DIFFUSION CHARGING EFFECTS ON PM10 COLLECTION EFFICIENCY IN A WIRE-PIPE ESP UNDER MULTI-FIELD COUPLING

The PM10 collection efficiency of a wire-cylinder electrostatic precipitator (ESP) was studied by means of a developed mathematical model, including the multi-field coupling between the gas flow field, the particle dynamic field and the electric field, and applying the Deutsch–Anderson Equation. The diffusion charging mechanism was considered due to the diameter range of PM10. The investigated variables were the applied potential, the gas velocity and the particle distribution. Numerical results indicate that the diffusion charging has significant influence on PM10 removal performance and the effect is more obvious with the decreasing diameter, and that the PM10 collection efficiency increases with the increasing applied potential and the increment of collection efficiency is more significant at lower voltage. Moreover, as the gas velocity at inlet decreases, collection efficiency of PM10 will increase. Finally, the overall efficiency increases with increasing the mean diameter of particle distribution. The research results can provide theoretical and technical references for the design of novel ESP aiming at economy and environment protection.

In this paper, a numerical simulation was carried out to analyze the effect of diffusion charging on collection efficiency of a wire-plate electrostatic precipitator (ESP). The gas flow field, the electric field, and the particle field with diffusion charging considered were included in the numerical model, which was implemented by using the commercial FLUENT software. Collection efficiency was calculated by using the Deutsch-Anderson equation, in which the parameters were provided from the numerical solutions. The simulated results indicate that the effect of diffusion charging on the wire-plate ESP particle collection increases with the decreasing particle diameter, as well as applied potential, particularly for PM10. Moreover, with the increasing applied potential, collection efficiency of PM10 is obviously improved, and the effect extent first increases and then decreases with particle size increasing, but also, the size range of particle, which is significantly affected by the diffusion charging, becomes smaller. As gas velocity becomes higher, the effect of diffusion charging on grade efficiency also becomes more obvious, and the affected range of particle size gets much larger, however, at the price of grade efficiency decreasing. Using collection efficiency as reference variable of judgment, instead of particle charge, may lead to a more accurate estimation of the aforementioned size range, which can provide a great help in the further analysis of fine particle collection.

In this paper, a numerical model was developed to describe the wire-plate electrostatic precipitator, commonly called electronic air cleaners. Electrostatic precipitator have been widely used to control particulate pollutants, which adversely affect human health. In this model, the complex interactions between fluid dynamics, electric fields and particle dynamics are considered. Therefore different approach methods are used in this study for each field, Eulerian reference frame was used for the fluid flow field and the electric field, Lagrangian reference frame used for the particles trajectories. In order to describe corona phenomena around high voltage electrode, electric field and ion current density field in electrostatic precipitator are numerically calculated using the iterative method for corona discharge model suggested by Kim (2010). The most important concept in electrostatic precipitator is the electric force applied to particles through the particle charging phenomena. The charge acquired by the particle in the corona region was obtained by combining the field charge, the diffusion charge and the time available for charging being the residence time of the particle in the corona region. In order to simulate more accurately, the charging model suggested by Lawless (1996) is used for the charging phenomena of particles by corona discharge because this model was designed to predict combination effect of diffusion charge and field charge. The diminution of particle concentration along the collection plate was derived from Deutsch’s theory, and migration velocity of the particle was developed from the condition that the magnitude of Coulomb force is equal to that of Stoke’s resistance force. This model is implemented by UDF in commercial software Fluent and validated with experimental and numerical results from literatures. CFD results had been compared with various experimental data obtained by Penney&Matick, Parasram and Kihm. Our results shows good agreement in terms of distributions of electric potential, current density, electrohydrodynamic flow pattern, and particle trajectories as well as corona current and collection efficiency. From this simulation, the effect of wire arrangement on electrostatic precipitator characteristics and particle charging are investigated. Both inline and staggered arrangements of wire electrode have been considered for fixed values of gas velocity equal to 2m/s. Applied voltage on wire electrode varies 6∼13kV and particle diameter is 4μm. For low voltage condition, staggered arrangement of wire electrode caused the turbulent effect so that collection efficiency increase more than inline arrangement. However, collection efficiency decrease in high voltage condition because electric force applied on particles passing between the wire electrodes is canceled out by both side wire electrodes.

The collection efficiency of many electrostatic precipitators (ESPs) must be improved in order to fulfill the particulate emission standard, which has become increasingly rigorous. According to classical collection theory, decreasing the gas velocity for an electric field is one of the most effective methods of enhancing the collection efficiency. Thus, we proposed a side-flow ESP, which, compared to its conventional counterpart, requires a far lower gas velocity for the electric field when the gas flow rate is constant. First, using aerodynamics, a theoretical expression of the collection efficiency and dust re-entrainment lift force was derived for a turbulent flow. The calculations indicated that a greater increase in efficiency was achieved by decreasing the gas velocity, which potentially reduces dust re-entrainment, than by lengthening the electric field. To confirm the advantages of the side-flow ESP, the collection efficiency and dust re-entrainment were measured in comparison experiments with a conventional ESP, in which talc powder with a medium diameter of 11.6 µm was used as the test dust. The dust concentration at the inlet averaged ~1400 mg m–3, and the strength of the electric field ranged approximately from 3 to 4 kV cm–1. The experimental results indicated that the emitted concentration and the dust re-entrainment were 45% and 42% lower, respectively, for the side-flow ESP than for the conventional ESP at the same gas flow rate.

The collection of low resistive particles from diesel engine emission is difficult by the conventional electrostatic precipitators (ESPs). The low resistive diesel engine particles are detached from the collection plate, which causes the dust reentrainment, resulting in poor collection efficiency. The new electrohydrodynamically (EHD) assisted ESP (EHD ESP) was developed to minimize reentrainment. The EHD ESP utilizes the ionic wind, combined with electrostatic force to transport the charged particles into the pocket zone attached to the collection plate effectively. The pocket zones are designed as zero electrostatic field, i.e., no electrostatic repulsion force acting on particles, so that no reentrainment takes place. The electrode position was designed to achieve an optimum electrode location with respect to pocket position, which was determined by observing EHD streamlines and particle collection efficiency. The particle size dependent collection efficiency was obtained using scanning mobility particle sizer (SMPS).

One of the electrostatic precipitator (ESP) applications is the cleaning in superhighway tunnels. In superhighway tunnels, contaminants in the air are carbon particles, NO/sub x/ etc. The ESP consists of the precharger and collecting section. The ESP can little remove NO/sub x/, when corona discharge is used as a precharger. The autors suggest a barrier discharge-type precharger instead of corona discharge. In this work, experiments were carried out using small two-stage type ESP with barrier discharge. As a result, it was shown that the particle collection efficiency and NO/sub x/ removal rate were high in barrier discharge type ESP.

To study the PM10 collection in a wire-plate ESP, a numerical model was built and performed by FLUENT software. Deutsch-Anderson Equation was subsequently applied to collection efficiency calculation. The numerical results under different gas velocities at inlet and particle distributions indicate that the collection efficiency of PM10 increases with a decrease in gas velocity, and that the increment of grade efficiency will become bigger if particle diameter gets smaller. By comparing with a decrease in gas velocity, diffusion charging mechanism is found to be valuable for PM10 in aspect of collection. As two parameters of Rosin-Rammler distribution decrease, grade efficiency will increase, especially for fine particles, and a larger positive effect the diffusion charging mechanism will have on collection efficiency, but overall efficiency will decrease. Finally, the effect of particle distributions on grade efficiency is much smaller than that of diffusion charging mechanism or the gas velocity at inlet.

An electrostatic precipitator (ESP) has such advantages as high collection efficiency of high concentration particles and low pressure loss. However, it can cause the re-entrainment phenomenon, and the hole-type ESP is considered to suppress this phenomenon. The authors developed a parallel-plate hole-type ESP after investigating the characteristics of collection efficiency with the hole-type electrode having a double-cylinder. In this study, the authors examined the right conditions for particle collection and electrode configuration. In the case of large particles, the collection efficiency decreased when the discharge current increased. However, with respect to the particle size of 300nm or less, the greater collection efficiency was achieved when the discharge current increased. The collection efficiency was improved by moving the position of the needle electrode toward the ESP outlet. The collection efficiency also reached the highest point when the hole diameter was 5mm.

In this work, the effect of electrohydrodynamic (EHD) secondary flow on the wire-plate type electrostatic precipitator partial collection efficiencies for submicron and ultrafine dust particles in the range from 0.01 μm to 10 μm have been calculated based on the cross-sectional averaged values of numerically obtained ion density, electric field and gas velocity and coupled together with particle surface charge models based on the modified diffusion and field particle charging rates. The results show that the particle surface charge of particles between 0.01 μm and 1 μm are not significantly influenced by EHD secondary flow, however, EHD flow do perturbing the local surface charge and local particle collection efficiency. The net effect on the partial collection efficiency is not significant for the submicron size particle range.

Collection and charging characteristics of particles in an electrostatic precipitator with a wet membrane collecting electrode

The fine particles are considered a significant environmental pollution problem. The wet electrostatic precipitators (ESPs) are now used most widely among the advanced fine particle removal technologies. The rigid glass fiber reinforced plastics (FRP) collectors modified with hole-slotted structures were used to enhance the water film distribution over their surfaces. This paper experimentally investigated the effects of the hole-slot patterns and the operating parameters on the particle removal performance in wet ESPs using the hole-slot-structured collector. The voltage-current characteristics and the energy consumption among the different pattern collectors were discussed. Besides, the effects of the water film on the particle number concentration classification and the effects of the applied voltage and the specific collecting electrode areas (SCA) on the relative collection efficiency (RCE) were investigated and discussed. The experiment results show that the corona current of three hole-slot structured FRP collectors was higher than that of the conventional rigid collectors (CRC) at the same voltage. Using the FRP collectors with circular holes and horizontal slots could decrease the energy consumption and using the FRP collectors with vertical slots increases the energy consumption compared with the CRC when the collection efficiency was lower than 92.5%. The water film could increase the collection efficiency of the fine particles and prevent the fine particles cross through the holes and the slots. The hole-slot-structured FRP collectors were appropriate for the working conditions of high SCA and long residence time. When the water film in the hole and slotted structures was incomplete, the fine particles could pass through the collector plate to form secondary back-mixing. Implications: The wet ESPs are used most widely among the advanced treatment technologies of the fine particles’ removal. The rigid glass fiber reinforced plastics (FRP) collectors modified with hole-slotted structures were used to enhance the water film distribution over their surfaces. The hole-slot-structured FRP collectors were appropriate for the working conditions of high SCA and long residence time. When the water film in the hole and slotted structures was incomplete, the fine particles could pass through the collector plate to form secondary back-mixing.

This paper explores the collection efficiency of Electrostatic precipitator (ESP) of a laboratory scale model which is geometrically similar to the industrial ESP of a local coal based power station. The variable fly ash velocity is used to determine the collection efficiency of an ESP. The applied voltage and current are being kept constant during different field test. Moreover a test run will be conduct to see the collection efficiency without applying high voltage. The experimental result shows that ESP collection efficiency is higher at lower fly ash velocity and suddenly decreases after a range of velocity. To confirm the validity of collection efficiency a mathematical model has been built and verified with experimental data. In addition, a test run is conducted without the high voltage (HV) supply to see the ability of capturing fly ash particles by keeping all other parameter same. The peak efficiency of ESP found in this experiment is 99.87%.

This paper describes a strongly coupled calculation procedure for the particle dynamics in electrostatic precipitators (ESPs) subjected to the applied magnetic field with the statistical particle size distribution taken into account. The turbulent gas flow and the particle motion under external forces are modeled by using the commercial computational fluid dynamics code FLUENT. Numerical calculations for the gas flow are carried out by solving the Reynolds-averaged Navier-Stokes equations, and the turbulence is modeled by using the turbulence model. An additional source term, which is obtained by solving a coupled system of the electromagnetic field and charge transport equations, is added to the gas flow equation to capture the effect of the electromagnetic field. The particle phase is simulated by using a discrete phase model. Different kinds of particles which follow the Rosin-Rammler distribution were simulated under different conditions, and the influence of the magnetic field density on the capture of a fine particle was investigated. In order to show the dust removal effect, the collection efficiency and the escaped particle size distribution were discussed in case of different applied magnetic fields. The particle trajectories inside the ESP were also given under the effect of both aerodynamic and electromagnetic forces. Numerical results in the wire-duct ESP show that the collection efficiency increases with the increase of the applied magnetic field, that the particle trajectory is more visible to the direction of the dust collection duct, and that the collection efficiency varies smoothly when the absolute value of the applied magnetic field trends to a certain size. Furthermore, the average diameter of escaping particles decreases and the dispersion for dust particles in different sizes increases with increasing applied magnetic field, and particle sizes are linearly decreasing with the magnetic field before the particle diameter is up to a certain size.

Effect of obstructions on the particle collection efficiency in a two-stage electrostatic precipitator

ABSTRACTThe fine particles are considered a significant pollution problem. The wet electrostatic precipitators (ESPs) have advantages of efficient collection of the fine particles with lower pressure drop and eliminating reentrainment. The wetting properties of the collector surfaces have significantly important effect on wet ESPs’ stable and secure operation. The modified rigid collector (MRC) was modified by coating specific vinyl ester resin composites and loose glass fiber cloth over the conventional carbon steel in a certain way. The rigid collector surfaces before and after modification had been characterized by scanning electron microscopy (SEM) and interface tensiometer. The effect of operating temperatures on the wetting properties of the rigid collector surfaces before and after modification was investigated. The temperature range was 40~90 °C, and the wetting properties contained liquid holdup, surface flow rate, film rate, average film thickness, and critical saturation time. The modified rigid collector surface exhibited excellent wetting properties at the operating temperatures. The fine particles collection performance compared among the MRC, the conventional rigid collector (CRC), and the flexible collector (FC) in the wet ESPs was investigated. The effects of the applied voltage, the water film, corona power, and the specific collecting area on the fine particles collection were evaluated. The modified rigid collector provided high fine particles collection effect with lower energy and water consumption.Implications: To improve the submicron particles collection efficiency and decrease energy and water consumption, the formation of uniform water film over the collector surfaces has been widely studied. The modified rigid collector was modified by coating specific vinyl ester resin composites and loose glass fiber cloth (ERGF) over the conventional carbon steel (CCS) in a certain way. The modified rigid collector surface exhibited excellent wetting properties. The wet ESPs by the modified rigid collector exhibited significantly higher particles collection efficiency than by the conventional rigid collector.