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Dynamic variation and influence factors of the particle filtration by a gas-solid fluidized bed
|關鍵字:||fluidized bed filtration|
particle size distribution
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研究結果顯示，當相對溼度增加，因顆粒間毛細管力較大，電廠灰的收集效率隨之提昇，且床體內收集的飛灰總量也相對跟著增加；收集在床內的顆粒因磨損作用再被淘失離開床體，造成飛灰的過濾效果呈現隨著時間增加而遞減的動態變化現象；將過濾飛灰的床質顆粒加以再生，並返送入流體化床可改善此一問題且可以達成連續操作的目標。當操作溫度為低溫(36℃)時，小顆粒容易凝聚成大顆粒，在低溫較高的質量流率情況下，大粒徑顆粒被淘失離開床體，造成總去除效率降低。在300℃、400℃及 500℃高溫情況下，飛灰的總收集效率隨溫度上升而遞增，此係因為高溫下，顆粒的磨損作用較為劇烈，且在低質量流率情況，大顆粒不易離開床體而提高去除效果。就原始飛灰及流體化床出口飛灰的粒徑分布資料分析，低溫下次微米顆粒的去除效率介於85-99%，高溫下則由初始時的95-99%遞減到操作末期的40-60%。高溫下操作初期的高效率說明了擴散效應的影響，然而高溫下熱應力造成飛灰在床內磨損而形成較多的微小顆粒，使得去除效率明顯隨時間而遞減。4-7 μm顆粒的去除效率相對較低，低溫時可維持在99-85%，高溫下則由99%隨時間遞減到約17-19%。小顆粒在高溫時有較低的去除效率，說明擴散作用的影響相對於熱應力的影響則較不顯著。另就時間序列分析，收集效率隨時間增加而逐漸降低，顯示被收集在床內的飛灰會因淘失作用而由流體化床再次逸出。在奈米微粒過濾部份，實作上分別利用石英砂及活性碳為床質顆粒濾除80 nm的SiO2及Al2O3結果顯示，以石英砂過濾SiO2及Al2O3顆粒的去除效率並無明顯差異，其效率介於85-92%，且不隨操作時間而變；而活性碳因有較大的比表面積，因此原本預期有較佳的去除效率，但實際操作以活性碳過濾Al2O3時，隨時間的增加，其去除效率卻由88%遞減到約80%；而過濾SiO2時，其去除效率更由開始時的80%快速下降到最後僅有40％，呈現與預期相反的結果，推測可能是不同性質的顆粒在流體化床內形成分層(segregation)現象所致。
對40及4 μm的SiO2、Al2O3及Fe2O3顆粒過濾分析顯示，40 μm的較大顆粒雖可藉由慣性撞擊機制與床質顆粒有較佳的接觸機會，但更會因為撞擊時的反彈效應造成極低的去除效果，然而若顆粒間有較強的凝聚作用力，可超越反彈效應時，反而會有較高的去除效果。因此對較大顆粒的去除，增加顆粒間的附著力為重要的關鍵。對粒徑介於1-10 μm的顆粒，顆粒間主要的作用力為凡得瓦爾力，顆粒可藉由截留作用而被去除。次微米或奈米微粒的去除，則係受到擴散機制的影響，反彈效應影響較輕微。整體分析後發現，提高被過濾顆粒與床質顆粒的接觸，並增加顆粒間的附著力以減少反彈效應的影響，將可確保流體化床具有極佳的過濾效果。|
Fluidized beds are widely used in industrial processes because its high thermal/mass transfer coefficient, high gas/solid and solid/solid contact area and continuous operation. The fluidized bed is also considered as a gas cleaning technology at high temperature. The fly ash is collected by the bed materials in the fluidized bed by the mechanisms of inertial impaction, interception, diffusion and electrostatic force etc. When the fly ash particles entered a fluidized bed, the gas fluiddynamic characteristics and interaction force influenced the contact between fly ash and bed materials, the removal efficiency was varied. The fly ash particles could be attritrd and elutriated from the fluidized bed, increasing the exit concentration in the exhaust gas. The particle size distribution of the fly ash in the exit was also changed because the fly ash particle was abrased or coagulated with other fly ash particles or with bed materials. In this sstudy, a bubble fluidized bed was applied to filter the particles in the gas stream.The topics included: (1) the influence of the relative humidity on the removal efficiency of fly ash. (2) The relationship between the removal efficiency and particle size distribution with the the diffusion mechanism and the thermal stress at high temperature. (3)The filtration of the particles of varied chemical compositions and particle sizes was studied. (4) Filtration of nano-particles by a gas-solid fluidized bed. (5) The effect of inter-particle forces on removal of the particles. Experimental results showed that the removal efficiency increased with the relative humidity. The total mass of the collected fly ash was also raised with the relative humidity, revealing the effect of capillary force on particle filtration. The filtration of particles by a gas-solid fluidized bed was a dynamic process because the removal of particles involved a balance between the collection and the elutriation of particles, which processes are both time-dependent. Therefore, the withdrawn of the accumulated bed material and the injection of new material were important in a continuous filtration of the fluidized bed. The overall collection efficiency was lower at low temperature than that at high temperature which was attributed to the coagulations of the small particles. The large particles were strongly elutriated because of the high mass flow rate at low temperature. At high temperature, the large particles were easily abrased to small ones. Moreover, the large particles were lightly elutriatd because of the low mass flow rate, raised the collection efficiency. For the grade collection efficiency of the varied size, the submicron particles were removed with the efficiency of 85-99% at temperature of 40℃. At high temperature, the removal efficiency of submicron particles decreased from 95-99% initially to 40-60% at the end of the test. The high efficiency at the beginning of the filtration revealed the effect of the diffusion mechanism. However, the particles were violently attrited to a small ones because the rising thermal stress at high temperature. The falling efficiency with time was attributed to the increasing of the attrited small particles with time. The removal efficiency of the particles of 4-7 μm was lower than that of different sizes. The efficiency was maintained at 85-99% at 40℃, however, it fell from 99% to 17-19% at high temperature. The effect of diffusion was not as important as the thermal stress at high temperature. The differences between the collection of 80 nm SiO2 and Al2O3 particles were not apparent. The removal efficiency was 85-92 %, which was independent on the operation time. When the activated carbon was used as bed material, the removal efficiency declined from 88% to 80% with the operating time. The efficiency of SiO2 particles decreased greatly from 80 % to 40% at the end. The differences are attributed to the extent of segregation in the fluidized bed. As for the filtration of SiO2, Al2O3 and Fe2O3 particles of an average size of 4 and 40 μm, the experimental results showed that the contact of 40 μm particles with bed material was better than that of 4 μm because of the strong inertial impaction. However, the rebounce of the 40 μm particles decreased the removal efficiency greatly. If the adhesion force between the particles and bed material overcame the rebounce effect, high removal efficiecvy was observed for large particles. The dominant interparticle forces was Van der Waals force for the particles in a size of 1-10μm, the particles were mainly collected by the interception mechanism. The removal of submicron and nanoparticles was dominantly determined by diffusion, not the rebounce of particles. The high filtration efficiency of a fluidized bed was attainable if the contact between the collected particles and bed material was raised and the rebounce effect was minimized by increasing the adhesion between particles.
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