CO2 Liquefaction at Sea: A Storage and Offloading Solution Aboard Ships Employing Carbon Capture "We Captured the CO2, Now What?"

The maritime industry faces increasing pressure to reduce greenhouse gas (GHG) emissions in compliance with the International Maritime Organization (IMO) regulations, specifically those outlined in MARPOL Annex VI. Much research in Marine and Offshore Carbon Capture and Storage (CCS) has focused on capturing CO2 from the engine exhaust stream. Storage of CO2 aboard is a problem in its own right. Storing CO2 as a compressed gas would require large volumes and high pressures, resulting in lost cargo-carrying capacity. Storing CO2 as a liquid reduces the storage volume to less than 0.2% of the gas volume. For marine and offshore applications, CO2 liquefaction is a must. To address this challenge, this paper explores the design of an onboard CO2 liquefaction skid aimed at liquefying CO2 after it is captured by a separate system from ship exhaust. The system's primary objective is to convert gaseous CO2 into liquid form, significantly reducing its volume and enabling efficient storage and handling. (Open AI, 2024) Most research has primarily focused on large-scale CCS systems for land-based plants. This includes the liquefaction of CO2 loaded onto liquid CO2 carriers (LCO2 carriers). The concept of installing a CO2 liquefaction plant contained within a single skid onboard a vessel is entirely novel. Since this innovation can offer a technological solution to reduce GHG emissions from ships and help to implement a plan of corrective actions for Carbon Intensity Index (CII) poorly rated ships, it is anticipated to garner significant interest from ship owners seeking to comply with the new International Convention for the Prevention of Pollution from Ships (MARPOL) regulations, Annex VI entered into force on 1 November 2022. Developed under the framework of the Initial International Maritime Organization (IMO) Strategy on Reduction of GHG Emissions from Ships agreed in 2018, these technical and operational amendments require ships to improve their energy efficiency in the short term and thereby reduce their greenhouse gas emissions. The proposed plant is designed to handle a flow rate of 1.5 to 2 tons per hour of CO2 captured from ship exhaust at 120°C and atmospheric pressure. The liquefaction process is based on thermodynamic principles, primarily by raising the pressure to 15 barg and cooling the CO2 to −43C, which ensure CO2 is at liquid state at 15 barg. While liquid CO2 is used in various industries where product quality is critical, particularly in the food sector, this study does not focus on quality aspects. However, filtration and drying systems are implemented to protect compressor components from damage caused by moisture and dust. This paper details the thermal processes encountered during liquefaction, including heat transfer dynamics, cooling methods, and energy consumption. It further discusses the equipment required for efficient operation, such as compressors, heat exchangers, and storage tanks, providing key sizing parameters to ensure optimal performance in a marine environment. (Open AI, 2024) The study also comprehensively analyzes process flow diagrams (PFD) for the CO2 liquefaction skid. These diagrams highlight the system layout, operational flow, and control mechanisms required for safe and efficient onboard CO2 liquefaction. The proposed solution contributes to reducing the environmental footprint of shipping operations, offering a practical pathway to meet IMO's stringent GHG reduction targets. (Open AI, 2024)