Both address the same decarbonisation pressure but with very different prerequisites. Carbon capture and fuel switching are not opposites to be played against each other – they are different tools for different situations. The debate is too often conducted ideologically: fuel switching as the “right” path, carbon capture as a delaying tactic. This simplification helps no one who must make real fleet decisions.
The starting point differs for every vessel. A newbuild with 25 years of operational life ahead has different options from a 15-year-old ship expected to serve another 10 years. A container ship in a liner service with fixed terminals faces different challenges from a tramp bulker with changing destinations. An operator with access to methanol bunkering infrastructure at core ports has different possibilities from one primarily calling at ports in developing countries.
Carbon capture acts on the existing propulsion train and avoids a portion of emissions through downstream capture. Fuel switching changes the fuel and thus emissions at the source. Both approaches carry costs, risks and dependencies on external infrastructure. The skill lies in choosing the more appropriate path for each unit – or a combination.
When existing fleets have no realistic alternative in the short term, OCCS offers a pragmatic way to reduce regulatory costs without replacing the propulsion train. The typical scenarios:
Vessels with 8–15 years remaining service life: For these units, a full fuel switch is often not economically justified. A methanol retrofit costs USD 10–25 million depending on vessel size; an LNG retrofit even more. If the remaining service life is too short to amortise that investment, OCCS with CAPEX of USD 8–15 million may be the better economic choice.
Missing bunkering infrastructure: Methanol bunkering is available in approximately 20 ports worldwide in 2026; ammonia bunkering practically nowhere. For vessels on routes without access to alternative fuels, OCCS is the only short-term option for emission reduction.
Dual-fuel uncertainty: The ongoing discussion about well-to-wake accounting could reduce the attractiveness of certain alternative fuels. LNG, for example, has a questionable GHG balance when methane slip is considered. OCCS bypasses this problem because it acts directly on the exhaust stream.
Regulatory time-buying: OCCS can serve as a bridging measure until IMO regulations and fuel infrastructures mature. Operators currently uncertain about which fuel will prevail in the long term can use OCCS to meet short-term compliance requirements without committing to a specific fuel pathway.
When larger investments are due anyway and the fuel pathway has reliable infrastructure, fuel switching is typically the strategically superior choice. The scenarios where fuel switching is clearly preferable:
Newbuilds: For a newbuild with 25+ years of service life, choosing an alternative fuel from the outset is the logical decision. The additional cost for a dual-fuel system (typically 10–20 % of the vessel price) amortises over the lifetime if the ETS price remains stably above EUR 60/t.
Mid-life retrofit with long horizon: For a 10-year-old vessel with 15+ years of expected remaining life, a methanol retrofit can pay off, especially if the route includes ports with methanol bunkering. The higher CAPEX compared to OCCS is compensated by longer-term utilisation and potentially higher emission reduction.
Available infrastructure: For vessels that regularly call at Singapore, Rotterdam, Shanghai or Houston, methanol bunkering is already reality. Here, the greatest weakness of OCCS – dependence on CO2 offloading infrastructure – is eliminated.
Long-term regulatory perspective: Fuel switching addresses emissions at the source. Under the IMO Net-Zero Framework, which targets a global fuel standard (GFS), fuels with low well-to-wake intensity are favoured. OCCS, by contrast, must fight for regulatory recognition of captured volumes – a risk that fuel switching does not carry.
Do not only compare emission impact, but also remaining service life, CAPEX and port dependency. A structured comparison should cover the following dimensions:
CAPEX comparison: OCCS (USD 8–15 million for Capesize) vs. methanol retrofit (USD 15–25 million) vs. LNG retrofit (USD 20–35 million). For newbuilds: dual-fuel premium (USD 5–15 million depending on size and fuel).
OPEX comparison: OCCS has ongoing costs for solvent, CO2 handling and maintenance (estimated USD 30–60/t CO2). Fuel switching has additional fuel costs (methanol approximately 50–100 % more expensive than VLSFO on an energy basis) and potentially higher maintenance costs for dual-fuel systems.
Risk profile: OCCS carries port infrastructure risk and regulatory recognition risk. Fuel switching carries fuel price risk and availability risk. Both have technical risks, but dual-fuel engines are significantly more mature than OCCS systems.
Flexibility: OCCS is theoretically removable and leaves the propulsion train unchanged. A fuel switch is a more permanent conversion but offers greater regulatory certainty in the long term.
Time horizon: For vessels with less than 8 years remaining service life: neither OCCS nor fuel switch – efficiency measures and speed optimisation are more economical. For 8–12 years: evaluate OCCS as a bridge. For 12+ years or newbuilds: fuel switch preferred.
From the marine engineer’s perspective, the two pathways differ fundamentally in their impact on the vessel:
OCCS integration: The existing propulsion system remains unchanged. Modification concerns the exhaust train (absorber integration), the engine room (pumps, controls, solvent tanks) and the deck (CO2 storage tanks, offloading connections). Classification requires a new notation but no redesign of the main engine. Yard time is typically 4–8 weeks.
Fuel-switch integration (methanol example): The main engine is modified or replaced. For MAN engines, a conversion kit is available that converts the existing engine to dual-fuel operation. Additionally, new fuel tanks are required (methanol requires approximately 2.5 times more tank volume than HFO due to lower energy density), a new fuel pre-treatment system and modified control software. Yard time is 8–16 weeks. Classification requires a complete reassessment of the fuel system.
Parallel operation: Some operators consider installing OCCS as a first measure and performing a fuel switch later. This scenario is technically possible but economically questionable, as the OCCS system becomes obsolete upon fuel switching. It may nevertheless make sense if the fuel switch is planned in 8–10 years and the OCCS system reduces ETS costs in the interim.
Crew implications: Both pathways require training. OCCS demands understanding of chemical processes (amines, corrosion, pressure systems). Fuel switching requires understanding of the specific risks of the new fuel (methanol: toxicity and invisible flame; ammonia: toxicity; LNG: cryogenic hazards and methane slip). STCW requirements are adjusted accordingly.
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