Yoshiyasu Ehara

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.

Diesel PM Collection for Marine and Automobile Emissions Using EHD Electrostatic Precipitators

The collection of low resistive particulate matter (PM) generated from marine and automobile diesel engines have been known to be difficult by conventional electrostatic precipitators (ESPs). The collection efficiency for three types ESPs, namely, the conventional dc-energized ESP, the trapezoidal-waveform-energized ESP (TW ESP), and the electrohydrodynamically assisted ESP (EHD ESP) were investigated. Low resistive PMs are detached from the collection plate by the electrostatic repulsion force caused by induction charge resulting in particle reentrainment. The EHD ESP utilizes ionic wind combined with an electrostatic force to transport the charged particles into the zero electric field zone (pocket zone) attached to the collection plate effectively. The conventional dc-energized ESP showed good collection efficiency for particle sizes less than 300 nm where adhesion force was dominated over electrostatic repulsion force but showed a severe reentrainment for particle sizes greater than 1000 nm, while the TW ESP suppressed the particle reentrainment for larger particles but still showed negative collection efficiency. On the other hand, the EHD ESP showed an excellent collection efficiency for particle sizes up to 1000 nm and a significant reentrainment suppression was observed even for particle sizes greater than 2000 nm.

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.

Electrohydrodynamically Assisted Electrostatic Precipitator for the Collection of Low-Resistivity Dust

The collection of low-resistivity particles is difficult by the conventional electrostatic precipitators (ESPs). The low-resistivity diesel engine particles are detached from a collection plate, which causes dust re-entrainment, resulting in a poor collection efficiency. The new electrohydrodynamically (EHD)-assisted ESP (EHD ESP) was developed to minimize re-entrainment. The EHD ESP utilizes the ionic wind, combined with electrostatic force to transport the charged particles effectively into the pocket zone attached to the collection plate. The pocket zones are designed as zero electrostatic field, i.e., no electrostatic repulsion force acting on particles by induction charge, so that no re-entrainment takes place. The electrode position with respect to pocket position influences the EHD streamlines and particle collection processes. The optimum electrode position was determined by observing the collection transport process into the pocket zone and collection efficiency. The particle-size-dependent collection efficiency was evaluated using a scanning mobility particle sizer and a particle counter.

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 EHD-enhanced plasma electrostatic precipitator

The novel device is able to not only collect the low resistive particles generated from diesel engine but also incinerate captured particulates within the same reactor. The new electrohydrodynamically (EHD)- enhanced plasma ESP (EHD plasma ESP) is to utilizes the ionic wind to transport the charged particles effectively into the low electrostatic field region where electrostatic repulsion force acting on particles by induction charge becomes very small, resulting in reentrainment suppression. At the same time, collected particulates are incinerated by ozone and oxidation product of NO2 in the flue gas by the surface discharge plasma, so that no particle handling device is required. The particle-size dependent collection efficiency and particle incineration were evaluated by the Scanning Mobility Particle Sizer (SMPS) and particle counter (PC). This device is particularity important applications for automobile and marine diesel engine emission control.

Electrohydrodynamically-Assisted Electrostatic Precipitator for Collection of Low Resistive Dust

The collection of low resistive particles from diesel engine emission is difficult by the conventional electrostatic precipitators (ESPs). The low resistive diesel engine particles are detached from the collection plate, which causes the dust reentrainment, resulting in poor collection efficiency. The new electrohydrodynamically (EHD) assisted ESP (EHD ESP) was developed to minimize reentrainment. The EHD ESP utilizes the ionic wind, combined with electrostatic force to transport the charged particles into the pocket zone attached to the collection plate effectively. The pocket zones are designed as zero electrostatic field, i.e., no electrostatic repulsion force acting on particles, so that no reentrainment takes place. The electrode position was designed to achieve an optimum electrode location with respect to pocket position, which was determined by observing EHD streamlines and particle collection efficiency. The particle size dependent collection efficiency was obtained using scanning mobility particle sizer (SMPS).

Effect of applied voltage frequency on NOx removal rate for a superimposing discharge reactor

Abstract We are interested in the removal technique of NO x gas exhausted from a diesel engine with the use of several electric discharge methods. Several reactors have been proposed in order to remove NO x by electric discharge. Nomura et al. have proposed the superimposing methods of surface and silent discharges. These methods suggested the usefulness for the removal of NO x and the generation of ozone. We have exhibited in this paper, the removal of NO x in dry gas similar to a gas exhausted from a diesel engine with the use of superimposing discharge methods. We present the accelerating effects of the superposition of discharges and the frequency of electric source onto the NO x removal process. The experimental results indicate a high acceleration of NO x removal rate. This high acceleration was performed by the extension of discharge space and the injection of high electric power into the reactor due to the discharge superposition and high frequency.

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.