A 240-metre-long container ship called the Sounion Trader recently completed a test of an onboard carbon capture system as it cruised around the Persian Gulf. It is one of a small but growing number of ships trying to reduce their climate footprint by capturing and storing their carbon dioxide emissions onboard – but finding space for tonnes of CO2 is a challenge.

“You’re miniaturising a system that was designed for huge power plants,” says Roujia Wen at Seabound, the UK-based start-up behind the Sounion Trader’s test run.

Shipping is responsible for around 3 per cent of global CO2 emissions. To reduce that, shippers are using cleaner fuels, lubricating hulls with bubbles to improve fuel efficiency and even turning back to sails. But near-term options to reach the industry’s pledge of net-zero emissions by 2050 are limited .

Another possibility is capturing ships’ emissions and storing them onboard, but it faces major obstacles. One is supplying the energy to recharge the chemical sorbents used to absorb CO2. Tristan Smith at University College London says some existing systems increase fuel use by a third just to catch half of CO2 emissions.

The systems, and the carbon they capture, also take up room on board that would normally be used for valuable cargo. “Space is an issue,” says Jasper Ros at TNO, a research organisation in the Netherlands. “Especially when you’re talking about long voyages.” Each tonne of combusted fuel forms around 3 tonnes of CO2, says George Mallouppas at the Cyprus Marine & Maritime Institute. When it is captured and stored, the added mass can affect a ship’s stability and reduce its fuel efficiency.

Wen says Seabound’s small-scale tests captured around a tonne of CO2 per day. That is a small fraction of the ship’s overall emissions, but she says the full-scale system will be able to capture as much as 95 per cent of a ship’s CO2.

To save energy, Seabound moves part of its process onshore. On the ship, exhaust is looped through a calcium oxide sorbent, which reacts with CO2 to form solid calcium carbonate pebbles. The company then waits to recharge the sorbents until the pebbles are offloaded at port for permanent storage. The trade-off is space. Seabound’s approach means a ship must carry tanks of sorbent along with every tonne of captured CO2. Still, Wen says the company aims to retrofit 1000 ships for carbon capture by 2030.

A Dutch company called Value Maritime is taking a similar approach, using a liquid amine sorbent to capture CO2 and then recharging it offshore. Yvette van der Sommen at Value Maritime says 26 ships are now using its system alongside existing sulphur pollution-scrubbers to capture up to 40 per cent of CO2 in exhaust, although the process hasn’t yet been certified by a third party. She says the company has sold some captured CO2 to greenhouses to fertilise plants, but much of it remains stored in tanks at ports.

Such systems could appear attractive to cut emissions now, says Smith. But the rapid scale-up of cleaner shipping fuels may soon make them obsolete – unless they can achieve very high rates of capture at a low enough cost. “Shipping faces a very short time to decarbonise, because it has started so late,” he says.