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Solid sorbents systems are a promising technology for Direct Air Capture (DAC) systems that offer significantly lower energy requirements and greater modularity compared to liquid sorbents. However, challenges remain in material durability during high-temperature regeneration phases as well as higher initial costs.

Summary​

Solid sorbents offer a promising approach for capturing CO₂ directly from air, using materials that trap or chemically bind with carbon dioxide molecules. These systems work through a cyclical process of absorption using fans to move air through sorbent-coated membranes, followed by vacuum-assisted CO₂ release through heat application. Compared to liquid alternatives, solid sorbent systems show potential for lower energy requirements and greater modularity, though challenges remain in material durability during high-temperature regeneration phases as well as higher initial costs. Current implementations like Climeworks' Orca plant in Iceland demonstrate operational capabilities at scale, with newer projects aiming for significantly increased capture capacities such as the under construction Project Cypress in Louisiana, USA.

Solid sorbents are one of the two main methods employed in Direct Air Capture (DAC) systems, with the other being liquid sorbents. A sorbent is defined as a material that can be used to capture, absorb, or adsorb a substance such as a gas or liquid from its surrounding environment. Solid sorbents achieve this by passing air through the solid sorbent material, which collects the CO₂.

Solid sorbents are a relatively recent development in the field of DAC, which has begun to show promise in addressing the largest issue with liquid sorbents, namely the high energy requirement in the desorption/regeneration phases 1. Solid sorbent systems generally consist of a fan connected to a sealed air contactor, and the systems typically are less complex than liquid sorbents. Contained within the air contactor is a porous membrane, similar to a sponge or air filter, with the solid sorbent material attached to the surface of the porous material. Solid sorbents can be broken down into two categories: physisorption and chemisorption. Physisorption is the process where the CO₂ is trapped within the micropores of a solid sorbent material, while chemisorption is the process where the CO₂ chemically bonds with the solid sorbent material using amines or other chemical compounds. Amines are alkaline substances that readily react with CO₂, which is acidic in nature.

A typical cycle for a solid sorbent DAC system is as follows 1 2:

Absorber​

At this step, air is passed through the solid sorbent material in the air contactor via a fan. The CO₂ in the air binds to the solid sorbent material while the 'cleaned' air is released from the system.

Vacuum Pump (intermediary step)​

Here, the air contactor is sealed, and a vacuum pump evacuates all the remaining air in the system. This is done to ensure that the CO₂ that is desorbed from the solid sorbent material is not mixed with any other gases in the air.

Desorption/Regeneration​

The desorption phase is where the CO₂ that is bound to the solid sorbent material is released. This is done usually by heating the air contactor to 100°C – 400°C, generally through the use of steam which is the main energy input for solid sorbent systems. This is also the stage where the solid sorbents experience the most degradation when they are exposed to high temperatures and moisture.

Vacuum Pump (intermediary step)​

In this final step, a vacuum pump is activated to evacuate the released CO₂ from the system, where it is collected and can be transported for utilization or storage. After the air contactor has cooled back down, it is ready for its next cycle.

Conclusion​

Solid sorbents are a promising technology for DAC systems, with the potential to be more energy efficient than liquid sorbents and are the general direction of development for DAC systems. Solid sorbent systems also tend to be smaller and modular, meaning that they can be mass-produced and rapidly installed in locations that are optimal for storage or utilization 2. One of the leading companies in the field of DAC using solid sorbents is the Swiss company Climeworks. Climeworks operates multiple plants in Iceland and elsewhere in Europe, and is constructing a plant in Louisiana, USA, known as Project Cypress, which is designed to capture 1 million tons of CO₂ per year 3. Since 2021, Climeworks has operated the Orca plant in Iceland, which has a capture capacity of 4,000 tons of CO₂ per year.

Sources​

Footnotes​

  1. Sodiq, A., Abdullatif, Y., Aissa, B., Ostovar, A., Nassar, N., El-Naas, M., & Amhamed, A. (2023). A review on progress made in direct air capture of CO₂. Environmental Technology & Innovation, 29, Article 102991. https://doi.org/10.1016/j.eti.2023.102991 ↩ ↩2

  2. Ozkan, M., Nayak, S. P., Ruiz, A. D., & Jiang, W. (2022). Current status and pillars of direct air capture technologies. iScience, 25(4), Article 103990. https://doi.org/10.1016/j.isci.2022.103990 ↩ ↩2

  3. Climeworks. (2024). Our plants. Climeworks. https://climeworks.com/our-plants ↩