Simulation and Optimization of SCR System for Direct-injection Diesel Engine

Transcription

1 Sensors & Transducers 04 by IFSA Publshng, S. L. Smulaton and Optmzaton of SCR System for Drect-njecton Desel Engne Guanqang Ruan, Zhendong Zhang, Jnrun Cheng Shangha Danj Unversty, No. 88, Wenjng Road, Mnhang Dstrct, Shangsh, 0045, Chna Unversty of Shangha for Scence and Technology, No. 56, Jungong Road, Yangpu Dstrct, Shangsh, 00093, Chna Tel.: E-mal: Receved: 4 July 04 /Accepted: 30 October 04 /Publshed: 30 November 04 Abstract: The turbo desel SCR system has been researched and analyzed n ths paper. By usng software of CATIA, three-dmensonal physcal model of SCR system has been establshed, and wth software of AVL-FIRE, the boundary condtons have been set, smulated and optmzed. In the process of SCR system optmzng, t manly optmzed the pray angle. Compare the effects of processng NO to obtan batter optmzaton results. At last the optmzaton results are compared by bench test, and the expermental results are qute consstent wth smulaton. Copyrght 04 IFSA Publshng, S. L. Keywords: SCR, Smulaton, Optmzaton, Experment.. Introducton The desel engne gets hgher thermal effcency, lower fuel consumpton and CO emssons than the gasolne engne. Presently the desel engnes are appled n all of the heavy trucks and most of the lght truck. However, as the standard of desel emssons get to be more and more rgorous all around the world, emsson control of desel partculate matter (PM) and NO x becomes the key to the development of desel engne technology. Usng hgh pressure njecton technology can reduce PM emssons of combuston n cylnder, and then take the Selectve Catalytc Reducton (SCR) technology to control NO x as the man way of electronc-control desel engne exhaust gas processng []. The SCR technology path s shown n Fg.. But there are some problems n the matchng between the SCR and domestc desel engne, the optmzaton of SCR s mmnent. In ths artcle, wth the software of FIRE, t has smulated the SCR process of exhaust, optmzed the structure parameters of SCR and verfed by experment at the same tme. Fg.. SCR technology path. 68

2 . The SCR Model.. Model Analyss The man parameters of SCR shape are: nput termnal dameter D, connectng ppe dameter D, connectng boss small termnal dameter D3 and bg termnal dameter D4, catalytc converter cavty forepart dameter D5 and posteror dameter, D6, output termnal dameter D7, output boss small termnal dameter D8 and bg termnal dameter D9, cavty mnmum dameter D mn and maxmum dameter D max, maxmum depth T max. Use CATIA to establsh SCR sketches, and then set up 3-dmenson model of the SCR as shown n Fg. below based on the stereoscopc model functon of CATIA. Fg.. SCR three-dmensonal model... The Dvson of Grd Export the 3-dmenson model of the SCR catalytc reactor from the CATIA, save as STL fle. Open FIRE and mport the model. Choose the selectons, set the arway nlet and outlet. Choose the demand of the grd, and then mplement. Check the qualty of the grd to ensure correct. FIRE s powerful software, n the smulaton of SCR, there are three major steps: frst s the dvson of grd; second s to mport grd, set condtons, namely flud propertes, boundary condtons, the control equaton. The last s calculaton. As shown n Fg. 3 s the meshng result of the SCR catalytc reactor n FIRE. Fg. 3. The SCR grd..3. The Intal Boundary Condtons ) The ntal condtons. In ths paper, t has smplfy the desgn of the ntake and exhaust and SCR reactor, sets the reacton tme as s, calculaton range s set from 0 s to s. In the SCR module, the effcency of catalytc reacton has a lot to do wth tme, t need choose of step wdth accurately, because the software needs to add a certan number to repeat operatons n the process of numercal calculaton, the step wdth n ths artcle s set as shown n Table. Table. Step wdth settngs. Reacton tme/ºca Step wdth/(s/step) ) Boundary condtons. Boundary condtons can be dvded nto boundary condtons on velocty and boundary condtons on temperature, and the boundary condtons on velocty can be dvded nto sldng, noslp and wall functon law boundary condtons. Generally no-slp boundary condtons are sutable for vscous flud. In ths chapter, n the smulaton process, SCR mport, export, the wall of converters can be treated as the fxed walls, so choose no-slp boundary condtons. In temperature boundary condtons, the boundary of the SCR can be dvded nto four parts of the mport, export, reactor front wall and back wall. In ths paper, the temperature boundary s processed as constant temperature n model calculaton. The determnaton of SCR ntal temperature boundary condtons can be based on experence, set the temperature of the nlet (BND_nlet) as 400 C, the front wall (BND_wall) and the back wall (BND_wall_SCR) of reactor are taken as a fxed boundary, set the temperature as 300 K, set the temperature of nlet (BND_oulet) as 300 K, cylnder lner wall temperature (BND_Lner) as K. 3. The Establshment of Model 3.. Mathematcal Model In the catalytc converter there are manly two parts of the spray zone and the catalytc reacton zone [], assume that chemcal reactons occur only n the latter zone. Ths artcle selects the porous meda model smulate catalytc reacton zone of ceramc honeycomb carrer. The gas flow characterstcs accord to compressble vscous flud Naver-Stokes equaton [3], N-S equaton for short. Multdmensonal numercal smulaton of gas flow s the numercal soluton to ths equaton. In the smulaton model, the conservaton equatons referred to n ths paper are: the mass conservaton equaton, energy conservaton equaton, momentum conservaton equaton and component balance equaton, n addton to the above basc conservaton equaton, as the model nvolves the gas flow ssue, t also can use turbulent flow model and state equaton of deal gas. It s worth mentonng that the whole process s n steady state, namely t s beleved that the flow state s tme-varyng, all the calculaton use the turbulence model and SIMPLE algorthm [4]. Equaton () s the mass conservaton equaton: ρ + = x j ( ρu j ) 0, () 69

3 where u j s the velocty component n j drect m s ; ρ s the densty, 3 kg m. Equaton () s the momentum conservaton equaton: ( ρu ) ρ τ j + ( ρuu j) = + + g f, () x x x j j where p s the hydrostatc pressure[ P a ] ; g s the acceleraton of gravty m s ; τ j s the vscous stress tensor effect on practcal control volume surface of flud[ P a ]. Equaton (3) s the energy conservaton equaton: ( ρh ) h + ( ρuh0 ) = ( Γ ) + S, 0 0 j h h xj xj xj (3) where h s the total stagnaton enthalpy, u h= cpt +, J [ kg K] ; S h s the chemcal reacton and element strength of outsde radant heat. Equaton (4) s the component balance equaton: ml ( ρml) + ( ρujml) = ( Γ l ) + R, l x x x j j j (4) where y a s the chemcal component qualty fracton of unt volume; D a s the dffuson coeffcent of components, m s ; ρω s the source term of the chemcal reacton rate. Equaton (5) s the state equaton of deal gas: p ρ RT a = = nrt, a (5) ma where n a s the molar mass of components kg mol ; R s the gas constant J [ kg K] ; T s the gas temperature [ K ]. In the process of movement, droplets wll be affected by a varety of resstance, the resstance greatly depends on the spatal dstrbuton and moton of droplet, the droplets n catalyst reactor are manly affected by ar drag force and gravty, the momentum balance equaton of droplets s as follows: ρ ρ dud. 3 g CD g = u g. u d. ( u g. u d. ) + g dt 4 ρd Dd ρd, (6) where u d. s the droplet velocty component n drecton; u g. s the gas flow velocty component n drecton; ρ d, densty respectvely; droplets; g s the gravtatonal acceleraton. ρ g are the droplet and gas D d s the dameter of the Equaton (7) s the nstantaneous poston of the droplet: dx dt d, = u, (7) d, where X d, s the droplets poston vector n drecton. 3.. SCR Catalysts Model The ceramc honeycomb carrer researched n ths paper large number of channel, each channel wdth s very small, only about mm. researchng drectly the flow of each channel s complex, and even mpossble, so we do need hypothess for the model and smplfy t. Ceramc honeycomb carrer s of porous medum, equvalently tube bundles composed of a lot of tubules arrayed parallel. All gas phase flud n dfferent tubule flow along the carrer from the begnnng to termnal n the same axal velocty. The flud only has an axal velocty component; the other velocty component s zero. Accordng to Hammel model, calculate the gas pressure loss, and then accordng Darcy s law, put forward extended model. Ths management can make t more smlar to the actual stuaton, closer to the test data [5]. p ρ = α μ v ς v v x, (8) where μ s the dynamc vscosty; α s the vscosty loss coeffcent ( refer to the X, Y, Z drecton); ς s the nertal loss coeffcent; v s the local velocty components; ρ s the flud densty. Due to the hgher exhaust temperature, evaporaton of water makes urea soluton separate out urea, the temperature reaches meltng pont, and urea generates ammona and socyanc acd by pyrolyss reacton [6]. NHCONH NH3+ HNCO, (9) Because socyanc acd s steady out of touch wth catalyst, so t can be beleved that the socyanc acd hydrolyss reacton occurs only wth the catalyst, wthout catalyst urea occurs only pyrolyss reacton [7]: HNCO + HO NH3+ CO, (0) 70

4 Accordng to Eley-Rdeal mechansm t can be concluded that SCR reacton rate s expressed [8] as follows: NH + 4NO + O 4N + 6H O, () 3 4. Smulaton and Optmzaton Analyss of the Model 4.. The Smulaton of the Orgnal Machne Model and Optmzaton Model Catalytc converter shell s the support of catalytc system, and t s the physcal boundary condtons of catalytc converter system, t plays a crucal role on practcal applcaton of catalytc converter system. The shell should be not only of certan stffness, strength, but also meet the demand that thermal expanson coeffcent should be as lower as possble, and the shell has stronger corroson resstance, at the same tme, aerodynamcs should be fully consdered n the structure desgn, to mnmze the engne exhaust arflow resstance, and help the catalytc converter system mprove the effcency of the catalytc purfcaton [9]. So ths paper ams at optmze the nner structure of the catalytc converter, manly optmze the spray angle, the smulaton of the spray angle of nozzle s as shown n Table. Table. Spray angle optmzaton settngs. Orgnal model Frst tme Second tme 79 (deg) 77 8 Set the ntal condtons and boundary condtons and smulate, analyze the results of SCR processng NO wth dfferent spray angle. In order to better analyze the effect of combuston of SCR, take SCR nto smulaton n 0.35 seconds. The smulaton results are as shown n Fg. 3, the results SCR processng NO are as shown n Table 3. Table 3. NO emssons comparson. Orgnal Second Frst tme model tme Spray angle NO/ppm The Analyss of Smulaton In Fg. 4, t can be seen that when the spray angle s 77 degrees, the SCR processes the most exhaust, and t s evdent n Table 3. The SCR system gets the mnmum NO resdual quantty n spray angle of 77 degrees, so the optmal value of SCR spray angle s 77 degrees. Orgnal model spray angle 79 Optmal model spray angle 77 Optmal model spray angle 8 Fg. 4. SCR smulaton results. 5. Experments 5.. Test Equpment To verfy the correctness of the smulaton results, two knds of SCR systems wth dfferent spray angle are tested on bench. Use the CW60-800/ T490 dynamometer, and FSTCPUMApen.. engne numercal-control system to control the workng condton, AVL CEB-II exhaust analyzer s used to measure the conventonal gaseous emssons of NOx, HC and CO. Use AVL SPC47 partal flow dlute partcle samplng devce to acqure exhaust partcles, wegh t for PM total mass. Expermental testng method s based on two knds of workng condtons, one s the steady-state crculaton workng condton, and the other one s transent cyclc workng condton. The dameter n SCR s 6.67 cm, and length 38. cm, wth total volume of L, the BASF catalyst s used n postprocessng system. Temperature sensor n catalysts nlet and outlet s PT00. Meterng pump adopts urea meterng pump of Grundfos Company, t can acheve hgh-precson njecton of lqud nto the exhaust post-processng system. Urea njecton electronc control unt (DCU) communcates wth engne ECU through CAN [0]. The engne model s shown n Table 4, dynamometer devce and emssons test devce s as shown n Fg. 5. 7

5 Table 4. Cummns engne performance parameters. Item Parameter Type of engne 6 cylnder, nlne drect njecton, 4 stroke, turbocharged nter-cooled Total dsplacement /L 6.7 Compresson rato 7.5 Rated power /kw(r/mn) 9 (600) Maxmum torque /Nm(r/mn) 840 (500) Ar ntake Ar compressor ar reservor Meterng pump CAN DCU Urea tank tlet temperature sensor Outlet temperature sensor Lqud level sensor Dynamometer Engne exhaust shaft Nozzle SCR atalytc converter CAN Fg. 5. Urea SCR post-processng test system schematc dagram. 5.. Expermental Results and Analyss In the ESC test cycle, the NO emsson volume fracton comparson after urea SCR emssons processng n the specfc condtons wth dfferent spray angle s as the followng Fg. 6. It can be seen that n most of the ESC test cycle, the results wth urea njecton angle of 77 degrees s better than that of the orgnal machne wth urea njecton angle of 79 degrees. NOx(ppm) operatng mode 9 0 Spray angle 77 Spray angle 79 3 Fg. 6. NOx emsson volume fracton n ESC condtons. In the ETC test cycle, the comparson of NO emsson volume fracton wth dfferent urea spray angle after SCR emssons processng s as Fg. 7. It shows that n most of the ETC test cycle, the results wth urea njecton angle of 77 degrees s better than that of the orgnal machne wth urea njecton angle of 79 degrees. 7

6 Spray angle 77 Spray angle NOx(ppm) Tme(s) Fg. 7. NOx emsson concentraton curve n ETC condton. Table 5. Emsson expermental results. Urea spray angle Conclusons Test workng condton NOx g/(kw h) ESC 3. ETC.3 ESC 3.3 ETC.87 In the case of optmzng SCR urea spray angle structure, smulate ts effect on processng NOx emsson. In the smulaton dagram and the results, t can be seen that optmal structure s of great help to mprove the ablty of SCR exhaust processng. Fnally wth the SCR test bench, the results are verfed. The optmal structure can mprove the performance of the SCR exhaust processng, and t provdes a drecton for SCR system desgn and performance test research. Acknowledgements Project Supported by Foundaton Project of Shangha Danj Unversty (Project No. XKJC0) and Foundaton Project of Shangha Muncpal Educaton Commsson (Project No. E3-3SY4). References []. N. Jeuland, J. B. Dementhon et al., Performances and durablty of DPF (desel partculate flter) tested on a fleet of Peugeot 607 taxs: fnal results, Socety of Automotve Engneers, 004, SAE Paper []. Hu Jng, Zhao Yan Guang, Chen Tng, Chen Zhen, Zhang Yun-Long, Shua Sh-Jn, Wang Jan-Xn, Development of urea dosng control unt for SCR after-treatment system of heavy duty desel engne, Chnese Internal Combuston Engne Engneerng, Vol. 3, No., 0, pp. 8-. [3]. VDl-Warmeatlas, Bereclmungsblatter fur den Warmenbergang, Veren Deutscher lngmeure, edtor, 6 th edton, VDbVerlag, Dusseldorf, 99. [4]. L. Lett, I. Nova, S. Camurr, E. Troncon, P. Forzatt, Dynamcs of the SCR-deNOx reacton by the transent-response method, Amercan Insttute of Chemcal Engneers, Vol. 43, Issue 0, 997, pp [5]. E. Troncon, I. Nova, C. Cardell, et a, Modellng of SCR catalytc converter for desel exhaust aftertreatment: Dynamc effects at low temperature, Catal Today, Vol. 05, Issue 3-4, 005, pp [6]. Da Yu Wang, Sheng Yao, Mark Shost, Ammona sensor for closed-loop SCR control, Socety of Automotve Engneers, 008, SAE Paper [7]. Monca Calatayud, Basma Mgug, Chrstan Mnot, Modelng catalytc reducton of NO by ammona OVeF V05, Surface Scence Reports, Vol. 55, Issues 6-8, 004, pp [8]. Zhang Wen-Juan, Shua Sh-Jn, Dong Hong-Y, Wang Zh, Wang Jan-Xn, 3D modelng of flow, spray of reductant and surface chemcal reacton n desel urea SCR-NOx catalytc converter, Transactons of CSICE, Vol. 5, Issue 5, 007, pp [9]. Crstan Cardell, Reactvty of NO/NOx-NH3 SCR system for desel exhaust after treatment: Identfcaton of the reacton networks a functon of temperature and NO feed content, Appled Catalyss B: Envronment, Vol. 70, 007, pp [0]. Hu Jng, Zhao Yan-Guang, Chen Tng, Chen Zhen, Zhang Yun-Long, Shua Sh-Jn, Wang Jan-Xn, Study of control strategy for urea-scr aftertreatment system of heavy duty desel engne, Chnese Internal Combuston Engne Engneerng, Vol. 3, Issue, 0, pp Copyrght, Internatonal Frequency Sensor Assocaton (IFSA) Publshng, S. L. All rghts reserved. ( 73