Wet ESPs for Meeting Boiler MACT - Technical Paper

WET ESPsFORMEETINGBOILERMACT J a n u a r y 2 0 0 4New NESHAPS regulations to bepromulgated in February 2004will require Maximum AvailableControl Technology (MACT) to be applied toa wide range of industrial boilers andprocess heaters. These new regulations mayrequire significant reductions in emissionsof particulate matter (including heavy metals), hydrogen chloride and carbonmonoxide. In addition, restrictions on mercury emissions may be required.One of the predominant control technologyoptions to comply with MACT standards is electrostatic precipitation and, in particular, wet electrostatic precipitation.The purpose of this paper is to introducewet precipitation technology and to discussthe application of this technology to thecurrent MACT standards for boilers andprocess heaters.HISTORYElectrostatic precipitation for the separation ofentrained particulate matter has been recognized asa practical and useful industrial technology since thetime of the first Wright Brothers flight. The installationof the first commercial-scale electrostatic precipitatorwas at a smelter in California in 1907. It is somewhatironic that this first installation was a wet precipitatorbecause today, most electrostatic precipitators aredry, and wet applications are in the distinct minority. THE PRECIPITATION PROCESSWet or dry, the process of electrostatic precipitationworks like this: A high-voltage corona dischargeimposes an electrical charge on an entrained particle.The particle is then “pushed” electrostatically to anadjacent surface of opposite charge. As shown inFigure 1, gases in the vicinity of a high-voltage negative discharge electrode form a plasma (glow)region when the imposed voltage reaches a criticallevel. Free electrons in this region are then repelledtoward the positive (ground) surface, and finally collide with gas molecules to form negative ions.Particles in the area become charged by the ionspresent and are then driven to the positive (ground)electrode by the electric field.FIGURE 1Once captured on the ground electrode the particlesare then removed by either mechanical rapping as ina dry precipitator or by irrigation in a wet precipitator.This process is described analytically by the Deutschequation:E = 1 – e(-A?/Q)Here, E = efficiency, A = area of collecting surface, ? = velocity of particle migration to the collectingsurface and Q = gas flow rate.The Deutsch equation is both an analytical (derivedmathematically) and an empirical equation. In theempirical form ?, the migration velocity, becomesthe effective migration velocity and may be used asa practical parameter for the sizing of precipitatorsusing real world data.Consideration of this relationship shows that as theratio of the collecting area to the gas flow (A/Q)grows the more efficient the precipitator will be. It also shows that ?, the migration velocity, is amajor determinant of the size or efficiency of the precipitator; one cannot be considered without theother. In other words, it is a mistake to compare thesize of a precipitator of one design with the size ofanother precipitator of a distinctively different design.WET OR DRY PRECIPITATIONWet precipitation is characterized by operation atthe dew point of the gas stream. Alternatively, dryprecipitation is characterized by operation above thedew point. Thus, the contrast: in the dry process thecollected material is dry and must be removed fromthe collecting surface by mechanical means. Thismechanical removal is normally known as rapping.In the wet process the material collected is wet and removal is accomplished by flushing or continuous irrigation. The choice between wet and dry is critical. While awet ESP could be applied to virtually any particulatecontrol application, the cost of a dry system is normally less. Thus, wet ESPs are usually applied only when dry systems can’t be used. Most of theseapplications can be characterized as one or more ofthe following:• The gas stream is already wet• Collection of liquid or condensable particulateis necessary• Fires or explosions are a concernExamples of common wet ESP applications are sulfu-ric acid mist collection, wood dryers and any fine-par-ticle source where wet scrubbing for gas absorptionis required.W e t E S P s f o r M e e t i n g B o i l e r M A C T Geoenergy a division of A.H. Lundberg AssociatesTYPES OF WET ESPsWhile dry ESPs are generally all of the same style (i.e. plate-type, cross-flow) wet ESP designs can varywidely. These design variations can be generallygrouped into the following categories: 1) flow direction– up, down or sideways 2) collecting electrode type –tubes or plates and 3) method of irrigation – self-irrigating, continuously wetted or intermittentlyflushed. Knowing when to use a particular configurationis one of the most critical wet ESP application decisions. Comments on this issue are given below.Flow Direction—Because the gas stream entering theelectrostatic zone of a wet ESP will always containboth fine particles and larger water droplets, the sizeclassifying effect of the electrostatic precipitationprocess must be carefully considered. What thismeans is that the relatively large water droplets willbe precipitated first with the smaller particulatebeing precipitated further downstream. Thus there isa big difference in the way up flow and down flowdesigns operate.In the down flow mode water droplets are collectedat the entrance of the electrostatic zone so that thecollecting surface tends to stay wet. In the up flowmode the water collected at the entrance drainsdown so that the top of the collecting surface will befree of liquid water. (In either case the bulk gas willbe 100% saturated, however.)(A note on cross flow designs: These perform thesame classifying effect as up and down flow units.The result is that the leading edge of the collectingelectrodes will stay wet with the trailing edge tending to be free of liquid water.)As a result, down flow designs are best suited forapplications involving sticky/resinous or combustiblematerials as are encountered in wood drying. On theother hand, up flow designs are good for applicationsinvolving inert, liquid or water-soluble particulate;these can be easily flushed off on a periodic basis.The final result is this: In industrial boiler applicationswhere sticky particulate is not an issue, upflow is the choice due to the inherent economy of this arrangement. Collecting Electrodes—There are two basic collectingelectrode types: plates and tubes. Among tube-typewet ESPs the tubes can be round, hexagonal orsquare. Round tubes offer the most uniform electric field distribution with minimum welds andmechanical stresses. Hexagonal and square tubes are very space efficientin that common walls are employed in the tube bundle. They have the disadvantage, however, ofhaving more welds and crevices, making thesedesigns more susceptible to corrosion. Mechanismssuch as crevice and stress corrosion are much morelikely to be a problem in tubes with flat sides. Finally,hexagonal and square tubes will corrode at doublethe rate of round tubes because both sides of thetube are exposed to the process stream.Wet ESPs with any of these tube electrodes could beemployed for boiler service. However, care must betaken to understand the potential for corrosion ifhexagonal or square tubes are being considered.Particular attention should be paid if chlorides arepresent; chlorides in a low-pH environment are particularly aggressive from a corrosion standpoint.Flat plates, whether employed in vertical or cross-flow designs can be effective. They do not, however,offer the same electrostatic performance as tubes.As a result, more collecting area is normally requiredin wet ESPs which utilize plates than in those utilizing tubes. Also, like hexagonal and square tubes,plates are susceptible to corrosion from both sides.Irrigation—Irrigation of collecting surfaces is the waythat collected particulate is removed in a wet ESP.Irrigation can be accomplished in three ways. First,some gas streams will self irrigate because the particulate is already a liquid. Sulfuric acid mist collection is an obvious example of self-irrigation.At the other end of the spectrum are continuouslyirrigated wet ESPs. These devices employ a continuousfilm of water to remove the collected particulate.Either gravity overflow through a network of distributors or hydraulic/pneumatic sprays are usedto keep a film of water flowing on the collecting surfaces. With this approach, treatment of the irrigat-ing water to insure adequate cleanliness is necessary.The third method for irrigating the collecting surfaces is intermittent flushing. Here, the collectingelectrodes are sprayed periodically to rinse away thecollected particulate matter. This approach can besuccessfully employed in any configuration as long asthe particulate matter will rinse off readily. Mostpower boilers are such applications.W e t E S P s f o r M e e t i n g B o i l e r M A C T Geoenergy a division of A.H. Lundberg AssociatesMEETING MACT WITH A WET ESPThe coming MACT regulations will require controlsto most solid fuel fired boilers and process heaters.In many cases, these add-on devices are already inplace and all that operators need worry about is per-forming the necessary testing to show compliancewith MACT.In some of these cases existing control equipmentwill not comply with MACT requirements. In thesecases operators will need to assess whether theyshould improve the performance of the existingequipment, replace it or add on new equipment.Unfortunately, there is no one-size-fits-all solution tothis problem; each situation should be evaluatedindividually.However, in the case of boilers that are already beingtreated with a wet scrubbing system a wet ESP maybe a very attractive option for meeting MACT. Thereare several reasons for this. First, meeting MACT particulate requirements is well within the demonstrated performance capability of wet ESPtechnology. In most instances the wet ESP efficiencyrequirements will be well below 90%, an easilyattainable level.Second, the capital cost of an add-on wet ESP will bein the same range as the cost to add energy toimprove the efficiency of an existing scrubber, if notless. This includes fan upgrades for higher pressuredrops, upgrading electrical systems and the necessaryboiler downtime to do the work. In addition, anadd-on wet ESP will cost less to operate than anupgraded wet scrubber.Finally, the wet ESP can be erected with the boiler inoperation and tied in to the scrubber outlet with abrief outage.CONCLUSION The upcoming MACT requirements may be easierand less costly to meet than one may think. One reason for this is the availability of wet electrostaticprecipitation technology that has been proven fornearly 100 years. In many cases a properly designedwet ESP will be the most cost effective solution tothis regulatory requirement.W e t E S P s f o r M e e t i n g B o i l e r M A C T BELLEVUE OFFICE13201 Bel-Red RoadBellevue, Washington 98005Tel: 425-283-5070e-mail: sales@lundbergassociates.comwww.lundbergassociates.com*Greater than 10 million BTU/hour heat inputSourceParticulateMatter or 0.07 0.20Total SelectedMetals 0.001 0.001HydrogenChloride 0.09 N/AMercury 0.000007 N/ALarge solid fuelfired unit*Small or limiteduse solid fuelfired unitMACT Compliance Levels (lb/MM BTU)