Why Capture CO2 on a Ship?

6 min read

What you'll take away Explain the case for treating the exhaust instead of changing the fuel, outline how post-combustion capture works, and state the honest limits of the argument.

Every mainstream route to shipping decarbonisation asks the owner to change the fuel: methanol, ammonia, LNG, biofuel blends. Each of those means new or converted engines, new bunkering arrangements, new crew competencies, and a bet on a supply chain that does not exist at the scale the fleet would need. Onboard carbon capture (OCC) starts from the opposite end. Keep the engine. Keep the fuel. Treat the exhaust.

The logic of treating the exhaust

The argument rests on what the world fleet already is: tens of thousands of ships burning fuel oil or gas oil through diesel machinery that is proven, efficient, and bunkerable in every port on earth. A ship delivered now can trade for twenty-five years; whatever fuel transition eventually arrives, an enormous stock of conventional tonnage will be at sea throughout it. OCC offers that stock a way to cut what leaves the funnel without touching what happens inside the engine.

The chemistry it borrows is old. Post-combustion capture, separating CO2 from a flue gas after the fuel has burned, has run in power stations and process plants for decades. ABS's open whitepaper Insights into Onboard Carbon Capture surveys the technology families and is a fair map of the field: absorption by a chemical solvent is the mature route, with membranes, adsorption and cryogenic separation behind it. Nothing about capturing CO2 is speculative. What is genuinely new is doing it on a moving, space-limited platform that must then sail with the product.

What the ship actually does

In outline, the plant takes exhaust downstream of the engine, cools and cleans it, strips out the CO2, compresses and liquefies it, and stores the liquid in tanks until a port can take it. Two things make this harder at sea than ashore. First, marine exhaust is dilute: CO2 is only a few per cent of the gas by volume, most of the rest being nitrogen, so the separation works against a weak concentration gradient and consumes real energy. Second, an onshore plant pipes its product away; a ship must give up deadweight and space to carry hers, voyage after voyage.

The combustion is untouched, every stage sits downstream of the engine nitrogen and the rest → funnel Engine exhaust Cool and clean Capture Liquefy and store only a few % CO2 gas conditioning CO2 separated out compress and cool Port reception then storage mostly missing on board ashore Keep the engine, keep the fuel, treat the exhaust, then find somewhere ashore to put the product.
Onboard capture in outline: the engine and its fuel are unchanged, and every stage works on gas that has already left them. The dashed block is the one no owner can buy: a port able to receive the CO2, which lesson 4 takes up.

The honest framing

OCC attracts strong views, and both sides deserve a fair statement.

The case against: the capture plant runs on the ship's own energy, so a tonne captured is never a tonne saved: the ship burns extra fuel to power the capture, and that extra fuel makes extra CO2. The Maersk Mc-Kinney Moller Center's open study, The Role of Onboard Carbon Capture in Maritime Decarbonization, worked this through for container, bulk and tanker cases and found that even with high onboard capture rates, the well-to-wake greenhouse-gas reduction lands around 55–60%, the fuel's upstream emissions and the energy penalty eat the difference. Add that the shore side of the chain barely exists, that regulations do not yet straightforwardly credit the captured tonnes, and that every euro spent on capture is a euro not spent on future fuels, and the sceptical position is coherent.

The case for is equally plain: 55–60% well-to-wake is a larger reduction than any drop-in measure offers a conventional ship, it applies to tonnage that exists rather than tonnage that must be built, and captured CO2 of fossil origin, once permanently stored, is genuinely out of the atmosphere. For an owner whose ships are young, large, and committed to conventional fuel, the realistic comparison is not OCC versus a green newbuilding: it is OCC versus doing nothing structural at all.

Where this course goes

The next two lessons open the machinery: how amine absorption and membrane separation work, why the advertised capture rate is not the effective one, and what the plant demands from the ship in space, steam, power and tanks. Lesson 4 follows the CO2 ashore: the half of the chain the owner does not control. Lesson 5 asks the question that decides everything: does a captured tonne count where money is charged for emissions? Hold that question through every lesson; it is the one the whole subject turns on.

Check yourself

1. What is the core proposition of onboard carbon capture?
2. Post-combustion capture separates the CO2 at which point?
3. Why is a ship's exhaust comparatively hard to capture from?
4. Why is a tonne of CO2 captured not a tonne of CO2 saved?
5. The Maersk Mc-Kinney Moller Center study found that even a well-integrated capture ship achieves a well-to-wake GHG reduction that is
6. Which of these belong on the honest list of OCC's open problems?

Select all that apply.

7. Post-combustion CO2 capture is a new technology invented for ships.
8. The MMMCZCS study put the well-to-wake GHG reduction of a ship capturing at high onboard rates in a band of roughly 55–60%. Enter a value in that band.
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