Shutdown and Startup of a SOFC/GT Hybrid System

During shutdown and startup trips, the SOFC/GT hybrid system and its components must be protected from critical incidents such as anode oxygen exposure, excessive temperature gradients and carbon deposition. A further task is to minimise the need for auxiliary equipment and resources. The paper shortly presents a previously published detailed model of a pressurised SOFC hybrid cycle for the analysis of transients and control strategies. Shutdown and startup procedures are proposed based on the possibility to use the GT system for temperature control of the SOFC. Air flow and SOFC system inlet temperature are controlled by using a combustion device upstream the turbine, turbine exhaust gas throttling and a variable bypass around the recuperative heat exchanger. During startup, a small amount of hydrogen for the ignition phase of the SOFC is used. A considerable nitrogen demand for anode flushing has been detected, though it is uncertain how much is required to safely protect the anode from oxygen exposure. In the simulation, shutdown takes app. 2 hours and startup takes app. 5.5 hours. It is, however, uncertain how quick temperature variations the SOFC can withstand. A more flexible control strategy would allow the system to follow a low load immediately after startup initiation by utilizing the gas turbine.