Development and Simulation of an Innovative Planar Stack Design-Combining a Solid Oxide Fuel Cell (SOFC) With an Allothermal Steam Reformer

Abstract

Increasing energy demands, limited resources, pollutants, and CO2-emissions caused by the use of fossil fuels require a more efficient and sustainable energy production. Due to their high electrical efficiencies as well as fuel and application flexibilities, high temperature fuel cells offer great potential to meet the demands of the future energy supply. The fuel gases hydrogen and carbon monoxide, which are electrochemically convertible in solid oxide fuel cells (SOFCs), have to be generated by reformation or gasification of hydrocarbons, or in the case of pure hydrogen, as fuel gas, by electrolysis. For these generating processes energy is required. This generally leads to a deterioration of SOFC-system efficiencies. At state of the art combined processes, the reformation or gasification reactor and the SOFC are usually separated. The heat required for the endothermic reforming is generated by partial oxidation (POx) of the supplied fuel or by using the waste heat of the exhaust gases. At the Institute for Heat- and Fuel-Technology of the Technische Universität Braunschweig, an innovative planar SOFC-stack-design with indirect internal reforming and without bipolar plates was developed. Due to the thermal and material couplings, the SOFC-waste heat can be directly used to supply the endothermic reforming process. Additionally, a part of the hot anode off-gas, consisting mainly of water vapor, is recycled as a reforming agent. Therefore, based on the principle of the chemical heat pump, depending on the fuel used, system efficiencies of more than 60% can be achieved, even though the SOFC itself reached only an electrical efficiency of approximately 50%. Additionally, due to the cascaded SOFC structure resulting in high fuel utilization, postcombustion of the waste gases is no longer necessary. Because of the SOFC membrane allowing only an oxygen-ion flow and thus representing an air separation unit and the SOFC design without the mixing of anode and cathode flows, a simple CO2-separation can be realized by condensing the water vapor out of the anode off-gas. Another advantage of the newly developed stack design is its parallel interconnection, which leads to higher reliability concerning single stack levels. The aim of the work was a first dimensioning of the new stack design for natural gas as a fuel and its energetical analysis concerning operation and feasibility. With the simulation program developed, the theoretical feasibility of the concept and a high electrical efficiency of about 60% were proven.