CAMBRIDGE, MA -- Capturing carbon dioxide from industrial plants is an important strategy in the efforts to reduce the impact of global climate change. It's used in many industries, including the production of petrochemicals, cement, and fertilizers.
MIT chemical engineers have now discovered a simple way to make carbon capture more efficient and affordable, by adding a common chemical compound to capture solutions. The innovation could cut costs significantly and enable the technology to run on waste heat or even sunlight, instead of energy-intensive heating.
Their new approach uses a chemical called tris — short for tris(hydroxymethyl)aminomethane — to stabilize the pH of the solution used to capture CO2, allowing the system to absorb more of the gas at relatively low temperature. The system can release CO2 at just 60 degrees Celsius (140 degrees Fahrenheit) — a dramatic improvement over conventional methods, which require temperatures exceeding 120 C to release captured carbon.
"It's something that could be implemented almost immediately in fairly standard types of equipment," says T. Alan Hatton, the Ralph Landau Professor of Chemical Engineering Practice at MIT and the senior author of the study.
Youhong (Nancy) Guo, a recent MIT postdoc who is now an assistant professor of applied physical sciences at the University of North Carolina at Chapel Hill, is the lead author of the paper , which appears today in Nature Chemical Engineering.
More efficient capture
Using current technologies, around 0.1 percent of global carbon emissions is captured and either stored underground or converted into other products.
The most widely used carbon-capture method involves running waste gases through a solution that contains chemical compounds called amines. These solutions have a high pH, which allows them to absorb CO2, an acidic gas. In addition to traditional amines, basic compounds called carbonates, which are inexpensive and readily available, can also capture acidic CO2 gas. However, as CO2 is absorbed, the pH of the solution drops quickly, limiting the CO2 uptake capacity.
The most energy-intensive step comes once the CO2 is absorbed, because both amine and carbonate solutions must be heated to above 120 C to release the captured carbon. This regeneration step consumes enormous amounts of energy.
To make carbon capture by carbonates more efficient, the MIT team added tris into a potassium carbonate solution. This chemical, commonly used in lab experiments and found in some cosmetics and the Covid-19 mRNA vaccines, acts as a pH buffer — a solution that helps prevent the pH from changing.
When added to a carbonate solution, positively charged tris balances the negative charge of the bicarbonate ions formed when CO2 is absorbed. This stabilizes the pH, allowing the solution to absorb triple the amount of CO2.
As another advantage, tris is highly sensitive to temperature changes. When the solution full of CO2 is heated just slightly, to about 60 C, tris quickly releases protons, causing the pH to drop and the captured CO2 to bubble out.
"At room temperature, the solution can absorb more CO2, and with mild heating it can release the CO2. There is an instant pH change when we heat up the solution a little bit," Guo says.
A simple swap
To demonstrate their approach, the researchers built a continuous-flow reactor for carbon capture. First, gases containing CO2 are bubbled through a reservoir containing carbonate and tris, which absorbs the CO2. That solution then is pumped into a CO2 regeneration module, which is heated to about 60 C to release a pure stream of CO2.
Once the CO2 is released, the carbonate solution is cooled and returned to the reservoir for another round of CO2 absorption and regeneration.
Because the system can operate at relatively low temperatures, there is more flexibility in where the energy could come from, such as solar panels, electricity, or waste heat already generated by industrial plants.
Swapping in carbonate-tris solutions to replace conventional amines should be straightforward for industrial facilities, the researchers say. "One of the nice things about this is its simplicity, in terms of overall design. It's a drop-in approach that allows you to readily change over from one kind of solution to another," Hatton says.
When carbon is captured from industrial plants, some of it can be diverted into the manufacture of other useful products, but most of it will likely end up being stored in underground geological formations, Hatton says.
"You can only use a small fraction of the captured CO2 for producing chemicals before you saturate the market," he says.
Guo is now exploring whether other additives could make the carbon capture process even more efficient by speeding up CO2 absorption rates.
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The authors acknowledge Eni S.p.A. for the fruitful discussions under the MIT–Eni research framework agreement.
