Simulation and Analysis of Multi-scale Transport Phenomena and Catalytic Reactions in SOFC Anodes

Abstract

Solid oxide fuel cell (SOFC) has been considered as one of the most efficient power generation devices for the coming decades. There are various physical phenomena appearing in SOFC in multi-length and -time scales, such as multi-component gas-phase species/charge flow, thermal energy and mass transfer. Meanwhile, generation and consumption of gas- and surface-phase species together with electric current production are involved at various active sites. Catalytic reforming reactions of hydrocarbon fuels in SOFC anode are strongly coupled with the transport processes making the physical phenomena more complicated.An effective SOFC electrode design is to correctly balance each of the transport processes and the involved reactions. To deeply understand the multi-scale chemical reactions and transport processes in the anode, a fully three-dimensional numerical calculation method (CFD approach) is further developed and applied. The calculated domain includes the porous anode, fuel gas flow channel and the solid interconnects.By calculating fractions of Ni catalyst vacancies and surface-phase species coverages, the gas-phase species/heat generation and consumption related to the internal reforming reactions of methane and the electrochemical reactions of hydrogen have been implemented. The variable thermal-physical properties and transport parameters of the fuel gas mixture have also been taken into account. Furthermore, the heat transfer due to the fuel gas diffusion is implemented into the energy balance based on multi-component diffusion models. A multi-step heterogeneous steam reforming reaction mechanism based on the micro and detailed reaction mechanisms of Ni/YSZ is employed in this study. The surface reactions include 42 irreversible elementary ones accounting for the steam reforming, the water-gas shift reforming and Boudouard reactions. This microscopic reaction model describes the adsorption and desorption reactions of 6 gas-phase species and surface reactions of 12 surface-adsorbed species.The predicted results show that the catalytic reactions take place at most regions of the porous anode, and a gradual CH4 distribution profile and incomplete CH4 conversion at the anode exit are observed. Transport processes of the fuel gas-phase species and temperature distribution are affected by both the internal reforming reactions and the electrochemical reaction. Parameter studies reveal that both low operating temperature and big permeability can increase the available nickel catalyst surface fraction in the anode. Such detailed reaction scheme is much more complicated to incorporate in SOFC modelling but helpful to predict surface-phase species profiles over Ni-catalyst surfaces.