Fitting the Plant into the Ship

A capture plant is not an exhaust accessory. It is a small process plant, columns, heat exchangers, compressors, a refrigeration train and pressure vessels, grafted onto a platform that was optimised without it. This lesson is about what that graft costs the ship, before a single tonne is credited anywhere.

The mass problem comes first

Start with the arithmetic that surprises people. Burning one tonne of conventional marine fuel produces roughly 3.1 tonnes of CO2, because combustion adds the mass of atmospheric oxygen to the fuel's carbon. A ship that captures 75% of her emissions is therefore making about 2.3 tonnes of product for every tonne of fuel she burns: the thing she must store outweighs the thing she bunkers. Tank capacity, deadweight and stability all answer to that ratio, and on a long voyage it, not the capture rate, often sets the design.

Stored as a gas, that CO2 would need impossible volumes; liquefied, it becomes dense enough to carry. So every OCC ship runs a liquefaction train, compression plus refrigeration, and stores the product in insulated pressure vessels at low temperature. Those tanks are the most visible part of the installation: cylindrical, heavy, and competing directly with cargo for space and deadweight.

Space, steam and power

Above the tanks sits the plant itself. The absorber column wants height and proximity to the funnel, which puts it in or beside the casing: real estate every ship type prices differently. A container vessel loses slots; a tanker loses deck area aft; a bulker may have the friendliest geometry.

Then the standing loads. Lesson 2 established that solvent regeneration wants steam-grade heat: exhaust-gas heat recovery supplies part of it, and an auxiliary boiler burns fuel for the rest. Compression, liquefaction, fans and pumps draw electrical power on top, which usually means running more generator capacity. The plant is, in effect, a new consumer that never sleeps while the main engine runs: the shipboard shape of the energy penalty.

What class asks of the design

None of this is unregulated territory. ABS publishes Requirements for Onboard Carbon Capture and Storage, ClassNK maintains its guidelines (lesson 2 met their October 2025 revision), and the Indian Register of Shipping issued its own Guidelines on Onboard Carbon Capture and Storage. The common threads are what a marine engineer would expect once the hazard is understood. CO2 does not burn, but it does not need to: it is an asphyxiant denser than air, so a leak pools silently in bilges, voids and stairwells. The rules therefore dwell on gas detection, ventilation of enclosed spaces, pressure relief, and materials: low-temperature steels for tanks and piping that see refrigerated liquid. Add integration questions no onshore plant faces: sloshing in partly filled tanks, hull motions and vibration, and keeping the plant's failure modes from touching the machinery the ship needs to sail.

The capex reality

Now the number that disciplines the whole conversation. The Maersk Mc-Kinney Moller Center's study of onboard capture priced full installations, capture plant, liquefaction, storage, at roughly 26–70% of the cost of the newbuilding itself, around 40% on average, all at the same 82% capture rate. Read that range slowly. At the bottom end, an LNG-fuelled VLCC, an owner is buying a quarter of a second ship; at the top, a small bulk carrier, most of one. The spread is set by ship size and fuel, not by capture rate: large tankers and container ships absorb the plant most cheaply and small bulkers worst, and the study found that capturing less did not make the plant much cheaper, so the cost per tonne is lowest at the maximum rate.

For a retrofit, add yard time out of service and the naval-architecture work of finding weight and stability margin in a hull that was never asked for it. For a newbuilding, the honest alternative is "capture-ready": reserving space, structure and power margin so the plant can come later, once, and this is the hinge, there is somewhere to put the CO2 and a rule that pays for it. Those two conditions are the next two lessons.