Fundamental Behavior of Direct-Coupled Submicrosecond Pulse Energization in Electrostatic Precipitators

The fundamental characteristics of the direct-coupled pulse energization in electrostatic precipitators (EP's) are investigated in a test rig comprised of a coal pulverizing facility, a furnace, a gas cooling system, and an EP. The EP consisted of a corona electrode with barbed wires and a collection electrode with 300-mm duct spacing. This simulates the type of EP normally used in a coal-fired power station. The pulse-energized collection field shows a very high collection performance of more than 99 percent collection efficiency and the performance enhancement compared to the conventional dc energization, causing very severe back corona, becomes as large as H = 2.50.H is the enhancement factor in terms of the Deutsch migration velocity in the case of the very high resistivity dust of rd = 2 x 1013 ?cm at the gas temperature of Tg = 150°C. Even at the lower level of the dust resistivity at Tg = 110°C, where no back corona occurs in the dc-energized collection field, the performance enhancement is as large as H = 1.21. The mechanism of such great performance enhancement is likely to be the result of the very uniform and very intense formation of corona plasmas on the wires as a result of the very fast rise in the pulse voltage applied and a special sawtooth waveform of the operating voltage between the corona and collection electrodes.     

The effect of high-voltage waveforms on ESP current densitydistributions

The efficiency of an electrostatic precipitator (ESP) collecting high resistivity dust is degraded by maldistribution of corona current in the interelectrode space. The current distribution can be improved by applying novel high-voltage waveforms to the discharge electrodes. Online measurements are presented of collecting plate current density distributions in a precipitator collecting fly ash from a coal-fired electric generating plant. The precipitator was operated with both round-wire and barbed-strip discharge electrodes. The current density distributions are compared for three high-voltage waveforms: conventional, pulse, and intermittent energization. A direct relationship is established between the ability of the waveforms to distribute useful values of corona current and the collection efficiency of the ESP     

Electrostatic Precipitator Pilot Studies on a Cement Plant Clinker Cooler

Some pilot studies into the removal of particulates from the exhaust of a cement plant clinker cooler are described. A two-stage electrostatic precipitator was employed, consisting of a conventional plate-wire precipitator closely followed by a three-stage grid-type precipitator, to handle 12 000 actual cubic feet per minute (acfm) of exhaust gas. To offset the high electrical resistivity of the clinker dust, moisture was added by spraying water directly onto the clinker bed in order to alleviate the problems caused by back corona. Overall collection efficiencies in excess of 90 percent were achieved, and in excess of 40 percent by the grid precipitator section alone. It is estimated on the basis of previous experience that, by using a twelve-stage grid precipitator section, overall collection efficiencies greater than 99 percent are possible. Overall treatment length would be appreciably less than for a conventional plate-wire precipitator having the same efficiency.     

Modeling the pre-breakdown V-I characteristics of an electrostaticprecipitator

A novel mathematical model for determining the electrical characteristics in a dc energized duct type electrostatic precipitator is described. The method is a quasi-analytical one, based on solving the current continuity equation by finite-difference method and Poisson's equation by variational principle with the help of Rvachev functions (R-functions). The methodology described represents a valuable design tool for simulating and comparing the voltage-current characteristics of different wire-plate precipitator configurations before optimizing the geometric parameters namely shape of the corona wire, shape of the collection electrodes, wire cross-section, wire-wire and wire-plate spacing. The proposed method will be useful in trying innovative ideas in the design aspect of a wire-duct precipitator. Other significant features of this method are reduced problem domain, less memory space, and faster convergence. The proposed method has been validated with published experimental results and the agreement is excellent. A comparison of electrical characteristics has been made for different sizes and shapes of corona wire and also for various configurations of the wire-plate precipitators     

Pulse energization system of electrostatic precipitator forretrofitting application

A pulser module has been developed for electrostatic precipitators for upgrading the performance of existing plants. The module is inserted between the existing DC power supply and corona electrodes. A sawtooth voltage appears on the corona electrodes at a switching frequency, including at its leading edge a transient LC oscillation with a very sharp first peak and fast-decaying amplitude. This acts as the submicrosecond pulse energization, producing very active negative streamers in uniform distribution along the entire length of corona wires of the conventional construction. The average of the sawtooth voltage produces a DC field between the corona and the collecting electrodes. This direct-coupled pulse energization indicates in present laboratory tests exactly the same collection performance as the hitherto most effective submicrosecond pulse energization using a coupling capacitor and a DC bias voltage. A specific feature of the present pulse energization system is its simplicity in construction and low initial and operating costs     

Pilot-plant testing of a novel electrostatic collector for submicrometer particles

A novel electrostatic collector CAROLA (Corona Aerosol Abscheider) for gas cleaning from submicrometer particles is described. The CAROLA concept is based on particle charging by corona discharge and subsequent particle removal in the grounded part of the collector. CAROLA collectors for fine oil mists and for fly ash were tested. The influence of the operation conditions on corona discharge was studied. It is shown that the CAROLA electrostatic collectors have high fractional removal efficiencies (>98% for particles >1 /spl mu/m and 95%-98% for particles with sizes 0.3-1 /spl mu/m), low operating voltages (10-20 kV), and low pressure drop (<200 Pa). The collection of charged particles without external electric field and the compact design provide a cost-effective solution for the removal of submicrometer particles from industrial off gases.     

215MW机组电除尘器的节能改造

根据电除尘器的除尘工作特性,结合电除尘器节能试验,利用节能优化控制方式能较大幅度地降低电除尘器运行电耗,提高经济效益。分析了新型电除尘节能技术的原理、技术特点,介绍了浙江浙能镇海发电有限责任公司4台215 MW机组电除尘器的改造与应用情况。     

The Effect of Back Corona in a Laboratory Scale Electrostatic Precipitator

The results are presented of a study on the influence of back corona on the performance of a laboratory scale wire-plate precipitator, with the plate to plate spacing as a parameter. Back corona is introduced into the precipitator by increasing the ash resistivity, attained by increasing the ash-laden gas temperature from 21 ° C (a no back discharge condition) to 104 ° C. Performance is evaluated in terms of the Deutsch migration velocity and using the particle concentration measurements made at the inlet and outlet of the precipitator with an optical counter. Results for the four different plate spacings with no back discharge show a time-independent migration velocity and current density, with higher migration velocities for the wider plate spacings. Results with back discharge show an initially rapid decrease in migration velocity with time, accompanied by a rapid increase in the current density, with greater decreases for the wider plate spacings. The behavior of the precipitator with and without back discharge is consistent with the space charge model interpretation of the wide-plate spacing precipitator.     

A Wet-Wall Electroinertial Precipitator: A Highly Efficient Air Cleaner for Cotton Dust

A wet-wall electroinertial air cleaner has been developed to control and remove dust generated during the processing of cotton textile fibers. The combination of electrostatic and inertial forces provides for high-efficiency collection of fine cotton dust, and the wet wall is used to continuously flush out the precipitated dust, thus avoiding problems of dust reentrainment or fire hazard. The air-cleaner design consists of a vertical tube equipped with a coaxial charging wire and an air entrance tangential to the periphery of the tube so as to inpart internal rotational flow. Dust entering the tube is charged by corona from the coaxial wire and is attracted to the tube wall by a combination of both the electrical and the centrifugal forces. A film of water introduced at the upper end of the tube flows down the inner wall and washes collected dust down the tube wall to a discharge sump. Data is presented on the efficiencies of a 4-in and an 8-in diameter precipitator operating at upper limits of about 200 ft3/min and 800 ft3/min, respectively, with artificially produced cotton dusts.     

Capture of fine particles on charged moving spheres: a newelectrostatic precipitator

It was verified experimentally that electrostatic cleanup of fly ash dust is possible using the scrubbing action of charged copper particles with diameters 10-100 times the size of the dust particles. Results show that target efficiencies in excess of unity (up to 1.8) are possible as a result of the induced charge on the target particles. Power consumption to the precipitator was confirmed to be very small (~5×10^-4 W). Values of the total collection efficiency of 0.65 (65%) were obtained for a precipitator zone length of 1.2 cm (in the direction of air flow) at air velocities of about 3 cm/s. This value decreased with increasing air flow rate. Collection efficiencies up to 0.99 (99%) are projected with a longer test section of at least 5.3 cm in length for low air velocities of about 3 cm/s, an electric field strength of 10.91 kV/cm, and 114.5 μm copper particles     

Three-dimensional (3-D) model of electric field and space charge inthe barbed plate-to-plate precipitator

A finite-element/method-of-characteristics model of three-dimensional (3-D) electrode geometries with corona discharge is used to predict space charge density, current density, electric potential and electric field in point-to-plane, single-barb plate-to-plane, and hexagonal multiple-barbed plate-to-plate electrostatic precipitator (ESP) geometries. Although a modification of Peek's formula for the hyperboid-to-plane was initially used to establish a boundary condition at the edge of the corona, predicted total current did not agree with measured values. As a result, it was necessary to use measured current-voltage characteristics to establish the space charge density at the outer surface of the corona sheath. An additional problem in modeling point discharges is specification of shape and size of the corona sheath. Both the authors' results and much earlier work by Trichel suggest that the thickness of the corona sheath cannot be automatically neglected     

新型双极电晕放电器的放电特性

为了得到放电参数更好的电晕放电器,解决传统电晕放电器在施加单极性高压电源时只在高压电极上放电,而接地极上并不放电的情况,采用了一种新型针—针式结构的双极电晕放电器,在施加单极性高压电源时,放电器内能同时在高压电极和接地电极产生正、负两种不同极性的电晕放电。实验研究直流高压与脉冲高压下新型双极电晕放电器的伏安特性、放电现象和离子风时,在平均场强约3.4kV/cm条件下采用粒子图像测速仪(PIV)测得电场内正、负离子风的核心区风速约2m/s。与针—板式电晕放电器相比,新型双极电晕放电器的工作电压降低,而输出功率和电流密度明显提高。     

Modeling of Corona Characteristics in a Wire-Duct Precipitator Using the Charge Simulation Technique

The charge simulation technique has been adapted to model the electrostatic and the corona characteristics in clean air of a duct-type electrostatic precipitator. The study involves the evaluation of the electric potential, electric field, and charge density in the interelectrode space as a function of corona current. The results show good agreement with published experimental data. The method developed can be applied to other geometries in the presence of space charge. The commonly used assumption that the space charge affects the magnitude but not the direction of the electric field is shown to be inadequate for large values of corona current. Also, the effect of using different values for the mobility of negative ions is presented.     

Semipilot plant pulse energized cold-precharger electrostaticprecipitator tests for collection of moderately high resistivity flyashparticles

The performance of an electrostatic precipitator (ESP) to collect moderately high resistivity flyash has been tested under pulse/precharger energization using the semipilot scale integrated electrostatics combustion flue gas (IECFG) cleaning system at Ontario Hydro's 640 MJ/h Combustion Research Facility Center. The pulse energization enhanced the performance of the existing de-energized wire-plate electrostatic precipitator in collecting moderately high resistivity (ρ~10¹⁰ Ω·cm) flyash. A 26% improvement in particle collection efficiency and 30% energy saving were obtained with pulse energization, with moderately high resistivity flyash generated by burning high-sulfur (3.7%) Nova Scotia coal with limestone conditioning. The cold precharger specially designed to suit the three-stage wire-plate ESP has also been tested for its ability to improve the performance of ESP in reducing the back corona. With an additional 30% of energy supplied to the precharger it was possible to obtain up to 40% enhancement in collection efficiency. In general an enhancement factor of 1.8-2.8 was obtained with dust loading ~2 g/m³. A thyratron switched pulse power supply was used for the ESP     

气压湿度对负直流电晕特性影响的研究

为得到负直流电晕特性随着气压湿度的变化规律,建立了考虑气压湿度影响的负直流电晕起始电压的物理模型,利用模拟电荷法和计算表面光电子数目的方法求解了负直流电晕起始电压。在人工气候罐中,利用棒–板电极,测量了不同气压湿度下的负直流电晕起始电压和电晕电流。计算和试验结果表明:电晕起始电压随气压下降、湿度升高而减小,主要原因分别是有效电离系数增大导致的电离区域的扩大和高场强区域内碰撞电离能力的增强。当直流电压和电晕起始电压的比值一定时,电晕电流随气压下降、湿度升高而减小;当直流电压一定时,电晕电流随气压降低而增大,随湿度升高而减小。电晕电流和直流电压、电晕起始电压关系式中的系数C随湿度增大的不同变化趋势和正负离子与水分子结合状态的差异有关。     

电除尘器效率降低原因分析及对策

针对新余发电有限责任公司电除尘器除尘效率下降的情况,分析其存在的主要问题,简要介绍所采取的改造措施:将相应的电除尘器一电场阴极线更换为管型芒刺线;阳极板下部加装加强板;阴极振打轴承改为托滚式轴承等。改造实施后电除尘器除尘效果明显改善,可靠性得到提高。本电除尘器的改造经验对其他电厂解决同类问题具有一定借鉴意义。     

电除尘器效率降低原因分析及对策

针对新余发电有限责任公司电除尘器除尘效率下降的情况,分析其存在的主要问题,简要介绍所采取的改造措施:将电除尘器一电场阴极线更换为管型芒刺线;阳极板下部加装加强板;阴极振打轴承改为托滚式轴承等。改造实施后电除尘器除尘效果明显改善,可靠性得到提高。电除尘器的成功改造经验对其它电厂解决同类问题具有一定的借鉴意义。     

离子风对静电除尘器中颗粒物脱除的影响机制研究

静电除尘器中电晕放电产生的离子风是影响颗粒迁移和沉积的重要因素,通过建立多过程耦合的静电除尘器数值模型,研究了不同极配型式静电除尘器的电场分布特性及离子风对空间流场和颗粒物脱除的影响规律。结果表明,针刺电极电场强度和离子电荷密度的最高值均略高于芒刺电极,但是芒刺电极的静电场分布更加均匀。在外加电压为60k V时,针刺电极和芒刺电极的离子风最高流速分别可达7.91m/s和4.62m/s,离子风会促进荷电颗粒向收尘极板运动,同时也会形成近壁高速区造成堆积颗粒的不均匀分布并导致二次扬尘。由此,针对不同形式静电除尘器提出了合理调整放电电极结构方法,进行离子风调控以强化颗粒物脱除。     

Electrostatic precipitation

Electrostatic precipitators have been used widely in industry, and play an important role in environmental protection. An electrostatic precipitator (ESP) can be operated with a high collection efficiency and a low pressure drop. Recently, an ESP also has been used for cleaning indoor air. In this review, principles of electrostatic precipitation, such as particle charging, migration velocity of charged particles and collection efficiency, are described. The performance of the ESP deteriorates by abnormal phenomena, including back corona for treating high resistivity dust, abnormal re-entrainment for low resistivity dust, and corona quenching for fine dusts. To cope with these phenomena, new technologies have been developed. Pulsed energization is a technique which copes with high resistivity dusts, and this results in lower power consumption. Using pulsed energization, non-thermal plasma can be generated and chemical reactions can be promoted for treating gaseous pollutants such as NO and volatile organic compounds. Wet ESP can also remove dusts and gaseous pollutants simultaneously. These new advancements will widen the field of application of electrostatic precipitation. Some novel applications of ESP, such as removal of dioxin from incinerators, are also included in this review