In 2015, nearly 200 countries agreed: they would reduce their emissions of carbon dioxide and other greenhouse gases to limit warming of the earth’s atmosphere to well below 2 degrees Celsius.

The Paris Agreement actually aims for 1.5 degrees above pre-industrial levels to avoid potential catastrophic changes to our climate. But it’s become increasingly clear to climate scientists and policymakers that just reducing emissions is not enough.

“We now know that we probably should have stopped putting massive amounts of CO₂ in the air 10, 20, 30 years ago to prevent the climate from getting above 2 degrees C,” said Chris Jones, a chemical engineer at Georgia Tech. “Now we've waited so long to reduce our emissions that we need to develop technologies that are referred to as negative emissions technologies that remove CO₂ from the atmosphere.”

Jones was one of a handful of scientists who co-authored a landmark National Academies report in 2018 that outlined a variety of approaches to negative emissions. Agricultural practices and forest management are options — essentially using nature’s ability to grab carbon dioxide out of the air and lock it away in plants and soil. But Jones said we’ll need quicker and more direct approaches.

“We could plant billions of trees to do this, but there's not enough available land. And the trees don't grow fast enough for us to do this quickly enough to slow global warming at the rate required,” said Jones, John F. Brock III School Chair in the School of Chemical and Biomolecular Engineering (ChBE). “That's where direct air capture comes in: It's a chemical engineering way of designing a process that takes CO₂ out of the air.”

Direct air capture is a bit like a massive household air purifier — but for the globe. Systems would pull air across specially designed filter materials with molecules that grab CO₂. When the filters are saturated, they’re cleaned, and the carbon dioxide is pumped underground for storage in the very places we’ve extracted oil and natural gas over the decades.

It’s a technology proposed only in 1999, with companies launched in 2008, and is now quickly becoming a reality, according to another Georgia Tech chemical engineer, Matthew Realff.

Just Like in Real Estate, Location Matters

Jones works at the first step of the carbon-capture process: the molecules that link with CO₂ to pull it out of the air. One of the areas he’s become most interested in is customizing different materials for different locations and climates.

“A molecule or a material that we studied five years ago maybe failed operating at 80 degrees Fahrenheit. That would not work in Atlanta or in Florida, but maybe it works really well if we go to 50 degrees F or 20 degrees F, and we can deploy it in Montana,” Jones said. “We're starting to think more about whether we might have advantaged materials or advantaged processes in particular locations.”

“One of the key cost drivers is, how long can you make these things last?” Lively said. “We know what the answer should be: If you can get them to where they largely maintain their performance for a year to a year and a half, then you're in good shape. We're just not sure if we can get there yet.”

Creating fibers for use in a canister, filter, or other device has become a fairly mature technology, Lively said, so the team is working now to move some of their materials into the commercial market. He’s also experimenting with 3D printing approaches that can create more complex structures with the materials — though at a cost.

“The nice part about 3D printing is you can get really complex structures that you can't get with fibers. But the manufacturing process is fundamentally slower,” Lively said. “The complexity of the device is going to have to make up for the slower rate at which you can make it, because the slower rate will mean higher cost.”

When A Double-Negative is a Positive

Another emerging idea would pair direct air capture systems with natural gas combined-cycle power plants. These plants use gas to turn a turbine and generate electricity. The waste heat from that process is then used to create steam that turns another turbine and generates more power.

Realff, Jones, and Lively have partnered with ChBE’s Fani Boukouvala and Joe Scott on a project funded by the Advanced Research Projects Agency-Energy to instead use that steam to do conventional post-combustion carbon capture and to power a direct air capture system.

“With the combination of those two systems, we can get natural gas combined-cycle plants that can operate and be net negative in their carbon dioxide,” Realff said.

In fact, he said, the reduction in carbon from such a system would be enough to wipe out the carbon emissions of another power plant operating at full capacity.

“For power companies, it gives you a way to get to essentially zero carbon by modifying, say, roughly half your fleet of natural gas combined-cycle plants.”

Jones pointed to solar energy as a useful analogy for getting to a future with widespread removal of atmospheric carbon. Initially, the technology was promising but too expensive to be practical when it debuted in the late 1970s and early ‘80s. Now, it’s cheap and common. The difference with direct air capture is the timeline: We can’t afford to wait 40 years to scale.

“Today, direct air capture from the initial startup companies costs between, say, $600 and $1,000 a ton. The target, in order to have it be widespread deployed all over the world, is about $100 a ton,” he said. “We can't have a five- or 10-year technology testing period; it's got to be a year or 18 months so that we can build it, operate it, learn from it, revise it, build it, operate it, learn from it. We need to do that five or 10 times to get the cost down.”