Study On Flow Characteristics In Circulating Fluidized Bed With Draft Tube

Treatment of organic waste gases by circulating fluidized bed(CFB)adsorption has the advantages of continuous operation and convenient recovery of the desorbed organic gas.However,the concentration of organic waste gas handled in industry varies greatly from a few hundred ppm to tens of thousands ppm,which demands higher flexibility of circulating fluidized bed(CFB).The shortcomings of circulating fluidized bed(CFB)system as uneven flow structure and unstable circulation can be overcome by the addition of a draft tube,improving in-bed particle distribution,solids flux adjustment,circulation control and operation flexibility.Therefore,research on the flow characteristics of circulation fluidized bed(CFB)with a draft tube was of great significance.In this paper,a draft tube was installed in a lab-scale conventional circulating fluidized bed(CFB)device.The device was designed and fabricated with an expansion section with the inlet diameter of 300mm and a 7-meter-high riser with a diameter of 100mm(d).A new type of optical fiber probe is also developed for this study to measure the particle concentration and velocity,with the advantage of wider particle range and higher sensitivity.Firstly,the single-factor method was used to investigate the effect of the draft tube diameter and the installation height on the solid mass flux.The results showed that under the same operating conditions,when the draft tube diameter was increased,the solid flux increased first and then decreased.Under the same operating conditions,when the installation height of the draft tube increased,the solid flux ramped up when the installation height increases from 0.9d to 1.2d,and t decreased when the installation height increases further from 1.2d to 1.8d.Secondly,the influence of operating conditions on the distribution of axial local solid holdup was investigated by using multi-factor interaction method.The results indicated that,under the same conditions of other operating conditions,when the flow ratio of aeration air to total air quantity was in the range of 0 to 0.5,the solid flux increased at first up to 300 kg/(m2·s)and then decreased.The solid holdup increased and its axial distribution was more uneven when the solid flux was increased or the superficial gas velocity was decreased.Finally,the experiment also compared the effects of two types of particles on the solid flux and the axial distribution of the solid holdup.The results suggested that under the same experimental conditions,the solid mass flux of type-A particles was larger than the type-B particles.The effects of the two types of particles on the axial distribution of the solid holdup were basically identical.This distribution revealed a feature that particles were concentrated at the bottom and diluted on the top.While in the radial distribution,the type-A particles had lower overall solid holdup and were more evenly distributed than type-B.