Effects of anode fuel recirculation on SOFCs fuelled with biogas

Biogas is one of the most interesting renewable sources and, in particular, landfill biogas, which has a low heating value, can be used in a solid oxide fuel cell (SOFC) in order to produce electricity, at higher efficiency with respect to internal combustion engine (ICE) or other types of fuel cell, and heat. An SOFC working with carbon-containing fuels needs a reforming agent, such as steam, carbon dioxide or oxygen, in order to avoid the carbon deposition, which leads to a fast degradation of the cell. Pre-mixed biogas, which consists in biogas with the same composition of the real one, but mixed directly through mass flow controller (MFC), in order not to have impurities, and real biogas, which needs a pre-cleaning, have been used. A new way to avoid carbon formation is to substitute the reforming agent by applying the recirculation of the anode off-gases again at the cell inlet. The aim of this thesis is to prove that recirculation allows avoiding carbon deposition, even if the reforming agent is not used, and increasing the electrical efficiency of the cell. It has been developed first using computer simulation in order to understand the general relationship between the recirculation ratio (RR), which means the amount of anode off-gases recirculated, and the carbon activity and so to identify the boundary conditions, and secondly through experiments in the laboratory. The second part has been divided in two section: firstly a "fictitious" recirculation has been applied, where the amount of recirculated gases has been taken from the simulation and directly injected at the anode making use of MFC; secondly the real recirculation, using a pump, has been performed. For each simulation and test a planar cell of 16 $cm^2$ operating at 750$^\circ C$ has been employed, with different types of biogas: pre-mixed landfill (low methane content), pre-mixed wastewater (high methane content) and real landfill. Cell performance and degradation were analyzed considering different RR, comparing not only the results obtained from simulations and tests but also the differences with respect to the biogas composition. Simulations and experiments show a decrease of carbon activity proportional to the RR, both for landfill and wastewater, while a decrement of degradation rate and a rise of efficiency (from 2 to 5 percentage points) in inverse proportion to RR are observed.