Particle attrition phenomena in a fluidized bed

Abstract

In many fluidization processes, the attrition of particles is a serious problem. To simulate the attrition, it is necessary to consider the attrition characteristics of materials, the breaking energy supplied by the system, and the distribution of attrition rates among the particles. The size distribution of the attrited fines is an essential element of the attrition characteristics. For some materials, a unique fine distribution suggests that the quantity of attrition may be directly proportional to the breaking energy supplied by the system. Based on the concepts of energy transformation and constraint, this study analyzes the results of attrition in fluidized beds with porous distributors in order to understand the relationship between the rate of breaking energy and the mechanical conditions. To simulate the distribution of attrition rates for a multicomponent mixture, a ‘surface-reaction’ model is proposed. The model states that the attrition rate for each component is proportional to its surface area, and is also a function of the interactions between different materials. Therefore, for particles of the same material but of different sizes, the fractional loss due to attrition is larger for the smaller particles. If one material has the dominant fraction of the surface area in a mixture, then, under the same magnitude of breaking energy, the attrition rate of this material is almost independent of its particle size. However, for a material with only a minute fraction of the total surface area, its attrition rate is inversely proportional to its average particle size. Experimental results from various studies are used to verify our distribution model. The model suggests that larger coal and smaller limestone are more desirable to reduce attrition in a coal combustor.