Adsorption and High-Grade Heat Regeneration

Capture Mechanism

Solid

Furthest Progress*

TRL 6

Highest Risks

Energy Environment

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Method Overview

Definition:

CO2 adsorption onto a solid mineral sorbent (where CO2 is captured on the surface of a solid material) followed by a desorption process in which high-grade heat (600-1200 °C) is used to separate CO2 from the mineral.*

Example:

Crushed calcium oxides chemically bind with CO2 in air to form limestone, which is then processed at high temperatures to release concentrated CO2 for storage and regenerate the calcium oxides.*

Advantages:

Sorbent materials (generally CaO or its hydrated form Ca(OH)2) are derived from minerals like limestone (CaCO3), a low-cost abundant feedstock ($10-50/ t CaCO3). The minerals must be crushed and processed (e.g., chemically treated to become more porous or mixed with additives for cycle stability), but are still much cheaper than synthesizing sorbents like MOFs.
This approach can reduce the energy required for air contacting because its CO2 capture rate is lower – it gains no added efficiency by flowing more air through.
This approach is modular and can be scaled as needed. For instance, sorbents are generally stacked in vertical trays to form contactor units, and multiple contactor units are integrated with a calciner. Modularity also accommodates iterative improvements.

Disadvantages:

Calcination at ~900 °C to release CO2 from CaCO3 and regenerate CaO is extremely energy-intensive (minimum of ~4 GJ/t of CO2 released). 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.
Particle sintering during regeneration impacts CO2 capacity over repeated cycles by lowering the surface area, significantly reducing the number of cycles over which the sorbent is effective. Additives can reduce sintering and improve cycling stability but also limit CO2 capacity.
Slow CO2 capture kinetics (days to weeks to reach saturation), relative to other approaches. These materials have low surface areas, especially when pelletized and offer limited active sites to react with CO2. When CaO is hydrated to Ca(OH)2, CO2 adsorption kinetics can be improved.

* 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

High risk: Sorbent regeneration by calcination is extremely energy intensive, but heat recovery is possible.

Energy needs meet our criteria for “very high risk” (>5 GJ/tCO2 and needs high-grade heat). However, since the capture and sorbent regeneration alone require approximately 6-9 GJ/tCO₂ and there is the possibility of heat recovery in the calcination and hydration steps, we assigned this a lowered risk rating of "high risk." The full DAC+S pathway will require additional energy (e.g., compression, transport, and storage).

Self-reported energy requirements of DAC companies (GJ/tCO2): Airhive: 7.2, Heirloom: 9.0.

Material Requirements

Low risk: No rare materials are required.

Beyond basic construction materials (e.g., steel, cement), this technology relies on abundant minerals that can be reused (CaCO3 or MgCO3).

Cost Requirements

Medium risk: Technology can be modular across certain components with cost estimates in the $100-500 range.

We base our rating on cost estimates for FOAK plants and the quality of these estimates. We apply different cut-offs for modular/non-modular technologies 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): Terraform Industries: $250.

Land Requirements

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

This approach has a higher footprint per capture capacity than other DAC approaches, as some solutions require large land areas, (~6000 km²/GtCO₂/y to expose and weather minerals), but optimal designs, such as the use of vertically stacked trays, can reduce those requirements. The footprint is larger if land use for energy production is considered.

Self-reported land requirements of DAC companies (km²/GtCO₂/y): Heimdal: 2900.

Water Requirements

Medium risk: Large volumes of water are needed to enable mineralization.

This DAC technology requires large amounts of water (2.5-5 t of H₂O per t CaO) as humidity is needed for the process to work. However, the water used can be recycled. The amount of water may vary depending on mineral used (CaCO3 or MgCO3). High water requirements restrict deployment to locations with low water stress indices.

Environmental Impact

High risk: No / incomplete LCA data available.

No LCA currently exists for this technology but is in progress. Apart from impacts associated with energy, water and material requirements, minimal to no other impacts are anticipated.

Co-Location of DAC and Storage

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

The DAC unit shouldn’t be situated in dry and hot environments due to high water requirements. However, there is sufficient geological storage in water-rich areas so co-location should not be a problem.

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. Capture occurs in a controlled environment and geological injection has established monitoring standards.

Key Innovations

Because the sorbent material in this approach is widely available and low cost, companies are primarily focused on tackling the substantial energy required to regenerate it, or compensate for its high regeneration energy by improving the sorbent's CO2 capture performance. Companies are innovating in Material, Process, and Equipment design to meet these goals.

Material Design:

By in large, companies purusing this approach all utilize CaO/Ca(OH)2 as their sorbent because it is a widely available material with well-understood CO2 reactivity. Thus, innovation in materials design primarily consist of strategies to improve its porosity, and by extention its CO2 capture kinetics (days to weeks to reach saturation), as well as its cycle stability upon calcination. Numerous synthesis procedures, chemical treatments, and additives to accomplish these goals have been studied but little information exists about companies' activities.

Process Design:

This method is well-positioned to integrate with industries like cement production that can utilize carbonate materials, namely CaCO3, in their products. Forming carbonate materials as a final step without reusing them in repeated cycles avoids the energy-intensive process of regeneration but requires a constant supply of fresh sorbent.
Other companies are pursuing novel process designs - either simplying the process to lower capital and operating costs, or chosing more intensive processes that may enhance CO2 capture efficiency.

Equipment Design:

Calcination is very energy-intensive so other energy efficiencies like as using passive air contacting are important focuses.
Besides being energy-intensive, calcination in other industries like cement manufacturing often relies on oxy-fired, natural gas fed equipment that releases CO2, which must be stored alongside captured CO2. Some DAC companies are pursuing electric calciners or integrating zero-emission kilns to remain carbon negative.

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.