Investigations into the Effects of Biofuel Contaminants on Solid Oxide Fuel Cells

Solid Oxide Fuel Cells (SOFCs) have gathered considerable attention as a clean, highly efficient conversion device for the production of both electricity and heat from fuels. Due to their high operating temperatures, SOFCs do not require pure hydrogen as fuel, but exhibit a high fuel flexibility, a major advantage concerning the high cost of hydrogen production. Nonetheless, certain requirements apply in terms of fuel purity. The presence of contaminants such as sulfur compounds, siloxanes, halogens, volatile organic compounds and tars within the fuel stream, even at trace levels can significantly reduce the lifetime of key components such as the fuel cell stack and reformer. Cleaning technologies can remove harmful impurities from the biofuels so as to meet the cleanliness requirements of SOFC stacks, though these come at higher cost. The purpose of this study is to provide guidelines regarding the maximum impurity concentrations in the biofuels which SOFCs can tolerate. This work has two principal aims: (i) to identify any threshold tolerance limits of different contaminants, if they exist and (ii) to investigate degradation mechanisms caused by contaminants. Siloxane D4, hydrogen chloride and sulfur were selected as biogas impurities. In addition, the effect of toluene (a common impurity in wood gasifier product gas) was also investigated on the performance of Ni-YSZ SOFCs. Short tests lasting 100-500 h were carried out to study the influence of the selected impurities on state-of-the-art Ni anode-supported SOFCs. Experiments were performed on single cells provided by Topsoe Fuel Cell (TOFC) and SOLIDpower and in some cases on 11-cell short stacks, provided by TOFC. Contamination tests were performed on simple hydrogen feed and more complex feeds such as simulated biogas or reformed biogas. Furthermore, in the cases of contamination with siloxane D4 and hydrogen chloride, single cell results were compared with those of short stacks. In-situ (electrochemical impedance spectroscopy- EIS) and ex-situ (electron microscopy and chemical analysis) characterization methods were used to identify the prevailing degradation mechanisms. Suggestions were made, which Ni-YSZ anode processes are affected by each contaminant. Siloxane deposit as SiO2 and block electrode pores and TPB (triple-phase-boundary) electrochemical reaction zone; HCl adsorbs on Ni particles to hinder the electrochemical reaction; sulfur, both from H2S and thiophene, adsorbs reversibly on Ni and leads to an immediate sharp cell voltage drop, which remains small at low S-concentrations. It was found that the complete removal of siloxanes is necessary, that S-compounds are acceptable to a level of 0.5 ppm and that there is no need to remove HCl and tar compounds from the biogas, for the concentration levels in which they are expected to be found in anaerobic digester gas.