Environment

MIT team makes a case for direct carbon capture from seawater, not air

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MIT proposes mobile seawater carbon capture plants on boats
MIT
MIT proposes mobile seawater carbon capture plants on boats
MIT
The system could be integrated with any existing infrastructure that processes seawater, such as a desalination plant
MIT
Left: schematic of the device. Middle: optimizing the current density and electrode gap. Right: cost breakdown of the highly efficient electrochemical cell
MIT
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The oceans soak up enormous quantities of carbon dioxide, and MIT researchers say they've developed a way of releasing and capturing it that uses far less energy than direct air capture – with some other environmental benefits to boot.

Pulling greenhouse gases out of water is an odd-sounding idea, but the oceans are the planet's number one carbon sink, and direct air carbon capture has pretty serious problems: it costs a lot, and uses a lot of energy. According to IEA figures from 2022, even the more efficient air capture technologies require about 6.6 gigajoules of energy, or 1.83 megawatt-hours per ton of carbon dioxide captured.

Most of that energy isn't used to directly separate the CO2 from the air, it's in heat energy to keep the absorbers at operating temperatures, or electrical energy used to compress large amounts of air to the point where the capture operation can be done efficiently. But either way, the costs are out of control, with 2030 price estimates per ton ranging between US$300-$1,000. According to Statista, there's not a nation on Earth currently willing to tax carbon emitters even half of the lower estimate; first-placed Uruguay taxes it at US$137/ton. Direct air capture is not going to work as a business unless its costs come way down.

It turns out there's another option: seawater. As atmospheric carbon concentrations rise, carbon dioxide begins to dissolve into seawater. The ocean currently soaks up some 30-40% of all humanity's annual carbon emissions, and maintains a constant free exchange with the air. Suck the carbon out of the seawater, and it'll suck more out of the air to re-balance the concentrations. Best of all, the concentration of carbon dioxide in seawater is more than 100 times greater than in air.

Previous research teams have managed to release CO2 from seawater and capture it, but their methods have required expensive membranes and a constant supply of chemicals to keep the reactions going. MIT's team, on the other hand, has announced the successful testing of a system that uses neither, and requires vastly less energy than air capture methods.

Left: schematic of the device. Middle: optimizing the current density and electrode gap. Right: cost breakdown of the highly efficient electrochemical cell
MIT

In the new system, seawater is passed through two chambers. The first uses reactive electrodes to release protons into the seawater, which acidifies the water, turning dissolved inorganic bicarbonates into carbon dioxide gas, which bubbles out and is collected using a vacuum. Then the water's pushed through to a second set of cells with a reversed voltage, calling those protons back in and turning the acidic water back to alkaline before releasing it back into the sea. Periodically, when the active electrode is depleted of protons, the polarity of the voltage is reversed, and the same reaction continues with water flowing in the opposite direction.

In a new study published in the peer-reviewed journal Energy & Environmental Science, the team says its technique requires an energy input of 122 kJ/mol, equating by our math to 0.77 mWh per ton. And the team is confident it can do even better: "Though our base energy consumption of 122 kJ/mol-CO2 is a record-low," reads the study, "it may still be substantially decreased towards the thermodynamic limit of 32 kJ/mol-CO2."

The team projects an optimized cost around US$56 per ton of CO2 captured – although it's not fair to compare that directly against full-system direct air capture costs. The study cautions that this does not include vacuum degassing, filtration and "auxiliary costs outside of the electrochemical system" – analyses of which will have to be done separately. Some of these, however, could potentially be mitigated by integrating the carbon capture units in with other facilities, for example desalination plants, which are already processing large volumes of seawater.

The system could be integrated with any existing infrastructure that processes seawater, such as a desalination plant
MIT

There are some other benefits too; increased carbon buildup in the ocean over recent years has already caused problems with acidification, threatening coral reefs and shellfish. The alkaline output of this process, if directed where it's needed, could help redress the balance.

The team has a practical demonstration project planned for sometime in the next two years, and says there are plenty of things that still need work. For one, the researchers would love to be able to separate the gas out without a vacuum system. And mineral precipitates are fouling the electrodes on the alkalinization side, so there's plenty of progress yet to be made.

The study is open access in the journal Energy & Environmental Science.

Source: MIT

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15 comments
Steve Pretty
Coastal nuclear power plants draw and filter large volumes of sea water for cooling. After use it is warm (which should help free CO2?). Anyone looking at using this resource for sea water decarbonisation - and maybe seawater hydrogen production as covered earlier this week?
jzj
For my own edification, and now yours, I did some research, and here is the take-away. Oceans are more alkaline than acid. An acid ocean is bad for most sea life.
Carbon dioxide (CO2) hits the ocean, and forms unstable carbonic acid (H2CO3), which then forms either bicarbonate (HCO3) -- an acid -- or carbonate (CO3). Carbonates are good for sea life and good for long term carbon capture (little critters use carbonate for their shells and are either eaten (good for sea life) or die and fall to the ocean floor which is good for carbon storage). The more carbon dioxide in the atmosphere, the more bicarbonate builds up in the ocean, making the ocean more acid. Therefore, removing bicarbonate, as with this MIT process, is good.
Last thought about the MIT thing: how are they going to use renewable electricity on a ship to drive this process? Land-based carbon dioxide capture uses solar/wind/geothermal renewable energy, which seems an obvious necessity, given how much electricity is needed to drive the energy-intensive carbon capture process.
vince
Wonders through chemistry abound the senses. Sadly the chemistry mankind usually introduces into the world do more harm than good. Good to see a good process for a change.
michael_dowling
DAC is not dead yet. There is a proposal to spread basalt rock dust on farm fields world wide,which could soak up ~ 2 billion tons of CO2 yearly,with no addtional energy required. Farmers already have the equipment to do the spreading,and could be paid to spread the rock dust. https://www.sheffield.ac.uk/research/rock-dust Another scheme would see large ponds to grow fast growing algae which could also remove additional bllions of tons of CO2. https://www.yankodesign.com/2022/04/18/the-worlds-largest-algae-growth-pond-uses-nature-based-technology-to-capture-co2-emissions/
Metal Organic Kraftwerk
Sounds like this could be a good fit for offshore wind farms, they could turn off-peak energy into carbon removal, decreasing the acidity of surrounding seawater and improving the latent ability for their bases to act like artificial reefs.
Gregg Eshelman
The oceans aren't acidic. They're not "acidifying". The PH of seawater is slightly to the alkaline or basic side of neutral and that's barely budged in all the time it's been measured.
TechGazer
Instead of separating the CO2 and having to compress it or react it, wouldn't it be better to convert the carbonate or bicarbonate into a solid form? Lots of marine lifeforms learned that trick long ago, so it's not that difficult, and we have plenty of examples to learn from. I vaguely recall a story about putting wire mesh in the ocean, applying DC, and having calcium carbonate build up on it (making useful structures, such as artificial reefs). Would that leave the ocean more acidic? Could genetic engineering make shell-forming organisms better able to build and maintain their carbonate shells? Another thought: check the remains from ancient periods where ocean CO2 levels were very high, to see whether some organisms managed to adapt to grow/maintain their shells. That would show whether it's possible, and maybe even provide some DNA showing how it was done.

Industry certainly wants to make a profit selling CO2 capture/storage services, but it might be cheaper to reduce consumption. I expect that governments (taxpayers) are subsidizing CO2 production via energy consumption subsidies, and now plan to subsidize CO2 capture/storage. If instead we increased the cost of energy, people would be motivated to reduce consumption, and thus CO2 production. Check your electricity bill: how much of the bill is actual energy cost--maybe 10-20%? That doesn't encourage energy use reduction. It also doesn't encourage installing home grid-tied solar/wind generation, since you're still paying all those administrative and transmission costs/fees while being paid very little for your energy.
Slowburn Fan
This all seems so silly. Just plant some trees.
Slowburn Fan
Just plan some trees. When they get big, cut them down and bury them. And plant some more.
DavidB
I’m curious where all the “captured” carbon dioxide will be stored. I seem to have missed that part of the story.