Research On The Capture Technology Of Fine Particles In Electrostatic Precipitator

Electrostatic precipitators(ESP), despite its wide applications in industrial dustcollection devices, is encountered with the challenge in capturing fine particles inthe size range of0.1-1μm due to strict environmental standards. Previously, the totalefficiency of ESP as high as99%somewaht covers up the problem of seriouspenetration of0.1-1μm particles. Aiming the technical improvement of ESP formeetinng strict standard on PM2.5, the field test, theoretical analysis and lab-scaleexperimental techniques were performed to the explorethe problems in the PM2.5capture and the methods to enhance efficiency.First, on the basis of the governing equations of ESP as well as the mechanism ofparticle transport and capture in ESP, the dimensionless equations were derived and thedominant factors of key process were determined. The theoretical analysis indicatesthat the particles in the size range of0.1-1μm has the lowest electric migration velocityand the velocity of electrohydrodynamic(EHD) flow is comparable to the main flow.The influence of the space charge density couldn’t be neglected. In the boundary layerclose to collector plate, the drag force and the coulomb force from the deposited layerare close to the electric force, leading to a challenge in capture.Then, the electrical low pressure impactor(ELPI) with a sampling system was usedto measure online size distribution and average charge capacity in the range from30nmto10μm in aerodynamic diameter. The results showed that the particle numberdistribution has a peak near0.1μm, and it exhibits a bimodal distribution with a peakaround1μm that stands for the coarse mode in circulating fluidized bed boiler. Thecharging amount of particle larger than0.5μm is near saturation, while those of the onesunder0.5μm are below the theoretical value. The test also examined the effect of laststage electric field in the ESP, gas temperature and the intermittent power supply on theparticle number concentration and average charging amount of fine particles. Themulti-stage dilution sampling system containing the isothermal dilution in the first stageis effective for accurately measing the number concentration, but ineffective formeasuring the charging distributions of particles. The combination of diffusion dryerand small percentage dilution is proved to be much better for the latter. Finally, a laboratory scale ESP has been established on the basis of thedimensionless factors, and the changes of particle number concentration and chargedistribution along the ESP and the influence of strength of the electric field on fineparticle capture were studied. The charge increases along the flow direction of ESP forall particle size, which multiplies near the discharge electrodes and decreased betweenthe adjacent discharge electrodes. The number concentration doesn’t decayproportionally. There is a capture-effective zone in the middle of the ESP, andsubsequent increase in length had little effect for grade efficiency. For different particlesizes, the sensitive range electric field strength is different. For fine particles, thesensitive strength of the electric field was near the corona inception range, while forcoarse particles it was higher. The capture efficiency of high dust concentration is lessthan the low dust concentration situation, while there is little difference in chargingcapacities for different dust concentrations because the gas anion dominant the spacecharge density. The approaches to remove dust boundary layer efficiently and suppresscorona wind are effective for PM2.5capture enhancement.