Charged spray generation for gas cleaning applications

The paper presents results of experimental investigation of properties of charged sprays generated by two types of pressure atomizers with charging by induction. Among other possible methods of charged spray generation, the induction charging has been considered due to its most practical importance. The goal of this research is to optimise the charging process with respect to obtain droplets of required size and charge for their application for exhaust gas cleaning from submicron particles in electrostatic scrubber used for the removal of PM from Diesel engine exhausts. Electrostatic scrubbers use electrostatic forces in order to deposit fine charged particles onto oppositely charged droplets.     

Bipolar charging of dust particles under ultraviolet radiation

The photoemission charging of dust particles under ultraviolet radiation from a xenon lamp has been investigated. The velocities of yttrium dust particles with a work function of 3.3 eV and their charges have been determined experimentally; the latter are about 400–500 and about 100 elementary charges per micron of radius for the positively and negatively charged fractions, respectively. The dust particle charging and the dust cloud evolution in a photoemission cell after exposure to an ultraviolet radiation source under the applied voltage have been simulated numerically. The photoemission charging of dust particles has been calculated on the basis of nonlocal and local charging models. Only unipolar particle charging is shown to take place in a system of polydisperse dust particles with the same photoemission efficiency. It has been established that bipolar charging is possible in the case of monodisperse particles with different quantum efficiencies. Polydispersity in this case facilitates the appearance of oppositely charged particles in a photoemission plasma.     

Nanoparticle Charging in a Twin Hewitt Charger

A new unipolar charger for aerosol nanoparticles has been developed. In this twin Hewitt charger two corona discharge zones are connected by a charging zone where the nanoparticle aerosol flows. Ions move into the charging zone alternating from each corona discharging zone by means of a square-wave voltage. The operation parameters of the device have been experimentally investigated at standard conditions with the goal to optimize the extrinsic charging efficiency in N₂ carrier gas. It has been found that there exists an optimal length of the charging channel for each gas flow rate through the charger which minimizes losses of charged particles and at the same time having a sufficient large n _iont-product. Extrinsic charging efficiencies of some 30% for particles with a diameter of 10 nm are obtained.     

Study on both discharge and charging of the earthed atomizing corona discharge and the influence of magnetic fields on them

In the paper, the influences of water flux on both discharge current and onset voltage were studied. Both charging and capturing particles of atomizing corona discharges were investigated when the magnetic field was used or not. The charge number of droplets and their sizes were calculated after some parameters were measured by Millikan oil drop instrument. In addition, the capturing ability of atomizing corona discharge pre-charger with magnetic field was compared with the traditional pre-charger. Eventually, the charging mechanism of atomizing corona discharge with magnetic field was analyzed through the above-mentioned experimentation and comparison. The result shows that the smallest onset voltage will appear with water flow increase in the atomizing corona discharge, and that the ion concentration between electrodes is the highest in the atomizing corona discharge charger with magnetic field than any other pre-charger, which is conducive for charging dust particles. Hence the new pre-charging technique is promising for capturing fine aerosol particles in electrostatic precipitators.     

Neutron yield when fast deuterium ions collide with strongly charged tritium-saturated dust particles

The ultrahigh charging of dust particles in a plasma under exposure to an electron beam with an energy up to 25 keV and the formation of a flux of fast ions coming from the plasma and accelerating in the strong field of negatively charged particles are considered. Particles containing tritium or deuterium atoms are considered as targets. The calculated rates of thermonuclear fusion reactions in strongly charged particles under exposure to accelerated plasma ions are presented. The neutron generation rate in reactions with accelerated deuterium and tritium ions has been calculated for these targets. The neutron yield has been calculated when varying the plasma-forming gas pressure, the plasma density, the target diameter, and the beam electron current density. Deuterium and tritium-containing particles are shown to be the most promising plasmaforming gas–target material pair for the creation of a compact gas-discharge neutron source based on the ultrahigh charging of dust particles by beam electrons with an energy up to 25 keV.     

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.     

Experimental investigation on charging characteristics and penetration efficiency of PM2.5 emitted from coal combustion enhanced by positive corona pulsed ESP

The electrostatic precipitator (ESP) has been extensively used for collecting aerosol particles emitted from coal combustion, but its collection efficiency of PM2.5 (Particulate matter whose aerodynamic diameter is less than 2.5 μm) is relatively low due to insufficient particle charging. The positive pulsed ESP is considered to enhance particle charging and improve collection efficiency. A laboratory-scale pulsed ESP with wire-plate electrode configuration was established to investigate the particle charging and penetration efficiency under controlled operating conditions of different applied impulse peak voltages, impulse frequencies, dust loadings and residence times. The results show that most particles larger than 0.2 μm are negatively charged, while most particles smaller than 0.2 μm are positively charged. For a given operating condition, the particle penetration efficiency curve has the highest penetration efficiency for particles with a diameter near 0.2 μm, and there is always a negative correlation between the particle penetration efficiency and the average number of charges per particle. Under the same operating conditions, the particle penetration efficiency decreases with increasing impulse peak voltage and impulse frequency, but increases as the dust loading increases. The results imply that residence time of 4 s is optimum for particle charging and collection. PM2.5 number reduction exceeding 90% was achieved in our pulsed ESP.     

The correlation between corona current distribution and collection of fine particles in a laboratory-scale electrostatic precipitator

An analysis of the electrostatic gas cleaning fundamental phenomenon shows an essential influence of discharge electrode construction on the gas cleaning process efficiency.In the physical model tests there were used rigid discharge electrodes with corona emitting elements of various geometries. Different constructions of discharge electrode were tested in the aspect of discharge current uniform distribution on collecting electrode surfaces. Measurements of discharge current distribution has been carried out for discharge electrodes with different spike shapes and in different electric field geometry. The research aim was to determine the optimal discharge electrode construction ensuring high collection efficiency of fine particles. Collection efficiency measurements of selected fly ash samples (from coal fired boilers) were carried out on a laboratory testing bench in a horizontal electrostatic precipitator model.     

Three-dimensional numerical studies on the effect of the particle charge to mass ratio distribution in the electrostatic coating process

Charge to mass ratio is a crucial parameter that governs the behavior of particle trajectories in a charged cloud of particles. The complex nature of the charging process limits our ability to accurately determine the charging level when particles of varying size are present. Using a numerical approach, it is possible, however, to take into account predefined values for this parameter. In this paper, the average charge to mass ratio and the distribution of the charge to mass ratio in the coating of a flat target were systematically varied to demonstrate their effect on the motion of the charged particles. The results show that the transfer efficiency increases as the average charge to mass ratio increases. It was found that the transfer efficiency is a weak function of the average particle size in the range tested and that it increases as the width of the size distribution increases.     

In situ oxygen plasma cleaning of a PECVD source for hard disk overcoats

Reducing the thickness of the diamond-like carbon protective overcoat to a thickness of about 2–3 nm is one major key to increase the recording density of magnetic disk drives. Plasma-enhanced chemical vapor deposition (PECVD) deposited carbon layers have been shown to be denser and harder than those produced by conventional sputter deposition. One key problem of PECVD deposited carbon is the contamination of the carbon film by particles produced inside the carbon source after long-time operation. This particle production limits the runtime of the source drastically. To avoid this particle generation the source was cleaned by an intermittent in situ oxygen plasma process. The cleaning efficiency was investigated by recording the pressure change inside the source during the cleaning process caused by the CO production. The ratio of the cleaning time and the deposition time shows no significant dependence on the deposition time. An almost linear increase of the ratio with the acetylene flow was observed. This results from a higher deposition rate at higher acetylene flow, leading to a higher contamination inside the source. A strong dependence of the cleaning rate on the oxygen flow in the cleaning process was measured. More oxygen leads to a strong decrease of the needed cleaning time. Adding Ar gas to the oxygen discharge shows no improvement of the needed cleaning time. The cleaning process seems to be dependent only on the amount of reactive oxygen species in the discharge . PACS 62.20.Qp; 52.50.Dg; 52.77.Bm; 52.80.Pi     

Comparison of influence of fluidization time on electrostatic charge build-up in the bubbling vs. slugging flow regimes in gas–solid fluidized beds

Electrostatic charge generation poses significant problems in some commercial gas–solid fluidized bed reactors such as those in gas-phase polyolefin production. Understanding the contributing factors to charge generation is important in determining the charge generation mechanisms, leading to the development of methods to reduce or prevent this phenomenon. This work focused on determining the effect of fluidization time on particle charging and the amount of particle adhesion on the fluidization column wall in both the bubbling and slugging flow regimes. The charging effect was investigated for particles in three regions of the fluidized bed: elutriated fines, bulk particles inside the bed, and particles adhered to the column wall. The particles size distribution, mass and charge were measured for all three regions. Fluidization was carried out with polyethylene resins from an industrial reactor; times of 15, 30, 60, 120, 180, and 360 min were evaluated. Increased fluidization time decreased the amount of particles mass collected in the bulk region and increased those adhered to the column wall during the velocities tested in the bubbling flow regime. Whereas the quantity of particles in each region was not affected by fluidization time for the velocities examined in the slugging flow regime. Bipolar charging was observed with relatively smaller particles becoming predominately positively charged and larger particles becoming predominately negatively charged. Each region of the bed affected the magnitude of net q/m, with elutriated fines having the largest magnitude, followed by those adhered to the column wall, and finally those in the bulk of the bed. Charge saturation was attained for fluidization times greater than 60 min for particles in the bulk and along the column wall for all gas velocities. However, extended fluidization times were required with the entrained fines in bubbling flow; whereas charge saturation of fines in slugging flow occurred shortly after the onset of fluidization. Mean particle diameter for each measurement region was not impacted by the fluidization time for any of the gas velocities tested. The bed hydrodynamics was found to definitely have an impact on the particle–wall fouling where the particle layer continued to develop on the inner column wall as fluidization time increased for those velocities in the bubbling regime while comparatively less impact on particle layer growth was observed in the slugging flow regime. In addition, the bubbling flow regime resulted in particle layers formed on the column wall to be longer and thinner whereas those formed in the slugging flow regime were shorter and thicker.     

Measuring surface area of airborne titanium dioxide powder agglomerates: relationships between gas adsorption,diffusion and mobility-based methods

Inhalation toxicology studies generally use the Brunauer, Emmett, and Teller (BET) gas adsorption method to measure total surface area of particles whereas occupational exposures are more readily measured by real-time mobility-based surface areas or active surface area measured with diffusion charger-based instruments. Three surface area measurement methods were studied: filter-based inert gas adsorption (BET method), diffusion charging, and mobility-based methods. The goal of the project was to investigate and develop a correlation between the measurement methods. The experimental design consisted of measuring surface area in a series of five trials for each of two powder types, fine and ultrafine titanium dioxide with primary particle sizes of 440 and 20 nm, respectively, and two aerosol concentrations. Diffusion charger instruments tended to underestimate the total particle surface area measured by the BET, but were well correlated with mobility-based surface areas obtained from a scanning mobility particle sizer. Filter-based gas adsorption methods and diffusion charging methods provide different but valuable information on total and active surface areas of particles, respectively. Results indicate they should not be used as predictors of one another.     

Corona discharge ion sources for fine particle charging

Charging of aerosol droplets and solid particles is applied in many industrial processes such as electrostatic painting, particle separation and electrostatic precipitation. In most of charging devices, electrical discharges are used as a source of ions, which are deposited onto the particles. In the present paper, the charging process by ionic current in alternating electric field was optimized experimentally. Alternating electric field charger was used as a charging device in these experiments. The current voltage characteristics of electrical discharge in this device, and the charge imparted to the particles were determined. The level of charge was measured at the outlet of the charger and was compared to the Pauthenier limit for different supply voltages, and frequencies. MgO powder was used as a source of particles in these experiments. It was noticed that higher supply voltage of the charger gives higher level of particle charge, but at the same time, the particle deposition on the charger elements was increased, decreasing the particle penetration. A compromise between these two tendencies is therefore necessary. As a result we have proposed a criterion maximizing the total charge born by the particles which is a product of relative particle charge and particle penetration.     

Experimental study on electrostatic removal of high-carbon particle in high temperature coal pyrolysis gas

A high-temperature electrostatic precipitator (ESP) presents a good solution for hot gas cleaning, which can remove fly ash from pyrolysis gas at temperatures higher than the tar dew point. In this paper, the characteristics of negative DC corona discharge in air and simulated coal pyrolysis gas were studied. The removal of coal pyrolysis furnace fly ash (ash A) was investigated and compared with that of coal-fired power plant fly ash (ash B) in ESP with a temperature ranging from 300?K to 900?K. The current density of simulated gas was higher than that of air under the same discharge voltage and at different temperatures. The simulated gas also had a higher spark voltage and a lower onset voltage compared with air. The fractional collection efficiency of ash A was lower for particles with diameters of larger than 0.1?µm at high temperature, compared with ash B. A lower collection efficiency in simulated gas was obtained for particles with diameters of less than 0.1?µm compared with air. The collection efficiency of submicron particles in simulated gas was usually higher than it in air, especially for particles with diameters of less than 0.04?µm. In simulated gas, the overall collection efficiency of ash A was obviously lower than that of ash B, especially at high temperature. From 300?K to 700?K, the collection efficiencies of both ash samples were as high as above 93%, but the collection efficiency of ash A in simulated gas decreased to 78.7% at 900?K.     

Enhanced fine particle collection by the application of SMPS energisation

Over the past decade or so the health problems associated with the inhalation of sub micron particles from industrial processes has taken prominence and has lead to the stricter emission legislation, such as the US PM 2.5 approach. Generally most forms of control equipment readily handle and collect particles greater than 1 micron diameter, however, those less than 1 micron diameter are very much more difficult to collect.In the case of electrostatic precipitation, which involves both particle charging and migration under the influence of an electric field, the larger particles, generally greater than 1 micron are charged by collision with the ions and electrons present in the inter electrode area. It will be shown that the charge on these particles is proportional to the radius squared and its migration velocity proportional to the voltage squared, both reducing with particle size. The very small particles however, are charged by a diffusion processes and migrate under the influence of Brownian motion, which increases as the particle size decreases. The result of this is that a typical particle size/efficiency curve indicates a significant penetration window in the 0.8–0.2 micron diameter range, which coincides with the change from collision to diffusion charging of the particles.Because of this penetration window, should an existing precipitator operating under optimum electrical conditions, not comply with fine particle emission requirements, the conventional performance enhancement scenario, since the charging and precipitation operating conditions have been already optimised, would be to increase the precipitator's plate area, a very expensive solution. It will be shown, however, that the replacement of the conventional mains energisation system by an SMPS approach in an existing ESP will enhance the collection efficiency of particles, particularly in the penetration window, as a result of the increase in both operating field voltages and currents.The SMPS approach was applied to a 2 field ESP dealing predominately with sub micron fume, which the Client wished to enhance the performance to enable higher recycle rates, while still complying with his emission permitting. This was initially assessed using PALCPE? (Proactive Approach to Low-Cost Precipitator Enhancement), which indicated a significant reduction in the fine particle emissions was achievable by operation under SMPS Operation. An SMPS unit was subsequently fitted to the outlet field of this precipitator and the operating data will be examined in detail. With the outlet field under a mains rectification energisation system the overall emission was ～25 mg/Nm³, which after installation of the SMPS unit reduced to less than 15 mg/Nm³.     

Millimeter-wave scattering from neutral and charged water droplets

We investigated 94 GHz millimeter-wave (MMW) scattering from neutral and charged water mist produced in the laboratory with an ultrasonic atomizer. Diffusion charging of the mist was accomplished with a negative ion generator (NIG). We observed increased forward- and backscattering of MMW from charged mist, as compared to MMW scattering from an uncharged mist. In order to interpret the experimental results, we developed a model based on classical electrodynamics theory of scattering from a dielectric sphere with diffusion-deposited mobile surface charge. In this approach, scattering and extinction cross-sections are calculated for a charged Rayleigh particle with effective dielectric constant consisting of the volume dielectric function of the neutral sphere and surface dielectric function due to the oscillation of the surface charge in the presence of applied electric field. For small droplets with radius smaller than 100 nm, this model predicts increased MMW scattering from charged mist, which is qualitatively consistent with the experimental observations. The objective of this work is to develop indirect remote sensing of radioactive gases via their charging action on atmospheric humid air.     

The Simulation of the Electrostatic Spray Painting Process with High‐Speed Rotary Bell Atomizers. Part I: Direct Charging

High‐speed rotary bell atomizers are widely used in the painting industry for high quality applications. They provide a highly uniform film thickness with reasonable transfer efficiency due to the additional electrostatic field supporting the droplet transport towards the target. A basic requirement for this type of paint atomizer is a fine and reproducible atomization of a large variety of different paints, ranging from solvent‐based materials to highly non‐Newtonian water‐borne systems. Furthermore, a broad range of paint flow rates must be covered. The present contribution summarizes investigations aiming to completely model the electrostatically supported spray painting process by means of CFD. In part I, so‐called direct charging atomizers, where high voltage is applied directly to the rotating bell, are considered. Here, charging of the droplets takes place at the bell edge and corona effects can be neglected. A powerful commercial code, in the present case Fluent in its current releases, has been extended to account for the electrostatic field and the space charge effect due to the charged paint droplets. As input conditions, the air flow from the shaping air orifices and measured droplet sizes and velocities close to the bell edge using phase‐Doppler anemometry and Fraunhofer diffraction were taken. Also, LDA measurements in front of the target were performed, yielding comparative data of the airflow field. In general, numerical and experimental results are in good agreement. This is especially true for the final film thickness on the target and the transfer efficiency, i.e. the amount of paint solids finally deposited on the target. The agreement was achieved using a droplet charge of 5% of the droplet size dependent Rayleigh limit. These results serve as a basis for a complete painting process simulation for complex work pieces, e.g. whole car bodies, in the future. This task, however, can only be successfully completed performing unsteady calculations with moving atomizers along given robot paths.     

Investigations of Droplet‐Plasma Interaction using Multi‐Dimensional Coupled Model

Recently developed multi‐dimensional coupled fluid‐droplet model is used to investigate the behavior of complex interaction between the liquid precursor droplets and atmospheric pressure plasma (APP). The significance of this droplet‐plasma interaction is not well understood under diverse realm of working conditions in two‐phase flow. In this study, we explain the implication of vaporization of liquid droplets in APP which are subsequently responsible to control major characteristics of surface coating depositions. Coalescence of water droplets is more dominant than Hexamethyldisiloxane (HMDSO) droplets because of its sluggish rate of evaporation. A disparity in the performance of evaporation is identified in two independent mediums, such as gas mixture and discharge plasma using HMDSO precursor. The length of evaporation of droplets is amplified by an increment of gas flow rate indicating with a reduction in the gas temperature and electron mean energy. In particular, the spatio‐temporal density distributions of charged particles show a clear pattern in which the typical nitrogen impurity ions are primarily effective as compared to other helium ionic species along the pulse of droplets in APP. Finally, we contrast the behavior of discharge species in the pure helium and He‐N2 gas mixtures revealing the importance of stepwise and Penning ionization processes. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of conductivity on corona discharge current from a water droplet and on ejection of nano-sized droplets

The influence of the conductivity of a water droplet formed at a capillary electrode on the negative corona discharge and production of nano-sized droplets was investigated. Conductivity of a water droplet was adjusted from 1 μS/cm of deionized water to 48 mS/cm of nitric acid water solution. The size distribution of nanometer sized water particles yielded at a disruption of a Taylor cone was measured. The higher conductivity of a droplet, the larger corona pulses appeared and the more number of charged droplets was generated.     

Studies of laser-induced removal mechanisms for tokamak-like particles

The removal of metallic and carbon particles is a great issue in the framework of the ITER (International Thermonuclear Experimental Reactor) project. Indeed, the presence of these particles in the vessel of a tokamak leads to safety risks.The laser process seems to be a very promising solution for this cleaning. However, the process optimization requires a good knowledge of the removal mechanisms. For this purpose, we investigated the influence of beam parameters, such as laser pulse duration and wavelength, on the cleaning efficiency. In this paper, two kinds of particles are chosen to be studied, carbon aggregates and tungsten droplets, because they are typical of dust collected in tokamak.The results show an influence of beam parameters on the tungsten particles removal efficiency (PRE), whereas this influence is not significant for carbon particles.To help the understanding of the removal mechanisms, substrates and particles were inspected by scanning electron microscopy, before and after the irradiation.We will see on this paper that even if carbon and tungsten particles strongly absorb the different laser wavelength used, the removal mechanisms of these particles are very different.