Effect Of Air Leakage From Standpipe On Efficiency Of A Cyclone Separator

A cyclone separator is one of the most important part of a circulating fluidized bed boiler. The cyclone separates high temperature ash particles from the furnace ensuring sufficient combustion of coal particles and stability of particle size. Particles after separation enter the refeed valve though a standpipe connecting dust outlet of cyclone and form a material sealing section. If the settings of air volume in refeed valve is not appropriate or the material sealing section is not steady, air will be probably to enter the cyclone though the standpipe. This phenomenon is so-called air leakage from standpipe. It effects the performance of cyclone, especially the separation efficiency, but there are few correlative researches now. Therefore it is significant to study on the influence of the leakage from the standpipe on the separation efficiency of a cyclone.The experiment part is taken on a small cyclone separator at room temperature in order to study separation efficiency, fractional collection efficiency and pressure drop at different inlet gas velocity, different inlet particle concentration and different air leakage velocity. The numerical simulation part is taken with a CFD software Barracuda in. The flow models are large eddy simulation(LES) and multiphase particle-in-cell model(MP-PIC). This part focuses on gas flow field, the pressure field and the particles distribution.The experiment results show that the pressure drop of the cyclone separator increases significantly as the inlet gas velocity increase but it is almost not affected by the air leakage velocity. The separation efficiency decreases when there is air leakage. The rate of decline is gentler at lower air leakage velocity than at higher air leakage velocity and it is gentler at higher inlet gas velocity than at lower inlet gas velocity. It is observed that the separation efficiency is at a very high level at lower air leakage velocity and higher inlet gas velocity. It is a general rule that the separation efficiency is higher when the inlet particle concentration is higher, but at lower inlet gas velocity it does not apply. The fractional collection efficiency of fine particles decreases obviously and the air leakage influences fine particles more than coarse particles.The simulation demonstrates that the pressure drop, separation efficiency and fractional collection efficiency show good consistency with the experiment result. The influence of air leakage on the flow field inside the cyclone separator can be ascribed to three aspects. Firstly, the swirling flow in the cyclone can be weakened with air leakage. Secondly, the turning point of swirling flow from downwards to upwards will move higher in the cyclone separator, resulting in larger upward flow region. Thirdly, higher pressure area will be generated correspondingly in the conic section of the cyclone separator. Besides, particles can be entrained upwards by the leakage air, the solid concentration in region that are far away from the wall of the cyclone will increase and the internal solid circulation are enhanced with higher solid inventory in the cyclone separator, which are disadvantageous to the separation function of the cyclone separator.