Design Challenges for Combined Cycles With Post-Combustion CO₂ Capture

In the present climate of uncertainty about CO₂ emissions legislation, owners and power plant planners are looking into the possibility of accommodating “add-on” CO₂ capture and sequestration (CCS) solutions in their current plant designs. The variety of CCS technologies currently under development makes it a very challenging task. Nevertheless, it is evident that the new generation of combined cycles must address the CO₂ capture issue. This discussion concentrates on the more mature post-combustion CCS technologies, such as chemical absorption, and the associated equipment requirements in terms of layout, integration within the generating plant, and auxiliary power consumption. The analysis specifically addresses combined cycle plants, where the capture process must accommodate low CO₂ concentration in the exhaust gases (around 3%). Several plant configurations and various operational scenarios are evaluated. The issues related to balance-of-plant systems, including water treatment and availability and redundancy criteria, are also examined. The paper discusses the option to increase CO₂ concentration by recirculating some of the exhaust flue gases. The impact of recirculation on the performance and operation of major gas turbine components (compressor combustion, turbo-machinery) is analyzed, as is the effect of the additional auxiliary loads needed to cool the flue gases prior to reinjection in the gas turbine inlet. Since the post-combustion CO₂ process requires substantial steam flows, the paper provides several design options for the steam turbine that address the need for large steam extractions. Finally, the paper presents the results of studies conducted by Bechtel in which a neutral but proactive technical approach was applied in evaluating a variety of post-combustion CO₂ capture technologies.