Exergy Analysis of a Gas Turbine Cycle With Steam Generation for Methane Conversion Within Solid Oxide Fuel Cells

Abstract

The combination of fuel cells with conventional mechanical power generation technologies (heat engines) promotes effective transformation of the chemical energy of fuels into electrical work. The implementation of solid oxide fuel cells (SOFCs) within gas turbine systems powered by natural gas (methane) requires an intermediate step of methane conversion to a mixture of hydrogen and carbon monoxide. State-of-the-art Ni-YSZ (yttria-stabilized zirconia) anodes permit methane conversion directly on anode surfaces, and contemporary designs of SOFC stacks allow this reaction to occur at elevated pressures. An exergy analysis of a gas turbine cycle integrated with SOFCs with internal reforming is conducted. As the efficiency of a gas turbine cycle is mainly defined by the maximum temperature at the turbine inlet, this temperature is fixed at 1573K for the analysis. In the cycle considered, the high-temperature gaseous flow from the turbine heats the input flows of natural gas and air, and is used to generate pressurized steam, which is mixed with natural gas at the SOFC stack inlet to facilitate its conversion. This technological design permits avoidance of the generally accepted recirculation of the reaction products around the anodes of SOFCs, which reduces the capacity of the SOFC stack and the entire combined power generation system correspondingly. At the same time, the thermal efficiency of the combined cycle is shown to remain high and reach 65–85% depending on the SOFC stack efficiency. The thermodynamic efficiency of the SOFC stack is defined as the ratio of electrical work generated to the methane oxidized (through the intermediate conversion). For a given design and operating condition of the SOFC stack, an increase in the thermodynamic efficiency of a SOFC is attained by increasing the fuel cell active area. Achieving the highest thermodynamic efficiency of the SOFC stack leads to a significant and nonproportional increase in the stack size and cost. For the proposed steam generating scheme, increasing the load of the SOFC stack is accompanied by a decrease in steam generation, a reduction in the steam to methane ratio at the anode inlet, and an increased possibility of catalyst coking. Accounting for these factors, the range of appropriate operating conditions of the SOFC stack in combination with steam generation within a gas turbine cycle is presented.

Thermodynamic Analysis of Solid Oxide Fuel Cell Gas Turbine Systems Operating with Various Biofuels

“…[4,[10][11][12]. Leucht et al [20] studied the dynamic performance of an SOFC-GT system through various operation strategies in order to be able to follow a particular load for the system.…”

“…Efficiencies of the order of 70-80% are expected from such a hybrid system [4,5,[10][11][12]14]. Optimization strategies have also been studied for SOFC-GT integration mostly dealing with system configuration [15,16].…”

“…The system exhaust temperature was brought down from 381 to 130.27°C. Granovskii et al [12] studied methane conversion in an SOFC-GT system using exergy where the maximum thermodynamic efficiency reached as high as 70-80%. Bavarsad [28] performed energy and exergy analysis of a NG fed SOFC-GT system based on parameters like pressure ratio and flow rates.…”

“…In fuel cells electrochemical processes of electron transfer is nearer to reversibility and hence has much higher thermodynamic efficiency depending on temperature. Among the various fuel cell types we consider the SOFC because of its high thermodynamic efficiency [10][11][12]. The main advantage lies in the high grade heat available at the fuel cell exhaust (1,073-1,273 K).…”

Thermodynamic Modeling of Solid Oxide Fuel Cell-Gas Turbine Combined Cycle Power Plant

“…This consists of two simultaneous processes; conversion of methane to synthesis gas (internal reforming) and electricity generation via oxidation of the synthesized gas (Cocco, et al, 2007). The cogeneration efficiencies of fuel cell systems have also been evaluated and compared with exergy methods (Granovskii, et al, 2008).Zhu and Kee have revealed the impact of fuel utilization factor on SOFC efficiency using a detailed electrochemical model and generated efficiency maps which give the range of methane-steam mixtures for maximum efficiency (Zhu and Kee, 2006). The exergy analysis of combined cycle power generation unit gives the details of exergy efficiency, exergy loss and exergy destruction for individual components in the plant and also for the overall plant (Reddy and Mohammed, 2007).The pressure ratio and the turbine inlet temperature are considered as the key parameters in the analysis of combined cycle power plant (Srinivas, et al, 2006).The main objective of the present work is to investigate the performance of SOFC-GT plant using diesel and gasoline as fuels and to evaluate the exergy losses in each component of the plant.…”