Assistant professor, Faculty of Mechanical Engineering, Shiraz University, Shiraz, Iran **

Transcription

1 Amrkabr Unv. of Technology Aerospace Engneerng Dept. The Thrd Fuel & Combuston Conference of IRAN Tehran - IRAN Feb. 010 FCCI LEAN HYDROGEN/AIR OXIDATION OVER PLATINUM IN A DIMENSIONAL MICROREACTOR M.H. Akbar *,, M. Andsheh Tadbr **, A.H. Sharafan Ardakan ** * Assstant professor, Faculty of Mechancal Engneerng, Shraz Unversty, Shraz, Iran ** M.Sc. Student, Faculty of Mechancal Engneerng, Shraz Unversty, Shraz, Iran ( Correspondent author s E-mal: h-akbar@shrazu.ac.r) ABSTRACT Oxdaton of hydrogen n ar wll produce large amounts of ntrc oxdes f the temperature of the products s more than about 1800 K; the actvaton of thermal mechansm for NO producton s the man cause of ths. One envronmentally compatble method for low-no x combuston of hydrogen s ts catalytc oxdaton. In ths nvestgaton, a two-dmensonal mcro-reactor for catalytc oxdaton of hydrogen over platnum s smulated, and the effects of nlet mxture equvalence rato on the product temperature and NO producton are studed. The reactor dmensons are consdered to be 00 μm n wdth and mm n length. Governng equatons for conservaton of mass, momentum, energy and the chemcal speces are solved usng an n-house CFD code. The results form ths nvestgaton ndcate that for equvalence ratos (φ) of up to 0.3, the amount of NO n the exhaust gases s less than 1 ppm. Increasng φ to 0.3 leads to 1. ppm NO mole fracton, whle at φ = 0.34 there wll be more than 1000 ppm NO n the exhaust gases. In addton, the average reactor wall temperature s 91 K at φ = 0.3, that can be used to generate the requred heat for fuel processng n small-scale fuel cells. Keywords Hydrogen-ar mxture, catalytc oxdaton, lean combuston, low NOx. INTRODUCTION Catalytc oxdaton of hydrogen s an emergng method for heat generaton n whch there can be no or lttle NOx generaton. Nonetheless, heat generaton form ths reacton wll cause the mxture near the reacton zone to heat up. Increased temperature of the mxture wll actvate NO producton mechansms n the gas phase. In the combuston of fuels that contan no ntrogen, NO s formed by three major chemcal mechansms that nvolve ntrogen from ar ncludng thermal, NOntermedate, and Fenmore mechansms [Turns 000]; there s new evdence for the possblty of a fourth route for NO producton [Bozzel 1995, Harrngton 1996]. The thermal mechansm s sgnfcant at hgh temperatures (over 1800 K), whle the Fenmore mechansm s mportant n rch combuston. In addton, the Fenmore mechansm s ntmately lnked to the combuston of hydrocarbons. The NOntermedate mechansm s mportant n fuel-lean, low temperature combustons [Correa 199]. Hellsng et al. [1991] studed heterogeneous knetcs of hydrogen-oxygen reacton over platnum. Konnov et al. [001] examned NO formaton rates for hydrogen combuston n strred reactors. They used explct expressons of NO formaton rates and modeled hydrogen combuston n the temperature range Nam et al. [003] studed heterogeneous gnton temperature of H/O/N mxture on polycrystallne platnum for a wde range of 1

2 Amrkabr Unv. of Technology Aerospace Engneerng Dept. The Thrd Fuel & Combuston Conference of IRAN Tehran - IRAN Feb. 010 FCCI composton at the atmospherc pressure. They proposed a new surface knetc model of hydrogen oxdaton on platnum by modfcaton of the tradtonal surface reacton model. Appel et al. [004] nvestgated catalytc combuston of hydrogenar mxtures over platnum and valdated varous hetero/homogeneous chemcal reactons. In the present study, fuel-lean combuston of hydrogen s modelled n a catalytc mcro-reactor. The dmensons of the reactor are 0. mm n wdth and mm n length. The nlet mxture temperature and velocty are consdered to be 300 K and 1.6 m/s, respectvely, whch correspond to a Reynolds number of about 80 (based on the channel hydraulc dameter). Ar composton n ths study s assumed to be 78% ntrogen, 0.74% oxygen and 1.6% argon. Varous equvalence ratos are set for the nlet mxture and the effects of ths parameter are studed on the wall temperature and NO formaton. CHEMICAL KINETICS In ths study, two global reactons are consdered: hydrogen oxdaton (R1) and NO formaton (R). R1: H + ½ O HO H = 4 kj/mol R: ½ N + ½ O NO H = 90.3 kj/mol Separate mechansms for the oxdaton of hydrogen (R1) are mplemented on the reactor surface and n the gas phase. The one-step mechansm of Schefer [198] s used for the heterogeneous catalytc reacton, wth the correspondng rate equaton gven by Eq. 1. R s H exp( / ) 3, RT (1) For the homogeneous reacton, the mechansm suggested by Marnov et al. [1995] s mplemented whch s gven by Eq.. R v 0.5 H O exp( / ) 13, RT () In the above relatons, [H] and [O] are hydrogen and oxygen concentratons n mol/cm 3, R s the unversal gas constant (8.314 j/molk), and T s the temperature n K. Snce the combuston s fuel-lean and the temperature of the reactor doman s less than 1800 K, only the NO-ntermedate mechansm for NO producton s expected to occur. Thus, the global rate equaton for R by ths mechansm s adopted from Konnov [001]. He derved the rate equaton for NO formaton n a hydrogen/ar mxture from 8 elementary reactons whch are tabulated n Table 1. In hs dervaton, he used the steady state assumpton for the NO concentraton and obtaned Eq. 3 for NO formaton reacton. R v, (( k (k K / k ) K 1/ [ O ] 1/ [ H k4k3k[ H ][ N ][ O ][ M ] 1/ 1/ [ O ] ( k k ) K [ H k K 8 7 1/ ] ) 1/ ] 1/ )[ H O] (3)

3 Amrkabr Unv. of Technology Aerospace Engneerng Dept. The Thrd Fuel & Combuston Conference of IRAN Tehran - IRAN Feb. 010 FCCI In ths equaton k s the reacton rate of reacton n Table 1, and Kj s the equlbrum constant of reacton j (j=1,). Table 1 Elementary reactons for NO formaton, unts are cm 3 -mole-cal-k, k=at n exp(-ea/rt) No. Reacton A n Ea Source 1 O + H = OH + H ,85 [Baulch et al. 199] H + O = OH + O ,843 [Baulch et al. 1994] 3 H + OH = HO + H ,300 [Baulch et al. 199] 4 N + O + M = NO + M ,640 [Rohrg et al. 1996] 1 5 NO + O = N + O ,00 [Hanson et al. 1984] 6 NO + O = NO + NO ,630 [Hanson et al. 1984] 7 NO + H = N + OH ,750 [Bozzell et al. 1994] 8 NO + H = NH + NO ,155 [Bozzell et al. 1994] 1 Enhanced thrd body effcences (relatve to Ar): O=1.4, N=1.7, NO=3.0, HO=1. GOVERNING EQUATIONS AND SOLUTION METHOD Model Assumptons The flow regme n ths artcle s assumed to be steady, ncompressble and lamnar. The gas mxture s treated as an deal gas wth varable propertes based on the local composton. The gas flow mxture conssts of 6 speces: H, O, N, HO, Ar and NO. Furthermore, Dufour and Soret effects are neglected n the speces mass dffuson. Governng Equatons The governng equatons are conservatons of mass, momentum, energy and chemcal speces, as well as the deal gas mxture equaton. The contnuty equaton for a steady-state flow n a Cartesan coordnate s gven by Eq. 4. ( u ) 0 (4) x The Conservaton of momentum s the Naver-Stokes equaton, gven n Eq. 5. ( u u j ) p u (5) x x x x j Eq. 6 gves chemcal speces conservaton for the th speces. The term n Eq. 6 s the molar producton rate of speces whch s dfferent on the surface of the reactor and n the gas phase regon accordng to Eqs ( uyk ) Y k D eff kmwk x x, x (6) j Conservaton of energy s mplemented to calculate the temperature, and s gven n Eq. 7. ( u c pt ) T k H Rr, (7) x x x j 1 3

4 Amrkabr Unv. of Technology Aerospace Engneerng Dept. The Thrd Fuel & Combuston Conference of IRAN Tehran - IRAN Feb. 010 FCCI In ths equaton Rr s Rs on the reactor surface, and Rv n the nternal computatonal cells, and (=1,) s the reacton number. Gas mxture densty s computed usng the deal gas relatonshp for a mult-component mxture, gven n Eq. 8. P MWmx RuT The mxture molecular weght n the above equaton s computed from the followng. 1 MWmx MW Y MW The effectve mass dffuson coeffcent, Deff, n Eq. 6 s computed usng the followng. 1 Deff, j j D j j Here Dj s the bnary dffuson coeffcent. The bnary mass dffusvty s calculated by the relatonshps gven by Red et al. [1987]. The methodology s based on the Chapman-Enskog theoretcal descrpton of a bnary mxture of gases at low to moderate pressures. In ths theory, the bnary dffuson coeffcent for the speces par and j s gven by Eqs (8) (9) (10) D T j 1 P MW j j j 1 MW 1 1 MWj D MW (1) j j (13) D * * * * (14) T exp T exp T exp T T k B T (15) * j (11) Table gves the values of hard-sphere collson dameter,, as well as Lennard-Jones energy,, for varous speces nvolved n our calculatons. The constant pressure specfc heat, dynamc vscosty, and thermal conductvty of the gas mxture are computed usng Eqs c p, eff cp, Y (16) eff Y (17) keff k (18) It s noted that the values of constant specfc heat of each speces vares wth temperature because of the hgh temperature gradents across the reactor. Table 4

5 Amrkabr Unv. of Technology Aerospace Engneerng Dept. The Thrd Fuel & Combuston Conference of IRAN Tehran - IRAN Feb. 010 FCCI Hard sphere collson dameter and Lennard Jones energy parameter for each speces Speces Α k B (K ) H O HO N Ar NO Numercal Approach The non-lnear, coupled governng equatons are solved usng a fnte volume method based on the SIMPLE algorthm. The convectve terms are dscretzed usng the power-law scheme, and the dffuson terms usng a second-order central dfference scheme. The resultng algebrac equatons are solved by a pont-by-pont teraton method. The nlet boundary condtons are the velocty, temperature and composton of the mxture. The reactor walls are assumed adabatc. At the outlet, the pressure s fxed. RESULTS AND DISSCUSSION In ths artcle, smulaton results for a two-dmensonal, steady-state, mcro-reactor for catalytc oxdaton of hydrogen are presented. The computatonal doman conssts of 3,150 fnte volume cells wth unform grd nodes. However, to select ths fnal doman dscretzaton, a grd study s performed for the base-lne condton on three dfferent grds: a fne grd of nodes (1,900 cells), a medum grd of nodes (3,150 cells) and a coarse grd of nodes (1,650 cells). The smulaton results for the wall temperature along the reactor usng these three grds are compared n Fg. 1. These results ndcate that the medum and fne grds predct temperatures that are vrtually ndstngushable on ths dagram. It s therefore admssble to use the medum grd n the subsequent smulatons. In order to valdate the present code, a comparson s made n Fg. between the predcted wall temperature and the correspondng expermental data by Appel et al. [004]. The nlet velocty n ths case s set to 1.6 m/s, nlet temperature 313 K, nlet equvalence rato 0.8, and the channel hydraulc dameter and length are 13.1 mm, 300 mm, respectvely. The numercal results exhbt a good agreement wth the correspondng expermental data. 5

6 The Thrd Fuel & Combuston Conference of IRAN Amrkabr Unv. of Technology Aerospace Engneerng Dept. Tehran - IRAN Feb. 010 FCCI Tw (K) x43 150x1 110x Relatve length Fg. 1. Grd study of the smulaton results Tw(K) Current Study Experment Relatve length Fg.. Valdaton of the smulaton results Smulaton results for catalytc oxdaton of hydrogen-ar mxture are gven now for the baselne condtons whch are shown n Table 3. Table 3 Base-lne condtons at the nlet of the reactor Inlet condton Value Tn 300 K φ 0.3 un 1.6 m/s % H O 18.65% H O 0.0 N 69.7% Ar 0.89% 0.0 NO 6

7 Amrkabr Unv. of Technology Aerospace Engneerng Dept. The Thrd Fuel & Combuston Conference of IRAN Tehran - IRAN Feb. 010 FCCI Contours of H, O, HO and NO mole fractons for these condtons are presented n Fg. 3, and those for the temperature are gven n Fg 4. It s obvous from these results that the hydrogen oxdaton reacton mostly occurs by the heterogeneous reacton on the reactor surface, where the catalyst exsts. (a) (b) (c) (d) Fg 3. Mole fracton contours for base-lne condtons: (a) H, (b) O, (c) HO, (d) NO Fg. 3(d) ndcates that the NO concentraton s neglgble n the frst half of the reactor length, but t ncreases n the second half-length of the reactor where the temperature rses above 800 K. Fg 4. Temperature contours for the base-lne condtons It s seen n Fg. 4 that the temperature s ncreased on the reactor wall surfaces snce the surface reactons are domnated. Fg. 5 shows the outlet NO mole fracton versus the nlet equvalence rato. NO mole fracton s approxmately zero up to an equvalence rato of 0.3; t then ncreases consderably as the equvalence rato s ncreased further. The reason for ths behavour s that by ncreasng φ, 7

8 Amrkabr Unv. of Technology Aerospace Engneerng Dept. The Thrd Fuel & Combuston Conference of IRAN Tehran - IRAN Feb. 010 FCCI the temperature of the mxture n the reactor wll ncrease, as shown n Fg. 6. Hence, the NO formaton rate wll ncrease rapdly for φ > 0.3. NO (ppm) 1.00E E E E E E E E-15 Equvalence rato Fg 5. Varaton of NO mole fracton versus nlet equvalence rato Tw (K) Equvalence rato Fg 6. Varaton of the average wall temperature versus nlet equvalence rato CONCLUSION In ths nvestgaton, a two-dmensonal, steady-state, mcro-reactor for hetero/homogeneous oxdaton of lean hydrogen-ar mxtures s modelled. The behavour of the results from ths numercal smulaton has a good agreement wth the expermental data. The results from ths smulaton ndcate that n order to reduce the NO mole fracton n the exhausted gases, equvalence rato of the nlet feedng gas should not be more than 0.3. In ths condton, the NO mole fracton s less than 3 ppm. The average wall temperature, whch s acheved at ths equvalence rato, s 91 K and can be used generate the requred heat for a mcro-scale fuel reformer of a fuel cell system. REFERENCES 8

9 Amrkabr Unv. of Technology Aerospace Engneerng Dept. The Thrd Fuel & Combuston Conference of IRAN Tehran - IRAN Feb. 010 FCCI Appel, C., Mantzaras, J., Schaeren, R., Bombach, R., Inauen, A. [004], Catalytc Combuston of Hydrogen Ar Mxtures Over Platnum: Valdaton of Hetero/Homogeneous Chemcal Reacton Schemes, Jornal of Clean Ar, Vol. 5, pp Baulch, D.L., Cobos, C.J., Cox, R.A., Esser, C., Frank, P., Just, T., Kerr, J.A., Pllng, M.J., Troe, J., Walker, R.W. and Warnatz, J. [199], Evaluated Knetc Data for Combuston Modelng, Journal of Physcal Chemcal Reference Data, Vol. 1, pp Baulch, D.L., Cobos, C.J., Cox, R.A., Frank, P., Hayman, G., Just, T., Kerr, J.A., Murrells, T., Pllng, M.J., Troe, J., Walker, R.W. and Warnatz, J. [1994], Summary Table of Evaluated Knetc Data for Combuston Modelng: Supplement 1, Journal of Combust and Flame, Vol. 98, pp. 59. Bozzell, J.W. and Dean, A.M. [1995], O + NNH: A Possble New Route for NOx Formaton n Flames, Internatonal Journal of Chemcal Knetcs, Vol. 7, pp Correa, S.M. [199], A Revew of NOx Formaton under Gas Turbne Combuston Condtons, Combuston Scence and Technology, Vol. 87, pp Hanson, R.K., Salman S., In: Gardner Jr. W.C., edtor [1984], Combuston Chemstry, Sprnger, New York. Harrngton, J.E., et al. [1996], Evdence for a New NO Producton Mechansm n Flames, 6 th Symposum (Internatonal) on Combuston, The Combuston Insttute, Pttsburgh, PA, pp Hellsng, B., Kasemo, B. and Zhdanov, V.P. [1991], Knetcs of the Hydrogen Oxygen Reacton on Platnum, Journal of Catalyss, Vol. 13, pp Konnov, A.A., Colson, G. and De Ruyck J. [001], NO Formaton Rates for Hydrogen Combuston n Strred Reactors, Journal of Fuel, Vol. 80, pp Marnov, N.M., Westbrook, C.K. and Ptz, W.J. [1995], Detaled and Global Chemcal Knetcs Model for Hydrogen, 8 th Internatonal Symposum on Transport Propertes, San Francsco, CA. Nam, C.H. and Shn, H.D. [003], Knetc Model of the HO Surface Reacton on Platnum at Hgh Partal Pressure, Reacton Knetcs and Catalyss Letters, Vol. 80, No. 1, pp Red, R.C., Prausntz, J.M. and Polng, B.E. [1987], The Propertes of Gases and Lquds, McGraw-Hll, New York. Rohrg, M., Petersen, E.L., Davdson, D.F. and Hanson, R.K. [1996], Measurement of the Rate Coeffcent of the Reacton CH+O Products n the Temperature Range 00 to 600 K, Internatonal Journal of Chemcal Knetcs, Vol. 8, pp

10 Amrkabr Unv. of Technology Aerospace Engneerng Dept. The Thrd Fuel & Combuston Conference of IRAN Tehran - IRAN Feb. 010 FCCI Schefer, R.W. [198], Catalyzed Combuston of H/Ar Mxtures n a Flat Plate Boundary Layer: II. Numercal Model, Combuston and Flame, Vol. 45, pp Turns, S.R. [000], An Introducton to Combuston: Concepts and Applcatons, nd edton, McGraw-Hll, Sngapore. 10