Development of Fuel-Flexible SOFC

Abstract

Efficient utilization of low-grade biomass resources (bio-wastes) such as sewage sludge, agricultural residues, livestock manures and garbage in a sustainable way must bring about a promising solution against global environmental issues. Biogas, mixture of CH4 and CO2, produced by anaerobic fermentation of bio-wastes is a useful fuel for power generation in a community abundant in biomass resources. For the biomass power generation, generally gas engine (GE) systems are applied, but then their low electrical efficiency of 20-30 % attributed to Carnot limitation and energy losses caused by multiple energy conversions can become critical problems for small-sized distributed power generation. Solid oxide fuel cell (SOFC), highly efficient energy conversion device, is one of the most promising candidates as a distributed power generator in the next generation, because fuel processing can be proceeded within the cell to produce syngas which will be directly converted to electricity without combustion process, in other words not through thermal energy. This operation mode is called direct internal reforming (DIR) operation. Hydrocarbon fuels including biogas can be converted into electrical energy with 2-3 times higher efficiency in comparison to GEs. In this study, for the global dissemination of SOFC, simplification of fuel processing was studied for further enhancement of energy conversion efficiency and creation of compact system, focusing on biogas as a fuel. With the above motivation, flexible paper-structured catalyst (PSC) has been developed. Especially, hydrotalcite (HT)-dispersed PSC (HT-PSC) exhibits high tolerance for sulfur poisoning in the methane dry reforming, and therefore it was applied to biogas-fuelled SOFC as a reaction field of the internal reforming or pre-reformer. In this study, planar-shaped compact reformer in which HT-PSC was embedded (PSC-reformer) was developed. The PSC reformer was applied as a pre-reformer to a stack testing fuelled by simulated biogas (equimolar mixture of CH4 and CO2). The PSC-reformer exhibited stable CH4 conversion leading to stable stack operation with power density nearly equal to that for pure H2 under the same load current during which residual CO2 can contribute to the reduction of anodic activation loss in the low current density region. In the high current density region, H2 amount is boosted up through water gas shift (WGS) reaction between CO and H2O produced by reforming and electrochemical oxidation of H2, respectively, which has positive impact on the stack operation. For further enhancement of the catalytic activity of PSC to suppress carbon formation during dry reforming, core-shell structure catalyst (Ni/CeO2) was prepared and dispersed in the fiber network. The reformate composition with CO/H2 > 1 suggests that Ni particles with the CeO2shell can promote reverse Boudouard reaction, resulting in the considerable suppression of carbon deposition. Moreover, in comparison with a conventional PSC, Ni loading amount can be suppressed to get a certain methane conversion probably due to the oxygen storage capacity of ceria. Technology of the fuel-flexible SOFC is now studied in the Vietnam-Japan International joint research project, “Project for Sustainable Development of Rural Area by Effective Utilization of Bio-wastes with Highly Efficient Fuel Cell Technology”, under the SATREPS program supported by Japan Science and Technology Agency (JST) and Japan International Cooperation Agency (JICA). Purpose of this joint research is to develop and demonstrate an SOFC-integrated energy circulation system for the environmental conservation and the stable energy supply in Mekong Delta region. Sludge accumulated at the bottom of a shrimp pond will be pumped out and fed to methane fermentation reactor to produce biogas. This biogas is supplied to SOFC system to generate electricity which will be used for the shrimp culture. In this system, PSC technology will be applied to increase energy conversion efficiency.