Catalytic Combustion of Biogas-Hydrogen Mixtures in a Packed Bed Reactor

Transcription

1 Iranan Journal of Chemcal Enneern Vol. 7, No. 2 (Sprn), 2010, IAChE Catalytc Combuston of Boas-Hydroen Mxtures n a Paced Bed Reactor S. A. Shahamr, I. Werzba Department of Mechancal and Manufacturn Enneern, Unversty of Calary 2500 Unversty Drve, NW, Calary, AB Canada, T2N 1N4 Abstract Catalytc combuston of lean premxed mxtures of boas/ar and boas/hydroen/ar on Pt n a paced bed reactor has been nvestated usn detaled surface chemstry. Non-equlbrum, adabatc and one dmensonal flow assumptons have been adopted for smulaton of the reactor. The nteracton between two phases of porous medum s ncorporated nto the model va heat and mass transfer factors. Radaton heat transfer has been taen nto account usn effectve thermal conductvty. All thermo-physcal propertes of the as phase are consdered to be temperature and concentraton dependent. Two enery equatons for two phases of the porous medum alon wth speces transport equatons are solved n the model. A mechansm of mult-step surface reactons, sutable for oxdaton of lean mxture of methane/ar, has been adopted from lterature and employed for smulatons. Ths mechansm ncludes 36 snle step reactons wth 11 surface and 6 as phase speces. Smulatons have been conducted for oxdaton of dfferent boas/ar mxtures for the nlet temperature rane of 700K to 900K at atmospherc pressure. The effect of the addton of hydroen to the boas/ar mxtures has also been nvestated. It s shown that hydroen mproves oxdaton of methane. Keywords: Boas, Catalytc, Hydroen, Detaled Chemstry 1- Introducton The use of catalytc systems for the oxdaton of common hydrocarbon fuels has been of crucal nterest n recent decades. Ths technoloy maes t possble to acheve a complete combuston of very lean fuel-ar mxtures at lower flame temperatures n combuston devces [1]. The complete combuston of such a mxture of fuels at lower temperature also results n lower NOx emsson. And ths fact has made the catalytc combuston of lean fuel-ar mxture an envronmental approach to combust fossl fuels. Moreover, ths technoloy enables the utlzaton of low heatn value fuels such as boas and landfll as, consdered as waste ases, and thus maes an addtonal source of enery. The catalytc oxdaton of lean methane-ar mxtures demonstrates a hh level of effcency at a cost whch s Correspondn author: sa.shahamr@ucalary.ca 3

2 Shahamr, Werzba compettve to other avalable technoloes (e.. thermal ncneraton) [2]. Methane s a major component of boas and landfll as and has a reenhouse effect up to 23 tmes worse than that of carbon doxde. Therefore oxdaton of these ases also attenuates the reenhouse effect of ases released nto the atmosphere [3]. Althouh methane s abundant n nature, t s very hard to react n the as phase. Many attempts have been made to mprove the oxdaton rate of methane n ar. Much expermental and analytcal evdence have reported that hydroen can be an effectve addtve to methane, permttn faster combuston rates and the employment of leaner mxtures n as-phase reactve combuston systems [4,5]. Also, t has been shown that catalytc oxdaton of hydroen can occur at very low temperatures (300K) for platnum [6-8]. Therefore, snce hydroen s very reactve n the presence of catalysts t may also be used to mprove the rate of catalytc oxdaton of other aseous fuels [7,9]. Intensve theoretcal and expermental studes have been conducted to mprove the performance, controllablty and understanddn of the catalytc systems. Because of the complexty of the physcal and chemcal phenomena nvolved and the ntmate coupln between them, most of the exstn theoretcal models for catalytc reactors are not vald over a wde rane of operatonal condtons, althouh these models are rather complcated. Most of the research was conducted wth monolth type reactors. Ths type has receved much attenton for ndustral applcatons and consequently for expermental and theoretcal studes [3,10,11]. However, a paced bed reactor can also be consdered as one deal alternatve for several ndustral applcatons where the pressure drop s not an ssue [12,13]. Ths s because 1) n comparson to the other reactor types, ths reactor type s capable of rendern a hher fuel converson rate at hh space veloctes, 2) t has a hh specfc eometrc surface area, 3) t has small thermal conductvty and therefore the catalyst reaches a lower temperature level, whle the reactor performance s not compromsed, 4) t has very hh mechancal and thermal resstances, and 5) t can be easly manufactured and altered. In comparson to monolth there s much less nformaton avalable for modeln paced bed reactors. The nformaton even narrows when t s necessary to nclude detaled chemstry n smulatons. Prmarly smplfed models of catalytc reactors benefted from one-step or rather smplfed chemstry models to descrbe the surface reactons [13,14]. However, due to chanes n rate lmtn steps wth temperature and composton n most catalytc oxdaton systems, detaled surface reacton mechansm s essental to capture the underlyn physcs and develop predctve models for reactor smulatons. Bzz et al. ncorporated detaled surface chemstry n a model for smulaton of catalytc partal oxdaton of methane on Rh n a fxed bed reactor [15]. They studed the effect of space velocty and CH 4 /O 2 rato on the selectvty of CO and H 2 and predcted an optmum rato for CH 4 /O 2 n whch a reactor yelds the maxmum converson of CH 4. Determnaton of flow pattern nsde the reactors s also another mportant factor nfluencn the accuracy of the model. The need for employn an adequate model for 4 Iranan Journal of Chemcal Enneern, Vol. 7, No. 2

3 Catalytc Combuston of Boas-Hydroen Mxtures n a Paced Bed Reactor predctn flow pattern ncreases because of computatonal lmtatons for solvn flow n the pores of an unstructured porous medum [13,14,16]. Especally f heat and mass transfer and chemcal reactons nfluence the flow pattern. Therefore, adequate heat and mass transfer coeffcents must be employed to obtan the temperature and speces concentraton n the bul of the flow and n the vcnty of the catalyst surface to calculate the reacton rates more accurately [16,17]. Treatn ether the structured or unstructured porous medum usn the volume averae method for enery calculatons has been proved to produce erroneous temperature profle n the catalytc reactors, snce the amount of heat released n one phase s much dfferent from that of other phases, and also the thermophyscal propertes of the as phase may dffer drastcally from that of the sold phase [18]. Ths smplfcaton has also been proved to brn about ncorrect predcton of nton temperature [13]. Thus, two-phase treatment for the reactors often s adopted. In ths wor oxdaton of lean mxture of boas (CH 4 /CO 2 ) and boas/hydroen n a catalytc paced bed type has been smulated. Therefore, precautons have been taen n order to adopt an adequate mechansm for surface reactons and sutable models for heat and mass transfer. The set of overnn equatons of the model have been solved usn the commercal software, FLUENT [19], alon wth a number of modfyn subroutnes. 2- Physcal descrpton of the problem A premxed, preheated homoeneous fuel/ar mxture enters nto a cylndrcal paced bed, ntally at unform temperature. The fuel and oxyen reach the catalyst avalable stes and are adsorbed, each covern ts share of the catalyst surface. The adsorbed speces reacts on the surface and the heat released from the reactons chanes the temperature whch n turn affects the reacton rates. A porton of the heat ncreases the sold temperatures and the rest s transferred to the as phase. Ths process contnues untl a steady state condton for both phases s acheved. The heat receved n the as phase may be hh enouh to promote the reacton n ths phase. Several modes of heat transfer contrbute to the transport of the heat from the sold surface; convectve heat transfer between the aseous mxture and pellets, axal heat conducton and radaton between the pellets. Convecton heat transfer provdes a coupln between as and sold phase temperatures. Moreover, the heat conducton redrects the heat from the downstream secton of the bed to ts upstream part. Also, radal heat transfer may exst n the reactor f there s heat loss from the reactor wall or non-unformty n the flow n the radal drecton. However, the present model does not nclude radal heat transfer n the whole reactor snce the reactor s assumed to operate under adabatc condtons and the flud flow s unform. Ths maes the problem smpler and provdes a better opportunty for employn mult-step chemcal netcs. F. 1 shows a schematc daram of the physcal model under consderaton for the paced bed. Iranan Journal of Chemcal Enneern, Vol. 7, No. 2 5

4 Shahamr, Werzba Fure 1. Schematc daram of the paced bed It s consdered that as and sold are not n local thermal equlbrum. Therefore, separate enery equatons must be solved for each phase. Gaseous radaton s nelble compared to the surface radaton of the pellets. Thermophyscal propertes of the as speces are a functon of the local temperature and concentraton and the propertes of the bed are unform and temperature ndependent. Snce the mxtures under study are lean, dlute approxmaton s adopted for the dffuson of speces. Pressure s assumed to reman constant n the reactor snce the reactor lenth s short and also space veloctes are not very hh. 3- Surface chemstry Several surface reacton mechansms have been proposed for partal or complete oxdaton of methane on the surface of Pt. Hcman and Schmdt developed a mechansm comprsn 19 elementary surface reactons for oxdaton of methane on the surface of platnum [20]. The one-step dssocatve adsorpton of methane to C(s) and H(s) whch appears n ths mechansm seems to be rather non-realstc. However, they obtaned ood areement wth the experment usn the mechansm. Deutschmann et al. proposed a mechansm consstn of 23 elementary steps ncludn a herarchcal dssocaton of CH 4 to C(s) and H(s) va CH 3 (s), CH 2 (s) and CH(s) [9]. They also ncluded the dependency of actvaton enery, on catalyst surface coverae, for hydroen and oxyen desorpton from Pt surface. They used ths mechansm to predct catalytc nton of methane n a monolth reactor. Snce pre-factors (.e stcn and pre-exponental coeffcent) and actvaton eneres of elementary reactons are not ndependent of the coverae of the catalyst surface, there have been attempts to nclude ths effect n recently proposed mechansms. Zerle et al. postulated a mechansm for partal oxdaton of ethane on Pt [21]. The methane oxdaton of ths wor was used by Quceno et al. to propose a mechansm for partal oxdaton of methane on Pt surface [22]. The steps n ther mechansm are almost the same as those of Deutschmann s [9]. The dependence of pre-factors and actvaton eneres on coverae of the catalyst surface has also been taen nto account by Ahalayam et al. [23]. They derved a reacton mechansm for oxdaton of methane, hydroen and carbon monoxde on the surface of platnum. BUI-QEP method was used for optmzaton of the pre-factors and actvaton eneres whle tan thermodynamc consstency nto account. Intal values for the pre-factors were taen from transton theory and those of actvaton eneres were taen from lterature. Most of the surface reacton mechansms only nclude C1 surface speces and elmnate C2 adsorbed hydrocarbons snce there s no suffcent thermodynamc and netc nformaton avalable about these speces [21], [24]. Chou et al. proposed a C1 mechansm for oxdaton of lean methane-ar mxture [25]. They added one extra step to Deutschamnn s mechansm to account for the dssocatve adsorpton of methane n the 6 Iranan Journal of Chemcal Enneern, Vol. 7, No. 2

5 Catalytc Combuston of Boas-Hydroen Mxtures n a Paced Bed Reactor presence of oxyen. Ths step s responsble for catalytc self nton of methane n oxyen domnated coverae for catalyst surface. The surface reacton steps used n the present wor comprses 36 reactons between 11 surface speces (CH 3 (s), CH 2 (s), CH(s), C(s), CO(s), CO 2 (s), H 2 O(s), OH(s), H(s), O(s) and Pt(s)) and 6 as phase speces (CH 4, H 2, O 2, CO, CO 2 and H 2 O). Reacton steps are chosen based on the steps whch have been used n Deutschmann s [9], Ahalayam s [23] and Chou s [25] mechansms, developed for the oxdaton of lean ar-methane mxtures at low and moderate temperatures (oxyen domnated surface coverae). Pre-factors and actvaton eneres for these steps have been taen from the wor of Zerle et al. [21] and Quceno et al. [22]. A number of modfcatons for the reacton steps and netc coeffcents have been performed n order to match the results of the mechansm wth the avalable expermental data [6,7]. The use of coverae dependent pre-factors and actvaton eneres for some of the reacton steps maes the mechansm sutable for a wder rane of operatn temperatures snce the catalyst oxyen free and oxyen domnated remes demand dfferent actvaton eneres for the reacton steps [23]. Table 1 llustrates the reacton steps and correspondn netc coeffcents used for smulatons. Table 1. Surface Reacton Mechansm of Oxdaton of Methane on Platnum. Unts: S 0 [-], A [S -1 ], E a [J/mol], µ [-], ε [J/mol] S 0 or A β Ea µ ε Adsorpton Reactons 1 H2+ PT(s)+ PT(s) => H(s)+ H(s) H(s) 2 O2+ PT(s)+ PT(s) => O(s)+ O(s) 0.07 (300/T) CH4+ PT(s)+ PT(s) => CH3(s)+ H(s) CH4+ O(s)+ PT(s) => CH3(s)+ OH(s) 1.36E H2O+ PT(s) => H2O(s) CO2+ PT(s) => CO2(s) CO+ PT(s) => CO(s) H+ PT(s) => H(s) O+ PT(s) => O(s) OH+ PT(s) => OH(s) Desorpton Reactons 11 H(s)+ H(s) => PT(s)+ PT(s)+ H2 1.0E H(s) 12 O(s)+ O(s) => PT(s)+ PT(s)+ O2 1.0E O(s) 13 H2O(s) => H2O+ PT(s) 4.5E CO2(s) => CO2+ PT(s) 1.0E CO(s) => CO+ PT(s) 1.0E CO(s) 16 H(s) => H+ PT(s) 6.0E H(s) 17 O(s) => O+ PT(s) 1.0E O(s) 18 OH(s) => OH+ PT(s) 5.0E O(s) Surface Reactons 19 H(s)+ O(s) => OH(s)+ PT(s) 3.5E OH(s)+ PT(s) => H(s)+ O(s) 2.0E O(s) 21 H(s)+ OH(s) => H2O(s)+ PT(s) 5.5E H2O(s)+ PT(s) => H(s)+ OH(s) 3.1E O(s) 23 OH(s)+ OH(s) => H2O(s)+ O(s) 2.0E H2O(s)+ O(s) => OH(s)+ OH(s) 2.7E O(s) 25 C(s)+ O(s) => CO(s)+ PT(s) 1.0E CO(s)+ PT(s) => C(s)+ O(s) 1.0E CO(s) 27 CO(s)+ O(s) => CO2(s)+ PT(s) 1.0E CO(s) 28 CO2(s)+ PT(s) => CO(s)+ O(s) 1.0E O(s) 29 CO(s)+ OH(s) => CO2(s)+ H(s) 2.72E O(s) 30 CO2(s)+ H(s) => CO(s)+ OH(s) 2.72E CH3(s)+ PT(s) => CH2(s)+ H(s) 3.4E CH2(s)+ H(s) => CH3(s)+ PT(s) 8.4E H(s) 33 CH2(s)+ PT(s) => CH(s)+ H(s) 2.0E C(s) 34 CH(s)+ H(s) => CH2(s)+ PT(s) 8.4E H(s) 35 CH(s)+ PT(s) => C(s)+ H(s) 8.4E H(s) 36 C(s)+ H(s) => CH(s)+ PT(s) 3.4E Iranan Journal of Chemcal Enneern, Vol. 7, No. 2 7

6 Shahamr, Werzba Snce only lean mxtures of CH 4 /Ar at low and moderate temperatures were to be nvestated, reacton R3 was ncluded n the mechansm [25]. Ths reacton becomes mportant at the oxyen free reon on the catalyst surface for dssocatve adsorpton of methane on platnum. In other words, t s responsble for the onset of methane oxdaton on the surface and leads to actvty of R2. All reactons are rreversble and the necessary thermodynamc propertes for surface speces (.e. heat of adsorpton and desorpton) and heat released from surface reactons have been adopted from [26]. 4- Model equatons The model comprses one dmensonal transent equatons of mass and enery conservaton. These equatons are descrbed as follows: Contnuty equaton ρ ( ρ u + t x ) = 0 Speces mass balance n the as phase 2 Y, Y, Y, + u = D,m 2 ρ ρ ρ t x x m, + a ρ (Y Y ) + M R ε v s,,, Speces mass balance on the catalyst surface m, Y, Ys, ) = M Rs, (1) (2) ρ ( (3) Enery balance n the as phase ρ 2 T T T c + ρcu = 2 t x x (4) h + a(t T ) + MR H ε v s, Enery balance for sold phase ρ 2 s s rad scs = s t x 1 T T q h x ε a (T T a v ) + M R 1 ε H Ideal as law pm v s s, (5) = ρ RT (6) Snce only the lean mxtures of fuel-ar were consdered, the dffuson coeffcents, D,, m n the equatons of speces balance were obtaned usn dlute mxture assumpton and netc theory. However, the dffusve mass transfer n the as phase s not snfcant n the lontudnal drecton, n comparson to the convectve mass transfer n ths drecton [15]. Ths s because the Peclet number s hh (> 500) for all the operatn veloctes. The conducton and radaton heat transfer n porous meda n the lontudnal drecton have been taen nto account usn effectve thermal conductvty [27]. Adabatc condton was adopted for the reactor walls snce the expermental data were obtaned for a well nsulated reactor, ncludn an extra heater for compensatn the effect of possble heat losses [6,7]. Thermal nteracton between the sold and as was taen nto account usn convectve heat transfer coeffcent, h, whch was obtaned from heat transfer factor, follows [17] J H h = ρuc p μ c p 2 / 3 J H, as (7) 8 Iranan Journal of Chemcal Enneern, Vol. 7, No. 2

7 Catalytc Combuston of Boas-Hydroen Mxtures n a Paced Bed Reactor Where J s computed from H Re ϕ, 0.01 < Re < 50 J = (8) H Re ϕ, 50 < Re < 1000 The mass transfer factor m, n the equatons of speces mass balance for as phase and on the surface of the catalyst s computed n an analoous way to calculate the heat transfer coeffcent, h. Ths s performed usn the Chlton Colburn analoy [28] J D = where m, ρ v x μ ρ D m. 2 / 3 (9) Re ϕ, 0.01 < Re < 50 J = (10) D Re ϕ, 50 < Re < 1000 In equatons 8 and 10 Re number s defned as ρ u Re = (11) a ϕ μ v where ϕ s the pellets shape factor and equals 1.0 for sphercal pellets, 0.91 for cylndrcal pellets and 0.81 for flaes [17]. In ths wor 0.91 was assned to ths parameter snce cylndrcal pellets were used n the smulatons. Boundary condtons The follown boundary condtons were employed for smulatons, Ts At x = 0 : T = T, nlet, h( T Ts ) = s, x = (12) Y Y, nlet T T Y s, At x = L : = 0, = 0, = 0 x x x (13) The reactor wall was consdered to be adabatc, therefore there s no need to nclude heat transfer n the radal drecton. Surface chemstry equatons Chemcal reactons on the catalyst surface appear n the form of a source term n the speces balance equaton for the surface and enery conservaton equaton of the sold phase (eq. 3 and 5). s the rate of R s, depleton or creaton of speces on the surface. If a set of elementary reactons comprses N s speces and K s elementary reactons are nvolved n the reacton mechansm, then one may wrte K N j s + Ns s, = ν f [ X j ], ( = 1,..., N ) + N s R = 1 j= 1 ν ' (14) n ths equaton [ X j ] s the molar concentraton of speces j on the surface and f s the forward rate and s calculated from f = A T β exp N s μ ( E RT ) Θ exp( ε Θ RT ) = 1 (15) where ε and μ are coverae dependence parameters and Θ s the fracton of the catalyst surface covered by the adsorbed speces. Surface coverae of speces, Θ, and surface molar concentraton of ths Iranan Journal of Chemcal Enneern, Vol. 7, No. 2 9

8 Shahamr, Werzba speces [ X ] relate va Γ, that s surface ste densty, [ X ] Γ Θ = (16) Γ s the number of moles of catalyst per unt area and depends on the lattce structure of the catalyst. The determnaton of evoluton of surface speces concentraton requres a system of ODE dθ dt = Γ R s,, ( 1 ) =,...N s (17) At present, contrbuton as phase reacton effect s nelected snce the reactor operates at atmospherc pressure and the resdence tme of the as n the reactor s not more than 10 ms [9,14]. Numercal Scheme Solvn the overnn equatons (equatons 1-6) for paced bed has been carred out usn the commercal software FLUENT [19]. In order to ntroduce surface reacton rates, the correspondn heat release n the sold phase, and also to embed an extra enery equaton for ths phase of paced bed and speces balance equaton on the surface, a number of modfyn subroutnes have been developed and used. To mprove the numercal accuracy of dscretzaton of spatal and temporal dervatves, QUICK scheme and second order method have been used, respectvely. The stff nature of surface chemstry equatons s due to a wde rane of rates for dfferent reacton steps. Therefore, a stff system nterator s requred to be employed for ths purpose. The system of ODE for surface reactons, equaton 17, s treated by a coupled stff solver. Ths allevates the rorous numercal stffness of the chemstry system of equatons whch needs to be solved at each computatonal tme step of flow. Moreover, proper ntal estmates for coverae of the surface speces can allevate the stffness of the ODE system n the very frst staes of the soluton. 800 computatonal cells have been used for one dmensonal spatal dscretzaton of the doman and a tme step of second have been used for smulatons of the paced bed reactor. 5- Results and dscusson Reactors specfcatons Smulatons were conducted for the oxdaton of a lean mxture of CH4/Ar, Ф=0.35, n the paced bed reactor. In order to valdate the model, expermental data from [6,7] have been used and all condtons and reactor propertes have been consdered to be the same. The fxed bed for whch smulatons were carred out has a lenth of 5 cm and a porosty of 0.4. The pellet densty, specfc heat and surface emssvty were respectvely 100 /m 3, 837 J/.K and 0.6. Thermal conductvty for Al 2 O 3 was chosen to be temperature dependent accordn to [7]. The approach velocty of the homoeneous mxture for all smulatons was 1.0 m/s at a reference temperature of 293K. Ths means that the physcal nlet velocty ncreases as the nlet temperature rses; however the mass flow rate stays constant. Pressure alon the reactor was assumed to reman constant and atmospherc, 89Kpa. The catalyst used was deposted polycrystallne Pt on Al 2 O 3 10 Iranan Journal of Chemcal Enneern, Vol. 7, No. 2

9 Catalytc Combuston of Boas-Hydroen Mxtures n a Paced Bed Reactor substrate n the form of cylndrcal pellets wth both a lenth and dameter of m. The reactor s embedded n a cylndrcal quartz tube wth an nsde dameter of 0.028m. Model valdaton As mentoned before, the surface netc model employed was adopted from lterature [9, 22, 24, 25]. The ste densty for the catalyst was assumed to be mol/cm 2. Snce there was no nformaton about the specfc surface eometry of the pellets, ths parameter was chosen as a fttn parameter to mnmze the devaton between the model and the experment. It was concluded that the total specfc surface eometry for actve catalyst stes for a fxed bed reactor s The comparson between the experment and the computatons for oxdaton of CH 4 and H 2, both wth equvalence ratos of 0.35, n the reactor s presented n F 2. The developed reactor model has also been valdated usn snle step lobal reacton for oxdaton of CH 4, CO and H 2 and ther mxtures on Pt and Cr 2 O 3 /Co 3 O 4 as catalysts under the same operatn condtons. The correspondn results are n ood areement wth avalable expermental data [6]. Converson, % H2 CH Inlet Temperature, K Fure 2. Comparson between experment [6,7] and calculatons performed wth surface netc model presented n Table1. calculatons, experment. Combuston of Boas n the paced bed reactor Usn ths model, converson of CH 4 n three dfferent mxtures wth CO 2 (CH 4 / CO 2 : 2/3, 1/1 and 3/2), each wth 3 dfferent equvalence ratos (0.2, 0.3 and 0.4) has also been predcted. The correspondn compostons of the resulted mxtures have been presented n Table 2. F. 3 shows the predcted fuel converson for the nlet temperatures rann from 700K to 900K. It can be seen that for the same equvalence rato the converson of fuel remans almost constant when the amount of CO 2 s chaned n the mxture. Ths s because CO 2 molar concentraton of s less than 0.06 n all mxtures, therefore t has no pronounced effect on specfc heat of the mxture. Table 2. CH 4 /CO 2 /Ar mxture composton Ф CH 4 / CO 2 = 2/3 CH 4 / CO 2 = 1/1 CH 4 / CO 2 = 3/2 CH 4 CO 2 O 2 N 2 CH 4 CO 2 O 2 N 2 CH 4 CO 2 O 2 N Iranan Journal of Chemcal Enneern, Vol. 7, No. 2 11

10 Shahamr, Werzba Fure 3. Converson of CH 4 as a functon of nlet temperature; for three ratos of CH 4 /CO 2 n fuel composton wth equvalence ratos of 0.2, 0.3 and 0.4 It should be mentoned that for catalytc oxdaton of very lean mxtures of methane/ar, the addton of dluents has no snfcant effect on the fuel converson, provded that the concentraton of fuel s ept constant and the mxture s stll lean. It can be seen that concentraton of methane nfluences the oxdaton rate much more than the rato of CH 4 /CO 2 n the fuel mxture. Ths s why a zero order Arrhenus rate wth respect to oxyen can be used for lean mxtures when a snle step lobal reacton s adopted for a model [14]. F. 4 demonstrates the effect of replacn ar wth CO 2 as a dluent when the concentraton of CH 4 s ept constant and the mxture s stll lean. The effect of hydroen addton on the oxdaton of boas n the reactor was also nvestated usn the model. Table 3 ncludes the concentraton of fuel/ hydroen/ ar mxture consdered for smulatons. The concentraton of hydroen was ept dentcal, 0.01, for all mxtures and the equvalence rato of the fuel was altered by chann the equvalence rato. The correspondn results for the effect of hydroen addton to the oxdaton of CH 4 are presented n F. 5. Fure 4. Effect of addton of dluent, CO 2, on catalytc oxdaton of methane n the reactor Table 3. CH 4 /CO 2 /H 2 /Ar mxture composton CH 4 / CO 2 = 2/3 CH 4 / CO 2 = 1/1 CH 4 / CO 2 = 3/2 ф CH4 CO2 H2 O2 N2 CH4 CO2 H2 O2 N2 CH4 CO2 H2 O2 N Iranan Journal of Chemcal Enneern, Vol. 7, No. 2

11 Catalytc Combuston of Boas-Hydroen Mxtures n a Paced Bed Reactor 100 CH4/CO2: 2/3 100 CH4/CO2: 3/ CH4 Converson, % CH4 Converson, % Inlet Temperature, K Inlet Temperature, K Fure 5. Methane converson n the reactor as a functon of nlet temperature Φ= 0.2, Φ= 0.3 and Φ=0.4, Thc lnes show the converson for the mxtures ncludn 1% H 2 H 2 s oxdzed completely at very low nlet temperatures (F. 2) and does not nfluence the process of CH 4 oxdaton chemcally snce the mxtures are lean. Therefore, the role of hydroen addton to the mxture s to release heat and ncrease temperature alon the bed. From F. 5 t s perceved that hydroen can mprove the oxdaton process of methane on the catalyst surface. Ths effect s more pronounced around lht off temperature. The reason s that, around ths temperature the senstvty of CH 4 converson to temperature s rather hh. Ths s why hydroen mproves the oxdaton of methane more snfcantly around ths temperature. In ths case, aan the concentraton of CH 4 n the mxture s an mportant factor and the converson rate ncreases as the concentraton of methane ncreases. 6- Conclusons It was shown that the process of catalytc oxdaton of methane and hydroen n the paced bed reactor at ntal temperatures of K can be predcted usn detaled surface chemstry. The effect of dluent was proved to be less snfcant for lean condtons, provded that the concentraton of fuel n the mxture s constant. Moreover, ncreasn the amount of methane n the mxture mproves the reacton rate on the catalyst surface. However the converson rate of methane for three dfferent mxtures of CH 4 and CO 2 (CH 4 /CO 2 : 2/3, 1/1 and 3/2) are very close at the same nlet temperatures and equvalence ratos. It was also shown that the oxdaton of methane can be mproved usn a small amount of hydroen n the fuel/ar mxture for the same value of the mxture equvalence rato. Iranan Journal of Chemcal Enneern, Vol. 7, No. 2 13

12 Shahamr, Werzba 7- Acnowledement The authors ratefully acnowlede the fnancal support by NSERC durn the course of ths research. Thans also o to Adam Depa for provdn expermental results. 8- Nomenclature Roman Symbols β temperature exponent ε porosty φ shape factor Γ surface ste densty, mol/m 2 μ vscosty, N.s/m 2 θ surface coverae fracton ρ densty, /m 3 υ stochometrc coeffcent Letters enthalpy of speces, J/ H j H, j D convectve heat & mass transfer factor conducton heat transfer coeffcent of as, W/m K f forward Arrhenus rate, m convectve mass transfer coeffcent of speces, 1/m 2 s K, K s number of steps n as and surface L R s t T u x M reactor lenth, m speces producton rate on surface, mol/m 2 s tme, sec temperature, K velocty, m/s x coordnate, m molar mass of speces. /mol N, N s number of as and surface speces p pressure, Pa q rad Radatve heat flux, W/m 2 R unversal as constant, J/.K R speces producton rate n as phase, mol/m 3 s X molar concentraton, mol/m 3 mass fracton of speces Y Subscrpt eff effectve, j, ndex s as phase surface, sold References [1] Forzatt, P., "Envronmental catalyss for statonary applcatons", Catalyss Today 62, 51 (2000). [2] Everaert, K., Baeyens, J., "Catalytc combuston of volatle oranc compounds", Journal of Hazardous Materals B109, 113 (2004). [3] Pablo, M., Muel, A. G., Salvador, O., Fernando, D. V., "Combuston of methane lean mxtures n reverse flow reactors: Comparson between paced and structured catalyst beds", Catalyss Today 105, 701 (2005). [4] Bar, S., "Effect of carbon doxde on the performance of boas/desel dual-fuel enne, Renewable Enery". 9(1-4), 1007 (1996). [5] Karbas, M., Werzba, I., "The effects of hydroen addton on the stablty lmts of methane jet dffuson flames", Int Journal of Hydroen Enery. 23(2) 123 (1998). [6] Werzba, I., Depa, A., "The catalytc oxdaton of heated lean homoeneously premxed aseous-fuel ar streams", Chemcal Enneern Journal. 91(2-3), 287 (2003). 14 Iranan Journal of Chemcal Enneern, Vol. 7, No. 2

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14 Shahamr, Werzba methane catalytc combuston nsde a monolth honeycomb reactor usn multstep surface reactons", Combuston Scence and Technoloy, 150 (1), 27 (2000). [26] [27] Modest, M. F., Radaton Heat Transfer, McGraw-Hll, New Yor, NY, (1993). [28] Chlton, T. H., Colburn, A. P., "Mass transfer (absorpton) coeffcents: Predcton from data on heat transfer and flud frcton". Ind. En. Chem., 26, 1183 (1934). [29] Shahamr, S. A., Werzba, I., "Combuston of lean fuel-ar mxtures nvolvn hydroen wthn a catalytc paced bed reactor", hydroen power - theoretcal and Enneern Solutons - Internatonal Symposum, Hypothess VII, March 27-30, Merda, Mexco (2007). 16 Iranan Journal of Chemcal Enneern, Vol. 7, No. 2