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.     

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.     

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。     

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.     

Possible reuse of Egyptian controlled cement dust

Possibilities for the reuse of controlled cement dust, collected from the hopper of the electrostatic precipitators installed on the different cement kilns of two of the three biggest companies for Portland cement production located to the south of Cairo, have been investigated. Twenty-one factors representing the physical properties and mineral and chemical composition of the cement dust were estimated and discussed. The results have been compared with the corresponding figures of three Portland cement companies in the United Arab Emirates and also with the British Standard Requirements of ordinary Portland cement. The study found that the cement dust can be reused after some feasible treatments, such as increasing the fineness and tricalcium silicate ratio and decreasing its alkalinity. Consequently, in this circumstance, pollution prevention at source is proving to be a boon as it can not only bring down pollution levels but also add to the companies' profits through efficient use of resources.     

Precipitator Gas Flow Distribution

An electrostatic precipitator, furnished on a 500-Mw tan-gentially-fired steam generator burning coal, was to collect 99.6% of the fly ash being generated in the furnace of this unit. Preliminary measurements indicated that the installation was not meeting its collection guarantee. Precipitator outlet flue dust distribution indicated that poor gas flow distribution might be one of the causes of the low performance. When observation ports were installed in the outlet flue to permit a direct visual evaluation of the problem, it was found that high velocity jets were present in the collecting electrodes, and that hopper sweepage was present. A model study along with velocity distribution measurements of the installation were authorized. Flow analysis indicated that the field and model correlated well. Extensive changes were made in the model, which eliminated or minimized the high velocity jets and hopper sweepage. After these changes were made in the field installation, it was able to achieve the required dust collection efficiency.     

Atmospheric Emissions from Oxidation Ponds

The electrodes in industrial precipitators collect many tons of dust daily, and the efficient transfer of this dust burden to the hoppers is a challenging problem in mechanical engineering. Many varieties of devices have been tried; hammers, vibrators, scrapers, water flushing, gas blasting, etc. Impact devices for this purpose are usually called “rappers.” Laboratory experiments described in this paper show that normal (perpendicular) rapping is more effective than shear rapping; that thick dust layers are more easily removed by rapping than thin ones; that rapping becomes easier with increasing temperature, within limits; and that the electrostatic forces acting upon the precipitated dust layer play an important role. Quantitative data are shown graphically. The relation of these results to previous studies by other investigators is discussed.     

Inhibition and promotion of trace pollutant adsorption within electrostatic precipitators

Among the technologies available for reducing mercury emissions from coal-fired electric utilities is the injection of a powdered sorbent, often some form of activated carbon, into the flue gas upstream of the particulate control device, most commonly an electrostatic precipitator (ESP). Detailed measurements of mercury removal within ESPs are lacking due to the hazardous environment they pose, increasing the importance of analysis and numerical simulation in understanding the mechanisms involved. Our previous analyses revealed that mercury adsorption by particles suspended in the gas and mercury adsorption by particles collected on internal ESP surfaces are not additive removal mechanisms but rather are competitive. The present study expands on this counterintuitive finding. Presented are results from numerical simulations reflecting the complete range of possible mass transfer boundary conditions representing mercury adsorption by the accumulated dust cake covering internal ESP collection electrodes. Using the two mercury removal mechanisms operating concurrently and interdependently always underperforms the sum of the two mechanisms’ individual contributions.

Implications: The dual use of electrostatic precipitators (ESPs) for particulate removal and adsorption of trace gaseous pollutants such as mercury is increasing as mercury regulations become more widespread. Under such circumstances, mercury adsorption by particles suspended in the gas and mercury adsorption by particles collected on internal ESP surfaces are competitive. Together, the two mercury removal mechanisms always underperform the sum of their two independent contributions. These findings can inform strategies sought by electric utilities for reducing the usage costs of mercury sorbents.     

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.     

An Interpretation of the Deteriorative Performance of Hot-Side Precipitators

A severe problem has developed with respect to the efficiency of certain hot-side electrostatic precipitators. The problem is manifested by a time dependent degradation of performance in which back corona related to a condition of high ash resistivity is the influencing factor. Performance can be restored by thorough cleaning of the precipitator only to experience the degradation once again. This paper hypothesizes an explanation for these observations. Experiments with both high and low resistivity ashes using resistivity test cells and a laboratory corona discharge device have been run to prove the validity of the suggested explanation. Data substantiate the idea that in the precipitation process a thin layer of ash more or less permanently remains on the collection plates and that the sodium ions serving as charge carriers in the conduction process through the layer are depleted leaving a thin layer of ash having extremely high resistivity. This phenomenon is called electrode polarization, sodium depletion, or an ash layer space charge depending on one's scientific discipline. Suggestions of remedial steps which may be of benefit in counteracting this difficulty are given.     

Technical Manual onSingle Retort Underfeed Stokers

Most of the recent work on the effects of cement-kiln dust has been confined to Germany and results differ considerably. There appears to be little doubt that naturally deposited dust from certain cement plants is responsible for leaf injury to deciduous and coniferous species and occasionally for death of the latter. Injury results from the combination of a relatively thick crust deposit and the toxicity of alkaline solutions formed when dusts are deposited in the presence of free moisture. The results of several hand-dusted field experiments, however, are not so conclusive; dusts are reported as either harmful, harmless, or even indirectly beneficial. In the present laboratory investigation, the comparison of CO₂ exchange in the leaf between dusted and nondusted leaves, as well as occurrence of obvious tissue damage, were used as the criteria of dust effects in short term experiments. The results demonstrated that the finer particles of certain cement-kiln dusts collected from electrostatic precipitators do interfere with CO₂ exchange and in some cases cause considerable leaf injury. The results further suggest that calcium content alone may not be the only indicator of whether a dust might be injurious, and that much more needs to be known about the effects of the interaction of chemical composition, particle size, and deposition rate.     

The Smogless Automobile

Demands for high performance and reliability of electrostatic precipitators for collection of fly ash from low sulfur fuels has led to rapid escalation of sizes and uncertainties in sizings of cold-side precipitators. This has led to utilization of the so-called “hot-side” precipitator. The underlying concept of hot-side precipitation is the avoidance of the necessity to operate the precipitator under high resistivity conditions. Data on in-situ measurements of resistivity of low sulfur fuel ash, as well as performance parameters of a number of operating installations, will be reviewed. These data will demonstrate the reduced sensitivity of hot-side precipitator sizing to fuel conditions. Other advantages of hot-side precipitators will be discussed.

Operating experience with hot-side precipitators has focused on structural problems which are peculiar to the larger, higher temperature installations. The nature and solution of these problems will be discussed. General comparative economics of hot-side and cold-side precipitators as they relate to fuel properties will be reviewed.     

Use of membrane collectors in electrostatic precipitators.

Membrane collection surfaces, developed and patented by researchers at Ohio University, were used to replace steel plates in a dry electrostatic precipitator (ESP). Such replacement facilitates tension-based rapping, which shears the adhered particle layer from the collector surface more effectively than hammer-based rapping. Tests were performed to measure the collection efficiency of the membranes and to quantify the potential improvements of this novel cleaning technique with respect to re-entrainment. Results indicate that even semiconductor materials (e.g., carbon fibers) collect ash nearly as efficiently as steel plates, potentially indicating that collection surface resistivity is primarily dictated by the accumulated ash layer and not by the underlying plate conductivity. In addition, virtually all sheared particles separated from the collecting membranes fell within the boundary layer of the membrane, indicating extremely low potential for re-entrainment.     

Performance of a Pulse-Jet Filter At High Filtration Velocity II. Filter Cake Redeposition

Pulse-jet filter cleaning is ineffective to the extent that collected dust redeposits rather than falls to the hopper. Dust tracer techniques were used to measure the amount of redeposition in a pilot scale pulse-jet filter. A mathematical model based on experimental results was developed to describe dust transfer from bag to bag, redeposition on the pulsed bag itself, and migration to the dust hopper. Dust redeposition upon the pulsed bag increased markedly with increasing filtration velocity, whereas migration and redeposition on bags adjacent to the pulsed bag decreased. For high velocity pulse-jet filters to operate at lowest possible pressure drop, filter cake redeposition must be minimized.     

Use of Membrane Collectors in Electrostatic Precipitators

ABSTRACT

Membrane collection surfaces, developed and patented by researchers at Ohio University, were used to replace steel plates in a dry electrostatic precipitator (ESP). Such replacement facilitates tension-based rapping, which shears the adhered particle layer from the collector surface more effectively than hammer-based rapping. Tests were performed to measure the collection efficiency of the membranes and to quantify the potential improvements of this novel cleaning technique with respect to re-entrainment. Results indicate that even semiconductor materials (e.g., carbon fibers) collect ash nearly as efficiently as steel plates, potentially indicating that collection surface resistivity is primarily dictated by the accumulated ash layer and not by the underlying plate conductivity. In addition, virtually all sheared particles separated from the collecting membranes fell within the boundary layer of the membrane, indicating extremely low potential for re-entrainment.     

BOP Air Cleaning Experiences

The basic oxygen process of steel manufacture evolves a large volume of gas at high temperature during the oxygen blow. This gas contains finely divided oxide particles that are removed in gas-cleaning equipment. Either electrostatic precipitators or water scrubbers are commonly used for the purpose. These devices are quite efficient but breakdowns, usually in the auxiliary equipment, are frequent. Maintenance problems arise primarily from the high temperature of the gases which warps and cracks the plates in the hood that draws the gases to the cleaning device, and from erosion of piping caused by abrasive particles moving at high velocity. This paper presents a number of case histories of BOP gas-cleaning installations, details maintenance problems, and discusses corrective measures that have been adopted. It gives reasons why merely appropriating a large sum of money for cleaning devices does not always guarantee full abatement of air pollution from the basic oxygen process.     

Optimization of Height of Release of Air Pollutants

A number of key projects in the Environmental Protection Agency (EPA) particulate R&;D program having applicability to industry are presented. For electrostatic precipitators (ESP) there is presented the result of work on large diameter discharge electrodes which provide a decrease in penetration of up to a factor of 4 when compared to conventional small diameter electrodes. Also discussed is the multistage ESP which provides a collection efficiency that would require a collecting plate area 4 or 5 times larger with conventional ESP technology. The E-SOX technology makes use of the multistage concept to free up space in the ESP for SO₂ removals of up to 90%. Electrostatically augmented fabric filtration provides a reduction in pressure drop of about 5 0% as compared to conventional fabric filtration. Wind tunnel modeling of windbreaks for material storage piles indicates a potential for providing engineering design data that would allow significant emission reduction caused by wind erosion     

The Efficiency of Electrostatic Precipitators Under Conditions of Corona Quenching

The suppression of corona by particle space-charge is of considerable importance in electrostatic precipitators dealing with medium to high concentrations of particulates. However, the effect of the dust concentration on collection efficiency has found no direct answer in the literature. In addition to the expected reduction in corona current due to low mobility dust particles, the presence of these charged particles has two other main effects: 1. The electric field in the vicinity of the discharge electrode is weakened and hence the concentration of ions originating in the ionization zone and forming the charging current is decreased. 2. The resulting space-charge build-up causes an increase in the field strength adjacent to the collecting surface of the precipitator. The importance of each of these effects on the collection efficiency will be dependent on the relative decrease in particle charge as compared to the increase in the collection field. Experiments were carried out under both positive and negative corona with aerosol concentrations having specific surfaces in the range 0 to 44 m²/m³. These results showed: 1. For low values of corona current densities, as the specific surface area increases, the efficiency decreases. In this cqse, the charge per particle decreases as the particle concentration increases and becomes far below the normal charge attainable. Here the increase in the collection field is more than counteracted by the jarge reduction in particle charge. 2. For higher values of initial corona current densities, as the particle specific surface area increases, the efficiency either increases slightly or stays constant, in spite of major reductions in the measured corona current. In this case there should also be a reduction in the charge per particle with the increase in particle concentration, however, this is apparently offset by the increase in the collection field strength. Analysis of the results, coupled with an interpretation of existing theories, indicates that a major parameter that must be considered is the ratio of the initial corona current density and the specific surface of the particles.     

雾化电晕净化烹调油烟技术

对雾化电晕放电极雾化过程、荷电液滴的捕集原理、自清洗作用进行探讨,在相同电压下雾化负电晕放电电流高于干式负电晕放电电流,极间大量荷电液滴具有很高的荷质比,并高速向极板驱进,对烟尘具有静电凝并和动力凝并除尘作用,具有高效净化油烟、自清洗极板的功能,此项净化烹调油烟的新技术,优于传统油烟净化技术,具有很好的应用价值和前景。     

雾化电晕净化烹调油烟技术

对雾化电晕放电极雾化过程、荷电液滴的捕集原理、自清洗作用进行探讨,在相同电压下雾化负电晕放电电流高于干式负电晕放电电流,极间大量荷电液滴具有很高的荷质比,并高速向极板驱进,对烟尘具有静电凝并和动力凝并除尘作用,具有高效净化油烟、自清洗极板的功能,此项净化烹调油烟的新技术,优于传统油烟净化技术,具有很好的应用价值和前景.