Absorption and High-Grade Heat Regeneration

Capture Mechanism

Liquid

Furthest Progress*

TRL 7

Highest Risks

Energy
Water

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Method Overview

Definition:

CO2 absorption with an alkaline solvent (where CO2 is dissolved or diffused in a liquid to form a solution), followed by a desorption process in which CO2 is mineralized out of the solution and then regenerated by high-grade heat (700-900 °C).*

Example:

Fans pull air over a potassium hydroxide solution that chemically binds with carbon dioxide in air to form potassium carbonate that then is regenerated with calcium hydroxide. This produces mineralized carbon dioxide that is processed at high temperatures to release concentrated carbon dioxide for storage.*

Advantages:

The raw materials for aqueous hydroxide-based DAC (e.g., CaCO3, NaOH, KOH) are cheap and readily available. KOH is generally preferred due to its stronger CO2 affinity and favorable carbonate solubility.
Aqueous hydroxide solutions react strongly with CO2 and offer fast absorption kinetics.
Much of the equipment required for this approach (e.g., calciner, slacker) is well-developed and widely used in other industries.

Disadvantages:

Calcination at ~900 °C to release CO2 from CaCO3 and regenerate CaO is extremely energy-intensive (~4 GJ/t of CO2 captured). High-grade heat is also more challenging to source from intermittent renewables, however oxy-fired kilns can be integrated with CO2 storage alongside captured CO2.
Large volumes of water are consumed during this process, particularly via evaporative cooling within the air contactor. Deployments may be limited to areas with low water stress indices.
A high capital investment is required to acquire the necessary equipment (e.g. contactor, calciner, slacker).

* 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: Solvent regeneration by calcination is extremely energy intensive.

Energy needs meet our DAC criteria for "very high risk" (>5 GJ/tCO2 and needs high-grade heat). Capture and solvent regeneration alone require approximately 6-10 GJ/tCO₂. The full pathway will require additional energy (e.g., compression, transport, and storage).

Self-reported energy requirements of DAC companies (GJ/tCO2): Carbon Engineering: 8.8.

Material Requirements

Low risk: No rare materials are required.

Beyond basic construction materials (e.g., steel, cement), this solution requires raw material inputs that are relatively abundant (KOH and CaCO3 solutions).

Cost Requirements

Medium risk: Technology is modular across certain components with cost estimates at $100-500/tCO2.

Rating is based on cost estimates for FOAK plants and the data quality used for these estimates. Different cut-offs for modular/non-modular technologies were applied, as modular approaches are comparatively less expensive and easier to scale. This technology meets our criteria for “medium” risk (FOAK: >10 ktCO2; $100-500/tCO2 plus available plant data).

Self-reported cost requirements of DAC companies ($/tCO2): Carbon Engineering: $94–232.

Land Requirements

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

These DAC plants have minimal footprint per capture capacity (~16 km²/GtCO₂) compared to other CDR solutions and can be deployed on most types of land. The footprint is larger if land use for energy production is considered.

Self-reported land requirements of DAC companies (km²/GtCO2): 1PointFive: 232 (STRATOS plant working footprint).

Water Requirements

High risk: Large volumes of water are evaporated when operating the plant.

This DAC technology requires large amounts of water (~5 GtH₂O/GtCO₂), which restricts deployment to locations with low water stress indices. However, water requirements vary depending on local conditions.

Self-reported water requirements of DAC companies (GtH₂O/GtCO₂): Carbon Engineering: 4.7.

Environmental Impact

Low risk: No environmental impacts anticipated beyond effects from energy and water intensity.

LCA exists for this technology. Methane leakage during transport is a potential risk, if natural gas is the source of high-grade heat. Apart from impacts associated with energy, water, and material requirements, minimal other impacts are anticipated.

Co-Location of DAC and Storage

Low risk: Co-location of DAC and storage is relatively easy.

The DAC unit should not be situated in dry and hot environments due to the high requirements for water. However, there is sufficient geological storage in water-rich areas, so co-location should not be a problem. Easy co-location with storage is also favorable due to high CO2 purity from the DAC unit.

Secure Storage for 100+ Years

Low risk: Storage is durable.

Geological sequestration or in-situ mineralization (primary storage options) are considered permanent storage approaches.

Path to MRV at Scale

Low risk: Measurement to inform MRV is straightforward.

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

Key Innovations

This field is dominated by Carbon Engineering who pioneered the approach, although CO2 capture by aqueous bases and causticization using calcium had been previously studied. While process varients have been investigated by the company and in preceeding research work, no alternatives to Carbon Engineering have been established (1PointFive acquired Carbon Engineering in 2023).

As the sole DAC technology developer within this approach, Carbon Engineerings's material, process, and equipment design encompasses current innovation in the industry. Selected characteristics are outlined below.

Material Design:

Absorbent - Carbon Engineering favors KOH due to its stronger CO2 affinity and favorable carbonate solubility, which avoids contamiating precipitated CaCO3 with other carbonate species.
Caustic - CaOH derived from limestone is a cheap and abundant caustic used to precipitate captured CO2 as CaCO3. Due to the high decomposition temperature of CaCO3, other caustics such as sodium tri-titanate have also been investigated to isolate different carbonates with lower decomposition temperatures.

Process Design:

Alternative configurations - Carbon Engineering has reported that it is developing an all-electric variation of its current cycle that eliminates methane input, as well as an all liquid-phase regeneration process based on the company's membrane-enhanced thermal swing DAC process. These varients will all incorporate Carbon Engineering's air contactor technology.

Equipment Design:

Contactor - Carbon Engineering has extensively developed its proprietary air contactor, which borrows components and design from conventional cooling towers. The crossflow design involves passing fan-driven air orthogonal to downward streams of aqueous potassium hydroxide flowing over thin plastic surfaces.
Calciner - Carbon Engineering utilizes an oxy-fired calciner to decompose CaCO3 and release CO2. Natural gas combustion produces pure CO2 that is then added to DAC-sourced CO2. Methane is an effective fuel source to provide high-grade heat, however, pure O2 is required and must be produced by a supplimental air separator unit, raising energy requirements. Some other DAC companies requiring high-grade heat like Heirloom plan to use renewably-powered electric kilns in future facilities.