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- Chanouian, Serg, 1995-
(författare)
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Mixing Time and Decarburization Reactions in Side-blown Metallurgical Converters : A Practical Approach using CFD and Thermodynamics
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The side-blowing Argon Oxygen converter (AOD), known for its intense gas stirring and turbulentnature, poses complex fluid dynamics and thermodynamic challenges. Modeling has played asubstantial role in the development of metallurgical converters, particularly in understanding jetbehavior, mixing, flow patterns, and chemical reactions. Flow characteristics and mixing time arerecognized as crucial factors that enhance the efficiency and decarburization rate in metallurgicalreactors. However, to the best of the author's knowledge, no prior study has investigated the impactof mixing time on the decarburization reaction. While most studies suggest that reducing mixing timeis beneficial, it is reasonable to assume that there might be a point at which further reduction inmixing time does not lead to an increase in reaction rates. Adjustments like tilting the converter orrepositioning the nozzles could improve decarburization efficiency by altering pressure conditions andmixing. This study aims to explore how these factors affect the decarburization reaction in side-blownconverters through modeling. The work has been done in a few steps resulting in differentsupplements.Side-blowing water model experiments were carried out to investigate how a vessel inclination wouldaffect the mixing time. The results showed a clear increase in mixing time when higher inclinationangles (14°) were applied. However, studying the non-reacting water models could only give insightto mixing efficiency and not provide information about decarburization efficiency.A numerical model capable of integrating mass and heat transfer with high temperature chemicalreactions was developed to aid in this investigation. First, the model was applied to an ascending gasbubble in liquid steel. The effect of pressure was investigated by injecting the bubble in different bathdepths. It was shown that a mere oxygen bubble injected at the nozzle position under industrialconditions did not decarburize efficiently, rather dissolved into the steel. Only pressure levels at thebath surface could maintain gas as a stable phase and decarburize efficiently.With high grid resolutions the model consumed a lot of computational time calculating equilibriumlocally in each cell with gas and liquid present. Therefore, a more practical approach was taken tostudy the AOD converter that showed high agreement to the first decarburization step whencomparing against two industrial heats. It was shown that with a coarse Computational Fluid Dynamic(CFD) solution the model could be practical, yet fundamental. In the study it was also found that nochromium oxidation was found in one of the heats at the beginning of the process when the initialcarbon content was high. The trends were compared against an industrial online process model andshowed similar behavior.With further developments, the model was tested with different treatments of the thermodynamiccoupling, including reactions limited by turbulence in an intensely stirred side-blown reactor. Themixing time was shown to have an insignificant effect on the decarburization rate. The system wasgoverned by thermodynamics and gas supply rate.Overall, this work developed a general model capable of coupling chemical reactions with CFD. Theuse of this model led to the conclusion that an inclination of the vessel within practical operationalangles would not benefit the decarburization rate in the early stages of decarburization. Withincreased mixing times and small pressure variations from the lowered bath height, the benefits todecarburization might not be worth compared to the engineering challenges posed by such changes.Even relocating the nozzle would require large and unpractical height differences to acquire thepressure decrease needed to benefit thermodynamically.
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| 2. |
- Lu, Yu-Chiao, 1995-
(författare)
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Application of Hydrochar for Low-CO2 Emission Steel Production
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Steel is an indispensable material of the modern society and yet the production of steel is one of the largest anthropogenic CO2 emission sources on the planet. The conventional blast-furnace-basic-oxygen-furnace (BOF) process is responsible for generating 85% of the steel industry’s total CO2 emissions, which is the result of a high coal consumption rate for the reduction of iron ores and for providing the heat necessary for the high-temperature process. In order to meet the climate goal set by the Paris Agreement, the iron and steel industry must drastically decrease its CO2 emissions and aim at achieving net-zero emissions by 2050. Bioenergy is a form of renewable energy, and if it is managed sustainably throughout its life cycle, it can be considered carbon-neutral. Replacing fossil fuels with biofuels consumed during the steelmaking processes is one way to decrease CO2 emissions. However, this approach has not been widely adopted by steelmakers over the world due to the high price and the limited availability of wood-based biofuels. Hydrochar is a coal-like solid material that is produced from the hydrothermal carbonization (HTC) of biomass. It has attracted great interest from steelmakers due to its coal-like properties and the fact that it can be produced from a wide range of organic waste streams that can be found in almost every country. Previous studies focused on the use of hydrochar for the blast furnace process. This thesis therefore examines the potential use of hydrochar in the direct-reduction-electric-arc-furnace (DR-EAF) process, and particularly in two applications where the use of fossil coal is difficult to abate—the coal-based direct reduction of iron ore and the carburization of liquid steel in the EAF. This thesis begins with a characterization study of a hydrochar produced from lemon peel waste (LPH) and its comparison with a fossil reference material (anthracite) and two bio-reference materials (charcoal). The results reveal that LPH is a highly volatile material that is characterized by a low fixed carbon content and a medium calorific value. The volatile matter of LPH consists of gas, tar, and aqueous liquids, and contains approximately half of the total carbon and energy content of LPH. On the contrary, charcoal, anthracite, and the pyrolyzed char of LPH (PLPH) hardly emit any volatiles and are stable up to a high temperature (1200 °C). These materials are characterized by high fixed carbon contents and high calorific values, which makes them ideal fuel, carburizers, and reducing agents. On the other hand, LPH seems to be more efficient when it is applied in areas where its volatile matter content could be utilized to an advantage, such as to provide heating energy and to reduce metal oxides. Next, two hydrochars (produced from lemon peel and rice husk) were tested for coal-based direct reduction and their performance were compared to that of anthracite. Hematite-carbon mixtures prepared with varying fixed-carbon-to-oxygen ratios (C/O) were heated in nitrogen atmosphere up to 1100 °C for direct reduction. The hematite in briquettes with molar C/O ratios greater than 1.0 were completely reduced to metallic iron, whereas briquettes with C/O ratios equal to 0.4-0.5 were reduced by 63-86%. It was confirmed that the volatile matter released by the carbonaceous materials and the organic binder reduced hematite up to a maximum of 35% but the utilized fractions of the volatile matter were quite low (12-56%). As a result, the reduction of hematite was dominated by carbothermic reduction which involved fixed carbon. Thus, the efficiency of a carbonaceous material as a reducing agent for the coal-based direct reduction processes is still predominantly determined by its fixed carbon content. Then, LPH was tested for carburization of liquid iron in a laboratory setup under an inert atmosphere and its performance was compared with that of charcoal. Iron-carbon briquettes, which have higher apparent densities than the carbonaceous material itself, were utilized as carburizers with an aim to improve the carbon’s penetration depth in the liquid iron. The briquettes were experimented in two different ways to simulate the carbon addition practices in an EAF. With the first method, the briquettes were slowly heated from room temperature up to 1600 °C, which simulates the loading of carbon into an EAF at the beginning of a heat via a scrap bucket. With the second method, briquettes were directly charged into a pool of liquid iron. The results reveal that the carburization yield is predominantly determined by the fixed carbon content of the carbonaceous material, and when a more aggressive carbon addition method (e.g. direct charging) was used, there were additional carbon losses which lowered the yield. In the final part of the thesis, two types of hydrochars (those produced from orange peel and green waste) and an anthracite were applied for carburization tests in a pilot-scale EAF. Carbonaceous materials were either top-charged into the EAF at the beginning of a heat, or injected as powder via a lance directly into liquid steel after scrap meltdown. The results show that hydrochar and anthracite has a similar carburization yield (based on fixed carbon) when the same carbon addition method was used, and the carburization yields achieved by top-charging were higher than that achieved by lance injection. Based on the results obtained in this thesis, three main conclusions are drawn. Firstly, hydrochar can completely replace fossil coal as a reducing agent for the direct reduction of iron ores and as a carburizing agent in the EAF process. However, it is more efficient to use pyrolyzed hydrochar than to use pristine hydrochar since the fixed carbon content of the material mostly determines its substitution ratio for anthracite. Secondly, some negative impact of the ash content of hydrochar has been identified in this study. For example, the reduction rate of hematite-carbon composite mixture is lowered by the hindering effect of ash on carbothermic reduction. Furthermore, ash increases the slag volume and decreases the slag’s basicity in the EAF. Hydrochars produced from fruit peel wastes (lemon peel, orange peel) have lower ash contents than hydrochars produced from plant wastes (rice husk, green waste) and are more suitable to be applied directly in steelmaking processes. Lastly, the substitution of anthracite with charcoal or hydrochar lowers the total amount of sulfur introduced into the EAF. The increase in the amount of phosphorous introduced into the EAF resulting from the addition of hydrochar can be resolved either by controlling the amount of hydrochar added, or by lowering the phosphorous content of hydrochar through additional impurity reduction treatment following the HTC process, which should be investigated in future studies.
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