Study Of Indoor Air Cleaning By Corona Discharge Coupling Photocatalysis

This thesis systematically studied the voltage-current relationship in DC corona discharge with wire electrode as the emitting electrode and adopted an indoor air cleaning unit having its core technology characterized by needle-plate DC corona discharge. Taking indoor air purification as the principal goal, removal experiments and application tests aiming at suspended particles/inhalable particles capture, airborne bacteria and virus sterilization, and harmful chemical pollutant gases purification were carried out respectively. According to the conditional experiments results, optimized electrode structure and running parameters were determined, the rules were discussed and the mechanisms were analyzed. Based on the above results, the purification tests were carried out in closed rooms. Finally the air cleaner has advantages of simultaneous, integrated and efficiently removal of particles, pathogenic bacteria and virus, and harmful gases.It has been summarized that indoor air pollution is complicated, mainly including particle pollution, microorganism pollution, volatile organic compounds and other chemical pollution. What's more, these pollutions have cross actions in ambient air. Current purification technologies mainly include infiltration, electrostatic collection, ozone sterilization, and etc. However, these technologies are undesirable in respect of small-size particle capture, treatment efficiency, extensiveness of harmful gases and bacteria and virus, and integrated capability of removing complicated pollutants. The development tendency shows that non-thermal plasma induced by corona discharge combining photo-catalyst has more advantages in indoor air purification. Thus, these two technologies were introduced, and finally an air cleaner featured with these two technologies was proposed.The voltage-current characteristics of corona discharge in emitting wire electrode configuration were studied systematically. According to the current and voltage values under different experiment conditions, the relationship of current and voltage was deduced as I=AV2(V-Vc)/L[R/ln(R/r0)]2 in corona discharge in emitting wire electrode configuration. Theoretical analysis was made and it was believed that the positive and/or negative ions form the current in migration area. The c-V curves provided evidence for distinguish the discharge stages as initial stage, stable stage and transitional stage to pre-breakdown. Compared to unipolar discharges, the ion concentrations of migration area are doubled in bipolar discharges; and the electron concentrations in ionization area are also doubled. The recombination of positive and negative ions can be neglected. Considering greater injective energy density and better discharge stability of corona discharge in emitting needle electrode than in wire electrode, the optimized discharge reactor of needle electrode was determined to be the core unit in the air cleaner.This new air cleaning unit was used to capture inhalable particles (Ips) in air when it was passed through the discharge reactor for one time. The results showed that better capture efficiency of IPs by corona discharge was obtained with an increase in the size of particle diameters Dp and decrease in the air flow rates Q. The values of CADR significantly increased with an increase in both Dp and Q. The reasons why this unit possessed higher capture efficiency were revealed by theoretical analysis. Then the indoor applicatio tests for suspended particle capture and IPS removal were carried out based on above study. It was indicated that the concentration of suspended particles would meet the national standard in short treatment time when initial concentration was high; and when initial concentration was low, it also would be decreased by one magnitude. Desirable capture efficiencies and CADR values of IPS with different diameter ranges were obtained. Compared to unipolar discharge, needle-plate bipolar corona discharge had better treatment result for IPs whose size was less than 1μm. It was proved that the cleaning unit was characterized by rapid and efficient capture of suspended particles and IPs and desirable CADR value.The airborne bacteria were sterilized by this air cleaning unit when the air was passed through the discharge reactor for one time. Results showed that the sterilization efficiency is increased with an increase in treatment time and applied voltage. The smaller the air flow rate was, the better the sterilization effect would be obtained. The optimal air velocity in discharge field and corresponding optimal air flow rate per unit volume of the reactor was determined. In addition, the sterilization mechanism was discussed and it was concluded that high-energy electrons and active particles play dominant roles in the sterilization process. The application tests results showed that after 30min treatment the bacteria concentration met the national standard of China and after 60min treatment it met the cleanliness requirements ofⅢarea identified by the ministry of health of China. Also, the PIV and HSV-I viruses were killed by this cleaner. The results showed that needle-plate corona discharge is capable to kill these viruses rapidly and efficiently. It was believed that the combination of UV light, high-energy electrons and radicals produced in discharge process is responsible for viruses killing.Multiple gas-phase pollutants were generated to simulate the air pollution in closed rooms respectively and the purification tests were carried out. Results showed that this cleaning unit has extensive and universal effect on gas pollutants removal; the efficiency and the effect were both better than other air cleaner. The purification effect of this cleaning unit on typical formaldehyde degradation was studied in detail. Results showed that higher treatment air flow rate and greater discharge power would benefit the formaldehyde removal; positive corona discharge has better effect than negative corona discharge. Qualitative analysis of the formaldehyde degradation mechanism and radical reactions were conducted. Finally non-thermal plasma combining with photo-catalyst technology has been proposed and utilized in purification of pollutant gases produced by burning cigarettes. The results of NTP and PPC treatment on pollutant gas when passed through the unit were compared and the rules were analyzed. It was proved that PPC method possessed synergetic effect in burning cigarette pollutant gas purification and the pollutants were removed by plasma chemical reaction and photo-catalysis reactions induced by high field intensity field, electrons, ions and UV lights.