Small-Scale Biogas-SOFC Plant: Technical Analysis and Assessment of Different Fuel Reforming Options

This paper investigates simulation results of the thermodynamic performance of a 25 kW_el small-scale solid oxide fuel cell model fuelled with biogas. Hereby, biogas is produced from a predefined group of substrates, namely, livestock effluents, energy crops, agricultural waste, and organic waste. For the analysis, average methane (CH₄) content is assumed to lie between 50 and 60 mol % as large seasonal and daily variations are observed which are independent of used matter. The main biogas contaminants are sulfur compounds, mostly in the form of H₂S. Sulfur leads to fast catalyst deactivation in the reformer and fuel cell, which is why an effective gas-cleaning system is established. For this, ZnO and activated carbon are the most practical gas-cleaning solutions for small-scale plants. The energy model of the solid oxide fuel cell plant is designed and analyzed through detailed energy and mass balance calculations throughout all system components and streams. The above-mentioned model is set up in in such a way that different gas reforming options can be analyzed and compared with each other in which steam, partial oxidation, and autothermal reforming are included. A comprehensive electrochemical model of the solid oxide fuel cell stack based on data from literature is applied in order to account for polarization losses under varying operating conditions. The system analysis shows that the highest electric efficiency of 56.55% based on a lower heating value is achieved under steam reforming. This value lies around 15 percentage points above average electric efficiencies of biogas engines based on the lower heating value. The highest total plant efficiency (electric plus thermal) of 74.14% is reached under partial oxidation reforming, as exothermic reforming reactions increase thermal output of the plant. Within the parametric study, it is concluded that due to low electric efficiency and high sensitivity to biogas composition, autothermal reforming is suboptimal reforming option.