The present work focuses on mineral reactions and slag formation of LKAB olivine iron ore pellets (MPBO) subjected to reducing conditions in the LKAB experimental blast furnace (EBF). The emphasis is on olivine reactions with surrounding iron oxides. Many factors influence the olivine behaviour. The study was performed by use of micro methods; optical microscopy, micro probe analysis, micro Raman and Mössbuer spectroscopy and thremodynamic modeling. During manufacturing, in oxidising atmosphere at high temperature (1350°C), olivine alterations occur through slag formation and rim reactions with iron oxides and other additives. To be able to describe olivine behaviour in the rather complex blast furnace reduction process one has to consider factors such as reactions kinetics, reduction degree of iron oxides, vertical and horizontal position in the furnace and reactions with alkali. Samples were collected from the EBF both from in shaft probing during operation and from excavation following quenching of the EBF. The initial slag forming olivine consist of primary forsterite – (Mg1.9Fe0.1)SiO4 – with inclusions of hematite and an amorphous silica rich phase, a first corona with lamellae of magnesioferrite, olivine and orthopyroxene, a second corona of amorphous silica and magnesioferrite. During reduction in the upper shaft in the EBF (700-900°C) Fe3+ reduces to Fe2+. The amorphous silica in the second corona absorbs alkali, Al, Fe2+, Mg, and Ca and form glasses of varying compositions. The lamellae in the first corona will merge into a single phase olivine rim. With further reduction the glasses in the second corona will merge with the olivine rim forming an iron rich olivine rim and leaving the elements that do not fit into the olivine crystal lattice as small silicate glass inclusions. Diffusion of magnesium and iron between olivines and iron oxides increase with increasing temperature in the lower shaft of the EBF (750-1100°C). In the cohesive zone of the EBF (1100-1200°C) Fe2+ is not stable any longer and Fe2+ will be expelled from the olivine as metallic iron blebs, and the olivine will form a complex melt with a typical composition of alkali-Al2O3-MgO-SiO2. Alkali plays an important role in this final olivine consumption. The quench time for samples collected with probes and excavation are minutes respectively hours. A study of the quench rate’s effect on the phases showed no differences in the upper shaft. However, in the lower shaft wüstite separates into wüstite and magnetite when wüstite grows out of its stability field during slow cooling of excavated samples. There is also a higher alkali and aluminium deposition in the glass phases surrounding olivines in excavated pellets as a result of alkali and aluminium gas condensing on the burden in the EBF during cooling. Coating applied to olivine pellets was studied in the EBF with the aim to investigate its behaviour, particularly its ability to capture alkali. The coating materials were kaolinite, bauxite, olivine and limestone. No significant reactions were observed in the upper shaft. In the lower shaft a majority of the phases were amorphous and reflecting the original coating compositions. Deposition from the EBF gas phase occurs and kalsilite (KAlSiO4) is found in all samples; coating used for binding alkali is redundant from a quality perspective.

The influence of ladle-slag additions on the BOF-process performance were investigated in plant trials. The aim of the study was to recycle ladle slag from secondary steelmaking to the LD-converter to save lime and improve the slag formation. More specifically, two plant trial campaigns covering in total 83 heats, whereof 47 with ladle-slag additions and 36 without ladle-slag additions, were performed.  Slag and steel sampling of the process were performed at tapping as well as during blowing at 15, 35, and 65% of the total blowing time. During the first campaign, ladle slag was added through the chute and lime reductions were made manually to correct for the ladle-slag addition.  In the second campaign, a development of the approach was made to suite a normal production practice. More specifically, the ladle slag was added through the weight-hopper system and implemented in the process-control system. In this way, the lime additions were reduced automatically by approximately 260 kg per heat. Moreover, the heat balance was compensated with a reduction in the iron-ore consumption. Additionally, the lance program was modified and the lance was lowered in the initial stages of the blow.  On the positive side, it was found that no demerits in the metallurgical performance of the process occur when ladle slag is recycled to the BOF-process. Furthermore, only slight affections on the slag composition were found, mainly with respect to the Al2O3 and FeO-content. In addition, the ladle slag was shown to melt during the initial stages of the blow. This contributed to an increased slag weight both during the blow and at tapping. However, a negative effect on the blowing time was experienced during the trials. Although, this effect was more pronounced during the first campaign and could be reduced with a controlled heat balance during the second campaign. / QC 20110503

There is a consensus worldwide that the share of renewable energy sources should be increased to mitigate climate change. The strive to increase the renewable energy fraction can partly be met by an increased utilization of different biomass feedstocks. Many of the "new" feedstocks puts stress on certain challenges such as air pollution emissions and operation stability of the combustion process. The overall objective was to investigate, evaluate, and explain the effects of fuel design and combustion control - fuel engineering - as primary measures for control of slag formation, deposit formation, and fine particle emissions during biomass combustion in small and medium scale fixed-bed appliances. The work in this thesis can be outlined as having two main focus areas, one more applied regarding fuel engineering measures and one more fundamental regarding the time-resolved release of ash forming elements, with particular focus on potassium. The overall conclusion related to the abatement of particle emissions and slag formation, is that the release of fine particle and deposit forming matter can be controlled simultaneously as the slag formation during fixed-bed biomass combustion. The methodology is in this perspective denoted “fuel engineering” and is based on a combined approach including both fuel design and process control measures. The studies on time-resolved potassium release showed that a Macro-TG reactor with single pellet experiments was a valuable tool for studying ash transformation along the fuel conversion. The combination of dedicated release determinations based on accurate mass balance considerations and ICP analysis, with phase composition characterization by XRD, is important for the understanding of potassium release in general and time-resolved data in particular. For wood, the results presented in this work supports the potassium release mechanism from "char-K" but questions the previously suggested release mechanism from decomposition of K-carbonates. For straw, the present data support the idea that the major part of the potassium release is attributed to volatilization of KCl. To further explore the detailed mechanisms, the novel approach developed and applied in this work should be complemented with other experimental and analytical techniques. The research in this thesis has explored some of the challenges related to the combined phenomena of fuel conversion and ash transformation during thermochemical conversion of biomass, and has contributed with novel methods and approaches that have gained new knowledge to be used for the development of more effective bioenergy systems.

[pt] Um modelo de descarburação e formação de escórias foi desenvolvido e aplicado ao processo de fabricação de aço em forno elétrico a arco de 120 ton de capacidade, com carga de ferro gusa e de sucata ferrosa. O carregamento de carbono foi significativamente variado para testar a consistência do modelo, considerando a cinética de oxidação do carbono, oxidação do fósforo e de redução do óxido de ferro. Gusa e coque foram empregados como fontes mais relevantes de carbono, resultando na entrada de 15 a 35 kg carbono/ton. As taxas de fusão do gusa e da sucata governam a disponibilidade dos elementos mais relevantes tais como carbono, fósforo e silício em solução, portanto, afetam as taxas de descarburação e de formação de escórias. A principal fonte de fósforo na carga ferrosa é o gusa. Desta forma, a evolução do teor de fósforo na fase metal mostrou-se importante para as estimativas das taxas de fusão do gusa, uma vez que o fósforo pode ser empregado como traçador adicional ao carbono. Modelos cinéticos envolvendo as reações do fósforo e silício operam simultaneamente com os modelos cinéticos referentes às reações do carbono e do ferro. Integrações numéricas associadas a um algoritmo de gradientes reduzidos generalizado foi empregado para o sistema não linear com restrições, de forma a determinar a maioria dos parâmetros cinéticos do modelo. A taxa de fusão global da carga de sucata foi maior do que a taxa de fusão aparente do gusa. Supõe-se que, o gusa apesar de ter relações geométricas desfavoráveis à transferência de calor em relação à sucata, poderia fundir mais rapidamente influenciado pelo seu baixo ponto de fusão. Entretanto, devido à formação de camada solidificada a partir da massa líquida na qual é imerso, é provável que mesmo fundido posteriormente, ocorra um processo de encapsulamento temporário, conferindo-lhe uma taxa aparente de fusão mais baixa. A constante cinética da reação de descarburação quando o teor de carbono é inferior ao carbono crítico de 0.19 por cento em massa e pelo menos 60 por cento da carga ferrosa estão fundidas, foi estimada em 0.74 min-1, taxas comparáveis às obtidas em aciaria a oxigênio. A principal fonte de oxigênio para oxidação do ferro é disponibilizada por lanças supersônicas. Estima-se que 20 por cento do oxigênio injetado via lanças sejam consumidos para a formação de óxido de ferro. Entretanto, cerca de 31 por cento e 26 por cento do oxigênio oriundo de injetores de póscombustão podem contribuir na formação de óxido de ferro ou são captados pelo sistema de exaustão de gases, respectivamente. Os resultados indicam que em torno de 15-30 por cento do carbono injetado podem não reagir no forno, sendo removidos com a escória. Adicionalmente ao estado de não-equilíbrio no sistema Fe-C-O observado, a dispersão nas estimativas de carbono solúvel na fase metal também pode ter sido influenciada pela intensidade de penetração da injeção de coque. O algoritmo proposto se constitui num promissor simulador de práticas que visam otimizar o rendimento metálico do ferro, a partir da dependência da cinética de redução do óxido de ferro com sua atividade química na escória. / [en] A decarburization and slag formation model was developed and applied to a steelmaking process based on scrap and pig iron mixes melted in a conventional AC electric arc furnace (EAF) with 120 ton capacity. The amount of carbon input was varied significantly in order to evaluate the model consistency regarding mainly the kinetics of carbon oxidation, phosphorus oxidation and iron oxide formation and reduction. Pig iron and coke were used as sources of carbon, resulting in variation of total carbon input in the range of 15 to 35 kg carbon/ton. The pig iron and scrap melting rates determine the availability of the most relevant elements such as carbon, phosphorus and silicon in solution in Fe-C melts, and therefore, affecting the decarburization as well the slag formation rates. The pig iron is the main source of phosphorus in the ferrous charge. Hence, the evolution of the phosphorus content in the metal phase is important to predict the pig iron melting rate, since phosphorus can be used as a tracer element in addition to carbon. Kinetic models regarding phosphorus and silicon were applied simultaneously to kinetic models of carbon and iron reactions. A numerical integration method supported a generalized reduced gradient algorithm for non-linear and constrained system (GRG) was applied to determine most of the kinetic model parameters. The scrap melting rates were found to be higher than pig iron apparent melting rates. This is expected that, even though the heat transfer issues related to significant differences in the area to volume ratio compared to scrap, pig iron may melt faster influenced by its low melting point. However, a solidified shell maybe created from the hot heel where pig iron is immersed, even when further melting occur, Fe-rich carbon melts could be encapsulated temporarily and present lower apparent melting rate. The decarburization rate parameter, when at least 60 percent of the charge is melted, was estimated as 0.74 min-1, when carbon content is lower than the critical carbon 0.19 percent wt, which is similar to the rate range observed in oxygen steelmaking facilities. Around 31 percent and 26 percent of the oxygen input through post combustion injectors were addressed to iron oxidation and to the off-gas system, respectively. The main source of oxygen taking part of iron oxidation is available from supersonic lances. Approximately 20 percent of the oxygen input through lancing are consumed to form iron oxide. The results also indicate about 15-30 percent of the injected carbon may not react and leave EAF during slagoff. In addition to the observed non-equilibrium state in Fe-C-O system, the dispersive behavior of the prediction of soluble carbon content in the metal phase could also be influenced by the intensity of penetration coke. The model framework is a promising tool to work preliminarily in what-if process scenario builder as a static model for iron yield optimization, regarding the kinetics of iron oxide reduction reaction and the proposed dependence on its chemical activity in the slag phase.