Maritime carbon capture becomes interesting where existing vessels still have many operational years ahead but no realistic fuel switch is feasible. This concerns a substantial part of the world fleet. Of the roughly 60,000 merchant vessels above 5,000 GT, most are designed for conventional fuels. Conversion to methanol, ammonia or LNG requires multi-million investments and is technically limited on many units.
OCCS fills a strategic gap: it enables emission reductions on existing vessels without altering the propulsion train. This is particularly relevant when regulatory pressure rises in the short term – EU ETS has required allowances for maritime emissions since 2024, FuelEU Maritime has been reducing permissible GHG intensity since 2025 – but alternative fuel infrastructure is not yet widely available.
The question is not whether carbon capture is technically possible – that has been demonstrated in several pilot projects – but where and under which conditions it makes economic and logistical sense. Not every vessel, not every route and not every operator benefits equally. A blanket recommendation for or against OCCS would be unprofessional. What is possible is a systematic classification by vessel type, trading area and operational framework.
Suitable candidates are vessels with predictable trades, high standardisation and sufficient space reserves. The segments can be ranked by their OCCS suitability:
Bulkers (Capesize, VLOC): High suitability. Large deck areas, consistent load profiles, often fixed routes (e.g. Australia–China, Brazil–Europe). The main engine runs over long sea passages at constant load, enabling stable exhaust conditions and thus predictable capture rates. Main limitation: cargo loss due to additional weight is marginally relevant for bulk commodities but not negligible.
Tankers (VLCC, Suezmax, Aframax): Medium to high suitability. Similar advantages to bulkers, but with the added complexity of inert gas systems and stricter safety requirements for additional equipment on tankers. The arrangement of OCCS components must account for SOLAS requirements for oil tankers, particularly deck zoning.
Container ships (Neo-Panamax, Ultra-Large): Limited suitability. Available space aft of the accommodation block is often tight on container vessels. At the same time, engine power is very high (50,000–80,000 kW), requiring correspondingly large OCCS installations. For vessels in fixed liner services with regular calls at the same terminals, it may still be economic – particularly if port infrastructure for CO2 offloading exists.
RoRo/ConRo: Limited suitability due to space constraints, but advantaged by frequent port calls. Short-sea routes with regular offloading could work provided port infrastructure exists.
Offshore units: Low suitability. Irregular port visits, variable load profiles and limited space make OCCS impractical for most offshore units.
Operators should assess available waste heat, space, integration into the exhaust system and CO2 offloading. These four factors form the basis of any serious feasibility study:
Waste heat: Solvent regeneration requires heat at 120–140 °C. On most vessels with two-stroke main engines, this is available from the exhaust gas boiler and jacket cooling. The critical question is whether enough remains after deducting demand for heating, fresh water generation and any existing scrubber. A thermal balance of the vessel is the first step.
Space: OCCS components (absorber, stripper, heat exchangers, tanks, pumps) require 100–300 m³ of volume depending on the capture rate. On a Capesize bulker, this is often feasible in the area of the accommodation block and steering gear decks; on a container ship it may cost several TEU of slot space. A detailed GA study (General Arrangement) is indispensable.
Exhaust system: The OCCS system must be integrated into the existing exhaust ducting – downstream of the turbocharger, ideally after a scrubber or economiser. Pressure drops caused by the absorber must not impair engine performance. On vessels with SCR catalysts, the exhaust train becomes particularly complex, as both SCR and OCCS require optimal temperature windows.
CO2 offloading: The entire chain depends on the availability of offloading infrastructure in the ports called. Currently, only a few ports in Europe (Rotterdam, Antwerp, Bergen) offer such facilities, and often only as pilot projects. Operators must check whether reliable offtake points exist or are expected in the foreseeable future on their regular routes.
Caution is warranted when carbon capture is presented as a blanket bridging technology. Not every application makes sense, and the risks are systematically downplayed in the sales communications of many providers.
Missing port infrastructure: Investing in OCCS without ensuring that the ports called can accept CO2 risks a system that produces CO2 (through parasitic energy demand) but cannot offload it. In the worst case, the CO2 must be carried across multiple voyages, binding tank capacity and reducing freight revenue.
Regulatory uncertainty: It is not yet conclusively settled how captured CO2 is credited under the various regulatory frameworks. Under the EU ETS, there is not yet a harmonised methodology for recognising OCCS measures. Under FuelEU Maritime, the treatment of capture systems is also not definitively regulated. Those who invest millions based on assumptions about future recognition carry a considerable regulatory risk.
Lock-in effects: An OCCS system ties up capital and space on the vessel for 10–15 years. If a practicable fuel switch becomes available during that time, the OCCS system may become a stranded investment. Operators should therefore define an exit strategy: can the system be dismantled? What costs arise? Can the space then be used differently?
Crew competence: OCCS systems require chemical and process engineering knowledge that is not self-evidently present in a traditional engine room crew. Training costs and the organisational effort for crew qualification are frequently underestimated.
The question of where OCCS makes sense can be translated into a technically structured evaluation matrix. The following six criteria should be quantified for every vessel-route combination:
1. Thermal balance: Available waste heat minus existing consumption (scrubber, economiser, steam system) yields the thermal budget for OCCS. On a Capesize with MAN 6S80ME-C at 75 % MCR, sufficient waste heat is available for 50 % capture; for 70 % it becomes tight.
2. Space balance: A GA analysis must quantify the available space on deck, in the engine room casing and in the steering gear area. Access routes, maintenance zones and sightlines must be considered. An amine-based 50 % absorber requires approximately 150 m³; for 70 %, approximately 250 m³.
3. Route analysis: The average sea passage duration between ports with offloading capability determines the required CO2 storage capacity. At 14 days and daily capture of 50 t, 700 t of CO2 must be stored – as liquid CO2 at -20 °C and 20 bar, this represents approximately 700 m³ of tank volume.
4. Port frequency: The more frequently a vessel calls at ports with CO2 infrastructure, the smaller the onboard tanks can be and the lower the capacity loss. Liner services with weekly calls in Rotterdam are clearly advantaged over tramp bulkers.
5. Engine profile: Vessels with consistent load profiles (70–85 % MCR over long passages) offer more stable exhaust conditions than units with frequent load changes. OCCS performance correlates directly with load consistency.
6. Remaining service life: At least 8–10 years for acceptable amortisation. With shorter remaining life, efficiency measures (slow steaming, propeller optimisation, hull coating) are more economical.
For operators seriously evaluating OCCS, the following steps are recommended:
Step 1 – Fleet segmentation: Identify the vessels in your fleet with the highest OCCS suitability: long remaining service life, fixed routes, available waste heat, sufficient space. Do not start with the most difficult case but with the most promising one.
Step 2 – Port chain assessment: Map the ports called and check which offer or plan CO2 offloading. Speak with port authorities – many have roadmaps that are not yet public.
Step 3 – Thermal pre-study: Commission a thermal balance of the selected vessel. An experienced marine engineer can complete this in 2–3 days and it provides the basis for deciding whether a detailed feasibility study is worthwhile.
Step 4 – Vendor discussions with critical scrutiny: Speak with at least two OCCS providers. Request reference data from real sea deployments, not just laboratory data. Ask for net capture rates (after parasitic energy demand) and for practical experience with CO2 offloading.
Step 5 – Engage class early: Contact your classification society already during the pre-study. DNV, LR and ClassNK have developed OCCS guidelines; an early conversation clarifies notation requirements and avoids later surprises.
The suitability of OCCS varies considerably by trading area. In European waters, where EU ETS and FuelEU Maritime apply, the regulatory incentive is highest. At the same time, port infrastructure is developing fastest here. Rotterdam has announced plans to provide commercial CO2 offloading capacity by 2028; the Porthos storage project aims to store CO2 beneath the North Sea.
In Asian waters, the regulatory pressure from regional emissions trading systems has been largely absent so far. The IMO framework will apply here as well, but with binding obligations at the earliest from 2028–2030. Meanwhile, port infrastructure in Singapore and Shanghai, whilst technologically advanced, does not have CO2 offloading on the short-term agenda.
For operators with mixed routes, a pragmatic approach emerges: deploy OCCS only on legs where both regulatory benefit and offloading capability are available. On voyages to Asia, the system could be shut down to avoid parasitic energy consumption. This flexibility must, however, be accounted for in system design and operating procedures.
A structured decision requires three levels of analysis:
Level 1 – Strategic: Does OCCS fit the long-term fleet strategy? Or is it a detour that diverts resources from a more robust solution? Operators planning to switch to methanol or ammonia in 5–7 years should assess whether the OCCS investment can be amortised within that timeframe.
Level 2 – Technical: Are the four prerequisites (waste heat, space, exhaust system, offloading) met for the specific vessel? If any one is absent, OCCS is not suitable for that vessel.
Level 3 – Economic: Is the business case robust under different ETS price scenarios? A realistic stress test should calculate with allowance prices of EUR 50, 80 and 120/t and show sensitivity to off-hire duration and capacity loss.
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