Electrostatic Precipitator Maintenance Survey

In the latter part of 1974, the TC-1 Committee of the APCA undertook a survey of four (4) major user industries of electrostatic pre-cipitators. The purpose of the survey was to establish the users’ degree of satisfaction with this equipment from an operational and a maintenance viewpoint. Specific areas of maintenance requirements were investigated as well as the nature of difficulty experienced. The 174 responses received covered user experience with 243 precipi-tators of various manufacturers. This paper reports on the results of this survey and gives the statistics derived. The conclusions reached should be beneficial to the user and the manufacturer in the areas of product improvement and maintenance.

This is the first of this type of survey handled by the TC-1 Committee, and it will be followed by Fabric Filter and Scrubber information gathering questionnaires. Problem areas so defined will be enlarged upon in future seminars planned under APCA auspices.     

Electrostatic Precipitator Applications in the Paper Industry

A survey of paper industry applications of electrostatic precipita-tors reveals problem areas and compares operators' experience on both tile shell and heated steel jacket units. In addition, items such as dew point operation, corrosion, electrode cleaning, wire breakage, etc. are discussed.     

Dust Reentrainment,Gas Distribution and Electrostatic Precipitator Performance

Field observations indicating that uniform gas flow at the precipitator outlet may not result in best performance led to a study of how reentrainment and changes in gas distribution within a precipitator affect performance. A computer model developed in the study predicts that an improvement over uniform flow performance is possible by using controlled nonuniform gas distributions at both the inlet and the outlet faces of the precipitator. The model was used to study how changes in precipitator side view geometry affect performance and offers explanations for the reduction in precipitation constants experienced with larger installations.     

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.     

Ozone Generation in an Electrostatic Precipitator With a Heated Corona Wire

The body of information in this paper is directed to individuals involved in the design of electrostatic precipitators or of any devices utilizing the corona discharge where the ozone generation is considered to be a problem. A method that gives as much as 85 % reduction in the ozone generation per unit corona current for a given corona wire is described. The electrical parameters governing the formation of O₃ in an electrical discharge are summarized. It is shown that a reduction in the average gas density in the ionized sheath of a corona discharge should lead to lower O₃ generation. Experiments were carried out with an a.c. heating current being used to heat a corona wire in a cylindrical geometry corona section. It was found that as the heating power applied to the wire increased, the O₃ generation per unit corona current decreased for both positive and negative corona. An experimental formula was obtained that relates the heating power to the O₃ generation with and without heating for positive corona. This formula was based upon experimental work carried out on corona wires of different materials and sizes and at flow conditions typical of two-stage electrostatic precipitators. The reduction in the O₃ concentration is believed to be caused primarily by the reduced corona power dissipated in the ionized sheath under heating conditions. These results have great practical importance for improving the performance of the two-stage type electrostatic precipitator as it allows the corona current and hence charging efficiency of the unit to be increased without exceeding the O₃ limits.     

Electrostatic Precipitator Performance from the Point of View of Gas Treatment Time

This paper describes a risk assessment methodology for preliminary assessment of municipal sludge incineration. The methodology is a valuable tool in that it can be used for determining the hazard indices of chemical contaminants that might be present in sewage sludge used in incineration. The paper examines source characteristics (i.e., facility design), atmospheric dispersion of emission, and resulting human exposure and risk from sludge incinerators. Seven of the ten organics were screened for furtherinvestigation. An example of the calculations are presented for cadmium.     

Recent Electrostatic Precipitator Experience with Ammonia Conditioning of Power Boiler Flue Gases

Motivated by heightened recent interest, Koppers Co. has been experimenting with ammonia conditioning of power boiler flue gases for the purpose of improving the precipitability of the emitted fly ash. Chemical reactions resulting from ammonia injection are postulated. Measurements on three pulverized coal and two cyclone fired boilers, all of which emit acidic ash, are described. In all five cases, considerable but varying, increase in precipitator power input and collection efficiency resulted when gaseous ammonia in the amount of 1 5 ppm was injected between the economizer and air preheater. The conditioned fly ash showed decreased acidity and inconsistent change in electrical resistivity. Unless air heater temperatures were unusually high (>400°F), tendency of the air heater to plug was an additional, but unwanted, result. At one station with high air heater outlet temperature ammonia injection has been adopted as a permanent solution to community pressure for reduction of stack discharge. Ammonia injection downstream of the air heater produced no effect. Future plans are presented to continue the program beyond results described here.     

Results of Field Measurements of Industrial Particulate Sources and Electrostatic Precipitator Performance

This paper presents results of source size distribution measurements over the size range from 0.1 μm to 5 μm for six classes of particulate sources and fractional efficiency measurements on five full scale electrostatic precipitators and one pilot scale precipitator. The precipitators all showed moderately high to high particulate collection efficiencies for particles having diameters larger than a few micrometers or smaller than a few hundredths of a micrometer and a minimum in collection efficiency for particles having diameters of a few tenths of a micrometer.     

Calculation Of Electric Field Strength Distributions For New Electrostatic Precipitator Discharge Electrode Designs

New collecting and discharge electrode configurations are being used in electrostatic precipitators. In order to mathematically model preclpitators with these arrangements, it is necessary to develop reliable models of the field strength distributions of these electrodeplate geometries. This paper presents the concept of an equivalent wire diameter and attenuation functions, permitting use of the well-known wire-plate calculations. Experimental data presented in this paper shows the approximation techniques are sufficiently accurate.     

Control of Diesel Gaseous and Particulate Emissions with a Tube-Type Wet Electrostatic Precipitator

Abstract

In this study, experiments were performed with a bench-scale tube-type wet electrostatic precipitator (wESPs) to investigate its effectiveness for the removal of mass- and number-based diesel particulate matter (DPM), hydrocarbons (HCs), carbon monoxide (CO), and oxides of nitrogen (NO_x) from diesel exhaust emissions. The concentration of ozone (O₃) present in the exhaust that underwent a nonthermal plasma treatment process inside the wESP was also measured. A nonroad diesel generator operating at varying load conditions was used as a stationary diesel emission source. The DPM mass analysis was conducted by means of isokinetic sampling and the DPM mass concentration was determined by a gravimetric method. An electrical low-pressure impactor (ELPI) was used to quantify the DPM number concentration. The HC compounds, n-alkanes, and polycyclic aromatic hydrocarbons (PAHs) were collected on a moisture-free quartz filter together with a PUF/XAD/PUF cartridge and extracted in dichloromethane with sonication. Gas chromatography (GC)/mass spectroscopy (MS) was used to determine HC concentrations in the extracted solution. A calibrated gas combustion analyzer (Testo 350) and an O₃ analyzer were used for quantifying the inlet and outlet concentrations of CO and NO_x (nitric oxide [NO] + nitrogen dioxide [NO₂]), and O₃ in the diesel exhaust stream. The wESP was capable of removing approximately 67–86% of mass- and number-based DPM at a 100% exhaust volumetric flow rate generated from 0- to 75-kW engine loads. At 75-kW engine load, increasing gas residence time from approximately 0.1 to 0.4 sec led to a significant increase of DPM removal efficiency from approximately 67 to more than 90%. The removal of n-alkanes, 16 PAHs, and CO in the wESP ranged from 31 to 57% and 5 to 38%, respectively. The use of the wESP did not significantly affect NO_x concentration in diesel exhaust. The O₃ concentration in diesel exhaust was measured to be less than 1 ppm. The main mechanisms responsible for the removal of these pollutants from diesel exhaust are discussed.     

Fine Particle Removal Performance of a Two-Stage Wet Electrostatic Precipitator Using a Nonmetallic Pre-Charger

ABSTRACT

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 μm 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/m2 Zukeran, A., Ikeda, Y., Ehara, Y., Matsuyama, M., Ito, T., Takahashi, T., Kawakami, H. and Takamatsu, T. 1999. Two-Stage-Type Electrostatic Precipitator Re-entrainment Phenomena under Diesel Flue Gases. IEEE. Trans. Ind. Appl, 35: 346–351. [Crossref], [Web of Science ®] , [Google Scholar].

IMPLICATIONS Current two-stage electrostatic precipitators (ESPs) have several technical problems such as a drop in collection efficiencies by small-particle re-entrainment during rapping and corrosion of metallic electrodes of the ESPs by corrosive gases. This paper evaluates a novel two-stage ESP that uses a nonmetallic pre-charger and water film collection plates to avoid the above mentioned problems of other ESPs. This ESP can be used not only for industrial applications but also for residential purposes because it has a high removal performance for fine particles with low ozone generation and maintains its efficiency due to the continuous cleaning of the collection plates with water film.     

A Model to Predict the System Performance of an Electrostatic Precipitator for Collecting Polydisperse Particles

Abstract

This paper presents a model for predicting the performance of an electrostatic precipitator (ESP) for collecting polydisperse particles. The particle charge was obtained by modifying Cochet’s charge equation; the particle size distribution was approximated by a lognormal function; and then the statistic method of moments was employed to obtain a set of the first three moment equations. The continuous evolution of the particle size distribution in an ESP is easily taken into account by the first three moment equations. The performance of this model was validated by comparing its predictions with the existing data available in the literature. Effects of the particle size distribution on the ESP performance were examined, and the results indicated that both overall mass and number efficiencies are lower for inlet particles with a larger mass median diameter and a higher geometric standard deviation. The methodology introduced may be applied to develop design criteria and determine optimal operating conditions of an ESP for improving the collection efficiency of the submicron particles.     

Precipitator Equipment

The purpose of this section is to set forth the main elements and features of the equipment used for the collection of fly ash. Hardware and structural details of a routine nature having little or no bearing on basic performance are outside the scope of the paper.     

Precipitator Design

The various design philosophies and methods used in the engineering design of precipitators for fly ash are reviewed and assessed in light of current stringent environmental standards. The basic precipitator size and electrical parameters are individually analyzed and related to particle and flue gas properties. Actual precipitator design practice is illustrated by data for a wide cross section of power plant installations.     

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.     

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.     

Some Factors in Minimizing Precipitator Costs

This paper is directed to those individuals involved in design of electrostatic precipitators. The Deutsch-Anderson model is usually employed by industry for the design of electrostatic precipitators. The so-called process design variable in this approach is the total capture area in the precipitator. Unfortunately, little is available on the equipment design of this unit, i.e., the geometric arrangement of the plates that constitute the capture area and the external dimensions of the physical structure that houses the precipitator components. These are important economic considerations, and it is to this subject that this paper is directed. It is relatively easy to predict equipment costs for “off-the-shelf” electrostatic precipitators; it is more difficult to closely predict the cost for a custom-made unit, which is more often the case encountered in practice. Once the capture area is calculated, the total precipitator cost becomes a strong function of the outer casing and outer accessories of the physical system. In this paper, a model is presented that can help minimize precipitator cost. An illustrative example complements the development of the model.     

Advanced Electrostatic Collection Concepts

Of the advanced electrostatic collection concepts studied, those employing water droplets or filters have demonstrated enhanced performance and should be considered for future applications. Electrostatic collection with water drops shows high removal efficiencies for 0.5 µm particles which are difficult to capture. Electrostatic collection with filters shows the potential for operation at either lower pressure drops or higher filtration rates.