(130d) Retrofitting Direct Air Capture Systems to Natural Gas Combined Cycle and Nuclear Power Plants

As CO₂ concentrations continue to rise rapidly due to increased use of fossil fuels resulting from industrialization, it is necessary to combat the effects of global warming by not only curbing the amount of CO₂ being released, but also by re-capturing the CO₂ already in the atmosphere. This process, called direct air capture, or DAC, is challenging because the concentration of CO₂ in ambient air is far more dilute than in flue gas, thus making it harder to perform DAC than post-combustion capture. It requires a sorbent that can adsorb CO₂ at low partial pressures, as well as an abundant source of power to run the DAC system. In this work, we have designed and optimized a direct air capture system that can be retrofitted to a natural gas combined cycle (NGCC) power plant. Tetraamine-appended Mg₂(dobpdc)(3-4-3) metal oxide framework (MOF) is chosen as the solid sorbent material because it has demonstrated that it can effectively capture CO₂ at both ambient air and flue gas concentration. These MOF sorbents are packed into a series of fixed bed reactors through which CO₂-rich gas is flown and adsorbed by the sorbents. The beds regenerate via temperature swing adsorption assisted by steam drawn from the NGCC power plant. The benefit of retrofitting the DAC system to an NGCC plant lies in the fact that it can power the DAC system, while the DAC system in combination with a membrane CO₂ capture system will scrub CO₂ from both the flue gas exhaust from the plant, as well as performing direct air capture. Running in two different operational modes, the DAC system has the two-pronged advantage of cleaning up the NGCC plant’s tailpipe (Mode 1), as well as performing DAC (Mode 2) during the off-peak operation hours of the plant, where electricity produced during this period is used to run the DAC system. This can lead to overall negative emission of CO₂ from the power plant. However, the results from this study show that the solid sorbent system, when combined with a membrane capture system, can achieve 98.7% and 99.2% capture of CO₂ during Modes 1 and 2 of operation respectively, thus achieving near-net zero carbon footprint from the NGCC plant instead.

In order to achieve net-negative emissions from a power plant, we propose that the DAC system be retrofitted to a nuclear power plant, since the operation of a nuclear plant does not produce CO₂, thus allowing for negative net CO₂ emissions. The DAC system does not need to be in constant operation, but rather can be operated when it is economically advantageous, likely at off-peak times when it is more profitable to capture CO₂ than to supply electricity to the grid. When the DAC system is in operation, steam and electrical energy from the nuclear plant will be used to power the DAC system. Extracting steam from the nuclear plant will decrease the efficiency of the nuclear plant, thus making the design and optimization of this system a complex challenge. This nuclear-powered DAC system has not yet been designed, and there are many factors at play. This work will discuss the factors that influence the design and optimization of a nuclear-powered DAC system along with the modeling efforts underway to address this challenge. Aspects of the DAC system, such as sorbent material and bed design, should be chosen to optimize performance under the constraints of the nuclear plant steam conditions. There are also a number of factors that must be taken into consideration pertaining to the integration of the DAC system and the nuclear plant, including the type of nuclear reactor (e.g. large light water reactor, small modular reactor, etc.), the system integration (e.g. turbine tapping, turbine bypass, waste heat, etc.) and the system control (e.g. grid demand vs economic demand). Modeling efforts are underway to better define this system with the goal of identifying important system parameters such as the amount of CO₂ that can be captured, the impact on nuclear plant efficiency, and an operating pattern that is profitable.