Upgrading versus reforming: an energy and exergy analysis of two Solid Oxide Fuel Cell-based systems for a convenient biogas-to-electricity conversion

Abstract Aiming at designing biogas-to-electricity advanced systems, Solid Oxide Fuel Cells are promising candidates. They benefit from scalability on plant sizes that suit anaerobic digesters potentialities. For biogas-Solid Oxide Fuel Cells applications, the implementation of an external pre-reformer is usually considered. However, the possibility to perform direct fuel feeding to the Solid Oxide Fuel Cell offers new opportunities towards the realization of lean systems, which are competitive especially on small-scale installations (i.e. on-farm biogas-to-electricity conversion). In this frame, scientific literature is rather poor and, to cover this gap, system simulations are called for two reasons: first, to demonstrate the potential efficiency gain of new concepts; second, to provide a meaningful support for long-term experimental investigation on Solid Oxide Fuel Cells operated upon direct feeding of unreformed biogas. For that, the current study compares two system designs for biogas utilization into Solid Oxide Fuel Cells. The conventional one realizes biogas steam reforming prior the fuel cell, while the novel concept is based on direct feeding of partially upgraded biogas by means of carbon dioxide-separation membranes. As main outcome of the study, the system equipped with carbon dioxide-separation membranes achieves better performances than its conventional competitor does, scoring 51.1% energy efficiency and 52.3% exergy efficiency (compared to 37.2% and 38.6% respectively exhibited by the reformer-based system). Because of the lack a high endothermic process steps, the membrane-based system is also convenient whether heat recovery is required, producing a combined heat-and-power efficiency of 74.8% versus 47.0% obtained in the other system. Moreover, the results of a sensitivity analysis of the impact of membrane and reforming operating parameters on the overall system performances justify the convenience of adopting the solution of biogas direct feeding. Even in the hypothesis of a poorly performing membrane and an optimized reformer, the membrane-based system exhibits a gain in the system energy and combined heat-and-power efficiency of 25.2% and 34.9% respectively, with regard to the reforming-based concept. The forcefulness of this result is reinforced by a preliminary evaluation of capital expenditures, which represents a further economic advantage beside the economic revenue coming from a higher energy conversion efficiency.