Two-stage vs. two-field electrostatic precipitator

The paper presents comparison of the collection efficiency of semi-industrial, two-field electrostatic precipitator and two-stage electrostatic precipitator comprising an electrostatic agglomerator. The two-stage electrostatic precipitator was converted from two-field electrostatic precipitator, in which the first field was replaced by unipolar electrostatic agglomerator. The investigated electrostatic precipitator was of parallel-plate type, with spiked-wire discharge electrode between sigma type collection electrodes. In the unipolar agglomerator used in this system, the particles were charged by ion current and forced to oscillate by alternating electric field. The oscillatory motion of particles perpendicular to the gas flow causes the collision between particles and deposition of small particles onto the larger ones. The agglomerated particles were next collected by one-field electrostatic precipitator.The collection efficiency of two-stage electrostatic precipitator for PM2.5 and PM1 particles was 99.1% and 98.8%, compared to 99% and 98.1% of two-field electrostatic precipitator, respectively. The most important result of this research is that the replacement of the first field of electrostatic precipitator by an electrostatic agglomerator does not change the overall collection efficiency of the system but the power consumption of electrostatic agglomerator can be 10–50 times lower than by the replaced field of electrostatic precipitator.     

Comparison of different electrode arrangements for gas decomposition efficiency using experimental method

The environmental pollution is a central issue in the present industrial societies. Within that the air pollution and the removal of hazardous components of flue and exhaust gases are very much important.In this paper the target is to decrease of the NO_x emission by means of a technology similar to that is used in the electrostatic precipitators. In most of the papers dealing with this technique cylindrical precipitator is used as a discharge chemical reactor, and fast rising electric discharges are applied for energizing the reactor. In the industry the over helming majority of the electrostatic precipitators are plate type one.In the cylindrical precipitator the discharge electrode is parallel with the gas flow, and the corona discharge filament is perpendicular to both of them. In the case of plate type industrial electrostatic precipitator the discharge electrodes are positioned vertically, and the flow of the flue gas is horizontal. Consequently, the discharge filaments are mainly perpendicular to both the flow and the discharge electrode.In cylindrical precipitator the decomposition of NO_x is done in one filament very soon, but there is no chance to modify the byproducts with a new pulse, because the energization is the same for the whole length of the discharge electrode.In the present paper a cylindrical precipitator, a plate type precipitator with horizontal electrode, and a plate type precipitator with vertical electrode were tested. The total length of the discharge electrodes of all of the precipitators was the same.The results of the NO_x decomposition were experimentally determined, and the differences between the precipitators were investigated. The cylindrical and the plate type precipitators with vertical electrodes had shown basically similar decomposition rate, while the plate type one with horizontal discharge electrode had proven inferior to the others.     

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.     

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.     

Enhancement of submicron particle electrostatic precipitation using dielectric barrier discharge in wire-to-square tube configuration

The aim of the present parametric study is to enhance the performances of a wire-to-square tube electrostatic precipitator (ESP) for the collection of submicrometer particles using dielectric barrier discharge (DBD). The input parameters under study are: the high voltage waveform, the wire electrode diameter, the collection electrode dimensions (width, discretization and number of collection sides) and the tube cross-section. The electrical measurements show that the discharge mode of the ESP is rather homogeneous. The particle collection efficiency as determined from aerosol spectroscopy measurements is higher at high applied voltage and within a certain frequency range. The parametric study of the ESP points out that using thicker wire electrodes as well as collection electrodes with different number of sides does not deteriorate the ESP performance. However, the penetration decreases with larger or discretized collection electrodes and larger tube cross-sections.     

Characterization of brush type discharge electrodes and impact of enhanced corona discharge on operation of electrostatic precipitators

This project presents the results of investigation of current/voltage characteristics of brush type discharge electrodes (BTDE) in tube type electrostatic precipitators and the effect on operation. Experimental investigations were conducted with discharge electrodes of different wire diameter and different brush diameter. The effect of electrode geometry on current/voltage behavior was recorded. Corona current with brush type discharge electrodes was modeled and compared with experimental data. Brush type discharge electrodes produce an enhanced corona current compared with wire type discharge electrodes. Limited enhanced corona has improving effect on collection efficiency. An adjusted correlation was therefore deduced from experimentally obtained current/voltage data with BTDE.     

The influence of fly ash physical and chemical properties on electrostatic precipitation process

Flue gases emitted from coal fired power plants are mainly cleaned with electrostatic precipitators (ESP). Increased awareness of the effects of atmospheric pollution and tightening legislation force precipitator producers to increase their ESP efficiency, especially when collecting very fine particulates (PM2.5). The fly ash characteristic parameters are formatted during combustion process and its depend on the coal type as well as boiler parameters and combustion conditions. Due to that many series of tests were done to investigate the influence of fly ash physical and chemical properties on ESP operation.     

Electrohydrodynamic flow patterns and collection efficiency in narrow wire-cylinder type electrostatic precipitator

Recently, narrow electrostatic precipitators (ESPs) have become a subject of interest because of their possible application for the cleaning of the exhaust gases emitted by diesel engines. Diesel engines emit fine particles, which are harmful to human and animal health. There are several methods for decrease particulate emission from a diesel engines, but up to now, these methods are not enough effective or very expensive. Therefore, an electrostatic precipitation was proposed as an alternative method for control of a diesel particulate emission.In this work, results of electrohydrodynamic (EHD) secondary flow and particle collection efficiency measurements in a narrow wire-cylinder type ESP are presented. The ESP was a glass cylinder (300 mm × 29 mm) equipped with a wire discharge electrode and two collecting cylinder-electrodes. A 0.23 mm in diameter and 100 mm long stainless-steel discharge wire electrode was mounted in the center of the cylinder, parallel to the main flow direction. The collecting electrodes were made of stainless steel cylinders, each with a length of 100 mm and inner diameter of 25.5 mm. An air flow seeded with a cigarette smoke was blown along the ESP duct with an average velocity of 0.9 m/s.The EHD secondary flow was measured using 2-dimensional particle image velocimetry (PIV) method. The PIV measurements were carried out in the wire electrode mid-plane, perpendicularly to the wire and the collecting electrodes. The results show similarities and differences of the particle flow in the wire-cylinder type ESP for a negative and a positive DC voltage polarity.The collection efficiency was calculated from the fractional particle concentration. The fractional particle concentration was measured using the optical aerosol spectrometer. The results of the fractional collection efficiency confirmed the common view that the collection efficiency of fine particles in the ESP increases with increasing voltage and it is higher for negative voltage polarity and decreases when decreasing particle diameter.     

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.     

Modern electrostatic devices and methods for exhaust gas cleaning: A brief review

Conventional electrostatic precipitators (ESPs) have been modernized over the last few decades. In recent years, many new methods of construction have been proposed with the goal of increasing cleaning efficiency, particularly for particles in the submicrometer size range. Adding electrical forces to traditional filters has also resulted in an increase in their collection efficiency for removing dust particles. This paper reviews modifications to ESPs aimed at increasing overall collection efficiency, as well as electrostatically assisted non-electric gas cleaning devices such as cyclones, fibrous filters, and granular-bed filters assisted by electrostatic field or ionization current.     

Submicron particles trajectory and collection efficiency in a miniature planar DBD-ESP: Theoretical model and experimental validation

In this paper, the collection efficiency of a plane-to-plane dielectric barrier discharge electrostatic precipitator is investigated experimentally and theoretically using a new model. A parametric study is carried out to evaluate the effects of the applied voltage amplitude and frequency on submicron particles motion and collection within the size range from 0.18 to 0.7 μm. Results show that the amplitude of the particles oscillatory motion increases with the voltage and the particles diameter which increase their collection. The collection efficiency decreases at low frequencies because of the low charge of particles and at high frequencies because of particles fast oscillation.     

Laboratory studies of back-discharge in fly ash

The back-discharge is a type of discharge that takes place in the presence of corona discharge and occurs at an electrode covered with a dielectric layer of resistivity higher than about 10⁸ Ω m. Back-discharge can be observed in electrostatic precipitators when dust covering the collection electrode has low conductivity. In this paper, the studies of back-discharge generated in ambient air, in point-to-plane geometry with the plate electrode covered with fly ash are presented. The discharge is characterised in terms of its visual forms, current–voltage characteristics, and light emission spectra. Three forms of back-discharge were investigated: glow discharge, streamers, and low-current back-arc discharge. The current of the back-arc discharge was only a few milliamps. The discharge was stabilised by a high series resistance. It was noted that the voltage of ignition of the back-discharge for negative polarity is lower than for a positive one. Spectroscopic measurements of emission spectra provided information on elements present in the discharge column. The elements present in the fly ash, including toxic metals, can be re-entrained into the gas as particles or can be emitted as ions or neutrals during the discharge, and can decrease the collection efficiency of electrostatic precipitators. These elements were detected in the emission spectra. The effect of the discharge on the fly ash layer was also discussed. It was observed that sinter-like leftovers remain in the dust layer after a back-arc discharge.     

Finite volume method for calculation of electrostatic fields in electrostatic precipitators

This paper presents a numerical method for calculation of coupled electric and space-charge density fields in electrostatic precipitators. It is based on the finite volume discretization of the solution domain by arbitrary polyhedral control volumes and employs an iterative segregated solution procedure of the resulting set of algebraic equations, amounting to a simple, accurate and efficient numerical technique. The method is tested on a number of cases for which analytical solution, numerical and/or experimental results exist. Also, shown are the results of calculation of a 3D model of electrostatic precipitator with spike discharge electrodes.     

Properties of biomass vs. coal fly ashes deposited in electrostatic precipitator

This paper presents comparative experimental studies of the morphology and elemental composition of fly ash particles from coal- and biomass-fired boilers, deposited in each stage of 3-stage electrostatic precipitators (ESPs). It was shown that fly ash morphology, its physical properties, and the percentage of elements in the fly ash taken from each stage of ESP depend on the kind of fuel. The biomass fly ash contains many irregular large particles, which are pieces of unburned wood. Bulk density of biomass fly ash is on average lower than that of coal fly ash, and drastically decreases in the second and third stages of ESP. The resistivity, measured at electric field of 4 kV/cm, of fly ash from biomass-fired boilers is much lower than that from coal, and can be below 10² Ω m, whereas from coal, except the first stage, varies in the range from 10⁷ to 10¹⁰ Ω m. The low resistivity of coal fly ash in the first stage of ESP results from high carbon content, and of biomass is probably an effect of additional high percentage of potassium, calcium and sodium sulfates. The percentage of Si, Al, Na, Fe, and Ti in fly ash from coal-fired boilers is much higher than from biomass, and in the opposite, the percentage of Mg, K, Ca, Mn, Mo, S, Cl, and P in biomass ash exceeds that in coal fly ash. Potential detrimental effects of biomass combustion products (salts, acids, tar) leaving the boiler on the construction elements of the electrostatic precipitator, including electrodes and HV insulators have been discussed in this paper. It was concluded that the long-term effects of biomass co-firing on the electrostatic precipitator performance, including the collection efficiency, have not been sufficiently studied in the literature and these issues require further detailed investigations.     

Study of a new electrostatic precipitator with asymmetrical wire-to-cylinder configuration for cement particles collection

In this paper, a new electrostatic precipitator (ESP) with asymmetrical wire-to-cylinder configuration is investigated experimentally and numerically. The main objective is to evaluate the collection efficiency of high resistivity particles.The electrical measurements show that the corona discharge behavior is similar to that obtained in symmetrical wire-to-cylinder configuration. Results show that the collection efficiency can reach 95% in the case of negative corona discharge.In order to understand the particle trajectories inside the ESP, the experimental results are compared with numerical simulation by using a coupled model. Numerical results indicate that particles can be collected on the collecting electrode backside.     

Novel wet electrostatic precipitator for collection of fine aerosol

A novel wet electrostatic precipitator (WESP) is designed for effective control of fine aerosol from humid gases. It operates on the principle of unipolar particle charging in the corona discharge and particle precipitation under the field of their own space charge. The new precipitator is characterized by high gas velocity in the ionizing stage. Tests were carried out for gas with (NH₄)₂SO₄, HCl and (NH₄)Cl aerosol at particle number concentration up to 5·10⁷#/cm³ and mass concentration 10–1000 mg/Nm³. For test conditions one-field WESP ensures mass collection efficiency 90–97% and two-field electrostatic precipitator up to 99%.     

Electrohydrodynamic flow and particle collection efficiency of a spike-plate type electrostatic precipitator

In this work, the results of electrohydrodynamic (EHD) secondary flow and particle collection efficiency measurements in a spike-plate type electrostatic precipitator (ESP) are presented. The EHD secondary flow was measured using 2- and 3-dimensional particle image velocimetry (PIV) method under the negative DC voltage. The PIV measurements were carried out in several cross-sectional planes along and across the ESP duct. The results show a complex and turbulent flow structure in the ESP. The EHD secondary flow significantly depends on applied voltage and measuring cross-sectional plane position in respect to the spike tip. The partial collection efficiency of the ESP was measured for negative and positive DC voltage. The particle concentration with and without discharge was measured at the ESP exit using an optical aerosol spectrometer.     

Use of electrostatic precipitation for excess ion trapping in an electrical aerosol detector

In this paper, the technique of electrostatic precipitation was used to remove excess ions from a mixture with charged particles before collection on a filter in a Faraday cup electrometer of an electrical aerosol detector. The ion precipitator part of the detector was designed, constructed, and evaluated. An analytical model was developed to investigate ion and particle transports due to diffusion and space charge effects inside the ion precipitator. Experimental investigations were carried out for positive ions, the positively applied voltage at the wire electrode ranged from 10 to 150 V, ion flow rates ranged from 5 to 15 L/min, and the radial distance of the inlet was 0.15 and 14 mm at a fixed separation between the wire and outer electrodes. The calculation results showed that all charged particles of 10 nm in diameter could pass through the ion precipitator smoothly without precipitation at the outer electrode. For all ion flow rates, an increase in ion trap voltage produced an increase in ion collection efficiency of the precipitator. Experiments confirmed that the efficiency of the ion precipitator could increase to 99% at an ion trap voltage larger than 100 V for all ion flow rates.     

CFD modelling of flue gas particulates in a biomass fired stove with electrostatic precipitation

The biomass fired stoves have been used in medium and large scales applications from several years and are utilizing electrostatic precipitator technology. Biomass based technologies are considered as renewable energy source and less harmful to the environment. The combustion of biomass generates a high concentration of flue gas particulates. The most of the flue gas particulates in the exhaust gas can be filtered through an electrostatic precipitator. In this work, a computational fluid dynamic (CFD) model has been developed for analysing the trajectory of particulates in a small scale domestic stove using biomass material. It is considered that electrostatic precipitator is based on an approach where both charging and precipitation of particulates takes place within the same set of electrodes. The precipitator is mounted in a vertical chimney of diameter 180 mm containing a central high voltage corona source. The model is based on biomass combustion models utilising a Eulerian–Lagrangian reference. The developed CFD model demonstrates the efficiency of the removal of charged particulates of the flue gases and also the interaction of the electric field in a semi turbulent flow together with the combination of the ion wind. Also it includes the effects of space charge within the system. In the modelling, modifications have been made to the grounded cylindrical collector of electrostatic precipitation through a re-design to include a series of inclined baffle plates for improving the particulates' collection efficiency.     

Electrical characteristics of electrostatic precipitator with a wet membrane-based collecting electrode

Electrostatic precipitators (ESPs) with the wet membrane-based collecting electrode play an important role on the flue gas cleaning process. However, the mechanism researches on the excellent collection efficiency of the membrane-based ESPs are insufficient. This paper aims at characterizing the excellent collection efficiency of the ESPs in the aspect of the electrical characteristics. The discharge current density and distribution of the metal and wet membranes collecting electrode were measured using the boundary probe method under different conditions. The differences of the discharge current density and distribution between the wet membranes collecting electrode and the metal one were discussed in detail. In addition, the effects of applied voltage, distance between the electrodes and discharge electrode construction on the difference of the discharge current density between the wet membranes electrode and the metal one were also presented. The results show that the discharge current density is strongly increased by the wet membranes electrode, the increased discharge current density is the main reason for the excellent collection efficiency of the membrane-based WESPs.