Available online at ScienceDirect. Procedia Engineering 102 (2015 )

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1 Available online at ScienceDirect Procedia Engineering 102 (2015 ) The 7th World Congress on Particle Technology (WCPT7) Numerical Modelling o ESP or Design Otimization Baoyu Guo a*, Aibing Yu a, and Jun Guo b a Lab or Simulation and Modelling o Particulate Systems, School o Materials Science and Engineering, University o New South Wales, Sydney, 2052, Australia b Exerimental and Research Centre, Fujian Longking Co., Ltd., Longyan, , China Abstract Electrostatic reciitators (ESP) are widely used or collection o articulate matters. In the resent aer, a Comutational Fluid Dynamics (CFD) model, based on Eulerian-Lagrangian ramework, is used to simulate the gas-article low under electric ield. A tyical two-stage ESP is chosen as a case study and its collection erormance is assessed under a series o modiied geometric conditions, including inlet iework, erorated late, addition o low bales, and satial extent o electric ield. The results show that the collection eiciency or PM2.5 can be increased by 20% ater otimization The Authors. Published by Elsevier by Elsevier Ltd. This Ltd. is an oen access article under the CC BY-NC-ND license (htt://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and eer-review under resonsibility o Chinese Society o Particuology, Institute o Process Engineering, Chinese Selection Academy and o eer-review Sciences (CAS). under resonsibility o Chinese Society o Particuology, Institute o Process Engineering, Chinese Academy o Sciences (CAS) Keywords: Particle Flow; Electrostatic Preciitator (ESP); Numerical Modeling 1. Introduction Electrostatic reciitators (ESP) are widely used in coal-ired ower generation lants or removing ine articulate matter. ESPs rove to have a high mass eiciency and the advantages o large handling caacity with low ressure loss. However, the major roblems arise rom the increasing demand or collection eiciency o ine articles (e.g., PM2.5) and urther requirement on device otimization. An industrial ESP is not only structurally comlicated, as shown in Figure 1, but also involves comlicated transort henomena, including electric ield, gas-article low, and strong interactions among them. Firstly the corona is generated at the small needle ti o the emitter electrode, which controls the distribution o the electric * Corresonding author. address: baoyu@unsw.edu.au The Authors. Published by Elsevier Ltd. This is an oen access article under the CC BY-NC-ND license (htt://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and eer-review under resonsibility o Chinese Society o Particuology, Institute o Process Engineering, Chinese Academy o Sciences (CAS) doi: /j.roeng

2 Baoyu Guo et al. / Procedia Engineering 102 ( 2015 ) ield intensity and ion charge in the collection assage [1]. Then non-uniorm electric current induces ionic wind that aects the turbulence level and article diusion and re-entrainment [2,3]. Particles are charged and driven to the collection lates to build u cake layers. Numerical simulations o these rocesses are becoming oular, but most o them so ar have ocused largely on the local corona region or seciic henomena [4-6]. A ull scale ESP rediction tool with all these key henomena considered together is rare. To date, in the ESP design, engineers still rely on the over-simliied Deutsch equation [7]. Following the earlier develoment o an integrated model to describe the major transort henomena [8], the current aer will urther investigate the model alicability in the design otimization o wire-late tye ESP. Nomenclature b d Cc C D D E F g k k 0 m P q q e T t U,u V μ μ t 0 ion ion mobility diameter o article Cunningham correction actor drag coeicient ionic diusivity E-ield intensity orce gravitational acceleration turbulent kinetic energy Boltzmann constant mass o a article ressure article charge unit electron charge temerature time velocity electric otential density gas dynamic viscosity eddy viscosity turbulence eddy requency electric ermittivity ion sace charge density dialectic constant o articles 2. Method The model is develoed based on the ramework o ANSYS-CFX (release 14), combined with user subroutines. The Poison equation o otential and current continuity equation are solved or the electric ield and sace charge density. The Eulerian-Lagrangian method is used or the gas-article low. The k- based Shear Stress Transort (SST) model is chosen or turbulence. The major governing equations are given below. Electric ield 2 V ion / 0 (1)

3 1368 Baoyu Guo et al. / Procedia Engineering 102 ( 2015 ) ( be ) D ) 0 (2) ion ion E V (3) Gas low ( ρu) 0 (4) 2 T ( ρu U) ( P ρk ) [( μ )( U ( U) )] F (5) 3 μ t a1k μ t ρ (6) max( a1ω, SF2 ) Particle low EHD m du dt = CDπd ρ u u ( u u ) πd g(ρ ρ ) qe Fdis (7) 8 6 Particle charging rate [9] dv d q v 3w ( w), v 3w ex( v 3w) 1 2 3w v 1 ( w), 3w v 3w 4 3w v 3w v ( w), v 3w ex( v 3w) 1 (8) where qq e Ed qe b v, w, ion t q, ( w) min( 1,( w 0.475) ) 2 0 d k0t 2 2k T 0 0 Electro-hydra dynamic (EHD) secondary low (or ionic wind) is treated as additional turbulence roductions, which are added in the turbulence equations as sources. This treatment eectively accounts or the enhanced global mixing o gas low, and thus aects the overall low attern to some degree, without considering the detailed local low structures. This, however, has an imlication or the article low modelling. For article low, the eddy interaction model with random walk method is used outside the electric ield [10]. In the electric ield, the turbulent disersion orce is switched o in the resent study, or the ollowing reason: the EHD low in the ESP is believed to adversely aect the overall collection eiciency [11,12]. When the EHD low is treated as turbulence rom macroscoic viewoint, diusion o susended article will be romoted and subsequently the wall collision requency will be dramatically increased, which will lead to over-rediction o the collection eiciency. This is contradictory to the general knowledge o EHD eect as mentioned above. Nevertheless, without resolving the detailed local article characteristics, the current hybrid treatment aears to give statistically sensible redictions o the overall article collection eiciency, which should be suicient or the urose o design otimization based on comarative studies.

4 Baoyu Guo et al. / Procedia Engineering 102 ( 2015 ) Result and discussion 3.1. Conditions A two-stage ESP model is shown in Fig.1. Two erorated lates with orosity = 0.5, 0.4 resectively are used in the diuser. Ten vertical vanes are attached to the erorated lates on the back side. The rectangular chamber (6m long, 1.25m wide, 2.2m high) includes two electric ields (irst and second electric ield) in series searated by a middle way barrier in the lower art. 64 barbed wire electrodes o 8mm in diameter are restricted to a narrow region in height. Four hoers are at the bottom with artition lates in the irst and the last ones. Geometric symmetry allows only one hal o the ESP to be considered. The gas low rate is seciied at the inlet such that the mean velocity in the collection ield is about 1.0 m/s. Perorated late Guide vanes Collection late Diuser assembly Barbed wire electrode Hoers & artition late Inter-stage bale Fig.1. Model geometry or a two-stage ESP Fig.2. Rendered sace charge density distribution Tyical result or base case Electric ield and sace ion charge density are solved locally in a unit cell model around a single barb on the wire [13]. The ield distribution obtained is then imlemented in the ESP model via a modular aroach using eriodical symmetry conditions (as visualized in Fig.2). Very comlicated low attern is ound in the collection chamber (Fig.3). The articles tend to go downwards ater exiting the diuser. The reasons are a), to wall and the bottom wall have dierent sloe angles (30 and 45 resectively); b), a large ree sace exists at the lower art o the chamber; c), the ie bend and transition o the cross section in the inlet ie work adversely aect the low uniormity to the diuser. The simulation shows a large size recirculation in the irst electric ield. A small ortion o articles may luctuate to the to above the electric region, whereas most o the articles go downward around the recirculation eddy, and then turn around uwards beore entering the second stage chamber. These articles have little chances o being charged in the 1 st electric ield. Fig.3. Gas low streamlines on the symmetry lane and article trajectories or the base case.

5 1370 Baoyu Guo et al. / Procedia Engineering 102 ( 2015 ) Fig.4 shows the collection eiciency on the collection lates or dierent article sizes. A theoretical analysis o article charging caacity and orce balance between the drag and electric orce on a single article indicates that the settling velocity is inversely roortional to the article size. Thereore, the collection eiciency is exected to increase with the article size in all the cases. However, the rate o change with resect to the article size varies or dierent article sizes. For ultra-ine articles, the eiciency increases slowly due to a smaller charging rate. It is also noted that the collection eiciency gradually aroaches a higher limit when article size increases beyond 4m. In this case, the articles have escaed largely via the by-assing low Eiciency d, m Fig.4. Collection eiciency as a unction o article size (Case B) Case studies Since PM2.5 is o ractical concern, the collection eiciency or article size o d = 2.5m is redicted in this section. The cases considered involve the ollowing modiications: inlet ie, bales, erorated late arameters and electric ield osition& extent. Reasons or the emissions can be diagnosed by showing the trajectories o the emitted articles that exit the system through the outlet (Fig.5). The color is scaled to the article surace charge density in unit [As/m 2 ]. For the original design, Case A, the articles ater the diuser sread widely in sace. Nearly all the articles that enter the 1 st electric ield, desite small in number, are successully collected. The collection rate on the late is highly uneven and localized, with large ortion o the collection area being wasted Fig.6(a). It is the large recirculating eddy that revents articles rom entering the electric ield directly. Collection occurs mainly in Stage- 2. For the articles that exit the outlet, the electric charge on the articles is low in average (7.4e-3 [As/kg]), and also varies widely, which indicates inadequate and uneven article charging conditions. Due to highly localized sace charge density, ree ion charges never aear over certain sace (e.g., in the close roximity o the wire surace or the barbed wire electrode) in the electric ield. Thereore, a small number o articles may haen to ass these so-called dead zones without being charged. In Case B, the inlet ie is relaced by a straight square duct. This eliminates the eddy behind the original transition art, so that the inlet velocity roile to the diuser becomes almost uniorm. As a consequence, the downward momentum ater the diuser is reduced, directing more articles to the 1st electric ield. The collection rate is increased by 74% in the irst stage and the total eiciency is u by 10 ercentage oints. In the revious cases, many articles ater the diuser are moving into uer and lower arts in the irst stage chamber. This can be imroved in Case C, by adding a low bale underneath and blocking a art o 2 nd erorated late at the to margin (0.1m). This modiication signiicantly reduces the by-ass article low, thus increasing the collection eiciency rom 76.7% to 83.7%. However, since the velocity in the irst electric ield increases, not all the articles have suicient residence time to charge to their ull caacity and to deosit in the 1 st stage. These ailed articles retain the charges obtained and roceed into the second stage.

6 Baoyu Guo et al. / Procedia Engineering 102 ( 2015 ) In Case C, the charged articles that have escaed rom 1 st stage tend to concentrate at the uer edge o the 2 nd electric ield. This can be imroved by u-shiting the 2nd stage electric ield, or alternatively (i.e., Case D), extending the 2 nd electric ield u by 0.225m (or the distance o three needles). Then the total collection eiciency can be increased rom 83.7% u to 91.6%, although at the sacriice o extra ower consumtion. The average charge carried by the escaed articles is [As/kg] comared with 7.4e-3 [As/kg] in the base case. In addition, nearly all the articles have been highly charged with small deviation. These suggest that the articles charging conditions have imroved. Moreover, the deosited dust layer covers lates in all the electric ield with less bias Fig.6(b). However, many articles manage to escae via a ree sace above the second electric ield in Case D, so local modiications in this region may hel the collections even urther. To summarize, the original inlet ie assembly (ie bend, transition section) in lace o a straight square duct has negative eect. A solid late margin (to block the oriices) at the to o the second erorated late can reduce the by-assing above the electric ield. Without bales, most articles tend to deosit in Stage-2 in stead o Stage-1. Adding a single bale is able to reverse this trend. Adjusting the osition and/or extent o the electric ield region in stage-2 can imrove the total collection eiciency. Table 1. Collection eiciency o dierent arts. Case Stage-1 Stage-2 Hoers Total A % B % C %. D % Case A Case B Case C Case D Fig.5. Trajectories o escaing articles in the ESP beore and ater modiication.

7 1372 Baoyu Guo et al. / Procedia Engineering 102 ( 2015 ) Case A Case D Fig.6. Pattern o article deosition on the collection lates beore and ater modiication. 4. Conclusion A CFD model is resented or the simulation o the gas-article low in an industrial ESP. Particularly the collection erormances or ine articles are examined in several cases. For the original design, a large-scale eddy is ound in the irst-stage collection region, which turns the articles away and causes a waste o sace and electric ower. It is demonstrated that roer modiications can regulate the turbulent gas low attern, and subsequently the article behaviors. The redictions show that the article collection eiciency or PM2.5 can be otentially imroved rom the original 67% u to 91% ater modiication. By tracking article motions, ine articles may escae due to by-ass low and insuicient article charging. The numerical exeriments rovide useul guidance or the ESP unit otimization. Acknowledgements The work is suorted by Australian Research Council and Fujian Longking Co. Ltd. Reerences [1] N.Neimarlija, I.Demirdzic, S.Muzaerija, Finite volume method or calculation o electrostatic ields in electrostatic reciitators, J. Electrostatics 67 (2009) [2] A.Soldati, On the eects o electrohydrodynamic lows and turbulence on aerosol transort and collection in wire-late electrostatic reciitators, J. Aerosol Science 31(2000) [3] N.Farnoosh, K.Adamiak, G.S.P.Castle, 3D numerical analysis o EHD turbulent low and mono-diserse charged article transort and collection in a wire-late ESP, J. Electrostatics 68 (2010) [4] McLean, K.J. Electrostatic reciitators, IEE Proceedings 135 (A6) (1998) [5] K.S.P.Nikas, A.A.Varonos, G.C.Bergeles, Numerical simulation o the low and the collection mechanisms inside a laboratory scale electrostatic reciitator, J. Electrostatics 63(2005) [6] N.Farnoosh, K.Adamiak, G.S.P.Castle, Three-dimensional analysis o electrohydrodynamic low in a siked electrode-late electrostatic reciitator, J. Electrostatics 69 (2011) [7] K.R.Parker, Alied Electrostatic Preciitation, Sringer, [8] B.Y.Guo, S.Y.Yang, M.Xing, K.J.Dong, A.B.Yu, J.Guo, Towards the develoment o an integrated ESP model, I&ECR 52 (2013) [9] P.A.Lawless, Particle charging bounds, symmetry relations, and an analytic charging rate model or the continuum regime, J. Aerosol Sci. 27(1996) [10] A.D.Gosman, E.Ioannides, Asects o comuter simulation o liquid-uelled combustors. J. Energy 7(1983) [11] A.Soldati, On the eects o electrohydrodynamic lows and turbulence on aerosol transort and collection in wire-late electrostatic reciitators, J. Aerosol Sci. 31(2000) [12] K.S.P.Nikas, A.A.Varonos, G.C.Bergeles, Numerical simulation o the low and the collection mechanisms inside a laboratory scale electrostatic reciitator, J. Electrostatics 63(2005) [13] B.Y.Guo, J.Guo, A.B.Yu, Simulation o the electric ield in wire-late tye electrostatic reciitators, J. Electrostatics, 2014, (submitted)