dc.description.abstract | The technology of dual fluidized bed system has been proposed and successfully applied to biomass gasification to generate product syngas of high quality. A dual fluidized bed gasification (DFBG) cold flow system, equipped with pneumatic air as a fluidizing agent and silica sand as bed material, has been designed and installed at our lab in NCU, Taiwan. The unsteady characteristics of the air-silica sand flow in that system have been studied experimentally and numerically. Besides developing a two-dimensional computational fluid dynamics (CFD) model with the commercial software ANSYS FLUENT, experimental tests were simultaneously conducted with the same operating conditions as those of the CFD model to investigate the parameters affecting the system hydrodynamics. A combination of the Eulerian multiphase flow model and the kinetic theory of granular flows was applied to perform the unsteady behaviors of the air and sand phases during the entire process. The variations of the fluid flow behavior with different operating and geometrical conditions were initially observed and analyzed in this work. Accordingly, a parametric study was carried out for the major factors, such as fluidizing air inlet velocities and static sand bed heights, to determine their effects on the fluid flow characteristics. Some typical results were obtained for the solid flow patterns, pressure distribution, pressure drop, and sand circulation rate in different zones and over the height of the DFBG system.
The results of the sand volume fraction properly identified the bubbling and fast fluidization patterns formed in the gasifier and riser, respectively. The riser air inlet velocity and static sand bed height were found to considerably affect the distributions of sand volume fraction, mixture pressure and sand circulation rates, while the gasifier air inlet velocity insignificantly influenced to those profiles. The mixture pressures at the bottom regions were greater than those at the upper regions, which maintain the stable operations of gas sealing, solid separation, and solid circulation. It was also indicated that the total sand flow rates considerably increased with the increasing riser air velocity, while they did not significantly change with varying the fluidizing air velocity in the gasifier and the riser static bed height. It was also noteworthy that further increases of the initial sand bed height and the air inlet velocity in the riser were restricted at their maximum values, otherwise, an unexpected reverse flow possibly occurred to interrupt the pressure balance and normal operation of the system. In general, all affecting parameters should be appropriately controlled to ensure stable system operation. Although the modeling results relatively reproduced the experimental data, there still existed certain discrepancies between them due to the simplifications of the proposed model. All the obtained results are expected to provide valuable predictions for preventing undesired phenomena and for improving the designs and performances of practical DFBG plants. | en_US |