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     

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。     

单区静电除尘器捕集烟尘过程的数值模拟

通过得到烟尘的颗粒尺度谱(PSD)演变过程的详细信息来定量描述静电除尘器(ESP)对烟尘的捕集过程,此时需要同时考虑静电力、流体对颗粒的水平输运力和颗粒横向湍流和布朗扩散作用等烟尘捕集机制。发展事件驱动常体积法来求解考虑烟尘捕集的通用动力学方程以得到颗粒尺度谱的演变过程。对于典型的火电厂单区ESP进行的数值模拟结果表明：ESP对直径大于10mm的烟尘具有良好的捕集效果,也能有效脱除直径小于0.02mm 的烟尘,但是对粒径范围在0.02~10mm的烟尘的除尘效率相对较低;这种"U"型分级除尘效率主要是由于颗粒自身惯性力与静电力之间的相互竞争的结果;布朗扩散机制对任何尺度颗粒的捕集效果影响很小;减小气流输运速度和增加电场强度有利于任何粒径的烟尘颗粒的脱除。     

高风速复合式电收尘器模拟烟气净化实验研究

为了探讨自制高风速复合式电收尘器(HVCESP)的收尘效果,对其进行了模拟烟气粉尘净化实验研究并探讨了其收尘机理。以收尘效率η作为收尘指标,以粉尘初始质量浓度0ρ、工作电压U、有效收尘面积S及烟气风速v作为影响因素进行单因素分析,分别考查各因素对η的影响规律。实验结果表明,当烟气处理量qV=3780 m3/h,v=1.75 m/s,ρ0=5 g/m3,U=47 kV,有效收尘面积S=14.5m2,异极距H=0.17m,有效电场长度L=2.8 m时,η可达95%;当qV=1787 m3/h,v=0.81 m/s,而其它条件不变时,η可达99.54%。HVCESP除尘系统比传统电收尘器(ESP)体积缩小几倍,实现了除尘设备小型化,降低了其一次性投资。     

Mercury and other trace elements removal characteristics of DC andpulse-energized electrostatic precipitator

Removal of zinc-rich ash and mercury from a gas stream using prototype electrostatic precipitators (ESPs) has been investigated. Dust particles collected from the thermal plasma ash volume reduction system of the Matsuyama, Japan, municipal incineration plant were used to study the performance of an ESP to collect zinc-rich ash. The ash was analyzed by neutron activation analysis using the nuclear reactor at McMaster University, Hamilton, ON, Canada. Collection efficiency as high as 99.9% was obtained in the removal of Zn from the flue gas with DC energization of the ESP. The mercury concentration was measured using a cold vapor atomic absorption spectrometer after sampling (OSHA-140 method) from the flue-gas stream. As a function of concentration, the mercury collection efficiency was higher at a higher concentration of mercury. Higher collection efficiency was obtained under negative DC energization compared to other modes of energization of the ESP; however, power consumption under the pulse application was much smaller compared to DC energization     

Agglomeration of particles by ac corona discharge

Improving the collection efficiency for particles smaller than 1 μm on every precipitator is important. We sought to improve the collection of these particles on an ESP due to particle agglomeration. Particles are charged by ac corona discharge in a precharger and agglomerated by a dc electric field in an agglomerator downstream of the precharger. Diesel exhaust particles were used as particulate matter for the experiments. The distribution of particle size was measured using a particle counter and a scanning electron microscope. By these methods, particles as small as 0.01 μm could be counted. Results showed the agglomeration between particles at ac corona discharge operating mode. The concentration of particles smaller than approximately 0.35 μm decreases, and that of particles larger than approximately 0.35 μm increases in the agglomerator. The agglomeration rate increases with increasing applied voltage, then saturates. These results may be due to the size distribution and to decrease of concentration by agglomeration. © 1999 Scripta Technica, Electr Eng Jpn, 130(1): 30–37, 2000     

紧凑型混合颗粒收集器的数值模拟

单独的静电除尘器(electrostatic precipitator,ESP)和布袋除尘器均对于0.1~1 mm的亚微米颗粒具有较低的除尘效率。采用复合外加条件,即ESP和布袋除尘器串联的紧凑型混合颗粒收集器(compact hybrid particulate collector,COHPC),有望提高可吸入颗粒物的除尘效率。针对一典型300 MW机组的ESP,讨论了2种对其进行改造的技术方案,即对ESP本体进行改造和采用COHPAC,对改造前后ESP的除尘过程进行了颗粒群平衡模拟,并对电增强布袋除尘器的非稳态除尘过程进行了数值模拟,从而定量描述可吸入颗粒物在多种外加条件下的非线性和微尺度动力学演变规律。数值模拟结果表明,COHPAC比本体改造前后ESP具有更高的可吸入颗粒物(particulate matter,PM)除尘效率,整体质量和整体数目除尘效率可以达到99%以上。     

PM2.5分布指数对线管式电除尘器除尘效率的影响

为了确定线管式静电除尘器（ESP）捕集PM_2.5性能的改善途径，需探明PM_2.5分布指数（分布指数）对ESP脱除细微颗粒PM_2.5效率的影响规律。为此，建立了多场耦合作用下的理论模型和简化模型，分别从分级效率和综合效率的角度描述了离子风、磁约束作用下EPS对PM_2.5的脱除性能。结果表明：随着颗粒粒径的减小、分布指数的降低，PM_2.5分级脱除效率会显著升高；离子风的存在提高了PM_2.5的脱除效率，但削弱了由降低分布指数给PM_2.5脱除效率带来的提升幅度；磁场有效地提升了线管式ESP对PM_2.5的捕集性能，且强化了减小分布指数对脱除PM_2.5性能的促进效果。     

A new approach to calculation of flue gas scrubbing efficiency in electrostatic precipitators

The majority of existing approaches to computation of electrostatic precipitator (ESP) efficiency do not take into account particle reentrainment from collecting electrodes. The first attempts to take into consideration particle reentrainment were made only recently. However, the existing reentrainment models for ESP are not sufficiently substantiated. A general model, which takes into account precipitation of particles, entering the ESP inlet (direct precipitation), as well as precipitation of reentrained particles (secondary precipitation), is presented in the paper. The model describes physical processes in an ESP using three integral parameters. These parameters are direct precipitation coefficient, secondary precipitation coefficient, and reentrainment coefficient, which is the ratio of reentrained particle flux to precipitation flux. An improved technique for computation of direct precipitation that takes into account particle charge distribution function is proposed. The influence of gas flow turbulence on charge distribution function parameters, as well as on other parameters of precipitation phenomena, is taken into account. The method has low computational costs. The secondary precipitation coefficient is also computed using this method. The most difficult task is calculation of the reentrainment coefficient. We suggest using data of periodical ESP testing to determine the reentrainment coefficient. The analysis of experimental data indicates that a fly ash layer on collecting electrodes is of paramount importance. It has been shown that the flux per unit length of particles falling into hoppers is proportional to weight (thickness) per unit length of the fly ash layer. Distributions of precipitation flux, flux into hoppers, reentrained particle flux along the ESP duct, and particle penetration through the ESP are computed using experimental data. A novel model that is based on a flux balance equation for each length element of an ESP duct is presented. The model has the form of a connection between physical phenomena that take place on two successive length elements of the collecting electrode. The model qualitatively agrees with experimental data. However, correction of the secondary precipitation coefficient and the reentrainment coefficient is required for quantitative agreement. In this regard, a model of layer growth and determination of functional relations between layer weight, hopper flux, and particle penetration play a key role.     

内旋流流化床床内颗粒运动特性的试验研究

流化床床内颗粒的大规模循环不仅可以加剧了颗粒横向扩散,提高燃烬速率,实现燃烧的均匀性和稳定性,还将有利于促使不燃物质的定向移动排出炉外。文中采用冷态硫化床装置,应用分层取样技术对微倾斜布风板实现流化床内不均匀布风,床内物料混合与分层特性进行了系统的试验,研究床内颗粒扩散的动态特性和流化风速,颗粒密度以及颗粒粒度等因素对流化床内分层的影响,并通过求解扩散方程来分析布风不均匀性和布风倾角对风板附近物料扩散的影响。研究表明：增大流化风速和布风板倾角能够强化颗粒的横向扩散和流化床的内旋流程度,对大而重的颗粒存在一个最佳速度比,既能实现床内旋流又有防止颗粒的严重分层,并针对低风速区的分层现象,提出了一个关联式,该式的计算值与实验值吻合较好。     

Finite element method investigation of electrostatic precipitator performance

This paper aims at analysis of the monopolar ionized field in electrostatic precipitator. A numerical model for simulating precipitation of particles in electrostatic precipitator is discussed in this paper. It includes all essential phenomena affecting the process. An iterative finite‐element technique is used to solve Poison's equation. We proposed the introduction of a potential corresponding to the critical minimum ionization field directly in the finite element formulation as a Dirichlet condition. The theoretical migration velocity is obtained by balancing the drag force with the Coulomb force or electrostatic force acting on a particle. We used the model introduced by Cochet for predicting a particle charge. The model assumes that a particle of the same size attains an equivalent maximum amount of charge for a charging time equal to infinity. Particles influence the electrical field, flow field, electrical migration velocity, gas discharge, particle charge and collection efficiency. Copyright © 2014 John Wiley & Sons, Ltd.     

充分发展尾流中粒子运动机制的研究

运用直接数值模拟方法(谱元)对圆柱尾流(Re￡175)进行计算，计算结果与实验结果吻合良好。在获得精确流场基础上，运用拉哥朗日法对充分发展尾流中的粒子(St=4)运动进行研究，同时考虑粒子之间的相互作用。研究发现，粒子在充分发展尾流中集聚于相邻涡团之间区域，该现象很大程度上决定于流场的横向速度分布。此外，粒子扩散分布清晰显示涡团轮廓。从粒子与当地流场相对速度的分布分析中可以得到粒子在尾流中的运动规律：在相邻涡团反向旋转作用下粒子围绕涡团做往复运动。因此粒子在充分发展尾流中的运动可以描述为：一边绕涡团运动，一边随涡团向尾部运动。     

一种布置在ESP进口封头的微颗粒捕集增效装置研究和开发

微颗粒捕集增效技术是目前燃煤电厂PM2.5控制较为经济有效的技术之一,但适用场合有限。利用CFD软件,分析了电除尘器进口封头内三层多孔板对不同极性颗粒的掺混效果,分别计算了导流板、接电导流板、接电直板对不同极性荷电颗粒掺混的促进作用,并对其气流分布进行验证,开发出一种布置在电除尘器进口封头内的微颗粒捕集增效装置,并配置声波清灰。该技术已在国电谏壁电厂14号炉1 000 MW机组电除尘器上成功投运,提效程度显著。     

电除尘器除尘效率的探讨

对托电一期电除尘器存在的问题进行了全面分析,探讨了其除尘效率低下的主要原因,并就灰分的比电阻、比集尘面积、气流分布、阴阳极振打等方面进行了分析、研究,介绍了所采取的技术措施。     

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     

440t/h循环流化床电站颗粒物排放特性的实验研究

针对440t/h大型燃煤循环流化床电站锅炉，分别在电除尘器前后水平烟道进行颗粒物采样，研究不同燃烧工况变化（包括煤种、锅炉负荷、Ca/S和氧量等）对颗粒物排放的影响。分析结果表明：静电除尘器分级效率随着粒径减小逐渐下降，对亚微米颗粒收尘效率不足90％，排放的颗粒物中可吸入颗粒物占据较大的份额，一般在70％～90％左右；随着煤中灰分含量的增加，锅炉负荷的增加，颗粒物排放浓度逐渐增加，静电除尘效率下降；添加石灰石后颗粒物浓度明显增加，CaO对颗粒物凝并和团聚有一定作用，使得粗颗粒物所占烟尘总量的百分比增加，烟尘颗粒d(0.5)从35.25μm增大到48.50μm；燃烧气氛含氧量增大时，排放颗粒物的粒径逐渐减小，PM1、PM2.5和PM10总排放量都是增大的。     

大颗粒在循环床密相区运动规律的可视化研究

在用循环流化床焚烧原生垃圾时，密相区中大尺寸的不可燃物质较多，为了维持正常运行，需要采取有效排渣措施，及时地将这些不可燃物排出，因此需要对大且不可燃物在密相区中的运动规律进行研究。该文设计了一种利用可视化拍摄示踪颗粒运动轨迹，通过图象处理后利用大量统计示踪颗粒位置信息得到颗粒位置概率分布的方法。在建立冷态实验台的基础上，通过实验证明这种方法的可行性。同时发现，当流化风速较高（如2.23 m/s时）时，密度较小的示踪颗粒在实验台内会做抛射运动；随着风速的提高，示踪颗粒的抛射运动也会发生变化；而密度较大的示踪颗粒则只会沿着布风板滚到排渣口处停住。     

Automatic control and management of electrostatic precipitator

The efficient operation of an electrostatic precipitator (ESP) in practice depends upon many variables, such as charging method, particle size, gas flow, temperature, dust resistivity, etc. With air pollution control requirements becoming increasingly stringent, it is essential to closely monitor and accurately control the key parameters of an ESP control system. The efficient functioning of an ESP normally means minimizing power consumption and maximizing dust collection. Several control strategies can be adopted to meet this broad requirement. In this paper, a distributed control technique of an ESP, which uses the actual dust emission and boiler load as feedback inputs has been explained. The electrostatic precipitator management system, which is a system designed by Bharat Heavy Electricals Ltd., Hyderabad, India, to meet the above control strategies using the distributed architecture to achieve efficient ESP operation is also described     

Electrostatic Precipitation of Submicron Particles using a DBD in Axisymmetric and Planar Configurations

The aim of the present work has been to quantify the efficiency of Dielectric Barrier Discharges (DBDs) for the collection of submicron particles. The experiments were performed with incense smoke particles having a mean size of about 0.3 mum. An aerosol spectrometer was employed for characterizing the size distribution of these particles at the outlet of Wire-to-Cylinder (WC) and Plate-to-Plate (PP) DBD reactors. The collection efficiency was estimated for various applied voltages (6 - 26 kV, 1 - 2000 Hz), and airflow rates of 1.6 - 24 L/min. The discharge mode was diffuse, in the case of the axisymmetric configuration, and filamentary, in the case of the planar configuration. When flow rate increases, the discharge current and the electric power decrease in both cases. This effect is less pronounced with the planar configuration. Results obtained with the aerosol spectrometer show that the particle collection efficiency of both reactors is higher at high applied voltage and low flow rate; it decreases at high frequency because of particle oscillations, and at low frequency due to the intermittent nature of the discharge. The frequency range for which the collection efficiency is higher than 90% is wider in the case of WC reactor.