ISESP XVI – Budapest, Hungary, 2022
26 October 2022 (Day 1)
ESP School
Options for Precipitator Upgrades
Introduction to ESP Systems 101
Extended operation by selection of negative or positive corona
ESP for hygroscopic & clogging dust
The use of ESP to reduce mercury emissions and the cooperation of electrostatic precipitators with existing air pollution control systems
Understanding the Deutsch Equation
Safety & Risk Management of Electrostatic Precipitators
27 October 2022 (Day 2)
Technical Sessions
TS1 – ESP fundamentals
Collection of Carbon Particles in Diesel Exhaust Gas Using Intermittent Dielectric Barrier Discharge
This study investigated the performance of a novel electrostatic precipi- tator. This system consists of a charging section and collection section with glass plates that have discharge electrodes on them. Biased alternating current (AC) with high voltage was intermittently applied to the discharge electrodes to build up charges with polarity similar to the bias on the dielectric surface to create a strong electric field between the electrodes and the surface. This electric field attracts the soot charged in the charging section to the glass surface, where plasma can cause the oxidative decomposition of the collected soot. In this system, the intermittent mode of voltage application plays a key role in the collection of particulates. The collection efficiency of the reactor was estimated with an OFF duration in the range of 0–1990 ms and an ON duration of 10 ms. The surface potential distributions of the dielectric substrates used in the reactor were measured with respect to the time duration after the voltage application had been completed. Thus, it was found that the collection efficiency was influenced by the OFF duration, because the surface charges on the glass plates had a certain lifetime.
DIFFERENT TYPES OF DUST RESISTIVITY
It is generally accepted that dust resistivity in excess of 1011 Ω𝑐𝑚 leads to the formation of back corona and to a deterioration of ESP performance. Besides dust conditioning, the most popular approach for reducing problems with back corona is the application of pulsed corona systems.
The idea behind pulsing is very simply to reduce the E-field within the dust layer by reducing the time-averaged current density. However, experience shows that other high resistivity cases are resolved successfully with an opposing approach, using especially smooth HV from three-
phase rectifiers in combination with high current densities.
A detailed investigation into this discrepancy has revealed that high values of dust resistivity can be traced back to at least two different mechanisms.
“Standard” high resistivity dusts show the behaviour which is typical for dielectrics. High potential drop across the dust layer is produced by the injection and immobilization of charge carriers into the material. Resistivity is strongly dependent on time and corona polarity and inversely proportional to the root of current density. The dust layers reach a high level of space charge density.
In a sample of an “untypical” high resistivity dust we found that resistivity is independent of layer thickness and of corona polarity and does not show time effects. The dust layer does not accumulate any significant amount of space charge. The potential drop across the dust layer is nearly independent from current density, resulting in a resistivity which is inversely proportional to current density. With very high E-fields, the dust layer shows a progressive increase of current uptake with time, ending in a breakthrough. All these are the typical characteristics of a varistor material.
Our results show that different mechanisms of high dust resistivity do exist. Therefore an appropriate choice of the high voltage supply and its control should be based on a thorough analysis of the electrical properties of the dust. A clearly better separation efficiency will be achieved.
The gas denitrification has always been the most difficult environmental problem in cement industry. High Temperature Ultra-Clean Electrostatic-Fabric Integrated Precipitator (EFIP for short) adopts the alloy fiber filter media, the service temperature can reach above 400°C, and the dust removal before denitrification can be realized by working with SCR. A pilot-scale test bench integrated technology of High Temperature Ultra-Clean EFIP and denitration has been built in a power plant, achieving the outlet dust concentration of less than 10mg/m3 and the outlet NOx concentration of less than 50mg/m3. Since the application of the High Temperature Ultra-Clean EFIP in the 1750t/d baking furnace of an aluminum plant, the dust concentration and the operating pressure drop has been kept below 5mg/m3 and 500Pa respectively for a long term. This technology is very suitable for cement industry, which can realize the integration process of “high temperature dust removal + low dust denitration”, and solve the difficulties of denitration in cement industry.
TS2 – Advancements in power supply
The effect of Micro-Pulse Technologies power Supply on Electrostatic Precipitator performance
Technical Analysis and Extended Application of Variable Frequency Power Supply
This paper introduces the principle and circuit topology of a new-type variable frequency power supply. The variable frequency power supply changes the output impedance of the transformer through amplitude modulation and frequency modulation control to suppress the back corona phenomenon, which provides a new technical means for back corona control of ESP. The output ripple coefficient of the variable frequency power supply is small and the spark is turned off in real time, which can effectively avoid the continuous flashover of the superimposed pulse power supply. Meanwhile, it has a high reliability structure for an ideal basic DC high-voltage unit of pulse power supply. Additionally, the variable frequency power supply can also be used as the supporting power supply of ozone generator for low-temperature flue gas denitration. The power consumption of the ozone generator is less than 7.5kw/kgO3 at high ozone concentration, and the index is better than the Chinese industry standard.
TS3 – ESP modeling
Combustion of Carbon Black collected on an ESP by Microwaves Irradiation using Slit Plate
In order to apply the electrostatic precipitator to a ship, it is necessary to reduce the size of the cleaning system. In search for a solution, the effect of microwaves on combustion of the carbon black was investigated in the previous study. In order to put this microwave method to practical use, this study examined a new system in which carbon black located at 6 places was burned by microwaves passing through a slit plate. Furthermore, two types of plate were used. One had slits shaped apertures with the same width (Equal slit). The other had apertures with different width so that the microwave affect carbon black at each position in a uniform way (Stair slit). Although the combustion rates with the equal slit were between 23% and 45% at the location between 1 and 3, the rates at the locations between 4 and 6 were extremely low, which were less than 6%. The cause is that the absorption of the microwaves at the location near the irradiation port was greater than that at far location. When the stair slit was used, combustion rates were between 4% and 36%. Compared with the result with the equal slit type, the rates at the locations between 4 and 6 were higher, whereas the rates at the locations between 1 and 3 were lower. This is attributed to the decrease of the microwave power passing through the slit near the irradiation port, and the increase the power at the location far from the port. This results shows that the carbon black in the combustion part can be burned uniformly using the stair slit in a practical use. It will be investigated in the future how to increase the combustion rate.
Study of Effective Particle Charging Regions in a Corona Discharge in an Electrostatic Precipitator
Every electrostatic precipitator uses a corona discharge to charge the material particles to be precipitated, and two particle charging processes occur in it: field charging and diffusion charging. The aim of this study is to evaluate the dependence of particle charging on the number density of negative ions and the electric field distribution. For this work, we replaced the space-charge with the number density of negative ions. We used a wire–plate-type electrode structure for the calculation model, applying negative DC high voltage to the wire electrode, and grounding the plate electrode. We previously calculated the distributions of the electric field and of the number density of negative ions using a two-dimensional finite-element method. In the present work, we calculate both the field charging and diffusion charging processes based on those results. However, we did not include the negative ions lost during particle charging in these calculations. Our results show that electric field charging tends to occur directly under the wire electrode, while diffusion charging is more widespread. For a constant-power discharge, we found that, although increasing only the voltage increases the electric field that accelerates the charges, it decreases the space-charge density. We also found that larger particles tend to be charged closer to the grounded plate electrode than in the vicinity of the wire electrode.
The optimal arrangement of collecting plates and corona wires considerably impacts the collection efficiency of Electrostatic Pre- cipitators (ESPs). Many experts are striving to improve existing models in a variety of methods. This research aims to examine the effects of using multiple corona wires with W-type collecting plates on the prop- erties of electrostatic precipitators. Three different types of corona wire arrangements with W-type collecting plates were modeled and compared with Flat Plates (FPs) to evaluate the effect of using multiple corona electrodes on the electrical field distribution, space charge density distri- bution, current density distribution, and particle collection efficiency.
This paper clearly focuses on the capabilities of CFD tools, which can be effectively used to simulate the gas flow inside the ESP in order to obtain equal flow distribution among all ESP passes at different operating conditions. This is in line with the international ICAC standard which is governed by strict environmental norms. In order to meet this objective, 1:1 scaled three dimensional geometric model is simulated computationally for a typical 500 MW Power plant, which faces this uneven issues of flow distribution as well as high loading of ID fan. The CFD model contains the computational domain starting from the outlet of Air pre-heater to the inlet of ID Fan along with ESP’s for simulating the flow of flue gas. Developed 3D numerical model corresponding to the actual site layout is created, meshed and analyzed using ANSYS workbench. The results obtained using CFD are validated against the experimental results evaluated from the typical project site. Flow control in different ESP passes were achieved, along with reduction in pressure drop, which considerably reduces the loading of ID fan. The position of control dampers at each ESP outlet passes are simulated here in order to obtain equal flow distribution among all ESP passes for different cases like, while all ESP’s are in operation or during the pass isolation condition. The capabilities of CFD are fully extended here by simulating the actual site conditions, thereby eliminating the need for experimental setup, which involves more trial and error conditions and also being advantageous in simulating various flow parameters in a short time with cost effective solutions.
TS4 – Novel applications and new techniques
In this study, the effect of the electrode length and the gas velocity on the collection of diesel exhaust particle (DEP) in an electrostatic precipitator (ESP) with a high electric field, which does not use corona discharge, was investigated. The ESP was a double cylindrical configuration composed of a grounded cylindrical electrode and a high voltage application columnar electrode. The edges of the elec- trodes were chamfered round to relax the concentration of electric lines of force. The length of the electrodes was between 100 mm and 1,000 mm. The gas velocity in the ESP was between 1 m/s and 4.8 m/s. A negative DC high voltage was applied to the high voltage application electrode. Diesel exhaust gas was flowed into the ESP. The gas temperature was between 50 °C and 110 °C. The particle concentra- tions at the downstream side of the ESP were measured using a low volume air sampler, and the collection efficiency was calculated. As a result, the collection ef- ficiency increased with increasing the electrode length, and reached 51% at the length of 1,000 mm and the gas velocity of 4.0 m/s. The efficiency increased as the gas velocity decreased, and that was greater than 70% at the length of 470 mm and the velocity of 1 m/s. An observation using a scanning electron microscope showed that particles were collected on both positive and negative electrodes. Therefore, it was revealed that DEP contains both positively charged particles and negatively charged ones, whereby these can be collected.
Development of the Disinfecting Air Filter
Development of the Intelligent Flue Gas Conditioning Injection Rate Control
TS5 – Electrostatics and non-thermal plasma
Air Purification Systems for Large Scale Space Applied Electric Discharge Technology
Effect of electrostatic precipitation and agglomeration of particles on bag filter regeneration
Performances of a novel hybrid electrostatic filtration system HYBRYDA+ have been investigated in the paper. The semi-industrial scale hybrid system, of a flow rate of 6000 Nm3/h, comprised of conventional electrostatic precipitator, kinematic electrostatic agglomerator and bag filter. The agglomeration process in this unipolar agglomerator was due to the collision between larger particles and smaller ones in the AC electric field. The electrostatic precipitator and kinematic electrostatic agglomerator used upstream of bag filter resulted in higher collection efficiency of the system than a bag filter operating alone, and the frequency of bag filter regeneration has been reduced. The period of bag filter regeneration without electrostatic agglomerator was about 8 minutes, but it increased more than 20 times, to 165 minutes, when the electrostatic precipitator and agglomerator were in operation. The collection efficiency of this system was >99.998%. The combination of electrostatic precipitator and agglomerator mitigate also the problem of bag filter clogging by submicron particles, which are agglomerated with larger particles (>5 µm) in the process of agglomeration.
The basic properties and applications of corona discharges have been studied for a long time. The fundamental research covers a wide range of discharge parameters, including corona starting voltage, current values and its distribution, several modes depending on the applied voltage polarity (glow and streamer for positive coronas, Trichel pulse and pulseless glow for negative coronas), ionic wind induced EHD gas flow, and ozone levels. In recent years, we have developed the methodology for the investigation of these all characteristics under repetitive ramp and triangular voltage applications. In particular, recent advances in measurement methods have led to an in-depth the understanding of corona discharges. Here, a typical needle-to-plate electrode system is used as the discharge electrode. The electrical characteristics of both positive and negative corona discharges (e.g., the corona onset voltage, the voltage at which the corona discharge transfers its mode, the appearance of corona discharge modes, the average current-voltage (I-V) characteristics, and the corona discharge hysteresis) is automatically investigated by repetitive ramp and triangular voltages generated with a high voltage amplifier with a function generator. Optical study makes clear the corona emission by full color observation with optical emission spectroscopy. Furthermore, real color corona imaging is performed by using a highly sensitive digital camera equipped with a zoom
lens. Also the gas velocity flow field generated by ionic wind is observed by Schlieren technique. These extensive research opportunities will further advance our knowledge of corona discharges. This paper presents a broad overview of the latest corona discharge measurement technology we have developed.
TS6 – Particle and aerosol charging
In the study, attention is given to the development of a compact electrostatic precipitator (ESP) for small scale biomass combustion facilities. The pilot ESP was tested, being installed downstream the wood-chips and wood-pellets boilers and a wood-logs stove. A DC negative corona discharge was used for particle charging. The ESP was operated at voltages up to 22,1 kV and corona currents up to 2,1 mA. The ESP included a casing with gas input and output sections and a grounded removable ash-box installed at a casing bottom part. The ESP with elongated barbed HV electrodes, installed axially inside the gas input section, was characterised with low corona discharge power consumption and reduced particle mass collection efficiency. The next generation electrode systems were tested which included disk and quadrat form barbed HV electrodes, installed inside the ash box being used as an opposite electrode. The optimization of HV electrode system geometry enhanced the long-term operation stability of the ESP, ensuring mean mass collection efficiency of 75%.
Novel Compact Electrostatic Precipitators for Small Scale Biomass Combustion Facilities
Electrostatic precipitation is an effective pollution control technology for reduction of particulate emissions from small-scale biomass combustion facilities. The electrostatic precipitators (ESPs) ensure high mass and fractional collection efficiency at low pressure drop and ESPs have low operation costs. The aim of current work is the development and experimental study of a prototype of a compact ESP for reduction of particle emissions from wood combustion facilities with heat power capacity 4-100 kW. In the designed ESP, particle charging takes place in a corona discharge double-stage electrode system. Particle precipitation takes place under the influence of corona discharge electric field in the ionizing zones and under the influence of electro-hydrodynamic, space charge, thermophoretic and gas-dynamic phenomena in the ash box, installed in the bottom part of ESP casing. The compact ESP assumes manual cleaning of collected fly ash. The results of the study of the influence of gas flow rate, corona discharge power consumption and particle concentration on the ESP mass collection efficiency are discussed. The prototype of a compact ESP ensures mean mass collection efficiency of 76%.
THERMIONIC EMISSION – A KEY TO UNDERSTANDING HIGH TEMPERATURE ELECTROSTATIC PRECIPITATION
For high temperature corona discharges, the range of operation becomes narrower with increasing temperature, as the decrease of sparking voltage is much more pronounced compared to the drop of corona onset voltage. However, at high temperatures the thermionic emission of electrons allows the operation of ESPs at voltages below corona onset, which will avoid sparking.
In order to provide a better understanding and to allow a proper dimensioning of high-temperature ESPs operated in the sub-onset range, thermionic emission from real, oxidized metal surfaces was studied in a precise laboratory experiment covering the temperature range between 500 °C and 800 °C. The main findings are (i) A Richardson constant which is far (20- to 40-fold) lower compared to the theoretical value. (ii) Very low values of the work function, which are around 3 eV only. (iii) A very pronounced Schottky effect, due to surface roughening and local field enhancement.
Altogether, the operation of ESPs in the sub-onset mode opens a number of interesting perspectives: (i) Thermionic emission allows stable ESP operation in the sub-onset mode for a wide range of temperature, starting at about 700 °C. (ii) Particle charging by non-thermal (hot) free electrons is extremely fast and highly efficient, providing high charge levels already with a current density in the range of µA/m and short residence time. (iii) Therefore, ESPs operated with sub-onset thermionic emission have low power requirements and offer high energy efficiency.
Investigating the reliability of needle-plane experiments with dust layer
Parameter correlations and phenomena associated with electrostatic precipitators are often investigated using laboratory models. In the literature, there are many measurements on dusts, which are used to determine, e.g., current-voltage curves. In our research, we investigate corona currents in a needle-plane arrangement covered with dust and further consider the validity of these measurements.
TS7 – Industrial experience and case studies
The removal of nanoparticles from hot gas streams is a challenging task. On the other hand, there is a huge potential for heat recovery from waste gas incineration, glass furnaces, ceramic, metallurgical, pyrolytic and many other high temperature processes. Another relevant application is the separation of nanoparticles formed by condensation in thermal post-combustion processes, in order to achieve efficient heat recovery at high temperatures.
This case study evaluates the performance of a high-temperature electrostatic precipitator (HT-ESP) between 400 – 800 °C for both discharge polarities. The tubetype ESP with 150 mm diameter and 1500 mm length is operated isothermally. Two electrode designs are tested by separating flame-generated iron oxide nanoparticles from a flue gas atmosphere. The total number concentration in the raw gas is around 2·1013 m-3 with a temperature-dependent mode diameter of 20 – 40 nm.
Between 400 – 600 °C very high separation efficiencies above 99.998 % (number-based) were found with just 1.5 s of residence time and negative polarity, using a wire discharge electrode requiring a moderate current density of 1.0 mA/m² up to 500 °C. In fact, the separation is more efficient than at room temperature which is explained by the occurrence of electronic charging with free electrons. This leads to exceptionally high particle charges compensating the reduced operating voltage. At 700 °C and 800 °C, separation efficiencies of 99.9 % and 99.5 % respectively (number-based) were obtained, using a rod discharge electrode for optimal use of thermionic emissions. The specific input of energy required for 99 % separation efficiency at any temperature is less than 150 J/m³, except for 800 °C with 250 J/m³.
These findings clearly prove that HT-ESPs are a feasible and highly performing alternative for nanoparticle removal from hot gases at up to 800 °C.
Consideration of Frequent Low Loads Operation for Electrostatic Precipitator Installation
For the design of an installation for particulate removal from flue gases containing an Electrostatic Precipitator (ESP), associated flue gas ducts and dust handling system, there are a number of factors affecting the optimal configuration. Nowadays there are required installations with ESPs for ultra-low emissions and to be able to operate with greater variation in gas flows, chemical composition, operating temperatures and dust quality.
The detailed design of the installation must consider the technological and structural aspects.
Technological aspects comprise of operating parameters as flue gas volume flow in actual conditions, flue gas and particulate analyses, changes of temperatures, pressure and other process fluctuations to determine particle properties and risks of condensing gaseous compounds into liquid or solid state and its related effects as corrosion, sticky dust and final particulate emission.
The structural design must consider a combination of a number of forces generated by wind, snow and collected dust loads, under or over pressure in the installation and must include risks for exceptional conditions of temperature, pressure and earthquakes. Design features should be included to minimise any potential detrimental effects in order to ensure operational ability and safety. Typical models for the ESP and flue gas ducts designs accurately predict performance for balanced conditions for a relatively narrow operating window, but for enlarged ranges of operation loads – in the case that the volume flow of gases at maximum load may be 3 times higher than flows frequently achieved at low loads – there are more challenging tasks.
Effective use of wastewater in electrostatic precipitators
The paper presents the results of research conducted in industrial conditions on the injection of treated wastewater (or water) from the wet flue gas desulphurization (WFGD) installation before the electrostatic precipitator (ESP) for flue gas cleaning from pulverised coal boiler (PC-boiler) feed by hard coal. The installation uses the waste heat of the flue gas to eliminate the injected liquid by completely vaporizing it. At the same time contributing to the increase of ESP efficiency by conditioning the flue gas. The tests were carried out on an installation with an injection capacity of a maximum of 6m3/h of liquid (treated wastewater, process water or mixture of both), into the flue gas stream with an average value of approx. 125,000 Nm3/h. During the tests it was confirmed that the conditioning liquid evaporates. No increase in the concentration of typical pollutants in the exhaust gases, in particular SO2, HCl, HF, NH3, was found, as at the temperatures at which the injection process takes place (about 200C). A decrease in the dust concentration downstream of the ESP was observed as a result of favorable changes in the properties of the dust-gas medium after the injection of the conditioning liquid. The obtained results show that the presented installation for injection of treated sewage into the flue gas duct before the ESP enables the elimination of the sewage stream. In addition, it will improve the operation of the electrostatic precipitator without the need for significant and costly modernization, which is particularly important for the existing industrial facilities (e.g. power plants). Additionally, it is possible to reduce other pollutants (NOx, mercury) by adding appropriately selected liquid additives.
The Application of 3D Collaborative Design in Flue Duct Reconstruction of Coal-Fired Power Plant
This paper mainly discusses the project of flue duct reconstruction of a coal-fired power plant, using the 3D software collaborative design method, making full use of the existing 3D model data of various disciplines, to simulate the complex situation of the transformation site to the 3D design environment as far as possible, and to deal with the problems of collision interference that may occur on the site in the design stage. At the same time, the docking process of design, procurement, and production departments is optimized by this design method, which provides a favorable guarantee for the smooth and efficient execution of the project. Emphasis is placed on the application of this design method in the flue gas cooler and heater retrofit EPC project of Unit 4 (660MW) of Chongqing Songzao Power Co., Ltd.
