A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 2016 / The quality and grade of South African coal is declining simultaneously with depleting seams. This has a negative impact on power generation and the economics of coal mining and power production. The reason is that good quality coal is more difficult to mine and hence costly, thus affecting coal prices and the ability of mines to supply coal quality of the required specifications. There is a global environmental awareness around the CO2 greenhouse gas and its effect on global warming. Legislations are becoming more stringent in limiting the amount of greenhouse gases and air pollutants we produce. In power generation, the most prominent greenhouse gas is carbon dioxide (CO2) and the most prominent air pollutants are oxides of Nitrogen and Sulphur (NOx and SOx). Oxy-fuel combustion (OFC) is a process change that can reduce the production of CO2 by increasing the concentration of oxygen in combusting air. A study is presented here, that focuses on the application of this process (OFC) to South African coals. Three different coal types were studied and characterized by conventional proximate and ultimate analyses and further characterized and graded by more specialized analyses; petrographic analyses and the quantitative evaluation of minerals b scanning electron microscopy (QEMSCAN). The gasification of the coals was then modeled to determine, qualitatively, its magnitude in comparison to combustion (oxidation) in oxy-fuel combustion. However, when modeling and conducting experiments to determine this, it was found that existing empirical formulae used to quantify char burnout are not suitable for all South African types of coal. The formulae found in literature (for both oxidation and gasification) could only be applied to two of the three samples. For the two samples that were successfully modeled, it was found that reactivity in gasification was probable but not to a significant level. For the third sample that couldn’t be modeled successfully, a recommendation was made that a new model be developed to take into account the nature of low grade, high inertinite South African coal. This is required in order to successfully formulate the char burnout of South African coals and thus depict with certainty, the applicability of Oxy-fuel combustion on South African coals. Such a step would benefit the forthcoming studies on modeling the char burnout of South African coal and therefore contribute to addressing the challenge of declining coal quality in South Africa. / MT2017

A injeção de carvão pulverizado (pulverized coal injection - PCI) é uma técnica largamente utilizada nos altos-fornos pelas siderúrgicas brasileiras, seguindo uma tendência mundial, que busca reduzir o consumo específico de coque por tonelada de gusa e, em consequência, do custo do ferro gusa produzido. A combustão do carvão pulverizado ocorre sob pressões médias (de até 4 atm), temperaturas de chama elevadas (em torno de 2 000 °C), altas taxas de aquecimento (104 a 105 °C/s), tempo de residência muito curtos (inferiores a 40 ms), e é seguida pela gaseificação na presença de CO2. Como não existem métodos padronizados para a avaliação das características de combustão dos carvões para sua utilização em PCI, utilizam-se geralmente equipamentos de injeção de carvão em escalas laboratorial e de bancada, além plantas-piloto com este propósito. O objetivo principal deste trabalho é apresentar as etapas do desenvolvimento de um equipamento em escala de bancada realizado no Laboratório de Siderurgia da Escola de Engenharia da UFRGS (LASID-UFRGS), desde o projeto conceitual até a fase pré-operacional, e os resultados iniciais obtidos, para estudar as características de combustão dos carvões em condições bastante similares às que ocorrem nos altos-fornos. Os testes iniciais incluem a avaliação da combustão de um carvão brasileiro objetivando seu uso potencial em PCI, em substituição parcial de carvões importados para este fim. Optou-se por um projeto moderno, altamente automatizado, que inclui aquisição rápida de dados, com escala e conceito operacional adequados para estudos acadêmicos, de configuração vertical e que possibilitasse a operação no modo de injeção de uma amostra de carvão em pulso único, bem como uma adequação futura ao modo de injeção contínua de carvão. Algumas características relevantes do equipamento são a medição e aquisição ultrarrápida de dados termodinâmicos de pressão e temperatura em diversos pontos do sistema, permitindo o registro dos fenômenos transientes que ocorrem durante a combustão, a possibilidade de programação e controle de tempos, pressões e temperaturas para testes de combustão e de pirólise através de rotinas especialmente desenvolvidas para isto, bem como a coleta representativa dos produtos sólidos e gasosos resultantes da combustão para posterior análise. É ainda possível a filmagem da xvi combustão em modo ultrarrápido, permitindo correlacionar as imagens aos dados termodinâmicos registrados durante a combustão ao longo de um segundo, em intervalos de poucos milissegundos. Os resultados iniciais obtidos na fase pré-operacional demonstram o bom funcionamento do sistema, permitindo distinguir claramente a influência da variação de parâmetros operacionais como tipo de carvão, massa e de amostra injetada, pressão e temperatura de operação e composição dos gases oxidantes. / Pulverized coal injection (PCI) is a technique used in blast furnaces (BFs) by Brazilian steel industry, following a worldwide trend, to reduce coke consumption by ton of hot metal produced, and therefore reducing the overall cost. Burning of pulverized coal injected into tuyeres of BFs takes place under medium pressure (up to 4 atm), high flame temperatures (around 2 000 °C), very fast heating rates (104–105 °C/s) and very short residence times, less than 40 ms, followed by gasification in presence of CO2. Since there are no standard tests for evaluation of coal combustibility at PCI conditions, lab and bench scale coal injection rigs and pilot plants are usually employed for this purpose. This work shows the development steps of a bench-scale rig, built at the Iron and Steelmaking Laboratory of the School of Engineering - Universidade Federal do Rio Grande do Sul (LASID-UFRGS), from the conceptual design to the pre-operational step, as well as the initial results. This equipment permits to study the combustion characteristics of coals in conditions very close to those occurring in blast furnaces. Initial tests include the combustion evaluation of a Brazilian coal, aiming its potential usage for PCI, in partial substitution of imported coals for this purpose. The rig has a modern design and is highly automated. Its scale and operational concept is fitted for academic studies. It has a vertical configuration, to be operated with injection of a coal sample in a single pulse mode and is capable to be adjusted afterwards to continuous coal injection mode. Some relevant characteristics of the injection rig are: (1) the very fast measurement and acquisition of thermodynamic data of pressure and temperature in several points of the system, allowing the capture of transient phenomena occurring along the combustion process; (2) the possibility of programming and controlling time intervals, pressures and temperatures to perform combustion and pyrolysis tests, employing specially developed routines; and (3) the representative sample collection of solid and gaseous combustion products to be further analyzed. It is also possible to capture images of the combustion by a high-speed camera, allowing correlate the images, acquired during a time interval of one second, with the thermodynamic data collected in intervals of few milliseconds. xviii The good performance of the equipment was shown by the initial results obtained at the pre-operational phase. The experimental data clearly depicted the effect of operational parameters like coal type, injected sample mass, operational pressure and temperature, and oxidizing gas composition.

CoordenaÃÃo de AperfeiÃoamento de NÃvel Superior / Durante a queima do carvÃo mineral sÃo gerados vÃrios tipos de resÃduos ou subprodutos (produtos da combustÃo do carvÃo, ou PCCs). Esses produtos incluem as cinzas volantes, as cinzas pesadas ou de fundo, a escÃria da caldeira, e o gesso do FGD - Flue Gas Desulfurization.Embora alguns PCCs sejam reutilizados como matÃria-prima para a construÃÃo civil, a maioria deles à depositada em aterros ou represas de contenÃÃo. AlÃm de ocupar vastas extensÃes de terra, existe uma grande preocupaÃÃo com o fato de que produtos quÃmicos tÃxicos dos PCCs podem dissolver na Ãgua e essa Ãgua poluÃda pode lixiviar, contaminando tanto Ãguas subterrÃneas quanto superficiais. Este trabalho teve como objetivo fabricar e caracterizar peÃas cerÃmicas a partir de resÃduos da extraÃÃo do granito Asa Branca, com adiÃÃo de produtos gerados pela combustÃo do carvÃo de uma usina termoelÃtrica. Foram avaliadas a microestrutura e algumas propriedades fÃsicas e mecÃnicas dessas peÃas. Os materiais utilizados foram granito Asa Branca e o subproduto da combustÃo do carvÃo mineral ( fornecido pela Energia de Portugal â EDP), nas concentraÃÃes de 10%p, 20%p e 30%p. Para a caracterizaÃÃo das amostras foram utilizadas as tÃcnicas de: DifraÃÃo de Raios-X, FluorescÃncia de Raios-X, Microscopia EletrÃnica de Varredura (MEV), AnÃlise TermogravimÃtrica (ATG). Para a determinaÃÃo das propriedades fÃsicas foram realizados os ensaios de ContraÃÃo Superficial e MÃssica; AbsorÃÃo de Ãgua e Microdureza Vickers. Os resultados obtidos permitem concluir que o uso de produto da combustÃo do carvÃo mineral como reforÃo em peÃas feitas com pà de Granito Asa Branca à tecnicamente viÃvel. / During the burning of mineral coal a series of different residues or byproducts (coal combustion products, or CCPs) are generated. These byproducts include fly ash, bottom ash, boiler slag, and flue gas desulfurization gypsum. Although some types of CCPs can be used as alternative raw materials for construction products, the majority of them are deposited in landfills or surface impoundments. Besides the occupation of large areas there is a major concern with the fact that toxic chemicals from the CCPs can dissolve in water, and this polluted water can spread underground, contaminating both groundwater and surface waters. The present work investigated the fabrication and characterization of ceramic products made from Asa Branca granite powder with the addition of products generated by the combustion of mineral coal in a power plant. The microstructure and some physical and mechanical properties of these parts were evaluated. The materials used were Asa Branca Granite and a mixture of CCPs supplied by EDP (Energia de Portugal) which were added in the concentrations of 10, 20 and 30% wt. to the granite matrix. The following techniques were used for the characterization of the materials: X-ray diffraction, X-ray fluorescence, Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA). Determination of physical properties was performed by means of Surface Contraction and Weight Loss tests; Water Absorption and Vickers Microhardness. The results showed that the use of products generated by the combustion of coal as reinforcements for the parts made from Asa Branca Granite powder is technically viable.

<p>Mercury contamination in aquatic ecosystems is a concern as anaerobic aquatic sediments are the primary regions of methylmercury production in freshwater and coastal regions. Methlymercury is a bioaccumulative neurotoxin, and human exposure to methylmercury can result in impaired functioning of the central nervous system and developmental disabilities in children. To minimize the risk of human exposure to methylmercury, it is important to be knowledgeable of the various sources which can supply mercury to aquatic ecosystems as well as have a complete understanding of the biogeochemical processes which are involved in methylmercury production in aquatic systems. In this dissertation work, both mercury biogeochemical speciation in anaerobic aquatic sediments and sources of mercury to aquatic systems were addressed. </p><p>The biogeochemical speciation of mercury is a critical factor which influences the fate and transformation of mercury in aquatic environments. In anaerobic sediments, mercury chemical speciation is controlled by reduced sulfur groups, such as inorganic sulfide and reduced sulfur moieties in dissolved organic matter (DOM). The formation of mercury sulfide nanoparticles through stabilization by dissolved organic matter (DOM) was investigated in precipitation studies using dynamic light scattering. Mercury sulfide nanoparticles (particle diameter < 100 nm) were stabilized through precipitation reactions that were kinetically hindered by DOM. To further investigate the interaction between DOM and metal sulfides, similar precipitation studies were performed using zinc sulfide and a number of DOM isolates (humic and fulvic acids) representing a range of DOM properties. The results of these experiments suggest that the mechanism of metal sulfide particle stabilization may be electrostatic or electrosteric, depending on the nature of the DOM molecule.</p><p>The mercury that is methylated in aquatic systems enters these environments via a number of sources, including atmospheric deposition, landscape runoff and other industrial and municipal activities. In two separate field studies, two potential sources of mercury to aquatic systems were investigated: landscape runoff and coal combustion products. The mercury loading to aquatic environments from these sources and their potential for transformation to methylmercury were investigated.</p><p>Landscape runoff from a Duke University campus catchment (Durham, NC) was identified as a source of mercury to a stream-wetland. The source of mercury to the runoff was likely from a `legacy' source of mercury; the historic application of mercury fungicide compounds to turf grass during the 20th century. Downstream of the point where the runoff was discharged to the stream-wetland, methylmercury concentrations were detected in stream sediments (up to 11% of total mercury), suggesting that this legacy mercury could be transformed to methylmercury. </p><p>The environmental impact of coal combustion products (CCPs) with respect to mercury and methylmercury was also investigated in a river system (Roane County, TN) that was inundated with fly ash and bottom ash from the Tennessee Valley Authority Kingston coal ash spill in 2008. Elevated total mercury and methylmercury sediment concentrations (relative to upstream sediments) were detected in regions impacted by the ash spill, and our biogeochemical data suggested that the ash may have stimulated methylmercury production in river sediments.</p><p>The results of this dissertation work address the formation of mercury sulfide (along with zinc sulfide) nanoparticles in anaerobic aquatic sediments. In the current mercury methylation paradigm, dissolved mercury species such as Hg(SH)02(aq) and HgS0(aq) are assumed to be the only mercury species that are available for methylation. The results of this dissertation work suggests that in previous studies, HgS0(aq) may have been mistaken as mercury sulfide nanoparticles which may be formed in under supersaturated conditions (with respect to HgS(s)) where DOM is present. Mercury sulfide nanoparticles are a mercury biogeochemical species that has been largely ignored in the research literature and whose role in the mercury biogeochemical cycle and in mercury methylation remains to be investigated.</p><p> This dissertation work also identifies potential sources of mercury to aquatic systems, namely, landscape runoff and CCPs. Atmospheric deposition is currently considered to be the major source of mercury to inland aquatic water bodies compared to sources such as landscape runoff and CCPs. However, in the watershed studied in this dissertation, landscape runoff was identified as a larger source of mercury than atmospheric deposition, suggesting that these so-called `minor' sources may actually be major sources of mercury to watersheds depending on land usage, and should be considered in watershed models. Furthermore, the environmental hazards of mercury-associated with CCPs has typically been determined through leaching experiments, such as the Toxicity Characteristic Leaching Procedure (TCLP), which are not representative of environmental conditions and do not predict that CCPs may influence mercury methylation in aquatic sediments. Thus, in this dissertation work, we suggest that leaching protocols such as the TCLP should be re-evaluated. </p><p>Overall, this dissertation work will be useful in future studies examining mercury speciation and bioavailability to methylating bacteria in aquatic sediments, and the formation of metal sulfide nanoparticles in aquatic systems. Additionally, data on sources of mercury will be useful in developing policies for the regulation of these sources and in assessing the risk to human health from mercury methylation.</p> / Dissertation

<p>Coal fired power plants are ubiquitous in the United States and most developed countries around the world, providing affordable electricity to consumers. In the US, approximately six hundred power plants generate 136 million tons of Coal Combustion Residuals (CCRs) annually, encompassing fly ash, bottom ash, and flue gas desulfurization materials. The range and blends of CCRs varies substantially across coal-fired plants and depends on a unique set of circumstances for each plant and coal source. Current U.S. regulations mandate the installation of advanced capture technologies to reduce atmospheric pollution, but do not address the transfer and storage, or the potential impacts to water resources. Thus improved air quality is traded for significant enrichments of contaminants in the solid waste and effluent discharged from power plants. </p><p>This work examines the geochemical and isotopic characteristics of CCRs, as well as potential environmental impacts from CCRs. This investigation looks at several different aspects of CCR and environmental interactions from 1) the immediate impacts of the 2008 TVA coal ash spill in Kingston, TN, 2) the long-term (18-month) exposure of the spilled ash in the Emory and Clinch rivers, 3) impacts on waterways in North Carolina that receive CCR effluent from coal fired power plants, and 4) examination of boron and strontium isotopes of CCRs from leaching experiments and their application as tracers in the environment of the TVA spill and NC waterways. These investigations have illuminated several conclusions, including contact of surface water with CCRs leach high concentrations of leachable CCR contaminants, such as As, Se, B, Sr, Mo, and V in the surface waters; the dilution effect is critical in determining the concentration of contaminants from the CCRs in surface water (both at the spill and in waterways receiving CCR effluent); recycling of trace elements (such as As) through adsorption/desorption can impact water quality; and elevated boron and strontium concentrations, in addition to their isotopes, can trace CCR effluent in the environment. Combining the geochemical behavior and isotopic characteristics provides a novel tool for the identification CCR effluents in the environment.</p> / Dissertation

A dynamic mathematical model for simulation of atmospheric circulating fluidized bed combustors has been developed on the basis of first principles and empirical correlations. The model accounts for dense and dilute zone hydrodynamics, volatiles release and combustion, char particles combustion and their size distribution, and heat transfer from/to gas, particles, waterwalls and refractory. Inputs to the model include configuration and dimensions of the combustor and its internals, air and coal flows, coal analysis, all solid and gas properties, inlet temperatures of air, cooling water, and feed solids, size distribution of feed solids / whereas outputs include transient values of combustor temperatures, gas concentrations, char and inert hold-ups and their size distributions. The solution procedure employs method of lines approach for the governing non-linear partial differential equations and combined bisection and secant rule for non-linear algebraic equations. The initial conditions required for the model are provided from the simultaneous solution of governing equations of dynamic model with all temporal derivatives set to zero. By setting all temporal derivatives to zero, model can also be utilized for steady state performance prediction. In order to assess the validity and predictive accuracy of the model, it was applied to the prediction of the steady state behavior of Technical University of Nova Scotia 0.3 MWt CFBC Test Rig and predictions were compared with measurements taken on the same rig. Comparison of model predictions at steady state conditions revealed that the predictions of the model are physically correct and agree well with the measurements and the model is successful in qualitatively and quantitatively simulating the processes taking place in a circulating fluidized bed combustor.

Although products of coal combustion (PCCs) such as coal ash are currently exempted from classification as a hazardous waste in the United States under the 1976 Resource Conservation and Recovery Act (RCRA), the U.S. Environmental Protection Agency (EPA) is now revising a proposed rule to modify disposal practices for these materials in order to prevent contamination of ground- and surface water sources by leached trace elements. This paper analyzes several aspects of EPA’s scientific reasoning for instating the rule, with the intent of answering the following questions: 1) Are EPA’s cited values for PCC production and disposal accurate estimates of annual totals?; 2) In what ways can EPA’s leaching risk modeling assessment be improved?; 3) What is the total quantity of trace elements contained within all PCCs disposed annually?; and 4) What would be the potential costs and feasibility of reclassifying PCCs not under RCRA, but under existing NRC regulations as low-level radioactive waste (LLRW)? Among the results of my calculations, I found that although EPA estimates for annual PCC disposal are 20% larger than industry statistics, these latter values appear to be closer to reality. Second, EPA appears to have significantly underestimated historical PCC disposal: my projections indicate that EPA’s maximum estimate for the quantity of fly ash landfilled within the past 90 years was likely met by production in the last 30 years alone, if not less. Finally, my analysis indicates that while PCCs may potentially meet the criteria for reclassification as low-level radioactive waste by NRC, the cost of such regulation would be many times that of the EPA June proposed disposal rule ($220-302 billion for PCCs disposed in 2008 alone, versus $1.47 billion per year for the Subtitle C option and $236-587 million for Subtitle D regulatory options). / text

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / O aumento da produÃÃo de resÃduos gera a necessidade de tratÃ-los e/ou armazenÃ-los de forma adequada, minimizando ou eliminando os prejuÃzos ambientais. Nesta pesquisa, foram estudados os Produtos da CombustÃo do CarvÃo Mineral (PCCs) e borra branca de alumÃnio. Os PCCs foram obtidos durante a queima do carvÃo mineral em uma usina termoelÃtrica e a borra branca foi obtida durante o processo de recuperaÃÃo de alumÃnio. Com o objetivo principal de produzir um cimento a partir da moagem de alta energia, os resÃduos foram caracterizados quimicamente por DifraÃÃo de Raios X (DRX), FluorescÃncia de Raios X (FRX), Espectroscopia de Infravermelho (INFRA) e fisicamente por Tempo de Pega e ResistÃncia à CompressÃo. As anÃlises quÃmicas mostraram a presenÃa de uma das fases que constitui o cimento Portland, e outras duas fases semelhantes. No entanto, ao realizar as caracterizaÃÃes fÃsicas, os resultados mostraram a nÃo formaÃÃo de cimento. Por outro, foi observado que à possÃvel formar mulita a temperaturas mais baixas que a apresentada na literatura, utilizando moagem de alta energia e sinterizaÃÃo a 1200ÂC. A formaÃÃo de mulita a temperaturas mais baixas a partir de resÃduos apresenta grande potencial de utilizaÃÃo dos materiais estudados, na indÃstria cerÃmica. / The increased production of waste generates the need to treat them and / or store them appropriately, minimizing or eliminating environmental damage. In this research, we studied the Coal Combustion Products (CCPs) and white aluminum dross. The CCPs were obtained during the burning of coal in a power plant and the white dross was obtained during the recovery process of aluminum. With the main objective of producing cement using high-energy milling, the residues were chemically characterized by X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), Infrared Spectroscopy (INFRA) and physically by determination of Setting Time and Compressive Strength. Chemical analysis showed the presence of a phase which is observed in Portland cement, and two other similar phases. However, when performing physical characterizations, the results showed no formation of cement. On the other hand, it was observed that it is possible to form mullite at lower temperatures than the one presented in the literature using high energy milling and sintering at 1200ÂC. The formation of mullite from waste at lower temperatures presents great potential for use of CCPs and white aluminum dross in the ceramic industry.

Die Arbeit befasst sich mit der Flamelet Modellierung für die Verbrennung von Kohlenstaub. Dabei liegt der Fokus sowohl auf der detaillierten Betrachtung der Gasphasenchemie als auch auf der Interaktion der Kohle mit der Gasphase. Ziel der Arbeit ist die Entwicklung einer Methode für die Simulation großtechnischer Kohlestaubfeuerungen. Die energetische Umsetzung von Kohle läuft in drei wesentlichen Schritten ab: Verdampfung der Feuchtigkeit, Ausgasung der Kohle (Pyrolyse) und schließlich der Koksabbrand. Da die Struktur der Kohle als fossiler Brennstoff hoch komplex ist, existieren viele prädiktive, rechenaufwändige Modelle zur Beschreibung dieser Prozesse [1–4]. Diese Modelle können nicht direkt in numerischen Strömungssimulationen genutzt werden, dienen aber zur Kalibrierung einfacherer kinetischer Modelle. Diese in der Arbeit angewendete Prozedur wird in [5] beschrieben. Zur detaillierten Beschreibung des Abbaus der entstehenden höheren Kohlenwasserstoffe werden in der Simulation große Reaktionsmechanismen benötigt. Die Benutzung solcher Mechanismen ist mit großen Rechenzeiten verbunden und daher bleibt deren Anwendbarkeit auf einfache Anwendungsfälle beschränkt. Der Vorteil der Flamelet Modellierung besteht darin, dass unter bestimmten Voraussetzungen der komplette thermo-chemische Zustand, bestehend aus Temperatur, Druck und Zusammensetzung, mit nur wenigen charakterisierenden Kontrollvariablen abgebildet werden kann. Durch Vorgabe und Variation der Kontrollvariablen können diese Zustände mittels kanonischer Flammenkonfigurationen vorberechnet und in sogenannten Flamelettabellen abgespeichert werden. Für das klassische Flamelet / Fortschrittsvariablen Modell [6] wird der thermo-chemische Zustand über Mischungsbruch und Fortschrittsvariablen parametriert, dabei bestimmt der Mischungsbruch den Anteil an Brenn- stoff im Gemisch und die Fortschrittsvariable den Fortschritt der chemischen Reaktion. Die Kontrollvariablen werden in der numerischen Simulation transportiert, an Stelle der Energie- und Speziesgleichungen. Dies stellt für große Mechanismen eine dramatische Reduktion der zu lösenden Gleichungen dar. Der thermo-chemische Zustand ergibt sich per Look-up aus den Flamelettabellen. Im Zuge der Verbrennung trockener Kohle werden zwei Brennstoffe durch Pyrolyse und Koksabbrand freigesetzt. Für die Flamelet Modellierung bedeutet dies entsprechend je einen Mischungsbruch für Pyrolysegas und Produkte aus dem Koksabbrand. Neben der Fortschrittsvariablen wird ebenfalls die Enthalpie der Gasphase als Kontrollvariable benötigt aufgrund des intensiven Wärmeaustauschs zwischen Kohle und Gasphase. In der Arbeit erfolgt die Vorstellung der benötigten Transportgleichungen sowie die Beschreibung verschiedener Methoden zur Integration nicht-adiabater Zustände in Flamelettabellen. Dabei unterscheiden sich die vorgestellten Tabellierungstrategien hauptsächlich in der betrachteten Verbrennungsart. IV Erfolgt die Mischung von Brennstoff und Oxidationsmittel erst in der untersuchten Flammenkonfiguration, spricht man von Diffusionsflammenstrukturen; sind beide schon gemischt, so entstehen Vormischflammenstrukturen. Die Detektion solcher Strukturen erfolgt in der Arbeit anhand einer Flammenstrukturanalyse mittels Flammenmarker. Die prinzipielle Übertragbarkeit des Flamelet / Fortschrittsvariablen Modells auf turbulente Kohlestaubfeuerung wurde von Watanabe [7] gezeigt, jedoch ist die Bewertung der eingesetzten Flamelet Modellierung in Grobstruktursimulationen nicht ohne weiteres möglich. Deshalb werden zur Verifizierung der entwickelten Tabellierungstrategie in der Arbeit einfache Flammenkonfigurationen betrachtet, die es erlauben, direkte Chemielösungen mit den Lösungen der tabellierten Chemie zu vergleichen. Für den entsprechenden Vergleich erfolgt die Vorstellung zweier Analysen. Bei der a priori Analyse wird der thermo-chemische Zustand der detaillierten Lösung mit dem tabellierten Zustand verglichen. Für den Look-up werden dabei die Kontrollvariablen der direkten Chemiesimulation benutzt. Die a posteriori Analyse ist der Vergleich einer voll gekoppelten Rechnung unter Benutzung der Tabellierungstrategie mit der zugehörigen detaillierten Rechnung. Die erste untersuchte Konfiguration stellt eine Gegenstromanordnung mit vorgewärmter Luft und Kohlebeladung dar. Die Hauptergebnisse dieser rein numerischen Studie wurden bereits veröffentlicht [8] und es konnte die erfolgreiche Applikation der vorgestellten Tabellierungstrategie in dieser Anordnung für Tabellen basierend auf Diffusionflammenstrukturen gezeigt werden. Für die Validierung der detaillierten Rechnungen erfolgt die Nutzung experimenteller Daten [9, 10] für magere Methan-Sauerstoff-Stickstoff Mischungen in Staupunktströmungen. Es zeigt sich, dass diese Konfigurationen stark von den vorgemischten Gasflammen dominiert werden und somit Tabellen basierend auf Vormischflammenstrukturen einzusetzen sind. Die entwickelte Tabellierungsmethode ist in der Lage, auch diese Flammenstrukturen abzubilden. Abschließend wird numerisch eine Parametervariation hinsichtlich Einlassgeschwindigkeit und Kohlebeladung vorgestellt, um die Robustheit und breite Anwendbarkeit der entwickelten Tabellierungstrategie aufzuzeigen. Zusammenfassend konnte mittels Flammenstrukturanalyse für jede vorgestellte Konfiguration der zu verwendende Typ der Tabelle bestimmt werden. In den untersuchten Konfigurationen führte deren Anwendung zu einer guten Übereinstimmung mit den detaillierten Rechnungen. Damit legt diese Arbeit den Grundstein für weiterführende Betrachtung zur Simulation großtechnischer Kohlestaubfeuerungen.

>Magister Scientiae - MSc / Fly ash is the major waste material produced by power plants in the combustion of coal to generate electricity. The main constituents of fly ash are Si, Al, Fe and Ca with smaller amount of S, Mn, Na, K, and traces of many other elements such as Co, Cd, As, Se, Zn, Mo, Pb, B, Cu and Ni. Fly ash is usually disposed either by dry or wet disposal methods. These disposal methods have raised major environmental concerns due to the potential leaching of chemical species from the ash heap by ingress of rainfall and brine used to transport the fly ash to the dam. This study focuses on the changes in chemical composition, morphology and mineral phases due to weathering, of coal fly ash co-disposed with brine over 20 years at Sasol Secunda ash dump in Mpumalanga Province, South Africa. The design and operation of the Secunda ash dump presupposes that the ash dump may act as a sink for the salts which originated from chemicals used for normal operation in the plants. The majority of these salts come from the brines generated during desalination and raw water regeneration. The aim of this study is to ascertain if the ash dump could serve as a sustainable salt sink.Samples were drawn along the depth of two drilled cores (S1 and S3) from the weathered Secunda ash dump and analysed in conjunction with the fresh (unweathered) Secunda fly ash taken from the fly ash hoppers for comparative analysis. Scanning electron microscopy (SEM), X-ray diffractive (XRD) and X-ray fluorescence (XRF) spectrometry were employed to obtain a detailed morphological, mineralogical and bulk chemical composition of all the samples. Pore water analysis was used to determine the pH, EC and moisture content of fly ash samples. A five step sequential chemical extraction procedure was used to establish the geochemical association of particular elements with various mineral phases. The total acid digestion test was also used to determine the total elemental compositions of the Secunda fly ash samples. The SEM results showed that the fly ashes consist of irregular and numerous spherically shaped particles. Changes (encrustations, etchings and corrosion) in the morphologies of the weathered ash particles were also observed. The XRD results revealed quartz, mullite, lime and calcite as the major mineral phases. Other minerals identified in very minor quantities in the drilled Secunda ash core that were dried prior to analysis were halite, kaolinite, nitratine, bassanite, microline. and hydrophitte. These phases may have formed during sample handling. XRF investigation revealed that the major oxides present in the dumped ash samples were SiO₂, A₂2O₃, CaO, Fe₂O₃, MgO, Na₂O, TiO₂ and the minor elements present were K₂O, P₂O₅, SO₃ and MnO. The sum of the mean values of the % composition of SiO₂, Al₂O₃, and Fe₂O₃ was 70.19 %, and 72.94 % for the two drilled ash core samples (S1 and S3) respectively, and 78.67 % for the fresh ash which shows the significant alteration of the Si, Al and Fe content in the ash matrix over time. The fly ash is classified as Class F using the ASTM C 618 standards. The loss on ignition (LOI) which is an indication of unburned carbon or organic content was 4.78 %, 13.45 % and 8.32 % for the fresh ash, drilled ash cores S1 and S3 respectively. The high LOI values for the drilled ash cores could indicate high hydrocarbon content in the ash dump because of co-disposal practises where hydrocarbon waste are included in the brine stream for disposal on the ash. While the ash samples from the surface appeared dry, moisture content (MC) analysis showed that there is considerable water entrained in the fly ash dump. The fresh ash MC was 1.8 % while core S1 ranged from 41.4 – 73.2 %; core S3 ranged from 21.7 – 76.4 %. The variations in the MC values can be attributed to uneven flow paths due to inconsistent placement conditions or variations in ambient weather conditions during placement. The fresh fly ash (n=3) had a pH of 12.38±0.15, EC value of 4.98±0.03 mS/cm and TDS value of 2.68±0.03 g/L, the pH of the drilled ash core S1 (n=35) was 10.04 ±0.50, the EC value was 1.08±0.14 mS/cm and the TDS value was 0.64 ±0.08 g/L. Core S3 (n=66) had pH of 11.04±0.09; EC was 0.99 ±0.03 and TDS was 0.57 ± 0.01. The changes in pH values can be attributed to the dissolution and flushing out from the dump basic alkaline oxides like CaO and MgO These variations in pH values shows that the fly ash is acidifying over time and metal mobility can be expected under these conditions. The large decrease of EC in the drilled ash cores S1 and S3 compared to the fresh ash indicated a major loss of ionic species over time in the ash dump. The sequential extraction scheme revealed that the elements Al, Si, Ca, Mg, Ba, Sr, Fe, Mn, Na, K, As, Pb, Cr, Mo, Cu, Ni and Zn are present in Secunda fresh and weathered fly ash and are partitioned between the water soluble, exchangeable, carbonate, iron and manganese, and residual fractions of the coal fly ash. It also showed that the trace elements As, Pb, Cr, Mo, Cu, Ni and Zn do not show permanent association with particular mineral phases as a continuous partitioning between different mineral phases was observed in the weathered drilled core. Generally, all the elements had the highest concentration in the residual fraction. But it was evident that the labile phase (water soluble, exchangeable and carbonate fractions) had fairly high concentrations of Si (± 6.5 %), Al (± 6.5 %), Ca (±10 %), Mg (± 5.5 %), Ba (± 7.5 %),Sr (± 7.5 %), Na (± 12 %) and K (± 12 %) for the Secunda drilled ash core (S1 and S3) and fresh fly ash samples. This indicates that these species can leach easily upon water ingress and could pose a danger to the environment. Na and K had the highest concentrations leached out in the labile phase in all the ash samples. The amount of Na leached out of the drilled Secunda ash core in the labile phase was 13.21 % of 18584.26 mg/kg in the five geochemical phases of core S1; and 9.59 % of 11600.17 mg/kg in the five geochemical phases of core S3 while the fresh Secunda fly ash leached out 11.28 % of 16306.30 mg/kg of Na in the five geochemical phases. This study provided significant insight into the pore water chemistry, morphology, mineralogy and chemical composition and the elemental distribution pattern of the major and trace elements in the Secunda fly ash and weathered drilled Secunda ashm core S1 and S3. Though results from XRF analysis and the sequential extraction scheme shows that Na, K, S, Ca and Mg were slightly captured from the co-disposed brine by the Secunda fly ash, these species were however released in the labile phase. Hence there was no significant retention of these species in the ash dump. The amount of these species retained in the weathered ash were (0.26 % and 0.55 %) for Na, (0.02 % and 0.34 %) for K, (0.08 % and 0.06 %) for S, (0.94 % and 0.01 %) for Ca and (0.37 % and 0.96 %) for Mg in drilled ash cores S1 and S3 respectively. This poor retention of Na K, S, Ca and Mg which are major components of Sasol Secunda brine in the drilled ash cores S1 and S3 clearly shows the unsustainability of the Secunda fly ash dump as a salt sink.