Answer
Carbon Capture means removing CO₂ before it is released to atmosphere, then storing it for later offloading, utilization or permanent geological storage. On ships this is called Onboard Carbon Capture and Storage (OCCS).
Main technologies are:
Chemical absorption
- Exhaust gas passes through an absorber containing solvents, commonly amine-based.
- CO₂ is absorbed and later released by heating the solvent in a regenerator.
- CO₂ is then compressed/liquefied and stored onboard.
- This is presently one of the most practical post-combustion options for ships.
Membrane separation
- Exhaust passes across a selective membrane.
- CO₂ preferentially passes through the membrane and is separated.
- Compact arrangement is attractive onboard, but efficiency and membrane durability are important considerations.
Cryogenic separation
- Exhaust is cooled to very low temperature so that CO₂ can be separated and liquefied.
- Produces relatively pure CO₂ but requires significant refrigeration energy.
Mineralization
- CO₂ reacts chemically with minerals to produce stable solid carbonates.
- Advantage: avoids storing high-pressure/liquid CO₂.
- Disadvantage: large quantity and weight of solid material can be produced.
Energy Penalty & Fuel Consumption Impact: Running compression pumps, solvent reboilers, and liquefaction chillers requires additional electrical power (10% to 25% increased auxiliary load). The net CII reduction must account for the additional fuel burned to power the OCCS.
Storage & Payload Penalties: Storing LCO2 consumes substantial cargo volume and deadweight capacity. For every 1 tonne of fuel burned, approximately 3.11−3.20 tonnes of CO2 are generated; capturing 70% yields ∼2.2 tonnes of liquid storage requirement per tonne of fuel.
Solvent Handling & Degradation: Amine degradation products can be corrosive and toxic. Regular sampling of solvent quality, monitoring thermal degradation, and controlling hazardous waste sludge disposal are required.
IMO Accounting Framework: Under current IMO regulations, frameworks are being finalized to officially credit captured CO2 offloaded to port reception facilities within the IMO DCS and CII AER calculations.