Liquid sorbents are typically alkaline solutions that are highly efficient at capturing CO₂ from the air, but require multiple stages and high energy inputs (known as the 'Kraft' process) to release the CO₂ and regenerate the sorbent so it can be used again.

Summary

Liquid sorbents are alkaline solutions used in direct air capture (DAC) systems to chemically bind with atmospheric CO₂. While highly efficient at absorption, these systems require energy-intensive regeneration processes. The benchmark Kraft process can utilize a sodium hydroxide (NaOH) solution that reacts with CO₂ to form sodium carbonate, which is then treated with calcium hydroxide to regenerate the sorbent. The most energy-demanding stage involves heating calcium carbonate to release pure CO₂ for storage. Despite their efficiency, liquid sorbent systems face scalability challenges due to high energy requirements, particularly during regeneration. A prominent example is the currently under construction STRATOS plant in Texas, designed to capture 500,000 tonnes of CO₂ annually using this technology. Current research continues to focus on reducing energy demands while maintaining capture efficiency.

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Liquid sorbents are one of the two main methods employed in Direct Air Capture (DAC) systems, with the other being the use of solid sorbents solid 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. Liquid sorbents achieve this by passing air through the liquid sorbent material, which then collects the CO₂.

The technology for liquid sorbents has been in existence longer than solid sorbents and is similar to the sulfur and carbon dioxide 'scrubbing' process that has been used in industrial processes for many decades. A key challenge for all DAC systems is the very low concentration of CO₂ in the air (around 420 ppm in 2024), which necessitates the use of strong chemisorbent materials like liquid sorbents that have been observed to be more efficient in DAC systems ¹.

Liquid sorbents are typically an alkaline solution (also known as amines) that chemically bond with CO₂, with the current benchmark for liquid sorbents being aqueous solutions of potassium hydroxide (KOH) and sodium hydroxide (NaOH) ². The main challenge to the mass adoption of liquid sorbent DAC systems is related to the high energy requirement of their desorption and regeneration phases.

A typical cycle for a sodium hydroxide (NaOH) liquid sorbent DAC system known as the 'Kraft' process is as follows ²:

Absorber

This is the step where the CO₂ is absorbed into the NaOH solution and is shown by the following equation:

$$\begin{align*} \mathrm{2NaOH + CO_2 \rightarrow Na_2CO_3 + H_2O} \end{align*}$$

At this stage, two molecules of NaOH react with one molecule of CO₂ to form one molecule of sodium carbonate (Na₂CO₃) and one molecule of water (H₂O).

Causticizer or Precipitator (desorption)

The causticizer is where the CO₂ is released from the Na₂CO₃ and is shown by the following equation:

$$\begin{align*} \mathrm{Na_2CO_3 + Ca(OH)_2 \rightarrow 2NaOH + CaCO_3} \end{align*}$$

Here the Na₂CO₃ reacts with calcium hydroxide Ca(OH)₂ to form two molecules of NaOH and one molecule of solid calcium carbonate (CaCO₃). After this stage, the sodium hydroxide (NaOH) is cycled back into the absorber for the next cycle.

Calciner (desorption/regeneration)

The calciner is where the CaCO₃ is heated to a high temperature to form calcium oxide (CaO) and CO₂. This is the stage that includes the highest energy input in the cycle and is the main limiting factor in the efficiency of liquid sorbent DAC systems.

$$\begin{align*} \mathrm{CaCO_3 \rightarrow CaO + CO_2} \end{align*}$$

At the end of this stage, the CO₂ can be collected for utilization or storage.

Slaker (regeneration)

The slaker is where the CaO is reacted with water to form calcium hydroxide (Ca(OH)₂) so that it can be cycled back into the causticizer for the next cycle.

$$\begin{align*} \mathrm{CaO + H_2O \rightarrow Ca(OH)_2} \end{align*}$$

Conclusion

The process for liquid sorbent DAC systems is highly efficient at capturing CO₂ from the air, but the energy required for each phase, and especially the regeneration phase limits its scalability and adoption. There are alternative methods of liquid sorbent DAC systems that rely on different chemical solutions and processes, yet their energy requirements still remain very high in the desorption/regeneration phases ². The kraft process using liquid sorbents is the technology utilized by the currently under construction 'STRATOS' plant in Texas built by Carbon Engineering. The STRATOS plant is designed to capture 500,000 tonnes of CO₂ per year when it is fully operational ³.

Sources

Footnotes

1. Sanz-Pérez, E. S., Murdock, C. R., Didas, S. A., & Jones, C. W. (2016). Direct capture of CO2 from ambient air. Chemical Reviews, 116(19), 11840-11876. https://doi.org/10.1021/acs.chemrev.6b00173 ↩

2. 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

3. Carbon Engineering. (2024). Direct air capture. https://carbonengineering.com/direct-air-capture/ ↩