By Jian Yanjun and Fu Xiaming, CCS Shanghai Rules & Research Institute

Carbon Capture, Utilization, and Storage (CCUS) refers to the process of separating CO2 from industrial processes, energy utilization, or the atmosphere and achieving its emission reduction and/or obtaining side effects through engineering means. CCUS is currently the only technical option for realizing the low-carbon utilization of fossil energy, and it is also an important means for reducing future CO？ emissions, ensuring energy security, building an ecological civilization, and achieving sustainable development.

As a measure to achieve carbon dioxide capture and emission reduction onboard ships, the Onboard Carbon Capture System (OCCS) is an innovative technology that enables the shipping industry to achieve large-scale emission reductions based on existing traditional fossil energy. If carbon capture technology can be widely promoted and applied on ships, it will inevitably have a profound impact on the energy development of the shipping industry and the high-quality development of the shipbuilding industry.

I. Background of International Shipping Emission Reduction

In 2008, at the 58^th session of the Marine Environment Protection Committee (MEPC), the International Maritime Organization (IMO) changed the CO2 design index standard for new ships to the Energy Efficiency Design Index (EEDI) and enforced its implementation to control carbon emissions from new ships. Since its implementation in 2013, the Phase 3 requirements for EEDI for some ship types have been implemented ahead of schedule on April 1, 2022. Meanwhile, the IMO is discussing the implementation plan for Phase 4 of EEDI. The continuously increasing EEDI indicators pose increasingly severe challenges to ship design and construction.

In June 2021, the 76^th MEPC meeting adopted the amendments to Annex VI of the International Convention for the Prevention of Pollution from Ships (MARPOL), approving relevant technical documents regarding the calculation and rating method for the Carbon Intensity Indicator (CII) of existing ships. According to the MARPOL Annex VI amendments, starting from January 1, 2023, all operational ships of 5,000 gross tonnage (GT) and above (limited to ship types applicable to EEDI) are required to conduct annual CII calculations and ratings (rated on a scale of A-E, from best to worst). Ships rated E or rated D for three consecutive years are required to formulate improvement plans in their Ship Energy Efficiency Management Plan (SEEMP). The 83rd MEPC meeting held in April 2025 adopted the CII Reduction Factor Guidelines (G3) amendments, determining that the CII reduction factor will increase by 2.625% annually from 2027 to 2030, meaning a 21.5% reduction by 2030 compared to 2019, further tightening the rating requirements.

Currently, technical measures to reduce EEDI and CII mainly include hull form optimization (optimizing main dimensions, block coefficient, etc.), energy-saving technologies (wind-assisted propulsion, air lubrication, waste heat recovery power generation, anti-fouling coatings, etc.), new energy sources (such as wind, solar, fuel cells, etc.), and alternative fuels (such as LNG, methanol, ethane, ammonia, hydrogen, etc.).

With the further reduction of EEDI values and the tightening of CII rating requirements by the IMO, it will be difficult for ships continuing to use fossil fuels to meet compliance requirements using traditional energy-saving technologies and measures. As an innovative technology for the low-carbon utilization of fossil energy, carbon capture can help fossil fuel-powered ships meet emission reduction requirements after EEDI Phase 3 and also improve the CII rating of ships after they are put into operation.

II. Analysis of the Impact of Carbon Capture Technology on EEDI and CII

Capturing CO2 from ship exhaust gas falls under post-combustion carbon capture technology, which mainly includes processes such as CO2 separation, purification, compression, liquefaction, storage, and transfer/unloading. First, ship exhaust gas enters the capture module to separate CO2. The captured CO2 is then cooled, compressed, dried, and refrigerated in the purification and liquefaction module to be converted into liquid CO2, which is then transported to the storage unit. The captured and stored liquid CO2 is directly transferred to a dedicated CO2 carrier via the unloading unit or unloaded at a specialized port, and then supplied to processing plants as raw material for chemicals such as alkali and alcohol, or for geological and biological utilization.

Processes such as CO2 separation, compression, and liquefaction storage all consume energy. Therefore, capturing CO2 from ship exhaust is an energy-intensive process, and energy on board is basically derived from ship fuel, resulting in additional CO2 emissions. Only when the total amount of captured CO2 is greater than the additional CO2 emissions generated by the operation of the capture system itself will the application of this technology on ships present emission reduction benefits. To more intuitively reflect the CO2 capture capacity of the carbon capture system on board, the Ship Carbon Dioxide Net Capture Rate can be introduced, which is calculated as the ratio of the amount of CO2 captured by the ship per unit time to the total amount of CO2 generated on board during the same period.

The additional CO2 emissions generated by the OCCS due to its own operation mainly come from power generation sets supplying electricity and/or fuel-fired boilers providing heat. In EEDI calculation: For carbon emissions generated by the electricity consumption of the exhaust gas carbon capture system, it can be calculated based on the total power consumption under rated conditions multiplied by the average carbon intensity of the ship's power grid supplying the system. For carbon emissions generated by using fuel-fired boilers for heating, it can be calculated based on the fuel consumption rate and carbon conversion coefficient of the fuel-fired boiler serving the system under rated conditions. In CII calculation, the total annual CO2 emissions from the combustion of all fuels on the ship can be calculated based on annual DCS data, and then the annual amount of CO2 captured, unloaded, and stored by the OCCS is subtracted to obtain the ship's actual annual emissions.

The ship's capacity for storing CO2 is related to the total amount of CO2 that can be captured during the ship's voyage, thereby affecting the ship's actual CO2 capture capacity. At least within one voyage, only when the ship's storable CO2 volume is greater than the designed capture volume before unloading, can the emission reduction effect of the OCCS meet the designed target. When using tank volume to calculate the storable CO2 capacity, factors such as CO2 purity, density, and filling limits must also be considered. The design of CO2 storage tank volume can be estimated based on the total fuel consumption, its carbon conversion coefficient, and the ship's CO2 capture rate. For the value of total fuel consumption, a conservative approach is to take the total fuel stored on board; if it is assumed that the captured CO2 on board can be completely unloaded after calling at a port for each voyage, the total fuel consumption required for a typical voyage can be used.

As a revolutionary technology that can significantly reduce CO2 emissions, OCCS will be one of the key technical pathways for reducing greenhouse gas emissions from ships in the future.