One if by sea, two if by sky: the search for affordable CO2 capture technology

August 30, 2021 |

In a companion piece, “Where will we put all the CO2?” we looked at the potential storage solutions from geologic storage to limestone.

Here’s the math. According to a chart from the US Department of Energy, we need to bring 10 billion gigatons of carbon dioxide from the atmosphere. That’s enough, by volume, to fill 5 million Empire State Buildings, or 2 million Great Pyramids.

That’s why we focus on covering solids that utilize carbon dioxide as a component compound. Hence, limestone, which contains various iterations of calcium carbonate, is made utilizing CO2, and sequesters roughly 1.3 tons of CO2 per cubic meter. That’s 8,000 Empire State Buildings or 3,000 Great Pyramids. Still a lot, but much more feasible.

Ultimately the problem is less a debate over gas vs solids — but rather, in the case of limestone, where the 56 percent of the tonnage that comes from calcium oxide is going to come from. After all, a primary source of calcium oxide today is limestone itself, and setting up a limestone cycle where we break up limestone into CO2 and calcium oxide, is a very interesting thing but sequesters no CO2, it’s merely CO2-neutral.

So, a UCLA team has come up with an interesting angle. Why extract the CO2 from the sky if you can extract it from the sea?

Goes the theory, if you pull CO2 from the sea, the oceans will pull down the CO2 from the sky to maintain equilibrium. And, there’s plenty of calcium dissolved in the oceans. 

So far, so good.

The researchers outline their concept, dubbed single-step carbon sequestration and storage, or sCS2, in a paper published in the journal ACS Sustainable Chemistry & Engineering.

The proposed technology would incorporate a flow reactor — a system that continuously is fed raw materials and yields products. The seawater would flow through a mesh that allows an electrical charge to pass into the water, rendering it alkaline. This kicks off a set of chemical reactions that ultimately combines dissolved carbon dioxide with calcium and magnesium native to seawater, producing limestone and magnesite by a process similar to how seashells form. The seawater that flows out would then be depleted of dissolved carbon dioxide and ready to take up more. A co-product of the reaction, besides minerals, is hydrogen, which is a clean fuel.

“What’s nice about turning carbon dioxide into a rock is, it’s not going anywhere,” said Sant, who is a member of the California NanoSystems Institute at UCLA.

“Durable, safe and permanent storage is the premise of our solution,” added first author Erika Callagon La Plante, a former UCLA assistant project scientist who is currently an assistant professor at the University of Texas at Arlington.

“To mitigate climate change, we need to remove carbon dioxide from the atmosphere at a level between 10 billion and 20 billion metric tons per year,” said senior author Gaurav Sant, director of the UCLA Institute for Carbon Management and a Samueli Fellow and professor of civil and environmental engineering and of materials science and engineering at the UCLA Samueli School of Engineering. “To fulfill a solution at that scale, we’ve got to draw inspiration from nature.”

Affordability looks good, though not great

The researchers note:

The analysis yields an “all-in” gross capital cost on the order of $500 per tonne of CO2 mineralized and operational expenditure (OpEx, i.e., including energy costs and fixed operations and maintenance costs) on the order of $83 per tonne of CO2 mineralized. This yields a levelized cost of CO2 abatement $145 per tonne of CO2 mineralized.

Why sequestering carbon the other way is tough

Unsurprisingly, given the enormous magnitude of the carbon challenge, the researchers estimate that it would take nearly 1,800 sCS2 plants to immobilize 10 billion metric tons of carbon dioxide each year, with a cost in the trillions of dollars.

The name includes “single-step” to differentiate it from other concepts that require carbon dioxide from the atmosphere to undergo a multistep concentration process before it can be stored. And while some plans propose storing captured carbon dioxide in geological formations such as depleted natural oil and gas reservoirs, there is a risk of leaks returning that carbon dioxide into the atmosphere. By contrast, sCS2 is meant to durably store carbon dioxide in the form of solid minerals.

It’s not the “how” of climate change mitigation, it’s the “how do we afford it?”

We should be clear: Managing and mitigating carbon dioxide is foremost an economic challenge,” Sant said. “Many of today’s approaches for carbon management either require more clean energy than we can produce or are unaffordable. As such, we need to create solutions that are accessible and that will not impoverish the world. We have tried to use a lens of pragmatism to consider how we may be able to achieve synthetic interventions at an unprecedented scale, while considering the finite energy and financial resources we have.”

More on the team

Other co-authors of the study are UCLA’s Dante Simonetti, an assistant professor of chemical and biomolecular engineering; Jingbo Wang, a postdoctoral researcher; Abdulaziz Alturki, a Ph.D. graduate who is now an assistant professor at King Abdulaziz University in Saudi Arabia; Xin Chen, an associate development engineer; and David Jassby, an associate professor of civil and environmental engineering.

Print Friendly, PDF & Email

Category: Top Stories

Thank you for visting the Digest.