NUS researchers find way to store CO2 beneath the ocean floor

Professor Praveen Linga said that the NUS team is now building a bigger prototype to scale up its experiment with hydrates. ST PHOTO: NG SOR LUAN

SINGAPORE - Researchers from the National University of Singapore (NUS) have found a new way to accelerate the rate at which carbon dioxide (CO2) is sucked out of the atmosphere and stored beneath the ocean floor, in a bid to mitigate the harshest impacts of climate change.

This comes as Singapore has recently set a target to reach net-zero emissions by around the middle of the century, bringing to the fore such carbon capture and storage technologies as a tool to reduce CO2 emissions.

The ongoing project will be one of 12 funded by the Low-Carbon Energy Research Funding Initiative - where the Government has set aside $55 million to look into hydrogen and carbon capture, utilisation and storage.

Carbon capture technology works by capturing CO2 produced by large emitters such as power generation and petrochemical companies, compressing it into liquid form, and then injecting it deep underground for storage.

While much headway has been made in storing carbon in rock formations underground or in depleted oil and gas fields, the NUS team has found that CO2 can be stored beneath ocean floor sediments, and kept stable by the pressure created by seawater.

The Earth's lithosphere - its rocky outer part - and the deep ocean are two large carbon reservoirs on the planet, due to their ability to store carbon over millions of years, via a series of chemical processes.

They play a huge role in regulating the climate, as this means that the CO2 will not escape into the atmosphere and trap heat, which contributes to the climate crisis.

Professor Praveen Linga from the NUS' Department of Chemical and Biomolecular Engineering, told The Straits Times that when liquid CO2 is injected into the ocean, it reacts with water to form ice-like substances known as hydrates.

The change usually occurs at a temperature just above the freezing point of water at a certain pressure and can store as much as 184 cubic m of CO2 in each cubic m of hydrate.

Using a specially designed laboratory reactor to mimic the conditions on the ocean floor - where temperatures range between 2 deg C and 6 deg C, and pressures are a hundred times higher than that of sea level - the NUS team was able to show that the hydrates remained stable for up to 30 days.

"Creating a macro-scale reactor that could maintain such conditions was challenging and is one of the reasons why experiments to test the stability of CO2 hydrates were previously not possible," Prof Linga said.

The NUS team overcame this challenge using an in-house-designed pressurised vessel, lined with a silica sand bed, which imitated ocean sediments.

Asked about the possibility of the CO2 hydrates melting due to warming oceans as a result of climate change, Prof Linga said that it would take a 9 deg C rise in oceanic temperatures for the CO2 hydrates to dissociate, thereby releasing CO2 into the oceans.

He noted that this is "way more" than the 2 deg C above pre-industrial levels in warming that the world has been trying to avoid.

The NUS team used a specially designed laboratory reactor to mimic the conditions on the ocean floor, and showed that the hydrates remained stable for up to 30 days. ST PHOTO: NG SOR LUAN

Increasing CO2 in the ocean alters the chemistry of seawater, leading to acidification - which has negative impacts on marine life.

The 2015 Paris Agreement commits countries to limiting the global average temperature rise to well below 2 deg C above pre-industrial levels, and to aim for 1.5 deg C.

Scientists have said that crossing the 1.5 deg C threshold risks unleashing far more severe climate change effects on people, wildlife and ecosystems.

As the team is eyeing the deep oceans that are at least 1,000m below the sea surface, the temperature would be a lot colder than the ocean's surface, Prof Linga added.

While Singapore's oceans are too shallow to facilitate carbon storage, oceans in the region would have the required depth, he noted.

"We could capture the CO2 here, and then transport it around the region for storage, which is what countries in Europe are looking to do under the Northern Lights partnership - where they capture CO2 and send it to Norway for storage," he added.

The completed phase of the team's project was funded through the Singapore Energy Centre - a consortium of NUS, Nanyang Technological University, and industry partners such as Exxon Mobil.

Its findings were published in the scientific journal Chemical Engineering Journal in December last year.

Now that the team has been able to prove the stability of CO2 hydrates in the deep ocean, the next step would be to test its stability over a period of at least six months.

The team is now building a bigger prototype to scale up its experiment, Prof Linga added.

"We eventually hope to develop models that can predict the stability of CO2 hydrates thousands of years into the future, taking into account future scenarios, such as warming oceans, or other disturbances like an earthquake," he said.

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