Analysis of Flexible CO₂ Capture Over an Investment Life Using a Dynamic Electric Grid Model

Global focus on greenhouse gas emissions has led the United State’s legislature to discuss various strategies to reduce carbon dioxide (CO₂) emissions. With coal-fired plants responsible for roughly half of United States (U.S.) electricity generation and approximately 30% of the nation’s CO₂ emissions, coal-fired plants will be largely affected by any future CO₂ emission regulations. However, coal-based generation could continue to meet our electricity demands while complying with future CO₂ emissions restrictions with the addition of carbon dioxide capture and sequestration (CCS) technology. Most studies of CCS systems have demonstrated a permanent energy requirement of 11–40% of a plant’s output when operating continuously at a 90% CO₂ removal rate. This study, however, used a dynamic model of the Electric Reliability Council of Texas (ERCOT) electric grid to consider post-combustion CO₂ capture systems that can operate flexibly. Post-combustion CO₂ capture systems using chemical absorption and stripping are particularly suited for retrofitting existing plants and operating in a flexible manner. Flexible carbon capture allows plant operators to vary the energy used for CO₂ capture and compression in order to regain this generation capacity when desirable. Thus, flexibility can be used to choose the CO₂ capture rate that allows the most economical combination of operating costs, electricity price, and output levels. Furthermore, operating at lower CO₂ capture energy requirement levels and increasing output capacity during peak demand periods could dramatically reduce the amount of replacement capacity needed to replace potential output lost when CO₂ capture systems are in operation. This research uses an existing modeling framework of a dynamic hourly dispatch system to study the economic, environmental, and performance implications of flexible CO₂ capture over an investment lifetime. The effects of CO₂ prices, natural gas fuel prices, and replacement capacity costs were analyzed along with various operating strategies. The fuel mixture behavior and emissions effects are presented, showing that large emissions reductions can be achieved using the current ERCOT plant fleet with the addition of flexible CO₂ capture. An annual system-level cash-flow analysis is used to determine a net present value (NPV) for a group of CO₂ capture plants under a range of possible replacement capacity costs. If replacement capacity costs are accounted for, flexibility can improve the NPV of a CO₂ capture investment by substantially lowering the associated capital costs to replace output lost to CO₂ capture energy requirements.