利用包覆空氣提升奈米微粒充電器的充電效率

Abstract

為了提升奈米微粒在靜電集塵器內的去除效率以及增加微粒的充電效率以增進監測儀器對奈米微粒的偵測靈敏度，本研究設計了單一電極線及多電極線兩種單極微粒充電器，並在實驗室內針對不同粒徑的微粒在不同包覆氣體流量和不同電壓下，進行充電器之外在充電效率和靜電損失的測試。本研究在充電器內沿著內壁施加高速的包覆氣流以減少充電後微粒的靜電損失。在電暈電壓和電流之關係的測試結果發現，多電極線充電器的電壓之操作範圍介於+4.0~+10 kV，所對應的電暈電流範圍在+0.02~+119.63 □A；而單一電極線充電器的正與負電暈電流分別由+0.001~+1.817 □A和-0.004~-2.087 □A，在此範圍的操作電壓為±1.6~±2.4 kV。在不同操作條件下充電器對單徑10~50 nm氯化鈉微粒和2.5~20 nm銀微粒之充電效率和損失的試驗結果顯示，包覆氣體流速的增加可以降低微粒靜電損失並大幅提升充電器對奈米微粒的外在充電效率。 改變充電電壓可以找到ㄧ個對應到最佳外在充電效率的操作電壓，當多線電極充電器施加+9 kV的充電電壓，且在10 L/min之氣膠流量配合20 L/min之包覆氣體流量和21.26 m/s之包覆氣體流速的情況下，可得到此充電器對2.5~50 nm的微粒之最佳外在充電效率值2.86~86.3 %；而當單一電極充電器施加1 L/min之氣膠流量配合3 L/min之包覆氣體流量和5.32 m/s之包覆氣體流速的情況下，此充電器對2.5~20 nm的微粒之最佳外在充電效率則為3.07~71.1 %。在找到最佳條件後，使用串聯微分電移動度粒徑分析儀(TDMA)的方法測量奈米微粒的充電量，結果發現，粒徑小於10 nm以下之微粒最多帶一顆電荷，而50 nm之微粒則可帶五顆電荷，顯示粒徑對帶電量影響相當大。本研究的充電器可以減少靜電損失，但由於較小的微粒難以被充電而使外在充電效率無法大幅提升，將來進一步提昇10 nm以下微粒之充電效率有其必要性。

In order to enhance the charging and collection efficiency of nanoparticles in the electrostatic precipitator and to improve the sensitivity of monitoring instruments for nanoparticles, a single-wire charger and a multiple-wire charger were designed and tested for nanoparticles of different sizes at different corona voltages and sheath air flow rates in the laboratory. High-speed sheath air flow near the wall of the chargers were applied to reduce the electrostatic loss of nanoparticles to enhance the extrinsic charging efficiency. The applied voltage of the multiple-wire charger ranged from +4.0~+10 kV, corresponding to corona current from 0.02 to 119.63 □A, and the corona current varied from 0.001 to 1.817 □A and from -0.004 to -2.087 □A at operating voltage of ±1.6~±2.4 kV in the single-wire charger, respectively. Monodisperse NaCl particles of 10~50 nm and Ag particles of 2.5~10 nm in diameter were produced to test the performance of the chargers and to investigate the particle loss with different sheath flow rates, corona voltages and mean velocities of sheath air flow. The optimal efficiency in the multiple-wire charger was obtained at +9 kV applied voltage, 10 L/min aerosol flow rate and 20 L/min sheath air flow rate, and the highest efficiency in the single-wire charger was acquired at the aerosol flow rate of 1 L/min and sheath air flow rate of 3 L/min. The extrinsic charging efficiency increased from 2.86~86.3 % in the multiple-wire charger and from 3.07~71.1 % in the single-wire charger as the particle diameter increased from 2.5 to 50 nm, and from 2.5 to 20 nm, respectively. The TDMA (tandem-differential mobility analyzer) technique was used to measure the charge distribution of charged particles. It was found that a particle carried at most one charge when the diameter is less than 10 nm, while it can carry up to 5 charges when the diameter is greater than 50 nm. This indicates the great influence of particle diameter on the electrostatic charges. The nanoparticle chargers developed in this study is able to reduce electrostatic loss of nanoparticles. However, the extrinsic charging efficiency is still low due to low intrinsic charging efficiency, which needs to be improved further.