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.     

Porcelain insulators in electrostatic precipitator

Electrostatic precipitators (ESPs) are commonly the most used filtration technology at industrial environments considering that ESPs allow to have a high dedusting efficiency. ESP insulators are key components in a precipitator inasmuch as if they do not work properly, the efficiency decreases quickly, and even having an inadequate insulation can end up in a serious accident. Therefore, there are several recommendations given about the adequate material for each insulator type and also about how to maintain an ESP insulator in good working conditions.     

Novel electrodes of an electrostatic precipitator for air filtration

Using electrostatic precipitators (ESPs) in filtration systems results in higher system energy efficiency than fiber-based filters, but particle re-entrainment could lower the collection efficiency of ESPs. This paper demonstrates a novel ESP that utilizes foam-covered collecting electrodes to reduce particle re-entrainment and enhance collection efficiency. Particles that settle down within the pores of the foam are less likely to re-enter the airflow. Results show that foam-covered ESPs have 99 percent collection efficiency. Parametric plots demonstrate the effects of the key design variables, such as corona voltage, repelling voltage, and free airflow velocity on collection efficiency.     

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%.     

Filtration characteristics of fibrous filter following an electrostatic precipitator

This study presents the results of investigations of a hybrid electrostatic filtration system (HEFS), which combines an electrostatic precipitator (ESP) and a fibrous filter installed downstream of the ESP. The particles escaping from the ESP carry large amount of charge and this can increase the filtration efficiency of the fibrous filter. The filtration characteristics, including the efficiency, pressure drop and ozone generation, were investigated experimentally. The influence of system parameters, including the filter type, applied voltage, and distance between the ESP and fibrous filter on the overall efficiency were also studied. The measured results show that utilizing the non-high-efficient fibrous filter to remove the charged particle could provide a much higher efficiency without adding the pressure drop due to the electrostatic force. If the efficiency was similar, the ozone generated by HEFS was much lower than that of the single ESP. The results proved that filter efficiency increased with a higher applied voltage and higher initial mechanical filtration efficiency. The distance between the filter and ESP had no influence on the system filtration efficiency. The efficiency of filter in HEFS supplied with the positive voltage was slightly lower than for the negative voltage. In addition, the mathematical model was utilized to model the air filter efficiency in HEFS. The modeled and measured results agreed reasonably. Overall conclusion is that the HEFS could operate at a high efficiency with the lower applied voltage, ozone generation and pressure drop.     

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.     

Effect of air flow on corona discharge in wire-to-plate electrostatic precipitator

This paper analyses corona discharge in ambient air with laboratory-scaled wire-to-plate electrostatic precipitator (WPESP). The electric field is behind the electro hydrodynamic (EHD) flow in air. Its measurements provide complementary results for the corona discharge study because the classical theory based on the current and voltage data is unsatisfactory. Taking into account the dynamic air flow velocity is perpendicular to the active wires, measurement method of the positive and negative DC corona current density and electric field, has been introduced. It has been shown also that the dynamic air flow velocity modifies the current density and the electric field distributions on the planes surfaces of the WPESP.     

Charging and absorption characteristics of small particulates under alternative and electrostatic voltages in an electrostatic precipitator

The charge quantity of small particulates such as PM2.5 plays a key role in the collection efficiency of an electrostatic precipitator（ESP）. Under a single electrostatic voltage, it is difficult to charge and absorb small particulates. A new method of superimposing an alternative voltage on the electrostatic voltage is provided in this paper. Characteristics of small particulates are analyzed under alternative and electrostatic voltages. It is demonstrated that an alternative voltage can significantly improve the collection efficiency in three aspects： preventing anti-corona, increasing the charge quantity of small particulates, and increasing the median particulate size by electric agglomeration. In addition, practical usage with the superposition of alternative voltage is provided, and the results are in agreement with the theoretical analysis.     

Agglomeration of sub-micron particles by a non-thermal plasma electrostatic precipitator

Non-thermal Plasma agglomeration is presented as a promising process to reduce the number concentration of sub-micron particles in an acrylic duct, which included a saw-tooth electrode and a wire-plate electrostatic precipitator (ESP). The generated plasma by pulse-energized ESP, the particle agglomerations were controlled under operating conditions such as pulse voltages, pulse frequencies, dust loadings, and gas velocities. When gas velocity increased from 0.5 to 1 m s⁻¹ at 45 kV_p, 20 kHz, it was found that efficiency was increased. At gas velocity of 1 m s⁻¹, the sub-micron particle number reduction efficiency for all particle sizes was over 90% in ESP.     

Experimental study of the mass balance in a pilot industrial electrostatic precipitator

Simultaneous measurements of the mass collection rates by plates and hoppers have been realized along a pilot ESP in an industrial environment, together with on-line current, voltage, opacity measurements; they have been associated with on-site measurements of particle concentration and resistivity, and with records of the operating conditions of the both boiler and the electrostatic precipitator. Using these measurements, it has been possible to reconstruct the detailed mass balance across the precipitator, under different operating conditions, depending on corona current and rapping sequences. It has been quantified that the best performances are obtained under slow rapping conditions and for current levels around the back-corona threshold. The detailed mass balance combined with size distribution of the fly ash may be very useful for the validation of theoretical models, in much more detail than through previously available published data. Continuous mass transfer from the collecting plates into the hoppers, independent on rapping shots, has been also put in evidence.     

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.     

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.     

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.     

Removal of diesel particulate matter (DPM) in a tubular wet electrostatic precipitator

In this study, experiments were performed with a tubular wet electrostatic precipitator (wESP) to evaluate its effectiveness for the removal of mass- and number-based DPM emissions. A nonroad diesel generator utilizing a low sulfur diesel fuel (500 ppm_w) operating under varying load conditions was used as a stationary DPM emission source. The US EPA Method 5 “Sampling Method for Stationary Sources” and Method 1A “Sampling and Velocity Traverses for Stationary Sources with Small Stacks or Ducts” were adopted as reference methods for measurement of DPM mass concentration. An electrical low-pressure impactor (ELPI) was used to quantify the DPM number concentration in the diluted exhaust gas at each tested condition. The wESP was evaluated with respect to varying engine loads and to different operational control parameters, such as corona power and gas residence time, to determine their effect on overall removal efficiency. The results show that the removal efficiency of the wESP increased as the engine loads decreased principally due to an increase in gas residence time and a decrease in DPM concentrations. At a constant wESP voltage and engine load, the increase of gas residence time within the wESP led to a significant increase in total DPM removal efficiency. In addition, total DPM removal efficiency was found to be directly related to the corona power, with increasing removal efficiency measured for increases in corona power. The linear correlation of DPM effective migration velocity and superficial gas velocity was established and an empirical equation is given. The wESP appears to be a promising alternative method for control of mass-based as well as number-based DPM emissions.     

Electrohydrodynamic flow patterns in a narrow electrostatic precipitator with longitudinal or transverse wire electrode

Results of 2- and 3-dimensional Particle Image Velocimetry (PIV) measurements of the flow velocity fields in narrow electrostatic precipitators (ESPs) with either a longitudinal or transverse wire electrode are presented in this work. The obtained results confirmed that the particle flow in the ESP have a strongly 3D character mainly due to applied voltage and narrow cross section of the ESP duct. It was found that several vortices were formed along and across the ESP duct. The complex character of the flow in both ESP may considerably affect the particle collection efficiency of the ESP. This issue is under investigation.     

Speciation of zinc in secondary fly ashes of municipal solid waste at high temperatures

The evaporation aerosols produced during the vitrification process of municipal solid waste incinerators (MSWI) fly ash represent a potential environmental risk owing to their high content of heavy metals. In this research, high‐temperature heating processes were carried out on fly ashes collected from bag houses in a Chinese MSWI plant and the secondary fly ashes (SFA) were separately collected at three high temperatures (1273 K, 1423 K and 1523 K) below the melting range. Elemental analysis showed that high contents of both zinc and chlorine were present in these SFA samples and, according to the standard of the heavy metals industrial grade of ore, SFAs can be re‐used as metallurgical raw materials or rich ore. Moreover, as shown by XAS analysis and for different high temperatures, zinc environments in the three SFA samples were characterized by the same local structure of the zinc chloride. As a consequence, a zinc recycling procedure can be easily designed based on the configuration information.     

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.     

Laser flow visualization and velocity fields by particle image velocimetry in an electrostatic precipitator model

Although improving electrostatic precipitator (ESP) collection of fine particles (micron and submicron sizes) remains of interest, it is not yet clear whether the turbulent flow patterns caused by the presence of electric field and charge in ESPs advance or deteriorate fine particle precipitation process. In this paper, results of the laser flow visualization and Particle Image Velocimetry (PIV) measurements of the particle flow velocity fields in a wire-to-plate type ESP model with seven wire electrodes are presented. Both experiments were carried out for negative and positive polarity of the wire electrodes. The laser flow visualization and PIV measurements clearly confirmed formation of the secondary flow (velocity of several tens of cm/s) in the ESP model, which interacts with the primary flow. The particle flow pattern changes caused by the strong interaction between the primary and secondary flows are more pronounced for higher operating voltages (higher electrohydrodynamic numbernehd) and lower primary flow velocities (lower Reynolds number Re). The particle flow patterns for the positive voltage polarity of the wire electrodes are more stable and regular than those for the negative voltage polarity due to the nonuniformity of the negative corona along the wire electrodes (tufts).     

EHD flow in a wide electrode spacing spike–plate electrostatic precipitator under positive polarity

In this work, results of two- and three-dimensional particle image velocimetry (PIV) measurements of the flow velocity fields in a wide spacing spike–plate electrostatic precipitator (ESP) under positive polarity are presented. A DC voltage of positive polarity (up to 28 kV) was applied to the spike electrode. The average gas flow velocity was 0.6 m/s. The PIV measurements were carried out in four planes perpendicular to the plate electrodes. Three parallel planes passed along the ESP while one plane passed across the ESP duct. The results show that electrohydrodynamic (EHD) secondary flow with relatively strong vortices exist in the ESP. The EHD secondary flow pattern depends on applied voltage and measuring plane position in respect to the spike tip. The strongest vortices occur in the plane passing through the tip of the upstream-directed spike. These relatively strong EHD vortices may hinder collection of the particles in the diameter range of 0.1–1 μm in the wide electrode spacing spike–plate ESPs.     

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.