HomeEconomyFind out How carbon dioxide is pulled from seawater?

Find out How carbon dioxide is pulled from seawater?

Researchers around the world have been working for years to find effective techniques to remove carbon dioxide from the atmosphere as it continues to accumulate in the planet’s atmosphere. The ocean, on the other hand, serves as the planet’s main “sink” for atmospheric carbon dioxide, absorbing 30 to 40 percent of all the gas generated by human activity.

Another interesting option for reducing CO2 emissions, which could eventually lead to net negative emissions, is the prospect of pumping carbon dioxide directly from ocean water. This possibility has recently come to light. Although there are several businesses trying to break into this market, the concept has yet to lead to any widespread adoption, similar to air capture systems.

Now a team of MIT researchers say they may have found the key to a truly effective and inexpensive removal mechanism. The findings were reported this week in the journal Energy and Environmental Science in a paper by MIT professors T. Alan Hatton and Kripa Varanasi, postdoc Seoni Kim, and graduate students Michael Nitzsche, Simon Rufer, and Jack Lake.

Existing methods of removing carbon dioxide from seawater apply a voltage across a set of membranes to acidify the feed stream by splitting the water. This converts the bicarbonates in the water into CO2 molecules, which can then be removed under vacuum.

Hatton, the Ralph Landau Professor of Chemical Engineering, notes that membranes are expensive and chemicals are needed to control the overall electrode reactions at both ends of the beam, further increasing the cost and complexity of the processes. “We wanted to avoid having to introduce chemicals into the anode and cathode half-cells and avoid using membranes if possible,” he says.

Acidifies the water to convert dissolved inorganic

The team came up with a reversible process consisting of membraneless electrochemical cells. Reactive electrodes are used to release protons into the seawater supplied to the cells, releasing dissolved carbon dioxide from the water. The process is cyclical: First, it acidifies the water to convert dissolved inorganic bicarbonates into molecular carbon dioxide, which is collected as a gas in a vacuum.

The water is then fed to a second set of reverse voltage cells to recover the protons and turn the acidic water back to alkaline before being released back into the sea. Periodically, the roles of the two cells are reversed once one set of electrodes has been depleted of protons (during acidification) and the other has been regenerated during alkalization.

Reinjection of the alkaline water

This removal of carbon dioxide and re-injection of alkaline water could slowly begin to reverse, at least locally, the acidification of the oceans that has been caused by the build-up of carbon dioxide, which in turn threatens coral reefs and molluscs, says Varanasi, a professor of engineering. Reinjection of the alkaline water could be done through scattered outlets or far offshore to prevent local increases in alkalinity that could disrupt ecosystems, they say.

“We won’t be able to process the emissions of the entire planet,” says Varanasi. But reinjection can in some cases be done in places like fish farms that tend to acidify the water, so it could be a way to help counteract that effect.

Once the carbon dioxide is removed from the water, it must be disposed of, as with other carbon removal processes. For example, it can be buried in deep geological formations beneath the sea floor, or it can be chemically converted into a compound such as ethanol, which can be used as a transportation fuel, or into other specialty chemicals.

“You can certainly consider using the captured CO2 as a feedstock to make chemicals or materials, but you won’t be able to use all of it as a feedstock,” says Hatton. “You’re going to run out of markets for all the products you’re making, so regardless, a significant amount of captured CO2 will have to be buried underground.”

At least initially, the idea would be to connect such systems to existing or planned infrastructure that already processes seawater, such as desalination plants. “This system is scalable so that we can potentially integrate it into existing processes that already process ocean water or are in contact with ocean water,” says Varanasi.

There, carbon dioxide removal could be a simple addition to existing processes that already return vast amounts of water to the sea, and would not require consumables such as chemical additives or membranes. “With desalination plants, you’re already pumping all the water, so why not settle there?” Varanasi says. “A lot of the capital costs associated with how you move water and permitting, that could all be taken care of.”

The system could also be implemented by ships that would process water along their journey to help mitigate shipping’s significant share of overall emissions. There are already international mandates to reduce emissions from shipping, and “this could help shipping companies offset some of their emissions and turn ships into ocean scrubbers,” says Varanasi.

The system could also be implemented in places such as offshore drilling rigs or aquaculture farms. Ultimately, this could lead to the deployment of free-standing carbon removal plants distributed around the world.

The process could be more efficient than air capture systems, Hatton says, because the concentration of carbon dioxide in seawater is more than 100 times higher than in air. In direct air capture systems, it is first necessary to capture and concentrate the gas before it is regenerated.

“But the oceans are big carbon sinks, so the capture step has kind of already been done for you,” he says. “There is no capture step, only release. This means that the volumes of material to be handled are much smaller, potentially simplifying the entire process and reducing footprint requirements.

Research is ongoing, with one goal being to find an alternative to the current step, which requires a vacuum to remove the separated carbon dioxide from the water. Another need is to identify operational strategies that prevent mineral precipitation that can clog the electrodes in the alkalization cell, an inherent problem that reduces overall efficiency in all reported approaches.

Hatton notes that significant progress has been made on these issues, but that it is still too early to report. The team expects the system could be ready for a practical demonstration project within about two years. “The carbon dioxide problem is the defining problem of our life, of our existence,” says Varanasi. “Clearly we need all the help we can get.

Read Now:<strong>India’s GAIL is exploring up to a 26% equity stake in US LNG projects</strong>

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