Improved methodologies for studies of sulfur poisoning on steam reforming of Ni-YSZ sofc anodes

Abstract

Consulta en la Biblioteca ETSI Industriales (9221)

[EN] Solid Oxide Fuel Cells (SOFC) is one of the most important potential renewable energy sources. Their high fuel flexibility combined with their high efficiency leads to the possibility of using fossil fuel derivates as SOFC direct fuel. Natural gas is one of the most abundant fossil fuel available. SOFCs can use methane-containing compounds as direct fuel due to the presence of Ni in the most common anode cermet material (Ni-YSZ), as well as their high operating temperatures which allow to the in-situ conversion of methane into hydrogen via methane steam reforming and water gas-shift reactions. However, sulfur compounds as H2S are unavoidable in any fossil fuel, poisoning Ni-catalyst active sites even in very small amounts (ppm levels). Therefore, it is highly important for the development of this technology to increase the knowledge in the sulfur poisoning intrinsic kinetics over SOFC anodes. Furthermore, intensive research in developing sulfur tolerant anode materials has been developed at the symmetrical cell scale, but lack of research of infiltration at the full cell scale is still missing. Infiltration is successfully achieved at the SOFC scale by anode cermet pre-reduction. Consequently, developing a methodology to achieve a systematic testing of infiltrated pre-reduced SOFCs seems crucial. Within this objective, a new experimental rig was designed, constructed and validated along this thesis in order to perform intrinsic kinetics studies of sulfur poisoning over steam reforming and water-gas shift reactions together fulfilling the demanded safety requirements. The design and construction process included a new dosage panel and calibration of mass flow controllers, high water steam content generator system, U-shaped quartz glass reactor and its coupling into the experimental setup with tailor-made fittings and complete piping system suitably insulated to avoid condensation. Additional equipment for the quantification of outlet concentration was also installed based on PO2-sensor, condenser unit and gas chromatograph. Validation of the equipment was also studied and reported. Complementary, a methodology to systematically analyze sulfur effects on methanefueled pre-reduced SOFCs was studied. Pre-reduction protocols minimizing mechanical bending were successfully achieved for Ni-YSZ/YSZ, Ni-ScYSZ/ScYSZ and Ni-ScYSZ/ScYSZ/CGO anode half-cells, the latter with TZ3YE as anode support. Cell test setup and start-up procedure was also developed minimizing mechanical stress during testing start. Additionally, an overall electrochemical characterization of two pre-reduced cells, Ni-YSZ/YSZ/LSM-YSZ (PS-131959) and i/1 Ni-ScYSZ/ScYSZ/LSM-YSZ (RR-410-102), was performed. Overall performance under different operating conditions was quantified, and the different anode and cathode contributions to the impedance spectra were allocated. Finally, quantification of the sulfur poisoning effect on methane steam reforming conversion as well as electrochemical oxidation of H2 was obtained.