Carbon capture with low energy penalty: Supplementary fired natural gas combined cycles

Enhancing CO2 concentration in exhaust gas has been considered as a potentially effective method to reduce the penalty of electrical efficiency caused by CO2 chemical absorption in post-combustion carbon capture systems. Supplementary firing is an option that inherently has an increased CO2 concentration in the exhaust gas, albeit a relatively low electrical efficiency due to its increased mass flow of exhaust gas to treat and large temperature difference in heat recovery steam generator. This paper focuses on the methods that can improve the electrical efficiency of the supplementary fired combined cycles (SFCs) integrated with MEA-based CO2 capture. Three modifications have been evaluated: (I) integration of exhaust gas reheating, (II) integration of exhaust gas recirculation, and (III) integration of supercritical bottoming cycle. It is further showed that combining all three modifications results in a significant increase in electrical efficiency which is raised from 43.3% to 54.1% based on Lower Heating Value (LHV) of natural gas when compared to the original SFC. Compared with a conventional combined cycle with a subcritical bottoming cycle and without CO2 capture (56.7% of LHV), the efficiency penalty caused by CO2 capture is only 2.6% of LHV.

► Integration of exhaust gas reheating can increase supplementary fired cycle efficiency by 6.3% of LHV.
► Integration of exhaust gas recirculation can increase supplementary fired cycle efficiency by 1.6% of LHV.
► Integration of supercritical bottoming cycle can increase supplementary fired cycle efficiency by 2.7% of LHV.
► The minimum energy penalty cause by CO2 capture can be 2.6% of fuel LHV for supplementary fired cycle.