SIMULATION OF NATURAL GAS FLUIDIZED BED USING COMPUTATIONAL FLUID DYNAMICS

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1 o PDPETO, Campinas, SP de Outubo de 007 SIMULATION OF NATUAL GAS FLUIDIZED BED USING COMPUTATIONAL FLUID DYNAMICS Felix Seveino de Faias Junio UNICAMP) 5, ubens Maciel Filho UNICAMP) 5, Seio Lucena UFPE) 3 6, Jonandes Dias da Silva UFPE) 6 felix@feq.unicamp.b maciel@feq.unicamp.b 3 lucena@ufpe.b jonandesdias@yahoo.com.b 5 State Univesity of Campinas UNICAMP), CP 6066, Campinas, CEP , SP, Bazil, Phone: , Fax: Fedeal Univesity of Penambuco UFPE), ua Pof. Atu de Sá, S/N, Cidade Univesitáia, ecife, PE, Bazil, CEP , Phone: The inceasin availability of hih pefomance and paallel compute envionments has stimulated depth studies of complex eacto systems in chemical enineein. Usin computational fluid dynamics appoach, bette desciption and compehension of the most impotant phenomena takin place in the system can be accomplished, esultin in moe efficient eacto and pocess desin. Fluidized bed is a elevant system in many applications in pocess industies. Fluidized bed combustos ae impotant altenatives to conventional ones, due to the fact that they pomote lae contact aea between as and solids and intense as mixin thouhout the bed, leadin to cleane bunin and less pollutants emission, because the elatively low opeatin tempeatues and isothemal pofiles. In this wok, computational fluid dynamics simulation of a natual as fed fluidized bed was caied out. The simulated eacto bed was composed by inet paticles sand) with 350 µm. A two-stae kinetics model fo methane combustion was used. Natual as, the cleanest fossil fuel, was used in the simulated fluidized bed combusto, minimizin pollutant emissions even moe. Dynamic simulations wee developed with distubances imposed into pocess input vaiables. The behavio of the simulated combusto was analyzed, and then compaed to expeimental data fom liteatue. CFD, natual as, fluidized bed, fluid dynamics. INTODUCTION The inceasin poduction and availability of natual as in seveal pats of the wold ae ceatin bette conditions and oppotunities fo the use and development of new technoloies and applications fo its consumption. Amon the possible applications, it is detached the use of fluidized bed combustos in substitution to conventional ones. In the last yeas, a lot of eseach and investments have been made in this technoloy, due to the incease of the envionment estictions and constaints about the emission of pollutants and consequent need fo cleane altenative technoloies. Fluidized bed systems pesent inteestin themal popeties fo its application as combustos. The lae contact aea between as and solids pomotes hih ates of eney eneation duin the solid fuel bunin Kuo, 986) and vey efficient themal eney exchane between as and solid paticles. The Fluidized Bed Combusto FBC) has divese applications: eneation of themal eney, eeneation of catalysts, CO poduction in lae scale fo othe pocess feedin puposes, food pocessin, teatment of solid esidues dyin, ovehaulin, calcination and incineation), amon othes. The adjustment of the flows of ai and fuel as allows the contol of the wokin tempeatue in elatively low levels, C Pe et al, 998). In such low tempeatues, the emissions of NOx ae small, which epesent an impotant advantae of the fluidized bed system ove othe conventional combustos. Natual as is yet a elatively clean fuel when compaed with common liquid fuels; in fact, it is the cleanest fossil fuel. The choice of natual as as fuel in a fluidized bed combusto contibutes even moe fo low fomation of pollutants. With all these chaacteistics combined, the development of the technoloy of natual as fluidized bed combustos assues eat envionment and economic impotance. In this wok, a natual as fluidized bed combusto simulation is caied out usin computational fluid dynamics appoach thouht MFIX, a multiphase CFD code developed at the U.S. Depatment of Eney - National Eney Technoloy Laboatoy NETL) Syamlal et al., 993).. KINETIC MODEL Copyiht 007 ABPG

2 o PDPETO, Campinas, SP de Outubo de 007 A natual as combustion kinetic model can easily extend to moe than 50 elementay equations. Howeve, when simulatin dynamics of combustos and eactos, a simple mechanism appoach is often desiable, because its much bette computational pefomance. Considein a lobal two stae kinetic model, the methane combustion can be expessed by the followin mechanism: 3 CH + O = CO + H O CO + O = CO Dye and Glassman 973) consideed a model of two staes fo the oxidation of the methane in a eacto with tubulent flow and obtained the followin kinetic ate expessions: CH / T ) = 0 e [ CH ] [ ] ) O CO / T ) = 0 e [ CO][ H O] [ ] ) O This two staes model allows that the CO and CO pofiles alon the bed be obtained, estimatin the nonlinea behavio of the two steps lobal eaction. 3. CFD MODEL A fluidized bed computational model was developed based on the fluidized bed descibed by Pe et al. 998) usin the Multiphase Flow with Intephase exchanes MFIX) code. The simulated eacto consisted of a 80mm diamete and 800mm pipe with a pefoated plate distibuto with.8% poosity fee aea). Sand with 350 µm aveae diamete and 00 k/m3 specific mass was used as solid phase. A mesh id of 8x50 points was consideed fo a D Catesian coodinates simulation. Axial symmety was consideed. The bed was assumed to be isothemal and opeatin at 900 C. Chemical eaction, momentum and mass balances wee consideed usin the followin eneal equations Syamlal et al, 993): 3. Momentum balances Momentum balance fo as-phase: v ) + vv ) = S + t M m= I m + f 3) Momentum balance fo solid-phase: ) + v ) = S + + I ) sm sm sm sm sm sm sm sm sm sm m t l= 3. Mass balances Continuity equation fo the as-phase: ) + ) = N t n= 5) Continuity equation fo the solid-phase: n M l m I ml Copyiht 007 ABPG

3 o PDPETO, Campinas, SP de Outubo de 007 ) + ) = Nsm sm sm sm sm sm t n= 6) Species consevation equations CH, O, CO, CO, HO, N): t X n ) + X n smn v ) = 7) n smsm X smn) + smsmx smnvsm) = smn 8) t The esultin patial diffeential equations system was solved thouh the finite volume technique. The convective flux discetization uses Supebee method and the iteative aloithm is a modified extension of SIMPLE Syamlal, 998).. ESULTS AND DISCUSSION. Simulation The sand used as inet solid is econized as a oup B solid of Geldat classification. Unde this condition, classical fluidization theoy pedicts 0cm diamete as bubbles, fast ascendin as velocity in bubbles and a slowe one in the emulsion phase. These chaacteistics wee obseved in the simulation esults too, accodin to Fiue. In Fiue.a, a snapshot of bed void faction distibution is epesented. Fiue.b shows an axial as velocity pofile with scalable as velocity vectos, illustatin as eciculation and dispesion phenomena. Fom Fiue -b, it is possible to obseve hihe as velocities in bubbles than in emulsion lae vectos) and counte-cuent steams in the emulsion phase. This as flow patten pomotes intense mixin and tempeatue homoenization alon the dense phase of the bed, pomotin an isothemal pofile in a eal bed. Fiue - a) Void Faction pofile b) Axial as velocity pofile In Fiue, the dynamic oscillatoy behavio of the fluidized bed fo the cabon monoxide component afte a step in methane feed is shown. The oscillation amplitudes ae elated to bubble sizes and positions and as ciculations pattens at each time sample. Copyiht 007 ABPG

4 o PDPETO, Campinas, SP de Outubo de 007. Expeimental Validation Steady state simulation esults wee compaed to data fom liteatue Pe et al., 998). Aveae values of component mola faction pofile wee calculated ove a ane of five seconds in dynamic simulation esults and ae compaed to expeimental data in Fiue 3. The esults obtained by simulation pesented faste methane kinetics at the entance of the eacto. This can be attibuted to the isothemal assumption of as tempeatue alon the eacto and to the limitations of the simplified kinetic mechanism adopted. Bien et al. 003) point out that the pesent uncetainty in the kinetics law is a known souce of eos in simulation of the combustion of the natual as. It is expected that bette esults can be obtained with a bette kinetic law and thouh a non isothemal simulation. Fiue - Cabon Monoxide dynamic esponse pofile afte step in methane feed 5. CONCLUSIONS Fiue 3 Components mola faction pofiles Copyiht 007 ABPG

5 o PDPETO, Campinas, SP de Outubo de 007 Despite of the simplifications assumed isothemal bed, elatively small mesh id and two staes methane combustion kinetics mechanism), simulation esults wee able to poduce ood pedictions, both qualitatively and quantitatively fo the fluidized bed. This shows the suitability of the poposed appoach. In this wok, sand was used as inet paticle which is a cheape altenative to limestone. Howeve, the limestone has an inteestin popety of sulfu adsoption that can be exploed in futue simulation woks. 6. ACKNOWLEDGMENTS The authos wish to thanks UFPE/ANP PH-8 poam, FINEP and PETOBAS fo the financial suppot. 7. EFEENCES DYE, F.L.; WESTBOOK, C.K.; 973. Chemical kinetic modelin of hydocabon combustion. Po. Eney Combust. Sci., 98, Vol 0, pp -57. Peamon Pess Ltd. KUO, L.K Pinciples of Combustion. ed. New Yok, John Wiley & Sons, MOUSTOUFI, N.; CUI, H.; CHAOUKI, J.; 00. A compaasion of two- and sinle-phase models fo fluidized-bed eactos. Ind. En. Chem. es. 0: O'BIEN, T. J.; SYAMLAL, M.; GUENTHE, C.; 003. Computational fluid dynamic simulations of chemically eactive fluidized bed pocesses. Thid Intenational Confeence on CFD in the Mineals and Pocess Industies. CSIO, Melboune, Austalia PATANKA, S.V Numeical Heat Tansfe and Fluid Flow. ed. Seies in Computational Methods in Mechanical and Themal Sciences. Hemisphee Publishin Copoation. PE, P.; HEMATI, M.; MACHAND, B.; 998. Study on natual as combustion in fluidized beds: modellin and expeimental validation. Chemical Enineein Science, 53: M. SYAMLAL, W. ODGES, T.J. O'BIEN, MFIX Documentation: Theoy Guide, Technical Note, DOE/METC-9/00, 993. M. SYAMLAL, MFIX Documentation: Numeical Technique, Technical epot, DE-AC-95MC336, 998. Multiphase Flow with Intephace Exchanes MFIX). National Eney Technoloy Laboatoy NETL). U.S. Depatment of Eney. Copyiht 007 ABPG