宽间距—长芒刺静电除尘器收集公路隧道模拟粉尘效率研究

公路隧道中的粉尘颗粒物粒径范围分布较广,种类相对复杂,研究隧道中粉尘的去除有一定的现实意义。本研究采用静电除尘技术,通过模拟公路隧道不同情况下的各工况参数,考察了不同条件下静电除尘器的除尘效率。静电除尘器主体采用负直流高压的电极供电形式,电极采用宽间距-长芒刺结构,实验粉尘选取烟尘、水泥粉和磷镁矿石粉模拟隧道粉尘。通过改变电源电压、风速、粉尘浓度、粉尘粒径和粉尘比电阻,分别计算了各条件下的除尘效率。实验结果表明,当电源电压为8.4 k V、风速4 m/s、粉尘浓度27.59 mg/m~3、粉尘颗粒物粒径60.71μm、粉尘比电阻适中的条件下,静电除尘器的除尘效率最高,可达90%。宽间距—长芒刺结构的静电除尘器对隧道中粉尘的去除有较理想的效果。     

湿式静电除尘器电场特性及极配优化数值模拟

电极配置会影响湿式静电除尘器污染物脱除效率,对此,采用有限元法对湿式静电除尘器不同电极配置的空间电场进行数值模拟,分析了放电极针形、同极间距和供电电压对电晕区和收尘极附近电场特性和污染物脱除效率的影响。结果表明:放电极针形的顶部越平滑,收尘区的平均电场强度越大,且电场强度相对标准偏差越小,越有利于增强湿式静电除尘器对污染物的脱除效果,放电极最佳针形为平头;考虑运行电耗,放电极同极间距在340~380mm,供电电压为45~55kV时,湿式静电除尘器对污染物的脱除效果最佳。     

线-板式静电除尘器芒刺电晕线放电特性

芒刺电晕线具有优良的放电特性,在静电除尘器中得到了广泛应用。为研究芒刺电晕线的结构参数和极配参数对电晕线放电特性的影响,以常用的4种芒刺电晕线作为研究对象,建立了静电除尘器机理的实验台。从收尘极板上电晕电流密度大小和分布两个方面对芒刺电晕线的放电特性进行了实验研究,考察了芒刺高度h、光管直径d和异极距H对芒刺电晕线放电特性的影响。结果表明:相同工况下,6钉型芒刺的电晕电流密度较大,RS芒刺的电晕电流分布较均匀;芒刺电晕线的结构参数和极配参数对芒刺电晕线放电特性的影响,可以近似等效为平均电场强度对芒刺电晕线放电特性的影响;平均电场强度E和电晕电流密度J、电晕电流密度分布标准差σ之间满足二次函数关系。研究结果为芒刺电晕线的设计、静电除尘器的极配方式和电源选择提供了参考依据。     

水膜对静电除尘器内部微细颗粒分布及运动特性的影响研究

建立了一个静电除尘器微细颗粒分布与运动特性研究平台,通过改造的静电低压撞击器对电除尘器极板有/无水膜状态下的微细颗粒浓度变化进行实时测试,结合静电除尘器内部不同区域微细颗粒的受力分析,研究了水膜对静电除尘器内部颗粒的分布状况、运动轨迹、沉积情况、分级效率及力学行为等的影响。研究表明,水膜对近壁区微细颗粒的力学行为影响显著,均布水膜后近壁区的电场力、热泳力、浓度梯度力均有不同程度的增大,且颗粒与收尘极之间的相互作用转变为气固两相流与水膜之间的扩散过程;有/无水膜状态下静电除尘器内部微细颗粒浓度从中心到壁面均呈现递减的趋势,均布水膜后截面浓度梯度更大(特别是近壁区域)且随着气流的发展近壁面处的低浓度区增大;均布水膜后颗粒沿程浓度的变化主要由粒径为0.1~1μm范围内的颗粒浓度变化引起,浓度降低的最大幅度为30%;有/无水膜状态下微细颗粒物分级脱除效率的曲线形状和变化趋势相似,颗粒粒径为0.3μm时脱除效率最小,当颗粒直径减小或增大时,颗粒脱除效率迅速增大,水膜对小颗粒的脱除效果更显著,较无水膜状态下提高20%左右。     

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。     

基于粒子成像测速法的正、负电晕放电下线-板式电除尘器内流场测试

电除尘器(ESPs)内流场的变化对细颗粒物的捕集具有显著影响。为此,采用粒子成像测速技术(PIV)测试了线–板式电除尘器模型内的流场。该电除尘器模型外壳为有机玻璃材质;放电极为2根不锈钢圆线且间距可调;示踪粒子为艾灸烟,一次流速约为0.5 m/s;流场测试平面垂直于收尘极和放电极的中心位置;分别施加正、负直流高压进行实验。结果表明,随着电压的增大,正、负电晕放电产生的离子风使得一次气流流速增大,最大增速分别可达0.6 m/s和0.7 m/s。当电压约为±30 kV时,2个放电极之间产生了4个涡旋;且电压越大,4个涡旋分布越均匀对称;较大的放电极间距更利于4个涡旋的形成。这些涡旋的存在会严重阻碍电除尘器内细颗粒物的捕集。     

静电除尘器对烟气汞脱除作用的实验研究

介绍了由电厂静电除尘器对燃煤烟气汞脱除作用的现场测试得到静电除尘器进出口位置烟气汞的形态分布及静电除尘器对烟气汞的脱除效率。结果表明,静电除尘器对烟气汞的脱除效率为27.9%,以脱除烟气中氧化态汞和颗粒汞为主,对单质汞脱除效果较弱。飞灰中细颗粒(25μm～60μm)占到58.5%,其对烟气汞的吸附能力较强,使静电除尘器对烟气汞,特别是氧化态汞具有良好的脱除效果。采取措施使元素汞转化为氧化态汞,提高飞灰吸附汞的能力,是提高静电除尘器对烟气汞脱除效率的技术方向。     

线–板式电除尘器中RS芒刺电极放电特性研究

线–板式电除尘器中管状芒刺类电极几何形状复杂,其电场特性的数值计算面临几何建模的困难,而对电极放电特性的研究是建立合理几何模型的基础。针对实际RS芒刺电极,建立了实验平台,对电极放电特性进行了实验研究,观察确定了电极的放电部位,测量了电场伏安特性和极板电流密度分布,对比分析了电极不同部位的放电情况;利用半球面代替芒刺尖端,通过数值方法获得了电极表面静电场分布。研究结果表明,RS芒刺电极的放电位置主要集中在芒刺尖端区域;无芒刺尖端时,芒刺片会放电;无芒刺片时,芒刺管将放电;而极板电流密度分布与芒刺位置有显著对应关系,离芒刺越近,极板电流密度越大,且沿RS电极长度方向,极板电流密度分布并不均匀。     

440t/h循环流化床锅炉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,分别在静电除尘器(ESP)前后水平烟道进行颗粒物采样,研究不同燃烧工况变化(包括煤质、锅炉负荷、n(Ca)/n(S)和氧量)对颗粒物排放的影响.分析结果表明:静电除尘器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,使得静电除尘器前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.     

电极结构对离子风特性的影响及应用研究

为探讨电极结构对空气放电离子风特性的影响,我们设计了一种离子风发生器,通过改变电极形式、电极距离、放电间距等结构参数对离子风特性进行研究。我们还以大功率LED芯片为散热对象,对离子风发生器进行散热应用试验。试验结果表明:放电电极选用针电极时的最佳布置形式为1×11阵列,选用线电极时的最佳布置形式为电极距离1 cm;在相同条件下,网状接地极比孔板式接地极能获得更大的离子风速,其中20目网电极可以实现对发热物体最大幅度的降温,其离子风速可达到1.79 m/s。     

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.     

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     

Corona Discharge and Electrostatic Precipitation in Carbon Dioxide Under Reduced Pressure Simulating Mars Atmosphere

In this paper, the possibility of using electrostatic precipitation (ESP) to clean the gas above solar panels of modules on the surface of planet Mars is investigated. Results are presented on corona discharge in carbon dioxide gas under reduced pressure ranging from 5 to 10 mbar with different electrode configurations. The corona-discharge inception voltage and the threshold of back discharge have been measured for three electrode configurations. The charging of suspended particles of micrometer size in the gas by unipolar ions is examined. Under the considered reduced pressure, diffusion charging very likely dominates over field charging. The drift velocity of charged particles is then estimated and is found to be not drastically lower than in industrial precipitators for fine particles despite the much lower electric field which can be applied under reduced pressure. Finally, the results of a laboratory experiment examining the dust deposit onto photovoltaic cells are presented. It appears that ESP reduces the rate of a Mars analog dust deposit and might be used in order to increase the lifetime of solar panels during Mars missions.     

Particle Charging in Combined Corona-Electrostatic Fields

The association of several ionizing and nonionizing electrodes generates combined corona-electrostatic fields, characterized by space charge zones of well-defined extensions. In a previous paper, the authors presented an effective numerical method for the computation of such fields. The aim of this present work is to show how these results can be employed for estimating the charge acquired by insulating and conducting particles when passing through the space charge zones generated by various corona-electrostatic electrode geometries. The study is done under several assumptions that authorize the use of Pauthenier's formula. Diffusion charging can be neglected, the applied electric field is quasi-uniform in the vicinity of particles, and particle speed is low compared with that of air ions. The charging model takes into account the computed spatial distribution of the electric field and charge density. The computations were performed for various values of the geometrical parameters of the electrode system and of the particle transit time through the corona discharge zone. The results can be used for the design of the electrode system of any electrostatic process employing corona discharge fields.      

Experimental studies of free conducting wire particle behavior between nonparallel plane electrodes with AC voltages in air

When a spherical conducting particle exists in an electrode system where electrical gradient force acts along the electrode surface, the particle tends to move towards the high field region. The possibility of appearance of this phenomenon is experimentally investigated with wire particles, which can easily produce the corona discharge on their ends resulting the action of a reactive force on the particle by a corona wind. The results show that a hovering particle travels deeper into the high electric field regions regardless of the corona discharge. Moreover, it is found that wire particle with corona discharge hovers near the negative electrode under DC voltage in atmospheric air due to different characteristics of corona discharge by the polarity. But under the AC voltage, the particle hovers near the bottom electrode only and invades in the high field region deeper than under the DC voltage.     

Numerical Modeling of Conductive Particle Trajectories in Roll-Type Corona-Electrostatic Separators

Several attempts have already been made to simulate particle trajectories in roll-type electrostatic separators. However, the predictive value of the results is limited by an excessive number of simplifying assumptions regarding the electric field distribution, as well as particle charging and discharging mechanisms. The present work is aimed at improving the existing models by taking into account: (1) the non-uniformity of the electric field in the active zone of the separator and (2) the effect of spark discharges occurring between the electrodes. Based on previous observations, the conductive particles were assumed to lift-off when no longer exposed to corona discharge. The numerical simulations were performed for particles of various sizes. The electric field was computed in each point of the trajectory using a finite element program. It was found that: (1) some of the smaller particles impact the static electrode and are deviated to the middling compartment of the collector and (2) field annealing which accompanies spark discharges significantly affects the trajectories of conductive particles. The results of this study could guide the design of new electrostatic separation applications.     

Three-dimensional ionic wind and electrohydrodynamics of tuft/point corona electrostatic precipitator

The two-dimensional flow interaction between the primary flow and the secondary flow (often called an electric wind or ionic wind) in the wire-duct electrostatic precipitator (ESP) has been investigated theoretically and experimentally in the past. However, the analysis was limited to the two-dimensional, which is acceptable only for the positive polarity and small tuft spacing. The negative corona, commonly used for the industrial ESPs, generates tufts along the corona wire and the point coronas on the discharge wire and requires three-dimensional analysis. Three-dimensional electric field and space-charge density distributions, and the flow interaction between the primary flow and secondary flow, i.e., electrohydrodynamics were investigated. The computational results show that the secondary flow distribution consists of a donut-shaped ring from each tuft or corona point, which was predicted by the first author 16 years ago. When the primary flow exists, a pair of spiral rings, like Goertler vortices, is formed in the direction of the primary flow. The flow interaction was described using dimensionless number N/sub EHD/, which is the ratio of the ionic wind velocity to the primary flow velocity. The effects of particle motion in the electrohydrodynamic field in the tuft/point corona ESPs are discussed.     

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     

Unipolar charging of cylindrical insulating particles nearelectrode surfaces

Numerous papers have discussed the ionic charging of insulating spheres in uniform electric fields. However, in certain electrostatic technologies, such as separation and flocking, the particles are often cylindrical in shape, and they get charged on the surface of an electrode or in its proximity, so that existing formulas cannot be used. This paper addresses this problem from both a computational and an experimental point of view. The charge acquired by cylindrical particles of various dielectric constants was evaluated with an original computer program, based on the boundary-element method of field analysis. The computed results show that the position of the particle with respect to the electrodes changes the value of the saturation charge. The experimental setup simulated the charging conditions in a roll-type electrostatic separator. The unipolar space charge was generated by a needle-type electrode. An electrometer was used to measure the charge acquired by millimeter-size calibrated cylinders of polyethylene and polyvinyl chloride on a rotating roll electrode. The experimental results, which were in good agreement with the theoretical predictions, put forward a particle self-discharge effect, at field intensities beyond a well-defined threshold. This kind of information may guide the design of the electrostatic technologies based on the corona charging of granular matter