Comparison of the exergy efficiency of four power generation systems from methane using fuel cells

Abstract

Exergy analyses are carried out on four different solid oxide fuel cell (SOFC) systems using methane as the original fuel, with focus on exergy flows, efficiency and destruction. The four processes are (1) CH₄-SOFC, which is a CH₄ directly fuelled SOFC system with a CO₂ capture unit; (2) CH₄-SOFC-CLC, in which the CH₄-SOFC system is integrated with chemical looping combustion (CLC); (3) SMR-SOFC, i.e. a SOFC system using H₂ (H₂-SOFC) generated by steam methane reforming (SMR); (4) MC-SOFC-DCFC, which is a combined system of H₂-SOFC and a direct carbon fuel cell (DCFC) where H₂ and C are supplied by methane cracking (MC). Generally, the CH₄-SOFC and CH₄-SOFC-CLC processes which directly use CH₄ as the fuel of cells have higher exergy efficiency. MC-SOFC-DCFC reaches an overall exergy efficiency of 71.4%, which is 17% higher than that of SMR-SOFC (54.4%) due to the higher exergy efficiency of MC than SMR. The effects of operating parameters on the performance of CH₄-SOFC are also examined in detail. The results of this investigation demonstrate that the development of methane directly fuelled SOFC, decreasing its operating temperature and suitable capture of CO₂ are the key technologies to improve the energy conversion efficiency of methane fuelled SOFC systems.