Energy integration in solid oxide fuel cell systems under different steam reformer conditions: A design and control approach

Solid oxide fuel cell (SOFC) systems are considered promising alternative to conventional energy systems for both stationary and stand-alone applications. Their fuel flexibility provides the ability for coupling with other well known technologies, such as, natural gas steam reforming, coal and biomass gasification units. The SOFC high operating temperature shows the potential for energy integration, especially with highly endothermic processes, such as steam reforming, leading to a higher overall system's efficiency. In our previous work we proposed and analyzed two different energy integrated SOFC systems (incorporating an external steam reformer). In the first configuration, the hot effluent streams leaving the SOFC were utilized directly for energy integration, while in the second configuration, the effluent streams leaving the SOFC were catalytically oxidized in a burner to produce more energy. The open- and closed-loop analysis revealed interesting features and challenges for each configuration. In this study, we analyze the effect of the reformer design parameters (steam-to-carbon (S/C) ratio and operating temperature) on the steady-state performance of the first configuration described above. Our previous work showed that this configuration is more challenging in terms of open-loop dynamics (instability) and the requirement for an external utility (furnace). Additionally, the effect of the S/C ratio on the open-loop behavior is also investigated. Lastly, in order to compensate for the process nonlinearities, a nonlinear controller for the fuel cell temperature is derived and implemented. The simulations show that the S/C ratio has a greater impact on the steady state design rather than on dynamics and control. Also, a comparative evaluation between the nonlinear and linear controllers indicates a superior performance of the nonlinear controller even in the presence of modeling errors.