Numerical Simulation of an Electrostatic Precipitator

Transcription

1 Numercal Smulaton of an Electrostatc Precptator Anandkumar S Malpatl Department of Thermal Power engg. Vsvesvaraya Technologcal Unversty Gulbarga, Inda Vaykumar V Nagathan Department of Mechancal Engneerng BLDEA S College of Engneerng, Bapur,Inda ABSTRACT:- Optmzaton of the perforated plates of the electrostatc precptator at the nlet secton requres expensve testng wth much teraton. A numercal analyss can be employed to reduce the number of desgn teratons. For power statons, the gas velocty s preferably between 1 m/s and 1.5 m/s n the treatment area, hgher veloctes tend to scour the collected dust (ash) off the plates. The requred velocty s obtaned by correctng the hole sze of the perforated plates at the nlet secton of the electrostatc precptator. The present work descrbes the smulaton of flue gas flow through an electrostatc precptator and the optmzaton of the perforated plates of the electrostatc precptator at the nlet secton. A Fnte volume approach has been used and the pressure-velocty couplng s resolved usng the SIMPLE algorthm. In ths numercal analyss perforated plates are consdered as porous medum to smplfy the soluton. Comparsons are made between the avalable expermental results and the redesgned computatonal work. The expermental results show the velocty n the treatment area excess of 1.8 m/s, whch s undesrable from effcency pont of electrostatc precptator. Redesgn of the perforated plates usng computatonal work at the nlet part of the electrostatc precptator, shows the velocty of flue gas n the treatment area of about 1.2m/s, whch s more desrable. U Re u k p C 2 I D h t A p A f NOMENCLATURE Average velocty Densty of flud Reynolds number Absolute velocty Dynamc vscosty coeffcent Turbulent energy dsspaton Turbulent knetc energy Dsspaton functon Pressure Inerta Resstance coeffcent Turbulent ntensty Hydraulc Dameter Thckness Area of the plate (sold and holes) Free area or total area of the holes 1. INTRODUCTION There are varous devces used for sold-gas separaton n Power plants, such as 1. Gravtatonal Separators. 2. Bag house dust Collectors or Bag Flters. 3. Cyclone Separator. 4. Electrostatc Precptator. Of all the devces used for sold- gas separaton, electrostatc precptator fnds wde applcaton because of ts nherent advantage over all other devces. Electrostatc rs can handle large volume of gases from whch precptato sold partculates are to be removed. Ther techncal superorty les n low-pressure drop, hgh effcency for small partcles sze, and relatvely easy removal of the collected partculates [1]. Optmzaton of the Porous Plates of the electrostatc precptator at the nlet and outlet secton by experment requres much tme and t s expensve. A numercal analyss can be employ to reduce the number of desgn teratons. For electrostatc precptator the flue gas velocty n the treatment area should be between 1 m/s and 1.5 m/s, hgher veloctes tend to scour the collected dust (ash) off the plates [2]. Computatonal flud dynamcs (CFD) s an attractve desgn tool to analyze the Flud flow problems. Snce t has the potental to explan the flow physcs nsde the electrostatc precptator. Presently the most wdely used turbulence model n CFD codes s k-є [3]. Ths model gves comprehensve results that are consstent wth expermental data for most turbulent flows, and t s computatonally less tme and memory consumng than large eddy smulaton (LES) and Detached eddy smulaton (DES). The other approach s the Reynolds stress transport model RSTM that has a better accuracy than the k-є model and requres less computatonal effort than LES and DNS. 2. COMPUTATIONAL MODEL 2.1. Governng Equatons Steady, ncompressble, turbulent flows are governed by the Reynolds-averaged contnuty and Naver-strokes 291

2 equatons. The conservaton forms of these equatons n tensor notaton can be wrtten as follows: Conservaton of Mass: x U 0 Conservaton of momentum: x U U P x x U uu x Where, U and u are the components of the mean and fluctuatng veloctes, P s the mean pressure, and and are the flud densty and vscosty, respectvely. The numercal soluton of the above set of mean equatons s obtaned by ntroducng addtonal transport equatons for the Reynolds stresses represented by u'v'. These equatons ntroduce sx varables and ncrease the dffculty of solvng the system. Also, these equatons contan hgher order correlatons whch represent the processes of dffuson transport, vscous dsspaton and fluctuatng pressure-velocty nteractons and have to be approxmated by model assumptons n order to close the system of equatons. The Reynolds stresses are calculated by usng one of the followng two turbulence models. The standard k- model: The turbulent stresses are related to the mean velocty gradents va the turbulent vscosty, t. Ths relatonshp s named as Boussnesq approxmaton [3]. The Reynolds Stress Transport Model: In the RSTM, the Reynolds stresses are calculated from ther transport equatons and the concept of an sotropc eddy vscosty s not requred. Table 1. Comparson of expermental and computatonal porosty values The above table 1 shows the comparson of expermental and computatonal porosty values (redesgned porous plate) Numercal Soluton Procedure: The mean flow, turbulence transport equatons and porous meda equatons are solved numercally by usng the FLUENT Code [5], whch s a general-purpose solver for heat transfer and flud flow n complex geometres and has been ntensvely valdated aganst expermental data for many flow cases. The unstructured body ftted coordnates has been employed for meshng the complex geometry of a Electro statc precptator as n the present work. A fntevolume, non-staggered grd approach has been used and a second order upwnd scheme s appled for the space dervatves of the advecton terms n all transport equatons. The pressure-velocty couplng s resolved by usng the SIMPLE algorthm. Convergence of the soluton s assumed when the sum of normalzed resduals for each conservaton equaton s reduced to about 1 x 10-3 and the number of teratons s GEOMETRY AND BOUNDARY CONDITIONS Fg 1 shows the electrostatc precptator model employed n the present numercal work. The electrostatc precptator model conssts of nlet nozzle, treatment area, outlet nozzle and hopper. There are three perforated plates provded n the nlet nozzle, whch are equally spaced. The entrance of the outlet nozzle s also provded wth perforated plate. Plates Inlet Expermental Porosty values Computatonal porosty values (Redesgned) 1 50% 40% 2 3 Outlet 4 Upper 40% Upper 40% Lower 40% Lower 33% Upper Left 23 % Upper Left 23 % Upper Centre 33 % Upper Centre 33 % Upper Rght 23% Upper Rght 23% Upper 40% Upper 40% Central 23% Central 23% Lower 40% Lower 40% Fg.1. Mesh Model of an Electrostatc precptator Inlet velocty Desgned gas volume m 3 /hr = Inlet area, m 2 = 2.050* Velocty = Volume/ nlet area = / (2.050*1.320)*3600=15m/s. Inlet Temperature, 0 C = 140. Outlet Boundary condtons. Pressure outlet Desgn pressure mmwc =

3 DESIGN DATA Desgn parameter. Table.2. Desgn parameter FUEL 100% Indan coal. SI # 1. Desgn gas volume n m 3 /hr Temperature 0 C Dust type Boler fly ash 4. Maxmum nlet dust loadng gm/ Nm Outlet emsson from ESP mg/ Nm 3 <= Mosture n gas % V/V Unburnt carbon n flue gases, % W/W 8. Collecton area, m Specfc collecton area m 2 / m 3 / s Mgraton Velocty cm/s Desgn pressure mmwc ± Dust densty for dscharge Kg/ m SOFTWARES USED 10% assumed We used the Gambt as modelng software; t s flexble software for fluent solver. It has capablty of producng fne surface. Fluent s used for the analyss of the model. FLUENT provdes complete mesh flexblty, ncludng the ablty to solve the flow problems usng unstructured meshes that can be generated about complex geometres wth relatve ease. Supported mesh types nclude 2D trangular/ quadrlateral, 3Dtetrahedral/hexahedral/pyramd/wedge, and mxed (hybrd) meshes. FLUENT also allows refnng or coarsenng the grd based on the flow soluton. All functons requred to compute a soluton and dsplay the results are accessble n fluent through an nteractve, menu-drven nterface[5]. 5. RESULTS AND DISCUSSIONS Fg.2. Velocty vector representng varaton of flue gas velocty from Inlet to Outlet Fg 2 shows the velocty varaton of the flue gas passng through porous plate from the nlet of the electrostatc precptator to the outlet n the longtudnal plane. At the nlet, the velocty of the flue gas was 15 m/s, from the above plot, the velocty at the treatment area vares from m/s, and ths s due to the screenng of the gas flow by the porous plate. Some of the gas passes through the clearance provded at the bottom of the porous plate. The flow hts the frst hopper and enters nto the treatment area as shown n the fg. Ths flow wll affect the unform flow comng out from the porous plate. Introducton Ths chapter represents the results of numercal smulaton carred out to smulate the flow of flue gases passng through an electrostatc precptator, n order to reduce the velocty of flue gases to prevent the scourng of the dust collected over the collector plates and to provde more retenton tme to absorb more dust. The rectangular co-ordnate system was used for the presentaton of the results Plate1 1 Plate 2 Plate3 3 Fg.3. Velocty vector representng velocty at nlet to electrostatc precptator The velocty vector at the nlet secton of the electrostatc precptator shown n the fg 3. The nlet secton contans three perforated plates at the equal ntervals. The three porous plates, whch are provded wth dfferent porostes. The frst plate has the porosty value of 40 %. The second plate s splt nto two sectons. The upper secton has the porosty value of 40%, whereas the lower porton has 33% porosty value. The thrd plate s splt nto four sectons; the upper left secton has the porosty value of 23 %, the upper mddle porton has the porosty value of 33 %, the upper rght porton has the value of 23 %, the Bottom porton has the porosty value of 23 %. From the fg, the velocty at the end of the nlet secton s 1.51 m/s. 293

4 Fg 4 shows the velocty vector at the treatment area ncludng hoppers. The average velocty n the treatment area s 1.2 m/s, whch s sutable for the effcent operaton of the electrostatc precptator. Fg 7 shows the contour plot of velocty magntude before and after plate, one. The velocty before plate one was m/s and after plate one, was 5.68 m/s. Ths s due to the screenng of the flow. Fg.4. Velocty vector representng velocty at treatment Zone of electrostatc precptator Fg.7. Contour plots of Velocty magntude before and after plate 1 Fg 8 shows the contour plot of velocty magntude before and after plate two, the velocty before plate two was 2.54 m/s and after plate two, was 2.15 m/s.ths s due to the screenng of the flow. Fg.5. Velocty vector representng velocty at Inlet to outlet Fg 5 shows the velocty vector at the sectonal top vew of the electrostatc precptator, the flow ht the hopper and enterng nto the treatment area so that the velocty vector s not unform. The Velocty vector at the outlet porton s shown n the fg 6 the flow comng out the electrostatc precptator wthout any recrculaton wth the velocty of 15 m/s. Fg.8. Contour plots of Velocty magntude before and after plate Fg 9 shows the contour plot of velocty magntude before and after plate three, the velocty before plate three was 1.35 m/s and after plate, three was 1.32 m/s. Ths s due to the screenng of the flow. Fg.6. Velocty vector representng velocty at Outlet Fg.9. Contour plots of Velocty magntude before and after plate 3 294

5 Fg 10 shows the contour plot of velocty magntude n the treatment zones.e. treatment zone one, two, three. From the contour plot of the treatment zone one, the velocty magntude was found to be 0.79 m/s. The velocty magntude n the treatment zone two from the contour plots was found to be 0.95m/s. Smlarly, the velocty magntude n the treatment zone three was found to be 0.88m/s. Treatment Zone one Treatment zone two 6. CONCLUSIONS From ths work the followng conclusons are drawn A comprehensve lterature survey has been carred out on the detals of the electrostatc precptator. Usng the front-end commercal software Fluent, the fnte volume model for the electrostatc precptator was generated. Expermental results show the velocty n the treatment area of about m/s. The numercal smulaton result shows the velocty of 1.2m/s n the treatment area, whch s more sutable for effcent operaton of the electrostatc precptator as shown n fg 5.3. From the results obtaned the flow pattern were analyzed and mportant observatons are mentoned. The flow httng the frst hopper as shown n fg 5.1 and enter nto the treatment area whch affect the unform flow comng out from the perforated plates. Treatment Zone three Fg.10. Contour plots of Velocty magntude n treatment zone one, two and three 7. REFERENCES [1] Steam generator and auxlares -BHEL TRAINING MANUAL [2] GAN, G.H. and RIFFAT, S.B., (1997), "Pressure loss characterstcs of orfce and perforated plates". Exp. Therm. Flud Sc., 14, [3] H. K. Versteeg and W. Malalasekera An ntroducton to computatonal flud dynamcs The fnte volume method, Longman Scentfc & Techncal Publcaton. [4] Fluent User Gude. [5] SKODRAS, G., KALDIS, S.P., SOFIALIDIS, D., FALTSI, O., GRAMMELIS, P. and SAKELLAROPOULOS, G.P., (2006), "Partculate removal va electrostatc precptators - CFD smulaton". Fuel Process. Technol., 87, Fg.11. Contour plots of Velocty magntude before and after plate four Fg 11 shows the contour plot of velocty magntude before and after plate, four. The velocty before plate four was m/s and after plate, four was 1.25 m/s. 295