Calculating Collection Efficiencies for Electrostatic Precipitators

The body of information presented In this paper is directed to those Individuals involved In research on, or the design of, electrostatic precipitators. A model for calculating collection efficiencies— one based solely on physical principles and, as a result, one requiring no prior Information from pilot plant or field testing—has been employed to generate performance curves for twelve Industries presently using electrostatic precipitators. These industries Include the electric power, cement, pulp and paper, steel, chemical, and petroleum industries. The performance curves describe the predicted collection efficiency as a function of preclpltator length, plate-to-plate spacing and average field strength, and are presented In graphical, tabular, and equation form. Wherever literature values for a particular Industry are available, comparisons are made between the collection efficiencies generated by the new model and those based on actual preclpltator performance. The two are generally found to be In reasonable agreement.

The use of the Deufsch-Anderson model for preclpltator design Is also discussed. Results from the new model demonstrate that collection efficiency Is sensitive to particle size distribution, a complication that cannot be easily treated In the Deutsch-Ander-son equation which, In Its unmodified form, requires the use of a single representative particle size. It is concluded that the new model Is potentially a more realistic and viable approach to the prediction of collection efficiencies and as such should prove to be a valuable aid in electrostatic preclpltator design.     

Discharge Electrodes and Electrostatic Precipitators

Controlled fumigation experiments were conducted to determine the dose-response relationships for four species of urban trees exposed to sulfur dioxide. The species chosen were ginkgo, Norway maple, pin oak, and Chinese elm.

Results indicated that resistance to SO₂ increased among the species in the following order: Chinese elm, Norway maple, ginkgo, pin oak. Elm showed almost 100% leaf necrosis at exposures over 2 ppm for 6 hr, and severe chlorosis and necrosis at 0.25 ppm for 30 days. Fifty per cent leaf necrosis occurred on Norway maple at 3 ppm for 6 hr, and on ginkgo at 4 ppm for 6 hr, and both species developed moderate marginal chlorosis at 0.50 ppm for 30 days. Injury on pin oak was minor, even at 8 ppm for 8 hr, but at 0.50 ppm for 30 days, a slight overall chlorosis developed on the leaves.

The relative susceptibilities of the four species were the same in the long-term as in the short-term exposures. The shapes of the dose-response surfaces indicated that duration of exposure and concentration of the pollutant were of equal importance in producing injury on Chinese elm and probably on pin oak, but on Norway maple and ginkgo, concentration of SO₂ was of greater importance than the duration of exposure.     

Sizing and Costing of Electrostatic Precipitators

This is the second of a two-part article that reviews electrostatic precipitation theory, presents size estimating methods, and gives costing procedures for a variety of electrostatic precipitator (ESP) types and sizes. Part I of the article, which appeared in the April 1988 issue of JAPCA, discussed theory and sizing; this part presents costing. Information is given for estimating total capital investment including separate costs for the bare ESP (five types) and auxiliaries. Factors are given for installation and for indirect costs. Direct and indirect annual costs are discussed. An example problem is given.     

Maintenance and Operation Problems of Electrostatic Precipitators

A general background is offered to the potential sources and solutions of maintenance difficulties Involved with the electrostatic precipitator. The purpose of the discussion is not only to show the Inherent complexity of the situation, but to stress areas of problem identification and activity for the user of this equipment. The information contained in this paper should provide an overall base for the details covered in the subsequent papers of this conference.     

Wet Electrostatic Precipitators for Control of Submicron Particles

The application of wet electrostatic precipitators is rapidly gaining popularity. The emission regulations are becoming more and more stringent and they are being enforced. The emphasis on removal of fine particles and organic liquid droplets in the submicron range is increasing. In order to meet the required outlet loadings and opacities, the pressure drop that has to be applied across a conventional scrubber has increased exponentially. The wet electrostatic precipitator is emerging as an economic alternative by virtue of its very low power consumption and its potential for removal of submicron particles with efficiencies in the high nineties. Several applications of the wet electrostatic precipitator will be reviewed in this paper.     

Corrosion Problems and Solutions For Electrostatic Precipitators

Travel on unpaved public roads is the single largest anthropogenic source of emissions of airborne particles in the U.S. The average impact of this source on average annual mean total suspended particulate (TSP) concentrations varies from 0.05 μg/m³ (Alaska) to 11 μg/m³ (Pennsylvania). When compared by contributions to the variance in state to state TSP levels, their impact is as great as the impact of emissions from all conventional sources. Common emission control methods include paving, oiling, speed reduction, watering, and application of calcium chloride. Total annual emissions are, in most cases, most economically reduced by either speed control or paving, with expected average control costs of less than $0.50/lb of emissions reductions. For most roads with average daily traffic flows above 100 vehicles per day, paving is shown to control emissions at average costs of less than $0.05/lb. In some situations, the costs of paving are more than offset by reductions in maintenance costs. Thus it would appear that, even accounting for the order of magnitude lower impact on statewide average concentration per ton of particles emitted from open sources, control of dust emissions from unpaved roads offers an economical means for reducing ambient TSP levels.     

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.     

The Use of Electrostatic Precipitators on Municipal Incinerators

Despite this country’s scientific advances, most communities in the United States are still disposing of solid wastes the way they did 50 years ago. The problem is advancing faster than the solution. Less than half the cities in this country with populations over 2500 dispose of their wastes by an approved sanitary and nuisance-free method. Do you realize that every 60 seconds people in the United States drop 251 tons of trash into their garbage cans. At the end of the day, 362,000 tons have accumulated. This means each of the 190 million people in the US disposes of 43½ lb of rubbish daily. By the year 2000 population is expected to double, while the per capita rate of increase in refuse production rises about 2% annually.

As the population booms and spreads to the suburbs, and suburbs expand into further suburbs, we will rapidly use up the land once used for waste disposal. Thus, sanitary landfill sites will disappear because by the year 2000 three-fourths of our population will live in urban areas.

The answer to this enormous problem faced by large and small communities is central incineration. Only this system can provide a maximum reduction in the volume of refuse. It is for this reason that communities are turning to incineration as the best solution. However, a growing public awareness plus changing municipal, state, and federal laws necessitate the need for a modern incineration plant that incorporates the most advanced and proved method of air pollution control. Where efficiencies of 60 to 80% were acceptable in the past, 90 to 95% are sought at present, and soon 96 to 99% will be required.

This paper deals with the effective control of particulate emissions from municipal incinerator exhaust gases.     

A Technique for Sizing Electrostatic Precipitators for Highly Variable Fuels

Costs for as many as seven SO₂ control technologies were determined for each of the 66 existing or planned coal-fired utility units in the state of Illinois, and projected operations for each unit from 1983 through the year 2000 were obtained from production cost modeling runs or other data. The Sulfur Dioxide Emission Reduction Optimization Program (SEROP), a utility-specific marginal cost pollution control model developed at Wisconsin Power and Light Co., was used to calculate annual marginal control cost data for each individual utility, and statewide aggregate data. The potential market in emission reduction credits (ERC) was then analyzed for three acid deposition control strategies. It was determined that emissions trading sufficient to meet targeted emission levels for acid deposition control in an economically efficient manner will not occur under any of the strategies unless the desire for utilities to maintain control of their ERCs in future years is taken into account. This can only be accomplished in one of two ways: 1) leasing of ERCs, or 2) a “contingent” sale where the buyer agrees to sell back the ERC after a specified period of time. Other factors affecting the feasibility of ERC leasing are discussed.     

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.     

Backfitting Electrostatic Precipitators: Gas Velocity and Reliability Considerations

Abstract

The kinetic behavior of the toluene biofiltration process was investigated in this research. Toluene was used as a model compound for less water-soluble gas pollutants. The limiting factor in the overall toluene biofiltration process was determined by analyzing the effectiveness factor of the biofilm along the biofilter. Experiments were conducted in three laboratory-scale biofilters packed with mixtures of chaff/compost, D.E. (diatomaceous earth)/compost and GAC (granular activated carbon)/compost, respectively. A mathematical model previously proposed was verified in this study as being applicable to these biofilters packed with different filter materials. Both the experimental and theoretical results confirmed that the biodegradation rate along the biofilter followed the zero order, fractional order to first order kinetics as toluene concentration decreased. Moreover, at higher toluene concentration, biodegradation rate and mass flux of toluene were lower near the bottom of the biofilter due to substrate inhibition. Analysis of the effectiveness factor indicated that biofiltration of a less soluble compound such as toluene should not be operated at high gas flow rates (low gas residence times) due to the mass transfer limitation of such a system. At an approximate constant inlet toluene concentration of 0.9 g/m³, the toluene removal efficiency in these three biofilters would drop below 90% when the gas residence time decreased to 2.5, 2.5, and 2.0 min, respectively.     

Solving Problems in the Electrical Energization of Electrostatic Precipitators

Achieving and maintaining continuous, reliable performance in electrostatic precipitators depends critically upon the proper design, application, operation, and maintenance of the high voltage electrical energization system. Common problems among these factors are discussed, and effects on performance are illustrated from field experience. Practical recommendations for solving problems and for preventing their occurrence in the electrical aspects of precipitation are presented for effective use by designers, operators, and maintenance personnel.     

Operation and Maintenance of Fabric Filters

Operation and maintenance and performing correct system monitoring of fabric filters is discussed. The anticipation of future problems at the time of start-up and the necessity of maintaining correct records on the system to assist in later troubleshooting is stressed. When all is going well, the fabric filter requires little but routine maintenance on moving parts, which is usually well identified in the service manuals. Problems usually appear as excessive emissions, high pressure drop, or inadequate bag life. In order to find the cause of these problems, one must have maintained sufficient Information on the system to identify what changed and when. With this information, there are logical paths to follow to the proper solutions.     

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.