Gas Turbine Cycles With Solid Oxide Fuel Cells—Part I: Improved Gas Turbine Power Plant Efficiency by Use of Recycled Exhaust Gases and Fuel Cell Technology

In conventional energy conversion processes, the fuel combustion is usually highly irreversible, and is thus responsible for the low overall efficiency of the power generation process. The energy conversion efficiency of the combustion process can be improved if immediate contact of fuel and oxygen is prevented and an oxygen carrier is used. In a previous paper (Harvey et al., 1992), a gas turbine cycle was investigated in which part of the exhaust gases—consisting mainly of CO2, H2O, and N2—are recycled and used as oxygen-carrying components. For the optimized process, a theoretical thermal efficiency of 66.3 percent was achieved, based on the lower heating value (LHV) of the methane fuel. A detailed second-law analysis of the cycle revealed that, although the exergy losses associated with the fuel oxidation were significantly less than those associated with conventional direct fuel combustion methods, these losses were still a major contributor to the overall losses of the system. One means to further improve the exergetic efficiency of a power cycle is to utilize fuel cell technology. Significant research is currently being undertaken to develop fuel cells for large-scale power production. High-efficiency fuel cells currently being investigated use high-temperature electrolytes, such as molten carbonates (~ 650°C) and solid oxides (usually doped zirconia, ~1000°C). Solid oxide fuel cells (SOFC) have many features that make them attractive for utility and industrial applications. In this paper, we will therefore consider SOFC technology. In view of their high operating temperatures and the incomplete nature of the fuel oxidation process, fuel cells must be combined with conventional power generation technology to develop power plant configurations that are both functional and efficient. In this paper, we will show how monolithic SOFC (MSOFC) technology may be integrated into the previously described gas turbine cycle using recycled exhaust gases as oxygen carriers. An optimized cycle configuration will be presented based upon a detailed cycle analysis performed using Aspen Plus™ process simulation software (Aspen Technology, 1991) and a MSOFC fuel cell simulator developed by Argonne National Labs (Ahmed et al., 1991). The optimized cycle achieves a theoretical thermal efficiency of 77.7 percent, based on the LHV of the fuel.