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Effects of Tars on Solid Oxide Fuel Cells
von Michael HauserThe operation of solid oxide fuel cells (SOFCs) with bio-syngas from the gasification of biomass is a promising approach to highly efficient and sustainable power generation. At the same time, the coupling is challenging as several biogenic impurities in the bio-syngas have a negative effect on the anode of the SOFC. In order to be able to further improve SOFCs with regard to their robustness, it is necessary to show the existing weaknesses by means of systematic degradation through model contaminants.
The impacts of the tar compounds naphthalene and phenol on anode-supported solid oxide fuel cells were investigated experimentally. Single-cell and short-stack experiments were performed at 700 °C under load with simulated bio-syngas consisting of hydrogen, carbon monoxide, carbon dioxide, methane and water vapour. Naphthalene and phenol showed significantly different negative effects on the performance of the SOFC.
Naphthalene caused a pronounced voltage drop at all applied concentrations. It blocked electrochemical hydrogen oxidation as well as the reforming of methane and the shift of carbon monoxide. Using electrochemical impedance spectroscopy it could be shown that naphthalene initially inhibited mass transport to the three-phase boundaries and later also fuel transport through the anode substrate. The latter effect could be attributed to the absence of hydrogen from the catalytic conversion of methane and carbon monoxide in the anode substrate. The cell voltage increased again when the naphthalene supply was stopped.
Phenol, on the other hand, led to heavy carbon deposition and irreversibly damaged the structure of the anode substrate. With the exception of the highest dosed phenol concentrations this form of degradation was not visible in the electrochemical operational data. Metal dusting was identified as the underlying degradation mechanism which is caused by the high solubility of carbon in the nickel grains of substrate and anode.
The impacts of the tar compounds naphthalene and phenol on anode-supported solid oxide fuel cells were investigated experimentally. Single-cell and short-stack experiments were performed at 700 °C under load with simulated bio-syngas consisting of hydrogen, carbon monoxide, carbon dioxide, methane and water vapour. Naphthalene and phenol showed significantly different negative effects on the performance of the SOFC.
Naphthalene caused a pronounced voltage drop at all applied concentrations. It blocked electrochemical hydrogen oxidation as well as the reforming of methane and the shift of carbon monoxide. Using electrochemical impedance spectroscopy it could be shown that naphthalene initially inhibited mass transport to the three-phase boundaries and later also fuel transport through the anode substrate. The latter effect could be attributed to the absence of hydrogen from the catalytic conversion of methane and carbon monoxide in the anode substrate. The cell voltage increased again when the naphthalene supply was stopped.
Phenol, on the other hand, led to heavy carbon deposition and irreversibly damaged the structure of the anode substrate. With the exception of the highest dosed phenol concentrations this form of degradation was not visible in the electrochemical operational data. Metal dusting was identified as the underlying degradation mechanism which is caused by the high solubility of carbon in the nickel grains of substrate and anode.