ABS: LCO2 Transport Poses Challenges

The American Bureau of Shipping (ABS) has issued a publication exploring challenges of transporting LCO2 by sea on specialized ships, focusing on the impact of impurities.

The report “CO2 Impurities and LCO2 Carrier Design: Practical Considerations”, provides a brief overview of the role and the challenges of CO2 transportation within the broader carbon capture, utilization and storage (CCUS) value chain. Carbon capture, utilization and storage plays a crucial role in achieving climate goals, and CO2 transport is a key component for its deployment.

According to ABS, evolving design trends of LCO2 carriers are a response to the growing demand for CO2 transport. Still, impurities and the varying compositions of CO2 pose significant challenges to the health, safety and environmental impacts of CO2 transport and storage systems.  

Liquid carbon dioxide is the liquid state of carbon dioxide (CO2), which cannot occur under atmospheric pressure. Uses of liquid carbon dioxide include the preservation of food, in fire extinguishers, and in commercial food processes. Liquid carbon dioxide is being considered as a means of CO2 transportation for underground or subsea storage purposes. Due to its high density as a liquid, it is much more feasible to ship than as a gas.

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Shipping Transportation as Liquid – LCO2 carriers

With the global expansion of CCUS, transporting CO2 by ships presents a unique business case, particularly for remote emitters over long distances and smaller quantities. The importance of transporting CO2 by ship has been recognized at the European Union (EU) level, in the European Taxonomy for Sustainable Activities and the EU Emissions Trading System (ETS) Directive, as well as at a national level, such as the Dutch SDE++ subsidy scheme and the United Kingdom’s CCUS program.Furthermore, unlike the supercritical conditions required for transportation through pipelines, CO2 is currently transported as LCO2 with ships. At the range of temperatures and pressures used for LCO2 its volume is approximately 1/500th of the CO2’s volume at standard temperature and pressure.

Transfers from ship-to-ship or from ship to floating platform will require detailed analysis of the relative motion parameters to support the design of the transfer system, similar to recent developments for the liquefied natural gas (LNG) ship-to-ship transfer systems. On a global scale, the total cross-border movement of captured CO2 within the CCUS ecosystem via shipping could approach ~170 MtPA by 2050, driven by routes in both Asia Pacific and Europe.

Globally, the number of vessels required to facilitate this would range from 100 to 200, depending on realized shipping routes globally and the capacity of the vessels for Europe. The current fleet includes seven LCO2 carriers in service and six on order, with deliveries planned for 2025 to 2026.

The CO2 captured from various sources contains a variety of trace gases depending on the chemistry of the source’s feedstock, the carbon capture technology, the solvents or absorbents utilized, and the downstream clean-up technology deployed. As per the ISO document, recognized impurities in a CO₂ stream typically include the following:

• Oxygen (O₂)
  
• Water (H₂O)
  
• Nitrogen (N₂)
  
• Hydrogen (H₂)
  
• Sulfur oxides (SOx)
  
• Nitrogen oxides (NOx)
  
• Hydrogen sulfide (H₂S)
  
• Hydrogen cyanide (HCN)
  
• Carbonyl sulfide (COS)
  
• Ammonia (NH₃) 
  
• Amines
  
• Aldehydes
  
• Particulate matter (PM)
  

Two primary factors determine how impurities affect CCUS systems:

• Physical effects: Impact on phase behavior and transport conditions
  
• Chemical effects: Corrosivity, toxicity and interactions with materials
  

Role of CCUS in decarbonization

The International Energy Agency (IEA) highlighted the importance of CCUS in achieving global climate goals. It is one of the few technologies that can directly reduce CO2 emissions from heavy industries, such as cement and steel production, which are rarely decarbonized.

The rapid global deployment of CCUS technologies has accelerated in recent years, driven by increasing climate commitments and advancements in capture and storage systems.

In addition, according to data from the Global CCS Institute, the number of operational and planned CCUS projects worldwide has significantly grown, with over 300 projects now at various stages of development as of the end of 2024. This expansion highlights the importance of addressing technical factors that impact CCUS efficiency, including examining CO2 impurities.

As CO2 capture scales up across industries, understanding and mitigating the effects of impurities on transport and storage processes have become critical to ensuring the reliability and safety of these systems.

Key takeaways of the report: 

The increasing demand for CO2 transportation highlights the critical role of LCO2 carriers in supporting the global carbon value chain. While pipelines remain a well-established option, shipping offers unique advantages, especially for remote and geographically dispersed emitters. 

Design and Operational Considerations

• The design trend is to transport CO2 at lower pressures closer to the triple point using larger tanks and an increased total capacity.
  
• Operational pressures for LCO2 carriers are typically maintained within the 6 to 10 bar(g) range and away from the triple point to ensure safety and efficiency while avoiding dry ice formation.
  
• Type C tanks, with their robust designs, are the preferred option, but material selection must account for the low-temperature and high-pressure operating conditions.
  

Impact of CO2 Composition

• Carbon dioxide composition plays an important role in designing these vessels and associated systems. Most designs consider reasonably pure CO2.
  
• The presence of impurities significantly influences thermophysical properties, corrosion potential and cargo handling systems. Variations in CO2 sources and capture technologies, the solvents or absorbents, and the downstream clean-up technology require flexibility in design specifications.
  
• Non-condensable gases, such as H₂ and N₂, increase pressure and energy requirements, while reactive impurities like SOx and H₂S demand careful material selection to mitigate corrosion and operational risks.
  

Research Gaps

• A lack of comprehensive VLE data for shipping conditions complicates the prediction of CO2 phase behavior, particularly with mixtures containing impurities.

• Further research is needed to investigate the quantitative impact of impurities in the CO2 mixture on shipboard transportation. Without proven data, one safe approach during the design stages is to use appropriate simulation tools and equations of state to predict phase changes for a safe cargo operation and choose suitable materials.