Keeping CCUS safe: From start to storage

INTRODUCTION: A CLIMATE SOLUTION WITH COMPLEX RISKS

Reducing fossil fuel use will be a key step in cutting global emissions, however it alone is insufficient. There is broad consensus that carbon capture, utilisation, and storage (CCUS) will play a critical role in achieving net zero emissions. Though not a permanent solution to climate change, CCUS can serve as an important bridge by helping to manage emissions in the near term as low-carbon technologies and infrastructure are scaled up.

While the technology holds significant promise – particularly in hard-to-abate sectors such as cement, steel, and chemicals production – it also brings new operational, environmental, and regulatory challenges. This article examines the CCUS chain from a risk perspective. It begins by outlining the fundamental properties of CO₂, then examines each stage of the process, highlighting the specific hazards and uncertainties, and how they should be managed. The key stages of the CCUS process are shown in Figure 1.

Figure 1 – CCUS process

MEET THE MOLECULE: CO2 AND ITS HAZARDS

CO₂ is naturally present in the air at a concentration of approximately 0.04% and is generally harmless at low levels. At room temperature and atmospheric pressure, it is a colourless and odourless gas. However, at elevated concentrations, it can cause headaches, dizziness, confusion, and in severe cases, loss of consciousness. In the event of a leak, CO₂ – being heavier than air – can accumulate in confined or low-lying areas, displacing oxygen and creating a serious risk of asphyxiation, which can be fatal.

Figure 2 – CO₂ physical states (Ref. 1)

In CCUS operations, CO₂ is typically handled at or above its critical pressure of 73.82 bar, where it exhibits liquid-like properties (see Figure 2). In this condition, it is commonly referred to as being in the “dense” phase. When both critical pressure and critical temperature (31.04°C) are exceeded, CO₂ enters the “supercritical” phase, behaving like both a gas and a liquid. Significant hazards related to dense phase or supercritical CO₂ can occur when there is a sudden drop or complete loss of pressure, leading to rapid phase change and unstable thermodynamic behaviour. This can place extreme stress on equipment and pipelines and threaten the integrity of the system, making careful design, material selection, and pressure control essential

CAPTURE: WHERE THE RISK BEGINS

The first stage of CCUS is the capture of CO₂ emissions from large industrial processes. In traditional industries, such as power generation, chemical manufacturing and cement production, fossil fuels are combusted to produce heat or electricity. Capture can also be applied in energy transition contexts, for example at facilities using steam methane reforming (SMR) for hydrogen production, where natural gas is converted into hydrogen and CO₂.

There are three main capture methods:

• Post-combustion: CO₂ is removed from flue gases after fossil fuel combustion. Flue gas is typically passed through a chemical solvent, often an amine solution, which selectively binds CO₂. The CO₂ is then released from the solvent by heating, producing a concentrated stream. This method is widely used in power plants and can be retrofitted to existing facilities.
• Pre-combustion capture: Fossil fuel is partially converted to syngas through a process called gasification (for solids like coal) or reforming (for natural gas). The syngas, a mixture of hydrogen and carbon monoxide, is then treated to remove CO₂ before the remaining hydrogen is used as fuel. This method is generally more efficient than post-combustion but cannot be retrofitted to most existing plants.
• Oxy-fuel combustion: Fossil fuel is burned in pure oxygen instead of air, resulting in a flue gas that is mostly CO₂ and water vapour. After condensing the water, a concentrated CO₂ stream remains. This method is efficient but energy intensive.

Integration and retrofitting risks

Integrating carbon capture technology with existing facilities can be complex. Like any construction site, there are risks from lifting, handling and accidental damage. In a high-hazard facility, such as a power station, there may be an increased risk of accidental damage and subsequent fire and explosion risk to existing assets during retrofitting (Ref. 2).

Hazardous substances

Amine-based solvents, widely used in post-combustion carbon capture, operate by chemically binding CO₂ from flue gas. In simple terms, the CO₂ “attaches” to the amine in an absorber column and is later released when the solvent is heated in a stripper column, producing a concentrated CO₂ stream for compression and storage.

Compounds like monoethanolamine (MEA) are corrosive and can cause skin burns, eye damage, and respiratory irritation upon exposure. Additionally, amine degradation can release volatile organic compounds (VOCs), nitrosamines, and nitramines – some of which are toxic or potentially carcinogenic even at low concentrations. Understanding this basic chemistry helps explain why careful handling, ventilation, and monitoring are essential in capture facilities.

TRANSPORT: MOVING THE MOLECULE SAFELY

After capture, the CO₂ is transported to storage or utilisation sites. This is most commonly by pipeline, however trucks and trains can be used for small volumes and where pipeline transport is not feasible. Ship transport can also be used for long-distance transport offshore. This is an emerging market and there are currently only a very small number of dedicated CO₂ vessels, however the fleet is expected to grow rapidly over the next decade.

High pressure hazards

CO₂ is commonly transported via pipeline at high pressures – typically between 80 and 150 bar – to keep it in a dense or supercritical phase, enabling efficient flow. If a rupture or leak occurs, sudden depressurisation can lead to an explosive release. This may cause serious injury and damage to nearby equipment or structures.

In the case of a buried pipeline rupture, the rapid expansion of CO₂ can forcibly displace surrounding soil, potentially creating a crater. This can expose and damage adjacent buried infrastructure, such as water pipes, gas lines, or electrical conduits – resulting in a domino effect of cascading failures (Ref. 3).

Low temperature hazards

When dense or supercritical CO₂ is released into the atmosphere, it undergoes rapid expansion and cooling due to the Joule-Thomson effect. This process can lead to the formation of solid CO₂ (dry ice) and a cold, dense vapour cloud containing solid particles. Close exposure to this release can cause cryogenic burns or impact injuries. Inhaling air with solid CO₂ within a release cloud poses an additional risk of cryogenic injury to the respiratory tract. This hazard is typically confined to the immediate vicinity of the release but may be significantly intensified in enclosed spaces, where the cloud cannot easily disperse (Ref. 4).

Because the released CO₂ is heavier than air, it tends to accumulate near the ground, forming a low-lying visible cloud that can displace oxygen and pose a serious asphyxiation hazard. This property directly influences the design of CO₂ process plants and storage facilities. For example, floor-level gas detectors are commonly installed to monitor for CO₂ buildup, and ventilation systems often include low-level vents to prevent accumulation in confined areas.

Impurities

Captured CO₂ is rarely pure and often contains impurities like water vapour, sulphur oxides (SOₓ), nitrogen oxides (NOₓ), and carbon monoxide (CO). Water is especially problematic, as it can mix with CO₂ to form carbonic acid, a highly corrosive substance. Over time, this can degrade pipelines and increase rupture risk.

Case study – Satartia, Mississippi CO₂ pipeline rupture

In February 2020, a 24-inch CO₂ pipeline ruptured near the town of Satartia, Mississippi. Triggered by heavy rainfall and a landslide, the incident placed excessive axial strain on a pipeline weld, leading to its failure. Approximately 3,498 cubic meters of CO₂ were released, forming a dense plume and producing dry ice due to rapid depressurisation and cooling (see Figure 3), creating hazardous conditions for nearby residents. Over 300 people were evacuated, and 45 were hospitalised with symptoms related to CO₂ exposure and oxygen deprivation (Ref. 5).

Figure 3 – CO₂ pipeline rupture in Satartia, Mississippi. The white material visible is dry ice – solid CO₂ formed during rapid depressurisation and cooling

UTILISATION: THE MOST DIVERSE STAGE

Captured CO₂ has many applications. It is widely used in Enhanced Oil Recovery (EOR), where CO₂ is injected into oil reservoirs to improve extraction efficiency. A significant portion of the CO₂ remains trapped in the depleted reservoir, effectively serving as a form of geological storage.

CO₂ is also used in the production of chemicals, including synthetic fuels, the food and beverage industry (for carbonation and food packaging), and in the manufacture of building materials such as carbon-cured concrete.

While these applications offer valuable uses for captured CO₂, they also carry safety risks. All use cases share the core hazards associated with CO₂ – such as asphyxiation, high-pressure handling, and cold exposure. In addition, each application may introduce additional risks. For example, chemical manufacturing often involves flammable, reactive, or toxic substances, requiring strict handling procedures and control measures to prevent incidents like chemical spills, fires, or harmful exposure.

STORAGE: OUT OF SIGHT, NOT OUT OF MIND

The final stage is storage, where CO₂ is permanently contained in deep saline aquifers or depleted oil and gas reservoirs. Known as geological sequestration, this process ensures that the CO₂ is securely trapped underground and continuously monitored over time to detect and prevent any potential leakage.

Key risks include:

• Leakage pathways: CO₂ can escape through faults, fractures, or abandoned wells.
• Pressure buildup: Injecting CO₂ increases underground pressure, which can create or reactivate faults, potentially allowing CO₂ to migrate upward.
• Induced seismicity: The rise in subsurface pressure can trigger seismic events, such as small tremors or even earthquakes.
• Groundwater contamination: Leaks into underground water sources can form carbonic acid, leading to acidification and contamination of potable groundwater.

To safely manage these risks, careful site selection, pressure control, and long-term continuous monitoring are critical.

RISK MANAGEMENT ACROSS THE CHAIN

Risks are present across the entire CCUS process and must be effectively managed. The following considerations are essential as the deployment of CCUS continues to scale up globally.

CO2 hazard awareness from design to operation

The physical and chemical properties of CO₂ – particularly in its dense phase or supercritical state – must be fully understood and carefully managed throughout the design, construction, and operation of CCUS systems. For instance, the specific characteristics of CO₂, along with the presence of impurities, can increase the risk of leaks. Ensuring material compatibility is therefore essential to maintain the integrity of pipelines and equipment.

Comprehensive risk assessment and modelling

Comprehensive hazard identification and risk assessment are critical – particularly during the early design phase of CCUS projects – to identify potential scenarios that could lead to a CO₂ release. For example, in the case of the Mississippi pipeline rupture incident, the investigation revealed that local weather conditions and geological factors, such as mudslides, were not considered in relation to pipeline integrity (Ref. 5).

CO₂ release scenarios should be modelled to accurately predict dispersion patterns and fully assess potential consequences – particularly for nearby populations. Currently, the behaviour of CO₂ released in its dense or supercritical phase is not yet fully understood. The CCUS industry is therefore actively conducting research to develop and validate suitable dispersion models, supported by experimental studies at appropriate scales (Ref. 6).

CO2 specific emergency response

Emergency response plans should be developed with careful consideration of the specific properties and behaviours associated with CO₂ releases and must involve engagement with all relevant stakeholders.

In Mississippi, local emergency responders were not informed by the pipeline operator about the rupture or the unique safety risks posed by the CO₂ release. As a result, responders were forced to make assumptions about the nature of the hazard. Subsequent investigations identified a need for improved public and stakeholder engagement, and to ensure that both communities and emergency responders understand the appropriate actions to take in the event of a release (Ref. 5).

Regulatory frameworks

To ensure effective and proportionate risk management, regulatory regimes must reflect the specific hazards posed by CO₂ – particularly in its dense and supercritical phases – and establish clear, consistent requirements for design, operation, emergency planning, and public safety.

In the UK, oversight of the CCUS chain is split between two arms of the Health and Safety Executive (HSE), which are managing the evolving risks and closing historical regulatory gaps:

HSE Energy Division (Transportation and Offshore): This area saw the most direct legislative updates in April 2026. Under the Pipeline Safety Regulations, CO₂ is now formally classified as a “dangerous fluid” across all phases (gas, liquid, dense, or supercritical), subjecting pipelines to stricter statutory duties like mandatory Major Accident Prevention Documents (MAPD) and local emergency planning. Additionally, an amendment to the Application Outside Great Britain Order (AOGBO) explicitly extended HSE’s offshore jurisdiction from covering only gaseous CO₂ storage and injection to encompassing CCUS activities involving CO₂ in all chemical phases (Ref. 7).

HSE CEMHD (Onshore Processing): The Chemicals, Explosives and Major Hazards Division governs onshore infrastructure. While CO₂ does not qualify as a dangerous substance under onshore COMAH regulations, large inventories of amine-based capture solvents may trigger COMAH applicability depending on their classification and quantities. Similarly, CCUS facilities co-located with hydrogen production or other major hazard installations may fall within COMAH and require Hazardous Substances Consent as part of the planning and development process.

CONCLUSION: LESSONS FROM THE MOLECULE’S JOURNEY

CCUS is an important tool in the transition to a low-carbon future, but it is not without risks. To fully realise its benefits while minimising potential harm, it must be underpinned by effective risk management, robust safety standards, and comprehensive regulatory frameworks.

While CCUS is not a permanent solution to climate change, it can play a valuable role in buying time to scale up clean energy. However, this opportunity is time limited. Public acceptance depends on transparency and positioning CCUS as part of a credible path to full decarbonisation – not as a means to extend fossil fuel use.

Maintaining that trust will require openness, ongoing innovation, and a visible commitment to accelerating the transition to a sustainable energy future.

REFERENCES

1. Web article, Supercritical carbon dioxide.
2. Marsh, Carbon capture in the UK: risk considerations.
3. Gexcon, CO2 hazards in Carbon Capture, Utilisation, and Storage (CCUS).
4. DNV, Energy Transition Outlook: CCS to 2050, June 2025.
5. Pipeline and Hazardous Materials Safety Administration, Failure Investigation Report – Denbury Gulf Coast Pipelines, LLC – Pipeline Rupture/ Natural Force Damage, February 2020.
6. HSE, Assessment of the major hazard potential of carbon dioxide (CO2).
7. HSE, HSE response to consultation on Carbon Capture Utilisation and Storage and Offshore hydrogen production