Solid oxide semi-closed CO2 cycle: A hybrid power cycle with 75% net efficiency and zero emissions

Abstract

This paper presents a new hybrid cycle based on the integration between a pressurized solid oxide fuel cell (SOFC) and a semi-closed regenerative intercooled Brayton cycle using a CO2-rich stream as the working fluid. Nearly pure oxygen is used as oxidant for both the Brayton cycle combustor and the fuel cell. The cycle is conceived to produce electricity while capturing 100% of the produced CO2 using natural gas or other fuels suitable for SOFC fuel cells. If the maximum cycle pressure is above the CO2 critical pressure, the semi-closed Brayton cycle becomes a supercritical CO2 cycle with the related efficiency advantages. In this work, the cycle is modelled with Aspen Plus and its design variables are optimized to find the maximum electric efficiency using an ad-hoc optimization approach. In the case study assessed (natural gas thermal input of 500 MW), the optimized cycle, working at 40 MPa with a cooled expander, achieves an outstandingly high efficiency of 75.7% (LHV basis) with CO2 capture. The sensitivity analysis shows that similar efficiency values can be achieved even with less challenging operating conditions for both the Brayton cycle and fuel cell (maximum cycle pressure of 27.5 bar, uncooled turbine and fuel utilization factor of the fuel cell equal to 0.75).