Collection Efficiency Of Electrostatic Precipitator Research Articles

微粉炭火力用電気集じん器の性能に及ぼす操作温度および石炭灰性状の影響

Electrostatic precipitator (ESP) is a major dust collection technology for pulverized coal combustion boilers in Japan. Although the collection efficiency of ESP is over 99%, the performance of ESP is affected by dust properties, especially by the electric resistivity of dust. In Japan, many kinds of coal are imported from countries overseas and used in pulverized coal combustion boiler. The property of coal ash formed by the pulverized coal combustion is different by coal species. It is very important to develop a high performance electrostatic precipitator for various types of coal ash. In this paper, the influence of operation temperature from 363K to 623K on the performance of ESP for pulverized coal combustion boiler is investigated, and the relationship between the coal ash property including electric resistivity and alkalic metal concentration and the collection efficiency of ESP is obtained.

Electrode geometry effects on the collection efficiency of submicron and ultra-fine dust particles in spike-plate electrostatic precipitators

The collection efficiency of electrostatic precipitators for the submicron particles ranging from 0.1 to 1 μm and ultrafine particles smaller than 0. lμm is below the requirements of new PM2.5 emission regulations. In this work, numerical and experimental studies were conducted to examine the effect of discharge and collecting electrode geometries on the ion density and electric field profiles and consequently their effect on the particle surface charge and collection efficiency. The collection efficiency prediction was based on a modified Deutsche's equation after calculation of three dimensional electric field and ion density profiles. Whereas, the particle surface charge was obtained from diffusion and field charging models. Results show that the collection efficiency of fine particles for the spike-type discharge electrode when compared to the conventional wire-type was improved. Experimental validations were conducted on a bench scale electrostatic precipitator for total and partial collection efficiency of particles ranging in size from 0.01 to 20 μm and the results indicated that the model can be effectively applied for prototype design, modification, and scale-up of collecting and discharge electrodes.

Influence of flue gas conditioning on fly ash characteristics

In this technical note an effort has been made to investigate the influence of flue gas conditioning, FGC, on physical, chemical, mineralogical and morphological characteristics of the fly ash. FGC is a technique that involves addition of chemical additives to the flue gas, for increasing the ash collection efficiency of electrostatic precipitators, ESPs. The FGC also helps in reducing the amount of suspended particulate matter, SPM, which is released into the surrounding environment at thermal power stations. It is a well established fact that utilization of the fly ash, as a cementitious material, depends on its pozzolanic characteristics, which in turn pivots on its physico-chemico-mineralogical and morphological characteristics. However, how these characteristics get influenced due to FGC has not yet been reported in the literature. With this in view, ash samples from a coal-fired thermal power station in India, where flue gas conditioning is being employed, were collected and their physico-chemico-mineralogical and morphological properties were examined. Based on these studies efforts have been made to highlight the influence of FGC on ash characteristics.

Laboratory studies of back-discharge in fly ash

The back-discharge is a type of discharge that takes place in the presence of corona discharge and occurs at an electrode covered with a dielectric layer of resistivity higher than about 10 8 Ω m. Back-discharge can be observed in electrostatic precipitators when dust covering the collection electrode has low conductivity. In this paper, the studies of back-discharge generated in ambient air, in point-to-plane geometry with the plate electrode covered with fly ash are presented. The discharge is characterised in terms of its visual forms, current–voltage characteristics, and light emission spectra. Three forms of back-discharge were investigated: glow discharge, streamers, and low-current back-arc discharge. The current of the back-arc discharge was only a few milliamps. The discharge was stabilised by a high series resistance. It was noted that the voltage of ignition of the back-discharge for negative polarity is lower than for a positive one. Spectroscopic measurements of emission spectra provided information on elements present in the discharge column. The elements present in the fly ash, including toxic metals, can be re-entrained into the gas as particles or can be emitted as ions or neutrals during the discharge, and can decrease the collection efficiency of electrostatic precipitators. These elements were detected in the emission spectra. The effect of the discharge on the fly ash layer was also discussed. It was observed that sinter-like leftovers remain in the dust layer after a back-arc discharge.

Electrostatic precipitator collection efficiency and trace element emissions from co-combustion of biomass and recovered fuel in fluidized-bed combustion.

Particle and trace element emissions from energy production have continuously been subject to tightening regulations. At the same time, not enough is known on the effect of different combustion processes and different fuels and fuel mixtures on the particle characteristics and particle removal device operation. In this investigation, electrostatic precipitator fractional collection efficiency and trace metal emissions were determined experimentally at a 66 MW biomass-fueled bubbling fluidized-bed combustion plant. The measurements were carried out at the inlet and outlet of the two-field electrostatic precipitator (ESP) at the flue gas temperature of 130-150 degrees C. Two fuel mixtures were investigated: biomass fuel containing 70% wood residue and 30% peat and biomass with recovered fuel containing 70% wood residue, 18% peat, and 12% recovered fuel. The particle mass concentration at the ESP inlet was 510-1400 mg/Nm3. Particle emission at the ESP outlet was 2.3-6.4 mg/Nm3. Total ESP collection efficiency was 99.2-99.8%. Collection efficiency had a minimum in particle size range of 0.1-2 microm. In this size range, collection efficiency was 96-97%. The emission of the trace metals As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sb, Tl, and V was well below the regulation values set by EU directive for waste incineration and co-incineration.

Loading Characteristics of a Miniature Wire-Plate Electrostatic Precipitator

In this work, in order to investigate the particle loading effects on the performance of an electrostatic precipitator (ESP), simultaneous measurements of the dust cake thickness accumulated on the collection plates, ESP's collection efficiency, corona discharge characteristics, and ozone concentration were conducted experimentally. A laboratory scale single stage wire-plate ESP was used for the aerosol loading test. Two kinds of particulate matter, cement and aluminum oxide (Al 2 O 3 ), were generated by using a Palas Powder Disperser. A displacement meter was used to monitor the dust cake thickness accumulated on the collection plates. A scanning mobility particle sizer was used to measure the particle size distribution and number concentration upstream and downstream of the ESP. Ozone generated by the ESP was sampled 20 cm downstream of the ESP exit and measured with an ozone analyzer. The Dioctyl Phthalate (DOP) was also used as a liquid challenge agent in order to investigate the loading effects of liquid particles on the ESP performance. The results showed that when challenged with cement particles, the ion current decreased with increasing dust cake thickness under a constant electrical field strength. Moreover, the collection efficiency and ozone generated by corona discharge decreased as the loading test progressed. For example, when the dust layer was about 5 mm in thickness, the output current and the ozone concentration decreased about 33 and 44%, respectively, and the collection efficiency (300 nm particle) decreased about 4% at a fixed electrical field strength of m 4.2 kV/cm. However, the ion current increased as aluminum oxide particles deposited on the collection plates. The increase in ozone concentration and aerosol penetration was mainly due to the occurrence of back corona, evidenced by the existence of the caves on the surface of the dust layers. In the case of testing with cement particles, the ion current rises after about 20 min of loading test and then decreases with time, while ozone concentration increases synchronously.

Application of ESP for gas cleaning in cement industry — with reference to India

Electrostatic precipitators (ESP) are used for gas cleaning in almost every section of cement manufacture. Application of ESP is studied, keeping in view Indian conditions. The characterisation of dust emissions has been done for different units, such as rotary kiln and raw mill, alkali by-pass, clinker cooler, cement and coal mill, in terms of exit gas quantity, temperature, dew point, dust content and particle size. It is seen that all these characteristics have a wide range of variance. The ESP system must effectively deal with these variations. The fundamental analytical expression governing the performance of ESP, i.e. the Deutsch equation, and that for particle migration velocity, were analysed to predict the effect of major operating parameters, namely particle size, temperature and applied voltage. Whereas the migration velocity (and the efficiency) varies directly with the particle size, it is proportional to the square and square root of applied voltage and absolute temperature of the gas, respectively. The increase in efficiency due to temperature is not seen in dc based ESP, perhaps due to more pronounced negative effect on the applied voltage due to the increase in dust resistivity at higher temperatures. The effect of gas and dust characteristics on the collection efficiency of ESP, as seen in the industrial practice, is summarised. Some main process and design improvements effectively dealing with the problem of gas and dust characteristics have been discussed. These are gas conditioning, pulse energisation, ESP-fabric filter (FF) combination, improved horizontal flow as well as open top ESP. Generally, gas conditioning entails higher operating and maintenance costs. Pulse energisation allows the use of hot gas, besides reducing the dust emission and power consumption. The improved horizontal flow ESP has been successfully used in coal dust cleaning. The open top or vertical flow ESP has a limitation on collection efficiency as it provides for only one electric field.

Experimental study of electrostatic precipitator performance and comparison with existing theoretical prediction models

A laboratory-scale single-stage electrostatic precipitator (ESP) was designed, built and operated in a wind tunnel. As a first step, a series of experiments were conducted to seek the operating conditions for increasing the particle collection efficiency by varying basic operating parameters including the wire-to-plate spacing, the wire radius, the air velocity, the turbulence intensity and the applied voltage. As the diameter of the discharging wires and the wire-to-plate spacing are set smaller, the higher collection efficiency has been obtained. In the single-stage multiwire ESP, there exists an optimum wire-to-wire spacing which provides maximum particle collection efficiency. As the air velocity increases, the particle collection efficiency decreases. The turbulent flow is found to play an important role in the relatively low electric field region. In the high electric field region, however, particles can be deposited on the collection plates readily regardless of the turbulence intensity. The experimental results were compared with existing theories and Zhibin and Guoquan (Aerosol Sci. Technol. 20 (1994) 169–176) was identified to be the best model for predicting the ESP performance. As the second step, the influence of particle contamination at the discharging electrode and at the collection plates were experimentally measured. The methods were sought for keeping the high collection efficiency of ESP over elapsed time by varying the magnitude of rapping acceleration, the time interval between raps, the types of rapping system (hammer/vibrator) and the particle re-entrainment. The rapping efficiency and the particle re-entrainment were increased with increasing magnitude of rapping acceleration and time interval between raps. However, when the thickness of deposited fly ash layer is sufficiently high, the concentration of re-entrained particles starts decreasing abruptly due to the agglomeration force which can interact among deposited particles. The combined rapping system is found more effective for removing deposited particles than the hammer rapping system only.

A Microwave Technique for Online Monitoring and Control of Dust Layer Thickness Inside Electrostatic Precipitators

A procedure is proposed for real time non-intrusive measuring of the thickness of the ash layer on the plates of electrostatic precipitators (ESP) commonly used in energy plants. The method is based on the measurement of the phase shift of a surface electromagnetic wave, which propagates on the plate under test, equipped with input-output electromagnetic couplers.It is shown that the phase shift depends on the mean thickness of the ash layer. Results are reported that were obtained from a laboratory-scale experimental ESP model, using a microwave test signal at 2500 MHz.The experiments confirm the possibility of using the proposed technique for on line determination of the collection efficiency of ESP and also to prevent electrostatic discharge.

A Study on the Electrohydrodynamic Flow in a Rectangular Impactor with Positive Corona Discharge

ABSTRACT Visualization experiments and numerical simulations were performed to investigate the electrohydrodynamic flow in a rectangular slit impactor. Smoke was used to trace the How, while How visualization was carried out using a pulse laser sheet technique. This study focuses on the interactions between the impactor airflow (i.e., primary flow) and the electric wind due to the positive polarity of the corona discharge. The Reynolds number for the primary flow varies from 500 to 2500, and the applied voltage on the corona discharge wire ranges from 10 kV to 12 kV. Flow visualization shows that the positive corona discharge produces a stable electrohydrodynamic flow, and that the streamlines between the throat exit and the impaction plate become narrow with decreasing electric Froude number, FR. The axial velocity at the throat exit is accelerated due to the reduced flow cross-sectional area. Thus, as a result of the high electric body force, the shape of the streamline resembles a jet. It can be predicted that the critical Stokes number will decrease when a positive corona discharge is generated in the rectangular impactor. Furthermore, the electric wind may play a role in enhancing the collection efficiency of electrostatic precipitators. Finally, the experimental results and numerical simulations exhibit good qualitative agreement.

97/02094 Development of a stack plume opacity index for subbituminous coal-fired utility boilers

Powder River Basin subbituminous coals were burned using conventional and low-NO{sub x} combustion conditions in a drop-tube furnace equipped with a multicyclone ash collection device. Fine ash fractions (< 2 {micro}m in diameter) collected during the tests were analyzed using computer-controlled scanning electron microscopy (CCSEM). Advances in particulate sample preparation methods enabled the CCSEM analysis of individual ash particles with submicron diameters as small as 0.1 {micro}m. The fine ash samples produced from the conventional combustion of coal consisted of discrete spherical particles, whereas particle agglomerates were characteristic of the low-NO{sub x} ash samples. Particle-size distributions of the low-NO{sub x} fine ash fractions were coarser because of the agglomeration. Theoretical light-scattering calculations indicate that for a given coal, the ash produced in low-NO{sub x} conditions causes less opacity as compared to conventional combustion conditions. The following phases were abundant in the ashes: Ca aluminosilicate, Ca aluminate, aluminosilicate, silica, (Ca, Mg)O, CaSO{sub 4}, Na{sub 2} SO{sub 4}, and (Na, K)Cl. Primary mechanisms that produced the fine ash include the thermal metamorphism of small (0.1 to 5 {micro}m) mineral grains and the vaporization and subsequent condensation of organically bound Na, Mg, and Ca, Empirical equations for estimating the concentration of fine ashmore » produced from burning subbituminous coals were formulated into an opacity index based on CCSEM coal mineral and fine ash analyses and on drop-tube furnace testing results. The effects of ash electrical resistivity on electrostatic precipitator collection efficiency are also considered in the index.« less

A SEMI-EMPIRICAL APPROACH TO PREDICT THE TOTAL COLLECTION EFFICIENCY OF ELECTROSTATIC PRECIPITATORS

Using empirical data for the effective migration velocity, the Deutsch-Anderson equation has been widely used for calculation of the total collection efficiency and design of electrostatic precipitators (ESPs). Based on a normal particle size distribution, the present paper develops a semi-empirical equation for the total collection efficiency of ESPs. The equation is then simplified and extended to other particle size distributions. The simplified equation shows that the total collection efficiency increases with increasing mass median diameter of particles and with increasing electrical field strengths. The effective migration velocity is found to be directly proportional to the mass median diameter and to the electrical field strengths for particle charging and collection. Moreover, an example is given to illustrate how to use the present simplified model in the design of ESPs.

Electrical Agglomeration of Aerosol Particles in an Alternating Electric Field

A laboratory scale test system has been used to study the electrical agglomeration of charged aerosol particles to increase the collection efficiency of electrostatic precipitators. The system consists of test aerosol generator, aerosol charger, agglomerator chambers, and aerosol measurement equipment. Air atomizing nozzles and the TSI six-jet atomizer have been used as the test particle generators. The test particles have been charged by a corona discharge. Two types of agglomerator chambers have been investigated. In one agglomerator the gas flows between two parallel plates, across which the alternating high voltage is applied. The other agglomerator is a quadrupole structure with cylindrical electrodes positioned between the grounded plates. Particle concentration and size distribution measurements have been carried out downstream of the agglomerator with agglomerator voltage on and off. Particle concentrations and size distributions have been measured with differential mobility analyzer (DMA) and a Berner Low pressure impactor. Agglomeration caused about a 4%-8% decrease in the fine particle concentration when the total mass concentration was between 1 and 2 g/m[sup 3]. There was no difference between the results measured with the parallel plate and the quadrupole agglomerator. 19 refs., 7 figs., 1 tab.

Fly ash from a coal power plant: Correlation of elemental and structural composition with electrostatic precipitator collection efficiency

Fly ash samples produced during the combustion of a South African bituminous coal (AMCOAL), and collected at the inlet and outlet of a pilot ElectroStatic Precipitator (ESP) with a standard isokinetic integral probe, are analyzed for elemental bulk and surface and phase compositions. The ESP was operated with either continuous or pulsed energization over a wide range of Specific Collection Areas (SCA) and electrode spacing. The collection efficiencies varied from 92.5% to 99.8%. Relative enrichment of volatile species is evident upon increase of collection efficiency, even though the total amount decreases. Quantitative XRD analysis indicates the concentration of quartz, magnetite and lime decrease from the inlet to the outlet of the ESP.

An asymptotic description of electrostatic precipitation of charged particles in turbulent flow

The collection efficiency of electrostatic precipitators is calculated from the transport equations for constant eddy diffusivity and mean velocity. For the non-collecting wall, a zero-flux boundary condition is used, whereas for the collecting wall, a radiation-type boundary condition is used, which is derived from the constant-flux solution of the turbulent diffusion equation. Two approximate solutions are derived, one for small values of the ratio between precipitation and friction velocity and one for large values of the velocity ratio. The two approximate solutions derived are shown to be in good agreement with the numerical solution of the transport equation. An increase in the turbulent friction velocity is shown to enhance collection efficiency for small velocity ratios, but reduce it for large ones. An indication of the effect of dust layer roughness on the performance of an electrostatic precipitator is also considered and it is shown that for small values of the velocity ratio collection efficiency increases with roughness height, but remains unaffected by it for higher values of that ratio.

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.

Mechanisms Involved in Fly Ash Precipitation in the Presence of Conditioning Agents - A Review

Conditioning agents are usually used to improve the collection efficiency of electrostatic precipitators by causing a reduction in the resistivity of the fly ash. However, certain conditioning agents may have other effects including changing the electrical breakdown strength of the flue gas, space charge enhancement of the collecting field, increasing the mean particle size, changing the adhesion and cohesion properties of the ash, and changing the acid dew point of the flue gas. These mechanisms are reviewed and related to results reported in the literature for ammonia and triethylamine, two conditioning agents which appear able to affect one or more of the mechanisms mentioned above, in addition to the resistivity.

Advances in particle sampling and measurement

Seventeen papers were presented on improved instruments and techniques for measuring the size, number, and composition of particles in process streams. Recent studies on cascade impactors, cyclones, and diffusion batteries were reported. Several papers discussed methods for making in situ particle size or mass determinations through light scattering. Systems that have been developed to determine the collection efficiency of electrostatic precipitators, scrubbers, and bag filters were described.

Sonic agglomeration: a promising method of enhancing the collection efficiency of electrostatic precipitators

Experimental and theoretical studies of the effect of sound on the agglomeration of airborne particulates have been carried out to determine the feasibility of using sonic agglomeration as a means of enhancing the collection efficiency of electrostatic precipitators. Experiments were conducted using carbon black, kaolin clay, white lead, and fly ash aerosols. In each case, it was found that the rate of agglomeration increases with increasing concentration, sound pressure level, and exposure time, and with decreasing particle size. For each aerosol, there appears to be an optimum frequency at which the rate of agglomeration passes through a maximum. Experiments have also been conducted to determine if the efficiency of electrostatic precipitators, could be increased by pretreating the aerosol acoustically. Depending on the sound field conditions, the overall collection efficiency for the white lead aerosol was increased from 75% (on a mass basis) for the untreated aerosol up to as much as 90%. On the basis of these experimental results, it appears that sonic agglomeration has considerable potential for elimination of ultrafine particles prior to subsequent treatment using a conventional collection device such as the electrostatic precipitator.

Fractional collection efficiency of electrostatic precipitator for open hearth furnace trace metal emissions

Fractional collection efficiency of electrostatic precipitator for open hearth furnace trace metal emissions