JJMIE Volume 4, Number 1, Jan. 2010 ISSN 1995-6665 Pages 155- 162 Jordan Journal of Mechanical and Industrial Engineering Numerical Modeling of Coal Tire-Shred Co-Gasification Ilham Talab, Zaki Al-Nahari, Rana Qudaih, Isam Janajreh * Mechanical Engineering Program, Masdar Institute of Science & Technology (MIST), Sas Al Nakhl Area Abu Dhabi, Abu Dhabi, P.O.Box 54224,United Arab Emirates AbstractTires,plastics,cellulosicmaterials,i.e.,papersandcardboardsarerichinhydrocarbonyetlandfillingofthewasteof these materials is still practiced causing potential risk to our ecosystem through gas emissions (essentially CH4) and ground water leaching. Co-Gasification within the existing infrastructure of pulverized coal utility gasifiers is considered a practical near-termsolutionfortheserichhydrocarbonwastematerialswhileminimizingcapitalrequirementsandmaintainingthe highefficiency ofpulverizedcoalreactors.Systematic andnumericalmodeling ofcoal/tireshredfuelblendgasificationis presentedinthisstudy.Co-combustionandgasificationoftireshredandcoalisacomplexproblemthatinvolvesgasand particle phases, along with the effect of turbulence on the chemical reactions. Coal/tire shred gasification modeling involves the prediction of volatile evolution and char burnout from the co-pulverized coal/biomass particles along with simulation of thegasificationchemistryoccurringinthegasphase.Themathematicalmodelsusedforco-pulverizedcoal/tireshred particlegasificationconsistofmodelsforturbulentflow(RNGk-model);gasphasegasification(SpeciesTransport model);particlesdispersionbyturbulentflow(CloudTrackingmodel);coal/biomassparticlesdevolatilization(Constant Rate model); heterogeneous char reaction (Multiple surface reactions model); and radiation (Discrete Ordinates model). The coal was blended with 5, 10, and 20% tire shred (mass basis) for co-gasification. The effect of the percentage of tire shred blended with coal on the temperature distribution, products distribution, particle burnout rate, and pollutant emissions at the exit of the furnace will be presented. 2010 Jordan Journal of Mechanical and Industrial Engineering. All rights reserved Keywords: Cogasification, numerical modeling, systematic modeling, char-burnout.

1. Introduction* A relatively new technology for electricity production that isgainingprominenceintheworldisthatofgasification. Wastetiresandcoalco-gasificationwithintheexisting infrastructureofpulverizedcoalutilityboilersorgasifiers isviewedasapracticalnear-termmeansofencouraging renewableenergywhileminimizingcapitalrequirements andmaintainingthehighefficiencyofpulverizedcoal boilers/gasifiers[1-3].Thewideavailabilityofpulverized coalboilers(innumberandcapacity)translatesinto significantopportunitiesforwastetireutilizationevenat levelsofonly5to20%ofthermalinput.Coal/tireco-gasificationhasseveralbenefits:itisthefastestwayto increasetheuseofthehumongousquantitiesoftire disposed every year for electric power generation; it saves capital cost by utilizing existing plant infrastructure; and it offersenvironmentaladvantages,suchasreducingNox emissions Corresponding Authors. ijanajreh@masdar.ac.ae. [4].Co-pulverizedcoalandtireparticlesgasification modelingisacomplexproblemthatinvolvesgasand particlephasesalongwiththeeffectofturbulenceonthe chemicalreactions.Inadditiontosolvingthetransport equationsforthecontinuousphase(gas),thediscrete secondphase(sphericalsolidparticles)intheLagrangian frameofreferenceisalsopredicted.Discretephase modelingisusedforthepredictionofparticletrajectories and the individual conservation equations for the chemical speciesaresolvedutilizingthespeciesconvection-diffusion equation. 2010Jordan Journal of Mechanical and Industrial Engineering. All rights reserved - Volume 4, Number 1(ISSN 1995-6665) 156 2. Governing Equations and Solution Procedures Modelingofgasificationinvolvestheapplicationof conservationlaws,andaccountingforvolatileevolution, charparticlesburnout,andcouplingthehomogeneous chemistryoccurringinthegasphaseaswellasthe heterogeneous in the solid phase. Systematic Analysis Equations:Systematicanalysisorzero-dimension-modelingisan equilibriumanalysiswhichquantitativelyestimatesthe associated oxidizer and moderator feedstreams and syngas yieldsinthecaseofgasification.Itpredictsspeciesand determinestheneededoxygenandmoderator(CO2or H2O)permoleoffeedstockattheoperatingtemperature andpressureofagivengaisifier.Thereareseveral gasificationtechnologiesthatclassifiedaccordingtotheir operatingtemperatureaslow,intermediate,andhightemperaturesuchasfixedbed(movingbed)gasifier, fluidizedbedgasifierandentrainedflowgasifier,respectively.Thehighoperatingtemperature(~1250-1600oC)oftheentrainedflowgasifierreducesthe residence time that make them popular for high throughput application. Entrain gasifiers are amenable for equilibrium duetothefastchemicalreactionsandthecalculationsof syngascompositionarereasonablyaccurate[5].Whether accommodatingpulverizedsolidhydrocarbonorinslurry andwiththeadditionofoxidizer,thegoalofsystematic analysisistodeterminetheequilibriumspecies concentrationsaswellasthetemperatureandpressureof theproducts.Whilethereactionsconsistofseveral hundreds of steps and hundreds of species and radicals, the followinggeneralthreestepreactionsrepresentthe predominant reaction pathway. ) 1 ( / 75 2) . 1 ( / 173 2) . 1 ( / 1314 2 ) (2 ) (2 2 ) (,,,,,,c C Kmole MJ CH H C reaction n Methanatiob C Kmole MJ CO CO C reaction Boudouarda C Kmole MJ H CO O H C reaction gas waterr fr br fr br fr bkksolidkksolidkksolid ++ ++ + +

(1) Thefeedstock(whetherismixedorhomogenous) consumesalltheavailableoxygeninaseriesofthree heterogeneousreactions.Intheabovereactionsthe feedstock is represented by a solid carbon and the products arelimitedtosixspecies(unconvertedsolidC(solid), CO,CO2, CH4, H2, and H2O). The Methanation reaction is severalordersofmagnitudeslowerthantheBoudouard andWatergasreactions(SmoothandSmith1985).Each ofthesereactionequationsisindependent,andhasan associatedequilibriumequationintermsofeithertheir molar concentration or partial pressure as follow: b a d cp cbBaAdDcCp b ad ccRT T K T K andp pp pT K orB AD CT K +== =) ).( ( ) (..) (] [ ] [] [ ] [) (

(2) Where [X] indicates the molar concentration of species X,lowercasec,d,a,andbarethestoichiometric coefficients,andKcisgenerallyexpressedinArrhenius rate as: RTEr crre T A T k=|) ( (3) where Ar is the pre-exponent constant, is temperature exponentconstant,Eristheactivationenergy,Risthe universalgasconstant(R=8.313kJ/kmol.K),Tisthe absolutetemperature.AtequilibriumKcisexpressedin termsofGibbsfreeenergy(Ago=Aho-TAso)thatties1st and 2nd thermodynamic quantities to indicate reaction spontaneousity as: RTgcoe T kA= ) ( (4) Wherehoandsoarethestandardenthalpyand entropychangeofreactions.Thethreeelementalmass conservationequationsforC,HandOaddanotherthree equations to the three reactions equilibrium equations. The totalmolarfractionsandenergyequationwarrantthe solution of the system (6 species and any two variables of four: Temperature, pressure, and moderator oxidizer molar concentrationpermoleoffeedstock).Thesteadyformof the energy equation is written as: = =+ =product niit reac nii i iQ h n h n1tan1

(5) Wheretheenthalpyterms,h,includeenthalpiesof formation and sensible enthalpies. Continuous Phase and CFD Equations: ThecontinuousphaseisgovernedbyNavier-Stoke equation that associated with source term: ( ) ( )source diffusion advective rate TimeSx xux ti iii| ||| | +||.|

\|ccIcc =cc+cc(6) where is the density and upper case S is the source termsduetothedispersed/discretephase interaction. is thedependentvariablecorrespondingtodensity(),the densityvelocitymultiple(,ui),andthetemperature(T), representingtheconservationofmass,momentum,and energyrespectively.canalsorepresentturbulent scalars,i.e.turbulentkineticenergy(k)andturbulent dissipationrate().Thesetwoequationsinsteadystate flow regime are written as: kCx xxuxuxukCxuxkx xuxuxuxkuzi ztiijjiijtziii kti ijjiijtii221c coc cco ||.|

\|cccc+cc||.|

\|cc+cc=cc||.|

\|cccc +cc||.|

\|cc+cc=cc

(7) Therighthandtermsrepresentthegeneration,the diffusion, and destruction respectively. In these equations, tistheturbulentoreddyviscosity . Wheref andCareconstantsandCc /2k C ft=1z,C2z,ok, and oz, are empiricalconstants.Thetransportationofspeciesmiis written as:( ) ( ) ( )i iiit t m iii iiiS RxmSc Dxm uxmt++cc+cc=cc+cc/,

(8) Where Di,m is the diffusion coefficient.Sct is the turbulent Schmidtnumberwhichisaratiooftheeddyviscosityt totheeddydiffusivityDti,m.Thesetransportequations 2010Jordan Journal of Mechanical and Industrial Engineering. All rights reserved - Volume 4, Number 1(ISSN 1995-6665) 157 incorporateareactionsourceterm RiinadditiontotheSi which accounts for discrete phase interaction. The Ri term is governed by the stoichiometric reaction below: iNir ikkiNir iS v S vr fr b = =' ' '1,1,,, (9) The ith species production/destruction due to the reaction r is written as: ||.|

\| ' ' ' =[ [= ='NjNjvr j b r j f r i r i r i r ir j rjC k C k v v M R1 1, , , , , ,*,*,] [ ] [ ) (q (10) wherekisthereactionconstantdescribedinequation3, and[C]jisthemolarconcentrationofjthspecieraisedto stoichiometriccoefficientsandreactionorder,andMi is the molecular weight of species i. Discrete Modeling Justification: Gasificationprocessesaretypicallyturbulentandhence requiremodelingavoidingtheexhaustiveandnumerical intensive direct numerical simulation (DNS). For example thelengthscale,velocityandReynoldsnumberofgas turbinecombustor,afterburner,andutilityfurnaceare (0.1m,50m/s,250,000),(0.5m,100m/s,2,500,000),and (10m,10m/s,5,000,000)respectively.Thesmallest turbulent scale, known as Kolmogorov scale, denoted with is expressed as: 43Re .= L q

(11) where L is the characteristic length scale. Solving the flow field down to scale requires Re computational node for each dimension, Re 9/4 nodes for three dimensional, and it isimpracticalatRe=10,000(109nodes).Thediscrete Lagrangianmethodisusedforthesolidphase.Atlow volumefraction) (o theaverageparticledistanceis greaterthantwiceitsdiameter,therefore,particle-particle interactioncanbeneglected.Smallparticulateloading c c d d o o / (