Study On Hydrodynamic And Particles Segregation In The Loop Of Circulating Fluidized Bed

Circulating fluidized bed (CFB), as one of the best commercialized clean-coal technologies, has good advantages in gas-solid mixing, mass and heat transfer in the boiler. However, current researches and engineering experience show that lots of issues remained unsolved in the area, such as the measurement of solid circulating rate (Gs) in industry boiler, the impact of gas flow in standpipe on material balance in whole CFB system, particle segregation in the furnace and so on.In this paper, a cold test rig of CFB was constructed to study the gas-solid flow in the CFB loop. In the view of system loop, the impacts of operation parameters on the pressure drops of each component with narrow size range of particles were investigated. From the experimental results, the influence of Gs on each component's pressure drop was obtained. The relationship between riser pressure drop and Gs was drawn. The relationship helps to choose the pressure drop in the riser to ensure the fast bed flow regime in the riser, which can be used to guide the reconstruction of flow state. The pressure drop of the cyclone firstly decreases and then increases with solid concentration increasing. The transition point of the variation was influenced by the gas velocity and particle prosperity. The pressure drop of the horizonal section part of the Loop seal increases with Gs, while that of the vertical section part decreases. The overall pressure drop of Loop seal that indicates the ability of transporting particles decreases with Gs. Combination with the pressure gradient measurement, lacer optic probe and direct gas trace method, the flow state in the standpipe was also studied. The packed flow state in the standpipe was verified by the comparison of experimental result and the theoretical calculation. By adjusting the relative gas-solid slip velocity, different pressure gradient of gas-solid flow is achieved. The special feature with low seal height to provide high pressure head helps to maintain the mass and pressure balance of the whole loop system.Based on the above results, this paper also studied particles segregation with binary partices with different sizes in the CFB. Experimental results showed that segregation intensity in the riser becomes weaker with increaseing gas velocity and Gs. In the case of higher gas velocity (6.0 m/s) and higher Gs, segregation disappeares and the system reaches completed mixing state. With other published literatures, this paper proposed the exit geometry may have impact on segregation in the system and verified it through the experiment. Because of the inner separation of exit geometry, more coarse particles are easily separated and flow down back into the lower riser, comparing with fine particles. Therefore, the distribution of particle between the riser and standpipe was changed, which lead to more coarse particles in the riser than in the standpipe. At last, a CFB mass balance model with wide size distribution particles, including the segregation model and pressure balance mocel, was developed. The model predicting results were in good agreement with data in present study and and literatures, which verified the reliability of the model.