DAC-system-overview
Carbon Dioxide Direct Air Capture (DAC) System Overview
Summary
Direct Air Capture (DAC) systems remove CO₂ from ambient air through six key steps: air intake, dehumidification (when needed), CO₂ capture using liquid or solid sorbents, CO₂ release through desorption, sorbent regeneration, and CO₂ preparation for storage or use. While the basic process remains consistent across technologies, significant differences exist between liquid and solid sorbent methods - particularly in energy-intensive regeneration phases. Current research focuses on optimizing energy efficiency, especially in developing passive air contactors and improved regeneration processes. The technology's simplicity belies complex engineering challenges, with most advancements occurring in material science and process optimization for the desorption and regeneration stages.
Although CO₂ DAC technology is considered cutting-edge in the fight to combat climate change, the premise behind the technology is fairly basic. The goal of a DAC system is to capture CO₂ from the ambient air so that it can be transported, stored, or utilized. The exact process can vary between different technologies such as liquid sorbents or solid sorbents, but the overall process is similar. DAC is achieved by drawing air into a system, removing water from the air using a desiccant material if the air is humid, capturing the CO₂ using a sorbent material, and then releasing the captured CO₂ under pressure into a storage or utilization process.
Step 1: Draw Air into the System
The first step of a DAC system is to draw air into the system, often called an 'Air Contactor'. This is typically achieved using a series of fans, which are powered by an external energy source. It is a common misconception in media reporting on carbon capture technologies that the energy required to run the fans is a large portion of the total energy required to run a DAC system. In reality, the energy required to run the fans is a small portion of the total energy required to run a DAC system and is often negligible compared to other energy costs, and is therefore mostly ignored in the literature. However, much research is being done to develop what are known as 'Passive Air Contactors', which do not require any external energy source to draw air into the system and can simply draw CO₂ from the ambient air 1.
Step 2: Remove Water from the Air
This step is not vital for all DAC processes and depends heavily on the sorbent material used. Some sorbent materials and methods rely on dry air for optimal performance, while others require a humidified air stream. For the systems that do require dry air, it is important to include a dehumidification step after the air has been drawn into the system if the system is located in a humid environment 2.
Step 3: Capture the CO₂
This is the step where the CO₂ is captured using a sorbent material. This is achieved by passing the air stream over or through the sorbent material. In this phase, the CO₂ is either absorbed into the sorbent, as is the case with liquid sorbents, or it is adsorbed onto the surface, as is the case with solid sorbents. At the end of this phase, the air which has passed through the sorbent and has had its CO₂ concentrations reduced is released from the system. What is left in the system is either a liquid sorbent solution or a solid sorbent material that contains a higher concentration of CO₂ than it started with.
Step 4: Release the CO₂ (Desorption)
The release phase is where there is the greatest variation between different technologies. In the case of liquid sorbents, the solution undergoes a process where the CO₂ is released from the liquid sorbent material and is bound to a different material after which it can be further processed. This step often requires some energy input, after which the liquid sorbent can be reused 2.
In the case of solid sorbents, the process is much simpler. The air contactor, sometimes called the "CO₂ contactor", where the solid sorbent material is contained, is heated, usually through the use of steam, which is the main energy input for solid sorbent systems. This heating causes the CO₂ to desorb from the solid sorbent material, where it can then be collected. After this process, the solid sorbent material is ready for another cycle 2.
Step 5: Regenerate the Sorbent Material (Regeneration)
This step is where the sorbent material is prepared for its next cycle. This is an extensive process for liquid sorbent systems, which requires a high energy input to regenerate the solid pellets that are used to capture the CO₂ from the liquid sorbent. In the case of solid sorbent systems, this step is much simpler and usually only requires a relatively low energy input to regenerate the solid sorbent material and can be as simple as waiting for the sorbent material to cool down.
Step 6: Prepare CO₂ for Transport, Storage, or Utilization
This step is where the captured CO₂ is prepared for transport, storage, or utilization. Some DAC systems are designed for utilization of CO₂ at nearby agricultural plants, which only require low concentrations of CO₂ and therefore do not require much preparation. If a high purity of CO₂ is required, further purification and compression steps may be required to prepare the CO₂ for transport and storage 1 2.
Conclusion
The Direct Air Capture (DAC) process is simple on paper, but each of the steps in the process is being optimized and improved upon to make the process more efficient and cost-effective. The main advancements and breakthroughs in the fields of DAC are being made in the release and regeneration steps where new materials and processes are being discovered.
Sources
Footnotes
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Ozkan, M., Nayak, S. P., Ruiz, A. D., & Jiang, W. (2022). Current status and pillars of direct air capture technologies. iScience, 25(4), Article 104055. https://doi.org/10.1016/j.isci.2022.104055 ↩ ↩2
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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 ↩3 ↩4