去除奈米和微米微粒的高效率濕式靜電旋風集塵器

Abstract

乾式靜電集塵器的操作壓損低且除塵效率高，因此被工業界廣泛的使用。然而傳統乾式靜電集塵器在使用上有粉塵餅堆積、粉塵再揚起及背電暈等問題。為了改善傳統乾式靜電集塵器在使用上的缺點，本研究設計並製造一個可長期操作的高效率濕式靜電旋風集塵器(wet electrocyclone)，旋風器圓柱部分的內徑為25 cm，其放電電極是由7個圓盤電極所組成，各圓盤電極的最外緣呈尖銳齒狀(齒尖直徑為20 □m)且與旋風器壁面的距離均為4 cm，可產生電暈放電作用使微粒充電及靜電移除。為了解決收集壁面粉塵累積的問題，在旋風器上方設計四道循環的切線水流來連續清洗旋風器內收集壁上的粉塵，以達長期操作的目的。 實驗結果顯示，當操作電壓為21 kV、氣體流量為1000 L/min (停留時間=1.28 sec)~4500 L/min(停留時間=0.28 sec)時，效率會隨著氣體流量增加而降低。當氣體流量為4500和1000 L/min時，本濕式靜電旋風器對於粒徑範圍為20-800 nm的油酸微粒去除效率分別為78~92%和97~99%，對於粒徑範圍為20-800 nm的碳煙(soot)微粒的去除效率為92~99%和99~99.9%；對粒徑範圍為0.5~20 □m的氧化鋁微粒粒徑的去除效率則分別為75~99%和87~99.9%。三種測試微粒的最低去除效率均發生在粒徑100~200 nm之間，效率曲線呈U字型。在同一操作條件下，相同粒徑範圍內的集塵效率以比電阻係數低的碳煙最高, 其次為油酸, 最後為比電阻係數最高的氧化鋁最低。若壁面無洗壁水而以乾式靜電旋風器的方式操作時，以氧化鋁微粒負荷(0.37g/m3)負荷時間6小時後的油酸微粒的去除效率從78~92%降至55~85%，而本濕式靜電旋風器的去除效率僅下降2~3%，無乾式靜電器因粉塵累積在收集壁面而導致效率降低之問題。在實場測試結果中發現，本濕式靜電旋風集塵器在連續操作14天下仍然可以對微粒(MMAD為220 nm、濃度為20~50 mg/m3)維持90%以上的去除效率。因此本濕式旋風靜電集塵器可有效去除細微粒及奈米微粒, 可長時間連續操作，且操作成本低。

Electrostatic precipitators (ESPs) are widely used to remove suspended particles in the exhaust gas because they are capable of handling large flow rate with low pressure drop through the collection chamber and with high removal efficiency. However, particles accumulated on the collection electrodes, particle re-entrainment, and back corona all result in a decrease in particle collection efficiency in conventional dry ESPs. In order to solve these problems, this study design and develop a wet electrocyclone with high efficiency for long-term operation. The inner diameter of collection electrode is 25 cm, and the discharge electrode consists of seven circular discs with zigzag-shaped edges for particle charging and removal. The tip of the zigzag-shaped edges is 20 □m in diameter and the tip to the inner wall (collection electrode) spacing is 4 cm. Circulating water is used to clean particles deposited on the collection electrode surface in the wet electrocyclone for long-term operation. The experimental results showed that particle collection efficiencies decreased with increasing gas flow rate when the applied voltage and the gas flow rate were 21 kV and 1000 (residence time was 1.28 s) ~4500 L/min (residence time was 0.28 s), respectively. When the gas flow rate was 4500 and 1000 L/min, the efficiency of the present wet electrocyclone for oleic acid particles with the diameter of 20~800 nm was 78~92% (4500 L/min) and 97~99% (1000 L/min), respectively. For soot particles, collection efficiencies were 92~99% and 99~99.9% at the air flow rate of 4500 and 1000 L/min, respectively. When the gas flow rate was 4500 and 1000 L/min, the efficiency of the present wet electrocyclone for Al2O3 particles with the diameter of 0.5~20 □m was 75~99% (4500 L/min) and 87~99% (1000 L/min), respectively. The collection efficiency curves for three test particles are U-shape curve with the minimum efficiency for particles in the size range of 100~200 nm. Under a fixed operation condition and particle size range, collection efficiencies are the highest for soot particles due to its lowest specific resistivity, followed by oleic acid, and then by Al2O3 particles with the highest specific resistivity. After six hours of Al2O3 loading test, the collection efficiency of the dry electrocyclone for oleic acid particles was shown to reduce from 78~92% to 55~85% for particles from 20 to 800 nm in diameter. Under the same operation condition, the collection efficiency of the present wet electrocyclone for oleic acid particles was shown to reduce only slightly from 78~92% to 75~90%. It is demonstrated that circulating water can solve the problems associated with dry electrocyclone, such as the decrease in particle collection efficiency due to the accumulation of particles on the surface of the collection electrode. The field test results showed that the removal efficiency of the present wet electrocyclone for particles with MMAD of 220 nm and mass concentration of 20~50 mg/m3 was kept higher than 90% for continuous 14-day operation. In summary, the present wet electrocyclone is able to remove fine and nanoparticles efficiently for long term operation with a low operation cost.