Analysis of a Gas Turbine Almost Real Open Cycle Linked to the SOFC Module Behaviour

Abstract

Solid oxide fuel cells (SOFCs) are particular promising electrochemical generators for their high operating temperature that makes them suitable for integration with gas turbine cycles and cogeneration. The kind of fuel cell taken into account is the tubular SOFC technology, mainly developed by Siemens-Westinghouse. The SOFC module is fed with natural gas with internal reforming and internal air preheating. Elevated pressures are adopted in order to increase both fuel cell efficiency and power density. The inlet air temperature is set to 630 °C, and the inlet fuel temperature to 200 °C. The heat generated by irreversibility at the electrodes structure is given to the fuel and air flow, and partially lost by the module external carter. A significant fraction of the heat is used by the endothermic reforming reactions, thus reducing the requested cooling air flow and enhancing the fuel heating value. The SOFC module finally generates a single outlet stream: both cathode and anode flue gases (depleted air and depleted fuel) react burning in a combustion chamber. Sometimes it is possible flue gases to preheat the inlet air flow and afterwards to expand in a gas turbine driving the turboblower. The authors consider the matching between the SOFC module and a system consisting of two coaxial turbomachines, i.e. a turbine and a turboblower. They analyse a turboblower flowed by air led into the SOFC at 630 °C and separately a gas turbine driving the turboblower, but flowed by a different fluid consisting of SOFC exhausted gases. For sake of simplicity, despite the high turbine inlet temperature (TIT), they assume there is not cooling of the blades. In order to increase the efficiency of the whole system they have studied the variation of the main parameters of the two turbomachines, especially the compression ratio and they have found that only a correct employment of flue gases enthalpy can achieve a good efficiency of the power plant. Furthermore they have used NIST-JANAF coefficients to describe the polytropic processes both of the compression and of the expansion.