Prediction of the expansion of fluidized beds containing tubes

Abstract

In a fluidized-bed combustor one technique to reduce the output while retaining the bed temperature constant is to uncover tubes near the surface of the bed as the superficial velocity is descreased. A prediction of the bed expansion has been developed which is based on a first-principles model of gas flow through the bed. In a large-particle bed where the bubble rise velocity is of the same order as the minimum-fluidization velocity, the two-phase model for gas flow is in serious error. A large fraction of the gas flow is found to pass through bubbles which are erupting at the bed surface. The gas flow for this case has been predicted by using previously published results by the present authors of the multidimensional numerical solution for the bed containing a cavity at the surface. The predicated bed expansion vs superficial velocity was calculated for several different tube geometries. The results were compared to experimental results for these geometries which were measured in the MIT 1.2 m wide cold bed model of the TVA bubbling fluidized-bed combustor. The predicted bed expansion closely follows the measured results while predictions based on the two-phase model give poor agreement with the measurements. The model also gives good results for open beds and for the expansion of beds with an upper surface above the top banks.