Understanding the Chemical and Physical Processes of Liquid Solvent and Solid Sorbent Direct Air Capture

Direct Air Capture (DAC) technologies utilize distinct chemical and physical processes for CO2 extraction, primarily through liquid solvents or solid sorbents. Understanding their capture, regeneration, and energy demands is key to climate change mitigation.

Benjamin Carter

September 28, 2026 · 5 min read

Direct Air Capture (DAC) technologies are pivotal in addressing the escalating atmospheric carbon dioxide (CO2) emissions that contribute to global climate change. These systems are engineered to extract CO2 directly from ambient air, offering a crucial pathway toward achieving net-zero emissions. Fundamentally, DAC systems employ two primary methodologies: liquid solvents and solid sorbents. Each approach utilizes distinct chemical and physical processes for CO2 capture, followed by regeneration of the capture material and the release of concentrated CO2. A comprehensive understanding of these mechanisms is essential for sustainable technology professionals and investors evaluating the efficacy and operational nuances of these solutions.

Liquid Solvent DAC: Chemical Absorption and Regeneration

Liquid solvent-based DAC systems operate by drawing ambient air through a contactor where it interacts with a chemical solution designed to selectively absorb CO2. A widely recognized example of this method involves the use of an aqueous potassium hydroxide (KOH) solution. The capture process initiates as air passes through a "shower" of the liquid solvent. Gaseous CO2 molecules in the air stream chemically react with the dissolved potassium hydroxide. This reaction absorbs the CO2 into the liquid phase, forming potassium carbonate (K2CO3) and water. The chemical transformation can be represented as: 2KOH (aq) + CO2 (g) → K2CO3 (aq) + H2O (l). This step effectively removes CO2 from the air, which is then returned to the atmosphere with a reduced CO2 concentration. To regenerate the capture liquid and isolate the CO2, a series of subsequent chemical reactions are performed. The potassium carbonate solution, now laden with captured carbon, is reacted with calcium hydroxide (Ca(OH)2). This reaction leads to the precipitation of solid calcium carbonate (CaCO3) and, critically, regenerates the potassium hydroxide solution. The regenerated KOH solution is then recycled back to the air contactor to absorb more CO2, ensuring continuous operation. This regeneration step is chemically described as: K2CO3 (aq) + Ca(OH)2 (s) → 2KOH (aq) + CaCO3 (s). The precipitated calcium carbonate (CaCO3) is then subjected to a high-temperature thermal decomposition process known as calcination. This step typically occurs at approximately 900°C. During calcination, the solid CaCO3 decomposes, releasing a concentrated stream of CO2 gas and forming calcium oxide (CaO). The reaction is: CaCO3 (s) → CaO (s) + CO2 (g). The resulting calcium oxide (CaO) is subsequently "slaked" by reacting it with water to reform calcium hydroxide (Ca(OH)2), thereby completing the regeneration loop for the calcium-based compounds. Companies such as Carbon Engineering have implemented this liquid solvent method, which integrates calcium carbonate precipitation and regeneration, as noted in research on DAC system control. The energy demand for this carbonate regeneration, particularly the high-temperature calcination, accounts for a significant portion of the overall energy consumption in such systems.

Solid Sorbent DAC: Adsorption and Desorption Mechanisms

Solid sorbent-based DAC systems employ physical filters made of solid materials engineered to selectively bind with CO2 molecules from the air. In these systems, ambient air is directed through a bed or array of solid sorbent material. As the air flows through, CO2 molecules become bound to the sorbent, while other atmospheric components pass through unimpeded. The primary capture mechanism in solid sorbent systems involves chemical adsorption, where CO2 molecules form weak chemical bonds with the active sites on the sorbent material. These sorbents are often designed with specific chemical functionalities to enhance their affinity for CO2. Examples of such materials include amine-functionalized sorbents and metal-organic frameworks (MOFs), as highlighted by research on their suitability for direct air capture. The selective reactivity of these solid sorbent materials with CO2 allows for efficient capture from the dilute atmospheric concentration. Once the sorbent material reaches its CO2 saturation capacity, the system transitions into a regeneration phase. The purpose of this phase is to release the captured CO2 in a concentrated form and prepare the sorbent for another capture cycle. Regeneration typically involves applying energy, most commonly in the form of heat, or by reducing the pressure surrounding the sorbent, a process known as vacuum swing adsorption. Sometimes, a combination of both heat and vacuum is used. Applying heat or a vacuum provides the energy necessary to break the weak chemical bonds between the CO2 molecules and the sorbent material. This causes the CO2 to desorb, or detach, from the sorbent, releasing it as a concentrated stream. The U.S. Department of Energy explains that these systems use physical filters that chemically bind with CO2 molecules, and when heated or placed under vacuum, they release the CO2. Following desorption, the regenerated sorbent material is cooled or repressurized and then returned to the capture process to absorb more CO2 from the air, ensuring the continuous operation of the system.

Regeneration Methods and CO2 Output

Both liquid solvent and solid sorbent DAC technologies are designed to capture CO2 from the atmosphere and subsequently release it as a concentrated stream. However, the specific chemical and physical processes involved in their regeneration, and consequently their energy demands, differ significantly. In liquid solvent systems, particularly those utilizing potassium hydroxide, regeneration is a multi-step chemical process. It involves a series of reactions that first precipitate calcium carbonate, followed by a high-temperature calcination step to release the CO2. This calcination, occurring at approximately 900°C, is notably energy-intensive. Research indicates that the energy demand for carbonate regeneration accounts for over 90% of the overall energy consumption in such systems, significantly impacting their efficiency. The process also involves the slaking of calcium oxide back to calcium hydroxide to complete the chemical loop. Conversely, solid sorbent systems typically regenerate through more direct physical processes, such as heating the sorbent or applying a vacuum. These methods are designed to break the weaker chemical bonds formed during CO2 adsorption. While these processes still require energy, the specific thermal and pressure conditions can vary considerably depending on the type of sorbent material used. The U.S. Department of Energy notes that existing DAC systems use a combination of heat and vacuum to remove captured CO2 and return the chemicals or sorbents to the process. Despite these differences in regeneration mechanisms and associated energy profiles, both liquid solvent and solid sorbent DAC systems ultimately produce a concentrated stream of CO2. This concentrated CO2 output is then available for various downstream applications. One primary use is permanent storage in suitable underground geologic formations, which prevents its re-entry into the atmosphere. Alternatively, researchers are actively developing and exploring approaches for converting this captured CO2 into useful products. These products can include building materials, various industrial chemicals, or synthetic fuels, offering pathways for CO2 utilization rather than just sequestration.

Conclusion

Direct Air Capture technologies provide distinct yet complementary approaches for removing atmospheric CO2. Liquid solvent systems, exemplified by the potassium hydroxide method, rely on a series of precise chemical reactions, including the formation of potassium carbonate, precipitation of calcium carbonate with calcium hydroxide, and high-temperature calcination to release CO2. Solid sorbent systems, conversely, utilize specialized materials like amine-functionalized sorbents or metal-organic frameworks that chemically bind CO2 through adsorption, regenerating through the application of heat, vacuum, or both. While their capture and regeneration mechanisms differ in their chemical and physical specifics and energy requirements, both technologies successfully yield a concentrated CO2 stream. This output is crucial for either permanent geological storage or conversion into valuable products, thereby contributing significantly to global decarbonization efforts and the pursuit of a net-zero emissions future.