The effect of high-voltage waveforms on ESP current densitydistributions

The efficiency of an electrostatic precipitator (ESP) collecting high resistivity dust is degraded by maldistribution of corona current in the interelectrode space. The current distribution can be improved by applying novel high-voltage waveforms to the discharge electrodes. Online measurements are presented of collecting plate current density distributions in a precipitator collecting fly ash from a coal-fired electric generating plant. The precipitator was operated with both round-wire and barbed-strip discharge electrodes. The current density distributions are compared for three high-voltage waveforms: conventional, pulse, and intermittent energization. A direct relationship is established between the ability of the waveforms to distribute useful values of corona current and the collection efficiency of the ESP     

Corona inception in typical electrode configurations forelectrostatic processes applications

The corona inception electric field at the surface of a wire electrode is usually evaluated by an empirical formula established by Peek. That formula was found to be valid in several simple electrode configurations (wire cylinder, wire plate), which are typical to electrostatic precipitators. The aim of this paper is to extend the study to other situations encountered in electrostatic applications. The experimental setup specifically modeled the several electrode arrangements which are commonly used with roll-type electrostatic separators. It consisted of wire-type corona electrodes, connected to a regulated DC high-voltage supply, and a rotating roll electrode, connected to the ground. The effect of associating the corona wire to one or several tubular electrodes of various shapes and sizes was investigated. The experimentally determined corona inception voltage was used as input data of a boundary-element-method program for the electric field computation. The computed values of the electric field were compared with those given by Peek's formula for wire electrodes of the same radius. The derived conclusions can be of help in the custom design of the corona electrode arrangements for various electrostatic applications     

Characterization of Dual Corona-Electrostatic Electrodes for Electrostatic Processes Applications

The dual corona-electrostatic electrodes consist of one or several ionizing elements (wires, needles, and blades) attached to a metallic support. This paper analyzes the characteristic features of a particular type of such electrodes and formulates recommendations on their design and utilization. The experiments focused on several models of dual corona-electrostatic electrodes, the ionizing element being the edge of a thin metallic blade. The tested devices had similar cylindrical metallic supports but blades of different sizes. Thus, it was possible to investigate the influence of electrode geometry on corona onset voltage and spark-over threshold in configurations that are specific to industrial electrostatic processes. Most of the experiments were performed by using roll-type electrostatic separators. A special experimental setup was employed for the study of the distribution of corona current density at the surface of a plate collecting electrode. For the model having the blade edge closer to the metallic support, the corona onset voltage was higher, and the corona discharge affected a smaller area at the surface of the collector. These experiments are discussed in relation with the results of the numerical analysis of the electric field generated by the different electrode configurations. At a given voltage and distance between the ionizing element and the collector, the presence of the cylindrical support diminishes the electric field at the edge of the blade and distorts the field lines. These effects were exploited in the design of the electrode system of an electrostatic separator for the recycling industry.     

Optimal high-voltage energization of corona-electrostaticseparators

The selection of the high-voltage supply can play an important role in the optimization of electrostatic separation processes. This paper aims to evaluate the influence of the main high-voltage parameters (waveform, polarity, level) on the efficiency of electrostatic separation, in the case of insulation-metal granular mixtures. A roll-type laboratory high-tension separator was employed for the experimental study, and the tests were carried out with samples of granular materials taken from the technological flowsheet of a recycling plant for electric wire scraps. The oscillograms of the voltage and of the current across the separator proved to be of great use for studying the transition from corona to spark discharges. The experiments, performed under various operating conditions (roll speed, roll radius, high-voltage level, interelectrode distance), show the existence of a strong interdependence between these parameters, the frequency of spark discharges, and the efficiency of the separation process. The reported results suggest that monitoring the frequency of the spark discharges could be of use for controlling the optimum operating voltage for a given electrostatic separation application. Although the full-wave rectifier allows for lower operating voltages than the half-wave rectifier, its general effectiveness in electrostatic separation processes is superior. Good insulation-metal electrostatic separation can be achieved at either positive or negative polarity of the high-voltage supply, but negative electrode energization is recommended for most industrial applications     

Computational and Experimental Study of Ionic Space Charge Generated by Combined Corona–Electrostatic Electrode Systems

Numerical computation of the electric field intensity and space charge density in electrode systems consisting of ionizing and nonionizing elements, connected at the same direct current (dc) high-voltage supply and facing a grounded plate, is a difficult problem, which is of interest to several electrostatic processes applications. The aim of the present paper is to demonstrate the effectiveness of an original method of field computation in the analysis of the factors that influence the distribution of the ionic space charge in such combined corona–electrostatic electrode systems. The computations and the experiments were carried out for an ionizing wire of diameter 0.3 mm, located at different distances$h$(10–30 mm) from a tubular support of diameter 25 mm. Several interelectrode distances (20–45 mm) were simulated. The extension of the zone at the surface of the grounded electrode, which is affected by the space charge, diminishes when reducing the intervals between these elements of the electrode system, and, at similar applied voltage, the density of the corresponding corona current increases. The experimental data were in good agreement with the computed results, validating the accuracy of the numerical method of space-charge calculation in this special electrode configuration.     

Electrostatic separation of brass from industrial wastes

Previous studies have demonstrated that electrostatic separation can be successfully employed for the recycling of nonferrous metals from chopped electric wire and cable scrap. The aim of this paper was to investigate the possibility of using the electric field forces for the selective sorting of other granular mixtures, such as brass dross. Laboratory tests of electrostatic separation were carried out on three samples: 0.08-1 mm, 0.08-0.2 mm, and 0.2-1 mm, containing more than 66% of brass. Sample 1 was separated in a corona-electrostatic field, generated by a standard electrode arrangement: a grounded rotating roll electrode (diameter 150 mm) and two high-voltage electrodes (wire-type dual corona electrode+tubular electrode). Processing of the other two samples was carried out in a custom-designed separator comprising an extended corona field generated between a matrix-type multineedle corona electrode and a roll electrode of large diameter (250 mm). Chemical analysis of the products showed that more than 90% of the brass can be recovered with a purity higher than 95%. The extended corona field electrode arrangement proposed in this paper seems to be a promising solution for the effective recycling of other granular wastes containing copper, aluminum and their alloys     

Charging of insulating spheres on the surface of an electrodeaffected by monopolar ions

The charge acquired by an insulating sphere in a uniform monoionized electric field has been accurately evaluated by Pauthenier. In certain electrostatic applications, such as the electroseparation of mixed granular solids, the particles to be charged are on the surface of an electrode. Under these circumstances, Pauthenier's formula is no longer valid, because the field is nonuniform. This paper addresses this problem from both a computational and an experimental point of view. A numerical method of field analysis was employed for the evaluation of the charge acquired by spheres of various dielectric constants, on the surface of a plate electrode. The numerically computed values of the saturation charge in this situation were always greater than those given by Pauthenier's formula. The experiments were carried out on laboratory equipment provided with various types of corona electrodes. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (3 mm diameter) when subjected to the positive or negative corona discharge generated between these electrodes and a metallic rotating roll electrode (150 mm diameter) connected to the ground. The experimental data were in good agreement with the theoretical predictions; the saturation charge increases linearly with the applied voltage, up to a threshold at which the self discharge of the particle occurs. The efficiency of ionic charging was shown to depend on the type of corona electrode that is employed     

Analysis of the behavior of ions produced by pulsed coronadischarge

The motion of an ion cloud which is produced by a corona discharge with the high-voltage pulse of a short duration and is introduced into parallel plate electrodes was calculated numerically. The ion cloud initially placed at the tip of a needle electrode was simulated by a number of ring charges and the trajectories of individual ring charges were calculated by using electric field strength obtained with a charge simulation method. Not only movement of the ion cloud but also the waveform of the induced current flowing through a ring electrode located at the center of the parallel plate electrode were simulated and analyzed. The ion cloud with an initial diameter of 0.5 mm expands to 5 mm by electrostatic repulsion within 10 μs after the beginning of drift. During drifting toward the counter electrode, the ion cloud extends wider to a diameter of around 20-25 mm. The size and velocity of ion clouds agree with those estimated by experiments and the waveform of induced current obtained by experiments was reproduced by this simulation     

高压脉冲负电晕荷电喷雾试验研究

为减少化学农药带来的污染,采用了荷电喷雾新技术。首先对负高压脉冲电晕荷电机理进行了理论分析,再用网状目标法通过调节电压、极距、电极直径、电极数目等参数进行电晕荷电试验,比较各因素对电晕放电伏-安特性影响及荷电后雾化效果;测量不同电压下雾滴的索太尔直径;并用高速摄影拍摄雾滴带电射流破碎时的状态,得到射流破碎时的雾滴形状。理论分析和试验结果表明,电压越高,雾滴谱较窄,雾滴粒径变细,均匀度越高;揭示了不同影响因素下的雾滴荷电特性,为负高压脉冲电晕荷电喷雾技术的应用和设计提供了理论基础和试验数据。     

DC corona discharge from a wire particle floated with a microgap in parallel plate electrodes

Conductive particles existing in a high-voltage system could deteriorate the system performance owing to an electric discharge via the particles. In case when the particles exist in a dc high-voltage electrode system, the location of the particle would inevitably affect the discharge aspect. A corona-onset and breakdown voltage from a metallic wire particle were measured in detail. The wire particle has a length ranging from 6 to 12 mm, and a diameter of 0.25 mm. It was fixed between a parallel plate electrode with a spacing of 20 or 30 mm. When the particle was close to the negative electrode, a corona discharge occurred stably, and the corona-onset voltage was increased as the gap length between the negative electrode, and the positive end of the particle increased up to 0.8 mm. In contrast, when the particle was close to the positive electrode, the breakdown always occurred without preceding the corona discharge. At the middle region between the electrodes, the corona onset occurred at almost constant voltage.     

Unipolar charging of insulating spheres in rectified AC electricfields

Nonfiltered rectifiers and pulsed power supplies are often used for the energization of the high-voltage electrodes in various electrostatic installations. The aim of this paper is to investigate the ionic charging of insulating particles in the pulsatory electric fields specific to such applications. In a first set of numerical simulations, the space charge was considered constant in time, which means that the ion generation is not related to the voltage drop between the electrodes. A second set of simulations was carried out using the assumption that the space charge is generated by the pulsed corona from one of the electrodes. The computed results, which were found in good agreement with the experimental data, show that the amount of charge acquired by a particle depends on the following factors: (1) particle transit time through the electric field zone; (2) space charge density; and (3) ratio between the corona onset voltage and the amplitude of the variable voltage applied to the electrodes. These factors are discussed in correlation with the operating parameters of roll-type corona-electrostatic separators, but the conclusions are valid for a wider group of industry applications     

High-voltage electrode position: a key factor of electrostatic separation efficiency

Industry application of electrostatic separation technologies still faces a major difficulty: good results can be obtained only by adequate control of a multitude of operating parameters. The aim of the present paper is to analyze a key factor of electrostatic separation efficiency: the position of the high-voltage electrodes. Experiments were performed with two types of granular materials: chopped electric wire wastes and foundry sands. The electrostatic separator employed for the tests was provided with a wire-type corona electrode, associated - in some experiments - with a tubular-type electrostatic electrode, at various angular and radial positions, with respect to a rotating roll electrode connected to the ground. The experimental data are discussed in relation to the results of the numerical analysis of the electric field, carried out with a charge simulation program. They show that the outcome of the separation process (i.e., the weight percentage of the conductor and nonconductor fractions, as well as the purity of the recovered materials) depends on the configuration of the electrode system. The position of the electrodes affects both the particles charging conditions on the surface of the roll electrode, and the trajectories of the charged particles in the high-intensity electric field of the separator. Some recommendations could be formulated for the industrial application of the electrostatic separation technology.     

Charge-Decay Characteristics of Granular Materials Forming Monolayers at the Surface of Grounded Electrodes

Laboratory studies and in-field observations have shown that the charge-decay characteristics of the granular materials at the surface of the grounded roll electrode significantly influence the outcome of the electrostatic separation process. This paper validates an indirect method of charge-decay characterization, based on the measurement of the electrical potential at the surface of a monolayer of granular insulating material. The study was performed on three materials–-polyvinyl chloride, polyethylene, and rubber–-extracted from chopped electric wire wastes. The granules (characteristic size in the range 1–4 mm) were disposed on the surface of a grounded plate electrode (layer area: 100 mm $ times$ 100 mm; electrode area: 200 mm $ times$ 200 mm). A wire-type corona electrode, energized from a dc high-voltage supply, was employed for charging the granules. The potential due to the charge at the surface of the granular layer was measured with the capacitive probe of an electrostatic voltmeter connected to a personal computer. Data acquisition and processing were done using the LabView environment. The influence of particles characteristics and of ambient factors was studied. The findings enabled a more accurate modeling of discharging phenomena that affect the performances of electrostatic separators. The method can be easily adopted in electrostatic discharge studies for material characterization.      

Robust design of electrostatic separation processes

The aim of this paper is to analyze the robustness of the electrostatic separation process control. The objective was to reduce variation in the process outcome by finding operating conditions (high-voltage level, roll speed), under which uncontrollable variation in the noise factors (granule size, composition of the material to be separated) has minimal impact on the quantity (and the quality) of the recovered products. The experiments were carried out on a laboratory roll-type electrostatic separator, provided with a corona electrode and a tubular electrode, both connected to a dc high-voltage supply. The samples of processed material were prepared from genuine chopped electric wire wastes (granule size >1 mm and <5 mm) containing various proportions of copper and PVC. The design and noise factors were combined into one single experimental design, based on Taguchi's approach, and a regression model of the process was fitted. The impact of the noise factors could be estimated, as well as the interactions between the design and noise factors. The conditions of industry application of Taguchi's methodology are discussed, as well as the possibility of adapting it to other electrostatic processes.     

Stimulation artifact in surface EMG signal: effect of the stimulation waveform, detection system, and current amplitude using hybrid stimulation technique

The purpose of this study was to investigate the amplitude properties of the artifact generated on the recorded surface electromyography (EMG) signals during transcutaneous electrical muscle stimulation. The factors which were investigated are the shape of the stimulation waveform, the distance of the stimulating electrode from the recording system, the interelectrode distance of the detection system, the spatial filter used for signal detection, and the stimulation current amplitude. Surface EMG signals were recorded during electrical stimulation of the biceps brachii motor point with a linear adhesive array of eight electrodes. Electrical stimulation was applied with seven stimulation waveforms (mono- and biphasic triangular, sinusoidal, and rectangular), generated by a specifically designed neuromuscular stimulator with hybrid output stage. The stimulation peak current was linearly increased from 0 mA to the maximum tolerated by the subject. The detection systems investigated were single and double differential with interelectrode distances multiple of 5 mm. Two trials for each contraction were performed on three different days. The average rectified artifact values (both absolute and normalized with respect to the corresponding M-wave values) were computed to investigate the artifact amplitude properties. Results indicated that, while the artifact average rectified value, normalized with respect to the M-wave amplitude, depended on the distance of the detecting electrodes from the stimulation point, it did not depend on the stimulation waveform, on the current intensity, on the interelectrode distance, and on the spatial filter. It was concluded that, using hybrid stimulation techniques, the selection of particular stimulation waveforms, interelectrode distances, or spatial filters has a minor effect on the reduction of the artifact when recording M-waves.     

Charging and discharging of insulating particles on the surface of a grounded electrode

The aim of the present paper is to analyze the corona charging of millimeter-size insulating disks, as well as their discharging when they are no longer exposed to the action of an external electric field. The experiments were carried out on a roll-type electrostatic laboratory separator, equipped with a wire-type corona electrode, simulating the actual charging/discharging conditions in an industrial unit. Disks of various sizes were charged on the surface of the roll electrode, then the high voltage supplied to the corona electrode was turned off and the particles were collected in a Faraday pail, connected to an electrometer. The charge measurements were performed at various time intervals from high-voltage turn-off. In this way, the charge decay could be recorded and the discharge process fully characterized. The measured data show that the discharge process depends on the nature, size, and shape of the particles, as well as on the contact conditions between the particles and the grounded roll electrode. These data could guide the design of the electrostatic separation experiments that precede any new industrial application of this technology.     

Premises for the mathematical modeling of the combinedcorona-electrostatic field of roll-type separators

Corona and induction charging mechanisms are frequently associated in modern roll-type electrostatic separators. Various electrode configurations have been proposed and numerous attempts have been made in order to fully characterize them. This paper approaches this problem from a computational point of view. The boundary-element method is employed for analyzing the electrostatic field distribution generated by a typical arrangement, consisting of a wire-type corona electrode and an ellipse-profile nonionizing electrode. The computed results pointed out the effect of various parameters on the uniformity of the electric field near the ionizing element and at the surface of the grounded rotating roll electrode. These data can be used by the designer in order to improve the electrode configuration, produce a uniform field in the active zone of the separator, and reduce the corona inception voltage. At the same time, they validate two important premises for the mathematical modeling of the combined corona-electrostatic held of a roll-type separator: (1) Peek's law can be used for evaluating the electric field strength at the surface of the wire electrode at corona onset in any of the usual electrode configurations; and (2) the distribution of the electric field in the active zone of the separator is affected only by the geometry of the high-voltage electrode system     

Modeling of Corona Characteristics in a Wire-Duct Precipitator Using the Charge Simulation Technique

The charge simulation technique has been adapted to model the electrostatic and the corona characteristics in clean air of a duct-type electrostatic precipitator. The study involves the evaluation of the electric potential, electric field, and charge density in the interelectrode space as a function of corona current. The results show good agreement with published experimental data. The method developed can be applied to other geometries in the presence of space charge. The commonly used assumption that the space charge affects the magnitude but not the direction of the electric field is shown to be inadequate for large values of corona current. Also, the effect of using different values for the mobility of negative ions is presented.     

High-Voltage Monitoring in Electrostatic Separators

High voltage is known to be one of the main control variables in any electrostatic-separation process. From this perspective, the aim of this paper is twofold: to develop an effective high-voltage monitoring system and to demonstrate that it can be a useful tool for controlling the overall operating conditions of an electrostatic-separator system. A custom-designed virtual instrument was employed for processing the experimental data provided by a high-voltage probe, the output of which was connected to an electrometer. In several experiments, the output of the high-voltage probe was also connected to a digital oscilloscope, in order to obtain a better understanding of the variation of the electrode potential after a spark discharge. The laps of time without corona discharge and/or with low electric field intensities could thus be accurately determined, and the impact of the spark discharges on the outcome of the separation process evaluated. The dispersion of high-voltage measured values was found to increase in the presence of the material. The statistical analysis of the data revealed a significant correlation between the standard deviation of the high-voltage and the concentration of metal in the processed material. The results of this paper could be helpful for those seeking the optimization of the operating conditions for the electrostatic separation applications, in which the metal content in the feed materials exhibits substantial fluctuation with time     

多针电极结构双极电晕放电伏安特性

为提高电晕放电的能量密度和放电稳定性,提出了针阵列电极结构的双极电晕放电方式并研究了多针电极结构双极电晕放电的伏安特性。实验得出放电电流I随针尖半径a和电极间距d的增大而减小,随相邻针尖间距s的增大而增大,但当s≥40 mm时,相邻针尖的相互作用已很小,I几乎不变;d对火花击穿电压的影响较大,a对其的影响较小。由于电极结构的对称性,高压电极的极性对放电无明显的影响,正负电晕放电的伏安曲线和火花击穿电压均较为接近。将多针电极双极电晕放电电流I等效成电极间距为d/2的多针对板正、负电晕放电电流I1和I2相加,分析了I>I1+I2的原因,并推知其电离区内电子密度也有相应规律。