Overview of Chemical Capture Technologies

Chemical carbon capture refers to processes that employ chemical reactions to separate carbon dioxide (CO₂) from industrial exhaust streams before it reaches the atmosphere. These methods exploit the affinity of certain compounds for CO₂, forming intermediate chemical species that can be reversed or converted to stable products. While physical separation techniques like cryogenic distillation exist, chemical methods achieve higher selectivity and efficiency, especially when CO₂ concentrations in the flue gas are moderate to low (4–15% by volume). The core principle involves a chemical reaction between a capturing agent—liquid, solid, or metal oxide—and CO₂, followed by regeneration of the agent to release a concentrated CO₂ stream suitable for compression, storage, or utilization. Chemical capture is distinct from direct air capture (DAC), which deals with atmospheric CO₂ at ~0.04%, but many of the same reaction chemistries are applied with adjustments for concentration and scale. Industrial adoption has been driven by tightening emissions regulations, carbon pricing mechanisms, and the growing need for carbon dioxide removal to meet net-zero targets, as outlined by the IPCC Sixth Assessment Report.

Major Chemical Capture Techniques

Absorption Using Liquid Solvents

By far the most mature and widely deployed chemical capture method is absorption using liquid solvents, particularly amine-based solutions. In a typical post-combustion capture system, flue gas is passed through an absorber column where it contacts an aqueous amine solvent, most commonly monoethanolamine (MEA). The amine molecules react reversibly with CO₂ to form carbamate and bicarbonate species, effectively scrubbing the gas. The CO₂-rich solvent is then pumped to a regenerator (stripper) column, heated to around 120–150 °C, reversing the reaction and producing a nearly pure CO₂ stream. The regenerated solvent is recycled back to the absorber.

Amine scrubbing has been proven at large scale—the Boundary Dam CCS facility in Saskatchewan, Canada, has used this process since 2014 to capture ~1 million tonnes of CO₂ per year from a coal-fired power plant. However, drawbacks include high energy consumption for solvent regeneration (typically 2.5–4 GJ per tonne of CO₂ captured), solvent degradation from oxygen and other flue gas contaminants, corrosion of equipment, and amine emissions. To address these, next‑generation solvents have been developed: sterically hindered amines like 2‑amino‑2‑methyl‑1‑propanol (AMP) reduce degradation rates; blends of MEA with methyldiethanolamine (MDEA) improve energy efficiency; and water‑lean solvents such as chilled ammonia reduce the sensible heat burden. Ionic liquids and non‑aqueous solvents are also being researched for lower regeneration energy and reduced volatility. The Global CCS Institute’s Global Status of CCS 2023 notes that amine‑based absorption remains the dominant capture technology in existing CCS projects.

Adsorption Using Solid Sorbents

Solid sorbents offer an alternative to liquid solvents by capturing CO₂ through physical physisorption or chemical chemisorption on a porous surface. Key materials include zeolites, metal‑organic frameworks (MOFs), amine‑functionalized silicas, and activated carbon. The process typically employs temperature swing adsorption (TSA) or pressure swing adsorption (PSA). In TSA, flue gas passes over a bed of sorbent at lower temperature; the sorbent adsorbs CO₂ until saturated, then the bed is heated (e.g., to 80–150 °C) to release a concentrated CO₂ stream. In PSA, adsorption occurs at high pressure, and desorption at low pressure.

Solid sorbents offer several advantages over liquid amines: they avoid liquid handling, corrosion, and amine degradation issues; they can be tailored at the molecular level for high selectivity and capacity; and they may operate over a wider temperature range. For example, MOFs such as Mg₂(dobpdc) show exceptional CO₂ uptake at low partial pressures and can be regenerated with moderate heat. Amine‑grafted mesoporous silicas combine the chemoselectivity of amines with the stability of solid supports. A notable large‑scale demonstration is Svante’s solid sorbent capture technology used at the Climeworks Orca plant (also for DAC) and other industrial pilots. Challenges include sorbent attrition over many cycles, heat transfer limitations in packed beds, and the need for efficient heat integration during regeneration. Research is focusing on structured sorbents (monoliths, fibers) and moving‑bed configurations to improve throughput and durability.

Chemical Looping Combustion

Chemical looping combustion (CLC) is a fundamentally different approach that incorporates CO₂ capture directly into the combustion process. Instead of combusting fuel with air, CLC uses a metal oxide (oxygen carrier) to supply oxygen for combustion. The metal oxide is reduced to a lower‑valent state while oxidizing the fuel to CO₂ and H₂O. The reduced metal is then re‑oxidized in a separate reactor with air, regenerating the oxygen carrier and producing a nitrogen‑free exhaust (mainly CO₂ and water, which can be condensed to yield pure CO₂). Common oxygen carriers are iron, nickel, copper, manganese, and mixed oxides supported on inert materials like alumina or silica.

CLC eliminates the need for an energy‑intensive separation step because CO₂ is inherently isolated in the fuel reactor. It also avoids NOₓ formation because the fuel reactor operates without nitrogen. Pilot plants up to several megawatts have been demonstrated (e.g., the 3‑MWₜ plant at the Technical University of Darmstadt and the 1‑MWₜ unit in Vienna). However, challenges remain: attrition and agglomeration of oxygen carriers over many cycles, high temperatures (800–1000 °C) that strain materials, and the need for efficient solid transport between reactors. The U.S. Department of Energy’s National Energy Technology Laboratory has funded numerous CLC projects to overcome these barriers. While CLC is promising for power generation, it requires retrofitting or new‑build plants, limiting near‑term deployment in existing facilities.

Carbonation and Mineralization

Carbonation processes convert CO₂ into stable solid carbonates through reaction with metal oxides or silicates. This can be done ex situ, grinding minerals such as olivine (Mg₂SiO₄) or serpentine (Mg₃Si₂O₅(OH)₄) and reacting them with CO₂ under elevated temperature and pressure to form magnesium or calcium carbonates. Alternatively, industrial wastes like steel slag, fly ash, or cement kiln dust can serve as feedstocks. The resulting carbonates are thermodynamically stable over geologic timescales, providing permanent storage without the need for reservoir monitoring.

While mineralization avoids the injection challenges of geological storage and can be integrated with construction materials (e.g., carbon‑cured concrete), the reaction rates are slow and the energy required for grinding and heating is high. Current research aims to accelerate carbonation using catalytic additives, high‑pressure reactors, or indirect carbonation routes. Companies like CarbonCure inject CO₂ into wet concrete during mixing, achieving both CO₂ mineralization and improved compressive strength. The technology has been adopted by hundreds of concrete plants, but the total CO₂ captured per tonne of concrete remains modest (~10–30 kg). For large point sources, mineral carbonation may handle only a fraction of total emissions unless coupled with massive mining operations—hence it is often considered more suitable for niche waste valorization rather than primary capture.

Advantages and Limitations of Chemical Capture Methods

Chemical capture methods collectively offer high CO₂ removal efficiencies (often >90%) from industrial streams, even when CO₂ concentrations are low. They can be retrofitted to existing power plants, cement kilns, refineries, and steel mills, providing a bridge to lower‑carbon operations while renewable energy scales up. The captured CO₂ can be used as a feedstock for chemicals (urea, methanol, synthetic fuels) or injected into deep geological formations for permanent storage.

However, each technique carries limitations. Amine scrubbing suffers from a significant energy penalty—regeneration typically consumes 20–30% of the plant’s steam output, reducing net electricity output by 15–30%. Solid sorbents still face scale‑up challenges in terms of cyclic stability and bed heat management. Chemical looping requires entirely new reactor designs and has high capital costs. Mineralization is energy‑ and mining‑intensive, limiting its throughput. Furthermore, the upstream environmental impacts of producing solvents, sorbents, or oxygen carriers—including mining, energy, and water use—must be factored into lifecycle assessments. None of the methods are zero‑emission themselves; the net CO₂ avoided depends on the carbon intensity of the energy used for capture and compression.

Energy Penalty and Cost Considerations

The energy penalty is the single most critical economic barrier to chemical CO₂ capture. For post‑combustion amine scrubbing, the steam required for regeneration reduces a power plant’s net efficiency by about 8–12 percentage points (e.g., from 40% to 28–32% LHV). This translates to increased fuel consumption per net megawatt‑hour and higher costs for electricity. The levelized cost of CO₂ capture for amine systems is currently in the range of $50–80 per tonne for coal plants and $80–120 per tonne for natural gas combined‑cycle plants, according to the International Energy Agency (IEA).

Solid sorbent technologies aim for lower regeneration energy (1.5–3 GJ/tonne) by using waste heat or low‑temperature steam, potentially bringing costs down to $30–50 per tonne. Chemical looping combustion could theoretically achieve net electrical efficiencies comparable to conventional combustion while capturing CO₂ at marginal additional cost (<$20/tonne), but the technology is not yet commercially mature. Minimizing the energy penalty requires process integration: recovering heat from flue gases, using low‑grade steam for regeneration, and combining capture with waste‑heat recovery. The IEA’s CCUS in Clean Energy Transitions report emphasizes that sustained research and demonstration are essential to drive down costs.

Current Deployment and Large‑Scale Projects

As of 2024, over 40 large‑scale carbon capture facilities are operating or under construction globally, capturing nearly 50 million tonnes of CO₂ per year—a tiny fraction of global emissions (~37 billion tonnes). The majority use chemical absorption with amines. Key projects include:

  • Boundary Dam (Canada): Coal‑fired power plant, amine scrubbing, ~1 Mt CO₂/year, operational since 2014.
  • Petra Nova (USA): Coal‑fired plant (now mothballed due to economics), amine scrubbing, ~1.6 Mt CO₂/year for enhanced oil recovery.
  • Sleipner (Norway): Natural gas processing, amine scrubbing, ~1 Mt CO₂/year injected into saline aquifer, operational since 1996.
  • Gorgon (Australia): Natural gas processing, amine scrubbing, up to 4 Mt CO₂/year; has faced injection challenges.
  • Fortum Oslo Varme (Norway): Waste‑to‑energy plant, amine scrubbing, ~0.4 Mt CO₂/year (project under development for full scale).

Emerging projects using novel solvents or solid sorbents include the Pilot‑scale Advanced CO₂ Capture (PACC) facility in Alabama, testing multiple technologies, and the Carbon Clean modular amine wash system deployed at a cement plant in India. In the steel sector, the Steelanol project in Belgium captures CO₂ from blast furnace gas using microbial fermentation (biological, not chemical), but chemical capture is also being piloted by ArcelorMittal with amines.

Innovations and Future Directions

New Solvents and Sorbents

Research is accelerating on solvents with lower regeneration heat, higher oxidative stability, and reduced environmental impact. Phase‑change solvents (e.g., biphasic systems that form a CO₂‑rich solid or immiscible liquid layer) can reduce the sensible heat load by requiring only part of the solvent to be heated. Enzyme‑catalyzed capture, using carbonic anhydrase to accelerate CO₂ hydration to bicarbonate, operates at mild conditions and could lower energy use. Meanwhile, advanced MOFs and covalent organic frameworks (COFs) are being designed with open metal sites that strongly bind CO₂ while allowing regeneration at moderate vacuum. The challenge remains to scale laboratory breakthroughs to millions of tonnes per year.

Electrochemical Capture

A radically different approach uses electric current to drive CO₂ separation, either by modulating pH in an electrolysis cell (e.g., the pH‑swing process) or by electrochemically capturing CO₂ with quinone‑based electrolytes. Electrochemical capture can operate at ambient temperature and pressure, potentially reducing the thermal energy penalty and enabling continuous, modular operation. Companies like Verdox and Carbon Infinity are developing electrochemical systems for both point‑source capture and DAC. If costs fall below $50/tonne, electrochemical methods could transform the economics of carbon capture.

Integration with Utilization

Rather than separating CO₂ for storage alone, many future plants will couple capture directly with conversion to valuable products. Chemical capture methods that produce a high‑purity CO₂ stream are well suited for use in greenhouses, for carbonated beverages, or as feedstock for synthetic fuels via hydrogenation. However, the scale of utilization is far smaller than the volume of CO₂ that must be removed to meet climate targets. Therefore, the majority of captured CO₂ will still need permanent geological storage. Innovations in capture must align with build‑out of CO₂ transport and storage infrastructure.

Conclusion

Chemical methods for capturing carbon dioxide from industrial emissions represent a mature yet evolving toolkit essential for deep decarbonization. From the widely deployed amine scrubbing to emerging electrochemical processes, each technique balances efficiency, cost, and operational complexity. While no single method is a silver bullet—energy penalties, material durability, and scale‑up costs remain formidable—the trajectory of innovation offers clear pathways to lower‑cost, lower‑energy capture. Governments and industries must continue to fund research, demonstration, and deployment of these technologies alongside renewable energy and efficiency measures. Only through a portfolio of capture methods can the industrial sector make progress toward net‑zero emissions by mid‑century.