Indirect Internal Steam Reforming of Methane in Solid Oxide Fuel Cells

Studies in Surface Science and Catalysis J.J. Spivey, E. Iglesia and T.H. Fleisch (Editors) 9 2001 Elsevier Science B.V. All rights reserved.

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Indirect Internal Steam Reforming of Methane in Solid Oxide Fuel Cells P. Aguiar a, E. Ramirez-Cabrera b, N. Lapefia-Rey~, A. Atkinson b, L.S. Kershenbaum ~ and D. Chadwick ~ Department ofaChemical Engineering and bMatefials, Imperial College of Science, Technology and Medicine, London, U.K A model of indirect internal reforming of methane in SOFCs has been developed. Simulation of SOFC performance demonstrates that mass-transfer controlled catalytic steam reforming can alleviate local temperature minima at the reformer entrance. It is also shown on the basis of measured methane reforming rates, that the use of oxide catalysts such as Ce-Gd-O would lead to smooth temperature profiles. 1. INTRODUCTION The solid oxide fuel cell (SOFC) operates at high temperatures (700-1000~ and can use H2 and CO (and hydrocarbons) as fuel. Improved overall efficiency can be achieved by internal reforming, and it has been shown that there is sufficient heat available for the complete conversion of methane [1 ]. Direct internal reforming has proved elusive due to deactivation of the electrocatalyst [2]. Indirect internal steam reforming of methane requires efficient thermal coupling of the endothermic reforming reaction to the exothermic oxidation reactions. However, such coupling is not easy to achieve because of the mismatch between the high activity of steam reforming catalysts at typical SOFC temperatures, and the heat available from the fuel cell reactions. Significant local cooling can result leading to thermally-induced fractures of ceramic components. The simple solution of diluting the steam reforming catalyst to reduce its net activity is inadequate because of the inevitable carbon (and sulfur) deposition, which leads to catalyst deactivation [3]. In our approach to this problem, we have developed a model of indirect internal reforming that can be used to define the required catalyst performance and distribution. To meet the desired performance characteristics, we seek to develop oxide-based catalysts, which have a lower activity than conventional steam reforming catalysts while being highly resistant to carbon deposition, or to control the reaction rate by means of mass transfer. The latter can be achieved by the introduction of a diffusive barrier near the outer surface of the catalyst. Under mass transfer control the rate of the reforming reaction is reduced whilst maintaining the overall activity (per unit mass) in the face of possible deactivation [4]. However, it is possible that a combination of approaches may be needed to achieve the desired temperature profile and for ease of fabrication. The paper reports experimental results of steam reforming over an oxide catalyst, Ce0.9Gd0.102.x, and modelling of indirect internal reforming in an SOFC. Simulations of temperature and methane concentration profiles have been performed using kinetic data of a Ni steam reforming catalyst at various dilutions, measured rate data for the oxide catalyst, and mass transfer limited steam reforming.

502 2. EXPERIMENTAL

CGO with composition Ce0.9Gd0.~O2.x was supplied by Rhodia and was calcined for lhr at 1000~ in order to minimize sintering at the maximum reaction temperature. The BET surface area after calcination was 7.1m:g ~. TPRx (25~ "~) and isothermal reaction were performed in a quartz tube, flow reactor with QMS operating at atmospheric pressure with 1-5%CI-I4 in At. For steam reforming, gas passed through a water evaporator at 90~ followed by a condenser controlled at a lower temtmature to give CI-I4:H20 ratios between 0.6-5.5. Lines downstream were maintained at 70~ to prevent condensation. TPO was carried out in 10%O2/He at 10~ ~ atter cooling to room temperature under argon. 3. MODELLING

Indirect internal reforming in SOFCs is illustrated schematically in Figure 1. A steam reforming reactor model and a solid oxide fuel cell model are required to be coupled. As our focus of interest is optimisation of the reforming catalyst, we have chosen in this initial work to develop a steady-state model for indirect internal reforming based on a genetic configuration: an annular tubular geometry with a single, central reforming reactor channel packed with the reforming catalyst [5]. A conventional steady-state, heterogeneous, 2-d fixed-bed catalytic reactor model [6-8] has been used for the inner reforming reactor. A steady-state, 1-d model is used for the SOFC [9-

11].

Fig. 1 Schematic diagram of a solid oxide fuel cell with an indirect internal reformer The SOFC model comprises mass balances of the fuel and air channels, energy balances of the same gas channels and the solid structure (cathode, electrolyte, and anode), and an electrochemical model that relates the gas stream compositions and temperatures to the current density, overpotentials, and cell voltage. The chemical species considered are CH4, H20, CO, H2, and CO2 for the fuel, and 02 and N2 for the cathode gas. The molar flux in the gas channels is considered to be mainly convective in the flow dirr Simultaneous electrochemical conversion of H: and CO to H20 and CO: is accounted for, the electrochemical reactiom occurring only at the anode/electrolyte and cathode/electrolyte interfaces. On the anode side, it is assmned that the water gas shift reaction is at equifibrium [10]. The thermal flux in the solid structure is mainly conductive. In the gas channels, it is mainly convective in the gas flow direction and conductive from the channels to the solid parts. An additional convective heat transfer between the anode gas stream and the adjacent inner reformer is also considered. It is asstlmed that all reaction enthalpies are released at the solid structure [10,12]. Radiation is not

503 considered so far, despite the high temperatures in a SOFC system. The electrochemical model is based on the approach of Achenbach [10] which assumes the electrochemical reactions operate under kinetic control. The kinetic parameters for reactions at the anode and cathode [10] are used without modification. Full details of the combined model for the SOFC and internal reformer will be presented elsewhere [13]. The resulting system of differential and algebraic equations is solved using gPROMS (Centre for Process Systems Engineering) with the orthogonal collocation on finite elements method [14].

4. RESULTS AND DISCUSSION 4.1. Steam reforming activity of cerias Interest has centred on doped cerias because of the use of ceria as a key constituent of SOFC anodes. Gd increases the concentration of oxygen vacancies, while Nb increases the concentration of mobile electron carders. Results are given for Gd doped ceria (CGO) with composition Ce~.9Gdo.lO2.x.Nb-doped cerias have been investigated, but as these catalysts proved to have lower steam reforming activity the results are not included here. Reaction of CGO with dry CI-I4 at 900~ gives a H2/CO ratio of 2.14 demonstrating that the dominant reaction is (1) in agreement with published results on undoped ceria [ 15]. CeO2 + n CH4 = CeO2., + n CO + 2n H: (1) The activation energy was estimated to be 17610 mol~ and 155kJ mol l from the H2 and CO signals respectively in good agreement with the value 16010 tool"~ for ceria [15]. Steam reforming began at 670~ producing H:, CO with a small &mount of CO:. The rate of CI-I4 conversion increased rapidly with temperature passing through a maximum before reaching an approximately constant level at 900~ after about 140 min. The steady-state rate at 900~ was 8.9x10 5 molmin~gmq, which is about 10.5 of the rate over a Ni steam reforming catalyst. The reforming rate was proportional to the CI-I4 concentration, but independent of the steam concentration. The activation energy was determined to be 153kJ mol~. No carbon deposition could be detected by TPO after any of the experiments involving steam. The rate of CI-I4 steam reforming at 900~ and activation energy are approximately equal to the values for the reaction with dry 5%CI-I4. This suggests a steam reforming mechanism that is controlled by the reaction between CI-I4 and lattice oxygen in the CGO surface. 4.2. Simulation of indirect internal steam reforming The combined model of indirect internal steam reforming and SOFC described above was used to simulate temperature profiles along the reformer tube using various catalyst options. Interest centres on the occurrence of a local temperature minimum near the reformer entrance. The base case catalyst was a Ni steam reforming catalyst. The kinetics of the steam reforming and shift reactions are based on the work of Xu and Froment [7,8] as were the properties of the Ni catalyst. The length of the module was taken as 0.3m and the diameter of the steam reforming reactor was between 2 - 3ram The diameter of the steam reforming catalyst was 0.2ram Steam reforming of CH4 on the anode was neglected, as the concentration was very low in most cases. In all cases, inlet CI-h/H20 ratio was 0.5 with small amounts of CO and I-I2, and some CO2, air inlet temperature = 950~ current density = 4x103A/m2, fuel utilization = 0.75, and air ratio =10. The simulations demonstrate the local cooling associated with indirect internal reforming using the standard Ni catalyst diluted to 2xl 0 3, Fig.2a. With a fuel inlet of 900~ methane

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505 b rapidly converted in the first 20% of the reformer resulting in a temperature minimum. Greater catalyst concentration results in almost instantaneous CH4 conversion. Reducing the fuel inlet temperature while maintaining the air inlet temperature and catalyst content gives a smoother temperature profile, Fig. 2, but leads to an undesirable large temperature rise along the tube. Simulations using the measured rate data for the Ce09Gd0.~O2-x catalyst (but assuming the same form of rate equation as the Ni catalyst), demonstrated that there would be only about 30% methane conversion in the reformer. Increasing the activity to 10.4 of the Ni catalyst leads to a smooth temperature profile, Fig. 3, but a few percent of methane is not converted in the reformer. Although the activity is approaching an order of magnitude higher than the particular CGO catalyst studied, it is probably within the achievable range for oxide catalysts. A higher fuel inlet temperature than used here could be required. The effect of controlling the reaction rate by a mass transfer barrier is shown in Fig 4. The characteristics of the mass transfer barrier and active catalyst distribution within the catalyst particle were taken from Aguiar et al [4], where optimum values were determined to maintain a desired rate of reaction despite a reduction of the intrinsic catalyst activity by 50% because of deactivation. While the predicted temperature profile is much smoother, the characteristics of the barrier require further optimisation. It should be emphasised that an advantage of mass transfer limited steam reforming is the ability to tolerate a degree of catalyst deactivation.

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Fig. 4. Axial profiles using a standard diluted Ni catalyst and with a mass transfer barrier for a fuel inlet temperature of 900~ (a) Temperature: .____! Reformer - Ni cat., . . . . . . SOFC fuel channel- Ni cat., _ . . . . . . Reformer - Ni cat. & mass transfer,. .................. SOFC fuel channel - Ni cat. & mass transfer; (b) Reformer CI-I4 mole fraction: . _ Ni cat., . . . . . . . Ni cat. & mass transfer. Preliminary work has been carried out to develop the methodology of deposition of a gasdiffusion barrier around an active steam reforming catalyst. The method used for the membrane fabrication is based on Sol-Gel processing techniques, which are widely used in the ceramics

506 field for the production of oxides with a controlled microstructure, shape, density, and porosity. The barrier materials are based on commercially available sols of zirconia and ceria, which satisfy the requirement for high resistance to carbon deposition. The micro structure of the barriers prepared using the sols showed highly dense structures lacking the desired porosity. Porosity has been introduced by mixing the sols with material of larger particle size. Although preliminary results are very promising, the barrier fabrication process is still under optimisatiorL 5. CONCLUSIONS

Simulation of the performance of an indirect internal reforming SOFC has demonstrated that local temperature minima associated with rapid steam reforming over Ni (and by implication other metal) catalysts can be alleviated by mass transfer control. Oxide catalysts, such as Ceo.gGdo.lO2.x , carl also s m o o t h the temperature profile, but do not convert all the methane for the same inlet t ~ t u r e and reformer length. 6. ACKNOWLEDGEMENTS

The authors are grateful for financial support from the UK Engineering and Physical Sciences Research Council, the Department of Trade and Industry (DTI) and Rolls Royce Pie. The first author would also like to acknowledge financial support from the Portuguese agency FCT through fellowship PRAXIS XXI/BD/15972/98. E. Ramirez-Cabrera thanks CONACyT, Mexico, for the award of a study scholarship. 7. REFERENCES

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