Experimental investigation of direct internal reforming of biogas in solid oxide fuel cells

This work investigates the behaviour of planar solid oxide fuel cells (SOFCs) fed by two different fuel mixtures that simulate biogases coming from anaerobic digestion. The fuel mixtures are namely 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 that contains a mixture of methane and carbon dioxide with 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, such that bio-hydrogen is produced as the only effluent of the digester (a mixture of H2/CO2, with no traces of methane).Both bio-methane and bio-hydrogen have been directly fed to SOFC planar cells; in particular, an anode supported cell (ASC) with nickel and 8 mol% yttrium-stabilised zirconia (Ni-YSZ), and an electrolyte supported cell (ESC) with a 3YSZ (3 mol% yttrium-stabilised zirconia) electrolyte and a thin Ni-GDC (Ni/Gd-doped ceria) anode have been tested. The bio-methane resulted in carbon deposition in both cells without the addition of any oxidant to promote methane conversion in H2 and CO. The addition of addition of air, steam and CO2 as oxidants was successfully tested to produce a direct internal reforming of methane onto the anode surface. For each selected bio-CH4/oxidant mixture, a stable behaviour of the cell voltage under load conditions of 0.5 A cm−2 for the ASC and 0.3 A cm−2 for the ESC were observed for at least 50 h at 800 °C. The oxidant addition was demonstrated to be effective in preventing carbon-deposition and converting the methane into H2 and CO.The bio-hydrogen was tested and found not to need an oxidant. Degradation did not occur in a temperature range of 700–850 °C. This biogas was completely safe in terms of carbon-deposition at SOFC temperatures.An energy model of a complete SOFC system running on reformed bio-methane and on bio-hydrogen in its original composition has been developed. The results reveal that the best performances are obtained with the steam-reformed methane, where a DC electrical efficiency of over 41% is achieved.