Electrostatic Precipitator Using Induction Charging

Transcription

1 Katatani et al. 135 Electrostatic Precipitator Using Induction Charging A. Katatani 1, H. Hosono 2, H. Murata 2, H. Yahata 2, and A. Mizuno 3 1 Panasonic Environmental Systems & Engineering Co., Ltd., Japan 2 Panasonic Ecology Systems Co., Ltd., Japan 3 Department of Environmental and Life Sciences, Toyohashi University of Technology, Japan Abstract The authors have had an idea that the power consumption in electrostatic precipitators, ESPs, might be decreased drastically if particles are touching to a conductive electrode in electric field, and are charged by induction charging. One reason is the fact that, in a corona discharge, most of the energy of accelerated electrons near the discharge electrode is wasted by bombardment to neutral molecules without ionization. To verify this idea, the authors have carried out an experiment using two-stage-electrodes sets. The first set of electrodes is made of the flocking electrode covered with ACF (Activated carbon fiber). DC high voltage was applied to the electrodes set. The voltage value was adjusted to be below the corona starting voltage. The second set of electrodes made of flat plates works as a collector of charged particles. This is designated as a two-stage ESP with induction charging. Varying the gas velocity inside the ESP, the collection efficiency of room-particles was measured. The result showed that the ESP was able to collect particles without corona discharge. This study implies the possibility of ESPs which can minimize the electrical power consumption. Keywords Electrostatic precipitator, electrostatic flocking, re-entrainment, induction charging I. INTRODUCTION Electrostatic precipitators (ESPs) have been widely adopted for purifying exhaust from motor-vehicle-tunnels or others [1] [7]. Trend of the development of these ESPs has been to reduce the size by increasing the wind velocity. As a result, power consumption has been increasing recently, as shown in [8], [9]. There are studies on charge and sedimentation of fine particles. Some reports have shown that particles in dieselexhaust are charged either positively or negatively [10] [14]. Several studies have indicated that, in many cases, suspended particles in air have charge [15] [20]. For sedimentation of fine particles inside ESPs, gradient force in non-uniform electricfield plays an important role [21] [23]. To improve collection efficiency of ESPs, there is a study to use the electrodes with nylon-piles (prepared by electrostatic flocking) that generate strong gradient-force at the tip of piles [24]. Furthermore, there are studies reporting on the phenomenon of re-entrainment which is inevitable to ESPs [25], [26]. The re-entrained particles are electrically charged by induction charging [15], [27] [30]. Taking these reports into account, the authors have the idea that the power consumption of ESPs can be decreased drastically if induction charging is used. The purpose of this study is to clarify the process in which temporarily attached particles onto the electrostatic-flocking electrodes are re-entrained with induction charge, and are collected by the electric field. Through the experimental observation, the new-concept of ESP using induction charging is discussed. Corresponding author: Atsushi Katatani address: katatani.atsushi@jp.panasonic.com Presented at the 3rd ISNPEDADM 2015 (New electrical technologies for environment), in October 2015 II. METHODOLOGY The experimental ESP is composed of the charger in the first stage and the collector in the second stage. To prepare the electrode with fibers, electrostatic-flocking is used. The electrodeplates (flocked-plates, hereafter) are used as the charger. How to make this flocked-plate is described as follows. The electro-conductive glue (Three-Bond 3303G), which is mainly made from silver-paste and silicon-resin, is applied on each one side of stainless-steel-plate (SUS304, mm, 0.4t) as base-plates for electrostatic flocking. Activated carbon fiber (ACF, production code CO6343 made by TORAY) is cut into the length from approx. 0.1 to 3 mm by using scissors to make ACF piles. The ACF piles are thrown in a device for electrostatic flocking, to which dc voltage is applied. Then, some ACF piles contact to the adhesive glue on the surface by the effect of circulation-wind in the device, and are finally fixed to form the flocked-plates. A part of the craft-process of the flocked-plates is shown in Fig. 1. The layout/electric-circuit of the flocked-plates in the charger is shown in Fig. 2, where twenty-four flocked-plates are arranged parallel with the gap of 10 mm. Two plates at the end points are not flocked at all. In addition, the surfaces of twelve flocked-plates at leeward are facing to the opposite direction of the flocked surfaces at windward. The applied voltage to the charger was determined according to the concept to use induction charging that does not want corona-discharge from flocked-plates but does need the electric field strength as strong as possible. Thus, taking into account the V-I characteristics in Fig. 3, the voltage of -2.4 kv which realizes the maximum electric field strength without forming corona-discharge, was determined. The collector consisted of parallel electrodes, and was placed downstream of the charger. The shape of all the electrode-plates in the collector is mm (0.4t). Seven

2 136 International Journal of Plasma Environmental Science & Technology, Vol.10, No.2, DECEMBER 2016 Fig. 4. Schematic diagram of test equipment. TABLE I S PECIFICATIONS OF T EST E QUIPMENT Fig. 1. Crafting-process of flocked-plates. Items Details Duct (#1, 2, 4, 6, 7) W 121, H 140, L 200 mm (Inside) Charger duct (#3) Duct; W 121, H 32, L 180 mm (Inside) with slots for fixing electrode-plates Collector duct (#5) Duct; W 121, H 90, L 300 mm (Inside) with slots for fixing electrode-plates Fan (#8) MU1238A-11B (Oriental Motor), Quantity; 2 (tandem coupled), With a variable frequency controller High voltage power supply (#9) MODEL-600F (Pulse Electric), Max. output; DC -15 kv, 30 ma, Stability 0.005% High voltage power supply (#10) APH-10K5N (Maxelec Co.,Ltd.), Max output voltage; DC -10 kv, Max current; 30 ma, Ripple; 0.02% Particle counter (#11) KC-01E (RION), Light scattering, Range; 0.3, 0.5, 1, 2, 5 over µm, Sampling volume for individual measurement; sample-mode of 283 ml (per 34 s) Wind velocity meter (#12) Climomaster MODEL6531 (Kanomax), Mode; 1 s measuring & 10 times ave. Voltage meter Probe (#13) Digital multi meter type73303 (Yokogawa), Ratio; 1/1000 (FLUKE) Fig. 2. Layout of electrode-plates with electrostatic flocking in the charger. & Current meter (#14) Fig. 3. V-I characteristic of the charger. grounded-plates and six energized plates are alternately arranged with 10 mm gap. The applied voltage to the energizedplates is -9 kv, which does not generate corona-discharge. The schematic diagram of test equipment and the specification of test equipment are shown in Fig. 4 and Table I, respectively. The particles to be collected in this study were those suspended in the air of the laboratory room. By measuring the wind velocity with the wind velocity meter #12 Type (Yokogawa), Range; 0.1, 0.3, 1, 3 ma at the inlet of the inlet duct, the wind velocity in the charger duct #3 was adjusted with a fan-speed-controller in order to obtain four levels of wind velocity of 2, 5, 8 and 11 m/s. The concentration (count-concentration of particle diameter 0.3 µm over) of particles in the room-air was measured by the particle counter of #11, whose two sampling tubes at the inlet-duct and the outlet-duct were alternately switched, to calculate the collection efficiency. The collection efficiency was measured for the three cases of different applied voltages. Each case includes the efficiency with four levels of the wind velocity. III. D ISCUSSION A. Case 1: charger -2.4 kv, collector 0 kv The purpose of Case 1 is to evaluate the collection efficiency of the charger only. The measured result is shown in Fig. 5

3 Katatani et al. 137 Fig. 5. Collection efficiency of Case 1 (charger; -2.4 kv, collector; 0 kv). Fig. 7. Collection efficiency of Case 3 (charger; -2.4 kv, collector; -9 kv). Fig. 6. Collection efficiency of Case 2 (charger; 0 kv, collector; -9 kv). which indicates the collection efficiency of all the particlediameters of 0.3 µm over. As the dispersion of three-timesmeasurement is within ±1% to the average indicated in solid line, there is repeatability. The collection efficiency was less than several percent. B. Case 2: charger 0 kv, collector -9 kv The purpose of Case 2 is to evaluate the collection efficiency of the collector only. The measured result is shown in Fig. 6. As the dispersion of three-times-measurement is also within ±1% to the average in solid line, there is repeatability. The collection efficiency was less than several percent. At the point of wind velocity 2 m/s, the collection efficiency is almost 25%, which means the extremely-greater value compared to other levels of wind velocity. The one of reasons is that the stronger electric field exists due to forming non-uniform electric field around the vicinity-space of the edges of electrode plates. When the velocity reaches to 5 m/s, the collection efficiency decreases to almost 7% with diving. The one of reasons is that re-entrainment of collected particles increases due to the faster wind velocity. C. Case 3: charger -2.4 kv, collector -9 kv The purpose of Case 3 is to evaluate the collection efficiency of both the charger and the collector, which means the measurement with electric fields existing in both parts. That is; this is the evaluation for the two-stage-esp. The measured result is shown in Fig. 7. As the dispersion of three-times-measurement is also within ±1% to the average in solid line, there is repeatability, too. At the point of wind velocity 2 m/s, the collection efficiency is about 40%, which means the much-greater value compared to other velocity levels. This tendency is similar to Case 2 of using the collector only. Therefore, this is the characteristic strongly affected by the collector. The three cases (Case 1 of voltage to the charger only, Case 2 of voltage to the collector only and Case 3 of voltage to both ) should be synthetically discussed as follows. Let X% as the collection efficiency of charger only under certain wind-velocity condition and under certain particlediameter. Let Y % as the collection of collector only under the same conditions of velocity and diameter. At this moment, the combined collection efficiency of operating both charger and collector simultaneously can be calculated using (1), although it should be under the condition that both parts do not interfere in each other. Z = ( 1 ( 1 X 100 ) ( 1 Y 100 )) 100% (1) According to the above mentioned idea, the synthesized collection efficiency obtained from calculating the two cases of collection efficiency in operating charger only and collector only is shown as the broken line in Fig. 8. The solid line in the figure indicates the result from the actual measurement, which has already shown, for the comparison. Here, the following fact can be noticed. That is; although the calculation (the broken line) resembles the measurement (the solid line) in the velocity characteristics of collection efficiency, the measured characteristic is higher than the calculated one in all points. This means that the charger and the collector are not independent of each other in case of applying voltage to both parts. That is; the particles which have contacted the charger or been collected at the charger temporarily do re-entrain with being electrically charged by induction charging, then, the charged particles are collected in the second stage collector with strong electric field. This might be the reason of the higher characteristic of measurement. The reference [31] reports the aspects of particle-sediment/collection

4 138 International Journal of Plasma Environmental Science & Technology, Vol.10, No.2, DECEMBER 2016 can be seen on (a). On the other hand, small dust of 1 µm under on the ACF surface is found on (b). Although it seems that the ACF diameters of (a) and (b) slightly differ from each other, both diameters are within the ACF product-specification of diameter from 5 to 10 µm. Any other differences on both figures cannot be found. IV. C ONCLUSION Fig. 8. Comparison of collection efficiency between measurement and calculation (charger: -2.4 kv, collector: -9 kv). Fig. 9. The observation before/after using ACF. by using one-stage-esp without corona discharge-parts. The report includes that the re-entrained particles with induction charging can be re-collected in strong electric field even if corona discharge does not exist, which is similar to the result of this study. In order to confirm how much or less there is particleattaching to the surfaces of ACF on flocked-plates in case of using flocked-plates in ESPs, the observation with a SEM (scanning electron microscope, Hitachi S-3600N, gold evaporation, acceleration voltage 10 kv, magnification 3000) was done. The observation results are shown in Fig. 9. The aspect of an ACF surface on an unused flocked-plate is shown in Fig. 9(a). On the other hand, Fig. 9(b) displays the aspect of an ACF surface under the condition for the ESP to be operated for approx. 10 h in the room atmosphere with applying -2.4 kv to the charger. Comparing (a) with (b), no particle-attaching of room dust Using the charger composed of metal plates to be electrostatic-flocked with activated carbon fiber, a dc voltage which does not make them generate corona-discharge was applied to the charger. In addition, the collector to be composed of parallel flat-plates was installed at the position after the charger to form a two-stage-esp. Making room-particles in the atmosphere pass through the ESP, the collection efficiency was measured by using a particle-counter. As a result, the following points were found. 1) Under the condition of wind velocity 2 m/s, the collection efficiency on all particle-sizes of diameter 0.3 µm over was measured as approx. 40% (No generation of corona discharge). 2) The above-mentioned collection efficiency was greater than the synthesized efficiency calculated by using the measured efficiency of both charger only and collector only. 3) The reason is as follows. That is; re-entrained particles from flocked-plates in the charger might be electrically charged by induction charging, even in case of using chargers without generating corona-discharge. And the particles, which obtain electric charge by induction charging, can be collected at the leeward collector with strong electric field. These effects might have an influence on the higher measured efficiency compared to calculated efficiency. 4) Small room-dust of 1 µm under on the surface of the activated carbon fiber in flocked-plates was found. R EFERENCES [1] N. Sugita, Electrostatic precipitator: Collection efficiency in 2-stage electrostatic precipitator (in Japanese), Journal of the Institute of Electrostatics Japan, vol. 31, pp , [2] R. Brandt and I. Riess, Possibilities and limitations of tunnel-air filtration and portal-flow extractions, in the 13th International Symposium on Aerodynamics and Ventilation of Vehicle Tunnels (conducted by British Hydrodynamics Research Group), vol. 1, New Brunswick, NJ, August 2009, pp [3] PIARC Technical Committee C3.3 Road Tunnel Operations, A guide to optimizing the air quality impact upon the environment, the World Road Association (PIARC), [4] V. Ferro, H. Aigner, and C. Barbetta, Air filtration system in an urban tunnel, in the 12th International Symposium on Aerodynamics and Ventilation of Vehicle Tunnels (conducted by British Hydrodynamics Research Group), vol. 2, Portoroz, Slovenia, 2006, pp [5] T. Baba, H. Ohashi, F. Nakamichi, and N. Akashi, A new longitudinal ventilation system using electrostatic precipitator for long vehicular traffic tunnel, in the 3rd International Symposium on Aerodynamics and Ventilation of Vehicle Tunnels (conducted by British Hydrodynamics Research Group), Sheffield, UK, 1979, pp [6] T. Baba and K. Okano, Ventilation system of tsuruga tunnel, in the 4th International Symposium on Aerodynamics and Ventilation of Vehicle Tunnels (conducted by British Hydrodynamics Research Group), York, UK, 1982, pp

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