New generation of carbon dioxide traps could make carbon capture practical

By Robert F. ServiceMar. 24, 2021 , 12:45 PM

Windmills and solar panels are proliferating fast, but not fast enough to stave off the worst of climate change. Doing so, U.N. climate experts say, will also require capturing carbon dioxide (CO₂) from the tens of thousands of fossil fuel power plants and industrial smokestacks likely to keep belching for years to come. Today’s most popular approach for capturing CO₂ is too expensive for widespread use. But researchers are now developing a new generation of chemical CO₂ traps, including one shown this month to reduce the cost by nearly 20%. When existing U.S. tax credits are added to the mix, carbon capture is nearing commercial viability, says Joan Brennecke, a carbon capture expert at the University of Texas, Austin.

Today’s technology uses CO₂-grabbing chemicals called amines, dissolved in water. The problem is that once the amines capture CO₂, the greenhouse gas must be stripped off and stored so the amines can be reused. Releasing the CO₂ requires boiling the water and later recondensing the water vapor, which requires a vast amount of energy and increases the cost. Enter new “water lean” capture materials, including one described in the latest report. “This is a beautiful, very complete study,” Brennecke says.

For decades, researchers have worked to find ways to capture carbon from industrial emissions and either use it to make chemicals or store it underground. Last year, companies captured some 40 million tons of CO₂ emissions, and the additional 30 carbon capture facilities planned worldwide could up that figure to 140 million tons—still minuscule compared with current annual global emissions of some 35 billion tons. For carbon capture efforts to be scaled up by orders of magnitude, the U.S. Department of Energy projects that by 2035, the cost needs to fall from roughly $58 per ton with state-of-the-art water-based amines to $30 per ton.

Typically, water that contains amines is sprayed into the top of an exhaust tower. As the droplets fall through the gas, they sop up CO₂. At the bottom of the tower, the CO₂-rich liquid gets pumped into a separate vessel and heated to boil off the water. Then, applied pressure causes water vapor to condense, leaving a pure stream of CO₂ to be captured and stored. The condensed water is added back to the amines and piped to the tower for another round of CO₂ capture.

A cheaper cleanup

In 2009, David Heldebrant, a chemist at the Pacific Northwest National Laboratory (PNNL), sought a new approach: “The goal was to get away from the water,” he says. Over the next decade, he and his team created a collection of liquid organic solvents, eventually settling on one containing C0₂-grabbing amine groups with no need for water or dissolved capture agents. Organic solvents can release CO₂ when heated but, unlike water, need not be boiled and recondensed, potentially saving energy.

It wasn’t an instant success. Heldebrant’s team found that when the solvent captured CO₂, carbon-rich solids precipitated out, making the liquid viscous and difficult to pump. A collaboration with a team led by Robert Perry, a chemist at GE Global Research, revealed that when the amines bound CO₂, hydrogen atoms on solvent molecules became attracted to neighboring molecules, tying them together. So, the researchers tweaked the structure of the solvent, creating a molecule called 2-EEMPA. When the new solvent bound CO₂, the hydrogen bonds were more likely to form within individual 2-EEMPA molecules, rather than between neighbors, they reported in November 2020 in Energy & Environmental Science.

Now, in the March issue of International Journal of Greenhouse Gas Control, the PNNL team, together with researchers at the Electric Power Research Institute and the engineering giant Fluor, have published a detailed analysis showing that a full-scale coal-fired power plant using 2-EEMPA would require 17% less energy than today’s state of the art carbon-capture systems. That would drop the cost of CO₂ capture to $47 per ton, not including the cost of transporting and pumping the CO₂ underground. “It’s a very promising solvent,” says Marty Lail, a carbon capture chemist at RTI International. Next year, the PNNL team plans to test 2-EEMPA at a small 0.5 megawatt coal-fired carbon capture testbed in Alabama.

Other researchers have made progress with their own solvents. In 2014, Brennecke and colleagues developed an ionic liquid-based CO₂ absorbent that has been projected to capture carbon at about the same cost as 2-EEMPA. And Lail and his colleagues have devised their own low-cost, proprietary water-lean solvent, which they will begin testing at a power plant in Norway early next year. Organic capture solvents “have a real future,” he says.

They could also get a boost from policymakers. The United States already offers companies a tax credit of $50 for each ton of CO₂ they capture and store underground. And last week, a bipartisan group in Congress introduced a bill that would provide $4.9 billion for carbon capture projects. Both environmentalists and fossil fuel backers support the legislation, a rare alignment in today’s divided political landscape.