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Application of Discrete Element Method to investigate the coke particle breakage mechanism and the particle movement in the cohesive zone

Application of Discrete Element Method to investigate the coke particle breakage mechanism and the particle movement in the cohesive zone
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Strength is one of the most important properties for coke fed into blast furnace to maintain gas and liquid permeability in the lower zone. However, a lot of factors such as raw materials and coking conditions affect coke strength, so it is not easy to investigate the coke breakage mechanism. Also, conventional strength measuring method such as drum index requires lots of time and resources to get a result. Therefore, a numerical method, discrete element method (DEM), has been used to investigate the effect of micro-structure on the coke strength. Especially, pore structure is the most effective factor, so coke models having intentionally simplified pore structure are generated to investigate the crack developing mechanism. A coke model consists of hundreds or thousands element particles. Element particles are bonded by parallel bonds which are supposed to be broken only when strong force and moment exceeding target strength are applied. To measure the strength, indirect tensile test is applied to the lying cylindrical coke model. Also, 2-dimensional simulation is adopted to increase repeatability by decreasing calculation burden. At the previous study which investigating 3-dimensional coke models having randomly or regularly distributed round shape pores, the tensile strength (σc) was expressed by σc = 450•(1-p2D,Max)•exp(-7.3•p) where p is porosity of coke, p2D,Max is the maximum value among the 2-dimensional porosity. According to the equation, decreasing p is the most effective way to increase coke tensile. Also, if coke models have same porosity, the tensile strength is proportional to the coke matrix area of broken cross-section. In other words, the tensile strength is proportional to the number of broken parallel bonding. To investigate more precisely, the first study adopt anisotropic pore-structures and 2-dimensional DEM to increase repeatability. It is well known that applied stresses concentrate strongly at the small areas where pore’s shape changes significantly. Therefore, elongated pores’ direction and layouts are manipulated to intentionally arrange stress concentrating points. According to the result, stress concentrates strongly at a side of pore when its curvature is large, and that point has a high possibility to initiate a crack. When the applied stress reaches the coke strength, cracks are initiated at a stress concentrating point and developed following stress paths between macro-pores at the same time and independently. Generally, Strength has positive relation to the number of broken parallel-bondings like the result of previous study. However, stress concentrating pattern also affects the relation between strength and number of broken bondings, so a coke sample can have higher strength by decreasing stress concentration pattern even though it has smaller number of broken bondings. Besides the investigation about coke’s pore-structure, a group simulation of the cohesive zone is conducted. Rather than simulate a whole BF, this study just focuses on a few ore and coke layers entering into the ore melting zone, and observed the coke layers movement according to the iron ore reduction. Unlike usual simulation which surrounding boundaries are fixed and element particles are added or removed from the domain, this simulation fix the observer’s view to the particle flow and surrounding boundaries moves upward to avoid a problem that element particle’s adding and removing is not continuous and natural because this is a particle domain simulation. In addition, a particular coke particle is chased to get x- and y-directional stresses information during the group simulation. This information is used at the binary compression test to investigate single coke particle’s breakage behavior in the cohesive zone. The coke model consists of thousand element particles like the coke models used in the indirect tensile test of the first study. According to the simulation results, coke layers in the group simulation are deformed due to the ore melting zone and enlarged right wall. During the time, many voids are formed and disappeared in coke layers due to the coke movement. When a particular coke particle undergoes surrounding changes, it is reflected to x- and y-directional stress changes, immediately. When stress information is applied to a breakable and gasified coke sample, only abrasion is observed through whole simulation. The porous particle seems to be damaged more when high stress is applied rapidly rather than when moderate stress is applied continuously.
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