Study On Fluidized Characteristics In Bottom Ash Cooler Of A CFB Boiler

Fluidization deterioration and agglomeration often occur in the first wind box of the wind-water cooler used in the circulating fluidized bed (CFB) boilers in China, because of the limitations of coal quality and coal preparation systems. Replacing the traditional plate type distributor of the cooler with the pipe type one is deemed as an effective way to improve the fluidization and avoid agglomeration. Fluidization characteristics, especially the particle mixing in the new type cooler that operating in the bubbling bed condition is important for the cooler design and operation, and hereby has been studied in the thesis.A cold experimental rig was built with a section area of 125×250 mm and 1200mm in height. In the experiments, ash particles with diameters of 150-250μm and 1-2mm are chosen as the bed material to study the minimum fluidization velocity, mixing and segregation characteristics of particles.The experimental results showed that the minimum fluidization velocity of mixed particles was between those of the large and small particles, closer to that of smaller particles. The velocity increased with the increasing of mass faction of the larger particles in the bed material. As the fraction of larger particles increased, the velocity increased gradually as the fraction was of a small percentage, and then increased rapidly as the fraction exceeded a certain value. It was also found that the bed expansion ratio of bed material increased with the superficial velocity, first rapidly and then gradually. There is a noticeable discrepancy between the results obtained from present experiments and the empirical formula given in the literatures. In addition, the mixing intensity between two-component particles increased with the superficial velocity and the mass fraction of larger particles. Furthermore, the segregation of particles showed a spatial-dependent behavior. When larger and smaller particles were completely separated in the beginning of the experiment, the mixing of particles mainly completed in the first 10s, and segregation phenomena was hardly detected after 30s and the bed turned into full fluidization.Numerical simulation of fluidization with the two-component particles was conducted by using Fluent software to verify the experimental results and to make up for deficiency of some operating modes. The results of numerical simulation show that the increase of air velocity is in favor of the mixing of particles. The mixing and segregation of particles complete in a short period, e. g., in the first 10s. The results are in accordance with experimental data. For a fixed superficial velocity, simulation results showed that the uniformity of the mixing increases as the size deviation of two-component particles decreases. Moreover, the simulation found that the internal recirculation exists in the bubbling bed, with particles move upwards in center and downwards near the wall. In the cross section of the bed, particles move toward wall from the center region.