Wall-to-bed heat transfer in a circulating fluidized bed for the reduction of iron ore particles

A model is presented that describes the wall-to-bed heat transfer in a circulating fluidized bed (CFB) used for the prereduction of iron ore in the smelting-reduction iron-making process. The model incorporates the core-annulus type flow structure and the wall emulsion layer growing downward along the surface. Model predictions showed good agreements with measured data taken from the literature. The hydrodynamic behavior near the wall surface was able to.be properly described by the core-annulus flow structure. A higher heat-transfer coefficient with higher solid circulation flux was obtained in the upper part of the bed because of the heat input caused by the lateral diffusion of particles from the core. The predicted and measured data also showed the minima in the heattransfer coefficients in the lower part of the bed. Model predictions indicated that in the CFB for the reduction of iron ore particles, it is important to properly control the inlet temperature of the reducing gas, rather than that of the solid particles. The implications of the behavior of heat transfer in the CFB are discussed for the reduction of iron oxides.