For the effective utilization of biogas, a CH₄-CO₂ mixture obtained from the anaerobic fermentation of organic wastes, as an alternative fuel for power generation, high temperature solid oxide fuel cell (SOFC) is one of the most attractive technologies due to its considerably high efficiency of over 50 %. Capability of direct internal reforming (DIR) operation of SOFC can make fuel-processing unit more simple and compact, and can further increase efficiency because heat released during power generation can be recycled for the fuel reforming in the stack. When biogas is supplied directly to SOFC, H₂, main reactant for the electrochemical oxidation to generate electricity, is produced within the porous anode material through the simultaneous methane steam and dry reforming reactions, hereafter called methane multiple-reforming (MMR) process (Fig. 1) [1]. However, excess stresses thermally induced by the strong endothermic nature of the MMR process might cause electrolyte fracture. Therefore, the MMR process has to be properly controlled.

In this study, advanced concepts to enhance cell performance and reduce thermal stresses accompanied by DIR were studied. Fig. 2-a shows an in-cell reformer concept, a sheet of Ni-loaded paper-structured catalyst (PSC) [2,3] is placed on the anode to partially reduce CH₄conversion within the anode volume. Fig. 2-b shows a concept with thin gas-barrier mask made from dense YSZ on the anode surface, which can control reforming rate along fuel flow direction.

Current-voltage (I-V) curves for the concepts of Figs. 2-a and b were measured at 800 ^oC using a 20 x 50 mm² anode-supported cell (ASC) with the direct feed of simulated biogas mixture (CH₄/CO₂= 1), and then, a 3-dimensional CFD model of ASC coupling mass- and heat transfer, MMR and electrochemical processes was adjusted so that it can reproduce measured electrochemical behavior. For precisely predicting the consumption and production rates of gaseous species involved in the MMR process, the MMR was modeled using a method based on artificial neural network (ANN) (Inputs: CH₄, CO₂/CH₄, H₂O/CH₄, Temperature; Outputs: Consumption and production rates of CH₄ and H₂, respectively). Using the experimentally-verified CFD model, distributions of gaseous species in fuel channel, temperature, thermally-induced stresses in electrolyte and cell deformations were estimated.

This study revealed that electrolyte crack can occur at the middle region of the cell length while the minimum electrolyte temperature appears close to the fuel inlet. Performance enhancement with the in-cell reformer concept was confirmed, as shown in Fig. 3. Meanwhile, the concept using gas-barrier mask was found to be effective to reduce temperature gradient within the cell (see Fig. 4), as a result, increasing mechanical stability of DIR-SOFC.

References

1 Meng Ni, Int. J. of Hydrogen Energy 38 (36) (2013) 16373–16386.

2 Y. Shiratori, T. Ogura, H. Nakajima, M. Sakamoto, Y. Takahashi, Y. Wakita, T. Kitaoka, K. Sasaki, Int. J. Hydrogen Energy 38 (2013) 10542–10551.

3 Y. Shiratori, M. Sakamoto, J. Power Sources 332 (2016) 170-179.