Model for Fast Evaluation of Charging Programs in the Blast Furnace
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INTRODUCTION
DUE to increasing global competition in steelmaking and rising raw material prices, as well as a pressure to significantly decrease the emissions, the steelmaking sector faces enormous challenges in the future. For this reason, the unit processes in the chain from iron ore to finished products must be operated efficiently and without wasting primary resources. The unit process with clearly largest energy consumption in the chain is the blast furnace, where the iron oxides are reduced and melted into hot metal, which is converted to steel in the downstream basic oxygen process. The blast furnace acts are a huge counter-current heat exchanger and chemical reactor, in the upper part of which the burden (consisting of preprocessed and agglomerated iron ores as well as coke, the main energy resource) meets an ascending gas during its descent. The efficiency of the blast furnace mainly reflects how well the charged ironbearing burden has been reduced in the upper, or lumpy, zone, as a reduction in the lower part is associated with a simultaneous consumption of coke. The primary means of controlling the conditions in the lumpy zone, including the material flows, heat and mass transfer and chemical reactions, is to control the radial distribution of the burden: Due to differences in permeability and density of the changed materials, the gas-flow distribution is largely controlled by the burden distribution. Furthermore, the burden-layer distribution plays an important role in controlling the supply of gas from the lower parts of the blast furnace to the lumpy zone through the coke slits in the cohesive zone, where the iron-bearing burden softens and melts. The burden TAMOGHNA MITRA, Researcher, and HENRIK SAXE´N, Professor and Head of Laboratory, are with the Thermal and Flow Engineering Laboratory, A˚bo Akademi University, Biskopsg. 8, 20500, A˚bo, Finland. Contact e-mail: hsaxen@abo.fi Manuscript submitted December 20, 2013. Article published online August 12, 2014. 2382—VOLUME 45B, DECEMBER 2014
distribution also controls the thermal conditions in the shaft since ‘‘ore’’ has about four time higher density than coke, and thus, through the thermal flow ratio, strongly affects the temperature distribution in the counter-current heat exchanging zone. The burden distribution can be controlled by the charging equipment, which for furnaces with bell-less top means by selecting size and sequence of the material dumps, as well as the angles of the rotating chute. Often, to facilitate the decision making, the chute angles are discretized into a number of positions (typically about ten), between which the operator can choose. Still, the number of potential choices is enormous, as a charging sequence consists of 5 to 20 dumps. Furthermore, as the distribution of a ring on the burden surface depends strongly on the present burden profile, i.e., on the distribution of previously charged rings, the problem is nonlinear and complex. Therefore, it is obvious that a model that can guide the operators in the choice and design of charging
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