Theoretical evaluation of solid oxide fuel cell and gas turbine combined systems fed by ethanol

Abstract

To present the performance analysis of a solid oxide fuel cell (SOFC) system fuelled by ethanol. The suitability of using ethanol for SOFC system is compared with other attractive renewable fuels, i.e., biogas and glycerol. The simulation results show that the SOFC system fuelled by ethanol provides the highest electrical and thermal efficiencies and less emission of carbon dioxide. The thermodynamic analysis of ethanol steam reforming indicates that high temperature and steam-to-carbon ratio are preferred operating conditions so as to obtain high hydrogen yield. The hydrogen production of ethanol steam reforming is improved when an adsorption-membrane hybrid system is considered. It is found that a higher hydrogen product can be obtained at a lower operating temperature and that the formation of carbon is likely to be reduced when a carbon dioxide adsorption is combined with the hydrogen production. Considering the integration of the ethanol steam reformer and SOFC, the recycle of anode exhaust gas to the reformer can reduce the energy required for preheating steam and enhance the electrical efficiency of the system. However, when SOFC is operated at high fuel utilization, an increase in the recirculation ratio of anode exhaust gas degrades the SOFC electrical efficiency. To improve the performance of SOFC system, a hybrid SOFC system with a gas turbine (GT) is also studied. The results show that the electrical efficiency of the SOFC-GT hybrid system raises with increasing the operating pressure (2-6 bar); however, the heat obtained from GT exhaust gas is reduced, thereby the SOFC-GT hybrid system cannot be operated at an energetically self-sustaining condition. Thus, the pressurized SOFC-GT hybrid system with a cathode exhaust gas recirculation is proposed with the aim to minimize the requirement of an external heat for air preheating, which is a major unit that requires the external heat. The results show that the SOFC-GT hybrid system with recycling cathode-exhaust gas can be run at the energetically self-sustaining condition. It is also found that a high cathode-exhaust gas recirculation ratio decreases SOFC electrical efficiency. Finally, a heat recovery of the SOFC-GT hybrid system is studied. Two designs of the SOFC system with uses of a high-temperature recuperative heat exchanger and a recirculation of cathode exhaust gas are compared. The results indicate that the SOFC system with cathode gas recirculation can achieve higher system efficiency and specific work.