How Does Carbon Capture Work and Can It Reduce Emissions

How Does Carbon Capture Work and Can It Reduce Emissions?

Carbon capture is a group of technologies designed to prevent some carbon dioxide from entering the atmosphere. In the most common approach, equipment separates carbon dioxide from an industrial gas stream, compresses it, transports it through pipelines or ships, and injects it into deep underground rock formations for long-term storage.

The basic idea sounds simple, but the engineering is complex. Carbon dioxide is mixed with other gases, often at low concentrations, and separating it requires equipment, energy, monitoring, and carefully selected storage sites. Carbon capture can reduce emissions from certain industrial processes, but it is not a substitute for energy efficiency, renewable energy, electrification, or reducing fossil-fuel use wherever those options are practical.

What is carbon capture?

Carbon capture is the process of removing carbon dioxide, usually called CO₂, from a gas stream or from the atmosphere before it contributes to climate change.

The term often appears in three related forms:

- **Carbon capture and storage**, or CCS, captures CO₂ and stores it underground.
- **Carbon capture, utilisation and storage**, or CCUS, captures CO₂ and either stores it or uses it in another process.
- **Carbon dioxide removal**, or CDR, removes CO₂ that is already present in the atmosphere.

These approaches are not identical. Capturing emissions from a cement plant prevents some new CO₂ from reaching the atmosphere. Direct air capture removes CO₂ from ambient air. The first is an emissions-control technology, while the second is a form of atmospheric carbon removal if the captured CO₂ is permanently stored.

Carbon capture is particularly relevant to industries where emissions are difficult to eliminate completely. Cement production, steelmaking, chemicals, fertiliser manufacturing, refining, and some waste-to-energy facilities may produce emissions through both fuel use and chemical reactions.

How does carbon capture work?

A complete carbon-capture system usually involves four stages:

1. Capture the carbon dioxide.
2. Compress and condition it.
3. Transport it to a storage or utilisation site.
4. Store it permanently or use it in a controlled application.

The exact equipment depends on the source of the emissions, the concentration of CO₂, the pressure and temperature of the gas, and the required purity of the final product.

Stage 1: Capturing carbon dioxide

Industrial exhaust gases contain a mixture of substances. Depending on the facility, the mixture may include nitrogen, water vapour, oxygen, carbon dioxide, sulphur compounds, dust, and other pollutants.

A capture system separates CO₂ from this mixture. The main approaches are known as post-combustion capture, pre-combustion capture, and oxy-fuel combustion.

Post-combustion capture

Post-combustion capture treats exhaust gas after fuel has been burned. This is often considered attractive because it can potentially be added to an existing facility.

One common method uses a liquid solvent. The exhaust gas passes through an absorber column, where the solvent selectively reacts with CO₂. The solvent carrying the CO₂ is then heated in a separate unit. The heat releases a concentrated CO₂ stream, and the solvent is returned to the absorber for reuse.

This method is similar in principle to washing one substance out of a mixture. The difficulty is that the solvent must be regenerated repeatedly, and regeneration consumes considerable heat.

Pre-combustion capture

In pre-combustion systems, fuel is converted into a gas mixture before it is burned. The process can produce hydrogen and a concentrated stream containing CO₂.

Because the CO₂ concentration may be higher than in ordinary exhaust gas, separation can be easier. However, the process requires additional equipment and is generally associated with specially designed fuel-conversion or gasification systems rather than simple retrofits.

Oxy-fuel combustion

Oxy-fuel combustion burns fuel in oxygen rather than ordinary air. Since air is mostly nitrogen, removing nitrogen from the combustion process can produce an exhaust stream with a much higher concentration of CO₂ and water vapour.

The water can be condensed, leaving a more concentrated CO₂ stream. The main challenge is producing large quantities of oxygen, which requires energy and specialised separation equipment.

Solid sorbents and membranes

Not all capture systems use liquid solvents.

Solid sorbents are materials that bind to CO₂ on their surface. They can be regenerated by applying heat, reducing pressure, or using another process.

Membranes are thin materials that allow some gases to pass through more easily than others. A membrane system may separate CO₂ from the rest of a gas mixture using differences in permeability and pressure.

Other methods include cryogenic separation, mineral-based processes, and chemical looping. The best choice depends on the composition and operating conditions of the emissions source.

Stage 2: Compressing carbon dioxide

After capture, CO₂ is usually processed into a dense fluid before transport. The gas is dried, purified, and compressed to reduce its volume.

Removing water is important because water and CO₂ can form carbonic acid, which may increase corrosion risk in pipelines and equipment. Impurities also matter because they can affect pressure, phase behaviour, material compatibility, and the reliability of valves and compressors.

Compression requires energy. A facility must therefore produce additional power or heat to operate the capture system. This is known as the **energy penalty** or capture penalty.

For example, a power station equipped with carbon capture may need more fuel or electricity to produce the same net amount of useful power. Engineers must evaluate the entire system rather than looking only at the percentage of CO₂ removed from the exhaust.

Stage 3: Transporting carbon dioxide

Captured CO₂ must be moved from the capture facility to a storage site or industrial user.

Pipelines

Pipelines are suitable when a large and steady volume of CO₂ travels over a relatively predictable route. Engineers must consider:

- Pressure and temperature.
- Water content and impurities.
- Pipeline materials.
- Terrain and land use.
- Pressure relief and emergency isolation.
- Population density near the route.
- Expansion and contraction caused by temperature changes.

CO₂ pipelines operate differently from natural-gas pipelines because carbon dioxide has unusual phase behaviour. Pressure changes can cause rapid expansion, and the design must prevent unstable flow conditions.

Ships

Ships can transport liquefied CO₂ between locations that are not connected by pipeline. This may be useful when capture facilities are widely distributed or when storage sites are offshore.

Shipping requires liquefaction, temporary storage tanks, loading systems, specialised vessels, and receiving terminals. The full chain resembles other liquid-gas logistics systems, but the temperature and pressure requirements are specific to CO₂.

Road and rail

Trucks and rail transport may be used for smaller quantities or early-stage projects. They are generally less efficient for very large, continuous volumes, but they can provide flexibility where pipeline or ship infrastructure is unavailable.

Stage 4: Storing carbon dioxide underground

The most important form of permanent carbon storage involves injecting CO₂ into deep geological formations.

Potential storage sites include:

- Saline formations containing brine.
- Depleted oil and gas reservoirs.
- Certain basalt formations.
- Other suitable porous rock formations sealed beneath impermeable layers.

The CO₂ is injected at depths where pressure and temperature allow it to remain in a dense state. It occupies pores in the rock rather than forming a large underground lake.

Several mechanisms help retain the CO₂:

- **Structural trapping:** an impermeable caprock prevents upward movement.
- **Residual trapping:** some CO₂ becomes immobilised in small rock pores.
- **Solubility trapping:** CO₂ dissolves into underground brine.
- **Mineral trapping:** over time, CO₂ may react with minerals and form stable solid carbonates.

Storage sites must be carefully assessed. Engineers study rock porosity, permeability, pressure, faults, existing wells, groundwater, and the strength of the caprock.

How is stored CO₂ monitored?

Permanent storage requires monitoring before, during, and after injection. The purpose is to verify that the CO₂ remains within the approved storage complex and that pressure is managed safely.

Monitoring methods may include:

- Pressure and temperature sensors.
- Seismic surveys.
- Underground chemical sampling.
- Satellite or aerial observations.
- Groundwater monitoring.
- Well-integrity inspections.
- Computer models that predict plume movement.

A storage project also needs a measurement, reporting, and verification system. This helps determine how much CO₂ was captured, how much was transported, how much was injected, and whether any emissions occurred along the chain.

Monitoring is not a single final inspection. It is an ongoing engineering and regulatory activity.

What happens when CO₂ is used instead of stored?

Captured CO₂ can be used in industrial processes, but use does not always mean permanent removal from the atmosphere.

Possible applications include:

- Producing chemicals.
- Manufacturing synthetic fuels.
- Carbonating concrete.
- Producing building materials.
- Growing plants in controlled environments.
- Enhanced oil recovery.
- Creating polymers and other products.

The climate benefit depends on the complete life cycle. If CO₂ is used to make a fuel that is later burned, the carbon may return to the atmosphere. If it is mineralised into a durable building material, storage may last much longer.

Using CO₂ can therefore reduce emissions in some circumstances, but it should not automatically be described as permanent carbon removal.

Where can carbon capture be useful?

Carbon capture may be most useful in situations where emissions are technically difficult or expensive to eliminate through other methods.

Cement production

Cement manufacturing releases CO₂ not only from fuel combustion but also from the chemical conversion of limestone into clinker. Even if the kiln used low-carbon energy, part of these process emissions would remain.

Capture can address a portion of these unavoidable chemical emissions.

Steel and chemicals

Some industrial processes use carbon-containing materials as chemical inputs, reducing agents, or sources of high-temperature heat. Capture may help lower emissions while alternative technologies are developed and scaled.

Hydrogen production

Hydrogen made from natural gas can produce CO₂. Capturing and storing that CO₂ can reduce the emissions intensity of the hydrogen, provided methane leakage, capture performance, energy use, and storage are all carefully controlled.

Waste-to-energy

If waste contains biological material, capturing the resulting CO₂ may provide a potential pathway to net atmospheric removal when the captured carbon is permanently stored. The result depends on the waste composition and the full life-cycle emissions of the facility.

Direct air capture

Direct air capture removes CO₂ from ambient air. Because atmospheric CO₂ is highly diluted, the equipment must process very large volumes of air.

This makes direct air capture energy-intensive and costly. It may nevertheless have a role for residual emissions that are difficult to eliminate, especially if powered by genuinely low-carbon energy and connected to secure permanent storage.

What are the main advantages?

Carbon capture offers several potential benefits:

- It can reduce emissions from existing industrial facilities.
- It may address process emissions that cannot be eliminated through electrification alone.
- It can support lower-emission production of some industrial materials.
- Underground storage can provide long-term containment.
- Captured CO₂ can sometimes become a useful industrial feedstock.
- Capture systems may be combined with hydrogen, biomass, or waste-processing technologies.

The value of these benefits depends on actual performance. A project must account for capture rates, energy consumption, transport emissions, leakage, construction impacts, and the source of the energy used.

What are the main limitations?

Carbon capture also presents important challenges.

High energy demand

Separating CO₂ from a gas mixture and compressing it requires energy. If that energy comes from a carbon-intensive source, the climate benefit may be reduced.

High cost

Capture equipment can be expensive to install and operate. Costs vary significantly by industry, gas composition, facility size, energy prices, transport distance, and storage conditions.

Difficult retrofits

Existing facilities may have limited space, unsuitable steam systems, or equipment that must be modified extensively. Adding capture can affect production rates and maintenance schedules.

Storage availability

A capture project is not complete unless suitable storage is available. Storage sites must be characterised, permitted, connected to transport infrastructure, and monitored.

Risk of leakage

Geological storage is designed to be permanent, but the possibility of leakage must be assessed and managed. Old wells, faults, pressure changes, or poor site selection can create risks.

Dependence on policy and infrastructure

Large-scale carbon management requires regulations, monitoring rules, liability arrangements, transport networks, and financial incentives. Without these, projects may struggle to attract investment.

It does not remove the need for prevention

Reducing energy demand, improving efficiency, replacing fossil fuels, and avoiding unnecessary emissions generally remain essential. Capture should be evaluated as one tool within a broader emissions-reduction strategy.

How should readers evaluate carbon-capture claims?

When a company or project claims that it captures a certain percentage of emissions, ask what the percentage describes.

Important questions include:

1. Is the figure based on the capture equipment alone or the whole facility?
2. Does it include emissions from producing the additional energy?
3. What happens to the captured CO₂?
4. Is the CO₂ permanently stored or later released?
5. Are transport and compression emissions included?
6. How is storage monitored?
7. What happens if the facility stops operating?
8. Are methane leakage and upstream emissions included?
9. Is the project reducing emissions or merely moving them elsewhere?
10. Has the performance been independently measured?

A high capture rate at the exhaust stack does not automatically equal the same percentage reduction in total life-cycle emissions.

Key takeaway

Carbon capture works by separating CO₂ from industrial gases or air, compressing it, transporting it, and storing it underground or using it in products. It may be valuable for industries with unavoidable process emissions, but its real climate benefit depends on energy use, capture performance, transport, permanent storage, and reliable monitoring. Carbon capture is best understood as a specialised tool for difficult emissions, not as a replacement for efficiency, clean energy, and direct emissions reduction.

References

International Energy Agency. "Carbon Capture, Utilisation and Storage." https://www.iea.org/energy-system/carbon-capture-utilisation-and-storage

U.S. Department of Energy. "Carbon Capture." https://www.energy.gov/fecm/carbon-capture

U.S. Environmental Protection Agency. "Class VI Wells and Geologic Sequestration of Carbon Dioxide." https://www.epa.gov/uic/class-vi-wells-and-geologic-sequestration-carbon-dioxide

National Energy Technology Laboratory. "Carbon Capture." https://netl.doe.gov/carbon-management/carbon-capture

Intergovernmental Panel on Climate Change. "Climate Change 2022: Mitigation of Climate Change." https://www.ipcc.ch/report/ar6/wg3/

National Academies of Sciences, Engineering, and Medicine. "Carbon Dioxide Utilization." https://nap.nationalacademies.org/catalog/25232/gaseous-carbon-waste

International Organization for Standardization. "ISO 27914:2017 — Carbon Dioxide Capture, Transportation and Geological Storage." https://www.iso.org/standard/64148.html

U.S. Geological Survey. "Carbon Sequestration." https://www.usgs.gov/mission/usgs-energy-resources/science/carbon-sequestration

Global CCS Institute. "State of the Art: CCS Technologies." https://www.globalccsinstitute.com/

Royal Society. "Greenhouse Gas Removal Technologies." https://royalsociety.org/topics-policy/projects/greenhouse-gas-removal/