Basic Study of Anode Off-Gas Recycling Solid Oxide Fuel Cell Module with Fuel Regenerator

Abstract

In recent years, with the rise of global environmental protection activities, it is becoming necessary to reduce CO2 emissions in social activities. In the field of power generation, it is very important to capture CO2 emissions and increase the conversion efficiency from chemical energy to electrical energy. Because the increased electrical efficiency can lead to the increased power, it is likely to reduce the consumption of fossil fuels for power generation, resulting in reduced CO2 emissions. In our previous report, the effect of a fuel regenerator in a two-stage solid oxide fuel cell (SOFC) module on the electrical efficiency was studied. The results showed that the electrical efficiency was considerably improved by removing H2O and CO2 from the anode off-gas by the fuel regenerator. The highest electrical efficiency was obtained when 100% of H2O and CO2 were removed from the anode off-gas, and it was estimated to be a net alternating current (AC) of 71.8% (lower heating value (LHV)). Although it is necessary to maintain the module at a high temperature in order to realize the system, a 4.8 kW two-stage SOFC module with a fuel regenerator was developed and confirmed to have the electrical efficiency equivalent to a net AC of 66.0% (LHV) with the thermal self-sustainability. In 2020, a demonstration test using a highly efficient two-stage SOFC system has started. In this study, we focused on an anode off-gas recycling SOFC module with a fuel regenerator. The reason why we studied the module is that it can generate highly efficient power and capture a large amount of CO2 at the fuel regenerator. Therefore, the effect of the fuel regeneration on both the electrical efficiency and capture of CO2 was examined. The authors are presently not aware of any reports of examining the effect of the fuel regenerator, which removes CO2 and/or H2O extensively from the anode off-gas, on the electrical efficiency and CO2 capture in an anode off-gas recycling SOFC module. The calculation was carried out under the following conditions. CH4 was used as a fuel, and the ratio of steam to carbon of the inlet fuel was 2.5 or more in order to avoid the carbon deposition. The temperature of both the reformer and SOFC stack was set at 700°C. The area specific resistance and current density of the SOFC stack were assumed to be 0.383 ohm cm2 and 0.25 A cm–2, respectively. The lowest available fuel concentration, which is assumed to be the ratio of the partial pressure of hydrogen to water vapor (= p(H2) / p(H2O)), was set by calculating the outlet fuel concentration when operating a single-pass SOFC stack with a maximum fuel utilization rate of 80%. For estimating the system performance, the inverter efficiency and auxiliary device loss rate were assumed to be 95% and 6%, respectively. The Nernst voltage was determined from the inlet and outlet gas composition of the stack which is calculated by a process simulation software. The electrical efficiency was estimated from the cell voltage and fuel utilization rate. The highest electrical efficiency was obtained when both the fuel utilization rate and recycle rate were 100%, and the removal rates of CO2 and H2O at the fuel regenerator were 100% and 44.4%, respectively. The reason to remove only 44.4% of H2O from the anode off-gas is to keep the ratio of steam to carbon of the inlet fuel at 2.5. The electrical efficiency was a net AC of approximately 75.5% (LHV) with 100% of CO2 capture at the fuel regenerator. In addition, the CO2 removal rate was studied to reduce to 63.9% because there is a margin in the fuel concentration of the SOFC stack. Even if the removal rates of CO2 and H2O at the fuel regenerator were 63.9% and 44.4%, respectively, the electrical efficiency was estimated to be a net AC of approximately 74.9% (LHV), and the CO2 capture at the fuel regenerator was 100%. These results show that the anode off-gas recycling SOFC module can generate highly efficient power as well as the two-stage SOFC module when using the fuel regenerator. Furthermore, the anode off-gas recycling SOFC module has advantage of capturing CO2 at the fuel regenerator. Although it is not easy to generate highly efficient power with the thermal self-sustainability, it is expected that the anode off-gas recycling SOFC module with the fuel regenerator is developed and put into practical use in the future.