Model-based thermodynamic analysis of direct air capture units in repurposed power plant cooling towers

Abstract

To achieve the climate goals, the energy supply system must be sourced by renewable energy instead of fossil fuels. Nevertheless, hard-to-abate sectors require negative emission technologies (NETs) to counteract their emissions. Thus, NETs play a significant role across all future scenarios considered. Since natural NETs, such as afforestation, exhibit lower scaling potential, technological approaches like Direct Air Capture (DAC) represent promising alternatives. However, DAC faces major drawbacks in terms of high energy demands and high required air mass flows due to the low CO₂ concentration in ambient air ($\sim$400 ppm). This results in elevated costs per captured tonne of CO₂. Interestingly, the infrastructure of thermal power plants shares similarities with components of DAC units, in particular the cooling tower due to its handling of high air mass flows. As countries progressively shut down their coal-fired power plants, there is an opportunity to repurpose existing power plant infrastructure into DAC units.

Thus, this work investigates the opportunities and challenges of repurposing thermal power plant cooling towers as air contactors of DAC units with a potential of several million tonnes of CO₂ captured per year. The investigation focuses on the integration of an absorption-based liquid DAC process into a wet cooling tower. Therefore, the influence of the repurposed geometry of the cooling tower and its internal packing on the operational behavior of the air contactor is analyzed for the cooling towers of the coal power Niederaußem in Germany using a two-film theory-based model. It can be observed that the repurposed geometry of the absorber enables higher air velocities due to lower pressure losses. At the same time, the reduced travel depth in cooling towers causes a lower capture rate than in geometries optimized for DAC, ultimately resulting in 50–150 t${}_{{\mathrm{CO}}_2}$/a per cooling tower. Finally, a sensitivity analysis shows that the effect of the correlations of mass transfer and volume specific surface areas is not negligible.