Techno-economic study of a direct air capture system based on the carbonation of Ca(OH)2 plates integrated into cooling towers

Direct Air Capture (DAC) is a crucial Negative Emissions Technology (NET) for mitigating global warming. One of the main challenges in DAC processes is the high energy and economic costs associated with airflow systems in large-scale air contactors. Recently, there has been a growing interest in using hydrate lime to capture low concentrations of CO₂ (∼450 ppm) from the atmosphere, particularly at higher air relative humidity. Cooling towers, commonly used in various industrial units to cool process water, provide an ideal environment for hydrated lime-based DAC systems as they expose large flows of ambient air to water. This study assessed the feasibility of integrating vertically oriented parallel flat plates of Ca(OH)₂ into the upper section of an industrial mechanical draft cooling tower to simultaneously perform the dual tasks of water cooling and CO₂ capture from the passing air. Results of an unsteady-state Shrinking Core Model (SCM) showed that 425 cooling towers of 20×20 m² cross-sectional area accommodating 5-meter long Ca(OH)₂ vertical plates of porosity 0.55, thickness 2 cm, and distance between adjacent plates of 2 cm can achieve an annual CO₂ capture rate of 1 million tons if the carbonated plates are replaced 4 times a year (3-month cycle duration). By utilizing a scaled-up centralized calcination unit for the regeneration of the recycled sorbent, the estimated final cost of the proposed DAC process is around $93.5 to $269 per ton of CO₂ captured which makes the proposed system a cost-competitive alternative among the state-of-the-art DAC technologies.