Ships could store their CO2 emissions in the ocean

Ships could capture their own carbon dioxide emissions by bubbling exhaust through seawater and limestone, then pouring the water back into the ocean. This could save space and energy compared with other systems, but it is unclear what the environmental impacts might be.

The system takes advantage of a natural reaction between CO2 and calcium carbonate, also known as limestone. “The ocean has been running exactly this reaction for billions of years,” says Jess Adkins at Calcarea, the start-up behind the technique.

When seawater absorbs CO2, it becomes acidic enough to break down limestone. The dissolved rock then reacts with CO2 in the water to form bicarbonate minerals, which can remain stable in the ocean for millennia. This is one of the primary ways the planet removes CO2 from the atmosphere over long timescales.

For decades, Adkins and his colleagues studied how this dynamic affects organisms with shells or skeletons made of calcium, like corals, as the oceans become more acidic due to rising levels of atmospheric CO2. They realised that speeding up the rate at which limestone dissolved would transform more CO2 into stable bicarbonate – and one way to do this was to increase the concentration of carbon dioxide exposed to limestone. “You can make [the reaction] go an order of magnitude faster if you use pure CO2,” says Adkins.

The researchers have now designed a way to use this process to capture carbon from ships, which are responsible for about 3 per cent of all human-caused CO2 emissions and have limited options to reduce their footprint.

Adkins says tests in California demonstrated that two prototypes can convert at least 30 per cent of the CO2 in diesel engine exhaust into bicarbonate. They are now working with the research arm of Lomar Shipping, a global shipping company, to test the system on a ship.

The on-board test would involve compressing exhaust, then bubbling it through large volumes of seawater, using the movement of the ship as a water pump to save on energy. The more acidic water would then flow over crushed limestone to form bicarbonate, before being discharged back into the ocean.

Adkins says this technique doesn’t use up as much space and is more flexible than other approaches, which require storing captured emissions on board and offloading them at specialised ports. Still, he estimates the Calcarea system would take up about 4 per cent of the space on a large bulk carrier ship sailing on a long voyage.

Phil Renforth at Heriot-Watt University in the UK says the idea is interesting but could face a few problems. For one, he says the approach is unlikely to ever capture all the CO2 from the exhaust without impractically large reactors. As more options for low-emissions shipping fuels become available, that may prove to be a better option than capturing emissions.

“We also need to know a lot about the consequences of scaling this up,” he says. Discharging bicarbonates into the ocean wouldn’t be a concern because they are abundant in seawater, but he says other compounds in the exhaust could have negative effects on ecosystems.

Many ships already use systems that discharge sulphur pollution from exhaust into the ocean. But the agencies that regulate global shipping and international waters remain divided on how to address schemes to store CO2 in the sea.