應用於生質物快速熱裂解之氣泡流體化床反應器其熱傳遞現象之模擬分析研究

CFD modeling of hydrodynamics with heat transfer in bubbling fluidized-bed reactors applied to fast pyrolysis of biomass

丁公平(Cong-binh Dinh)

指導教授 ： 蕭述三

國立中央大學/工學院/機械工程學系/碩士(2014年)

摘要

本研究探討在生物質快速熱裂解過程中發生的流體力學和熱傳遞現象。透過結合尤拉多重流體模型和顆粒流的動力學理論之數值方法並將其應用到模擬氣泡流化床反應器的氣-固流動行為。在這項研究中，具有特定屬性的稻殼和石英砂在此分別作為生物質和熱惰性物質。由於這些系統的動態特性往往是混亂的，故該模型首先以先前的研究結果加以驗證其可行性，接下來就參數方面進行一廣泛的研究，以確定其對流動分佈和相位之間的熱傳遞的主要變量的影響。該組統御及組成方程式是由商業計算流體力學程式軟體，ANSYS FLUENT 13.0.0解得。在此，各個方面，如固相體積分率，速度，床體混合，粒子溫度，熱溫度，壓力，排除輻射熱傳遞機制也進行了詳細研究。從兩個不同的拖曳模式，即，Syamlal-O'Brien和Gidaspow結果的比較，也包涵在本論文中。結果發現，不同拖曳模型和預測壓降之間沒有顯著差異，而隨著增加的進氣速度而稍微的增加。壓降則隨著生物質初始體積分率的的減少而增加。
　固相體積分率和速度剖面的分佈分別可在不同的情況下獲得。雖然結果顯示流化床仍保持固有的混沌性質，但所獲得的總體趨勢仍然與以前的研究有良好的一致性。本結果還表明，該固體混合是受到入口氣體速度和生物質的初始體積分率的強烈影響。較佳的粒子混合可以通過增加入口氣體速度來獲得。
兩模式固體顆粒溫度被發現在床體底部和頂部附近會比較高。它隨著入口氣體速度的增加而增加，但是隨著生質物量初始百分比的增加而降低。發生在流化床內的熱傳遞是由固體的溫度，熱傳導係數的變化和表面熱通量的分佈所描述。這兩種熱傳遞量在密相床區被觀察到佔有一定主導地位，因為他們在流化床中強烈地取決於固體濃度。由於強烈攪拌，生質物的溫度分佈在床體區域相當均勻。不斷增加的進氣速度提升固體顆粒的混合，從而導致了顆粒間和壁面對於粒子之間的有效熱傳遞。模擬中的總表面熱通量和不同拖曳模型之間沒有顯著差異。
　總體而言，目前的結果是基於一些假設和受限於此工作過程中的時間和計算量的限制而以二維模式呈現。因此，這些結果不能充分地描述在流化床反應器中發生的熱物理過程。今後的工作應考慮到用更少的假設，建立一個完整的三維空間模型。此外，模擬與實驗結果的定量比較，亦可進一步驗證吾人所建議設置的實驗設備。

關鍵字

生質物快速熱裂解 ； 流體化床反應器 ； 流體動力學 ； 計算流體力學 ； 固體混合 ； 尤拉多重流體模型

並列摘要

The present study investigates the hydrodynamics and heat transfer phenomena that occur during the biomass fast pyrolysis process. A numerical approach that combines an Eulerian multifluid model and the kinetic theory of granular flow applied to simulate the gas-solid flow in a bubbling fluidized bed reactor. In this study, rice husk and quartz with specified properties have been used as biomass and inert material, respectively. Since the dynamics of these systems are often chaotic, the model was first validated the feasibility using previous findings, then an extensive parametric study was conducted to determine the effects of the major variables on the flow distribution and the heat transfer between the phases. The set of governing and constitutive equations was solved by the commercial computational fluid dynamics code, ANSYS FLUENT 13.0.0. Herein, various aspects such as solid volume fraction, velocities, bed mixing, granular temperature, thermal temperature, pressure, heat transfer mechanisms excluding radiation were studied in detail. A comparison of results from two different drag models, namely, Syamlal-O’Brien and Gidaspow, was also included in this thesis.
It was found that there was no significant difference between the predicted pressure drops for different drag models, whereas it slightly increased with the increasing inlet gas velocity. The pressure drop increased with decreasing the initial volume fraction of biomass.
The distribution of solid volume fraction and velocity profiles were obtained for different cases. Although the results revealed the inherently chaotic nature of fluidized bed, the general trends were still obtained which agree good with previous studies.
The results also indicated that the solids mixing was strongly influenced by the inlet gas velocity and the initial volume fraction of biomass. Better particle mixing could be obtained by increasing the inlet gas velocity.
The granular temperatures of both solids were found to be higher near the bottom and the top portion of the bed. It increased with the increasing inlet gas velocity but with the decreasing initial percentage of biomass.
The heat transfer happening inside the fluidized bed was described by the distribution of solids temperature, the variation of heat transfer coefficients and the surface heat fluxes. Both heat transfer quantities were observed to be dominant in the dense bed regions as they strongly depend on the solids concentration in the fluidized bed. Due to the strong mixing, the distribution of biomass temperature was fairly homogeneous in the bed region. The increasing inlet gas velocity promoted the mixing of solid particles, thus resulted in effective heat transfer between the particles and wall to particles. There was no significant difference between the simulated total surface heat fluxes for different drag models.
Overall, the current results were based on some assumptions and in two-dimensions only due to the limit of time and computational effort during this work. Therefore, those results could not describe the adequately thermo-physical processes occurring in the fluidized bed reactor. A full three-dimensions model with less assumptions should be taken into account in future work. Furthermore, a quantitative comparison of the simulations with the experimental results can be done to further validate the utility of the suggested setup.

並列關鍵字

biomass fast pyrolysis ； fluidized bed reactor ； hydrodynamics ； computational fluid dynamics ； solids mixing ； Eulerian multifluid model

參考文獻

[107] J.H. Lienhard IV, J.H. Lienhard V. A Heat Transfer Textbook. Phlogiston Press, 2006.

[37] Lindsey Teaters. A computational study of the hydrodynamics of gas-solid fluidized beds. Master Thesis, Virginia Polytechnic Institute and State University, United States, 2012.

[2] The IEA’s Bioenergy Task40 report. Available online at

[3] Zhang Y., Jin B., Zhong W. Experimental investigation on mixing and segregation behavior of biomass particle in fluidized bed. Chemical Engineering and Processing, 48 (2009a), 745-754.

[5] D. Gidaspow. Multiphase flow and fluidization: continuum and kinetic theory description. Academic Press, San Diego, 1994.