| Summary: | 博士 === 中原大學 === 化學工程學系 === 87 === In order to reduce the elutriation of excessive amount of particles from the bubbling fluidized bed (BFB), an integration of cyclonic combustor, cyclone and bubbling fluidized bed, which named vortexing fluidized bed combustor (VFBC), was developed. In the VFBC, a part of the air was introduced tangentially and distributed evenly into the freeboard of the combustor.The performance of a VFB can be characterized by the strong swirling gas flow in the freeboard which is the dominant factor for particle elutriation. However, although the study of combustion efficiency of VFBC has been the subject of a number of investigations in the last ten years, only few studies have reported the elutriation behavior. Therefore, it is important to understand the elutriation behavior of fine particles for the VFB.A systematic investigation on the gas flow field in the VFB cold models was reported. The gas velocity profiles within the freeboard with diameters of 0.19 m I.D., 0.29 m I.D. and a combination of 0.19 m I.D. and 0.29 m I.D. connected with an expansion section were measured by using a five-hole pitot tube. The vortex number (Vor) which defined as the ratio of the tangential velocity to the axial velocity of the swirling gas stream was proposed for representing the swirl intensity.When an expanded freeboard is used, an "recirculation flow zone"can be found at the upper part of the expanded section. The effects of various operation parameters were investigated in this study. Based on the results with an expanded freeboard section, the correlations for evaluating the index of the swirl intensity were also developed.The elutriation of fine particles from a fluidized bed is a very important issue in design of fluidized beds. In this study, the elutriation behavior of fine particles were investigated in the BFB and VFB, respectively. A new criterion, CEM, for distinguishing the elutriation mechanism of the first-order and the secondary-order was identified. The effects of various operation parameters, such as the primary air velocity, the effect of the secondary air, fine particles characteristics and the geometric structure of the gas exhaust tube were also investigated in this study. Based on the results from a vortexing fluidized bed, a correlations was presented to estimate the elutriation rate constant (Ki**). The Ki** was found to be a function of the gas property, the particle property, the Froude number, the Reynolds number and the vortex number.Moreover, the transport disengaging height (TDH) in a VFB were also studied. The freeboard of a VFB can be divided into the "entrainment zone"(FEZ) and the "effect zone"SEZ). The length of the SEZ was the dominant factor for elutriation in VFBs. Compared experimental data in the VFB and BFB, the effect of the swirling flow on the TDH was not conspicuous. The TDH was found to be a function of the primary air velocity, the ratio of the tangential air flow rate to the primary air flow rate, and the static bed height.
|
|---|
