Coupling effect of operating parameters on performance of a biogas-fueled solid oxide fuel cell/gas turbine hybrid system

The correlation between effectiveness of multiple operating parameters is a complicated issue and significant to the safe operation of solid oxide fuel cell/gas turbine (SOFC/GT) hybrid system. This paper presents a numerical analysis on biogas-fueled SOFC/GT hybrid system with a recirculation process using combustor exhaust gas. With consideration of safety constraints for critical components (fuel cell thermal crack, reformer carbon deposition, turbine blade overheat), the interaction mechanism of recirculation ratio, steam/carbon ratio and fuel/air ratio is studied from the perspective of thermodynamic analysis. Results show that the recirculation process could increase the electrical efficiency of system from 58.18% to 62.8%. However, for the safety consideration of SOFC, the acceptable recirculation ratio should be controlled between 0.17 and 0.32. Meanwhile, there exists a minimum point of turbine inlet temperature at the recirculation ratio of 0.3. With recirculation ratio switched from 0.1 to 0.3, impact of steam/carbon ratio variation on SOFC temperature gradient shrinks from 52.5% to 17.9% of the reference value. On the other hand, effect of fuel/air ratio variation on SOFC temperature gradient is promoted from 20.8% to 44.3% of the reference value, due to that oxidation reaction of biogas becomes a dominant factor. Based on these results, a parametric comparison is carried out to quantitatively describe the impact of recirculation process on the effectiveness of other parameters. Short discussion is conducted on parameter selection to acquire higher system efficiency and sufficient safety range, in which case the total output power of 176.8 kW and electrical efficiency of 61.78% could be achieved.