Getting greenhouse gases out of water sounds like a strange idea, but the oceans are the largest carbon sinks on Earth, and capturing carbon dioxide directly from the air is a pretty serious problem. It is expensive and consumes a lot of energy. Even more efficient air capture technologies require about 6.6 gigajoules, or 1.83 megawatt hours, per tonne of carbon dioxide captured, according to 2022 IEA figures.
Most of that energy is not used to separate the CO2 directly from the air, but rather is used as thermal energy to keep the absorber at operating temperature, or to efficiently carry out large volumes of air recovery operations. Used as electrical energy used to compress to a point where it can. But either way, costs are out of control, with estimated prices in 2030 between $300 and $1,000 per tonne. According to Statista, no country on Earth is currently willing to tax carbon emitters by even half of their low estimate. Uruguay, number one, imposes a tax of $137 per tonne. Direct air capture will not work as a business unless costs drop significantly.
It turns out there is another option: sea water. As the concentration of carbon in the atmosphere rises, carbon dioxide begins to dissolve in seawater. The ocean currently absorbs about 30-40% of all humankind’s annual carbon emissions and maintains a constant free exchange with the atmosphere. Sucking carbon out of sea water sucks more out of the air, rebalancing the concentrations. Best of all, the concentration of carbon dioxide in seawater is over 100 times higher than in air.
Previous research teams have successfully released and captured CO2 from seawater, but that method required expensive membranes and a constant supply of chemicals to keep the reaction going. Meanwhile, a team at MIT announced that it has successfully tested a system that uses neither and requires far less energy than the air recovery method.
and
In the new system seawater passes through two chambers. The first uses reactive electrodes to release protons into seawater, acidifying the water, turning dissolved inorganic bicarbonates into carbon dioxide gas, bubbled, and collected using a vacuum. The water is then pushed into a second cell with a reverse voltage, recalling protons and turning the acidic water back to alkaline before releasing it into the ocean. 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 a peer-reviewed journal Energy and Environmental ScienceAccording to the team, the technology requires an energy input of 122 kJ/mol, which corresponds to 0.77 mWh/ton by our calculations. And the team is confident it will do even better: “Although the fundamental energy consumption of 122 kJ/mol-CO2 is a record low, we are heading towards the thermodynamic limit of 32 kJ/mol. mol-CO2”.
The team projects an optimized cost of about US$56 per tonne of CO2 captured. Note that in this study, this does not include vacuum degassing, filtration, and “ancillary costs other than the electrochemical system.” These analyzes should be done separately. However, some of these could potentially be mitigated by integrating carbon capture units with other facilities, such as desalination plants that already process large volumes of seawater.
and
There are some other advantages as well. Increased carbon accumulation in the ocean over the last few years has already caused acidification problems, threatening coral reefs and shellfish. The alkaline output of this process can help correct balance if directed where it is needed.
The team plans a working demonstration project within the next two years, and says there is still a lot of work to be done. For one thing, researchers hope to be able to separate gases without a vacuum system. Also, mineral deposits foul the electrodes on the alkalizing side, so there’s still a lot of progress to be made.
This study is open access in journal Energy and Environmental Science.
Source: MIT
