Design Considerations for CO ₂ Liquefaction in Onboard Carbon Capture and Storage Systems

The maritime sector accounts for approximately 3% of global CO 2 emissions, and stricter regulations under the International Maritime Organization greenhouse gas strategy and the European Union Emissions Trading System are accelerating the demand for mitigation technologies.Onboard carbon capture and storage (OCCS) can reduce emissions from existing vessels without fuel switching; however, design criteria for post-capture liquefaction remain unclear.This paper reviews recent literature, industrial reports, and demonstration cases related to OCCS liquefaction and storage.The information was organized around three design challenges: impurity removal, liquefaction conditions including pressure and refrigerant selection, and liquefied natural gas (LNG) cold energy utilization.Each challenge was evaluated in terms of safety, energy demand, process simplicity, and operational feasibility.The results show that onboard impurity removal should target components that limit liquefaction and storage stability, whereas stringent product specifications are better addressed onshore.A liquefaction pressure of approximately 15 bar is identified as a practical option, considering energy consumption, storage stability, and equipment burden.Refrigerant selection requires a balanced consideration of efficiency, safety, and environmental regulations, and LNG cold energy can reduce refrigeration power, though auxiliary cooling is still required to manage operational variability.This paper provides practical design criteria for shipboard OCCS liquefaction systems.