Adsorption and Humidity/Water Regeneration

Capture Mechanism

Solid

Furthest Progress*

TRL 6

Highest Risks

Energy
Cost
Environment
Storage

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Method Overview

Definition:

Adsorption onto a solid sorbent (where CO2 is captured on the surface of a solid material) followed by a desorption step in which a swing in humidity (high humidity for regeneration) is used to separate CO₂ from the sorbent (i.e., a resin); further purification of CO2 may be necessary, depending on the water concentration in the product stream.*

Example:

CO2 is passively captured on a filter coated with resins that chemically bind with CO2 in air, then water is added, causing the filter to release the CO2 in an aqueous form and allowing the coated filter to be reused. Further treatment of the solution releases the CO2 as a gas for storage.*

Advantages:

After water capture causes CO2 desorption, the sorbent materials can be dried in ambient air where wind velocities facilitate adequate water evaporation or dried by a fan at ambient temperatures. Heat recovery from water condensation during the CO2 drying step could also be recycled to dry the sorbent. These approaches can substantially reduce the energy requirement of DAC by avoiding thermal regeneration.

Disadvantages:

The CO2 produced by this process is contaminated with water and requires additional conditioning to produce a CO2 stream of sufficient purity for geological storage.
This DAC approach is sensitive to environmental factors like wind speed and humidity, complicating its deployment. This may be addressed by conditioning the inlet stream, at the cost of additional design complexity.
This approach requires water and may be unsuitable outside of areas with low water stress. Additionally, the water used for desorption must be deionized to avoid contaminating the sorbent.

* Reproduced from The Applied Innovation Roadmap for CDR (2023) by RMI.

Company Overview

Plot of estimated funding vs. deployment status of companies utilizing this approach. Select data points to view company details. Only companies for which funding information is publicly available are included. Companies without funding information are tabulated with related details where relevant.

Summary of Deployments

View DAC deployments within this approach that have achieved or surpassed prototype scale. Planned deployments are included. Sort DAC deployments by company, scale, start of operations, and more. Because DAC is a rapidly evolving industry, this list may not be exhaustive.*

* Due to uncertain funding, plans for most DOE-funded DAC Hubs are not included in this analysis.

Risk Assessment

RMI's Applied Innovation Roadmap for CDR (2023) categorizes the risks associated with different classes of DAC technologies. Their assessment of this method is reproduced here and contextualized with current self-reported company data, where available.

Energy Requirements

Very high risk: There is no data available on energy needs to achieve high purity CO2.

Reported energy requirements are relatively low (1.2 GJ/tCO₂) due to passive contacting and no heat requirements for regeneration. However, more energy will be needed to adequately purify the CO2 product stream and the full DAC+S pathway will require additional energy (e.g., compression, transport, and storage).

Material Requirements

Low risk: No rare materials are required.

Apart from basic construction materials (e.g., steel, cement), this solution requires chemicals (ion exchange resin powder (I200) from polypropylene with ammonium) that need to be synthesized and purchased.

Cost Requirements

Very high risk: Cost data is incomplete.

A FOAK plant is expected at <10 ktCO2; however, plant data is not yet available. Net FOAK capture cost is expected to be >$600/tCO2 (excluding compression, transport, and storage).

Land Requirements

Low risk: Significant land areas are suitable for DAC siting.

Limited information is publicly available but, based on pilot plants (at 80 km²/GtCO₂), it is expected that the plant footprint will be very competitive with other DAC approaches and can be deployed on most types of land. The footprint is larger if land use for energy production is considered.

Water Requirements

Low risk: Modest volumes of water are needed for operation.

This process relies on water for the humidity swing. Water needs to be recycled for the water requirements not to become excessive. This approach should be deployed only in favorable environmental conditions in areas with low risk of water stress.

Companies co-producing water: Avnos

Environmental Impact

High risk: No / incomplete LCA data is available.

No LCA is currently available. Apart from impacts associated with energy, water and material requirements, minimal to no other impacts are foreseen.

Co-Location of DAC and Storage

Low risk: Co-location of DAC is relatively easy if high CO2 purities can be achieved.

From publicly available data, this approach can only be deployed in dry areas. However, CO2 storage is widely available. Therefore, easy co-location can be achieved if high CO2 purities can be obtained from the DAC unit.

Secure Storage for 100+ Years

High risk: Scalable, permanent options for low purity DAC are underdeveloped

Geological sequestration or in-situ mineralization (primary storage options) are considered permanent storage approaches. However, high CO2 purity DAC output needs to be achieved first, and there is low certainty as to whether durable or permanent storage for low purity DAC output is feasible.

Path to MRV at Scale

Low risk: Measurement to inform MRV is straightforward.

DAC+S measurement and monitoring is straightforward. Capture occurs in a controlled environment and geological injection has established monitoring standards.

Key Innovations

Moisture-swing adsorption offers a promising alternative to thermal DAC approaches by avoiding the substantial energy required for thermal regeneration; CO2 release in this approach is triggered by water adsorption, albeit with gentle heating and/or vacuum in some instances. Despite its potential, this method remains less developed due to its current sensitivity to local climate conditions and, in some cases, the low purity of its captured CO2 stream. To address these issues and others, companies are pursuing innovations in Material, Process, and Equipment design.

Material Design:

Novel Sorbent – This DAC approach typically involves amine-functionalized polymeric resins (e.g. Purolite A110). Some companies are pursuing new classes of sorbents like celulose-based materials derived from agricultural waste.
Water Production - The materials used to capture CO₂ in this DAC approach can also co-capture water from air without a pre-drying step. Some companies are leveraging this feature to co-produce fresh water.

Process Design:

Regeneration - Elevated humidity, often alongside gentle heating and/or vacuum, releases CO2 from these sorbents in a mixture with water. Because geological storage, the dominant sequestration method, requires pure CO2, an important focus is generating dry CO2. Some companies are studying variations of humidity swing to release CO2 as a purified stream.

Equipment Design:

Air Contactor - DAC requires flowing enormous volumes of air through a contactor unit to capture CO₂ and that can come at a high energy penalty, especially due to pressure drop across the contactor. Innovations in contactor design, such as using passive air contactors, seek to lower the energy required to filter out CO2.

This tool organizes several key characteristics of DAC companies for which public information is available based on these three categories. Search below by company name or search "Material", "Process", of "Equipment" to view all related company activities.