The Study On Fluidization Simulation And Agglomeration Behavior In Fluidized Bed

Due to the excellent characteristics in such as mixing, heat and mass transfer, the gas-solid fluidized bed (FBC) has been widely used in coal combustion. While engineers are still facing series challenges during the commercializing and scaling up of the pressurized fluidized bed combustion (PFBC) boiler. One of them is the serious agglomeration occurring in the PFBC developed by IHI, a Japanese boiler manufacturer. The agglomeration decreases the availability of the boiler. So far there is no effective prediction and detection method to prevent agglomeration except shutting down boiler to clean the slagging bed material. Similar problem happened in other PFBC or FBC boilers in China and aboard as well. The motivation of this dissertation is to investigate and simulate the hydrodynamic behaviors in a PFBC boiler with a bench-scale fluidized bed in laboratory, and to develop an effective method to predict the agglomeration in a FBC boiler. First, the present work studied and compared the existing scaling theory about the hydrodynamic behaviors of a fluidized bed. Because the bubble behaviors play an important role in gas-solid, solid-solid mixing and solid recirculation, which has the major impact on agglomeration, the Horio's scaling law that is mainly based on the bubble behavior, was adopted in the study. Secondary, based on the Horio's Model, a bench-scale fluidized bed was designed and constructed in the laboratory at Tsinghua University to simulate the existing pilot-scale fluidized bed (called original bed) at IHI. The methodology used for acquisition and analysis of the pressure fluctuation signals in the fluidized bed was proposed and developed. The experimental results showed that the method well predicts the bubble size and the bubble rising velocities in both the bench-scale and pilot-scale fluidized beds at Tsinghua and IHI respectively. The results validated the feasibility and the effectiveness of the experimental approach in mixing phenomena in bubbling fluidized bed. Therefore, the suggested Horio scaling law together with the acquisition and analysis of the pressure fluctuation signals in the fluidized bed are the effective tools to investigate the fluidization behavior in PFBC. After the validation, the experimental methodology was further applied to study the agglomeration mechanisms in fluidized bed. The experimental results showed that the Temperature VS Umf diagram solid particles can be separated into four zones by Umf and Ts , that are: fluidization zone, defluidization zone, agglomeration zone and non-agglomeration zone. This map provides an effective reference on normal operation and agglomeration avoidance of fluidized bed. The experiments on the measurement of the minimum agglomeration temperature (Ts) and the detection of the agglomeration appearance were also conducted. It is found with the pressure drop measurement in a high-temperature tube furnace can quickly detect the voidage variation inside the bed materials and is convenient to determine the minimum agglomeration temperature. The early agglomeration was successfully detected with the time-frequency analysis of pressure fluctuation signals, characterized by the parameters of the averaged amplitude of local major frequency and the local power weighted averaged frequency. The methodology developed in this dissertation is convenient, effective and prompt to early detection and prevention on the agglomeration in fluidized bed boilers.