Modeling and optimization of carbon-negative NGCC plant enabled by modular direct air capture

Natural gas combined cycle (NGCC) plants are the most prevalent source of electricity in the United States and are expected to continue playing a key role in the future energy market. However, they are also one of the main sources of CO2 emissions in the power market. Recently, the authors proposed a retrofit design for an existing NGCC to allow power generation with negative CO2 emissions by utilizing both post-combustion carbon capture (PCC) and direct air capture (DAC). The DAC unit captures CO2 directly from the atmosphere using low-grade heat from the NGCC via a novel heat integration scheme. The heat integration scheme and modular DAC design introduce significant flexibility, allowing the system to focus on either power generation or CO2 capture in response to electricity price changes. However, this creates a complex trade-off between the cost of DAC capacity and the enhanced operational flexibility it provides. In this work, we model the proposed retrofit systematically and co-optimize the design and operation of the system for an entire year under different electricity price signals and CO2 prices. The DAC operations are modeled with a novel formulation to account for the sorbent dynamics in its temperature swing cycles. The solutions for the resulting large-scale mixed-integer linear program show that a large DAC unit is preferred in most situations. The extra flexibility leads to longer dispatch time and higher profits compared with the base NGCC.