Answer
Onboard Carbon Capture System (OCCS)
An Onboard Carbon Capture and Storage system captures CO₂ from the ship’s exhaust gas before it is discharged to atmosphere, conditions the captured CO₂, stores it temporarily onboard, and later offloads it ashore for utilisation or permanent storage. (OCCS) reduce the vessel's net emissions, aiding compliance with IMO Carbon Intensity Indicator (CII), EEXI, and EU ETS requirements.
IMO is developing a dedicated regulatory framework for OCCS; work was advanced at MEPC 83 and MEPC 84, with the framework targeted for completion by 2028.
Typical system arrangement
For a conventional post-combustion amine-based system, the flow is:
Engine/boiler exhaust → cooling & cleaning → absorber → CO₂-rich solvent → stripper/regenerator → concentrated CO₂ → drying/purification → compression/liquefaction → CO₂ storage tank → shore offloading
The cleaned exhaust, with much of the CO₂ removed, is discharged through the funnel. Post-combustion capture is currently the most practical concept for retrofitting existing ships. DNV
Procedure / working principle
- Exhaust gas collection
Exhaust from the main engine, auxiliary engines or boiler is diverted to the carbon-capture plant. - Exhaust pre-treatment
The gas is passed through a cooler/ scrubber to cool and clean the contaminants such as particulates, SOx and moisture. This is important because contaminants can degrade the capture solvent. - Absorption of CO₂
Exhaust enters an absorber tower, generally from the bottom. An absorbent, commonly an amine solution (e.g., Monoethanolamine - MEA), is sprayed from the top. CO₂ is chemically absorbed into the solvent. Treated exhaust gas (low CO2) exits through the top stack. - Solvent regeneration
The CO₂-rich solvent is pumped to a stripper/regenerator and heated, often using waste heat or steam. Heating releases CO₂ from the solvent. The regenerated lean solvent is cooled and returned to the absorber. - CO₂ purification/drying
Released CO₂ is separated from water vapour and impurities. - Compression and liquefaction
CO₂ may be compressed and cooled into liquid CO₂ to reduce storage volume. - Temporary onboard storage
Liquid CO2 (LCO2) is stored under pressure and low temperature (typically 15–20 bar at −25∘C to −30∘C) in insulated IMO Type-C independent tanks or deck-mounted ISO tank containers. - Offloading ashore
In port, captured CO₂ is transferred through an approved connection to shore reception facilities for utilisation or permanent geological storage. IMO notes that captured CO₂ may be temporarily stored onboard before being offloaded for further transport, use or permanent storage.
Main hazards
CO₂ is an asphyxiant, so leakage into enclosed spaces is a major risk. Other hazards include high-pressure CO₂, very low temperatures from expanding/liquid CO₂, solvent toxicity/corrosion, machinery-space integration risks, storage-tank overpressure and loss of refrigeration.
Therefore we need CO₂ detectors, effective ventilation, pressure relief, ESD, PPE, emergency procedures, restricted-space entry controls and proper crew training.