Feasibility of bio-Hydrogen and bio-Methane from Anaerobic Digestion as SOFC Fuels

摘要

The following work addresses the issue of feeding a planar SOFC with two different kinds of mixtures simulating a biogas from anaerobic digestion, named bio-methane and bio- hydrogen. The first composition is the conventional one, where a biological process of fermentation is carried out to produce a gas containing essentially a mixture of methane and carbon dioxide, with some relevant traces of H2S and other organic sulphur compounds. The second mixture is representative of a biogas produced through a novel routine: a particular pre-treatment of the bacteria inoculum (generally clostridia bacteria) is performed in order to inhibit the methanogenic step in the fermentation process, and to produce bio-hydrogen as the only effluent of the digester (which is basically a mixture of H2/CO2, with no traces of methane) [1]. Aiming to a safe and durable operation of SOFCs, carbon-deposition and sulphur poisoning are well-recognized as the two major degradation issues for conventional anodes fed by hydrocarbon-based fuels. We analyse here the potential technical- energetics advantages/disadvantages, and the cell performance behaviour, derived from the selection of the conventional or the innovative fermentation process (bio-methane vs bio-hydrogen), assessing in particular the carbon-deposition issue. One of the main points is the necessity of providing a fuel processing step in case of bio-methane: therefore, we discuss several reforming processes of the bio-methane with lab experiments. Regarding the experimental session, the biogas feeding the SOFC has been in the following forms: i) bio-methane with steam addition, ii) bio-methane with air addition, III) bio-methane with CO2 addition and finally, iv) bio-hydrogen, as it is. Thus, the experiments aim also the investigation of different internal direct reforming processes on SOFC anodes. Polarization and durability experiments have been performed on these mixtures to specifically investigate carbon-deposition phenomena in a planar cell configuration. Finally, an energy model is built to analyse the electrochemical, but also energetic, behaviour of complete SOFC systems under the various types of biogas compositions and reforming typology.