Study On Motion Laws And Removal Efficiencies Of Fine Particles In Electrostatic Precipitator

Currently, particulate pollution is the major air pollution problem of China. With the introduction of various environmental policies, dust removal of coal-fired boiler as a major source of particulate emissions is facing unprecedented pressure and challenges. ESP technology is widely used in the dust removal of coal-fired boiler flue gas because of its many advantages such as high collection efficiency, low resistance, treatment of large volume flue gas. However, the active ESPs were designed and manufactured based on the old emission standards in China, have been unable to meet the new environmental requirements, so most of them face transformation, improving efficiency or elimination.Collection efficiency of fine particles is low in ESP, because the trapping of fine particles is affected not only by their charge but also by their motion laws in ESP. Ion wind generated during corona discharge will cause strong motion of fine particles in ESP, which is one of the important factors affecting the trapping of fine particles. Therefore, more and more researchers devoted to study on the ion wind. However, most studies had focused on the theoretical models and the experimental models were also large differences from the actual ESP, both failed to clarify the relationship between motion laws and removal efficiencies of fine particles. To solve this problem,2D-PIV visualization technique and ELPI quantitative analysis techniques of particulate matter were used to study on the motion and collection of fine particles induced by ion wind, thus revealing the quantitative relationship of motion laws of fine particles, ESP index with grade collection efficiencices. The main contents and conclusions are as follows:(1) Through a comparative study of the PIV tracer particles, and using BBO equation and refractive index formula verified moxibustion smoke particles had better following performance and scattering characteristics. Therefore, moxibustion smoke was selected as the tracer particles in this experiment.(2) Experiment on ion wind shows that the ion wind can be produced at very low voltages. The greater the extreme curvature of the discharge electrode and the applied voltage, the greater the intensity of ion wind.(3) Experiment on single-stage ESP shows that the motion laws of fine particles induced from positive corona is similar to negative corona. Four vortexes form around the discharge electrode between the plates at high field strength. Discharge electrode structure, number and spacing will affect the strength, distribution and abundance of the vortex.Grade collection efficiency of particles can be formulated as for the single-stage ESP. When Ea2 is less than 9(kV/cm)2, there is a positive correlation between grade collection efficiency and ESP index Ea2S, and the interference of ion wind on paticles collection is low; When Ea2 is greater than 9(kV/cm), there is no significant correlation. Charging of particles basically reached saturation and the trapping of particles is mainly influenced by the interference of ion wind.(4) Experiment on double-stage ESP shows that a large vortex forms in the collection zone after pre-charge of particles. Although there is no the interference of ion wind, the particles move circularly between the plates in the role of large vortices and are not easily captured.Grade collection efficiency of particles can be formulated as EaES for the double-stage ESP. When EaE is less than 7.5(kV/cm)2, there is a positive correlation between grade collection efficiency and ESP index EaES. Therefore, it is necessary to respectively control the field strengthes of pre-charge zone and dust zone.(5) Using thumbtack electrode and acrylic sieve plate electrode to simulate the occurrence of back corona in ESP. It can be found that the back corona weakens the ion wind and reduces the charging of particles, but the overall pattern of motion trail changes little.Grade collection efficiency of particles is still related with the ESP index when back corona occurs in ESP, which is similar to negative corona condition, but there is the significantly lower collection efficiency.(6) The ion wind induces turbulation in ESP because of the rotational component of electrohydrodynamic force derived from the change of current density transfering the electric field energy to turbulence. Through the analysis for the turbulent kinetic energy of vortex shows that the turbulence intensity in the vicinity of the vortexes intersection and the vortex center is stronger. The motion of fine partices is also stronger.