ESP粉尘收集性能的影响因素

为了进一步提高电除尘器的收尘效率,尤其是对高比电阻粉尘的收尘效率,依据非稳态静电收集理论,对影响电除尘器粉尘收集性能各项因素的作用程度及机理进行了进一步研究。实验研究了粉尘收尘效率与不同比电阻粉尘的最优极间距、最优工作电压、粉尘层厚度和比电阻之间的相互关系。研究结果表明,随着极间距的增加,对应比收尘极面积,对于不同比电阻粉尘的收尘效率的增加幅度是不同的,其中高比电阻粉尘的收尘效率增加的趋势更加显著;粉尘比电阻越高,所对应的最优极间距越大,宽间距电除尘器对捕集高比电阻粉尘具有一定优越性;在最优极间距条件下,粉尘比电阻越高,其所对应的最优工作电压越小;相对于正常比电阻粉尘,随极板沉积粉尘层厚度的增加,高比电阻粉尘的最佳收尘效率所对应的最优工作电压升高幅度较大,而且最优工作电压所对应的收尘效率下降显著。随粉尘比电阻的增大,电除尘器收尘效率逐渐降低,特别是当粉尘比电阻大于1011Ω.cm后,粉尘收尘效率显著下降。研究结果与非稳态静电收尘理论提出的观点相吻合,有助于透彻理解电场结构和运行参数与粉尘收集性能的关系,特别是对于今后研发提高高比电阻粉尘收集性能的针对性技术措施具有指导作用。     

Collection of Fly Ash with High Electrical Resistivity in a Pilot Plant Electrostatic Precipitator Preceded by the EPA/SoRI Precharger

It is well known that dust with high electrical resistivity is difficult to collect in electrostatic precipitators (ESP). The difficulty is primarily due to poor particle charging. This poor particle charging is not because particles having high electrical resistivity are intrinsically difficult to charge, but because back corona (which results from deposition of material on the collection electrode) produces a bipolar ion field. When ions of both positive and negative polarity are present in the charging region, the competing effects of the two, plus low values of electric field, produce low electrical charge on particles.     

Fine particle migration and collection in a wet electrostatic precipitator

Electrostatic precipitation is considered as an effective technology for fine particle removal. A lab-scale wet electrostatic precipitator (ESP) with wire-to-plate configuration was developed to study particle migration and collection. The performance of the wet ESP was evaluated in terms of the corona discharge characteristics, total removal efficiency and fractional removal efficiency. The corona discharge characteristics and particle removal abilities of the wet ESP were investigated and compared with dry ESP. Particle removal efficiency was influenced by discharge electrode type, SO₂ concentration, specific collection area (SCA) and particle/droplet interaction. Results showed that the particle removal efficiency of wet ESP was elevated to 97.86% from 93.75% of dry ESP. Three types of discharge electrodes were investigated. Higher particle removal efficiency and larger migration velocity could be obtained with fishbone electrode. Particle removal efficiency decreased by 2.87% when SO₂ concentration increased from 0 ppm to 43 ppm as a result of the suppression of corona discharge and particle charging. The removal efficiency increased with higher SCA, but it changed by only 0.71% with the SCA increasing from 25.0 m²/(m³/s) to 32.5 m²/(m³/s). Meanwhile, the increasing of particle and droplet concentration was favorable to the particle aggregation and improved particle removal efficiency.

Implications: This work tends to study the particle migration and collection under spraying condition. The performance of a wet electrostatic precipitator (ESP) is evaluated in terms of the corona discharge characteristics, total particle removal efficiency, and fractional particle removal efficiency. The effects of water droplets on particle removal, especially on removal of particles with different sizes, is investigated. The optimization work was done to determine appropriate water consumption, discharge electrode type, and specific collection area, which can provide a basis for wet ESP design and application.     

The Trielectrode Electrostatic Precipitator for Collecting High Resistivity Dust

High resistivity dust is normally the major problem in electrostatic precipitation. Even though the corona current is only a fraction of a microampere per cm² the resulting RI drop exceeds the breakdown voltage gradient of the dust layer. This breakdown or sparking through the collected dust causes the familiar back corona and premature sparkover. The Trielectrode Electrostatic Precipitator (TEP) is an effort to make precipitator performance insensitive to dust resistivity. The TEP (U. S. Patent 3,915,672) is based on the concept that the layer of collected dust behaves like capacitance and resistance in parallel so that the corona can occur in short pulses and yet maintain a moderately constant current through the dust layer. In between corona pulses the precipitating field is supplied by a third set of electrodes. Initial bench scale tests using high resistivity particulate indicate that the concept is sound. Recent testing has extended the effective operating range of the TEP for particulate resistivities of 10¹⁰-10¹³ ohm-cm indicating that the TEP is essentially insensitive to particulate resistivity and can give a constant high collection efficiency regardless of the type of coal burned in a utility application.     

Adhesive Behavior of Dust in Electrostatic Precipitation

In 2-stage precipitators particles are driven to the collecting plates by electrostatic forces but then the electrostatic force reverses and tends to pull the particles off so that dust is held on the collecting electrodes only by adhesion. In Cottrell or single-stage precipitators the corona current can provide a significant force tending to hold the collected dust to the electrode provided that the resistivity of the dust is 10¹⁰ ohm-cm or more. Adhesion is still essential in the collection of lower resistivity dust and is of vital importance in the transfer of dust from the collecting electrodes to the hopper. As the dust falls from the plates to the hopper it must be held in agglomerations or chunks. There are many peculiarities in the adhesive behavior of electrostatically collected dust. A better understanding of this adhesive behavior is essential if we are to improve the transfer of dust from the collecting electrodes to the hopper.     

Fine particle removal performance of a two-stage wet electrostatic precipitator using a nonmetallic pre-charger

A novel two-stage wet electrostatic precipitator (ESP) has been developed using a carbon brush pre-charger and collection plates with a thin water film. The electrical and particle collection performance was evaluated for submicrometer particles smaller than 0.01- 0.5 micrometer in diameter by varying the voltages applied to the pre-charger and collection plates as well as the polarity of the voltage. The collection efficiency was compared with that calculated by the theoretical models. The long-term performances of the ESP with and without water films were also compared in tests using Japanese Industrial Standards dust. The experimental results show that the carbon brush pre-charger of the two-stage wet ESP had approximately 10% particle capture, while producing ozone concentrations of less than 30 ppb. The produced amounts of ozone are significantly lower than the current limits set by international agencies. The ESP also achieved a high collection rate performance, averaging 90% for ultrafine particles, as based on the particle number concentration at an average velocity of 1 m/sec corresponding to a residence time of 0.17 sec. Higher particle collection efficiency for the ESP can be achieved by increasing the voltages applied to the pre-charger and the collection plates. The decreased collection efficiency that occurred during dust loading without water films was completely avoided by forming a thin water film on the collection plates at a water flow rate of 6.5 L/min/m(2).     

Purification characteristics of fine particulate matter treated by a self-flushing wet electrostatic precipitator equipped with a flexible electrode

A self-flushing wet electrostatic precipitator was developed to investigate the removal performance for fine particles. Flexible material (polypropylene, 840A) and carbon steel in the form of a spiked band were adopted as the collection plate and discharge electrode, respectively. The particle concentration, morphology, and trace-element content were measured by electric low-pressure impactor, scanning electron microscope, and energy-dispersive x-ray spectroscopy, respectively, before and after the electrostatic precipitator. With increasing gas velocity, the collection efficiency of fine particles (up to 0.8 μm in diameter) increased, while it decreased for particles with diameters larger than 0.8 μm. Increasing the dust inlet concentration increased the collection efficiency up to a point, from which it then declined gradually with further increases in the inlet concentration. The particulate matter after the wet electrostatic precipitator showed different degrees of agglomeration. The collection efficiency of trace elements within PM₁₀ was less than that of the PM₁₀ itself. Notably, the water consumption in the current setup was significantly lower than for other treatment processes of comparable collection efficiencies.

Implications: Wet electrostatic precipitators, as fine filtration equipment, were generally applicable to coal-fired plants to reduce PM_2.5 emissions in China. However, high energy consumption and unstable operation, such as water usage and spray washing directly in the electric field, seriously restricted the further development. The utilization of self-flushing wet electrostatic precipitator can solve these problems to some extent.     

C5–C12 volatile organic compounds at roadside,residential, and background locations in Ankara,Turkey: Temporal and spatial variations and sources

Abstract

Particle emissions of modern diesel engines are of a particular interest because of their negative health effects. The special interest is in nanosized solid particles. The effect of an open channel filter on particle emissions of a modern heavy-duty diesel engine (MAN D2066 LF31, model year 2006) was studied. Here, the authors show that the open channel filter made from metal screen efficiently reduced the number of the smallest particles and, notably, the number and mass concentration of soot particles. The filter used in this study reached 78% particle mass reduction over the European Steady Cycle. Considering the size-segregated number concentration reduction, the collection efficiency was over 95% for particles smaller than 10 nm. The diffusion is the dominant collection mechanism in small particle sizes, thus the collection efficiency decreased as particle size increased, attaining 50% at 100 nm. The overall particle number reduction was 66–99%, and for accumulation-mode particles the number concentration reduction was 62–69%, both depending on the engine load.     

旋风除尘器流场及浓度场实验与模拟

在研究旋风除尘器内气固两相的运动状况及分离机理方面,计算机模拟替代部分实验的方法能够优化设计旋风除尘器结构参数,提高其对微细颗粒的捕集效率,减少运行压力损失。本研究采用RSM模型和随机轨道模型对旋风除尘器内流场及浓度场进行模拟及实验。研究表明,旋风除尘器压力损失模拟结果与实验结果吻合较好,对于大于5μm的颗粒其捕集效率模拟结果与实验结果基本吻合;旋风除尘器外壁的颗粒浓度呈螺旋带状分布;如将排气管管径减少至原直径0.8倍,可使其对2μm颗粒捕集效率提高6.6%,但压力损失提高36.5%;颗粒的凝并作用有利于提高旋风除尘器微细颗粒的捕集效率。     

Cohesion of Precipitated Dust Layer in Electrostatic Precipitators

One of the important cohesive forces acting on the precipitated layer in an electrostatic precipitator Is that due to the effect of the electric field established in the air gaps between adjacent particles by the corona current as it flows through the layer. A theoretical expression for this force is developed for an idealized dust layer which takes into account the increase in effective contact area between adjacent particles due to elastic deformation and the limiting of the electric field in the airgap to a maximum allowable value. It is shown that the cohesive force is approximately proportional to the electric field established across the layer. This relationship is in general agreement with measurements made in the laboratory using industrially produced dust samples.     

Experimental studies on particle impaction and bounce: effects of substrate design and material

This paper presents an experimental investigation of the effects of impaction substrate designs and material in reducing particle bounce and reentrainment. Particle collection without coating by using combinations of different impaction substrate designs and surface materials was conducted using a personal particle sampler (PPS) developed by the University of Southern California. The PPS operates at flow rate of 4 l min^-1 with a 50% cutpoint of approximately 0.9 μm in aerodynamic diameter. The laboratory results showed that the PPS collection efficiency for particles larger than 50% cutpoint is strikingly low (e.g., less than 50%) when an uncoated open cavity made of aluminum was used as an impaction substrate. The collection efficiency gradually increased when Teflon tape, Nuclepore, and glass fiber filters were used as impaction surfaces, respectively. Conical or partially enclosed cavity substrate designs increased collection efficiency of particles of 9 μm up to 80–90%. A conical cavity with glass fiber filter used as impaction surface was identified as the optimum configuration, resulting in a collection efficiency of 92% at Stokes numbers as high as 15.4 (corresponding to 9 μm in aerodynamic diameter). Particle losses were low (less than 10%) and relatively independent of particle size in any design with glass fiber filter. Losses seemed to increase slightly with particle size in all other configurations. Finally, outdoor PM₁ concentrations obtained with the PPS (in its optimum configuration) and a modified micro-orifice uniform deposit impactor (MOUDI) with coated impaction stages were in excellent agreement. The mean ratio of the PPS-to-MOUDI concentration was 1.13(±0.17) with a correlation coefficient R²=0.95. Results from this investigation can be readily applied to design particle bounce-free impaction substrates without the use of coating. This is a very important feature of impactors, especially when chemical analysis of the collected particulate matter is desirable.     

泛比电阻电除尘器收尘性能实验研究

泛比电阻电除尘器是一种具有辅助电极和交错平板收尘电极的新型电除尘器.通过对其伏安特性及除尘效率的试验研究,并与传统线板式静电除尘器进行比较,结果表明辅助电极对电晕电流具有明显的抑制作用,在相同的外加电压下,泛比电阻电除尘器对高比电阻锅炉飞灰的收尘效率比普通的线板式电除尘器明显要高.     

Correlation of Dust Scrubber Efficiency

The grid-type electrostatic precipitator¹ evolved from some basic research into the collection of airborne charged dust particles in the wake of a flat, metallic, perforated plate under the action of an externally applied electric field. In the earliest experiments in the laboratory² a flat grounded perforated plate (or grid) was placed at right angles to an air stream moving at 6 ft/sec and an electric field was created on its downstream side by the placement of a second grid, bearing high negative potential, just downstream of the first. Negatively charged dust particles, carried along by the air stream through the openings in the first of the two grids, collected on the downstream face of the upstream grid as a result of the electric force acting on them in the direction opposing the air flow. Measured values of dust collection efficiency could not be accounted for simply by resolving the directly opposing mainstream air flow and electrostatic forces. The concept of vortex-enhanced precipitation was therefore proposed in order to explain the experimental results.     

Modeling of surface tension effects in venturi scrubbing

A modified model of venturi scrubber performance has been developed that addresses two effects of liquid surface tension: its effect on droplet size and its effect on particle penetration into the droplet. The predictions of the model indicate that, in general, collection efficiency increases with a decrease in liquid surface tension, but the range over which this increase is significant depends on the particle size and on the scrubber operating parameters. The predictions further indicate that the increases in collection efficiency are almost totally due to the effect of liquid surface tension on the mean droplet size, and that the collection efficiency is not significantly affected by the ability of the particle to penetrate the droplet.     

电除尘器比电阻软件测算研究与应用

为了进一步提高静电除尘器的收尘效率、降低能耗,对粉尘比电阻这一重要影响因素进行了研究。依据粉尘比电阻值定义,建立了收尘极板上粉尘层压降模型,提出了利用压降模型和伏安拟合特性曲线数学表征式来在线测算真实工况下粉尘层比电阻的实现方法。以480C型阳极板、RS芒刺阴极线极配形式进行了模拟验证实验,结果表明测算所得的比电阻数值与实际值相符。最后对现有电除尘控制系统中基于比电阻值的反馈控制方法做了简要介绍。     

High volume electrostatic field-sampler for collection of fine particle bulk samples

A high volume electrostatic field-sampler was developed for collection of fine particles, which easily can be recovered for subsequent sample characterisation and bioassays. The sampler was based on a commercial office air cleaner and consisted of a prefilter followed by electrostatic collection plates operating at 2.7 kV. The sampler performance was characterised for 26 nm to 5.4 μm-size particles in urban street air. The collection efficiency reached a maximum (60–70%) between 0.2 and 0.8 μm and dropped to ∼25% at 30 nm and 2.5 μm, respectively. After extraction in water, the particle loss was<2%. The extraction efficiency for dry lyophilised particulate matter was above 80%, allowing retrievement of ∼12 mg day⁻¹ in urban street air at PM₁₀ levels of ∼24 μg m⁻³. The ozone generating capacity of the corona discharge during operation was on the order of 10 ppb. A polycyclic aromatic hydrocarbons (PAH) degradation test using benzo[a]pyrene as a model showed that ∼85% was degraded after 24 h. However, similar results were observed when the corona discharge was switched off. Hence, the ozone and other corona discharge reactants do not appear to contribute considerably to PAH-degradation. The overall results show that the sampler type is a promising alternative to traditional sampling of fine particles for bulk analysis and bioassays. The main advantages are simple operation, high stability, high quantifiable particle recovery rates and low cost.     

Dust Collection Efficiency Analysis in a Two-Dimensional Circulating Granular Bed Filter

Abstract

Dust collection efficiency data were analyzed to determine better operating conditions for a two-dimensional circulating granular bed filter (CGBF). The dust collection efficiency in the granular bed was affected by the following operating parameters: the louver angle, the solids mass flow rate, and the particle size of the bed material. Experimental results showed that higher dust collection efficiency occurs when the solids mass flow rates were 20.34 ± 0.24, 21.50 ± 0.11, and 30.51 ± 0.57 g/sec at louver angles of 45°, 30°, and 20°, respectively. Optimal dust collection efficiency peaked with a louver angle of 30°. Average particle sizes of bed material by sieve diameters (μm) of 795 μm had higher dust collection efficiency than the average collector particle size of 1500 μm. Dust collection efficiency is influenced by bed material attrition phenomenon, causing dust collection efficiency to decrease rapidly. The dust collection efficiency analysis not only found the system free of design defects but also assisted in the operation of the two-dimensional CGBF system.     

Resuspension of allergen-containing particles under mechanical and aerodynamic disturbances from human walking

This study presents and develops a controlled and characterized method to explore the influence of specific occupant activity on the aerosolization of allergen-containing particles. Indoor allergen-related diseases are primarily inhalation sensitized and developed, suggesting an aerobiological pathway of allergen-containing carrier particles from dust reservoir to occupant respiration. But the pathways are not well understood or quantified. The influence of occupant walking on particle aerosolization is simulated by a system in which complex floor disturbances are deconvoluted into aerodynamic and mechanical components. Time resolved particle size distributions are measured for particles resuspended from representative samples of flooring materials and different types of floor disturbances in an environmentally controlled experimental chamber. Results indicate aerodynamic disturbances, relative to mechanical, dominate the particle resuspension behavior. Dust type, dust load and floor type showed marginal influences on a normalized surface loading basis. Humidity effects were not clear since during experiments the floor samples may not have reached moisture partitioning equilibrium with the controlled air humidity. Average resuspension rates ranged from 10⁻⁷ to 10⁻³ min⁻¹, having phenomenological consistency with previous, large room or chamber investigations, suggesting the method can be utilized to develop a database for particle resuspension rates.     

Measurement of the surface charge of ultrafine particles from laser printers and analysis of their electrostatic force

Ultrafine particles (UFPs) released from laser printers are electrostatically charged during the working processes of the devices, and the electrostatic force can obviously influence the dynamics of the particles. Due to the measurement difficulty and scarcity of relevant research, this issue was not reported. This study tried to address this issue through experimental measurement of the surface charge of UFPs and numerical investigation on the influence of electrostatic force on the dynamics of UFPs. A test chamber was set up to collect the UFPs, and the Scanning Electron Microscope was used to observe the morphologies of the UFPs. Based on the particle diameter and surface zeta potential, the surface charge of UFPs was calculated. The measurement results gave that particle emitted from laser printers are negatively charged and the average surface charge of particle emissions for four laser printers is in a range of about ?4.16 × 10^?17 to ?6.07 × 10^?17 C (～?260 to ?379 e). This paper also discussed the influence of electrostatic force on the dynamics of UFPs. According to the numerical investigation, it was found that, in the absence of electric field, the electrostatic force has to be considered when the surface charge is larger than 1 × 10^?16 C and when the UFP is very close to the wall with a distance of less than 0.01 m. These findings will guide constructively in predicting the dispersion and deposition of particles emitted from laser printers.