Exploring the use of Rotary Adsorption Systems for Direct Air Carbon Capture and Storage (DACCS) applications (Flexible Funding 2023)

Dr Laura Herraiz, Heriot-Watt University, was awarded funding in the UKCCSRC’s Flexible Funding 2023 call to look at “Exploring the use of Rotary Adsorption Systems for Direct Air Carbon Capture and Storage (DACCS) applications”.

Direct Air Carbon Capture and Storage (DACCS) is an essential technology for mitigating climate change by capturing CO₂ directly from the atmosphere. It is a critical component of the UK’s strategy to achieve net-zero greenhouse gas (GHG) emissions by 2050. Rotary adsorption systems, particularly those using Temperature Swing Adsorption (TSA), offer a promising approach for DACCS due to their compact design, scalability and ability to handle large volumes of ambient air, which contains low concentrations of CO₂ (~0.04%).

The rotary system’s continuous operation allows for cyclic adsorption and desorption, making it energy-efficient by minimizing pressure drops and ensuring rapid regeneration. The ability to rotate the adsorbent, and alternate between capturing CO₂ from air and regenerating the adsorbent using steam or indirect heating, is a key advantage. This system could reduce capital and operational costs by combining fast adsorption kinetics, efficient heat transfer and modular scalability.

Schematic representation of a Rotary Adsorption Wheel with steam regeneration

Despite these advantages, several key challenges remain:

1. Material Selection and Water Adsorption: The selection of adsorbent materials is crucial for the efficiency of DACCS. Materials must have high selectivity for CO₂ over nitrogen and water, but water vapor in ambient air complicates the process. Water competes for adsorption sites, particularly on hydrophilic materials, reducing CO₂ capture efficiency. This issue is exacerbated in humid conditions, where water can saturate the adsorbent, requiring significant energy for desorption. MOFs (Metal-Organic Frameworks) have been identified as promising materials due to their tunable properties, but finding water-stable MOFs remains a challenge.
2. Adsorption Isotherms: Modeling the co-adsorption of CO₂ and water accurately is difficult, especially since water adsorption isotherms often exhibit hysteresis. This leads to discrepancies in the amount of water and CO₂ adsorbed and desorbed at the same relative humidity, complicating regeneration and energy efficiency predictions.
3. Regeneration Process: Achieving uniform heat distribution for regeneration in large-scale rotary systems is technically challenging. Indirect heating may lead to inefficiencies, while direct heating with steam introduces complexities in managing the condensation of water and CO₂.

The development of this DACCS model, based on open-source software and collaboration with industry partners, is still ongoing. Overcoming these challenges is crucial for creating a scalable, cost-effective solution for removing CO₂ from the atmosphere and contributing to global net-zero targets.

Read more on Laura’s Flexible Funding 2023 project page.