| 摘要: | 產學界提出雙流化床系統技術，並已成功地應用於生物質氣化以產生高質量的產物合成氣。我們在台灣NCU的實驗室中設計並安裝了一個雙流化床氣化（DFBG）冷流系統，該系統以氣動空氣作為流化媒介，並以矽砂作為床層材料。以實驗和數值研究該系統中空氣-矽砂流動的非穩態特性。除了以商業軟件ANSYS FLUENT開發二維計算流體動力學（CFD）模型外，還使用與CFD模型相同的工作條件同時進行了實驗測試，以研究影響系統流體動力學的參數。將歐拉多相流模型與顆粒流動力學理論相結合，在整個過程中執行了空氣和砂相的非穩態行為。在這項研究工作中，初始時觀察並分析了不同操作和幾何條件下流體流動行為的變化。後續接著對主要因素進行參數研究，例如流化空氣入口速度和靜態砂床高度，以確定它們對流體流動特性的影響。對於DFBG系統不同區域的固體流型，壓力分佈，壓降和砂的循環速率，獲得了一些典型結果。
砂的體積分率的結果分別正確地確認了在氣化爐和立管中形成的鼓泡和快速流化模式。立管進氣速度和靜態砂床高度發現會顯著影響砂體積分率，混合物壓力和砂循環速率的分佈，而氣化爐進氣速度對這些分佈曲線的影響卻很小。底部區域的混合物壓力大於上部區域的混合物壓力，從而保持了氣體密封、固體分離和固體循環的穩定運行。同時顯示出總砂流量隨立管空氣速度的增加而顯著增加，而不隨著氣化爐中流化空氣速度和立管靜態床層高度的變化明顯變化。還值得注意的是，初始沙床高度和立管中空氣入口速度的進一步增加應被限制在其最大值，否則，可能會發生意外的逆流，從而中斷壓力平衡和系統的正常運行。通常，應適當控制所有影響參數，以確保系統穩定運行。儘管建模結果相對再現了實驗數據，但是由於所提出的是簡化模型，它們之間仍然存在某些差異。所有獲得的結果可期望為防止不良現像以及改善實際DFBG工廠的設計和性能提供可靠實用的預測。
;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. |
