Solid Oxide Fuel Cells (SOFC) is a technology with great potential. Its high efficiency makes it a relevant alternative to existing technologies for utilizing fossil fuels and its fuel versatility makes it invaluable in the transition from a fossil fuel based energy system to on based on renewable energy. The overall efficiency of a fuel cell system operating on natural gas can be significantly improved by having part of the steam reforming take place inside the SOFC stack. In order to avoid large temperature gradients as a result of the highly endothermal steam reforming reaction, the amount of internal reforming has to be carefully controlled. The objective of this thesis is to make such a careful control possible by examining the rate of internal steam reforming in SOFCs. The catalytic steam reforming activity of Ni-YSZ anode material was tested both in a packed bed reactor to determine intrinsic kinetics, and in a stack configuration to determine the rate observed under realistic SOFC conditions. The kinetic expressions obtained from respectively the packed bed measurements and the stack measurements are shown in Equations 3 and 4. Furthermore, a simple model was derived, which can accurately predict the steam reforming rate in a stack from the rate expression obtained from the packed bed experiments. During the experiments a previously unreported long term dynamic behavior of the catalyst was observed. After startup, the initial high reactivity was slowly reduced by a factor 5-10 over a period of several days or several weeks depending on operating temperature. It was also found that prolonged exposure to a H2O/H2 mixture without CH4 resulted in a reactivation of the catalytic activity up to the initial high level. It was attempted to account for this behavior through characterization with SEM, TEM, XRD and EXAFS.
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Nielsen, Jens Ulrik, Hendriksen, Peter Vang, Grunwaldt, Jan-Dierk
Technical University of Denmark, Department of Chemical Engineering, 2011