Combined Cycles With CO₂ Capture: Two Alternatives for System Integration

As Carbon Capture and Storage (CCS) technology has grown as a promising option to significantly reduce CO₂ emissions, system integration and optimization claim an important and crucial role. This paper presents a comparative study of a gas turbine cycle with post-combustion CO₂ separation using an amine-based absorption process with Monoethanolamine (MEA). The study has been made for a triple pressure reheated 400 MWe natural gas-fuelled combined cycle (NGCC) with exhaust gas recirculation (EGR) to improve capture efficiency. Two different options for the energy supply to the solvent regeneration have been evaluated and compared concerning plant performance. In the first alternative heat is provided by steam extracted internally from the bottoming steam cycle while in the second option an external biomass-fuelled boiler was utilized to generate the required heat. With this novel configuration the amount of CO₂ captured can be even more than 100% if the exhaust gas from the bio-fuelled boiler is mixed and cleaned together with the main exhaust gas flow from the combined cycle. In order to make an unprejudiced comparison between the two alternatives, the reduced steam turbine efficiency has been taken into consideration and estimated, for the alternative with internal steam extraction. The cycles have been modelled in the commercial heat and mass balance programme IPSEpro™ using detailed component models. Utilizing EGR can double the CO₂ content of the exhaust gases and reduce the energy need for the separation process by approximately 2%-points. Using an external biomass-fuelled boiler as heat source for amine regeneration, turns out to be an interesting option due to high CO₂ capture effectiveness. However the electrical efficiency of the power plant is reduced compared to the option with internal steam extraction. Another drawback with the external boiler is the higher investment costs but nevertheless, it is flexibility due to the independency from the rest of the power generation system represents a major operational advantage.