Abstract: In an SOFC stack system, a reformer supplies reformate to the stack. An anode tail gas portion is recycled into the reformer. An inline flowmeter indicates an apparent recycle flow rate. A map of anode tail gas composition as a function of fuel utilization efficiency U, reformer oxygen/carbon ratio O:C, and recycle percentage R is generated on a laboratory test bed at various recycle rates and fuel utilization rates. Compositions are sent through a flowmeter and a Coriolus mass flowmeter provide a Coriolus flow and sensor voltage function across the various compositions and flow rates to create an average curve and an average (reference) voltage at a given flow. A scale factor Z is calculated and used to determine a scaled sensor voltage X for use in an average flow polynomial curve fit equation to calculate tail gas mass flow rate at any flow value indicated by the flow meter.

Abstract: An SOFC fuel cell stack system in accordance with the invention including a recycle flow leg for recycling a portion of the anode tail gas into the inlet of an associated hydrocarbon reformer supplying reformate to the stack. The recycle leg includes a controllable pump for varying the flow rate of tail gas. Preferably, a heat exchanger is provided in the leg ahead of the pump for cooling the tail gas via heat exchange with incoming cathode air. A low-wattage electrical reheater is also preferably included between the heat exchanger and the pump to maintain the temperature of tail gas entering the pump, during conditions of low tail gas flow, at a drybulb temperature above the dewpoint of the tail gas.