COLD GAS ANALYSIS OF A WASTE-GAS INCINERATOR TO ENHANCE MIXING CAPABILITIES USING CFD

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1 HEFAT h Inernaonal Conference on Hea Transfer, Flud Mechancs and Thermodynamcs July 2014 Orlando, Florda COLD GAS ANALSIS OF A WASTE-GAS INCINERATOR TO ENHANCE MIING CAPABILITIES USING CFD Darband M. 1, Modarres M.R. 1 *, Schneder G.E. 2, and Ghana H. 3 *Auhor for correspondence 1 Deparmen of Aerospace Engneerng, Cener of Excellence n Aerospace Sysems, Sharf Unversy of Technology, Tehran, P. O. Box , Iran, 2 Deparmen of Mechancal and Mecharoncs Engneerng, Unversy of Waerloo, Waerloo, Onaro, N2L 3G1, Canada, 3 Arak-Shazand Bumen Producon Facory, Pasargad Ol Company, Mrdamad Boulevard, Tehran, P.O. Box , Iran Emal: mmodarres@ae.sharf.edu ABSTRACT Reducng he combuson lengh, mprovng he flame sably, generang suffcen vorcal recrculaon zones, and exendng he durably of furnace nsulaon are some reasons o arge swrlng flow n combuson chambers wh suffcen swrl characerscs. These reasons am o ncrease he speces resdence me and mprove he mxng qualy n he relaed combuson chambers. As s known, burnng he wase-gas maeral n ncneraors can produce oxc polluans. Neverheless, one man objecve n ncneraon combusor s o mprove he flow mxng characerscs. Our concern n hs paper s o mprove he mxng characerscs of one avalable ncneraor wh some unsuable polluon usng he CFD smulaon. We sudy he flow paerns and analyze he mxng qualy o monor he nfluence of wase-gas nle crosssecons and s poson no he ncneraor chamber. The degree of mxng s analyzed usng he sandard devaon parameer, whch deermnes he homogeney of he fuel mxure fracon n he ncneraor chamber. The mprovemen of mxng characerscs and ncreasng he resdence me n he ncneraor are wo mporan facors, whch are suably elaboraed and dscussed n hs paper. The mxng sudy s provded for cold gas assumpon. INTRODUCTION In hs paper, we address he analyss of mxng n an ndusral hydrocarbon-based wase gas ncneraor. The wase gases are produced n some ndusral uns; however, hey canno be released n amben due o her hgh polluon. So, s requred o re-burn or ncnerae hese by-producs o elmnae her polluans. Snce he wase gas enerng he ncneraor self s a low-grade fuel, can furher conrbue o a beer ncneraon process and self-burnng. The measuremens from he sack of hs ncneraor have shown ha he concenraon of polluans lke NO x and CO would be consderably hgh. So, needs mmedae aenon o reduce he mposed polluans suffcenly low. The observaons and smulaons show ha he shape of flame would be asymmerc despe a symmercal confguraon for he ncneraor and burner. These problems,.e., hgh polluan concenraons on one hand and he asymmerc shape of flame on he oher hand, promoed he ndusry o fnd some remedes o recfy hem. As a sde effec, an asymmerc flame would expose he ncneraor wall a hgh emperaure magnudes, whch s no conssen wh s desgn consderaons. Pas nvesgaons show ha he flow swrlng can sablze he flames wh complcaed flow paerns [1-9]. Some advanages of usng swrlng flows n combuson sysems are as follows [5]: 1. Reducng he combuson lengh by producng hgher raes of enranmen of amben flud. 2. Provdng fas mxng close o he ex nozzle and on he boundares of he recrculaon zones. 3. Improvng he flame sably as a resul of vorcal srucure n he recrculaon zones. 4. Mnmzng he flame mpngemen on he furnace wall, whch can reduce he manenance cos and exend he lfe for he furnace uns. Mxng s known as a general remedy o mprove he basc characerscs of an ncneraor. We also consder hs mporan arge and ry o mprove n our ncneraor usng hree-dmensonal smulaon of he ncneraor. Ths perms o sudy he flow paerns and analyse he urbulence nensy parameer, whose suable conrol can affec he qualy of 1177

2 ncnerang he relaed wase gas. The mxng level can be qualfed usng he sandard devaon parameer, whch deermnes he homogeney of fuel mxure fracon n he soluon doman. Generally, he flow swrl n ncneraor can boh decrease he chemcal reacon zone and ncrease he flame blow-off lms o nfny. Cho e al. [10] proposed a scheme o quanfy he degree of mxng. Km e al. [11] appled hs scheme o evaluae he desgn of a 2D ncneraor. They showed ha by means of flow characerscs, such as he pah of je enerng no he ncneraor, one can conrol he recrculaon zone and s mxng degrees. In oher words, he mxng s affeced by he varaon of momenum flux rae from he wase gas jes enerng as he man sream of ncneraor no he ncneraor. Ryu and Cho [12] showed ha changng he angle and dsance of enerng jes from a drecon parallel o he axs of ncneraor, whch s parallel o he flame elongaon, would have posve effecs on he mxng qualy and polluon conrol. There are a few deas o mprove he qualy of ncneraon n an ncneraor consderng he wo wase-gas and fuel-ar je sreams no he ncneraor [13-15]: 1. A wder dsrbuon of wase gas paerns n he ncneraor, whch can subsequenly mprove he wase gas ncneraon effecvely. 2. Choosng suable number, locaon, and he mass flow rae magnudes for he wase gas jes enerng no he ncneraor. 3. Choosng suable enrance angle wh respec o he flame and he wall of ncneraor for boh jes sreams. 4. Choosng suable cross secons for he enerng jes; preferably a saggered arrangemen. The man purpose of hs paper s o analyse he mxng qualy usng dfferen arrangemens of enerng jes, whch can n urn mprove he mxng qualy and ncrease he resdence me for he speces appearng n he ncneraor. NOMENCLATURE f [-] Mxure fracon f [-] Mean mxure fracon f [-] Mean mxure fracon flucuaons f 2 [-] Mxure fracon varance g [m/s 2 ] Gravaonal acceleraon k [m 2 /s 2 ] Turbulence knec energy N [-] Toal number of mesh cells locaed on a plane P [kg/m.s 3 ] Producon of urbulence p [N/m 2 ] Pressure p [N/m 2 ] Mean pressure R [kg/m.s 2 ] Reynolds sress T [K] Temperaure [s] Tme u x, [m/s] Velocy as a funcon of space and me u x, [m/s] Tme averaged velocy componen u x, [m/s] Flucuang velocy componen u [-] Scalar velocy u [m/s] Frcon velocy v [m/s] Velocy componen n y drecon y [-] Scalar coordnae (local wall Reynolds number) Specal characers [-] Mass fracon [N/m 2 ] Shear sress [-] Mxure fracon varance [kg/m 3 ] Densy [Pa.s] Dynamc vscosy [m 2 /s 3 ] Dsspaon rae of urbulence knec energy [m] Boundary layer hckness [-] Graden operaor. [-] Dvergence operaor [-] Tme-averagng operaor Subscrps fuel sec ox j w GOVERNING EQUATIONS Sream orgnaes from he fuel nozzle Sream orgnaes from he secondary feed o he combuson chamber Sream orgnaes from he oxdzer condu h elemen Componen of a varable n x drecon Componen of a varable n y drecon Wall Relaed o urbulence Mxure Fracon In case of non-premxed combuson, fuel and oxdzer ener he reacon zone hrough wo separae condus. Mxure fracon s a good parameer o monor he combuson process because makes possble o race he wegh of burned and unburned speces n he ncneraor. Mxure fracon can be regarded as a conservave scalar parameer. Usng hs parameer, we can reduce a reacng flow problem o a smple non-reacng mxng flow problem. Ths elmnaes he need for accounng he nonlnear mean rae equaons n our smulaons. The mxure fracon s defned as [13]: -, f - ox, fuel, ox where s he mass fracon of he elemen and he subscrps ox and fuel denoe he oxdzer and fuel sreams, respecvely. Modelng of non-premxed flames needs he soluon of ranspor equaons for one or wo conservable scalars, of whch one s he mxure fracon. In hs way, he equaons do no need o be solved for each speces ndvdually because he speces concenraons can be exraced from he predced mass fracon felds nsead. Havng hs n hand, he relaon beween emperaure, densy, and mole fracon of he speces can be readly obaned from he fuel mxure fracon. Fgures 1-3 show mole fracon of speces, mean densy and emperaure of mxure n erms of mxure fracon, respecvely. In he res of hs sudy, he hermochemsry calculaons and he neracon beween chemsry and urbulence are accomplshed usng suable probably densy funcon. (1) 1178

3 Temperaure (K) Speces Mole Fracon Mean Mxure Fracon Fgure 1 Temperaure versus fuel mxure fracon C2H4 C2H6 C3H8 CH2O CH4 CO2 CO H2 H2O H HCN HO2 N2 N2O NH3 NO2 NO O2 O OH Indeed, he mxure fracon s he mass fracon of an elemen, whch orgnaes from he fuel sream. If a secondary sream, e.g., fuel, oxdzer, or non-reacng flow, s avalable here he mxure fracon of secondary flow or secondary fuel sream can be obaned from f fuel fsec fox 1 Accordng o hs equaon, he summaon of all chemcal speces, conanng a reacng flow sysem,.e., fuel, oxdzer, and secondary sream as llusraed n Fg. 4. The relaon beween sreams, gven by Eq. (2), presens a rangular plane expresson. The maxmum value for hs plane s one a all s verces. In he space of mass fracons, hose values whch are locaed on rangle ABC would be vald. So, he wo mxure fracons of ffuel and fsec are no ndependen from each oher. Ther values are gven by Eq. (2). The assumpon of equal dffusvy coeffcens s vald for all speces n lamnar flow cases. However, n urbulen flow cases, he ranspor phenomenon would be domnan n general. So, he mplemenaon of hs assumpon for urbulen flow cases would be logc and accepable. (2) Mean Mxure Fracon Fgure 2 Mole fracon of speces versus fuel mxure fracon Densy (kg/m^3) Mean Mxure Fracon Fgure 3 Mean densy versus fuel mxure fracon Fgure 4 The relaonshp beween f fuel, f sec, and f ox. Favre mean averaged equaon for he mxure fracon s gven by f. v f. f S m (3) 1179

4 The source erm S m ncludes he mass ransfer rae from he gaseous phase. In order o close he urbulence-chemsry 2 equaons, he mxure fracon varance, f, s obaned from. v. 2 f f f C f C f k g d where f f f and ha he values of, C g, and C d are fxed a 0.85, 2.86, and 2.0, respecvely. In general, he nsananeous sae of flud hermochemsry depends on he mxure fracon value. So, he oher scalar quanes can be readly derved from he knowledge of mxure fracon whou solvng any ranspor equaons for hem. Turbulence Model We use he wo-equaon k urbulence model o perform our smulaons [8, 14]. Comparng wh oher urbulence models, he relably of acheved soluons and he hgh speed of convergence were wo reasons o choose hs model. As s known, he wo parameers of k (he urbulence knec energy) and ε (he dsspaon rae of urbulence flucuaons) can be derved from k u. k. k P 0 k 2 u.. C 1 P C 2 0 k k where he consans are C 0. 09, 1. 0, 1. 0, C , and C Addonally, he urbulence producon erm s calculaed from P u : u u T (7) Wall Funcon Implemenaon We use he sandard wall funcon n he vcny of walls. If we defne y 1 2 w and u u 1 2 w, where he scalar velocy u 1 2 can be obaned from [14] u y ; 30 w k (4) (5) y (n vscous sub-layer) (8) (6) u 0 x u p u u x x x x j x j uu j g T T0 (10) (11) where he las erm on he rgh hand sde of momenum conservaon equaon consders he mporan buoyancy effec, whch shows up due o he emperaure dfferences. Mxng Parameer We use he parameer (mxure fracon sandard devaon) o analyse he mxng performance n our ncneraor. I s defned as 1 N 1 N 1 f 2 f (12) where f s he mxure fracon a he h cell, whch s locaed n our arge plane. Ths plane s perpendcular o he ncneraor axs. Moreover, f s he mean mxure fracon of he cells co-locaed on one plane. N s he oal number of mesh cells on one plane. The governng equaons are dscrezed usng a secondorder upwnd mehod. We also use he SIMPLE algorhm o provde suable pressure-velocy couplng. SOLUTION DOMAIN AND GRID GENERATION The curren ncneraor s llusraed n Fgure 5. The ncneraor has hree npu sreams of ar, wase-gas, and fuelgas. The deals of hese sreams are provded n Table 1. The pressure s abou 82 kpa for all sreams. As he oule boundary condons, we assume a oal pressure of 82 kpa and emperaure of K a he ncneraor nles. Inle Table 1. The deals of ar, fuel, and wase-gas a he ncneraor nles. Temperaure (K) Mass Flow Rae (g/s) Hydraulc Dameer (mm) Componens (Mole Fracon) Wase- CH4 C2H6 C3H8 H2 N2 Gas Fuelgas *16 CH4 CO2 H2O O2 N2 Nozzles ln u Ey ; (n buffer layer) (9) Ar O N where s he Von-Karman consan, and E s he roughness parameer, whch s abou 9.0 for smooh wall surface cases. Conservaon Equaons The connuy and momenum conservaon equaons are respecvely gven by 1180

5 Fgure 9 A close up vew of furnace, burner Fgure 10 A close up vew of furnace nozzles Fgure 5 The prmve geomery of he wase-gas ncneraor Fgure 11 A close up vew of furnace cenral nozzles A 3D unsrucured erahedral prsm mesh was generaed hroughou he ncneraor. Fne mesh was used for he nozzle exs and especally a he hegh where he flame appears. The oal number of cells n hs mesh s nearly 640,000. Fgures 612 show more deals abou he mesh srucure and s qualy. RESULTS AND DISCUSSIONS As was elaboraed before, one approach o mnmze he polluon n a combuson chamber s o ncrease he mxng qualy. Good mxng process can lead o longer resdence me of speces n he chamber and consequenly a beer oxdaon of gaseous parcles and less hazardous polluon. For example, an mproper wase gas enrance can cause napproprae vercal vorces, whch n urn affec par of he reacng parcles and may expose hem o low emperaure zones near he wall of ncneraor, where he chance of oxdaon reduces serously. In oher words, exposng he reacng mxure o low-emperaures may resul n ncomplee burnngs. For example, he CO oxdaon process would sar when he combuson emperaures s suffcenly hgh,.e., C. The lowemperaure zones are prone o produce carbon-monoxde formaon. Changng he poson of wase gas nozzle o somewha a he upper level of he furnace boom and nclnng wh a 45 degrees upward angle, we would expec o earn some posve effecs lke, elmnang vercal vorces n he ncneraor and promong hem o horzonal ones, whch can smulaneously roae around he ncneraor axs. The resuls for he prmve ncneraor confguraon are observed n Fgs. 12 and 13. Fgure 12 shows he complee ncneraor Fgure 7 A close vew of wase-gas and ar nles Fgure 6 The complee ncneraor wh an unsrucured grd dsrbuon Fgure 8 A close vew of fuel and ar nles n - plane 1181

6 confguraon. Also, Fg. 13 shows he sreamlne paerns for he wase gas enerng no he ncneraor. As s seen, he flow paern exhbs fully asymmerc. As was menoned before, we nend o mprove he mxng process n he prmve ncneraor by replacng he wase-gas nflow from he boom of furnace o somewhere rgh a he boom par of furnace wall. As s seen n Fg. 15, he wase gas nle from he boom has been replaced wh wo nles a he upper par of ncneraor burner. These wo nles have an nclnaon angle of 45 degrees upward and 45 degrees counerclockwse wh he angen lnes on he furnace perphery. These wo new nles can have eher crcular or recangular shapes. Fgure 14 shows he wase-gas sreamlnes n he ncneraor consderng a recangular cross-secon for hese wo nles as well as square. A quanave comparson among dfferen wase gas nle cross-secons are llusraed n Fgs. 20 and 21 a several longudnal planes hrough he ncneraor. They mply ha he mxng qualy s a drec funcon of generaed flow paerns. One may readly conclude ha he wase-gas enrance hrough a recangular cross secon provde a beer mxng hroughou he ncneraor axs. Ths means ha, usng he recangular cross secon for he wase-gas enrance o he ncneraor would resul n a homogeneous mxng and desrable dluon zones. Fgure 14 The modfed ncneraor wh new poson for he wase-gas nles havng recangular cross-secon and an upward nclnaon a angle of 45 Fgure 15 Wase-gas sreamlnes hrough he modfed ncneraor usng wo new nle posons and drecons havng a recangular cross-secon shape Fgure 12 The prmve ncneraor confguraon Fgure 13 Wase-gas sreamlnes hrough he prmve ncneraor Fgure 16 The modfed ncneraor wh new poson for he wase-gas nles havng square cross-secon and an upward nclnaon a angle of 45 Fgure 17 Wase-gas sreamlnes hrough he modfed ncneraor usng wo new nle posons and drecons havng a square cross-secon shape 1182

7 Fgure 18 The modfed ncneraor wh new poson for he wase-gas nles havng crcular cross-secon and an upward nclnaon a angle of 45 Fgure 19 Wase gas sreamlnes hrough he modfed ncneraor usng wo new nle posons and drecons havng a crcular cross-secon shape. Unforunaely, he sandard devaon parameer does no gve any explc nformaon abou he speces resdence me. We know ha he resdence me nformaon could provde useful daa abou he effcency of our new desgned wase-gas nles. To enrch our sudy, we calculae he speces resdence me n he ncneraor usng he Lagrangan parcle rackng mehod, see Fg. 22. Ths fgure shows ha he use of recangular and crcular cross-secons would be more preferable han he square one for he wase-gas enrance nozzle confguraon. To reach a suable concluson, we ake no accoun he mxng qualy and sandard devaon of speces as well as he resdence me for he speces passng hrough he ncneraor o deermne he bes new wase-gas nle choce. Our comparsons show ha he frs prory should be gven o he one wh a crcular cross-secon. The nex prory s he square shape and he las s he recangular one. Naurally, a beer prory choce would auomacally resul n a beer combuson process n he ncneraor resulng a more complee oxdaon and less amben polluon. CONCLUSION We used he compuaonal flud dynamcs ool and suded he flow n an ncneraor. To mprove he mxng qualy n he ncneraor, we presened hree new wase-gas nle desgns wh crcular, square, and recangular cross-secons enerng no he ncneraor angenally and upward. They are locaed above he boom face of furnace. We showed ha he crosssecon of wase-gas enerng no he ncneraor would have mporan effec on he flow mxng qualy and he resdence me for each speces. I can be deduced ha he recangular cross-secon would mprove he mxng very effecvely especally near he ncneraor burner. Ths confguraon of nle sreams can cause homogeneous combuson chamber whch fuel sream parcles are propagaed unformly hroughou he ncneraor. On he oher hand, he crcular cross-secon desgn would ncrease he speces resdence me consderably n he ncneraor. Evdenly, he new desgns would preven a connuous mpngemen of wase-gas parcles o he furnace wall, and whch can effecvely reduce he manenance coss. Our sudy shows ha he change of orgnal wase-gas enrance, whch s from he boom of burner and along he axs of ncneraor, o wo separaed ones, whch ener from he boom of furnace wall wh angenal and upward drecon, wll mprove boh he mxng qualy and he speces resdence me. However, among dfferen wase-gas nle cross-secons, he one wh crcular cross-secon would perform much beer han he square and recangular ones, especally for hs ncneraor wh hgh CO concenraons a he oule of he furnace. For furnaces wh hgh NO polluons or even wh hgh operang emperaures, he recangular crosssecon confguraon s recommended for provdng more homogeneous mxng and dluon zones. ACKNOWLEDGEMENT The auhors would lke o acknowledge he gran receved from he Depuy of Research and Technology n Sharf Unversy of Technology and Pasargad Ol Company. Fgure 20 Sandard devaon for he dsrbuon of chemcal speces along he ncneraor axs 1183

8 Fgure 21 Sandard devaon of he mxure fracon versus he hegh of ncneraor magnfyng he dsance from he bed up o a hegh of 3500 mm Incneraor Hegh (m) Man Scheme Wase Gas wh Crcular Cross Secon Inerence Wase Gas wh Recangular Cross Secon Inerence Wase Gas wh Square Cross Secon Inerence [4] Surjosayo, Ad, and Nasr An, Fard, Sudy of Enhancng he Swrl Burner Performance on a Small Scale Bomass Gasfcaon, Inernaonal Journal of Engneerng & Technology, Vol. 11, No. 4, 2011, pp [5] Syred, N., and Beer, J. M., Combuson n Swrlng Flows: A Revew, Combuson and Flame, vol. 23, No. 2, 1974, pp [6] Gassoum, T., Guedr, K., and Sad, R., Numercal Sudy of he Swrl Effec on a Coaxal Je Combusor Flame Includng Radave Hea Transfer, Journal of Numercal Hea Transfer, Vol. 56, No. 11, 2009, pp [7] G. Sloan, Davd, J. Smh, Phlp, and Smoo, L. Douglas, Modelng of Swrl n Turbulen Flow Sysems, Journal of Progress n Energy and Combuson Scence, Vol. 12, No. 3, 1986, pp [8] ang, W., and hang, J., Smulaon of Mehane Turbulen Swrlng Flame n he TECFLAM Combusor, Journal of Appled Mahemacal Modelng, Vol. 33, No. 6, 2009, pp [9] Launder, B. E., and Spaldng, D. B., The numercal compuaon of urbulen flows, Numercal Predcon of Flow, Hea Transfer, Turbulence and Combuson, 1983, pp [10] Cho, S., Lee, J. S., Km, S. K., and Shn, D. H., Cold Flow Smulaon on Muncpal Wase Incneraors, 25 h Symposum on Combuson, Vol. 25, No. 1, Irvne, CA, 1994, pp [11] Km, S., K., Shn, D., and Cho, S., Comparave Evaluaon of Muncpal Sold Wase Incneraor Desgns by Flow Smulaon, Combuson and Flame, Vol. 106, No. 3, 1996, pp [12] Ryu, C. K., and Cho, S., Desgn Consderaon for he Cross Je Ar Mxng n he Muncpal Sold Wase Incneraors, ASME IMECE Symposum on Fre and Combuson Sysem, San Francsco, CA, 1995 [13] axn, S., heng, A., and hao, B., Numercal Smulaon of effec of Inle Confguraon on Square Cyclone Separaor Performance, Powder Technology, Vol. 210, No. 3, 2011, pp [14] Svahanu,. R., and Faeh, G. M., Generalzed Sae Relaonshps for Scalar Properes n Nonpremxed Hydrocarbon/Ar Flames, Combuson and Flame, Vol. 82, No. 2, 1990, pp [15] Launder, B. E., and Spaldng, D. B., Mahemacal Models of Turbulence, Academc Press, London, Resdence Tme (s) Fgure 22 Resdence me of chemcal speces along he ncneraor hegh REFERENCES [1] Elsayed, Khary, and Lacor, Chrs, The Effec of Cyclone Inle Dmensons on he Flow Paern and Performance, Appled Mahemacal Modelng, Vol. 35, No. 4, 2011, pp [2] Gronald, G., and Derksen, J.J., Smulang Turbulen Swrlng Flow n a Gas Cyclone: A comparson of Varous Modelng Approaches, Powder Technology, Vol. 205, No. 1-3, 2011, pp [3] Hrez, Raner, Genrc, Carolne, and Mdoux, Noël, Numercal Invesgaon of Swrlng Flow n Cylndrcal Cyclones, Chemcal Engneerng Research and Desgn, Vol. 89, No. 12, 2011, pp