Despite over a dozen direct air capture facilities currently operational globally, significant technological challenges remain, particularly regarding the high energy consumption required.
A study published on May 1 in the journal ACS Energy Letters by researchers from the University of Colorado Boulder and their collaborators highlights that a popular method aimed at reducing these energy costs is likely impractical. The research team, which includes scientists from the National Renewable Energy Laboratory in Golden, Colorado, and Delft University of Technology in the Netherlands, also suggests a more sustainable design for capturing CO2 and converting it to fuel.
"Ideally, we want to take CO2 out of the air and keep it out of the air," said Hussain Almajed, a Ph.D. student in the Department of Chemical and Biological Engineering and the study's lead author. "However, some of this CO2 can be recycled into useful carbon-containing products, which is why researchers have proposed different ideas of how we can achieve that. Some of these ideas look very simple and elegant on paper, but researchers rarely check whether they are practical and economical in industrial settings."
CO2 Capture Techniques
One common direct air capture technique involves using air contactors, which are large fans that draw air into a chamber containing a basic liquid. The acidic CO2 reacts with the solution, forming harmless carbonate (used in concrete) or bicarbonate (used in baking soda).
The Stratos facility, one of the largest direct air capture plants being built in Texas, utilizes this method.
Once CO2 is absorbed by the carbonate or bicarbonate solutions, it must be extracted from the liquid so the liquid can be reused. The captured carbon can then be converted into products such as plastics, carbonated beverages, and potentially even fuel for homes and aircraft with further processing.
However, to release the captured CO2, the solution must be heated to at least 900 C (1,652 F), a temperature that solar and wind energy cannot achieve. This process is usually powered by burning fossil fuels like natural gas or methane.
"If we have to release CO2 in order to capture CO2, it defeats the whole purpose of carbon capture," said Wilson Smith, a professor in the Department of Chemical and Biological Engineering and a fellow of the Renewable and Sustainable Energy Institute at CU Boulder.
Seeking Sustainable Solutions
Researchers are exploring alternatives. One idea, known as reactive capture, involves applying electricity to the carbonate and bicarbonate solutions to separate the CO2 from the liquid, allowing the liquid to capture more CO2 and creating a closed-loop system.
"Reactive capture is now the buzzword in the field, and researchers proposed that it could help save energy and costs associated with carbon capture. But no one really assessed whether that's realistic under industrial conditions," Almajed said.
To evaluate this, the team calculated the mass and energy outputs of the reactive capture units. They discovered that, in an industrial setting, electricity would not be sufficient to regenerate the liquid for further CO2 capture.
After five cycles, the liquid's ability to capture CO2 from the air significantly diminished.
The researchers proposed enhancing the reactive capture process with a step called electrodialysis, which splits additional water into acidic and basic ions, thus maintaining the liquid's capacity to absorb CO2. Electrodialysis can be powered by renewable electricity, offering a potentially sustainable method for converting captured CO2 into useful products.
More crucially, electrodialysis can produce CO2 gas that can be used to reinforce concrete.
"To me, turning CO2 into rocks has to be one of the leading solutions to keep it out of the air over long periods of time," Smith said. Concrete production is energy-intensive and contributes to 8% of global carbon emissions.
"This is solving multiple problems with one technology," he said.
Emission Reduction is Key
According to the Intergovernmental Panel on Climate Change (IPCC), carbon dioxide removal is essential for achieving net zero CO2 and greenhouse gas emissions globally and nationally.
More than 20 direct air capture plants are currently operational worldwide, with 130 more under construction.
However, Smith emphasizes that while carbon capture has its place, reducing emissions is still the most vital step to prevent the worst effects of climate change.
"Imagining Earth as a bathtub, with the running water from the faucet being CO2. The bathtub is getting full and becoming unlivable. Now, we have two options. We can use a little cup to scoop out the water, cup by cup, or we can turn the faucet off," Smith said.
"Cutting emissions has to be the priority."
Research Report:Closing the Loop: Unexamined Performance Trade-Offs of Integrating Direct Air Capture with (Bi)carbonate Electrolysis
Related Links
University of Colorado at Boulder
Carbon Worlds - where graphite, diamond, amorphous, fullerenes meet
| Subscribe Free To Our Daily Newsletters |
| Subscribe Free To Our Daily Newsletters |
