Numerical Simulation Of Flow Characteristics In High-Low Circulating Fluidized Bed

High-Low Circulating Fluidized Bed (HLCFB) adopts multiple bed layers and uses nonuniform air distributions between the main bed and the secondary bed to form the intense internal circulation in the dense zone. Under this condition, particles of different diameters can be combusted in divided beds. For this combustion method has many advantages including strong mixing of materials, stable combustion temperature, high combustion and desulfurization efficiencies, fuel adaptability and effective control of the production and emission of pollutants, the HLCFB has a great superiority and wide application prospects in the combustion of low-rank fuels and biomass, etc.The numerical simulation results of the gas-solid flow in the dense zone and the whole region of HLCFB by Fluent software are shown and discussed in this paper. Euler-Eluer two-fluid model, kinetic theory of particle and standard k-εtwo gas-processing model of turbulent flow are applied in the simulation. The influences of different operating conditions to the flow structure in the dense zone of HLCFB are studied, the center-core flow structure of the freeboard and the whole flow characteristics of the external cycle are also discussed.By analyzing the flow parameters such as the particle concentration, distribution of particle velocity and pressure of the internal circulation in the dense zone, some important conclusions are obtained. The high velocity air should not be less than 3.5m/s. The best low velocity air condition is 2m/s. The bed height difference of the main bed and sencondary bed should be kept at 500mm when the particle mixing and internal circulation quality are the most desirable. Partition walls have positive contributions to the quality of the internal circulation. Small particles will mainly distribute in the secondary bed while large ones mainly domain the bottom zone of the main bed with the process of flow. The biomass briquette particles also tend to accumulate in the main bed and the bulk biomass particles will mainly participate in the internal circulation of the secondary bed.The simulation results of the whole flow process in the HLCFB show that the furnace is clearly divided as two regions: the dense zone in the bottom region and the freeboard in the upper zone. The flow of particles forms the typical center-core flow structure in the freeboard, and particle clusters are forming and disintegrating constantly.