by Removing greenhouse gases from the water is a strange idea, but the oceans are the planet’s main carbon sink, and capturing carbon directly from the air has very serious problems: it costs a lot and uses a lot of energy. According to 2022 IEA figures, even the most efficient air capture technologies require about 6.6 gigajoules of energy, or 1.83 megawatt hours per ton of captured carbon dioxide.
Most of this energy is not used to directly separate CO2 from the air, it is either thermal 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 with ease. efficiency . But either way, costs are out of control, with price estimates per tonne for 2030 ranging between $300 and $1,000. According to Statista, there isn’t a nation on Earth currently willing to tax carbon emitters even at half the lowest estimate; The first place, Uruguay, rates it at US$ 137/t. Direct air capture will not work as a business unless your costs go down.
It turns out there is another option: sea water. As atmospheric carbon concentrations increase, carbon dioxide begins to dissolve in seawater. The ocean currently absorbs about 30-40% of all of humanity’s annual carbon emissions and maintains a constant free exchange with the air. Suck carbon out of seawater and it will suck more out of the air to rebalance the concentrations. Best of all, the concentration of carbon dioxide in seawater is more than 100 times higher than in the air.
Previous research teams managed to release CO2 from seawater and capture it, but their methods required expensive membranes and a constant supply of chemicals to keep the reactions going. The MIT team, on the other hand, announced the successful testing of a system that uses neither and requires far less energy than air capture methods.
MIT
In the new system, sea water passes through two chambers. The first uses reactive electrodes to release protons into seawater, which acidifies the water, transforming dissolved inorganic bicarbonates into carbon dioxide, which bubbles and collects in a vacuum. Then the water is pushed into a second set of cells with reverse voltage, calling those protons back and turning the water acidic into alkaline before releasing it back into the sea. Periodically, when the active electrode runs out 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 and Environmental Science, the team says their 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: “Although our baseline energy consumption of 122 kJ/mol-CO2 is a record”, says the study, “it can still be substantially reduced towards the thermodynamic limit of 32 kJ/mol-CO2 mol-CO2.”
The team projects an optimized cost of around $56 per tonne of CO2 captured – although it’s not fair to compare this directly to the direct costs of capturing air from the complete system. The study warns that this does not include vacuum degassing, filtration and “ancillary costs outside the electrochemical system” – which analyzes will have to be done separately. Some of these, however, could be mitigated by integrating the carbon capture units with other facilities, for example desalination plants, which are already processing large volumes of seawater.
MIT
There are some other benefits as well; Increased carbon buildup in the ocean in recent years has already caused acidification problems, threatening coral reefs and shellfish. The alkaline output from this process, if directed where it is needed, can help restore balance.
The team has a hands-on demo project planned for the next two years, and says there’s still a lot of work to be done. On the one hand, researchers would love to be able to separate the gas without a vacuum system. And mineral precipitates are fouling the electrodes on the alkalizing side, so there’s still a lot of progress to be made.
The study is open access in the journal Energy & Environmental Sciences.
Source: MIT
