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Storing carbon in the form of gas hydrates deep under water

Storing carbon in the form of gas hydrates deep under water

  • 20th Jun 2024
  • Mongabay

* A new study finds that the Bay of Bengal and the Indian Ocean can serve as potential carbon sinks by storing gas hydrates in depths beyond 2,800 metres.
* The findings can help India achieve its national decarbonisation and climate change goals for 2030.
* However, technologies are in their preliminary stage and require funding to be tested.

New research from India indicates that the Bay of Bengal and the Indian Ocean can potentially serve as storage sinks for large amounts of carbon dioxide in the form of gas hydrates. A gas hydrate is formed when gas molecules bind with hydrogen atom to form a network of water molecules, resembling ice.

Carbon can be permanently stored as gas hydrates in oceans beyond depths of 500 metres, according to the study by scientists from the Indian Institute of Technology (IIT), Madras, in Fuel journal.

Above 2,800 metres, liquid carbon dioxide is denser than seawater, and hence there is a natural gravitational barrier to its escape from water. "Thus, beyond 2,800 m. sea depth, carbon dioxide can be stored permanently in the form of liquid pool and solid hydrate," Jitendra Sangwai, a professor at the Department of Chemical Engineering at IIT-Madras and one of the authors of the research paper, told Mongabay-India.

The study focuses on the formation of carbon dioxide hydrates in bentonite clay slurry in seawater. The subsea clay sediments improve the mechanical and thermal stability of gas hydrates, which help for long-term carbon dioxide storage potential, he adds.

Ocean depth is key to carbon storage. Sequestration using gas hydrates is possible beyond a depth of 500 metres, but carbon's permanent storage space in the sea lies below 2,800 metres. Since not all oceans may have a depth of 2,800 metres with the possibility of permanent storage, oceans with depths of 1,000 metres can also be considered for carbon sequestration.

"The initial investigation delves into the kinetics of hydrate formation, examining factors such as conversion efficiency, gas absorption, and release using seawater," said Sangwai. The team's observations indicate that clay plays a significant role by improving conversion efficiency, facilitating gas absorption into hydrates, and prolonging hydrate stability.

"The capture of carbon dioxide can lead to the creation of environmentally friendly gas hydrates, resembling ice," says Sangwai. Approximately 150-170 cubic metres of carbon dioxide can be sequestered by one cubic metre of gas hydrate in the pores of subsea sediments under oceanic conditions beyond 500 metres sea depth, he adds.

"The key finding of this research is that it can help devise large-scale carbon dioxide storage and utilise the fullest potential of oceans to decarbonise the world without harming marine ecology," says Sangwai. This research can also help India achieve its national decarbonisation and climate change goals, which is to decarbonise energy to 50% and generate 500 gigawatts of fossil fuel-free energy by 2030.

From research to tech
"Oceans do a lot of carbon absorption. It is a natural process," says Madhavan Nair Rajeevan, vice chancellor of Atria University, Bengaluru, and former secretary, Ministry of Earth Sciences.

"But studies suggest we need more carbon absorption or carbon capturing," he says. There are a few options, and ocean sequestration is one of them, which, Rajeevan adds, is theoretically possible but difficult to implement. Rajeevan is also dubious about the viability of such initiatives.

Sangwai says he believes that further investments are needed to take the findings to a workable technology for the development of large-scale facilities for carbon capture, transportation, and sequestration. Nevertheless, existing oil and natural gas infrastructure can be repurposed for injection and storage purposes. "Establishing these infrastructures demands substantial initial investments, necessitating collaboration between industry and academia," says Sangwai.

The financial requirement could be reduced by identifying carbon-intensive industries such as steel, coal, cement, and power near the seashore and selecting a sequestration site not beyond 500 km from the land mass to avoid additional transportation costs.

The current financial projections for a single carbon capture and sequestration (CCS) cycle involving ocean hydrates - which involves capturing the carbon emissions at source, transporting it to the storage site, and storing it - range from $90-150; and costs vary depending on the geological location of the sequestration site, labour costs, and transportation expenses. "Considering these factors, government support, industry partnerships, and a minimum timeframe of five to six years are necessary to initiate a large-scale CCS programme in the subsea sediments of the Bay of Bengal," adds Sangwai.

Options for ocean carbon sinks
Carbon dioxide can also be stored in subsea sediments which will have less impact on marine ecology. The subsea sediments have tiny spaces that can hold the gas and over the period of time stored gas form ice-like gas hydrate crystals in the pores. This will further reduce the permeability of hydrate-bearing sediments and create a permeability barrier. "Oceans depths exceeding 1,000 metres present opportunities for carbon dioxide sequestration in subsea sediments," says Sangwai. Other factors also impact the process, such as an increase in temperature with ocean depth or 'geothermal gradient', along with sediment composition and porosity.

"Sequestration in subsea sediments is the most environment-friendly approach to sequester giga tonnes of carbon dioxide without affecting marine ecology," he adds. All subsea sediments contain clay that is deposited by the rivers over the course of millions of years. These clays may affect carbon dioxide hydrate formation.

The IIT-M findings, while focusing more on the chemical dynamics of seabed clay and carbon dioxide, add to previous reports of oceans' potential to act as carbon sinks, one of the key threads of discussions in UN climate change summits.

For example, a 2023 report on the role of oceans in climate change solutions says that ocean-based action can play a critical role in helping the world avoid the worst impacts of climate change by closing the emissions gap by as much as 35%, based on solutions that are ready to implement on a pathway that can limit global warming to 1.5 degrees C by 2050.

The report cites seven potential ocean-based options to reduce emissions, which includes the removal of marine carbon dioxide, and carbon capture and storage under the seabed. The others include scaling ocean-based renewable energy; decarbonising ocean-based transport; conserving and restoring coastal and marine ecosystems; utilising low-carbon food from oceans; decarbonising ocean-based tourism; and reducing offshore oil and gas.

According to a World Resources Institute report, such 'biotic' or biological-based approaches to help oceans sequester carbon "leverage photosynthesising organisms in seawater to take up carbon dioxide and store that carbon as biomass. For example, seaweeds can be grown and then sunk into the deep ocean or the seafloor, storing a portion of the carbon-rich biomass. Similarly, oceans can be 'fertilised' by adding nutrients such as iron to stimulate growth of phytoplankton which can take up carbon dioxide, convert it to biomass, and help sequester carbon. Another technique is 'artificial upwelling' by moving deeper, nutrient-rich water to the surface, rather than adding new nutrients, to spur the growth of ocean plant organisms.

The WRI report says there are also 'abiotic' methods that harness the physical or chemical properties of the ocean to remove carbon dioxide from the air and, hence, do not rely on marine organisms to sequester carbon. For example, one can increase the alkalinity of oceans by adding certain minerals that can help dissolve more carbon dioxide in seawater for storage. Another potential option is the use of electrochemical techniques that use electricity to increase alkalinity enhancement or directly extract carbon dioxide from seawater for storage on land.

It also cautions that "most ocean carbon removal approaches are at an early stage of development, with little or no testing done in the field."

"It is not yet clear how effective these approaches would be at removing carbon, how that could change depending on a project's scale or location, how long the carbon would remain sequestered, or what impact these approaches might have on ocean ecosystems or people whose livelihoods depend on them," it says.

Banner image: Bay of Bengal and the Indian Ocean can potentially serve as storage sinks for large amounts of carbon dioxide in the form of gas hydrates. Image by Ravikiran Rao via Flickr (CC BY-SA 2.0)