Onboard carbon capture offers a retrofit pathway for existing fleets. In an industry where the average commercial vessel lifespan spans 25 to 30 years, a substantial share of global tonnage sits in ships that still have ten or more operational years ahead – yet run on conventional fuels. A newbuild with alternative propulsion is not an option for these units; premature sale for recycling is rarely justifiable economically. It is precisely into this gap that onboard carbon capture and storage (OCCS) technology is advancing.
The basic idea is compellingly simple: CO2 is separated directly from the main engine exhaust stream, stored temporarily on board and offloaded in port. This would allow a portion of emissions to be avoided without fundamentally redesigning the propulsion train. The fact that several technology providers have now completed pilot projects at sea adds further momentum. At the same time, regulatory pressure – EU ETS since 2024, FuelEU Maritime since 2025, the IMO Net-Zero Framework in preparation – is driving the search for measures that can be implemented in the short term.
Yet expectations should remain realistic. OCCS is not a substitute for a long-term fuel strategy; at best it is a tool for the transition period. The decisive question is not whether the technology works, but under which conditions it is economically and logistically viable.
Technically relevant are space requirements, energy demand, heat integration, weight and tank arrangement. Anyone seriously evaluating OCCS must calculate these five parameters concretely for the specific vessel – generic manufacturer data is insufficient.
Space requirements: A typical amine-based absorption system needs an absorber column and a regeneration tower, heat exchangers, pumps and control equipment. On a container ship with limited deck space aft of the accommodation block, this can be tight. On bulkers with large, open deck areas, integration is often simpler – provided bridge sightlines are not compromised.
Energy demand: Solvent regeneration requires heat, typically at 120–140 °C. If this can be recovered from the exhaust gas boiler or jacket cooling, the parasitic energy consumption remains moderate. If additional steam must be generated, net efficiency drops noticeably – on some concepts by 10–15 % of main engine output.
Heat integration: Integration into the existing exhaust gas train is the most critical point. Scrubbers, economisers, SCR catalysts and OCCS units compete for the same exhaust stream and the same thermal energy. A clean heat management concept determines the real capture rate under operating conditions.
Weight and stability: The captured CO2 must be stored in pressure tanks or as a dissolved phase (e.g. in seawater). At high capture rates and on long sea passages, the stored mass can reach several hundred tonnes. This has direct implications for draught, freeboard, stability and cargo capacity.
Tank arrangement: The positioning of CO2 storage tanks affects the centre of gravity and must be coordinated with the classification society. Solutions using CO2 dissolved in seawater reduce pressure requirements but increase overall volume considerably.
OCCS becomes economically interesting when high emission costs meet fleets with longer remaining service lives. The calculation is simple at its core: CAPEX for the system plus ongoing OPEX (solvent, maintenance, energy losses, port charges for CO2 offloading) versus saved EU ETS allowances and avoided FuelEU penalty payments.
Current estimates for a retrofit project on a Capesize bulker range from USD 8–15 million CAPEX, depending on capture rate and system complexity. Operating costs are frequently underestimated: solvent losses, regular maintenance of absorber columns, CO2 offloading infrastructure in port and the efficiency loss from parasitic energy demand add up to an estimated USD 30–60 per tonne of avoided CO2.
Against this stand EU ETS allowance prices, which have moved between EUR 50 and 100 per tonne in recent years. From 2026, shipowners must cover 100 % of EU intra-voyage emissions. When the allowance price exceeds the marginal capture cost, a positive business case emerges – but only if the entire chain works, including reliable CO2 offtake in port.
What is missing from many economic assessments: off-hire time for retrofit (typically 3–6 weeks), cargo loss due to additional weight on board and uncertainty about the long-term CO2 offtake price. Those who calculate only the optimistic vendor scenarios underestimate the risk.
OCCS is best suited to vessels with predictable trades and sufficient space reserves. Three segments are crystallising in particular:
Large bulkers on fixed routes: Capesize and VLOC units that regularly trade between the same ports (e.g. Brazil–China, Australia–Japan) offer predictable offloading points and have deck space. Their consistent load profiles favour a stable capture rate.
Tankers in EU trades: VLCCs and Suezmaxes that regularly call at EU ports fall under the EU ETS. With a remaining service life of ten or more years and no realistic fuel-switch option, OCCS becomes a candidate for a transitional measure.
RoRo and ConRo vessels on short-sea routes: Units with frequent port calls could benefit from regular CO2 offloading, provided port infrastructure exists. Here, however, available space is often the limiting factor.
Less suitable are vessels in tramp trades with changing destinations, since CO2 offloading infrastructure will not be available at every port. Equally problematic are units with limited remaining service life, where the high initial investment can no longer be amortised.
The dominant process for maritime OCCS is chemical absorption using amine solutions, typically based on monoethanolamine (MEA) or proprietary solvents with lower regeneration energy requirements. The process operates in two stages: in the absorber, the amine binds the CO2 from the exhaust gas; in the stripper, the loaded solvent is regenerated at elevated temperature and the CO2 is released.
For maritime application, there are important differences compared to stationary onshore plants. Ship motions – rolling, pitching, yawing – affect the flow distribution within the columns. Column diameters and packing heights must be dimensioned to ensure uniform wetting of the packing even in heavy seas. Some providers use rotating packed bed absorbers (RPB), which are more compact and less sensitive to vessel motions.
As alternatives to amine absorption, membrane-based processes and calcium looping technology are being investigated. Membrane processes are more space-efficient but so far achieve lower capture rates (40–50 %) and are more sensitive to exhaust gas contaminants (SOx, particulates). Calcium looping uses limestone as a sorbent and produces high-purity CO2 but requires very high temperatures (850–900 °C) and is currently impractical for shipboard retrofit.
A frequently overlooked aspect is CO2 conditioning after capture. For transport and geological storage, the CO2 must be dried, purified and compressed or liquefied to transport pressure. Purity requirements depend on the offtaker – specifications for EOR (Enhanced Oil Recovery) differ from those for aquifer storage. These downstream requirements must be considered during system design.
Retrofitting an OCCS system fundamentally changes onboard operations. Engine room crews must learn to operate a new process system involving chemical solvents, pressure vessels and additional piping systems. This means training requirements, adapted SMS (Safety Management System) procedures and expanded risk assessments.
In port, a new operational step is added: CO2 offloading. This requires connections, safety perimeters and coordinated time windows with port infrastructure. In ports without appropriate facilities, the CO2 must continue to be carried, which affects tank capacity and cargo planning.
Maintenance of the OCCS system itself is non-trivial. Amine solutions degrade over time, heat exchangers foul, pumps and valves are subject to corrosion from the acidic environment. A realistic maintenance plan should include annual solvent changes, semi-annual column inspections and quarterly instrumentation checks.
For the superintendent this means: OCCS is not an “install and forget” system. It requires its own position in the PMS (Planned Maintenance System), dedicated spare parts on board and a reliable supply chain for consumables. Anyone who underestimates this risks the plant running at reduced performance or not at all after a few months.
The pilot projects to date – including those on vessels operated by Mitsubishi Shipbuilding, Wärtsilä and Value Maritime – provide the first robust data. Value Maritime has installed systems on several ships that dissolve CO2 in seawater and offload it in port. Measured capture rates are around 30–40 %, significantly below the theoretical maximum of amine absorption, but with the advantage of lower system complexity.
Mitsubishi tested an amine-based system on a coal bulker and achieved capture rates of approximately 50–60 % under real sea conditions. The key findings were: heat integration into the existing exhaust gas boiler worked better than expected, but ship motions reduced column efficiency by 15–20 % in heavy weather.
What all projects show in common: the technology works in principle, but real capture rates fall short of laboratory conditions. The logistics of CO2 offloading in port is the least developed part of the chain. And the economics depend heavily on external factors beyond the operator’s control – allowance prices, port charges, regulatory recognition of captured volumes.
Before an operator invests in a detailed feasibility study, five guiding questions help with pre-screening:
1. Remaining service life: Does the vessel still have at least 8–10 years of operation ahead? If not, OCCS is difficult to justify economically.
2. Fuel-switch option: Is there a realistic alternative (LNG, methanol, ammonia)? If so, this should be evaluated first, as it may be more robust in the long term.
3. Trading area: Does the vessel regularly call at ports that offer or are planning CO2 offloading? Without reliable offtake infrastructure, OCCS is not operationally feasible.
4. Space reserves: Is sufficient space available for absorber, tanks and auxiliary systems without unacceptably reducing cargo capacity?
5. Waste heat availability: Can the energy demand for solvent regeneration be predominantly covered from existing waste heat?
Only if all five questions are answered positively does a deeper technical-economic analysis make sense. In all other cases, operators should focus their resources on other measures – efficiency improvements, speed optimisation or preparation for a later fuel switch.
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