Collection efficiency of electrostatic precipitators by numerical simulation

A numerical simulation code has been developed to analyze the coupled electro-aerodynamic phenomena within an electrostatic precipitator operated in both the positive and negative corona modes. The space charge and current densities, as well as the static and space charge components of the electric field, are first obtained using the known cross-sections for ionization and negative ion formation. Data includes the effects of fluid flow. The above quantities (except the static field) are then corrected in a subsequent iteration for the presence of dust particles of known size distribution, charges predominantly by the field charging mechanism. The precipitator collection efficiency is calculated from the trajectories of the charged dust particles. For theory comparison, the collection efficiencies of model precipitators were measured by light-scattering techniques using oil drops as test particles. The variation of efficiency with wire-to-wire spacing will be discussed as a case study.     

The effects of corona electrode geometry on the operational characteristics of an ESP

Experiments were run in a pilot scale electrostatic precipitator (ESP) to determine the effects of corona wire-to-wire spacing on the operating conditions. Tests were run, using a reentrained low sulfur fly ash at both hot- and cold-side conditions. The effects of varying wire-to-wire spacing were determined. Results are given which show that varying wire spacing at cold-side conditions has little operational effect on the ESP while improved efficiency can be obtained at hot-side (low resistivity) conditions by reducing wire spacing. The increased efficiency results from a higher average operating voltage. The effects of back ionization are clearly demonstrated by a set of experiments in which dust was selectively removed from the wires or plate. These tests show that the lower operating voltage caused by back ionization is a combined effect of high resistivity dust on both the wires and plate.     

Tubular electrostatic precipitators of two-stage design

Although the “two stage” electrostatic precipitator concept was first developed in 1910, until recently most of their use was confined to in-plant air cleaning. In the last decade, plate type designs have been modified to make them suitable for industrial applications involving organic emissions, most notably in asphalt saturating, plastic curing, food processing, printing, textile finishing, and heat treating industries. However, plate type designs are inadequate in applications where very high loading and/or high particulate content are involved.Tubular design with wider spacing and higher voltages incorporates the best features of both the single stage and two stage type precipitators. These units have been used in molybdenum roasting, zirconium calcining, ammonia scrubbing of oxides of sulfur, meat broiling, foundry exhaust, etc.The paper discusses design parameters, field test data, and operating data on these units. Comparisons with single-stage type precipitators are made wherever applicable. Advantages and disadvantages of two stage precipitators are also discussed.     

Rapping systems for collecting surfaces in an electrostatic precipitator

Dust build-up on adjacent surfaces is often an unwanted effect in industrial processes handling pulverized materials. In the case of electrostatic precipitation, build-up of dust on the collecting surface is the purpose of the process. Removal of this build-up is sometimes difficult, but necessary to maintain the continuity of the process. Dry-process methods for dust removal include rapping by impact.It is the intent of this paper to provide information on the general subject of dust removal from plates, on rapping systems used in electrostatic precipitators, and on measurements and order of magnitude of rapping forces.Rapping acceleration test results obtained in the laboratory are expanded to cover full-scale collecting surface plates.Effects of plate design, plate height and length, method of support, rapping, hammer weight, and lift are analyzed.     

Design of high intensity ionizer—electrostatic precipitator systems

This paper presents the results of a comprehensive set of analytical and experimental studies concerning the application of high intensity ionization technology to retrofit a flyash electrostatic prepitator.Described in detail are the computer-aided methods of performing functional design for ionizer-precipitator systems, selected results of ionizer-precipitator flow model tests, and the chosen physical retrofit. Also presented is the projected performance of an ionizer-precipitator system, derived from two independent design procedures which take into account the effect of space charge and gas velocity distribution on system performance.Results of these studies confirm that high intensity ionizer technology can be applied to significantly improve the collection efficiency of conventional wire-plate electrostatic precipitators.     

Particulate collection in a high temperature cyclone

A new particle collection technique is analyzed and presented for its potential application in a high temperature, high pressure gas cleaning system. The technique is based on the collision and the aglomeration phenomena among the coal-ash particles when the cyclone is operated near the coal-ash fusion temperature. The percent increase of agglomeration rate is estimated by mathematical modeling for particles smaller than five microns in diameter. Particulate collection efficiency with or without agglomeration is presented. Experimental results in a high temperature cyclone are presented. The output dust loading varied from 0.025 to 5 grains per cubic foot (0.057–11 g/m³) as the input dust loading is increased from 4 to 35 grains per cubic foot (9.2–80g/m³) of gas flow.     

Electrostatic enhancement of moving-bed granular filtration

The electrostatic granular filter developed by Combustion Power Company combines a moving-bed filter with an electrostatic grid for high-efficiency collection of particulate. The coulomb charge occurring naturally on particulate from combustion or other processes in which tribocharging occurs is sufficient such that imposition of an electric field substantially enhances mechanical collection in the packed filter. A mathematical model is presented which accounts for the major parameters affecting electrostatic performance. Experimental data developed in an industrial unit retrofitted with electrostatic equipment and a pilot-scale filter demonstrates dramatic improvement in opacity and outlet loading; the industrial unit achieved 99% collection efficiency with less that 4 Iwd (1kPa) pressure drop. Data from the pilot-scale filter has shown that collection of micron-size particulate is especially enhanced with the electric field and has confirmed the 99% overall collection capability demonstrated in the industrial unit. The design of a 420,000 acfm (1200 m³/min) system being installed at Weyerhaeuser Company's Longview, Washington plant is also discussed.     

Control of fine particle emissions with wet electrostatic precipitation

This paper presents the results, both pilot and full-scale, of experience with the application of wet electrostatic precipitation technology for the control of fine particle emissions from industrial processes. Performance data involving the collection of such difficult-to-clean emissions as a sulfuric acid mist and recovery boiler salt fume are presented. The measured wet precipitator performance parameters (e.g., SCA, effective migration velocity) are compared to those reported in literature for dry precipitators operating on equivalent processes. Particular emphasis is directed toward examining wet electrostatic precipitator performance levels in controlling very high concentrations of submicron fume with the accompanying problem of space charge corona quenching. Relationships between these performance data and specific wet precipitator operating parameters such as operating voltage and current density are also examined.     

Precipitator upgrading and fuel control program for particulate compliance at Pennsylvania power and light company

Two cold side electrostatic precipitators handling low sulfur eastern bituminous coal flyash were upgraded from 94% to 99.4% efficiency in order to meet a particulate compliance limit of 0.1 lb/MMBtu (43 ng/J).The comprehensive upgrading was the result of a 2 year research and testing program during which various aspects of flue gas conditioning, electrical energization, rapping, coal quality control, and gas distribution were independently tested on three similar precipitators. The ensuing upgrading program included the installation of hardware for maximum performance and reliability as well as flue gas conditioning and a fuel ash control program.A cost and reliability analysis of the upgrading program as compared with the installation of an additional series precipitator is included.     

Condensational enlargement as a supplement to particle control technologies

An experimental study has been undertaken at the Colorado School of Mines to evaluate the feasibility of using condensational enlargement to enhance the collection efficiency of existing dust control technologies for sub-micron particulates. Although the study was begun to consider the problem of respirable dust control in the underground coal mine environment, this enhancement technique shows promise for a more general class of applications. In the experiments to be reported on, particulates are injected into a continuous flow thermal diffusion chamber and subjected to varying environmental conditions; saturation ratio (rel. humidity), temperature, and residence time were the parameters. The results show that condensation on a dust nucleus (≈0.1 micron) can bring it to a size of several microns in a sufficiently short period of time as to indicate engineering applicability of the concept.     

Precipitator energization utilizing an energy conserving pulse generator

The performance of a conventional two-electrode type precipitator collecting high resistivity dust can be improved by pulse energization. Pulses of a suitable duration and repetition rate superimposed on the DC voltage permit higher peak-voltage without sparkover, improve particle charging and current distribution, and allow independent regulation of the precipitator voltage and current. A considerable quantity of energy, however, is required for each pulse to charge the precipitator to the pulse voltage level. For high pulse voltages only a minor part of this energy is necessary for the discharge current in the precipitator. For reasons of economy, the recovery of the energy stored in the precipitator capacitance during each pulse is therefore extremely important for pulse energization of large precipitators. An energy conserving pulse generator with pulse transformer, a pulse initiating switch element, and a feed-back diode for the energy recovery is described. The design and the instrumentation of a pilot precipitator specially developed for the comparison of different kinds of precipitator energization are outlined. The ability of the applied pulse energization system for controlling the corona discharge current independently of the precipitator voltage is demonstrated. Further, results from field tests showing the performance of the system under operating conditions are presented.     

Evaluation of the George Neal electrostatic precipitator

The Electric Power Research Institute (EPRI) is currently supporting a major research program characterizing the performance of high efficiency electrostatic precipitators (ESP). One such effort evaluating the George Neal ESP of Iowa Public Service Company is described in this paper.Results show that under well tuned conditions the ESP overall collection efficiency was 99.7% at a specific collecting area of 745 ft²/kacfm (147 m²/m³/s) (520 MW) with associated mass concentrations of 0.025 lb/10⁶ Btu and stack opacity of 4.6%. The boiler outlet size distribution was found to be bimodal with submicron and large particle peaks at 0.2 and 5 microns diameter, respectively. Consequently, an apparent bimodal fractional efficiency curve results with efficiencies of 99.6%, 98%, and 90% measured for 20, 2 and 0.2 micron diameter particles, respectively. Rapping reentrainment losses were found to be insignificant except during episodes of high ash hopper levels resulting from a malfunctioning ash removal system, when large rapping puffs were observed. In addition, outlet emissions increased dramatically to 0.08 lb/10⁶ Btu (34 ng/J) during these periods, suggesting that an emission level more representative of daily operation lies somewhere between 0.025–0.08 lb/10⁶ Btu (10.8–34 ng/J).     

Pressure drop in electrostatic fabric filtration

Fabric filtration systems have been employed in industry for over a century with relatively few technological modifications. However, with the recent substantial increase in energy costs, conservation in energy consumption has become vitally important. As a result, the filtration systems of yesteryear may not be the best approach for future applications. Recently, an external electrical field was considered in fabric filtration of industrial dust with very promising initial results. An increase in the collection efficiency, particularly for fine particulates, and a decrease in the pressure drop was observed. In this paper the further results of an experimental program in the investigation of pressure drop in the electrostatic fabric filtration in industrial dust control are presented. The basic apparatus, a bench-scale electrostatic fabric filtration system, creates a representative dust cake under specific conditions of operating parameters and charge levels. The results clearly indicate that filter and dust cake resistance, or pressure drop, decreases substantially with the increased electrostatic field strength for all industrial dust samples tested, regardless of fabric type and other relevant parameters.     

Fabric filters versus electrostatic precipitators

Control of particulate emissions from pulverized coal fired steam generators is becoming a significant factor in the siting and public acceptability of large coal burning power plants. The particulate emission limit established by the EPA for new coal fired boilers is 0.03 lb/10⁶ Btu (13 ng/J) Possibly more restrictive than this is the state of New Mexico's particulate regulation which calls for no more than 0.05 lb/10⁶ Btu (22 ng/J) total, and no more than 0.02 lb/10⁶ Btu (9 ng/J) less than 2 microns in diameter. This paper will evaluate the effect of these stringent limitations on the technical feasibility and economics of dry particulate removal. Electrostatic precipitators have been the dominant particulate collection device in the electric utility industry for many years because of their low capital and operating cost. However, increasingly stringent emission standards have led to substantially higher costs for precipitators. These costs have increased sufficiently for fabric filtration to become a competitive alternative in achieving cost effective control. This paper will compare the economics and performance of fabric filtration with respect to conventional electrostatic precipitators. The paper will also address the preliminary evaluation procedures that should be followed in order to select the appropriate device for new or existing coal-fired boilers.     

A comparison of concentrations of polycyclic aromatic compounds detected in dust samples from various regions of the world

Settled house dust can be a source of human exposure to toxic polycyclic aromatic hydrocarbons (PAHs) through non-dietary ingestion and dermal contact. Information regarding the concentrations of various contaminants in house dust would be useful in estimating the risk associated with exposure to these compounds. This study reports on the surface loading, variability and distribution of PAHs in settled house dust collected from homes in three locations: Sumgayit, Azerbaijan; Shanxi Province, China; and southern Texas, United States. The highest PAH floor surface loadings were observed in China, followed by Azerbaijan and Texas. Median concentrations of high molecular weight (four ring and larger) PAHs ranged from a low of 0.11 microg/m(2) in Texas, to 2.9 microg/m(2) in Azerbaijan and 162 microg/m(2) in China. These trends in total surface loading and relative carcinogenicity indicate that the risk of health effects from exposure to PAHs in house dust is highest in the Chinese population and lowest in the Texas population. As anticipated, variability among dust samples from different houses within the same region was high, with coefficients of variation greater than 100%. Alkylated PAHs comprised 30-50% of the total mass of PAHs. Based on a comparison of the composition of specific components, PAHs in China and Azerbaijan were determined to be derived mainly from combustion sources rather than from unburned fossil fuels such as petroleum. These results, coupled with ongoing investigation of appropriate PAH exposure biomarkers in humans, will guide future efforts to identify ways to reduce exposures in the study areas.     

Critical factors in assessing exposure to PBDEs via house dust

Assessment of indoor exposure to polybrominated diphenyl ethers (PBDEs) requires a critical examination of methods that may influence exposure estimates and comparisons between studies. We measured PBDEs in residential dust collected from 20 homes in Boston, MA, to examine 5 key questions: 1) Does the choice of dust exposure metric-e.g., concentration (ng/g) or dust loading (ng/m2)-affect analysis and results? 2) To what degree do dust concentrations change over time? 3) Do dust concentrations vary between rooms? 4) Is the home vacuum bag an acceptable surrogate for researcher-collected dust? 5) Are air and dust concentrations correlated for the same room? We used linear mixed-effects models to analyze the data while accounting for within-home and within-room correlations. We found that PBDE dust concentration and surface loading were highly correlated (r=0.86-0.95, p<0.001). Average dust concentrations did not significantly differ over an 8-month period, possibly because home furnishings changed little over this time. We observed significant differences between rooms in the same home: PBDE concentrations in the main living area were 97% higher than the bedroom for decaBDE (p=0.02) and 72% higher for pentaBDE (p=0.05). Home vacuum bag dust concentrations were significantly lower than researcher-collected dust and not strongly correlated. Air (vapor and particulate phase) and dust concentrations were correlated for pentaBDE (p=0.62, p<0.01), but not for decaBDE (p=0.25). In addition, potential markers of BDE 209 debromination (BDE 202 and the BDE197:BDE201 ratio) were also observed in household dust samples. One vacuum bag sample contained the highest concentrations of BDE 209 (527,000 ng/g) and total PBDEs (544,000 ng/g) that have been reported in house dust.     

Aerosol filtration by a cocurrent moving granular bed: Penetration mechanisms

An experimental apparatus has been constructed to investigate aerosol filtration by a cocurrent moving granular bed filter (CMGBF). In a CMGBF, aerosol passes downward through a descending bed of dirty granules while clean granules are added continously at the bed top. The effect of an intergranular dust deposit on penetration in a CMGBF was investigated in a 2⁴ factorial experiment in which the control variables were superficial gas velocity (100 and 250 mm/s), bed depth (130 and 230 mm), granule size (2.1 and 2.7 mm), and the intergranular dust deposit expressed as percent by weight of collected dust in the bed (1% and 5%). All tests were conducted at ambient temperature and pressure using resuspended utility boiler fly ash as the test aerosol.Mass penetration data identified two major penetration mechanisms: (1) straight through penetration and (2) reentrainment due to granular motion. A static, intergranular dust deposit produced high filtration efficiency (>99%) at the low superficial velocity. When the bed was moving, reentrainment of collected dust was significant and accounted for about three-quarters of the penetrating dust. Particle size analyses reveal that the two penetration mechanisms are size dependent. Most particles larger than about one micrometer in diameter that penetrate the CMGBF do so by reentrainment after being collected initially. Virtually no particles smaller than about 0.3 μm in diameter penetrate by reentrainment; when these small particles penetrate, they pass straight through without being collected.     

复合构建湿地运行初期理化性质及氮的变化

研究了在间歇式进水条件下，复合构建湿地系统对氮的去除效果，阐述了湿度要、溶氧、pH等理化因子变化的原因。实验初期，系统尚处于不稳定时期，对N的去除不够理想。随着筚驼逐步步和稳定状态，对KN、NH4^ 、N、NO2^-有明显的去除效果，平均去除率分别为50％、66％和71％。硝化－反硝化作用是氨氮去除的主要途径。从总体上看，有植物系统中硝态氮的出水含量较对照系统同，说明间歇式进水以及植物的存在都有利于硝化作用的发生。实验发现，复合构建湿地在冬季仍能较好地改善水质，是一种有效的水链管理对策，对受污水体水质改善和水生态系统恢复具有重要意义。     

Collection characteristics of a double stage scrubber to eliminate the paint mist from a spray booth

A 4.5 m³/min laboratory paint spray booth was built adopting a double-stage scrubber with heavy oil as the scrubbing liquid. The relationship between the collection efficiency E and the pressure drop ΔP was studied using a cylindrical paper tilter and a Digital Dust Counter. Experimental results indicated that the collection efficiency in this system is in accordance with the collection efficiency equation based on the theory of inertial impaction. With water used as the scrubbing liquid, the factor K is about 0.5 for single-stage scrubbing, while it is 0.6 for double-stage scrubbing. At a pressure drop of 60 mm of H₂O, heavy oil used as the scrubbing liquid gained about 2.0% more in collection efficiency than water. The zigzag baffler equipped for removing heavy oil mist was also effective for the paint mist collector, and 99.1% of E was gained at a pressure drop of 135 mm H₂O.     

Electrical augmentation of granular bed filters

A study of the enhancement of the collection efficiency of granular bed filters by electrical means is reported. By applying electric fields of a few kV/cm (DC or AC) across a bed of insulating granules, the efficiency for submicron charged aerosol is greatly increased, to the point where the efficiency minimum normally observed with such filters in the size range 0.1–1 μm is removed. The performance of such filters is explored as a function of granule size, applied field, face velocity and charge state of the entering aerosol with both a bench-scale flow system (to 40 ft³/min; 1.1 m³/min) and a wind tunnel (to 600 ft³/min; 17 m³/min). Some theoretical estimates are made to identify the most likely physical mechanism of electrical enhancement, and a number of potential application possibilities are discussed.