The research assesses the feasibility of converting captured carbon into fuel

The struggle to reduce emissions is real.

Last year, the world emitted more than 37 billion metric tons of carbon dioxide, setting a new record high. As a result, absorbing CO2 from the atmosphere has become an increasingly popular idea. Governments around the world are using this technology, called direct air capture, to help them meet climate goals and avoid the worst consequences of climate change.

But despite the fact that more than a dozen direct air capture facilities are already up and running around the globe, the technology still faces major technological hurdles, including its high energy use.

In a study published May 1 in the journal ACS Energy Letters, researchers at the University of Colorado Boulder and collaborators found that a popular approach that many engineers are exploring to reduce those energy costs would, in reality, fail. The team, including scientists at the National Renewable Energy Laboratory in Golden, Colorado, and Delft University of Technology in the Netherlands, also proposed an alternative, more sustainable design for capturing CO2 and converting it to fuel.

“Ideally, we want to get CO2 out of the air and keep it out,” said first author Hussain Almajed, a Ph.D. student in the Department of Chemical and Biological Engineering. “However, some of this CO2 can be recycled into useful carbon-containing products, so researchers have proposed various ideas about how we can achieve this. Some of these ideas look very simple and elegant on paper, but researchers rarely check whether they are practical and economical in industrial settings.”

Shut off the gas

One of the most common approaches to direct air capture is the use of air contactors, essentially large fans that draw air into a chamber filled with a base fluid. CO2 is acidic, so it naturally binds and reacts with the solution to form harmless carbonate (main ingredient in concrete) or bicarbonate (ingredient in baking soda).

Stratos, one of the world’s largest direct air capture facilities under construction in Texas, uses this approach.

Once the CO2 is trapped in the carbonate or bicarbonate solutions, engineers must separate it from the liquid so that the liquid can be returned to the chamber to capture more CO2.

Meanwhile, the captured carbon can be turned into things like plastics, sodas, and even—with further processing—fuel to power homes and potentially airplanes.

But there is a catch. To release trapped CO2, companies must heat the carbonate and bicarbonate solution to at least 900˚C (1,652°F), a temperature that solar and wind power are unable to reach. This step is usually enabled by burning fossil-based fuels such as natural gas or pure methane.

“If we have to release CO2 to capture CO2, it defeats the whole purpose of capturing carbon,” said Wilson Smith, a professor in the Department of Chemical and Biological Engineering and a member of the Renewable and Sustainable Energy Institute at CU Boulder. .

Close the loop

Researchers are actively seeking answers. One idea, commonly known as reactive capture, is to apply electricity to carbonate and bicarbonate solutions, separating the CO2 and base liquor in the chamber. In theory, the recycled liquid can then capture more CO2, forming a closed system.

“Reactive capture is now the buzzword in the field, and researchers have proposed that it can help save energy and costs associated with carbon capture. But no one has really assessed whether this is realistic under industrial conditions,” Almajed said.

To do this, the team calculated the mass and energy output of the reactive capture units, based on the given inputs, to understand how well the overall system would perform. They found that in an industrial setting, electricity would not be able to regenerate the base fluid to re-capture more CO2 from the air.

In fact, after five cycles of carbon capture and regeneration, the base fluid could barely pull any CO2 from the air.

The team also suggested a change to the reactive capture process by adding a step called electrodialysis. The process splits the extra water into acidic and basic ions, helping to preserve the base fluid’s ability to absorb more CO2. Electrodialysis can run on renewable electricity, making it a potentially sustainable way to turn captured CO2 into useful products.

More importantly, electrodialysis can release CO2 gas, which engineers can use to strengthen concrete.

“To me, turning CO2 into rocks has to be one of the main solutions to keeping it out of the air for long periods of time,” Smith said. Concrete production is energy intensive and is responsible for 8% of global carbon emissions.

“It’s solving multiple problems with one technology,” he said.

The root of the problem

According to the Intergovernmental Panel on Climate Change (IPCC), a team of scientists convened by the United Nations, removing carbon dioxide “is required to achieve global and national targets for net CO2 and greenhouse gas emissions.”

Worldwide, more than 20 direct air capture plants are in operation with another 130 currently under construction.

But Smith points out that while carbon capture may have its place, reducing emissions is still the most critical step needed to avoid the worst outcomes of climate change.

“Imagine the Earth as a bathtub, with the water coming out of the faucet being CO2. The bathtub is filling up and becoming unlivable. Now, we have two options. We can use a small cup to remove the water, cup by cup, or we we can turn off the faucet,” Smith said.

“Reducing emissions must be the priority.”