Response analysis of saturable reactors and tap changer in an aluminium smelting plant

Aluminium smelters are one of the peculiar loads, which typically represent a series of electrolytic cells supplied by a multi-pulse rectifier system. In order to address the complexity involved in the modelling of a smelting plant, the impact of multipulse configurations and its connection to HV networks and control aspects of load current should be considered. This paper addresses the issues related to the control of DC current using saturable reactors for different alumina feeding mechanisms in a typical smelter. The saturable reactors are modelled as variable inductors of which the magnitude is controlled using a proportional-integral control scheme. A smelting plant with a twelve pulse rectifier system and variable DC load has been modelled in PSCAD®/EMTDC©. The plant is connected to the AC network through 220 kV/110 kV transformer with tap changing facilities to control the secondary voltage and associated load current of the smelter. The simulation results are reported for the control of DC current under varying load conditions

Multivariate statistical analysis of Hall-Héroult reduction cells : investigation and monitoring of factors affecting performance

Les cuves d'électrolyse utilisées pour la production aluminium sont soumises à des variations de la qualité des matières premières, à des perturbations diverses encourues en cours de production ou en cours de démarrage. Il est connu que ces perturbations ont un impact sur la durée de vie des cuves ainsi que sur l'efficacité de production, métallurgique et énergétique. L'amélioration des performances passe nécessairement par une meilleure compréhension des sources de variations. Plusieurs travaux ont été présentés jusqu'à présent par le biais d'études univariées entre les différents facteurs et les performances. Cependant, dans ces études, le comportement des cuves n'est pas étudié de manière multivariée, ce qui ne permet pas d'étudier les interactions entre les différentes variables. Cette thèse propose d'étudier les facteurs affectant les performances des cuves d'électrolyse, précisément la duré de vie, le rendement Faraday et la consommation énergétique, par le biais de méthodes statistiques multivariées (PCA et PLS). Premièrement, il est démontré que la durée de vie des cuves est expliquée à 72% en utilisant l'information provenant des préchauffages, des démarrages et de l'opération transitoire, démontrant ainsi l'effet de ces étapes sur la durée de vie des cuves. Cette étude est suivie d'une analyse des facteurs affectant l'efficacité de courant et la consommation énergétique des cuves. L'effet de la qualité de l'alumine, des anodes, des variables manipulées, et des variables d'états des cuves permet d'expliquer 50% des variations des performances. Cette étude démontre l'importance du contrôle de la hauteur de bain. Ainsi, une étude approfondie des facteurs affectant la hauteur de bain est effectuée. La composition du produit de recouvrement des anodes a un impact majeur sur la hauteur de bain. Malheureusement, il est présentement impossible de bien effectuer le suivi et le contrôle de cette composition puisque seulement quelques échantillons sont analysés quotidiennement. Afin de palier à ce manque, cette thèse présente une nouvelle approche, basée sur l'analyse d'image, pour prédire la composition du produit de recouvrement. Cette application faciliterait le suivi et le contrôle de la composition, ce qui améliorerait le contrôle de la hauteur de bain permettant ainsi d'améliorer les performances des cuves

Multivariate monitoring of individual anode current signals for anodic incident detection

L’aluminium est produit industriellement grâce à l’électrolyse. Ce procédé a lieu dans une cuve d’électrolyse et il consiste à injecter un courant électrique pour transformer l’oxyde d’aluminium en aluminium métallique et en dioxyde de carbone. Les anodes permettent le passage du courant à travers la cuve et fournissent également le carbone nécessaire pour la réaction électrolytique. Un incident anodique se produit lorsqu’une déformation se développe sur la surface inférieure d’une anode ou lorsque l’anode est placée trop basse dans la cuve, ce qui cause un court-circuit partiel à la position de l’anode affectée. Les incidents anodiques ont un impact négatif sur l’efficacité du courant de la cuve. La détection et la correction hâtives des incidents anodiques sont donc avantageuses d’un point de vue économique. L’objectif de cette étude est de concevoir un système qui est capable de détecter les incidents anodiques en temps réel, et ce plus rapidement que la technique standard actuelle. Pour ce faire, l’Analyse en Composantes Principales a été utilisée pour entraîner des modèles prédictifs développés à partir des signaux individuels de courant d’anodes et du signal de voltage de la cuve, dans le but de classifier les anodes selon l’erreur de prédiction au carré et la statistique T2 de Hotelling. Cette stratégie semble permettre de détecter des incidents anodiques grâce aux signaux individuels de courant. Toutefois, ce n’est pas le cas avec le signal de voltage de la cuve. La surveillance de la moyenne ou l’écart-type des signaux de courant à haute fréquence à l’aide d’un modèle de régression logistique semble aussi faciliter la détection des incidents anodiques.Aluminum metal is produced industrially in electrolysis cells, in which an electric current is used to transform aluminum oxide into metallic aluminum and carbon dioxide. Anodes are used to carry the current across the electrolysis cells and provide the carbon source necessary to drive the electrolytic reaction forward. Anodic incidents occur when an anode develops a spike or other deformation on its bottom surface or when the anode is set too low in the cell, causing the electrolysis cell to partially short circuit at the affected anode position. Anodic incidents have a deleterious effect on the cell’s current efficiency, making early detection and correction of anodic incidents economically advanta-geous. The objective of this study is to develop a real-time anodic incident detection system capableof identifying problematic anodes faster than the standard contemporary technique. Principal Component Analysis models were trained with individual anode current signals and cell voltage signals, and were subsequently used to classify anodes according to the squared prediction error and Hotelling’sT2 statistic. This strategy appears to enable anodic incident detection with individual anode current signals, but not with the cell voltage signal. Monitoring the signal mean and standard deviation of high-frequency anode current signals with a logistic regression model also appears to facilitate anod icincident detection

Advanced Alumina Feeding Control of Aluminium Smelting Cell

With the trend towards operating the Hall-Heroult process at higher amperages, there is an increasing number of difficulties in the effective control of the aluminium reduction cells, due to a greater variability in cell conditions spatially and temporally. The underlying reason is that the traditional cell control methods are developed based on the measurements of cell voltage and line current, which can only represent the overall cell conditions instead of those at a localised level. The technology of measuring individual anode current signals has been attracting increasing attention, since these signals reflect the information of localised cell conditions. However, the existing studies of using individual anode current signals in improving the control performances of aluminium reduction process have limitations. The individual anode current measurements are mainly used for process monitoring and fault diagnosis, rather than process control. Therefore, new cell control approaches need to be developed. This work investigates the applications of individual anode current signals in the aluminium reduction cells for the development of advanced cell control strategies, as well as the design of a novel semi-continuous alumina feeding system. The proposed cell control methods as well as the novel alumina feeder have achieved desired performances. This work starts with the development of an optimal alumina feed control algorithm which reduces the temporal variations in average alumina concentration by controlling it at desired targets. This control algorithm, incorporated with a state estimation algorithm, is validated on an industrial aluminium smelter, which shows enhanced process performances. These include a reduction in the temporal variation in average alumina concentration, whose standard deviation is reduced by at least five times. Motivated by the uses of individual anode current signals in monitoring local cell conditions, a multi-variable alumina feed control strategy is developed. Different from the traditional feed control method, this new approach utilises the individual anode current signals with the consideration of spatially distributed dynamics within the cell, achieving a uniform distribution of alumina concentration. Based on this new feed control method, a fault-tolerant control algorithm is also developed to reduce the risk of process abnormalities caused by feeder blocking. A Model Preditive Control based control strategy is developed to reduce the variations in anode current distribution. This control approach coordinates with the multi-variable alumina feed control, and takes into the consideration of process economies to optimise the economic benefit of the process as an objective function. One of the features of this approach is its flexibility to customise different objective functions by considering various constraints, including variability of alumina concentration, alumina feed rate and beam movement. However, the application of this approach is currently limited by the situation that not many smelters are equipped with equipped with individual ACD adjustment infrastructure. A novel semi-continuous alumina feeding system is also developed. This system comprises of a semi-continuous alumina feeder and an alumina distributor. It aims to reduce the spatial variations in alumina concentration by delivering alumina continuously through several feeding points spatially distributed in the cell. Furthermore, more accurate feeding rate is ensured by a feedback controller based on the real-time weight measurement of alumina being fed. A lab-scale alumina feeding system prototype is fabricated, based on which a number of experiments are conducted to validate its effectiveness. Some contents in this thesis have been redacted due to confidentiality agreement with industrial partner

Prediction of Low-Voltage Tetrafluoromethane Emissions Based on the Operating Conditions of an Aluminium Electrolysis Cell

Greenhouse gas (GHG) generation is inherent in the production of aluminium by a technology that uses carbon anodes. Most of those GHG are composed of CO2 produced by redox reaction that occurs in the cell. However, a significant fraction of the annual GHG production is composed of perfluorocarbons (PFC) resulting from anode effects (AE). Multiple investigations have shown that tetrafluoromethane (CF4) can be generated under low-voltage conditions in the electrolysis cells, without global anode effect. The aim of this paper is to find a quantitative relationship between monitored cell parameters and the emissions of CF4. To achieve this goal, a predictive algorithm has been developed using seven cell indicators. These indicators are based on the cell voltage, the noise level and other parameters calculated from individual anode current monitoring. The predictive algorithm is structured into three different steps. The first two steps give qualitative information while the third one quantitatively describes the expected CF4 concentration at the duct end of the electrolysis cells. Validations after each step are presented and discussed. Finally, a sensitivity analysis was performed to understand the effect of each indicator on the onset of low-voltage PFC emissions. The standard deviation of individual anode currents was found to be the dominant variable. Cell voltage, noise level, and maximum individual anode current also showed a significant correlation with the presence of CF4 in the output gas of an electrolysis cell

Zero-direct-carbon-emission aluminum production by solid oxide membrane-based electrolysis process

The traditional aluminum production process (Hall-Héroult process) involves electrolyzing the alumina dissolved in the molten cryolite salt. This process is energy intensive and emits massive amounts of CO2 and other greenhouse gases. The market demand of aluminum and the environmental impact of the current aluminum production process justify research and development of alternative electrolytic processes for aluminum production that can both reduce the cost and eliminate adverse environment impacts. Solid oxide membrane (SOM) based electrolysis process is an innovative technology that has been demonstrated to successfully produce many energy-intensive metals directly from their oxides in an efficient, economical and environmentally sound way. During the SOM electrolysis process, an oxygen-ion-conducting SOM tube made of ytteria-stabilized zirconia (YSZ) separates the pre-selected molten flux with dissolved metal oxide from the inert anode assembly inside the YSZ tube. When the applied DC potential between the cathode and the anode exceeds the dissociation potential of desired metal oxide, the metal is reduced at the cathode while oxygen ions migrate through the YSZ membrane and are oxidized at the anode. Employing the inert anode allows the oxygen to be collected at the anode as a value added byproduct. In this work, a zero-direct-carbon-emission aluminum production process utilizing SOM electrolysis is presented. The molten flux used in the electrolysis process is optimized through careful measurements of its physio-chemical properties. The liquidus temperature, volatilization rate, alumina solubility, aluminum solubility, YSZ membrane degradation rate and electrical conductivity of various flux compositions were measured, and the flux chosen for SOM electrolysis was a eutectic MgF2-CaF2 system containing optimized amounts of YF3, CaO and Al2O3. Laboratory scale SOM electrolysis employing the inert anode were performed at 1100 ~ 1200oC to demonstrate the feasibility of producing and collecting aluminum while producing pure oxygen as a byproduct. The aluminum product was characterized by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). An equivalent circuit model for the electrolysis process was developed in order to identify the polarization losses in the SOM electrolysis cell.2016-12-21T00:00:00

Recent Advances in Metal, Ceramic, and Metal-Ceramic Composite Films/Coatings

This reprint gathers works on various coating materials and technologies aimed at the improvement of materials’ properties, such as corrosion resistance or biocompatibility. Systematic studies demonstrate how the structure and morphology of coatings can change the mechanical, chemical and various functional properties of materials. The reprint contributes to the better understanding of various phenomena induced by metal, ceramic or composite coatings in core materials and, thus, it can help in the more rational design of the selected material’s properties in future studies by the application of coatings

Tribocorrosion of high-strength low-alloy steels and coatings used in offshore applications.

316 p.La presente tesis doctoral se centra en el estudio del comportamiento frente a tribocorrosión (corrosión y desgaste) de aceros de alta resistencia y baja aleación (HSLA, por sus signas en inglés) y recubrimientos utilizados en aplicaciones offshore.El uso de aceros HSLA como material estructural en aplicaciones offshore ha aumentado durante la última década, debido a la creciente demanda de aceros con mayor relación resistencia/peso. Sin embargo, la resistencia a corrosión en ambientes marinos de estos aceros es baja, lo que supone elevados costes de mantenimiento y reemplazo de estructuras dañadas. Además de corrosión, los componentes y estructuras en aplicaciones offshore también se encuentran sometidos a solicitaciones mecánicas, como el desgaste. Existe un efecto sinérgico entre corrosión y desgaste, ya que el material perdido cuando estos fenómenos actúan simultáneamente es mayor que cuando lo hacen por separado. Esto da lugar a pérdidas de material superiores a las consideradas en los códigos de diseño, lo que podría tener una gran repercusión en la integridad a largo plazo de componentes y estructuras. Por lo tanto, es necesario ampliar el conocimiento en el comportamiento frente a tribocorrosion de este tipo de materiales, para poder restablecer los márgenes de tolerancia en los códigos de diseño. La primera parte de la tesis se centra en el estudio de la tribocorrosión en aceros HSLA.Además, una de las mayores soluciones utilizadas para proteger estructuras y componentes de acero frente a la corrosión son los recubrimientos. Al igual que los materiales metálicos, los recubrimientos se encuentran sometidos a desgaste, pero su selección se realiza teniendo en cuenta únicamente su efectividad frente a corrosión. Por lo tanto, es necesario evaluar el comportamiento de recubrimientos frente a condiciones de tribocorrosión, y la segunda parte de esta tesis doctoral se centra en estos estudios. Además, se ha realizado la funcionalización de un sistema de recubrimiento comúnmente utilizado mediante distintas técnicas de modificación superficial, mejorando la resistencia a corrosión y desgaste del sistema original.IK4 Teknike

Tribocorrosion of high-strength low-alloy steels and coatings used in offshore applications.

316 p.La presente tesis doctoral se centra en el estudio del comportamiento frente a tribocorrosión (corrosión y desgaste) de aceros de alta resistencia y baja aleación (HSLA, por sus signas en inglés) y recubrimientos utilizados en aplicaciones offshore.El uso de aceros HSLA como material estructural en aplicaciones offshore ha aumentado durante la última década, debido a la creciente demanda de aceros con mayor relación resistencia/peso. Sin embargo, la resistencia a corrosión en ambientes marinos de estos aceros es baja, lo que supone elevados costes de mantenimiento y reemplazo de estructuras dañadas. Además de corrosión, los componentes y estructuras en aplicaciones offshore también se encuentran sometidos a solicitaciones mecánicas, como el desgaste. Existe un efecto sinérgico entre corrosión y desgaste, ya que el material perdido cuando estos fenómenos actúan simultáneamente es mayor que cuando lo hacen por separado. Esto da lugar a pérdidas de material superiores a las consideradas en los códigos de diseño, lo que podría tener una gran repercusión en la integridad a largo plazo de componentes y estructuras. Por lo tanto, es necesario ampliar el conocimiento en el comportamiento frente a tribocorrosion de este tipo de materiales, para poder restablecer los márgenes de tolerancia en los códigos de diseño. La primera parte de la tesis se centra en el estudio de la tribocorrosión en aceros HSLA.Además, una de las mayores soluciones utilizadas para proteger estructuras y componentes de acero frente a la corrosión son los recubrimientos. Al igual que los materiales metálicos, los recubrimientos se encuentran sometidos a desgaste, pero su selección se realiza teniendo en cuenta únicamente su efectividad frente a corrosión. Por lo tanto, es necesario evaluar el comportamiento de recubrimientos frente a condiciones de tribocorrosión, y la segunda parte de esta tesis doctoral se centra en estos estudios. Además, se ha realizado la funcionalización de un sistema de recubrimiento comúnmente utilizado mediante distintas técnicas de modificación superficial, mejorando la resistencia a corrosión y desgaste del sistema original.IK4 Teknike