A capture rate of 70 percent is only meaningful when it is clear under which load conditions it is achieved. The number sounds impressive, but its significance depends entirely on context. 70 % at 75 % MCR (Maximum Continuous Rating) under calm sea conditions is fundamentally different from 70 % across all operating states including manoeuvring, part load and heavy weather.
The CO2 concentration in the exhaust of a two-stroke marine diesel is typically between 4 and 6 vol% – significantly lower than in coal-fired power plants (12–15 %) for which most OCCS processes were originally developed. This means the absorber must process larger gas volumes to capture the same quantity of CO2. A capture rate of 70 % at maritime CO2 concentrations requires a considerably larger exchange area than in stationary plants.
Part-load behaviour adds another layer. When the engine operates at 50 % MCR, not only the exhaust volume changes but also its temperature and composition. Most OCCS systems are optimised for a specific design point and lose efficiency at part load. Serious providers therefore state an operating window – e.g. 70 % at 60–85 % MCR – rather than a blanket capture rate.
The definition of the reference value is equally critical: 70 % of exhaust emissions or 70 % of well-to-wake emissions? For the latter, the parasitic energy demand of the OCCS system itself would also need to be included, which can reduce the net capture rate to 55–60 %.
The higher the capture rate, the greater the increase in energy demand and system complexity. In amine-based systems, solvent regeneration is the largest energy consumer. To achieve 70 % capture on a Capesize bulker, 2–4 GJ of thermal energy per tonne of CO2 are required depending on the system. For a daily emission of 80–100 tonnes of CO2, this translates into a heat demand that claims a significant share of available exhaust heat.
If this heat cannot be fully covered from the exhaust gas boiler and jacket cooling, it must be generated by additional fuel consumption. The irony: to capture CO2, more fuel is burned and more CO2 is produced. In poorly designed systems, this rebound effect can push the net saving below 50 %.
In addition to thermal energy, the system requires electrical energy for pumps, fans, instrumentation and CO2 compression. On a vessel with already limited generator capacity, this may mean that an additional auxiliary diesel is needed – with corresponding space requirements, weight and its own fuel consumption.
The core economic question is therefore: what is the net CO2 gain after deducting parasitic energy consumption, and how does it compare to the avoided ETS allowance purchase? If the marginal capture cost exceeds the allowance price, the system is economically counterproductive – regardless of the technically achievable capture rate.
Ongoing costs, maintenance, port logistics and off-hire are frequently assessed too optimistically. Typical manufacturer presentations show CAPEX and a simplified OPEX calculation. What is missing is the complete operating cost comparison over the system lifetime:
Solvent losses: Amine solutions degrade through thermal stress and reaction with residual SOx in exhaust gas. Even with pre-treatment (scrubber), annual losses of 1–3 kg amine per tonne of CO2 are realistic. At 20,000–30,000 tonnes of CO2 per year, this adds up to considerable procurement costs.
Maintenance: Columns, heat exchangers and seals are exposed to chemical and thermal stress. Manufacturers often cite optimistic intervals. Realistically, annual inspections and a major column overhaul every 3–5 years should be expected.
Port logistics: CO2 offloading requires infrastructure that currently exists in only a few ports. Where it is absent, waiting times, detours or the necessity to carry CO2 to the next suitable port arise. Costs for port-side CO2 offtake are not yet standardised and vary considerably.
Off-hire for retrofit: The retrofit takes 4–8 weeks depending on complexity. In a market with day rates of USD 20,000–40,000 for Capesize bulkers, this means USD 600,000 to 2.2 million in lost revenue – on top of yard costs.
Capacity loss: The weight of the OCCS system and the stored CO2 mass reduces cargo capacity. On a Capesize bulker, 500–1,500 tonnes of cargo can be lost depending on the system and voyage duration. At freight rates of USD 8–15 per tonne, this adds up to significant revenue losses.
With a technically robust system, predictable port chains and sufficiently long remaining service life, a 70 % capture rate can still be economically viable. The prerequisites can be clearly defined:
The vessel must have a remaining service life of at least 10 years, ideally more. The route must regularly call at ports with CO2 offloading infrastructure – Rotterdam, Antwerp and selected Norwegian ports are leading the way. The exhaust system must provide sufficient waste heat to cover the parasitic energy demand without requiring an additional generator.
Under these conditions the figures can work: at an EU ETS price of EUR 80/t CO2, annual emissions of 25,000 t and a net capture rate of 60 % (after deducting the rebound effect), 15,000 allowances are saved annually – a value of EUR 1.2 million. With CAPEX of EUR 12 million and annual OPEX of EUR 400,000, the payback period is approximately 15 years. That is borderline, but becomes more attractive if the ETS price continues to rise or FuelEU penalties are added.
The real attraction of the 70 % mark lies not in the capture rate itself but in the regulatory signal effect. Systems that demonstrably achieve 70 % or more could be recognised in future IMO regulations as an equivalent compliance measure – a considerable competitive advantage for vessels that have no other option.
Achieving a stable 70 % rate under real sea conditions requires technical optimisation at several points. The most important lever is solvent chemistry. Conventional MEA (30 wt%) has a high CO2 loading capacity but also a high regeneration energy demand of approximately 3.5–4.0 GJ/t CO2. Newer solvents – sterically hindered amines, amino acid salts or phase-change solvents – can reduce this to 2.5–3.0 GJ/t, which can make the difference between economically viable and unviable.
The second lever is column dimensioning. Maritime absorbers must be more compact than their stationary counterparts, which increases the specific exchange area per unit volume. Structured packings with high wetting efficiency are standard; however, efficiency decreases in heavy seas. An over-dimensioning of 15–20 % relative to the design point is a pragmatic approach but increases weight and cost.
Thirdly, process control is critical. An adaptive control system that adjusts solvent circulation rate and regeneration temperature in real time to the engine load condition can stabilise the capture rate over a broader operating range. This requires reliable sensors (CO2 analysers at inlet and outlet, temperature probes, flow meters) and robust PLC programming.
A frequently underestimated factor is exhaust gas pre-treatment. Even vessels with scrubbers have residual SOx and particulate levels in their exhaust that degrade the solvent. A pre-separator or a cooled scrubber upstream of the absorber extends solvent lifetime considerably and stabilises the capture rate over time.
A 70 % capture system noticeably changes daily life on board. The engine room crew must monitor and maintain a chemical process system that, if mishandled, is corrosive, toxic (amine vapours) and temperature-critical. Training in accordance with the ISM Code and manufacturer-specific familiarisation are mandatory – and must be repeated at every crew change.
Bunker planning becomes more complex. In addition to fuel, lubricating oil and fresh water, solvent must now also be bunkered and CO2 tank capacity must be considered. On voyages of more than 14 days without an offloading opportunity, CO2 storage capacity can become limiting, which either intentionally reduces the capture rate or influences route planning.
For the superintendent, new KPIs emerge: net capture rate, solvent consumption, CO2 offloading volume and parasitic energy consumption as a proportion of main engine output. These data must be systematically recorded and reported to technical management – not least because regulatory recognition of captured CO2 volumes requires complete documentation.
The discrepancy between laboratory capture rates and real sea conditions is systematic and should be priced into every project evaluation. In controlled environments, 90 %+ is not uncommon; at sea, several factors reduce the rate: varying engine loads, ship motions, temperature fluctuations, solvent ageing and maintenance interruptions.
A realistic deduction for maritime deployment is 15–25 percentage points relative to the manufacturer’s specification under design conditions. Anyone who wants to achieve 70 % under real conditions therefore needs a system that delivers 85–90 % under design conditions. This has direct consequences for system dimensioning, weight and cost.
A further practical aspect: the first months after installation are typically the most difficult. The crew must familiarise themselves with the system, control parameters are readjusted and unforeseen interactions with existing systems (scrubber, economiser) occur. A realistic project plan should factor in a 3–6-month running-in phase with reduced capture rates.
The choice of target capture rate is a strategic decision with technical, economic and regulatory dimensions. Three scenarios help with orientation:
Scenario A – Minimum compliance: 30–40 % capture is sufficient to reduce EU ETS costs and lower FuelEU penalties. Simpler systems, lower CAPEX, less operational complexity. Sensible with shorter remaining service life or when a fuel switch is planned within 5–7 years.
Scenario B – Optimised compromise: 50–60 % capture as a balance between cost efficiency and regulatory impact. The system requires good heat integration but no additional generator. For most cases the most economically robust approach.
Scenario C – Maximum capture: 70 %+ capture for maximum regulatory impact and potential recognition as an equivalent compliance measure. Requires optimal conditions (waste heat, space, port infrastructure) and highest CAPEX. Only economically viable with long remaining service life and rising ETS prices.
The recommendation: for most operators, Scenario B is the most realistic starting point. Those aiming for Scenario C should only do so on the basis of a robust feasibility study that includes all five cost categories (CAPEX, solvent, maintenance, off-hire, capacity loss).
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