A Novel Method to Investigate Lateral Mixing of Solids in Bubbling Fluidized Beds

The fluidized bed technology is widely used in a large number of industrial processes due to its high heat and mass-transfer capability. When applied to combustion and gasification of solid fuels, the mixing of solids is of high importance since it controls the mass and heat transfer. Numerous works have been published in the last decades investigating the solids mixing phenomena, in particular with respect to its influence on heat conductivity. However, interpreting and extrapolating results from published data on solids dispersion in fluidized beds is often challenging since data were obtained under different conditions. Moreover, there is a lack of data measured under conditions representative for operation under industrial conditions. Fluid-dynamically down-scaled units can be a cost-efficient tool to perform studies under ambient conditions, while maintaining similar fluid-dynamic properties as under industrial conditions at elevated temperatures.

This work presents a novel experimental method to investigate the lateral mixing in bubbling fluidized beds. The method, which is applied to a fluid-dynamically down-scaled bed, is based on placing a heat source in the bed and monitoring the temperature field over the bed surface with a high-precision thermographic camera placed above the bed. Based on the temperature field recorded by the camera and the heat input into the system, effective heat conductivity values can be computed, which are directly related to the dispersion coefficients of the bed material. With this, the method is used to evaluate lateral mixing of solids under different operational conditions. An adjustable internal division wall is used to estimate how the lateral mixing depends on axial position along the bed height. The division wall provides an adjustable gap between the air-distributor and the division wall allowing particles, and thereby, heat to be transferred in the horizontal direction. Therefore, the axial mixing can be blocked over different parts of the bed height.

The fluid-dynamically down-scaled model used for the experiments resemble the bed of the Chalmers 12 MW boiler. The bed has a cross sectional area of 0.26m x 0.26m and consists of 60-μm bronze particles fluidized with ambient air, applying a bed height of 0.11 m. The bed is operated without the division wall and with the division wall applying gap heights of 0.02 m, 0.05 m and 0.11 m. The fluidization velocities range from 3 to 5 times the minimum fluidization velocity using two different air distributors with different pressure drop.

The results show that the lateral solids dispersion coefficients in the bed operated without division wall fall within the order of magnitude of values reported in literature, which is considered to validate the method presented in this paper. The dispersion coefficient increases with fluidization velocity in agreement with previous findings. Applying the division wall, it is seen that the dispersion coefficient is reduced with reduced gap between air distributor and division wall, i.e. there is an axial dependence in the lateral solids mixing.