Wet ESP

Periodic washing of the collecting tubes is done to remove the deposited particles and thus the active surfaces ate not masked. 

These features make the wet ESP a device better than the dry ESP, and one may find the higher initial cost worth it due to the improved performance. The life of downstream equipment improves due to better removal of erosive, corrosive particulate matter lay the wet ESP. 

Any electrode may be damaged if there is a chance of acidic particles with chloride content in the gas stream. In such cases, Hastelloy C-276 electrodes are used. 

Water spray prevents build up of pressure drop on the gas side. A separate filtration arrangement (filter press) for the circulation water maintains the system clean and the collected dust particles can be removed easily as solid cake fiom the filter press. A continuous bleed of circulation water can also be done to control build-up of the TDS. 

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Fig. 29-10. Wet ESP with plate collection electrodes. Source Oglesby, S., ]r., and Nichols, G. B., Electrostatic precipitators in "Air Pollution," 3rd ed., Vol. IV (A. C. Stem, ed.), p. 239, Academic Press, New York, 1977.

Blast furnace exhaust gases Particulate matter Multiple cyclone plus wet scrubber or wet ESP, two-stage wet scrubber... 

Applicable pollutants for this equipment are Particulate Matter (PM), including particulate matter less than or equal to 10 micrometers (/im) in aerodynamic diameter (PM,q), particulate matter less than or equal to 2.5 /im in aerodynamic diameter (PMj,), and hazardous air pollutants (HAPs) that are in particulate form, such as most metals (mercury is the notable exception, as a significant portion of emissions are in the form of elemental vapor). Wet ESPs are often used to control acid mists and can provide incidental control of volatile organic compounds. 

Other Considerations Dust resistivity is not a faetor for wet ESPs, beeause of the high humidity atmosphere whieh lowers the resistivity of most materials. Partiele size is mueh less of a faetor for wet ESPs, compared to dry ESPs. Mueh smaller particles ean be efficiently colleeted by wet ESPs due to the laek of resistivity concerns and the reduced reentrainment (Flynn, 1999). 

When the pollutant loading is exeeptionally high or consists of relatively large particles (> 2 /tm), venturi scrubbers or spray chambers may be used to reduce the load on the ESP. Much larger particles (> 10 /tm) are controlled with mechanical collectors such as cyclones. Gas conditioning equipment to reduce both inlet concentration and gas temperature is occasionally used as part of the original design of wet ESPs (AWMA, 1992 Flynn, 1999). 

Unlike dry ESPs, resistivity of the collected material is generally not a major factor in performance. Because of the high humidity in a wet ESP, the resistivity of particles is lowered, eliminating the "back corona" condition. The frequent washing of the pipes also limits particle buildup on the collectors. 

Another advantage is that wet ESPs can collect sticky particles and mists, as well as highly resistive or explosive dusts. The continuous or intermittent washing with a liquid eliminates the reentrainment of particles due to rapping which dry ESPs are subject to. The humid atmosphere that results from the washing in a wet ESP enables them to collect high resistivity particles, absorb gases or cause pollutants to condense, and cools and conditions the gas stream. Liquid particles or aerosols... 

Wet ESPs add to the complexity of a wash system, because of the fact that the resulting slurry must be handled more carefully than a dry product, and in many cases requires treatment, especially if the dust can be sold or recycled. Wet ESPs are limited to operating at stream temperatures under approximately 80 to 90°C (170 to 190°F), and generally must be constructed of noncorrosive materials (EPA, 1998 Flynn, 1999). 

For wet ESPs, consideration must be given to handling wastewaters. For simple systems with innocuous dusts, water with particles collected by the ESP may be discharged from the ESP system to a solids-removing clarifier (either dedicated to the ESP or part of the plant wastewater treatment system) and then to final disposal. More complicated systems may require skimming and sludge removal, clarification in dedicated equipment, pH adjustment, and/or treatment to remove dissolved solids. Spray water from an ESP preconditioner may be treated separately from the water used to wash the ESP collecting pipes so that the cleaner of the two treated water streams may be returned to the ESP. Recirculation of treated water to the ESP may approach 100 percent (AWMA, 1992). 

References 47 through 58 provide additional information on wet ESPs, design, and scale-up principles, as well as operational guidance. 

Wet ESPs are used in situations for which dry ESPs are not suited, such as when the material to be collected is wet, sticky, flammable, or explosive, or has a high resistivity. Therefore, producer gas streams with tar represent a problem. 

SO3 control is also an increasingly important issue for refinery FCC operators. Regional haze, PMj 5, hazardous air pollutants, and visible stack emissions all in some way relate to SO3. The present level of control achievable in wet scrubbers is limited. However, use of wet ESPs integral to the scrubber has been used commercially. 

Several refiners have recently dismantled older ESPs and built wet gas scrubbers with wet ESPs to remove SO3 at the stack. In these applications, the SCR unit can be designed to handle the catalyst fines as well as full range equilibrium catalyst during an upset condition. 

Wet ESPs have been used extensively in other industries. To date, there are three units installed on ECC units in North America. In all cases, SO3 and PM emissions... 

Anode bake plant wet air pollution control (open top ring furnace with wet ESP and spray tower)... 

Note the modem trend is to use WET ESP with special MOC like Hastelloy-variants for the wetted parts (electrodes). These have higher efficiency and less pressure drop... 

These features can make the wet ESP work better than the dry ESP. 

Another example of a commercial-scale hybrid cold gas cleaning application is provided by the Harbopre up-draft gasification plant in Denmark [33]. Here, the gas is first processed in a water scrubber, with further water/tar aerosol and dust removal being carried out in a wet ESP (Figure 11.7). This combined operation results in the contents of both tar and dust in the cleaned gas being brought to below 25 mg/Nm, at 40 C. 

Figure 11.7 Process scheme for the harboore gas cleaning system with water-based tar scrubber, wet ESP and TARWATC system. ("Source Reproduced from Ref [33])...

Harbo0re X Quench and wet ESP Sedimentation Waste water evaporation Combustion of residues in the plant... 

Wiener Neustadt X Quench and wet ESP Waste water evaporation Disposal of residues... 

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