Neutralization of charged insulating granular materials using AC corona discharge

The control of the residual electric charge carried by granular materials processed in various electrostatic installations is a prerequisite for the safe and efficient operation of the respective industrial equipment. The aim of the present work was to evaluate the neutralization efficiency of polyethylene granules exposed to an AC corona discharge from a wire-type electrode. The control variables and their domains of variations were the following: the amplitude and the frequency of the applied high voltage (16–18 kV, 20–400 Hz), the position of the corona electrode (3–7 cm above the ground plate that carries the sample) and the duration of the exposure to the corona discharge (4–10 s). The surface potential decay method was used for monitoring the charge carried by granular samples of PE before and after neutralization. Design of experiment methodology was employed to evaluate the influence of each of the above mentioned control variables and determine the optimum operation conditions. The efficiency of the neutralization was characterized by the ratio between the values of the surface potential before and after AC corona discharge exposure. The obtained results show that neutralization efficiency may be improved by increasing the frequency of the high voltage as well as by adequately correlating its amplitude with the inter-electrode spacing.     

Experimental observation of negative differential characteristic of corona discharge in ultraviolet spectrum

Corona discharge is a self-sustained discharge which appears at electrodes with a small radius curvature in gas insulation. An almost invisible glow occurs just above the inception voltage. Corona phenomenon is mainly used in electro-technological processes to obtain space charge for electrostatic precipitation, separation of different particles, electrostatic liquid or solid coating, neutralization of space charge, etc. All of these processes rely on a strong nonhomogeneous electric field generated by a point – plate electrode system. When the critical value of the applied voltage is reached, the ionization processes near the point electrode start and give rise to the current between two electrodes. If the pointed electrode is positive, it is possible to observe an anomaly of the current – voltage (I-U) characteristic for the point-plate space. It means that while the voltage is raising the current density decreases in a narrow voltage area (2–3 kV). The anomaly was technically named as negative differential conductivity (dI/dU < 0). Unstable current can have a negative influence on electro-technological processes. The anomaly was detected for different shapes and materials of the electrode as well as for various temperatures and distances between electrodes. An oxidation layer, which appears on the metal electrode, also influences the ionization processes near the pointed electrode and causes a decrease of a current. In this paper measuring of the discharge activity in a point – plate electrode system is presented. Ionization of gas atoms and molecules in a high electric field and the following recombination of electrons and positive ions in the corona region can give rise to high-energy photons which produce new electrons in the field of discharge. Corona discharges are detected by DayCor Corona camera which can register UV emission generated by corona in a day light. The experiment was conducted with various shapes of the pointed electrode and distances between the high voltage and the grounded electrode under applied direct voltage with positive and negative polarity.     

Modulation effect produced by adjacent AC voltage on the characteristics of negative corona current pulses in a wire-cylinder electrode configuration

In this paper, a wire-cylinder electrode system was designed to study the negative DC corona pulses due to the influence of AC voltage applied to an adjacent conductor. The corona onset voltage and distribution pattern and the parameters of the corona current pulses such as the rise time, pulse duration and the average pulse amplitude are systematically analyzed. The AC voltage is found to have an interesting modulation effect on the time interval of the current pulses. At last, based on a simplified ion cloud model, a full explanation is given to reveal the mechanism behind the influence of the AC voltage on negative DC corona.     

Experimental study of a modified dual-type high-voltage electrode for electrostatic separation applications

Several electrode arrangements have been proposed to enhance the efficiency of insulating materials charging by corona discharge. Recent studies pointed out that the presence of metal strips in the proximity of a dual-type high voltage electrode increases the total current measured at the surface of the collecting electrode, decreases the corona onset voltage value and enlarges the reparation of current density as well. The aim of this paper is to evaluate the benefits of using such an electrode arrangement for corona charging of non-conductive particulate materials in belt-type corona-electrostatic separators. The experimental study was carried out with samples of Aluminum and Polystyrene particles in the size class 125–250 μm. The presence of grounded strips reduces the electric wind, which is associated to corona discharge but not tolerated in such processes that involve micronized materials. At the same time, it improves the corona charging conditions of non-conductive materials and as consequence the overall efficiency of the corona-electrostatic separation process. The use of the new electrode configuration is characterized by both high recovery rates and better purities of the separated products.     

Sliding discharge in air at atmospheric pressure: electrical properties

Several studies have shown that a surface non-thermal plasma may be used as an electrofluidodynamic actuator for airflow control. For few years, we has been working on this subject, especially in the case of DC corona discharges and AC barrier discharges established at the wall of profiles. The present paper deals with a new type of surface plasma using a sliding discharge. This discharge, excited here by a negative AC voltage with a positive DC component, is created in a three-electrode geometry: one DC positive electrode and two negative AC electrodes at the same voltage. Then a barrier discharge is established between the positive electrode and the first negative one when a surface corona discharge or sliding discharge is generated between the positive electrode and the second negative one. In this preliminary study, the goal is to obtain a stable sliding discharge. Then the electrical properties of this discharge are observed and briefly discussed.     

Neutralization of charged dielectric materials using a dielectric barrier discharge

Safety and efficiency are two characteristics that must be satisfied by an electric charge neutralizer. The dielectric barrier discharge (DBD), which has the advantage of preventing arc transition, is an interesting tool to safely neutralize unwanted charge. This paper is aimed at studying the efficiency of neutralizing charged polyethylene (PE) granules by using a dielectric barrier discharge. During this study, several factors were considered such as the amplitude and the frequency of the AC voltage, the polarity and the charging mode of the samples, as well as the electrode configuration. Two DBD electrode configurations were considered: simple DBD and a DBD with installed metallic grid. The obtained results show that using the DBD can lead to excellent neutralization results when the grid is installed. With the appropriate voltage amplitude and frequency and with grid installed, the elimination of nearly 99% of the initial surface charges can be achieved. The metallic grid placed between the DBD electrode and the target enhances significantly the neutralization efficiency.     

Effect of ground electrode on charge injection and surface potential of corona charged polyethylene film

The surface potential decay measurement is a simple and low cost tool to assess electrical properties of insulation materials; therefore, understanding the physical mechanisms of the surface potential decay becomes necessary. With our recent space charge measurement results on corona charged samples, bipolar charge injection on corona charged samples had been observed. Based on this new fact, it is anticipated that the ground electrode should have significant effect during corona charging and subsequently decay processes. In the paper, low density polyethylene (LDPE) film with gold ground electrode was compared with LDPE film with aluminium ground to study effect of ground electrode on charge injection and surface potential decay processes. Charging current during the corona charging, surface potential decay and space charge dynamics after corona charging in the samples with either gold coated or aluminium ground electrode were measured. Differences have been observed for gold ground electrode when compared with aluminium ground electrode. Higher work function of gold electrode is responsible for the observed differences. A preliminary simulation has also attempted to show that the bipolar injection may take place in corona charged LDPE films.     

Two-stage electrostatic precipitator with co- and counter-flow particle prechargers

Lab-scale, two-stage electrostatic precipitation system comprising of precharging stage, in which PM2.5 particles are electrically charged, and collection stage, in which the charged particles are removed from the flowing gas by electric field, was investigated in this paper. Two types of electrostatic particle prechargers were compared with respect to the collection efficiency of the system: (1) co-flow precharger, in which ionic current was generated co-currently with the gas conveying the particles, and (2) counter-flow precharger, in which ionic current was generated oppositely to the flowing gas. In each case, the electrodes of precharger were supplied with DC or AC high-voltage in order to compare the effect of discharge mode on the collection efficiency of two-stage electrostatic precipitator. The collection stage was formed by two parallel-plate electrodes connected to DC high voltage source. Plate electrodes without discharge points (spikes) are corona-free electrodes, which prevent the collection stage from electrical discharges, and reduce the probability of back discharge ignition. The back discharge decreases collection efficiency of conventional electrostatic precipitators.It was concluded that the co-flow electrode configuration of the precharger, supplied with DC high voltage, has the highest total number collection efficiency for PM2.5 particles, higher than 95% and the mass collection efficiency larger than 99%. The counter-flow precharger provided only about 90% number collection efficiency of two-stage electrostatic precipitator. It was also shown that by AC electrode excitation, the collection efficiency of the system is lower than for DC supply. The two-stage electrostatic precipitators allowed obtaining higher fractional collection efficiency for PM2.5 particles than other conventional systems and can be recommended as highly effective devices for gas cleaning in power plants or cement industry.     

2-D electrohydrodynamic simulations towards zero offset voltage with corona ionisers

This paper reports results from a computational investigation of charge neutralisation with DC, AC, and pulse-DC corona ionisers using 2-D axisymmetric electrohydrodynamic modelling with an emphasis on minimising the offset voltage. The results showed that balanced DC ionisers can effectively neutralise the charge of a conductive plate; however, balanced AC and pulse-DC ionisers give rise to a certain offset voltage with an oscillation caused by alternate mono-polar space charges created near the ionisers resulting from ions emitted from the ionisers. In addition, we suggest a modification for AC and pulse-DC ionisers to minimise the offset voltage, including its AC component. Consequently, we found that continuously balanced ions emitted from ionisers which can create a quasi-neutralised charge distribution by positive and negative ions in the entire space at a steady state are of significant importance to make the offset voltage zero.     

Numerical simulation of a tribo-aero-electrostatic separation of a ternary plastic granular mixture

Granular materials, when fluidized by air or other gaseous medium, acquire electrostatic charge by particle-particle and particle-wall collisions. The effectiveness of particle tribocharging achieved with such fluidization process is crucial for establishing the feasibility of electrostatic separation of mixed granular solid wastes in the recycling industry. The aim of the present work is to introduce a simple mathematical model for simulating the outcome of a novel tribo-aero-electrostatic separation process involving mixture of three granular materials. The process is characterized by the fact that the charging of the granules is produced in a fluidized bed device, in the presence of an electric field. The mathematical model in this case assumed that the probability of a granule to be separated can be expressed using the normal distribution law, as a function of the number of impacts with granules belonging to the other classes of materials. The effect of the presence of a third species of particles was taken into account. Thus, it was possible to calculate the evolution over time of the mass of granules separated at the electrodes for different compositions of the granular mixture. The calculated results are in good agreement with the experiments.     

Behaviour of charged insulating particles in contact with a rotating roll electrode

The overall economic efficiency of standard industrial roll-type separators for granular materials can be improved by operation at higher velocities of the rotating roll electrode. The aim of this paper is to estimate how high this speed could be and still have a good separation. The answer to this question implied the calculation of the electric image force, which opposes the centrifugal force and sticks the corona-charged insulating particles to the rotating roll electrode. This force depends on the residual charge carried by the particles. By estimating the decay of this charge from surface potential measurements carried out on granular layers of insulating materials dispersed on grounded plate electrodes, it was possible to simulate the particle lift-off from the rotating roll electrode under various operating conditions. The results presented in the paper were obtained for fly-ash particles, but the numerical simulation methodology employed by the authors can be successfully applied for the optimisation of other electrostatic separation applications.     

Polarity reversal charging of polypropylene films under DC corona discharge

In this paper, a needle–plane electrode system and polypropylene (PP) films were used to study the DC corona charging of polymers. It was found that the charges injected into the PP films sometimes show different polarity to the applied DC voltage, called polarity reversal charging, which may results in invalidation charging. The pulsed electro-acoustic (PEA) space charge measurement showed that charges of different polarities have been injected into the PP films’ bulk, simultaneously. From these results, a mechanism of two charging processes of high-field injection and return-stroke injection has been introduced here. We presumed that the return-stroke injection can be boosted with the increase of corona voltages and thus reduces the net charge as well as even change the polarity of charges injected.     

A new dual-type DMA for measuring nanoparticles emitted from combustion engines

A new dual-type differential mobility analyzer (dual-type DMA) was developed in order to detect transient number concentrations of airborne nanoparticles with diameters centralized at around 10 nm (for nuclei mode particles) and 100 nm (for accumulation mode particles) in automobile exhaust gas. The apparatus divides the gas sample into two parts, and each part is sent through one of two coaxially nested sections for analysis. For the scanning mode measurement, the nanoparticles are charged by ²⁴¹Am and their size distributions are determined by varying the applied voltage over 2 min. The transient mode measurement, on the other hand, fixes the voltages for the two sections at peaks near 10 and 100 nm in order to monitor the transient behavior of the exhaust nanoparticles. The measurement principles and design of the dual-type DMA are detailed and the results for time response experiments are presented using model nanoparticles charged by a corona charger. The transient concentrations of the nuclei mode and the accumulation mode particles from a diesel engine were shown to be detected by this method, when ²⁴¹Am was used for charging the particles.     

A new dual-type DMA for measuring nanoparticles emitted from combustion engines

A new dual-type differential mobility analyzer (dual-type DMA) was developed in order to detect transient number concentrations of airborne nanoparticles with diameters centralized at around 10 nm (for nuclei mode particles) and 100 nm (for accumulation mode particles) in automobile exhaust gas. The apparatus divides the gas sample into two parts, and each part is sent through one of two coaxially nested sections for analysis. For the scanning mode measurement, the nanoparticles are charged by ²⁴¹Am and their size distributions are determined by varying the applied voltage over 2 min. The transient mode measurement, on the other hand, fixes the voltages for the two sections at peaks near 10 and 100 nm in order to monitor the transient behavior of the exhaust nanoparticles. The measurement principles and design of the dual-type DMA are detailed and the results for time response experiments are presented using model nanoparticles charged by a corona charger. The transient concentrations of the nuclei mode and the accumulation mode particles from a diesel engine were shown to be detected by this method, when ²⁴¹Am was used for charging the particles.This revised version was published online in August 2005 with a corrected issue number.     

Three-dimensional analysis of electrohydrodynamic flow in a spiked electrode-plate electrostatic precipitator

A three-dimensional numerical model has been created to evaluate the electrical and electrohydrodynamic characteristics of a single spiked wire-plate electrostatic precipitator. The hybrid Finite Element – Flux Corrected Transport numerical technique is used for solving the Poisson and current continuity equations to estimate the electric potential and ion charge density distributions in the precipitation channel. The fully three-dimensional turbulent airflow distribution is calculated using the commercial FLUENT software assuming a standard k–? turbulence model. A non-uniform corona discharge is assumed, as it is produced along the electrode in the form of a flat tape with some number of spikes. The EHD secondary flow pattern and its interaction with the main airflow in different planes along the precipitation channel are examined for different voltages applied to the corona spiked electrode. The numerical results are compared with experimental data published in the literature.     

Evaluation of a soft X-ray unipolar charger for charging nanoparticles

Three types of unipolar chargers (parallel multi-electrodes, single electrode, and single electrode with compact size) using the soft X-ray were constructed and their charging performance was evaluated by measuring positive, negative, and neutral fractions of size-resolved ultrafine particles (20–100 nm) with the Tandem Differential Mobility Analyzer (TDMA) technique. The unipolar charger with a single electrode and compact size showed the highest charge fraction with least particle loss probably due to lower electrostatic loss of ions among tested chargers. With positive voltage applied to electrode to remove negative ions, we found that the positively charged particles were 43, 52, 62, 69, and 75% for 20, 30, 50, 70, and 100-nm particles, respectively, and a few particles were negatively charged although their fraction increased with size (1, 2, 4, 5, and 6% for 20, 30, 50, 70, and 100-nm particles, respectively). The positive charge fractions were about three times higher than the values estimated theoretically from a bipolar charger. Also, based on comparison of current data with previously reported values using corona discharge unipolar charger, the soft X-ray charger showed better performance in terms of charging efficiency and penetration for particles (NaCl) currently tested in the particle size range of 20–100 nm.     

Positive discharge from a grounded electrode toward negatively charged particles cloud

In order to understand electrostatic discharges occurring between a grounded electrode and a space charge cloud, the positive discharges were experimentally caused by negatively charged particles cloud. The discharges were initiated by locating a grounded sphere electrode at the inside or outside of the charged powder particles blown by an air flow. The luminous aspect and the discharge current were observed for the grounded sphere electrode with various diameters. Positive streamer corona discharges extended from it. The luminous aspect, peak value of the discharge current and the interval of the discharge significantly depend on the diameter of the grounded sphere electrode as well as its position.     

Measurement and simulation of partial discharge in oil-paper insulation under the combined AC–DC voltage

Depending on unique operation characteristics for voltage applied on valve side winding of the converter transformers, it is extraordinarily significant to study the partial discharge (PD) behavior with oil-paper insulation under combined AC–DC voltage. Therefore, this paper investigated PD inception characteristics by pulse current methods with needle-plate electrode system under combined AC–DC voltage. Furthermore, 3D electric field distributions versus combined AC–DC voltage in different ratios were calculated by Finite Element Analysis (FEA). An experimental conclusion was drew that AC partial discharge inception voltage (PDIV) in pure oil would decrease linearly with the DC component increasing but the inception voltage with oil-paper insulation appeared to be independent of DC voltage and dependent of AC voltage. And 3D electric field distribution deduced from simulation provided a supplementary proof on the experimental results. Moreover, high speed photography was used to capture emitted light produced by discharge, estimate streamer velocity (1.8 km/s) and record streamer initiation and propagation process in oil gap. Previous studies have shown that the prebreakdown phenomena involving the generation and propagation of vapor channels through the oil could be divided into a three-stage process.     

Numerical simulation of the continuous operation of a tribo-aero-electrostatic separator for mixed granular solids

Numerical simulation has proved to be a powerful tool in the research and development of new electrostatic processes. In a previous paper, the authors have introduced a simple mathematical model for simulating the outcome of a novel tribo-aero-electrostatic separation process for binary mixtures of granular materials. The mathematical model assumed that the probability of a granule to be separated can be expressed as a function of the number of impacts with granules belonging to the other class of materials. The process is characterized by the fact that the charging of the granules is produced in a fluidized bed device, in the presence of an electric field. The aim of the present paper is to simulate the continuous operation of such a device at various feed rates. The evolution in time of the mass of granules collected at the electrodes has been computed for various compositions of the granular mixture. The effect of the walls was taken into account. The computed results were in good agreement with the experiments. They demonstrate that open-loop continuous operation of the separator is possible for a range of feed-rates that depends on the composition of the materials to be separated.     

Surface charge dynamics studied by the temporal evolution of the corona charging current

The decay of surface charges deposited on the dielectric material by the partial discharge (PD) activity has a great impact on the repetition of partial discharges. In this work, the effect of dielectric placed on the surface of ground electrode in a needle-plane configuration on the discharge activity was investigated, with the application of a periodic negative step voltage. The charge decay mechanisms on a corona charged dielectric surface were investigated based on a comparison between experiments and a FEM-based numerical model. The comparison indicates that the surface charges may decay due to different mechanisms depending on the applied stress.