GAS-TURBINE CYCLES WITH SOLID OXIDE FUEL-CELLS .1. 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 us ually highly irreversible, and is thus responsible for the low overall efficiency of the power generation process. The energy conversion eff iciency of the combustion process can be improved if immediate contact of fuel and oxygen is prevented and an oxygen carrier is used. In a p revious paper (Harvey et al., 1992), a gas turbine cycle was investiga ted in which pan of the exhaust gases - consisting mainly of CO2, H2O, and N-2 - are recycled and used as oxygen-carrying components. For th e optimized process, a theoretical thermal efficiency of 66.3 percent was achieved, based on the lower heating value (LHV) of the methane fu el. A detailed second-law analysis of the cycle revealed that, althoug h the exergy losses associated with the fuel oxidation were significan tly less than those associated with conventional direct fuel combustio n methods, these losses were still a major contributor to the overall losses of the system. One means to further improve the exergetic effic iency of a power cycle is to utilize fuel cell technology. Significant research is currently being undertaken to develop fuel cells for larg e-scale power production. High-efficiency fuels cells currently being investigated use high-temperature electrolytes, such as molten carbona tes (similar to 650 degrees C) and solid oxides (usually doped zirconi a, similar to 1000 degrees C). Solid oxide fuel cells (SOFC) have many features that make them attractive for utility and industrial applica tions. In this paper, we will therefore consider SOFC technology. In v iew of their high operating temperatures and the incomplete nature of the fuel oxidation process, fuel cells must be combined with conventio nal power generation technology to develop power plant configurations that are both functional and efficient. In this paper, we will show ho w monolithic SOFC (MSOFC) technology may be integrated into the previo usly described gas turbine cycle using recycled exhaust gases as oxyge n carriers. An optimized cycle configuration will be presented based u pon a detailed cycle analysis performed using Aspen PlusTM process sim ulation software (Aspen Technology, 1991) and a MSOFC fuel cell simula tor developed by Argonne National Labs (Ahmed et al., 1991). The optim ized cycle achieves a theoretical thermal efficiency of 77.7 percent, based on the LHV of the fuel.