Takeo Takahashi

Two-stage-type electrostatic precipitator re-entrainment phenomena under diesel flue gases

One of the applications of the electrostatic precipitator (ESP) is the cleaning of air to increase the visibility index in highway tunnels. Particles floating in air in highway tunnels are mainly carbon. Collection efficiency of a large particle diameter in an ESP often decreases when the ESP collects carbon particles which have low electric resistance. Collection efficiency often becomes negative in an experimental ESP. The negative collection efficiency means that the particle concentration flowing downstream is greater than that upstream in the ESP. This phenomenon is explained as the re-entrainment of particles. In this paper, experiments were carried out to investigate the cause of the decrease in efficiency of particle collection of the ESP. The time characteristic of the collection efficiency and the distribution of particle size on the collection electrodes were studied. Experimental results showed that the decrease in the collection efficiency was caused by re-entrainment of particles during the ESP operation. The effect of gas-flow velocity on the collection efficiency of the ESP was also investigated to study the cause of re-entrainment phenomena. The result showed that the re-entrainment phenomena depended on the gas-flow velocity.

Effect of electrode shape on discharge current and performance with barrier discharge type electrostatic precipitator

Abstract In general corona discharge is used as precharger in electrostatic precipitator (ESP). The corona discharge type electrostatic precipitator can remove diesel exhaust particles (DEP) at high efficiency but at low efficiency for NO x . We propose here a barrier discharge type ESP which gives a high removal efficiency for both NO x and DEP. To study the influence of electrode shape of precharger on the collection efficiency, we carried out experiments using metallic electrodes with/without punched holes as a barrier discharge electrode. As results, discharge current was higher for the electrode with punched holes than for plane electrode. The collection efficiency obtained with punched electrode was higher than that with at plane electrode. In the next step, additionally change of metallic electrode shape, and then discharge current and performance are evaluated.

Electrostatic Beneficiation of Coal Using a Cyclone-Tribocharger

A preliminary study is presented on a new method of separation of mineral inclusion from coal. The separation process involves triboelectrical charging of pulverized coal by a cyclone-tribocharger and electrostatic differential deflection of the coal. The separation tests for a pulverized Australian coal with 18.5 percent ash content using this separator yielded the following results: a cyclone with a copper wall gives the best separation performance; the percentage ash contents of particles collected on the anode and cathode of the separator are 26 percent and 11 percent, respectively; a coarse pulverized coal with 74-150 pm diameters showed a beneficiation rate better than that for a fine pulverized coal with diameters below 44 pim. In order to understand these results, the authors experimentally examined charging behavior of pure mineral particles fed to the cyclone-tribocharger with either copper or teflon walls and to a fluidization-charger. Avoidance of particle-to- particle collision and choice of cyclone wall material are important in raising the separation performance. On the basis of microscopic observation, the authors confirmed liberation of most mineral inclusion from coal crashed to particles of less than 70-¿m diameters.

Agglomeration of particles by ac corona discharge

Improving the collection efficiency for particles smaller than 1 μm on every precipitator is important. We sought to improve the collection of these particles on an ESP due to particle agglomeration. Particles are charged by ac corona discharge in a precharger and agglomerated by a dc electric field in an agglomerator downstream of the precharger. Diesel exhaust particles were used as particulate matter for the experiments. The distribution of particle size was measured using a particle counter and a scanning electron microscope. By these methods, particles as small as 0.01 μm could be counted. Results showed the agglomeration between particles at ac corona discharge operating mode. The concentration of particles smaller than approximately 0.35 μm decreases, and that of particles larger than approximately 0.35 μm increases in the agglomerator. The agglomeration rate increases with increasing applied voltage, then saturates. These results may be due to the size distribution and to decrease of concentration by agglomeration.

Suppression of particle deposition onto downstream wall in an AC Electrostatic Precipitator with neutralisation

Electrostatic Precipitators (ESPs) are used to decontaminate polluted environment around the tunnels. A conventional ESP has high collection efficiency but still has a problem in that its downstream walls are polluted due to particle deposition. The experiments carried out in this study have focused on how the particle deposition can be prevented by neutralising the gases. The collection efficiency as a function of particle diameter and the amount of deposited particles on the wall were studied on three types of the ESP: the universal ESP under DC operating mode (DCESP), the ESP under AC operating mode (ACESP) and the ACESP with gas-neutralising equipment (ACESP with neutralisation). Electrostatic field intensity and ion concentration were also measured to understand the state of neutralising the gases. The ACESP, an invention of the authors, prevents particle re-entrainment. The ACESP with neutralisation, which not only prevents the particle re-entrainment but also decreases the charge amount of the downstream gases, was most effective in decreasing the amount of deposited particles.

Development of high collection efficiency ESP by barrier discharge system

One of the electrostatic precipitator (ESP) applications is the cleaning in superhighway tunnels. In superhighway tunnels, contaminants in the air are carbon particles, NO/sub x/ etc. The ESP consists of the precharger and collecting section. The ESP can little remove NO/sub x/, when corona discharge is used as a precharger. The autors suggest a barrier discharge-type precharger instead of corona discharge. In this work, experiments were carried out using small two-stage type ESP with barrier discharge. As a result, it was shown that the particle collection efficiency and NO/sub x/ removal rate were high in barrier discharge type ESP.

Investigation of two-stage type electrostatic precipitator reentrainment phenomena under diesel flue gases

One electrostatic precipitator (ESP) application is cleaning the air and increasing the visibility index in highway tunnels. The main particles found in highway tunnels are made of carbon. ESP collection efficiency of a certain particle diameter is often negative when the ESP collects carbon particles of low resistance. This means that the number of particles downstream is greater than that of the number of particles upstream of the ESP. Generally, this phenomenon is explained by the reentrainment of particles. In this work, experiments were performed to investigate the cause of the ESP negative particle collection efficiency. Experimental results show that the ESP negative particle collection efficiency is caused by a reentrainment of particles during ESP operation. The effect of gas flow velocity on the ESP collection efficiency was also investigated to clear the cause of reentrainment phenomena. The result shows that the reentrainment may depend on gas flow velocity, because the ESP collection efficiency for the larger particles increases with increasing gas flow velocity.

The relation between shape of particles and collection efficiency by electrostatic precipitators

Electrostatic precipitator (ESP) has been used to decontaminated polluted gases and improve the visibility index in expressway tunnels. Lots of particles in expressway tunnels are made of carbon that has low resistivity. The collection efficiency of large particles decreases due to re-entrainment phenomena. Therefore, it is important to prevent re-entrainment. In this work, we discussed the method preventing particles re-entrainment, and devised transforming shape of the agglomerated particles on the collecting electrodes.

Polarity dependence of applied voltage to deposition of charged particles for electrostatic precipitator

Electrostatic precipitator (ESP) has been used to remove the particles and clean the air in a home, expressway tunnel, factory and thermal power station, etc. The performance of ESP has improved, however it has a matter that particles passed through ESP deposit on the downstream wall. The aim of this study is to prevent particle deposit on the wall of ESP.