How is CO₂ transported?

Like other gas products, carbon dioxide (CO₂) can be transported by a variety of methods, including pipelines, ships, trucks, and trains. The selected method of transport for carbon dioxide depends on several factors, ranging from distance, volume, and geographical considerations to regulatory requirements and project-specific constraints.

Carbon dioxide transport is a key part of carbon capture and storage (CCS) projects, taking captured CO₂ from the emission source to a storage site for long-term storage or utilization.

The United States already operates more than 50 CO₂ pipelines, covering around 8,000 km and transporting approximately 70 million tonnes of CO₂ a year, but infrastructure will have to increase by a factor of 100 globally if it is to meet plans to halve CO₂ emissions in the next 30-40 years.

CCS transportation infrastructure

A robust infrastructure is required to effectively support the transportation of CO₂ in CCS projects, including pipelines, storage terminals, compression facilities, and monitoring systems. Typically, pipelines form the backbone of the CCS infrastructure, connecting the emission sources to storage or utilization sites. The storage terminals then receive and condition the transported CO₂, injecting it into the storage reservoirs – usually underground geological formations.

Compression facilities may be used to compress the captured CO₂ to the desired pressure for transportation, while monitoring systems, such as sensors and remote monitoring technologies, help to ensure the safe and efficient transport of CO₂, detecting any leaks or anomalies during the transportation process.

Pipeline design and safety

Design and safety are crucial considerations for pipelines used in the transportation of CO₂. Pipeline materials must be chosen to prevent corrosion and leakage, with corrosion-resistant linings also a possibility. The pipeline diameter is determined based on the required CO₂ flow rate and to minimize the pressure drop along the pipe network. The pipeline route should be carefully planned as well, taking into account such factors as geological conditions, environmental impact, land use, and proximity to emission sources, storage sites, or utilisation facilities.

In terms of safety, pipelines for CO₂ transportation must adhere to the stringent standards and regulations established by relevant authorities, covering aspects such as pipeline design, construction, operation, maintenance, and emergency response protocols. CO₂ pipelines operate at high pressures to maintain the CO₂ in a dense phase for efficient transportation, so adequate pressure management systems are required, along with effective leak detection and monitoring systems, and emergency shutdown systems that can quickly isolate sections of the pipeline if necessary.

Storage site selection and characterisation

It is important to identify suitable geological formations for the long-term storage of CO₂, and to assess their capacity, integrity, and containment potential. It is estimated that the UK alone has 70 billion tonnes of potential CO₂ storage space in sandstone rock formations under the North Sea. Factors to consider include geological criteria (such as porosity and permeability), proximity to emission sources, and alignment to environmental, regulatory, and legal requirements.

Characterisation studies are used to assess the suitability of potential storage sites. These include geological surveys, a detailed analysis of reservoir properties, geochemical analysis to understand the potential for mineral trapping or chemical reactions, computer-simulated behavioural modelling, monitoring and verification plans, and a full risk assessment of the site.

Regulatory and policy considerations

Regulatory and policy considerations provide the legal framework, guidelines, and standards necessary to ensure safe and effective deployment of CCS technologies. CCS projects, including CO₂ transportation, typically require permits and licenses from relevant regulatory authorities at local, national, and potentially international levels to ensure compliance with environmental, safety, and operational standards. These may involve multiple agencies responsible for different aspects of CCS, including energy, environment, and mining authorities.

Cross-border transport of CO₂ is also an important consideration – for example, the UK’s 1996 London Protocol prohibits the export of CO₂ for storage, as this is regarded as “incineration at sea”. Under a 2009 amendment to this protocol, countries involved in cross-border transport of CO₂ must enter into a bilateral agreement before CO₂ can be exported from the UK.

CCS projects often undergo comprehensive Environmental Impact Assessments to evaluate potential environmental and socio-economic effects. Liability and financial assurance mechanisms need to be put in place to address the potential risks and liabilities associated with carbon dioxide transport and other CCS operations. These mechanisms require project developers to demonstrate their financial capacity to cover any damages or incidents that may arise during CO₂ storage or transportation, ensuring that adequate funds are available for site remediation or corrective actions.

Monitoring and verification of CO₂ transport

Monitoring and verification systems are implemented to ensure the safe and effective transportation of CO₂, and to detect any potential leaks or anomalies. There are a wide range of methods used, including pressure and temperature monitoring, flow measurement, leak detection using acoustic sensors or gas detection systems, satellite monitoring, observation by infrared cameras, and the use of tracers – such as stable isotopes – to identify any potential leaks or deviations from the expected transport pathway.

Advanced real-time monitoring systems integrate a mixture of these methods, including various sensors, data acquisition systems, and data analytics, to provide continuous monitoring of CO₂ transport. In addition, periodic inspections and integrity assessments are conducted to evaluate the condition of the pipeline and identify any potential areas of concern.

Economics of CCS transport infrastructure

The costs involved in transporting captured CO₂ from emission sources to storage or utilisation sites are typically associated with building, operating, and maintaining the necessary transport infrastructure. They can vary widely depending on regional factors, project scale, and technological advancements.

Capital costs include the expenses related to the initial design, construction, and installation of pipelines, compression facilities, and other associated infrastructure, and vary depending on factors such as the length of the pipeline, the terrain, the number and size of injection or extraction points, and the required capacity.

Operating costs cover the expenses incurred during the day-to-day operation of the carbon dioxide transport infrastructure, and typically include energy requirements for compression, maintenance and inspections, personnel, monitoring and control systems, and any associated administrative costs.

Government policies, financial incentives, and carbon pricing mechanisms can significantly impact the economics of CCS transport infrastructure. Subsidies, tax credits, grants, or carbon pricing mechanisms can help offset the capital and operating costs, making the project more economically viable.

Additionally, it is estimated that the price of reusing offshore oil and gas pipelines to transport CO₂ could be just 1% to 10% of the cost of building a new pipeline, creating a considerable saving.

Social and environmental impacts of CCS transport infrastructure

While CCS is a key component in efforts to reduce greenhouse gas emissions, it is important to consider and address the potential social and environmental implications associated with its infrastructure. Pipelines and associated facilities may require land acquisition and can result in disruption to local communities. There are also health and safety risks, including accidental releases of CO₂, pipeline leaks, and safety concerns related to compression facilities.

Engaging local communities and stakeholders throughout the development and operation of CO₂ transport infrastructure is crucial for ensuring social acceptance and minimising conflicts. In addition, there are positive effects beyond the environmental benefits of CCS, including employment opportunities, supply chain development, and local economic growth.

While CCS aims to reduce CO₂ emissions, the infrastructure itself may contribute to greenhouse gas emissions during construction, operation, and maintenance, so its carbon footprint needs to be carefully managed and minimized. Transport infrastructure can also impact natural habitats and biodiversity, particularly when pipelines traverse sensitive ecosystems, water bodies, or protected areas.

The infrastructure may require water for pipeline construction, facility operation, and dust suppression, affecting local water resources, and the operation of compression facilities can lead to air emissions, including particulate matter, nitrogen oxides, and other pollutants. leaks or accidental releases of CO₂ can have adverse environmental impacts, including acidification of water bodies, impacts on aquatic ecosystems, and potential risks to human health.

However, all these negative impacts can be overcome, or at least significantly mitigated, by effective management and monitoring, and the adoption of best practices to achieve sustainable and responsible CCS transport infrastructure deployment.

Methods of transporting CO₂

While there are several methods available for the transportation of carbon dioxide, pipeline networks and shipping are the primary options for both onshore and offshore storage. The choice of CO₂ transport method depends on various factors, including distance, project scale, terrain, regulatory requirements, and economic considerations. Pipeline networks are typically more cost-effective and commonly used for long-distance onshore transport. Shipping is often employed for offshore storage or when pipeline infrastructure is not feasible. The specific choice of transport method is determined through project-specific feasibility studies and assessment of technical, economic, and environmental factors.

CO₂ by pipeline networks

To transport carbon dioxide across long distances, the CO₂ is compressed and transferred through a network of pipelines, typically made of steel or composite materials. Onshore pipelines take CO₂ from the capture sources, such as power plants or industrial facilities, to the storage or utilization sites. They may span hundreds of kilometres, with multiple injection and extraction points along the route. Offshore pipelines transport the CO₂ from onshore sources to offshore storage sites, and are designed to withstand harsh marine conditions, often being buried beneath the seabed for protection. Pipelines are typically supported by compressor stations, strategically located along the pipeline network, to maintain the pressure required for effective CO₂ transport. Additionally, monitoring and control systems are installed to ensure safe operation, and to detect and respond to any CO₂ leaks or anomalies.

CO₂ shipping for offshore storage

Shipping can be used to transport CO₂ in liquid form to an offshore storage site and is typically employed when the distance between the capture source and storage site is relatively short, or when the storage site is located in deep waters where pipelines may not be feasible.

Specialized ships are used for transporting CO₂, with the carbon dioxide either stored in specialized cryogenic tanks as a liquid at low temperatures, or maintained in a supercritical state by maintaining both temperature and pressure above their respective critical points.

Loading and unloading facilities are required at both the capture source and storage site, to transfer CO₂ between the transport vessels and onshore/offshore storage infrastructure. These facilities include storage tanks, pumps, and connections for efficient transfer operations.

To ensure the safe handling, storage, and transportation of CO₂ during shipping, it is important to follow strict safety measures, including adherence to international regulations, rigorous vessel inspection and maintenance, emergency response plans, and crew training.

Ammonia as a medium for transportation

Ammonia has a high affinity for CO₂, readily absorbing it to form stable compounds such as ammonium carbamate or bicarbonate. By leveraging the absorption properties of ammonia, CO₂ can be chemically captured and stored as an ammonia-CO₂ complex, effectively reducing the volume of CO₂ for transportation through pipelines, shipping, or by other means.

Carbon capture

CO₂ can be captured from industrial flue gases using ammonia as a chemical absorbent. The flue gases containing CO₂ are directed into an absorber unit, typically a large vessel or tower. Here, the flue gas is brought into contact with a solution of ammonia, often in the form of an aqueous solution. A chilled process (between 2-10^oC) helps to achieve a high CO₂ capacity and avoid degradation issues.

Conversion to ammonium carbonate/bicarbonate

When the captured CO₂ reacts with ammonia in the presence of water, it forms ammonium carbonate (as a soluble compound) according to the following reaction:

CO₂ + 2NH₃ + H₂O → (NH₄)₂CO₃

This reaction is exothermic, so the released heat can be captured and used to generate electricity, or for other purposes.

This reaction can go in either direction depending on the conditions. In the absorber unit, the high concentration of CO₂ in the flue gas drives the reaction towards the formation of ammonium carbonate until an equilibrium is established.

A significant portion of the CO₂ from the flue gas is converted into ammonium carbonate through this process, while remaining gases such as nitrogen, oxygen, and other trace pollutants pass through the absorber unit and are released into the atmosphere.

The captured CO₂ can be immediately recovered from the ammonium carbonate through a process called stripping, typically by applying hot air or steam, or the ammonium carbonate can be used as a transport medium.

In some processes, captured CO₂ is converted to ammonium bicarbonate (NH₄HCO₃) instead. This behaves in a similar way to ammonium carbonate for carbon capture and transportation purposes, but is generally more stable and less corrosive than other ammonia-CO₂ complexes.

Transportation

Ammonia has gained attention as a potential medium for CO₂ transportation due to its ability to absorb and carry large quantities of CO₂ in a relatively compact form. Ammonia-CO₂ complexes can effectively reduce the volume of CO₂ for transportation, increasing pipeline capacity and allowing more CO₂ to be transported by ship, rail, or road.

Ammonium bicarbonate can be transported as a solid, while many ammonia solutions can be transported in liquid form, using specialized tanks or pipelines. By using ammonia as a carrier, the need for dedicated CO₂ pipelines, or the transport of large volumes of uncompressed CO₂, can be mitigated.

While ammonia is an efficient carrier, it is important to consider the handling and safety aspects. Ammonia is a toxic substance and should be handled with care to ensure the safety of personnel and the environment. Proper safety measures, training, and precautions are necessary, including adequate containment and emergency response systems.

The use of ammonia as a CO₂ carrier is an active area of research and development, and there are ongoing efforts to explore its feasibility and address the associated challenges.

Decomposition and storage

Once it has been transported to the storage site, the ammonia-based absorbent containing the captured CO₂ is processed, with the goal of separating the CO₂ from the ammonia in a controlled manner.

Typically, this is accomplished through thermal or chemical decomposition. For example, if the CO₂ has been transported in the form of ammonium bicarbonate, this compound is decomposed by heating it to release the CO₂ and ammonia (along with water). Heating the absorbent shifts the chemical equilibrium in favour of releasing the CO2.

Chemical decomposition methods involve the use of chemical reactions or catalysts to promote the separation of CO₂ from the ammonia-based absorbent.

Once separated out and purified, the CO₂ can then be stored in deep underground geological formations, like saline aquifers or depleted oil and gas reservoirs, where it will be trapped and prevented from entering the atmosphere.

Ammonia can also play a role in underground storage of CO₂. As captured CO₂ is injected into geological reservoirs, such as depleted oil and gas fields or deep saline aquifers, ammonia can be injected simultaneously to displace the CO₂ and ensure its effective containment.

Ammonia has a lower density and lower viscosity than CO₂, so it can act as a trapping agent, reducing the risk of CO₂ leakage from the storage site. It is also less likely to react with CO₂ or the surrounding geological formations, which can enhance the site’s long-term stability.