| Summary: | 碩士 === 國立交通大學 === 環境工程系所 === 100 === 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 ?慆 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 ?慆 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.
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