Wall-to-bed heat transfer in a circulating fluidized bed for the reduction of iron ore particles
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INTRODUCTION
CIRCULATING fluidized bed (CFB) reactors have several advantages, such as excellent heat and mass transfer, temperature uniformity through the whole reactor circuit, excellent thermal efficiency, low investment cost, and efficient pollution control. The CFB reactors have been used extensively in both catalytic and noncatalytic gas-solid reactions, such as in the catalytic cracking of crude oil, the combustion of low-grade fuels to meet strict environmental standards, and the gasification of wood and biomass. However, little work has been done on the behavior of fluidizing iron ore particles in the CFB for the purpose of iron ore reduction. In recent years, various smelting-reduction processes have been under development to replace the conventional iron-making process, i.e., the blast-furnace process. The smelting-reduction iron-making process may be required to satisfy criteria such as the use of different coals, simplified material preparation, hot metal with little impurities, independent process steps, closed energy system, efficient pollution control, and no generation of wastes. The smelting-reduction process that combines a smelting furnace and a CFB prereduction reactor has been highlighted as one of the promising processes that meets the aforementioned requirements.[1,2] For the prereduction stage, there is an increasing interest in operating a CFB system due to its applicability to prereduction of iron oxides. The overall reduction of iron oxides in the CFB is somewhat endothermic, and the reduction gas and iron ore particles are fed into the reactor at temperatures much lower than the wall temperature. In order to design CFB reactors, it is of great importance to properly calculate the wallto-bed heat-transfer coefficient. Many empirical correlations[3–5] for this purpose have been proposed in the Y.B. HAHN, Professor, and Y.H. IM, Graduate Student, are with the School of Chemical Engineering and Technology, Chonbuk National University, Chonju 561-756, Korea. Manuscript submitted November 1, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
literature, but the use of them is limited to the experimental conditions that they are based on. Furthermore, the correlations do not contribute to the understanding of the fundamental aspects of the heat-transfer mechanism. Hence, many investigators have proposed mechanistic models to predict the wall-to-bed heat-transfer process occurring in fluidized beds. The mechanistic models can be divided largely into single-particle models, emulsion-phase models, and a two-fluid model (TFM) approach. In single-particle models, the fluidizing medium is considered to be a continuous phase, and the solid particles to be a discrete phase.[6–11] The single-particle model first developed by Botteril and Williams[7] showed predictions that deviated considerably from the measured data. To improve the model predictions, they introduced a gas gap with a thickness of about 0.1dp between the particle and the wall surface. However, this assumption has been criticized by
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