The nonlinear steady state behaviour of solid oxide fuel cells (SOFCs) is investigated. It is found that the temperature dependence of the electrolyte conductivity has a very strong influence on the occurrence of multiple steady states, instabilities and the formation of hot spots. Two correlations from the literature for the electrolyte conductivity are studied in a lumped model and in a 1D spatially distributed model of a SOFC. The cases of galvanostatic operation, potentiostatic operation, and operation under a constant ohmic load are considered. The lumped model possesses a unique steady state under galvanostatic operation and up to three steady states under potentiostatic operation or under constant load. In the distributed model, three steady states may coexist under galvanostatic operation and up to five under potentiostatic operation. List of Symbols Bwidth of cell (m) c P molar heat capacity (J mol )1 K )1 ) C SE coefficient in Equation (24) Greek symbols a heat transfer coefficient (W m )2 K )1 ) b 1/2 coefficients in Equation (23) c pre-exponential kinetic factor (A m )2 ) g overpotential (V) h charge transfer coefficient k heat conductivity of the solid (W m )1 K )1 ) m stoichiometric coefficient q resistivity (W m) q S density of the solid (kg m )3 ) F electrical potential (V)

Recently, an approach involving electrochemical preferential oxidation (ECPrOx) of CO was suggested as having the potential to replace the PrOx concept for deep CO removal from reformate gas in proton exchange membrane (PEM) fuel cells. The first part of this paper deals with the characterization of such an ECPrOx unit from a reaction engineering point of view. Based on a spatially lumped, isothermal model, the qualitative selectivity-conversion behavior is discussed for varying feed flow rates and CO inlet mole fractions. A simple two-phase mechanism is suggested that explains the findings. The second part of the contribution considers qualitative questions on cascading of two ECPrOx reactors. The crucial importance of the configuration of their electrical connection is demonstrated and explained. While two cells connected electrically in parallel exhibit almost the same selectivity-conversion behavior in comparison with a single cell, an electrical series connection enables a considerable increase in the selectivity at the same CO conversion.

Forced periodic operation can lead to improved reaction processes. Such operation is studied for the example of the application of a cascade of connected adiabatic fixed-bed reactors to perform catalytic total oxidation. It is shown that the periodic operation of the configuration considered allows an autothermal operation. The concept can outperform classical reverse-flow operation concerning the reduction of slips of unconverted feed. A simple model based on the assumption of a hypothetical countercurrent of the solid phase is derived and compared with a standard dynamical model of the reactor cascade. By evaluation of steady state profiles and application of bifurcation theory, it is demonstrated that the reduced model describes the limiting case for an infinitely segmented cascade and allows assessment of important properties of the periodic process. It permits a fast and detailed analysis of limit point bifurcations, which result in curves enclosing the region of desired ignited states of the reactor. Using singularity analysis further helps to understand the influence of secondary parameters and to identify operational limits. Copyright © 2009 John Wiley & Sons, Inc. All Rights Reserved. [accessed November 25, 2009

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