Copernicann Letters Vol 1/2010 ISSN: X. Abstract. gas engine or directly combusted in boiler, which could lead to minimize the pollutants

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1 Copernicann Letter Vol 1/010 ISSN: 08968X Modelin of chemical and phyical procee in reactive porou bed (INDEX: /010 Copernican Letter Vol 1) Jacek Kluka 1), Dariuz Kardaś ) The Szewalki Intitute of Fluid Flow Machinery Polih Academy of Science jkluka@imp.da.pl 1), dk@imp.da.pl ) Abtract Thi paper contain the decription of pyrolyi and flow proce throuh porou media takin into account kinetic rate coefficient and equilibrium tate of pyrolyi. Mathematical equation of pyrolyi mechanim includin kinetic rate coefficient and equilibrium rate of pyrolyi, and equation like ma, momentum and enery and flow throuh porou zone are preented. Conidered model i baed on a imple eometry baed on downdraft aifier and analyed bain on CFD calculation of pyrolyi and flow of volatile. Numerical analyi of a a flow in an open ytem with ma ource i carried out by mean of Fluent. Calculation of the chemical and phyical procee in porou bed are performed for different poroity. 1. Introduction Bioma, which contain mainly an oranic ubtance, i a common renewable fuel in nature. The mot popular kind of bioma i wood, which i available almot everywhere in the world. Bioma i utiied in combution, aification and pyrolyi procee. Ga produced in aification proce can be ued for power eneration in. It i erie of endothermic reaction which do not involve reaction with oxyen [5,6]. The product of pyrolyi are combutible ae, tar and char. In the preented work the pyrolyi proce wa decribed by mean of one-tep lobal mechanim (Fi.1). a enine or directly combuted in boiler, which could lead to minimize the pollutant emiion and CO reduction. Thee advantae enerated coniderable interet in the aification of bioma. In a fixed bed aifier bioma contitute packed bed. The flow of volatile and ae in a aifier can be treated like a flow throuh porou media. Many invetiation [1,,3,4] turn to CFD modelin thermochemical converion of bioma in porou media becaue of the poibility of indepth look in hih temperature and hihly polluted reion and comparin it with experimental data. CFD reult could ive information on chemical and phyical procee in packed bed and could help to undertand experimental reult. Finally, numerical olution could be ued to optimize bioma aification proce.. Bioma pyrolyi Pyrolyi referred to bioma aification i the fundamental chemical proce which occur in aifier but it i till not completely comprehenive. Fi.1. Sketch of one-tep lobal mechanim of bioma pyrolyi. 16

2 Copernican Letter Vol 1/010 ISSN: X Takin into account that ma of bioma ample conit of ma of olid m and a phae m : m = m + m, (.1) where ma of olid m conit of ma of volatile m v and reidual ma m r. The chane of olid phae durin pyrolyi proce can be defined a function of kinetic rate coefficient k, period of time t and difference between olid ma of ample m and reidual ma m r : low heatin rate. The rate of pyrolyi can be defined a [9,10]: m Z = m,0 where: m ma of olid phae [k], m,0 initial ma of olid phae [k], For the very low heatin:, (.5) m = k ( m m ) t, r (.) T 0, (.6) where the reaction kinetic rate coefficient k i uually expreed by Arrheniu law [1,4]: E (- RT ) k = k0 e, (.3) pyrolyi rate Z can be function of temperature only and named a equilibrium rate of pyrolyi Z e(t) [9],[10]. If the pyrolyi proce i led with finite rate of heatin the pyrolyi rate and equilibrium rate are different: where: Ε enery activation [kj/mol], k 0 contant, Z( T) Z ( T). (.7) e R ideal a contant [ J/mol K], T temperature [K]. Accordin to literature kinetic rate coefficient can be alo calculated by mean of thermoravimetry meaurement and iven by [7,8]: m k =. (.4) t ( m m ) where m e in thi cae i a reidual ma of olid phae in certain temperature for very e After imple calculation the chane of pyrolyi rate can be expreed a a function of kinetic rate coefficient and deviation of pyrolyi rate Z from equilibrium tate: dz dt = k( Z Ze). (.8) The relationhip between pyrolyi rate Z and equilibrium rate of pyrolyi Z e i preented in Fi. 17

3 Copernicann Letter Vol 1/010 ISSN: 08968X C + H O = CO + H, (3.8) C + O = CO. (3.9) 4.Governin equation Fi.. Relationhip between pyrolyi rate Z and equilibrium rate of pyrolyi Z e. 3.Gaification proce Phyical and chemical procee in a reactive porou bed were decribed by the conervation law of ma, momentum and enery for olid and a phae and flow throuh porou media [1,11,14]. For the olid phae overnin equation are iven by: Bioma aification conit of oxidation and reduction procee [5,6]. In the oxidation zone volatile are burned with air to CO, CH 4, CO, H O and H in erie of exothermic reaction. The heat obtained in exothermic reaction i ued to releae volatile from bioma in pyrolyi proce. In the reduction zone reaction between char and CO and H O take place. All chemical reaction occurrin in aification proce could be of volumetric or urface type. The main homoenou and heteroeneou reaction which can be found in literature [1,6,1] are: CH4 + HO = CO + 3H, (3.1) CO + HO = CO + H, (3.) 1 H + O = H O, (3.3) CH 4 + O = CO + H O, (3.4) CO + H = CH4 + CO, (3.5) ρu + ρ = ε ρυ ( ε ρ υ U ) = α T + λ T And repectively for a phae: ρ υ ρ U + + ρ + ε ρ υ ( ε ρ υ υ ) ( ε ρ υ U ) = p + ε µ µ + S ( ) S, = p ( T ) + λ T m ( ) S. ( ) = S, m U (4.1) (4.) (4.3) ( ), υ + α ( ) + S, (4.4) p ( T T ) U (4.5) C + CO = CO, C + O = CO, (3.6) (3.7) 18

4 Copernican Letter Vol 1/010 ISSN: X where: ε poroity, υ fluid velocity [m/], ρ denity [k/m 3 ], S m ma ource term [k/m 3 ], p preure [Pa], µ vicoity [Pa ], S p ource term, reitance of olid to fluid flow in a porou medium, Τ temperature [K], λ thermal conductivity [W/mK], U internal enery [J], S U heat ource term [J] (due to heat releae durin chemical reaction), (lower ubcript) olid phae, (lower ubcript) a phae. To better undertand procee which occur in reactive porou bed preented model wa implified to one dimenional balance equation. Aumin that the olid phae i teady υ =0, o eq.(4.1),(4.) do not contain convection term. The poroity of a phae i defined a ratio of ae volume occupied by volatile V and the total volume V: Coniderin that the rate of pyrolyi (eq.(.5)) in followin form: m ε ρ Z = =, (4.8) m ε ρ, 0,0,0 where: ρ denity of olid phae [k/m 3 ], ρ,0 initial denity of olid phae [k/m 3 ], the ma ource in eq.(4.1) and (4.3) i iven by equation: dz ε (, 0ρ,0 = kε,0ρ,0 Z Ze) = Sm. (4.9) dt Aumin that the preure lo in the momentum equation (eq.(4.4)) i etimated by Darcy law [3], which i baed on the a averaed velocity, the ource term S p can be defined: S p µ ε = υ, (4.10) K V ε =. (4.6) V where: υ a velocity [m/], K permeability [m ], µ a vicoity [Pa ]. Takin into account definition of volume fraction of olid, the relationhip between poroity of a and olid phae fulfill formula ε ε = 1. (4.7) + The permeability can be alo decribed [11] with reard to poroity ε and particle diameter D: 19

5 Copernicann Letter Vol 1/010 ISSN: 08968X 3 ε K = D. 150 (1 ε) (4.11) tranfer wa decribed by P-1 model, which i the implet formulation of the radiation model. where: D bioma particle diameter [m], ε poroity of olid phae. 5.Model decription In preented work, the analye of the rate of pyrolyi proce, flow throuh porou media includin a temperature and velocity were preented. The eometry of CFD model which repreent a implified aification chamber, i illutrated in Fi. 3. It i aumed that the chamber i filled by two porou material in it riht part. In eneral poroity in porou zone 1 wa hiher than in porou zone. It wa aumed that beinnin of the proce cold a (300K) flow into the chamber. Fi.3. Reactor eometry and boundary condition. The heat i tranported to the porou zone by heatin wall, which wa kept in T=1400K. Other wall wa thermally tabilized in contant temperature (300K). Air (300K) enter inlet, flow throuh porou zone and cool material in zone 1 and. The heat i tranported by conduction in riht-left direction wherea oppoite direction of heat i caued by convection of ae. The radiative heat Decribed problem were treated a a non- tationary and reolved with uae of Any Fluent. For thi cae the ma, momentum and enery equation for aeou phae, and ma and enery equation for olid phae were calculated. Ma ource in ma balance equation (4.1) and (4.3) i dependent on temperature. Reitance of olid to fluid flow in a porou medium in momentumm equation (4.4) i dependent on a velocity and vicoity and zone permeability. Referred to eq.(4.9) the chane of pyrolyi rate depend on kinetic rate coefficient k, eviation of pyrolyi rate Z and equilibrium rate Z e. The equilibrium rate of pyrolyi Z e wa et by thermoravimetric experimental reult [7,8] and approximated by temperature function Z e(t). Kinetic rate coefficient k(t) wa determined baic on thermoravimetric experimental reult [7], which included reidual ma of olid phae m e in certain temperature. Kinetic parameter were et at: k 0=0.4-1, E/R=850K. When the pyrolyi rate reached equilibrium tate Z=Z e then pyrolyi proce wa topped. The initial ma fraction of volatile (ratio of volatile ma m v and the total ma of olid phae m ) wa et at Y v=0.5. The pyrolyi proce wa implemented by uer defined function. 6.Reult and dicuion Numerical analyi of pyrolyi proce wa calculated for two cae. In the firt cae (c1) poroity wa et at ε=0..8 for fixed bed of bioma (porou zone 1) and ε=0.4 for fixed bed of char (porou zone ). The diameter of bioma particle wa et at d=8 cm. In the econd cae (c) poroity wa et at ε=0.5 for 130

6 Copernicann Letter Vol 1/010 ISSN: 08968X fixed bed of bioma and ε=0..3 for fixed bed of char and the diameter of bioma particle wa et at d= cm. very low due to poroity. On the left part of chamber filled by ae the temperature increaed fater than in porou zone which i caued by a heatin by radiation from heatin wall. A ditribution of temperature for firt cae i illutrated in Fi.5. for the three repreentative point: x1-porou zone 1, x- midpoint,x3 - porou zone (Fi.3). Fi.5. The temperature ditribution of pyrolyi in porou zone. Fiure 6 illutrate the rate of pyrolyi in porou zone for cae1. The ditribution of temperature reached a maximum value at point x3 and the pyrolyi rate decreaed to minimum value Z=0.05, which repreented equilibrium rate Z e for maximum temperature, in t=950. In thi point the proce of devolatilization can be treated a finihed. For point x1 and x temperature did not reach maximum value o the rate of pyrolyi chaned ininificantly. The deviation of pyrolyi rate Z from equilibrium rate Z e in point x3 i preented in Fi.7. Fi.4. Contour of tatic temperaturee [K] for cae1 for the particular time tep. Fiure 4 illutrate the temperature front of pyrolyi. The fixed bed become heated 131

7 Copernicann Letter Vol 1/010 ISSN: 08968X Fi.6. The rate of pyrolyi in temperature function for cae 1. Fi.8. Comparion of temperature ditribution at point x1 for two cae of porou zone in fixed bed. Fi.7. The pyrolyi rate Z and equilibrium rate of pyrolyi Z e in temperature function at point x3 for cae1. Fi.9. Comparion of temperature ditribution at point x for two cae of porou zone in fixed bed. Comparion of two cae (c1, c) of zone temperature for point x1, x and x3 are preented in Fi Proce of heat propaation wa lihtly different in each cae. The temperature ditribution for cae 1 wa fater in each point which wa caued by hiher poroity than in cae. Fi.10. Comparion of temperature ditribution at point x3 for three cae of porou zone in fixed bed. The difference in ditribution of temperature for variou poroity provide to the different rate of pyrolyi which i illutrated for point x in Fi.11. The pyrolyi wa fater for cae c1 due to lower 13

8 Copernican Letter Vol 1/010 ISSN: X heat capacity of porou bed in thi cae and better tranport of radiation. Fi.13. Comparion of x-velocity at point x3 for two cae aaint the time. Fi.11. The rate of pyrolyi at point x for two cae of porou zone in fixed bed. The analyi of flow throuh porou media howed different influence of poroity on manitude of a x-velocity which i hown for point x and x3 in Fi.1 and Fi.13. For cae 1 the x-velocity wa hiher than for cae in point x and x3 due to hiher zone poroity and (with reard to eq.(4.10) and (4.11)) hiher permeability. For each cae increae of a velocity durin flow throuh porou zone wa oberved (Fi.14 and 15). It wa caued by increae of a temperature and production of additional ae in pyrolyi proce. Fi.14. Comparion of x-velocity at point x1,x and x3 for cae1. Fi.1. Comparion of x-velocity at point x for two cae aaint the time. Fi.15. Comparion of x-velocity at point x1,x and x3 for cae. 133

9 Copernican Letter Vol 1/010 ISSN: X 7.Summary A devolatilization model in porou medium ha been developed. The pyrolyi mechanim includin kinetic rate coefficient k and equilibrium tate of pyrolyi Z e were preented. Preented work howed that uer defined function make the modelin of pyrolyi proce in CFD calculation poible. The chane rate of pyrolyi can be decribed by function dependent on kinetic rate coefficient k and deviation of pyrolyi rate from equilibrium tate Z-Ze. In conequence ditribution of temperature in porou zone, the temperature front of pyrolyi wa enerated and the rate of pyrolyi wa variable in fixed bed. In thi cae different a velocity wa oberved. Increae of a velocity wa caued by heatin of porou bed and releaed volatile ource term in ma and enery equation. In ummary the numerical modelin of chemical and phyical procee in reactive porou bed can be ue to predict and analye the bioma pyrolyi. 8.Acknowledement [3] C. Brucha, B. Peterb, T. Nuba, Modellin wood combution under fixed bed condition, Fuel 8 (003) [4] A.A. Rotami, M.R. Hajaliol, S.E. Wrenn, A bioma pyrolyi ub-model for CFD application, Fuel 83 (004) [5] S. Porada, The reaction of formation of elected a product durin coal pyrolyi, Fuel 83 (004) [6]A. K. Rajvanhi, Bioma Gaification, Alternative Enery in Ariculture (1986) [7] J. Guo, A. C., Lua, Kinetic tudy on pyrolyi of extracted oil palm fiber. Iothermal and non iothermal condition, Journal of Thermal Analyi and Calorimetry, Vol. 59 (000) [8]O. Senneca, Kinetic of pyrolyi, combution and aification of three bioma, Fuel Procein Technoloy 88 (007) [9]D. Kardaś, Model zybkości pirolizy węla i biomay z uwzlędnieniem nierównowai termodynamicznej układu, Polka Akademia Nauk, Intytut Mazyn Przepływowych w Gdańku, Nr arch. 7/ ]S. Potrzednik, Ciepło odazowania paliw tałychmetoda określania, podtawowe właności, Karbo, Eneriochemia, Ekoloia, KSG, 39(9):0-8,1994. [11]FLUENT 6.3 Uer' Guide, September 006. [1]N. Prakah, T. Karunanithi. Kinetic Modelin in Bioma Pyrolyi A Review, Journal of Applied Science Reearch, 4(1): , 008. [13]H. Yan, R. Yan, H. Chen, D. Lee, C. Zhen, Characteritic of hemicelluloe, celluloe and linin pyrolyi, Fuel 86 (007) [14]Yan Y.B., Ch. Ryu, A. Khor, V.N. Sharifi, J. Swithenbank, Fuel ize effect on pinewood combution in a packed bed, Fuel 84 (005) The invetiation preented in thi paper have been obtained within the Reearch Project No. POIG /08. Model aroenery complexe a an example of ditributed coeneration baed on a local renewable enery ource. REFERENCES [1] Yiqun Wan and Lifen Yan, CFD Studie on Bioma Thermochemical Converion, Int. J. Mol. Sci. 008, 9, []Noorhelinahani Abu Bakar, Downdraft Gaifier Uin Computational Fluid Dynamic Software- Fluent, A thei ubmitted for the deere of Mater of Science