Conclusions
The aim of this study was to examine the capabilities of MCFC to act as a CO2 separator in natural gas combined
cycles. The reference NGCC is based on a dual pressure HRSG, with a rated power of 35.8 MW, a LHV efficiency
of 50.2%, and CO2 specific emissions of 397 kg/MWh.
The simulation results have shown that the addition of a MCFC fed by gas turbine exhaust gases markedly
increases the rated plant capacity, that reaches about 49.8 MW (+39%); although the efficiency remains almost
unchanged, the specific CO2 emissions without capture system reduces from 397 to 381 kg/MWh.
In order to increase the CO2 concentration at the cathode inlet and then the MCFC performances, different
configurations based on two gas recirculation options have been investigated.
In the first configuration (Case 2), 40% of the exhaust gas at the turbine outlet is cooled up to 35°C and
recirculated at the compressor inlet. As a result, the CO2 concentration at the cathode inlet increases to 5.7%,
enabling an efficiency gain of MCFC of about 1.2 percentage points (50.5%). However, with respect to NGCC
integrated with MCFC, the exhaust gas recirculation adversely affects the rated capacity of the gas (-5%) and steam
(-14%) cycles, as well as the size of MCFC (-5%). Hence the net plant rated power and efficiency reduce to 45.7
MW and 49.1% respectively, while the specific CO2 emissions increases to 383 kg/MWh.
In the second configuration (Case 3), 45% of the anode exhaust gas is recirculated at the cathode inlet, thus
increasing the MCFC efficiency up to 50.9%. Hence, due to the additional capacity of the MCFC and to the increase
of steam cycle power, the rated capacity of the integrated system reaches 63.2 MW, while the efficiency gains 3.6%
percentage points with respect to the baseline NGCC. Hence, the specific CO2 emissions without carbon capture
reduces from 397 to 350 kg/MWh.
The same configuration, if integrated with a carbon capture system (Case 4), has a slightly lower rated power
(60.6 MW), due to the energy requirement for CO2 compression and pays a net efficiency decrease of 2 percentage
points (51.8%). However, the specific CO2 emissions reduces to about 58 kg/MWh, with a carbon capture ratio of
about 85%.