Carbon Dioxide (CO₂) transport is the intermediary step in the carbon capture value chain, where CO₂ shipping is increasingly playing a more important role in global decarbonisation efforts via transport for CO₂ storage or utilisation. Although technical challenges remain in CO₂ shipping, the technologies for transport are maturing, making the business case more favourable for widespread adoption.
CO₂ Shipping
Carbon Capture is regarded as a necessary technology to be deployed in conjunction with zero-carbon energy to reach the IPCC 1.5C scenario by 2050 (IPCC). This means that global CO₂ emissions must be reduced by 5 gigatons per year - the equivalent to the total CO₂ emissions from about ten thousand factories and power stations. It is estimated that CCS can contribute to eliminating 14-17 percent of these emissions (SINTEF).
The transport of CO₂ by pipeline has been practised for three decades, and international standards such as ASME B31.4 are widely adopted. Liquid transportation systems for hydrocarbons, liquid petroleum gas, anhydrous ammonia and alcohols are subject to the widely applied Norwegian standard (DNV, 2000) with explicit mention of Carbon Dioxide. Globally, CO₂ shipping is growing to accommodate offshore geological storage. For decades commercial CO₂ shipping has served markets seeking food grade, or high-purity CO₂ for utilisation purposes. Although technical challenges remain in CO₂ shipping the technologies for transport are increasingly maturing, making the business case more favourable for widespread adoption. Marine based transport of liquid carbon dioxide is becoming more widespread due to maturing technology. CO₂ transportation by ship requires a pressure system to maintain the CO₂ in a liquid state. CO₂ carriers generally operate at conditions near the triple point – the temperature and pressure where CO₂ can coexist in thermodynamic equilibrium, or where the solid, liquid and gas states converge—but new research suggests that CO₂ can be shipped at varying pressure and temperature conditions to help integrate shipped CO₂ into the existing carbon capture and storage value chain.
Image of the conventional marine-based carbon capture value chain.Because liquid CO₂ can only exist at a combination of low temperature and pressures exceeding atmospheric pressure, CO₂ cargo tanks therefore should be pressurised or semi-refrigerated. The semi-refrigerated generally preferred for similarities with LPG carriers, and the design point of the cargo tank would be approximately –54C per 6 bar to –50C per 7 bar, which is near the triple point of CO₂.
For purposes of loading and unloading liquid CO₂ to ships, liquified CO₂ is discharged from intermediate storage tanks to the onboard cargo tanks with process systems suitable for high pressure and low temperature CO₂ handling. Before discharge, the onboard cargo tanks are prepared for the cargo with pressurised gaseous CO₂ to prevent contamination by humid air and the formation of dry ice. To prevent CO₂ diffusion resulting from heat transfer during transfer and storage additional refrigeration units are used to capture any potential boil-off and emissions from the CO₂ vessel. CO₂ vessels generally have a capacity of 800m3 upwards to 22,000m3, which is not massive in the grand scheme of CO₂ capture mitigation, but it is a superior option to decarbonise industries in remote areas over longer distances. The alternative to shipping is using a CO₂ pipeline, where there is a high-cost associated with maintaining stable temperature and pressure over long distances, hence shipping of smaller volumes over longer distances is more economical. Source: ssrccs_chapter 4-1 IPCC Carbon Dioxide Capture and StorageCO₂ shipping economics
Costs of a marine transport include many elements: apart from the investment for vessels, other investments include loading and unloading facilities, temporary storage and liquefaction units. Additionally, operational costs include labour, ship fuel, harbour fees and maintenance. According to a report by the IPCC, the optimal use of installations and ships in the transport cycle is crucial for the business case for CO₂ shipping. In some cases, extra storage facilities are required to account for any potential disruption to the marine based CO₂ value chain. In 2004 Equinor (then Statoil) estimated marine transport costs of 5.5Mt CO₂ per year by 17 20,000m3 tanker (upgraded LNG tankers) over a distance of 7600km per sailing, with liquefaction and loading/unloading costs to be USD 300million. The IEA conducted a comparable study demonstrating lower costs: for the same CO₂ cargo using 30,000m3 ships over a distance of 7600km the cost is estimated to be USD 35million – suggesting a stronger cost correlation with distance and cargo load. The implementation of carbon capture technologies in shipping will result in an increased need for infrastructure supporting off and on-loading of CO₂. Innovative technologies can successfully mitigate the environmental impact of CO₂ when refitting industries towards a sustainable future.
References: Hisham Al Baroudi, et al. “A review of large-scale CO₂ shipping and marine emissions management for carbon capture, utilisation and storage. Applied Energy, Volume 287 (2021). Accessed from: https://www.sciencedirect.com/science/article/pii/S0306261921000684 “Flow of CO₂ in Pipes,” SINTEF Blog (2015). Accessed from: https://blog.sintef.com/sintefenergy/ccs/flow-of-co2-in-pipes/ IPCC, Carbon Dioxide Capture and Storage Report (2005). Accessed from: https://www.ipcc.ch/report/carbon-dioxide-capture-and-storage/ “Shipping’s Future role in carbon capture and storage,” DNV (2022). Accessed from: https://www.dnv.com/expert-story/maritime-impact/Shippings-future-role-in-carbon-capture-and-storage.html
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