Abstract
For many industrial processes, dry electrostatic precipitators are employed to remove particulate matter from the waste discharge gases.  Recent concerns however, regarding the emission of fine particulate matter, most of which being respirable, is considered hazardous/injurious to health, has led to a rethink of using wet electrostatic precipitators (WESPs).  These can be either stand-alone units or as a hybrid dry/wet combination to achieve the necessary removal efficiency for these materials, some of which may be gaseous at normal process back end temperatures. As the precipitated material in WESPs is initially retained on a wetted surface, particle reentrainment and resistivity difficulties are mitigated, making them an ideal vehicle for fine particle collection at high efficiency.

A review of the various designs of WESPs will be presented followed by an examination of the advantages and disadvantages of each approach.  Finally, some typical examples of WESPs used as stand-alone units will be reviewed.

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This paper discusses the similarities and differences of the Horizontal Flow Wet Electrostatic Precipitators (HFWESPs) with conventional dry Electrostatic Precipitators (DESPs).  It also discusses the design flexibility; together with technical advantages that the horizontal flow unit offers over vertical flow tubular units for large gas flows, corona quenching and ultra low outlet emissions.

  References and experiences are included for a number of HFWESPs operating on various applications.

  The potential for usage of HFWESPs is increasing.  This type of precipitator is ideally suited to meet the requirement to catch sub-micron particles (PM 2.5), aerosols and fumes after wet scrubbers.  In addition, since it is a horizontal flow devices, it can be considered as a logical last field of an existing DESP, when space restrictions apply, to achieve required emissions not possible through ordinary extensions.

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Abstract
Over the past 10 years, Xcel Energy’s (Formerly Northern States Power) Sherburne County Station has been actively involved in the design and development of a wet electrostatic precipitator for the purpose of flue gas opacity control form at its units 1 and 2 _(1).  The plant has agreed to reduce the stack opacity to less than 20% by August 2001 as compared to the prevailing levels varying about 40% in the early 1990’s.

After a successful demonstration of a full-scale commercial wet ESP prototype on module 107, Sherburned County Generating Station purchased twenty three additional wet ESP modules from FLS  miljo Inc. under standard commercial terms to retrofit all of the modified scrubber modules on two 705 MWe units.  This wet ESP installation is the first of its kind in the world, and presently the largest application of wet ESP technology in coal-fired power plants.

At this writing, additional twenty wet ESP modules have been installed at Sherburne County Generating Station.  Average stack opacity is well below the 20% level required late this year, with only 80% of the wet ESPs in service.

In this paper we discuss the performance of wet ESPs in service at Sherburne County and compare it to well characterized performance data on conventional dry ESP technology to achieve similar results.

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Abstract
Eastman Chemical Company, Tennessee Eastman Division wished to upgrade its existing incineration facilities to meet future anticipated MACT regulations, which will be in effect on September 30, 2002 .  The wastes generated at Tennessee Eastman are primarily high BTU, organic materials that can be effectively managed through combustion.  For disposal of these wastes, Tennessee Eastman operates three incineration units, a liquid chemical destructor and two rotary kilns.  In addition, Tennessee Eastman co-manages some high BTU wastes and biological wastewater treatment sludges in coal fired boilers used to produce steam and electricity for the facility.

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Abstract
Computational Fluid Dynamics (CFD) is a well-established technology to model fluid flow. The incorporation of collection dynamics into the CFD capability could establish CFD as useful tool for electrostatic precipitator (ESP) efficiency predictions.

Eskom has applied CFD extensively for the successful implementation of skew gas flow technology to 8270 Mwe. Present research focuses on the manipulation of the particle distribution, the incorporation of electrostatic field forces and reentrainment. Case studies have been carried out, which demonstrate the working of the model regarding the capability of predicting the efficiency of ESPs.

The paper describes in principle how electrostatic forces and reentrainment have been incorporated into the CFD model.

The ultimate goal of the research is to develop an accurate predictive model, which takes all major collection parameters, including flow dynamics, into account. The model will be used to predict ESP performance and indicate optimal modifications to improve the performance of existing plant, rebuilds and new installations.

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Abstract
The aim of the present study is to investigate the gas flow distribution within a full Electrostatic Precipitators (ESP’s) with special focus on modeling the gas distribution screens present in the inlet and outlet cone sections. The three-dimensional flow distribution and the pressure distribution within an ESP is calculated by standard Computational Fluid Dynamical (CFD) tools giving high resolution of the calculation domain. The gas distribution screens are modeled by a newly developed source term model especially designed for FLS miljo screens. This model is linked to the standard CFD tool package and a principal approach is presented where different screen designs quickly can be evaluated. The ESP geometry studied includes inlet ducting, inlet cone section with gas distribution screens, precipitation section with collecting plates, hopper, outlet cone section with gas distribution screens and outlet ducting. The presented results include validation of the new gas distribution screen model, simulations based on FKS miljo central type ESP inlet design with focus on different screens configurations causing different gas distributions, and investigation of a skew inlet gas distribution versus the traditional uniform distribution. Finally movement of small particles are studied with special focus on re-entrainment in the hopper. The study of different screen configurations clearly demonstrates the flexibility of the FLS miljo screen design making it easy to implement a numerically obtained gas distribution at site. Moreover, calculations with small particles initiated at the bottom of the collecting plates indicates that many of these are re-entrained in the case of a uniform gas distribution and less are re-entrained in the case of a skew distribution with bottom peak velocities at the inlet and top peak velocities at the outlet.

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Abstract
This paper examines dust movement within a precipitator and how it leads to the optimization of gas flow distribution to improve precipitator performance. The information presented is based mainly on experience with precipitators installed in the coal-fired utility industry, some of which was gained recently with the application of Skewed Gas Flow Technology.

The examined utilizes a two-dimensional model of a vertical plane parallel to the gas flow. Analysis of dust movement indicates that it moves horizontally while particles are suspended in the gas flow until it is deposited to form a layer of dust on the precipitator collecting surfaces. When it is dislodged, agglomerated dust falls vertically downward under the influence of gravity until it is dispersed and re-entrained or until it falls below the treatment zone and is truly collected. The downward movement of dust is examined in some detail.

The model, which correctly predicted performance improvement with Skewed Gas Flow Technology, also predicts performance improvement with Precipitator Extraction Technology, which is presented conceptually in this paper.

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Abstract
Knowing, that ESP is a "typical fuzzy system" (S. Masuda-S. Hosokawa in Handbook of Electrostatics, chapter 21), including several phenomena and interactions, a new method was created to handle the complexity of ESP modeling. This method is based on fuzzy logic. The experiences industrial electrostatics and control engineering shows, that fuzzy logic is very effective in the handling of complex systems. In the present paper the applicability of the fuzzy logic based ESP modeling is discussed, where classical models can be applied with great difficulty.

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Abstract
The EPRI ESPert program with Automatic mode has been installed by six utilities at six different plants for monitoring and calculating the expected performance of twelve separate electrostatic precipitators. Every 15 seconds the program provides updated calculations of combustion gas composition, actual gas flow rates, calculated emission rates, calculated stack opacities, ash resistivity, total precipitator power per 1000 acfm and other parameters. Predicted opacities for each chamber of the precipitator and total stack opacity are calculated every 15 seconds. This paper presents examples of data acquired and generated by the program and discusses how the program can be used to ensure and improve electrostatic precipitator operation. The use of the program for controlling SO₃ injection rates is also discussed.

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Abstract
Electrostatic precipitators (ESPs) are efficient collectors of fly ash in a wide variety of industrial applications and operating conditions. The optimum performance is achieved, when the collecting plates and discharge electrodes are efficiently cleaned. The cleaning is critical for high resistivity ashes, where the electrical forces between the dust cake and the plate are high.

The deposited ash is usually removed by accelerating collecting plates and discharge electrodes with tumbling hammers. Cleaning is also associated with re-entrainment of ash to the flue gas causing emission peaks, which can be reduced by e.g. off-flow rapping, stepwise rapping or conditioning agents.

In this paper different cleaning technologies and measures to reduce re-entrainment are discussed in connection with experience from commercial ESP installations. The structural dynamics of different collecting electrode systems and rapping modes have also been investigated in the laboratory.

Heavy-duty bottom rapping is the most efficient rapping design for removal of medium to high resistivity dust cakes in ESPs. The rapping is often combined with reduced electrical forces during the cleaning. This can be achieved by an integrated control of energization and rapping.

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Abstract
One of the most commonly used devices for dust removal from industrial flue gases is the electrostatic precipitator (ESP). Dust particles collected in these devices need to be cleaned off the electrodes in adequate intervals to maintain the function of an ESP. The use of hammers rapping against the electrodes to clean them is common practice.

In principle there are many ways to rap the plates with a hammer as there is an almost infinite choice of locations and orientations to install the hammers. Also the hammers can be used for one single element or a set of electrodes. The hammers are typically tumbling hammers that are fixed to a rotating shaft or cylindrical hammers that are lifted by electromagnetic force. Once lifted the hammers usually fall down gravity driven and hit the electrodes via a suitable anvil.

Another aspect is the suspension of the electrodes, which has to be adjusted to the chosen type of rapping system. This interaction between rapping system and the electrode system influences the intensity of the oscillation of the electrodes. The most suitable way to determine the efficiency of a given configuration is to measure the acceleration of the electrodes in those sections where the dust has to be removed. Various types of rapping systems have been investigated to evaluate their efficiency. Acceleration measurements were performed with a frequency analyser. These measurements were done on collecting electrodes as well as on different types of discharge electrodes. Since electrodes installed inside an ESP are hardly accessible most of the measurements took place on a test rig, which allows measurements of full scale electrodes up to dimension of 15m x 5m for one set of electrodes. To validate the data from these tests additional site measurements were carried out.

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