Research On Particle Charging Mechanism And Coagulation Removal Of PM_2.5 By Pulsed Corona Discharge

Nowadays, the fine particulate matter (PM2.5) has become a major type of air pollutions in China, and causes serious damage to human health and atmospheric environment. Efficient removal and control of PM2.5 are widely studied at home and abroad. Coal-fired power plant is an important sources for PM2.5 pollution in our country. Traditional dust removing technologies has high total dust removal efficiency, however, the PM2.5 removal efficiency is low. Electric agglomeration technology which is considered to be an effective way for PM2.5 control can contribute to coagulation and removal of charged particles. The key of electric agglomeration technology is particle charging, which is limited in traditional direct current electric field.The pulsed corona discharge starts from electron avalanche, its streamer-channels penetrate between positive and negative electrodes. There are a large number of high-energy free electrons, positive and negative ions which can enhance particle charging, and promote coagulation of charged particles at the same time, and it will increase the backend dust removal efficiency as one of the most promising electric agglomeration technologies. But this technology for PM2.5 removal with high-efficiency is still based in the lab, owing to the fact that there is lack of a comprehensive understanding of the influences of experimental parameters and the model for charging mechanism is oversimplified in the pulsed electric field, relevant experimental and theoretical studies have been carried out in this dissertation.In this dissertation, the mechanism of particle charging is based on the theory of unipolar charging, simplifies the particle charging in pulsed electric field into electrons and positive ions charging at different times of pulse period, and simplifies the particle charging in direct current electric field into field charging and diffusion charging of unipolar ions. According to the relationship between the mean free path of electrons and ions and particle diameter, the models for particle charging in pulsed electric field and direct current electric field have been built with considering the influences of Coulomb force and image force respectively. The diffusion charging of high-energy free electrons in the electronic temperature is used to explain the particle charging with high-energy free electrons in pulsed electric field.In the experimental study, a laboratory scale wire-plate experimental platform based on the pulsed corona discharge combined with DC dust collection device is set up to investigate the influences of reactor structural parameters, discharge parameters, pulse shape and gas composition on particle charging and removal.First of all, the energy level and its spatial distribution characteristics of high-energy free electrons produced by pulsed corona discharge have been investigated. The experimental results indicate that the spatial distribution of high energy free electrons’ energy level is like a sector faced with plate electrodes, which is in accordance with the spatial distribution characteristics of secondary streamer. At the same time, increasing the relative humidity and decreasing the oxygen content concentration are beneficial to the production of high-energy free electrons.Next, the discharge characteristics and particle charge distributions in different electric field have been studied. Experimental results show that volt-ampere characteristics and particle charging in the negative DC electric field are superior to those in the positive DC electric field, and energy input in the pulsed electric field is superior to the DC electric field. Particles are charged with unipolar ions in the DC electric field, and in the pulsed electric field, particles with large diameter are negatively charged while small ones are positively charged. With the increase of pulse energy consumption in wire-plate reactor, the critical particle diameter for different polarity charging gradually increases. Increasing the pulse rise time and pulse width can improve the pulse energy input, improving the relative humidity and reducing the oxygen content concentration will have the same effect, thus particle charging will be enhanced. Among these factors, pulse width and oxygen content concentration have greater influences.Then in view of the reactor structure parameters and electrical parameters, four parameter and four level orthogonal experiments and single factor experiments have been carried out to optimize the particle charge distributions with small diameter in pulsed electric field. Experimental results indicate that wire-wire distance has most effects to particle charging, wire-plate distance has less effects and pulse frequency has least effects. The optimum combinations of experimental parameters for particle charging is that the wire-wire distance is over twice of the wire-plate distance, and the optimal pulse frequency is 300 Hz. Particles with 0.2 μm in diameter is most difficult to be charged in the pulsed electric field.Based on the above research, the coagulation removal of PM2.5 has been investigated in the system of pulsed corona discharge combined with DC dust collection device. The experimental results show that dust removal efficiency in the negative DC electric field is higher than that of positive DC electric field and pulsed electric field. The electric field combination system of DC- pulse-DC has high removal efficiency to particles with large diameter and sub-micron particles, particles with 0.2 μm in diameter have the lowest removal efficiency. Increasing the pulse rise time and pulse width and improving the relative humidity and reducing the oxygen content concentration can improve the removal efficiency of PM2.5, pulse width and oxygen content concentration have greater influence.Based on the research of laboratory scale experimental platform, high-efficiency PM2.5 removal system for coal-fired power plant has been designed. The electric field matching form is DC-pulse-DC. This system has higher particle removal efficiency for mass concentration and higher sub-micron particle removal efficiency for number concentration, and it has a good effect of atmospheric environment protection and high economic benefit.