecipitation mechanism. The originality of this new approach lies in the possibility to simulate the phenomenon in non equilibrium conditions and at the reactor scale. This modelling combined with the resolution of the population balance, enable to identify the influence of process parameters (flowrates, stirring speed…) on crystal size and ultimately on decontamination. To test this new modelling, simulations of the coprecipitation of strontium ions with barium sulphate have been performed in continuous and semibatch reactors. Thanks to these simulations, laws of the treatment efficiency variation as a function of several process parameters (mean residence time, stirring speed, BaSO4 concentration) have been determined and experimentally verified. This study leads to the determination of optimal treatment conditions. Three apparatus (recycling apparatus, fluidised bed and reactor/settling tank) providing these optimal conditions have been successfully tested and offered significant outlooks for the reduction of the volume of sludge produced by the process. Two new processes are patent pending

L'agglomération humide est une étape cruciale dans la technologie de mise en forme des poudres de par son fort impact sur la qualité finale des produits élaborés. Malgré son intérêt lors de l'élaboration de nombreux produits (aliment, pharmaceutique, génie civil,…), ainsi que les différents modes technologiques (malaxeurs horizontaux, verticaux, à faible ou fort de taux de cisaillement, lit fluidisé, …), elle reste insuffisamment maîtrisée de par la difficulté à proposer des diagrammes de fonctionnement opérationnels qui couplent les contributions croisées des procédés et des produits. Ce travail de thèse s'inscrit dans cette problématique et porte sur l'étude des mécanismes d'agglomération en lien avec les caractéristiques de la formulation et les paramètres opératoires. L'identification et la représentation du mode d'assemblage des particules au cours de l'opération de mouillage/malaxage est l'un des enjeux central de ce travail. A partir d'expériences menées avec deux types de poudres alimentaires (semoule de blé dur et poudre de lait), l'agglomération est induite soit par contacts humides en malaxeur à faible taux de cisaillement soit par des collisions entre les particules collantes en lit fluidisé. Les mécanismes d'agglomération humide de la semoule de blé dur développés dans un malaxeur à faible cisaillement sont décrits selon un processus de morphogénèse qui corrèle la taille des agglomérats à leur texture via une dimension fractale. L'influence des propriétés physicochimiques des liants liquides sur les mécanismes d'agglomération est étudiée au regard des paramètres du modèle fractal. L'étude fine des distributions des propriétés hydrotexturales et dimensionnelles des structures agglomérées a permis d'identifier leur « règle » d'agencement : association par même classe hydro-texturale et dimensionnelle. Les régimes d'agglomération en fonction du mode d'apport d'eau (débit faible, débit fort) sont étudiés en se basant sur la notion de flux d'atomisation adimensionné. En ce qui concerne l'agglomération de la poudre de lait en lit fluidisé, les mécanismes d'agglomération sont étudiés en suivant l'évolution des tailles et des teneurs en eau des échantillons prélevés durant l'opération de mouillage/séchage et par la suite l'étude de l'impact de certaines conditions opératoires et de formulation sur les propriétés des agglomérats finaux et les régimes d'agglomération. Un lien entre les procédés est discuté et ouvre sur une requalification des régimes d'écoulement granulaire ainsi que de la réactivité de surface des particules. / Wet agglomeration is a crucial step in the shaping technology of powders because of its strong impact on the final quality of the elaborated products. Despite its interest in the elaboration of many products (food, pharmaceutical, engineering,...) as well as different technological process (horizontal mixers, vertical, low or high shear rate, fluidized bed, ...), it remains insufficiently mastered due to the difficulty in establishing operating diagrams taking into account, in the same time, products and process contributions. This thesis focuses on the study of the agglomeration mechanisms related to the characteristics of the formulation and process parameters. The identification and representation of the way particles come together during the wetting / mixing operation is one of the central challenges of this work. From experiments with two different food powders (durum wheat semolina and milk powder), agglomeration is induced either by wet contacts in mixer at low shear rate or by collisions between sticky particles in a fluidized bed. Mechanisms of semolina wet agglomeration developed in a low shear mixer are described as a process of morphogenesis that correlates the agglomerates size to their texture via a fractal dimension. The influence of the liquid binder physicochemical properties on the agglomeration mechanisms is studied according to the values taken by the fractal parameters of the model. The detailed study of the distribution of size and textural properties of agglomerated structures make it possible to identify the rule that lead to their layout: the association is mainly possible between structures belonging to the same class concerning their hydro-textural and dimensional characteristics. Agglomeration regimes depending on the mode of water supply (low flow, high flow) are studied based on the notion of dimensionless spray flux. Concerning milk powder agglomeration in a fluidized bed, agglomeration mechanisms are studied by following the evolution of the size and water content of the samples taken during the operation of wetting / drying and thereafter studying the impact of some operational conditions and formulation on the properties of the final agglomerates and agglomeration regimes. A link between the processes (low shear mixer and fluidized bed) is discussed and opens onto a redefinition of granular flow regimes and the surface reactivity of particles.

L’objectif de ce travail est de concevoir, développer, mettre en œuvre et optimiser une technologie continue permettant une bonne maîtrise des réactions de précipitation, processus chimique rapide donnant naissance à une phase solide. Ce précipiteur est un réacteur multifonctionnel à lit fluidisé qui n’a pas connu le même essor que les cristallisoirs à lit fluidisé. Deux approches expérimentales ont été adoptées : (1) l’étude des phénomènes de mélange des réactifs et (2) l’étude de l’influence des conditions opératoires sur la précipitation d’un produit modèle. Le produit modèle choisi pour cette étude est la calcite, le polymorphe le plus stable du carbonate de calcium qui possède trois polymorphes : la vaterite, l’aragonite et la calcite. Cette dernière est obtenue via la réaction de précipitation entre les solutions de chlorure de calcium et de carbonate de sodium à une température de 20° C et en présence d’un agent complexant (citrate de sodium) pour orienter la forme cristalline recherchée. Une étude cinétique a été menée pour la détermination des cinétiques de nucléation et de croissance cristalline de la calcite en milieu citrate. Le réacteur à lit fluidisé, de volume de 10 L, constitué de deux zones : cylindrique de fluidisation et de classification des particules et cylindro-conique de décantation, a été conçu au Laboratoire des Sciences du Génie Chimique. L’étude du mélange global, réalisée à l’aide de la réaction de décoloration acide-base et l’étude hydrodynamique, réalisée à l’aide de suspensions de microbilles, ont permis de développer et d’optimiser ce réacteur multifonctionnel. La faisabilité de la précipitation de la calcite en lit fluidisé a été ensuite vérifiée. Les expériences effectuées en présence d’une charge solide conséquente ont permis de diminuer la sursaturation et de favoriser la croissance cristalline. Le précipité obtenu présente une distribution de taille relativement étroite et la forme des particules obtenues est sensiblement sphérique. Enfin, des essais de modélisation du réacteur à lit fluidisé ont été entrepris pour réaliser des simulations à l’aide du code de calcul commercial FLUENT / The objective of this work is to conceive, develop, implement and to optimize a continuous technology allowing a good control of the precipitation reactions, fast chemical process, giving birth to a solid phase. This precipitor is a multipurpose engine with fluidized bed which did not make the same great strides like fluidized bed crystallizers. Two experimental approaches were adopted : (1) the study of the mixing phenomena of the reagents and (2) the study of the influence of the operating conditions on the precipitation of a model product. The model product selected for this study is the calcite, the polymorphic most stable phase of calcium carbonate which has three polymorphs: vaterite, aragonite and calcite. The latter is obtained by the precipitation reaction between the calcium chloride and sodium carbonate solutions at a temperature of 20° C and a complexing agent (sodium citrate) to have the required crystalline form. A kinetic study was undertaken for the determination of the nucleation and crystalline growth kinetics of calcite in citrate medium. The fluidized bed reactor, having a volume of 10 L, consisted of two zones: cylindrical for fluidization and classification of the particles and cylindro-conical for decantation, was conceived at the Chemical Engineering Science Laboratory (LSGC). The study of mixing phenomena, performed using the decoloration acid-base reaction and the hydrodynamic study, carried out using suspensions of glass microballs, made it possible to develop and optimize this multipurpose reactor. The feasibility of the precipitation of calcite in a fluidized bed was then checked. The experiments carried out in the presence of an important solid content made it possible to decrease supersaturation and to support the crystalline growth. The precipitate obtained presents a relatively narrow size distribution and the particle shape is appreciably spherical. Lastly, the modelling tests of the fluidized bed reactor were undertaken to carry out simulations using the FLUENT commercial computer code