Carbon Capture

Removing CO₂ from the Atmosphere

Reducing greenhouse gas emissions is one of the biggest challenges in addressing climate change. While renewable energy can reduce future emissions, it cannot eliminate the billions of tons of carbon dioxide (CO₂) that are already released into the atmosphere every year from industries such as cement, steel, aviation, and power generation. Many of these sectors are difficult to fully decarbonize using existing technologies alone.

Carbon capture is a group of technologies designed to prevent CO₂ from entering the atmosphere or remove it after it has already been emitted. Rather than replacing renewable energy, carbon capture is increasingly viewed as a complementary solution that can help reduce emissions from industries where alternatives remain limited.

How Carbon Capture Works

Figure 1: Simplified steps of carbon capture [16]

Carbon capture technologies generally involve three main steps [10]:

Capture: CO₂ is separated from a gas stream, such as industrial exhaust or ambient air.
Transportation : The captured CO₂ is compressed and transported, usually through pipelines or by ship.
Storage or Utilization: The CO₂ is either permanently stored underground or converted into useful products such as fuels, chemicals, or building materials.

The first step, capturing the CO₂, is typically the most energy-intensive and expensive part of the process, making it the primary focus of current research.

Types of Carbon Capture

Post-Combustion Carbon Capture

This is currently the most widely deployed carbon capture technology.

After fossil fuels are burned, the resulting exhaust gases contain a relatively low concentration of CO₂ mixed with nitrogen, water vapor, and other gases. Chemical solvents,most commonly amine-based liquids, are used to selectively absorb the CO₂ [2]. The solvent is then heated, releasing concentrated CO₂ for storage while regenerating the solvent for reuse.

Advantages

Can be retrofitted onto many existing power plants and industrial facilities.
Relatively mature technology with decades of industrial experience.

Challenges

Requires large amounts of energy to regenerate the solvent.
Solvents gradually degrade and must be replaced.
Adds significant operating costs to industrial facilities.

Figure 2: Diagram of process of post-combustion carbon capture, highlighting how carbon dioxide must be separated from other gases [17]

Pre-Combustion Carbon Capture

Instead of removing CO₂ after fuel is burned, pre-combustion capture separates carbon before combustion occurs.

Fuels such as coal or natural gas are converted into synthesis gas (syngas), primarily composed of hydrogen and carbon monoxide. Through the water-gas shift reaction, carbon monoxide reacts with water to produce hydrogen and CO₂. Since the CO₂ is at a much higher concentration than in flue gas, it is generally easier to separate [2].

The remaining hydrogen can then be burned or used as a clean fuel.

Advantages

Higher CO₂ concentration makes separation more efficient.
Produces hydrogen that can be used as a low-carbon energy source.

Challenges

Requires entirely different power plant designs.
Expensive to retrofit existing facilities.

Figure 3: Diagram of process of pre-combustion carbon capture [18]

Oxy-Fuel Combustion

Traditional combustion uses air, which is mostly nitrogen. Oxy-fuel combustion instead burns fuel using nearly pure oxygen [2].

Without nitrogen present, the exhaust consists primarily of CO₂ and water vapor. After the water condenses, a highly concentrated stream of CO₂ remains, simplifying capture.

Advantages

Produces very high-purity CO₂.
Reduces the need for complex gas separation afterward.

Challenges

Producing pure oxygen requires large amounts of electricity.
Oxygen production equipment significantly increases system costs.

Figure 4: Diagram of process of oxy-fuel combustion [19]

Direct Air Capture (DAC)

Unlike industrial carbon capture, Direct Air Capture removes CO₂ directly from the atmosphere [13].

Large fans pull ambient air through systems containing either liquid solvents or solid sorbent materials that selectively bind CO₂. Once saturated, the materials are heated or exposed to reduced pressure, releasing concentrated CO₂ for storage or utilization.

Because atmospheric CO₂ concentrations are only about 0.04%, DAC requires processing enormous volumes of air, making it substantially more energy-intensive than capturing CO₂ directly from industrial emissions.

Figure 5: Depiction of DAC process [20]

Advantages

Can remove historical emissions already present in the atmosphere.
Can be deployed almost anywhere with access to clean energy and suitable storage.

Challenges

Extremely energy-intensive [5].
Currently one of the most expensive carbon removal technologies [5][13].
Requires abundant low-carbon electricity to achieve meaningful climate benefits.

Technologies Used to Capture CO₂

Researchers are developing a variety of materials that selectively capture CO₂ more efficiently than conventional solvents.

Liquid Solvents

Amine-based solvents remain the commercial standard because they react readily with CO₂. However, researchers are developing new solvent formulations that require less energy during regeneration and exhibit greater long-term stability.

Liquid-solvent direct air capture does not perform uniformly around the world. Local temperature, humidity, and water availability can significantly influence operating costs and net carbon removal, making site selection an important factor in the successful deployment of DAC technologies. This is explored in the publication “Liquid solvent direct air capture’s cost and carbon dioxide removal vary with ambient environmental conditions” [12].

Solid Sorbents

Instead of liquids, porous solids can physically or chemically trap CO₂ on their surfaces [13].

Examples include activated carbon, zeolites, functionalized silica, and advanced polymer materials. These materials can often be regenerated using lower temperatures, potentially reducing energy consumption [14].

Figure 6: Humorous depiction highlighting the many types of solid sorbents [14]

Metal-Organic Frameworks (MOFs)

Metal-Organic Frameworks (MOFs) are highly porous crystalline materials composed of metal ions connected by organic linkers. Their enormous internal surface areas allow them to selectively adsorb large amounts of CO₂ [4][15].

Figure 7: Schematic representation of important reported MOFs [21]

Researchers can precisely tune their pore sizes and chemical properties, making MOFs one of the most promising materials for next-generation carbon capture.

Current challenges include improving long-term stability, reducing manufacturing costs, and maintaining performance in humid industrial environments [6].

Membrane Separation

Selective membranes allow certain gases to pass through while blocking others.

By engineering membranes with high CO₂ selectivity, researchers hope to separate carbon dioxide continuously with lower energy requirements than solvent-based systems [3].

Although membrane technology has advanced rapidly, achieving both high permeability and high selectivity remains an active area of research [1].

What Happens to Captured CO₂?

Capturing carbon is only part of the challenge. Once collected, the CO₂ must either be permanently stored or converted into useful products.

Geological Storage

The most common long-term storage method injects compressed CO₂ deep underground into geological formations such as depleted oil and gas reservoirs or deep saline aquifers [7][8].

Over time, multiple geological mechanisms trap the CO₂, allowing it to remain stored for thousands of years if properly managed.

Figure 8: Depiction of options for geological storage of carbon dioxide [22]

Carbon Utilization

Rather than storing CO₂, researchers are exploring ways to transform it into valuable products [5].

Examples include:

Synthetic fuels
Plastics and polymers
Chemical feedstocks
Concrete and building materials
Carbon monoxide for industrial chemistry

These approaches seek to treat CO₂ as a resource rather than simply a waste product. However, many conversion processes still require significant amounts of clean electricity or hydrogen to become truly sustainable [7].

Figure 9: Various products that can be made from carbon dioxide utilization [23]

Current Research Directions

Carbon capture research extends far beyond simply improving existing equipment. Scientists are working to redesign nearly every aspect of the process.

Major research areas include:

Developing lower-energy capture materials.
Designing more stable solvents and sorbents.
Creating highly selective membranes.
Engineering scalable MOFs.
Integrating carbon capture with renewable electricity.
Combining carbon capture with electrochemical CO₂ conversion.
Lowering costs through improved manufacturing and process optimization.

Many researchers are also studying how carbon capture can be integrated directly into industrial processes instead of being added afterward as a separate system.

Challenges to Large-Scale Deployment

Despite decades of research, carbon capture remains expensive and energy-intensive.

Several barriers continue to limit widespread adoption:

High capital costs for building capture and storage facilities [9].
Large energy requirements, particularly for solvent regeneration.
Limited CO₂ transportation infrastructure.
Availability of suitable long-term storage sites [9][11].
Public acceptance of underground carbon storage [11].
Policy uncertainty and insufficient financial incentives.
Difficulty competing economically with simply emitting CO₂ where carbon pricing is absent.

Because of these challenges, many experts emphasize that technological innovation alone is not enough. Large-scale deployment also depends on supportive policy, infrastructure investment, and economic incentives that make capturing carbon financially viable.

Looking Ahead

Carbon capture is not a single technology but an evolving field that combines chemistry, materials science, engineering, geology, and economics. While it is unlikely to replace renewable energy or eliminate the need to reduce emissions at their source, it may become an essential tool for addressing emissions from industries that are difficult to decarbonize and for removing excess CO₂ already present in the atmosphere.

As researchers develop more efficient materials, improve system designs, and reduce costs, carbon capture has the potential to play an increasingly important role in achieving global climate goals. Its ultimate success, however, will depend not only on scientific advances but also on the policies, infrastructure, and societal investments needed to deploy these technologies at the scale required.

References

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