Performance Analysis of Solid-oxide Electrolysis Cells for Syngas Production by H 2 O/CO 2 Co-electrolysis

Transcription

1 A publcaton of CHEMICAL ENGINEERING TRANSACTIONS VOL. 57, 17 Guest Edtors: Sauro Perucc, Jří Jaromír Klemeš, Laura Pazza, Serafm Bakals Copyrght 17, AIDIC Servz S.r.l. ISBN ; ISSN The Italan Assocaton of Chemcal Engneerng Onlne at Performance Analyss of Sold-oxde Electrolyss Cells for Syngas Producton by H O/ Co-electrolyss Dang Saebea *a, Suthda Authayanun b, Yaneeporn Patcharavorachot c, Sopatta Sosuwan a, Suttcha Assabumrungrat d, Amorncha Arpornwchanop e a Department of Chemcal Engneerng, Faculty of Engneerng, Burapha Unversty, Chonbur 131, Thaland b Department of Chemcal Engneerng, Faculty of Engneerng, Srnakharnwrot Unversty, Nakorn Nayok 61, Thaland c School of Chemcal Engneerng, Faculty of Engneerng, Kng Mongkut's Insttute of Technology Ladkrabang, Bangkok 15, Thaland d Center of Excellence on Catalyss and Catalytc Reacton Engneerng, Department of Cehmcal Engneerng, Chulalongkorn Unversty, Bangkok 133, Thaland e Computatonal Process Engneerng Research Unt, Department of Chemcal Engneerng, Faculty of Engneerng, Chulalongkorn Unversty, Bangkok 133, Thaland dangs@eng.buu.ac.th Hgh-temperature sold oxde electrolyss cells (SOECs) are promsng technologes to store excess renewable energy generaton. In ths work, the mathematcal model of SOEC, whch can descrbe the behavour of a cathode-supported SOEC operatng for H O and co-electrolyss, s developed from mass balance, dusty gas model, and electrochemcal model. The valdated SOEC model s used to analyse the nfluence of the reversble water-gas shft reacton takng place on the cathode on the performance of the SOEC for syngas producton. The smulaton results show that the reverse water-gas shft reacton s hghly pronounced at the cathode surface due to hgh component and can contrbute to producton. The rate of water-gas shft reacton ncreases along the depth of the cathode to the three-phase boundary. At the three-phase boundary, an ncrease n operatng temperatures results n the enhancement of the rate of water-gas shft reacton. Addtonally, regardng the SOEC performance, the electrcal energy consumed for co-electrolyss n SOEC decreases wth ncreasng temperature because the actvaton overpotentals and ohmc overpotentals are lower. 1. Introducton The global energy consumpton has ncreased contnuously whereas fossl fuel as man energy has depleted. Presently, renewable energy sources, such as solar and wnd, have receved consderable attenton due to ther abundance and sustanablty (Saebea et al., 16). However, these types of energy are restrcted n tme and space, ntermttence, and ste specfc. Hydrogen producton technologes have been developed rapdly (Saebea et al., 14). The use of electrcty from solar cells or wnd turbnes to produce hydrogen va electrolyzers s a promsng way to convert ther unstable energes to a stable chemcal energy (Ebbesen et al., 1). Among varous types of the electrolyss cells, sold oxde electrolyss cells (SOECs) have receved consderable attenton because of thers hgh cency and hgh feed mpurty tolerance (Ferrero et al., 13). Addtonally, the SOECs are able to reduce an amount of by electrolyss of to. In general, the electrolyss consumes hgh electrcal power (Kazempoor & Braun, 15). However, the power consumpton of electrolyss can be reduced by smultaneously electrolyzng H O and because more can be generated va the reverse water-gas shft (RWGS) reacton (Menon et al., 14). Because the RWGS reacton s reversble, the reacton drecton depends on ts operatng condtons. As the electrochemcal reactons, knetc reactons, mass transfers, and charge transfers of H O/ co-electrolyss n SOEC are qute complcated. The understandng of prmary operatng parameters on physcal phenomenon n SOEC s requred to provde ts optmal condtons.

2 The am of ths study s to analyse the nfluence of the RWGS reacton takng place on the cathode and the SOEC performance for the syngas producton. The SOEC model s based on mass balances, the electrochemcal model, and the dusty gas model to explan the multple reactons and mult-component mass transport n SOEC. Addtonally, the nfluence of operatng temperatures on the cell performance s nvestgated.. Modellng of SOEC A planar sold oxde electrolyss cell (SOEC) s consdered n ths study, as shown n Fgure 1. The producton of H and n SOEC needs the electrcty nput n order to proceed the electrolyss reacton. H O and are converted to H and at the cathode/electrolyte nterface va co-electrolyss (Eqs (1) and ()). Oxygen ons produced at the cathode dffuse through the electrolyte to the anode and the oxygen occurs from the oxdaton reacton at the anode, shown n Eq(3). Moreover, the WGS reacton occurs at the cathode porous materal of SOEC and can be expressed n Eq(4). The WGS reacton as the equlbrum reacton over nckel catalyst can shft forward and backward reactons dependng on the operatng condton. Ths reacton has an mportant nfluence on the producton and can reduce the power nput n the SOEC (N, 1). Reducton reacton (cathode): - - H O+e H +O (1) - - +e +O () Oxdaton reacton (Anode): - - O O +4e (3) Water-gas shft reacton: +H O H + (4) The gaseous speces n ar and fuel channels transport n the electrode to the three-phase boundary. The RWGS reacton takes place n the porous cathode. The transports of gas molecules n the electrode are consdered as the one-dmensonal dffuson along the electrode thckness from the reacton-dffuson equaton, whch s gven by: ( yp ) N R RT t (5) where y s the molar fracton of component, s the electrode porosty, P s the total pressure, R s the reacton rate of component, and N s the molar flux of component. The molar flux of mult-component H -H O-- systems s represented by the dusty gas model, whch can express the relatonshp between molar fluxes, molar concentratons, pressure gradent, and concentraton gradents as: n N y N y N P dy D D RT dx, k j1, j j j j (6) where D k, s the Knudsen dffusvty of component and and j. Dj s the bnary dffusvty of component between Fgure 1: Schematc sde vew of a planar sold oxde electrolyss cell.

3 The net current densty s the sum of current densty for the H O electrolyss and electrolyss. The partton rato of the current densty of H O and at the electrode/electrolyte nterface s calculated from the normalzaton factor ( ), whch s gven by I I = (1- ) H H (7) where y y HO HO y (8) The dfference of thermodynamc potental among the electrode reactons s the reversble potental that can be calculated by the Nernst equaton as: E E 1 rev RT PH P O H E H ln F PHO 1 rev RT P PO E ln F P (9) (1) where E the open-crcut potental at standard pressure and P s the partal pressure of component. The cell performance can be evaluated from the cell operatng voltage (Eqs (11) and (1)). The actual cell voltage, V cell, s more than the reversble voltage. Ths s due to the overpotental and nternal resstance losses,.e., the ohmc overpotental ( ohm) and the actvaton overpotentals ( act). V E ( ) ( ) (11) cell rev H ohm H act H V E ( ) ( ) (1) cell rev ohm act The ohmc overpotental can be expressed as: ohm RT (13) where R T s the nternal resstance of cell related to the thckness and the conductvty as: R T de e (14) The actvaton overpotentals can be descrbed by the non-lnear Butler Volmer equaton, whch relates to the current densty by consderng the charge transfer at the three-phase boundary as a rate-lmtng step and the actvaton overpotentals. The actvaton overpotentals for the electrochemcal reducton of H O and are gven by (1 a) Fact,c cfact,c exp exp RT RT H H (15) af act,c (1 c ) F act, c exp exp RT RT (16) The Butler Volmer equaton for the anodc actvaton overpotental can be shown n Eq(17). F F RT RT a act,a c act,a O exp exp (17)

4 Cell voltage (V) where s the charge transfer cocent (usually taken to be.5), s the exchange current densty. The knetc and cell parameters can be found n the lteratures (Suwanwarangkul, 3; Janardhanan et al., 7; Narasmhaah & Janardhanan, 13). 3. Results and dscusson The SOEC for H O/ co-electrolyss s smulated based on the mathematcal equatons shown n the prevous secton. The SOEC model ncludng the mass balances and electrochemcal model, whch are the nonlnear and dfferental equatons, s solved by usng MATLAB. To valdate the SOFC model, the I-V characterstc curves of the SOEC from the model predcton s compared wth the expermental data performed at DTU Energy converson (Ebbesen et al., 1). Table 1 shows cell parameters used for model valdaton. Fgure presents the comparson between the smulated and expermental results. It can be seen that the results of model predcton shows good agreement wth the experment data. Table 1: Cell parameters for the model valdaton Parameters Value Parameters Value Porosty, (-).35 Cathode thckness, d c (m) 5 Tortuosty, (-) 5. Anode thckness, d a (m) 5 Average pore radus, r p (m) 3.56x 111 Electrolyte thckness, d e (m) K (Experment) 113 K (Smulaton) Current densty (A/m ) Fgure : Comparson between the expermental and model-predcted I-V curves of SOEC at 113 K and 1 bar. The valdated model was used to study the behavor and performance of SOEC. To study the nfluence of the operatng parameters on the SOEC performance for H O/ co-electrolyss, the nlet gas compostons are fxed at 4% H O, 4%, 1% H, and 1% and ar s ntroduced at the anode sde. To nvestgate the nfluence of the RWGS reacton on the cathode for the syngas producton from the co-electrolyss n the SOEC, the WGS reacton rate and the gas composton along the cathode thckness at varous temperatures are consdered n Fgure 3a and 3b, respectvely. From Fgure 3a, t can ndcate that the WGS reacton rates are negatve value at the cathode surface because of hgh and H O composton. The RWGS reacton has an mpact on the cell performance at a range of the cathode thckness between - m. Thus, the mole fracton of s hgher than that of H, as shown n Fgure 3b. The WGS reacton rates ncrease from the cathode surface to the three-phase boundary along the cathode thckness. Ths s caused by an ncrease n the amount of, resultng n a hgh drvng force of the WGSR n the forward drecton. When consderng the ect of temperature, the WGS reacton rate decreases wth ncreasng temperature at the cathode thckness range of - m whereas the ncrease n temperature enhances the WGS reacton rate at the thcker cathode greater than m.

5 act,h (V) act, (V) Cell Voltage (V) Ohm (V) Mole fracton H HO (a) Depth n the cathode m) (b) Fgure 3: Effect of temperature on (a) WGSR rate and (b) gas composton at 173 K as a functon of the depth n the cathode K K Current densty (A/m ) K K Current densty (A/m ) (a) (b) K K Current densty (A/m ) K K Current densty (A/m ) (c) (d) Fgure 4: Effect of temperature on (a) cell voltage; (b) ohmc overpotental; (c) actvaton overpotentals for H O electrolyss; and (d) actvaton overpotentals for electrolyss as a functon of current densty. The operatng temperature s a key factor affectng the SOEC performance. The ect of operatng temperature on the cell voltage as a functon of current densty for H O/ co-electrolyss s llustrated n

6 Fgure 4a. It s found that the cell voltage decreases wth an ncrease n the operatng temperature, resultng n reducton of the power supply for co-electrolyss n the SOEC. The ncrease n the operatng temperature can ncrease thermal energy demand. On the contrary, the electrcal energy demand subsdes and t causes to decrease the reversble voltage. The ect of the operatng temperature on the ohmc overpotentals as a functon of the current densty s llustrated n Fgure 4b. The ohmc overpotentals s lower at hgher cell temperature. Ths s manly due to the hgh conductvty of YSZ electrolyte at hgh operatng temperatures, resultng n a decrease n the cell resstance. Moreover, an ncrease n the operatng temperature has an ect on a decrease n the nternal voltage losses. Fgure 4c and 4d show the actvaton overpotentals of H O and electrolyss as a functon of the current densty, respectvely. It can be seen that the actvaton overpotentals decrease when the cell temperature ncreases. Ths results from the hgher rate of the electrode reacton wth a hgher temperature, leadng to the ncrease n the exchange current densty. When comparng the H O and electrolyss, the actvaton overpotentals occurrng n the electrolyss s hgher than that n H O electrolyss about 53.4%, 8.%, and 7.9% for 173, 113 K, and, respectvely (at the current densty of 1, A/m ). Thus, the ncrease n operatng temperature has hgh ect on dmnshng the actvaton overpotentals of electrolyss. 4. Conclusons In ths study, the electrochemcal model of a sold oxde electrolyss cell (SOEC) for H O and coelectrolyss s presented and the model predcton s verfed by comparng wth the experment data from the lterature. The valdated model s used to study the ect of a water gas shft (WGS) reacton n the cathode durng the H O and co-electrolyss on the SOEC performance. It s found that the WGS reacton rate depends on the gas composton and operatng temperature. An ncrease n the operatng temperature decreases the WGS reacton rate at the cathode surface, owng to hgh compostons. On the other hand, the component s hgher at thcker cathode, resultng n an ncrease n the WGS reacton rate wth ncreasng temperature. Moreover, the power supply requred for the co-electrolyss n the SOEC can be reduced by the ncrease of the operatng temperature. The actvaton and ohmc overpotentals decrease when the cell operates at a hgher temperature. Acknowledgements The authors would lke to express ther grattude for fnancal support from Burapha Unversty (No.15/559) and the Thaland Research Fund (No. DPG5883). References Ebbesen S.D., Knbbe R., Mogensen M., 1, Co-Electrolyss of Steam and Carbon Doxde n Sold Oxde Cells, Journal of the Electrochemcal Socety, 159, F48-F489. Ferrero D., Lanzn A., Santarell M., Leone P., 13, A comparatve assessment on hydrogen producton from low- and hgh-temperature electrolyss, Internatonal Journal of Hydrogen Energy, 38, Janardhanan V.M., Deutschmann O., 7, Numercal study of mass and heat transport n sold-oxde fuel cells runnng on humdfed methane, Chemcal Engneerng Scence, 6 (18), Kazempoor P., Braun R.J., 15, Hydrogen and synthetc fuel producton usng hgh temperature sold oxde electrolyss cells (SOECs), Internatonal Journal of Hydrogen energy, 4, Menon V., Janardhanan, V.M., Deutschmann, O., 14, A mathematcal model to analyze sold oxde electrolyzercells (SOECs) for hydrogen producton, Chemcal Engneerng Scence, 11, Narasmhaah G., Janardhanan V.M., 13, Modelng electrolyss n sold oxde electrolyss cell, Journal of Sold State Electrochemstry, 17, N M., 1, D thermal modelng of a sold oxde electrolyzer cell (SOEC) for syngas producton by H O/ co-electrolyss, Internatonal Journal of hydrogen energy, 37, Saebea D., Authayanun S., Patcharavorachot Y., Arpornwchanop A., Enhancement of hydrogen producton for steam reformng of bogas n fludzed bed membrane reactor, Chemcal Engneerng Transactons, 39, Saebea D., Authayanun S., Patcharavorachot Y., Arpornwchanop A., Performance evaluaton of lowtemperature sold oxde fuel cells wth SDC-based electrolyte, Chemcal Engneerng Transactons, 5, 3-8. Suwanwarangkul R., Croset E., 3, Performance comparson of Fck s, dusty-gas and Stefan-Maxwell models to predct the concentraton overpotentals of a SOFC anode, Journal of Power Sources, 9-18.