Large Scale Electrostatic Precipitator

Transcription

1 11th Internatonal Conference on Electrostatc Precptaton 164 4BStudy and Applcaton of Numercal Calculaton Method for Gas Flow Dstrbuton of Large Scale Electrostatc Precptator DANG Xaoqng, HU Hongsheng, MA Guangda, YAN Donge (School of Envronmental & Muncpal Engneerng, X an Unversty of Archtecture& Technology. X an , PR Chna) Abstract: The gas flow rate dstrbuton n ppelnes and the gas flow dstrbuton at the nlet secton of the frst electrc feld were numercally smulated wth CFD. Accordng to the smulated results, we ascertaned the collocaton schemes of gude vanes n ppelnes and on the plate of gas flow dstrbuton. Meanwhle, we valdated smulated results wth the model test. Smulated results were n accordance wth the model test, and met the requrement of the electrostatc precptator. Keywords: Electrostatc precptator; Gas flow dstrbuton; CFD; Flux dstrbuton 1 0BINTRODUCTION Gas flow dstrbuton s one of the mportant factors that affected the collecton effcency of ESP. The hgher desgn effcency of precptator s requred, the greater affecton the gas flow dstrbuton wll mae. Accordng to the lterature [1-3], the collecton effcency (only 60% or 70%) s less than the desgn effcency (about 99 0%) as a result of the poor gas flow dstrbuton of the nlet secton n the frst electrc feld.due to dfferent layout of ppelnes and dfferent shapes of nlet dffuser n the ESPs of power plants, all ESPs need to be determned ther desgn parameters of gas flow dstrbuton devce through the model test of gas flow dstrbuton accordng to current correlatve standards [4,5]. Studyng on the ssue of unformty of gas flow dstrbuton manly has two methods, ncludng model test and numercal smulaton. Model test can show the speed dstrbuton of gas flow n the nlet secton of the frst electrc feld, and have ntutonstc and credble results. Numercal smulaton can get the numercal soluton of quanttatve flow feld and descrbe the rule of gas flow. It avoded the blndness n the desgn process, reducng the desgn cycle [6,7].Through the numercal smulaton, the moton of PM10 were studed, and results ndcated that most of fne partcles were collected by the thrd electrc feld of ESP [8]. There has been examples of usng fluent software to calculate the dstrbuton of two-dmensonal flow feld n the model of ESP, whch solved the gas flow speed dstrbuton n the nlet secton of the frst electrc feld. The results of numercal smulaton were n accordance wth that of model test [9].For the structure of the ESP s complex, the prevous qualtatve descrpton of partculate moton and the accuracy of two-dmensonal calculaton can not fully meet the requrements of desgn. Ths paper had a three-dmensonal calculaton of the gas flow moton n a 660 MW ESP by usng computatonal flud dynamcs (CFD) methods, whch ncluded the gas flow rate dstrbuton between the two compartments of the ESP, the state of gas flow n the ppelnes before nlet trumpet, the speed dstrbuton of gas flow n the nlet secton of the frst electrc feld. Meanwhle, we made a model test, verfyng whether the relablty and accuracy of numercal smulaton can meet the requrement of ESP desgn. 1BNUMERICAL SIMULATION Actually, the flow feld n ESP was complex threedmensonal turbulent flow feld. The man flow chart of CFD method, whch was used to calculate the flow feld n ESP, was shown n Fg. 1. It contaned the establshment of geometrc model, generaton of grd, dentfed model, defned boundary condtons, gven control of the parameters for calculaton, calculaton and post-processng part. The establshment of geometrc model Generaton of grd Identfed model Defned boundary condtons Gven control of the parameters for calculaton Fg. 1 Calculaton Post-processng part Numercal smulaton processes of CFD.1 The Establshment of Geometrc Model of ESP The geometrc model of ESP was shown n Fg..The ppelnes from the outlet of the boler preheater to four nlet dffuser of the two ESPs were symmetrcal, the shapes of whch were the same.the two ESPs nstalled the same type of fan, the gas flow rate of whch were equal to each other. So numercal calculaton only need to smulate one ESP of them. However, due to the dssymmetrcal placement of ppelnes before the nlet trumpet, numercal smulaton stll needs to solve the problem on how to dstrbute gas flow rate n two compartments of one ESP.

2 Study and Applcaton of Numercal Calculaton Method for Gas Flow Dstrbuton of Large Scale Electrostatc Precptator 165 Fg. Geometrc model of ESP Numercal adopted gambt, the pre-processng software of fluent, to establsh geometrc model of the man part of ESP and the ppelnes from the outlet of boler preheater to the outlet of ESP. As a result that structure of the plate of gas flow dstrbuton s very complex, we vewed t as unform ar board on the assurance of calculatve accuracy, and made the ar-resstance of unform ar board be equal to that of porous plate.. Mesh Generaton Due to the complex structure of the ESP, t can not generate sngle structural grds. Ths paper adopted mxed structured and unstructured grd to generate the mesh n ESP. Fg. 3 showed a cross secton of grd map, and Fg. 4 showed a vertcal secton of grd map. Fg. 3 Fg. 4 Cross secton of grd map Vertcal secton of grd map.3 Defnton of Boundary Condtons Boundary condtons of mport and export were adopted boundary condtons of speed mport and pressure export respectvely. Meanwhle, gude plate and gude vane of gas flow, collectng plate, hopper baffles and trough board were used wall boundary condtons. The plate of gas flow dstrbuton s thn plate, usng the porous hoppng model..4 Calculaton Model The wnd speed s 0.5m/s 1.5m/s n the electrc felds of ESP, So the Reynolds number of gas flow s at least 104, and there s a hgh degree of turbulent state n the ESP. When smulatng the ESP, we selected the turbulent model, dd not neglect gas vscosty. We assumed that the ncompressble flow of gas was steady flow durng the entre calculaton process, and vewed the whole process as sothermal process. To solve the turbulent problem, we adopted standard model of dual equaton, and treated boundary wall wth wall functon. The control equatons of the whole process of numercal smulaton ncluded contnuty equaton, momentum equaton, the equaton and the equaton [10, 11]. Contnuty equaton: where: ν u w ( u ) + + = = 0 y w Momentum equaton: ρu + ρuu t (1) ( ) ( ) = + + ρg + F p s statc pressure; p τ () F contans other relevant source model tems, such as porous meda and the custom source tems. K turbulent netc energy equaton: μt ( ρ) + ( ρu ) = [( μ + ) ] + G ρ t σ (3) Dsspaton rate equaton :

3 11th Internatonal Conference on Electrostatc Precptaton 166 μt ( ρu ) = [( μ + ) ] + C1 σ G C ρ (4) where: uvw,, -gas velocty for varous components, m/s; ρ-the densty of flud,g/m 3 ; -the netc energy of turbulent flow, m /s ; Fg. 5 The gas flow feld n ESP -the dsspaton rate of the netc energy of turbulent flow, m /s 3 ; P -tme equlbrum pressure, Pa; μ -dynamcal vscous coeffcent of lamnar flow, Pa s; μ -dynamcal vscous coeffcent of turbulent flow, Pa s. t.5 Calculaton Method Durng the whole process of calculaton, we adopted the dscrete control equaton of fnte volume method, SIMPLE algorthm, the secondary upwnd scheme when usng dfference scheme of convecton tem..6 Results The relatve root mean square method (RMS), whch was wdely used n Amerca, was used as an estmated crteron to evaluate the unformty of gas flow dstrbuton n ESP. where: ( v v) n = 1 σ = (5) v nv -the gas velocty of measurng pont, m/s; v -the average gas velocty of the measurng secton, m/s; n-the number of ponts n measurng secton. The value of σ means the dscrete extent between the gas velocty of measurng ponts and average gas velocty. The greater σ value s, the worse unformty of gas flow dstrbuton n electrc feld wll be. After numercal calculaton, we found the value of σ s 0. n the nlet secton of the frst electrc feld.the gas flow feld n ESP and the dstrbuton of speed vector n nlet secton of the frst electrc feld were shown n Fg. 5 and Fg. 6 respectvely. Fg. 6 The dstrbuton of speed vector n nlet secton of the frst electrc feld 3 BMODEL TEST On the bass of smlar prncples, model test of flud dynamcs should mae both of prototype and model be smlar n geometry, moton and momentum. The geometrc rato of model test was 1:10. The scope of smulaton s the ppelnes from the outlet of boler preheater to the outlet of ESP and the man part of ESP(see Fg. 7).Consderng that colletng plates affect the gas flow dstrbuton and drecton of gas flow, we also had a numercal smulaton of them. The man ndex of model test was n lght of the unformty of gas flow dstrbuton n the nlet secton of the frst electrc feld. By adustng the devce of gas flow dstrbuton n nlet dffuse of ESP, the ndex of unformty of gas flow dstrbuton was acheved. 3.1 The Placement of Gude Plates n Ppelnes The results of numercal smulaton of gas flow feld n nlet ppelnes ndcated that when there was not any gude plates n ppelne, the gas velocty dfference between the left and rght ppelne was evdent. As a result that the state of gas flow dstrbuton n nlet ppelne drectly affected the unformty of gas flow dstrbuton n the nlet secton of the frst electrc feld, we need to set gude plates n nlet ppelnes to adust the state of gas flow n ppelnes. Accordng to the results of numercal smulaton, there are three parallel gude plates that placed n the ppelnes, the dstance of whch s the same to each other (see Fg. 7 from K drecton). Fnally, the model test adopted ths scheme. 3. Devce of Gas Flow Dstrbuton ninlet Dffuser The gude plates n nlet trumpet and gude vanes n the ESP model test were shown n Fg. 8. The plates of gas flow dstrbuton n nlet trumpet, wth equdstance of each other, were placed vertcally n the wdth drecton of the electrc feld, the open porosty of whch dd not change. By changng the number and spacng of the horzontal gude vanes to adust the speed dstrbuton of gas flow n the nlet secton of the frst electrc feld. The model test made a testng of gas velocty at 480 ponts wth QDF-B hot ball anemometer, wth a σ value of 0.17, achevng a good level of RMS standard

4 Study and Applcaton of Numercal Calculaton Method for Gas Flow Dstrbuton of Large Scale Electrostatc Precptator 167 scheme of numercal smulaton. (Fg. 7). Standard plot tube and tlt mcro-pressure measurer were used to the speed of 5 ponts n sectons of each ppelne durng the model test. After calculatng, we found the dfferences of gas flow rate were 1.7%, whch met the desgn requrement of ESP. 4. Unformty of Gas Flow Dstrbuton n Inlet of Electrc Feld The model test measured that the σ s 0.17 at the nlet secton of the frst electrc feld, and test results s shown n Fg. 9. The xy plane represents the nlet testng secton of the frst electrc feld, x-axs represents the wdth drecton of electrc feld, and y-axs represents a heght drecton of electrc feld. The speed dstrbuton of numercal calculaton results n the nlet secton of the frst electrc feld was shown n Fg. 10. It ndcated that most of the gas speeds were between m/s, meetng requrement of ESP standards. It showed that the proporton that gas velocty of testng ponts were less than 1.4 tmes of the average velocty was 99% at the same tme. Fg. 7 The test model. Fg. 9 The speed dstrbuton of model test n the nlet secton of the frst electrc feld Fg. 8 The placement of dstrbuton plates and gude vanes n nlet trumpet 4 COMPARISON BETWEEN THE RESULTS OF NUMERICAL SIMULATION AND THAT OF MODEL TEST 4.1 Dstrbuton of Gas Flow Rate Due to the dssymmetrcal placement of ppelnes before the nlet trumpet, the gas flow rate of two compartments of ESP had an evdent dfference. For adustment of the gas flow dstrbuton, t needs to be placed spltter plates n ppelnes. Accordng to the results of numercal smulaton, gude plates were placed at an angle of 7 to the horzon, (ant-clocwse rotaton) the length of whch s 150 mm. The scheme made the dfference of gas flow rate between the two compartments only be l%-1.86%. The layout of ppelnes n model test was n lght of the Fg.10 The speed dstrbuton of the numercal calculaton n the nlet secton of the frst electrc feld

5 11th Internatonal Conference on Electrostatc Precptaton CONCLUSIONS [1] By usng CFD numercal calculaton method, we can ascertan the structural parameters of gude plates and dstrbuton plates of gas flow n nlet ppelnes. It can decrease the model tests by utlzng the results of numercal calculaton. We found that the results of numercal calculaton were n accordance wth that of model test, and met the apprasal requrement of the performance of ESP. [] The results of the numercal calculaton not only reflects the velocty dstrbuton of the nlet secton of the frst electrc feld, but also drectly reveals the gas flow law n the electrc feld and n the ppelne. Numercal smulaton avoded the blndness of desgn process, and provded the basc scheme for optmzng the nternal structure of the electrostatc precptator. 3BREFERENCES 1. Q Junlan. The man problems and measures of ESP n the coal-fred power plant [J]. Thermal power generaton, 003, (11): (n Chnese).. L Zash. The selecton of nstallaton and operaton management n the ESP [M]. Beng: Chna Electrc Power Press, 005. (n Chnese). 3. H J Whte. Industral ESP [M]. Wang ChengHan Translaton. Beng: Metallurgcal Industry Press, (n Chnese). 4. DL / T , "coal-fred power plant operaton and mantenance of ESP[S]. (n Chnese) 5. DL / T , ESP [S]. (n Chnese). 6. ArthurG Hen. Electrostatc precptaton, dust movement and gas flow optmzaton [A]. Presented at the8th Internatonal Conference on Electrostatc Precptaton [C]. Brmngham, U S A: Southern Company Servces. Inc Publcaton Servces, Dang Xaoqng, Yang Hen, Zhao Xnr. Study of computatonal flud dynamcs (CFD) Methods for electrostatc precptator oblque ar dstrbuton [J]. Envronment Engneerng, 005, (5): (n Chnese). 8. Nels F. Nelsen, LefLnd. Numercal Modelng of gas dstrbuton n electrostatc precptators[a]. Presented at the 8th Internatonal Conference on Electrostatc Precptaton[C]. Brmngham. U.S.A: Southern Company Servces. Inc. Publcaton Servces Dang Xaoqng, Yuan Shengl, Yang Chunfang. ESP ar dstrbuton CFD method prelmnary study [J]. Thermal power generaton, 005, (3):1-14. (n Chnese). 10. Tao Wenquan. Numercal heat transfer [M]. X'an: X'an Jaotong Unversty Press, (n Chnese) 11. Wang Fuun. Computatonal flud dynamcs analyss - CFD software prncples and applcaton [M]. Beng: Tsnghua Unversty Press, 004. (n Chnese).