From waste to a solid mineral: a new technology that captures CO2 from the atmosphere and cleans up the planet

UW-Madison team looks to bring carbon capture to market

Researchers at UW-Madison have developed a way to pull carbon dioxide from the air and store the heat-trapping gas in solid particles that can be turned into building materials.

Now they are looking to scale their laboratory experiment into a business that can solve two of the biggest challenges to fighting climate change.

Known as Earth RepAIR, the project is built on concepts developed by Bu Wang, an assistant professor of civil and environmental engineering, and Rob Anex, a professor of biological systems.

The student-led team is among the finalists in a four-year, $100 million XPRIZE competition designed to spur development of carbon removal technologies. To be eligible for the grand prize the team must build a working plant that can capture 1,000 tons of carbon dioxide per year as well as model the costs at a scale of 1 million tons a year.

Anex said the team is developing a business plan and exploring partnerships with utilities and cement makers who could provide waste to store the carbon.

“There’s two products here,” Anex said. “One is the service of capturing CO2; the other is the carbonated residue, which replaces cement in one form or another.”

Last fall Earth RepAIR was awarded a $250,000 award in the XPRIZE student competition, which combined with support from the Wisconsin Alumni Research Foundation enabled the team to continue refining the technology.

Anex said a share of the $80 million final purse, to be awarded in 2025, would help turn the lab experiment into a viable business, though he stressed the prize itself is not the goal.

“Our goal is to capture carbon,” Anex said.

Earth RepAIR uses “direct air capture” to pull carbon dioxide from the atmosphere. While the chemistry is complicated, the process is simple: a fan blows air over a sodium hydroxide solution that absorbs carbon dioxide.

That solution is then mixed with a material like coal ash or cement kiln dust, where the carbon dioxide bonds with calcium to form calcium carbonate, commonly known as limestone. The reaction regenerates the hydroxide, which is cycled back to capture more carbon dioxide, and a carbonated powder that can be used in building materials.

A group of UW-Madison students have equipped a Little Free Library in a Downtown park with a solar-powered battery that people can use to charge their phones and other electronic devices.

Traditional direct air capture technology requires a lot of energy, which makes it hard to remove more carbon than it contributes, but Wang’s innovation allows for carbon molecules to be captured in ambient conditions, reducing energy consumption, emissions and costs.

“What this really is is a sponge that will take CO2 out of the air,” Anex said. “Ours is wonderful because carbon goes into the sponge and it stays there. Most people have a sponge and they have to wring it out to get the CO2 out.”

And there’s no need to concentrate the carbon dioxide.

“We can even do it with carbon dioxide that’s super impure, like air,” Wang said.

The carbonated material can be added to cement, though Anex said it could potentially be used as a cement substitute, which could yield even greater carbon savings since making Portland cement requires intense heat and releases carbon dioxide.

Carbon dioxide is released when fossil fuels and other organic materials are burned. Oceans and plants absorb much of the gas, but about 45% remains in the atmosphere, creating a sort of heat-trapping blanket.

That blanket has made the earth about 1.2 degrees Celsius warmer than pre-industrial times, and scientists warn it will get much warmer — with potentially catastrophic consequences — by the end of this century.

Replicated at scale, the reaction could be used to generate electricity without releasing pollutants or heat-trapping gases like carbon dioxide, providing a viable alternative to fossil fuels and helping achieve a carbon-neutral economy.

Limiting warming to 1.5 — or even 2 — degrees will require the elimination of nearly all greenhouse gas emissions by 2050. That means finding alternatives to coal, gas, petroleum and other fossil fuels as well as ways to take carbon out of the atmosphere, according to a recent report from the world’s top scientists.

“It doesn’t get us all the way there, but it’s helpful for sure,” said Greg Nemet, a UW-Madison professor and co-author of the report from the Intergovernmental Panel on Climate Change.

Anex said carbon capture technology will be needed in the near term to offset emissions from certain industrial processes, heavy transportation and aviation, which can’t be easily decarbonized, and in the long term to augment emissions reductions.

But Anex said the technology is not a “get out of jail free card” to continue burning coal and other fossil fuels to generate electricity.

“This is not a panacea,” he said. “We don’t want to become their excuse.”