'Rock Candy' Technique Offers Simpler, Less Costly Way To Capture Carbon Directly From Air

In this prototype carbon capture apparatus, a solution of potassium hydroxide is wicked up into polypropylene fibers; circulating air evaporates the water in the solution, concentrating it to very high levels. The white crystals are nearly pure potassium carbonate, formed from carbon removed directly from air. Credit: Dongha Kim / University of Toronto Engineering

University of Toronto Engineering researchers have discovered a new way of capturing carbon directly from the air—one that could offer significant cost savings over current methods.

The team calls their new technique evaporative carbonate crystallization. Because it is powered by passive processes such as capillary action and evaporation, it has the potential to eliminate some of the costliest steps required by existing carbon capture methods.

>> In Other News: Deep Sky and DACMA Join Forces to Develop End-to-end Carbon Removal Technology for At-Scale Deployment in Canada

"We've had the technology to capture carbon dioxide (CO2) from flue gases, or even directly from the air, for decades now," says Professor David Sinton, Interim Director of the University of Toronto's Lawson Climate Institute and senior author on a paper published in Nature Chemical Engineering that describes the new technique.

"There are even some full-scale plants in operation, but the criticism that the industry always gets—with justification—is that it's still just too expensive. So, we've oriented our team's approach around radical cost reductions, and that is what this new method of evaporative carbonate crystallization is all about."

Postdoctoral fellow Dongha Kim is the lead author on the new paper. He says that he was strongly motivated by a desire to simplify current state-of-the-art carbon capture systems.

"One way to capture carbon is to use a strongly alkaline liquid, for example, a solution of potassium hydroxide. When air makes contact with this liquid, the carbon dioxide in the air reacts to become dissolved potassium carbonate," says Kim.

"To speed up the reaction rate, you want to maximize the contact between the air and the liquid. In today's most advanced systems, this is done by increasing surface area: a thin layer of the liquid is flowed over a porous solid support material, with a honeycomb-shaped structure. Giant fans or blowers are used to push air across this thin liquid layer at about 1.5 meters per second."

Kim says that in many places in the world, prevailing winds are already faster than that: globally, the average is about 3 meters per second. This led him to think about ways to leverage those existing winds via a more passive system.

The design he came up with uses long strands of polypropylene fiber—essentially string. One end of the string is immersed in a solution of potassium hydroxide, which is slowly wicked up into the fibers.

When wind blows across the surface of the string, it evaporates the water in the solution, concentrating the dissolved potassium hydroxide to extremely high levels. That's where the advantages of this system come into play.

"Because we have a very thin layer of extremely concentrated potassium hydroxide, the rate at which it reacts with carbon dioxide speeds way up," says Kim.

"We can capture carbon at a much higher rate than with the more dilute solutions used in today's systems. On top of that, the potassium carbonate salt that we produce doesn't stay dissolved in solution—instead it forms a solid crystal right on the surface of the fibers."

The result looks a bit like rock candy, which can be made from highly concentrated sugar solutions via a similar evaporative process. The fact that the carbon is captured in this solid form leads to another advantage.

"In conventional systems, you need some way to remove the dissolved carbonate from the capture liquid so you can use it again," says Kim. "Typically, this is done by adding other chemicals, such as calcium, to create a non-soluble salt, which you then have to filter out.

"But because we have this highly concentrated solution generated by passive evaporation, we can go straight to the salt. We don't need to add calcium, and we don't need to filter it out; instead, we can just wash it off with water, producing a highly-concentrated potassium carbonate solution."

From here, an electrochemical process converts the potassium carbonate salts back into pure CO2 gas while simultaneously regenerating the potassium hydroxide, which can be reused. The CO2 gas can be stored, injected into underground wells or further processed into carbon-based fuels and chemicals such as methanol, ethanol, ethylene, etc.

In the paper, the team carried out a techno-economic analysis to evaluate how cost-competitive the new system might be if scaled up to industrial levels. They found that while the operating costs were similar to existing systems, the capital costs could be reduced by up to 40%.

"If you tour an industrial-scale carbon capture plant, the two biggest things you'll see are the air contactor, with the fans and blowers, and the chemical plant used to regenerate the capture liquid," says Sinton. "If you can eliminate both of those, you can save a lot of money."

There are still hurdles to be overcome. One is humidity: Kim says that the process is more efficient in dry air, rendering it more suitable for some environments than others. And more challenges may arise as the team works to build a pilot-scale plant to further validate the technology.

Still, the team feels that the current study demonstrates proof-of-concept, and that further refinements could continue to enhance its economic feasibility.

"It's hard to predict the ultimate cost, but what we do know for certain is that polypropylene fibers are already cheap and plentiful, and that passive processes are inherently simpler and less costly than active ones," says Sinton.

"Combine that with the scientific surprise, which is that our system creates a very thin layer of a super-concentrated solution that kicks the carbon-capture reaction into a higher gear, and it all adds up to a very promising approach."