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This article is from the In-Depth Report New Solutions for Clean Energy

Smokestash Industry: ARPA-E Seeks Breakthroughs in Carbon Capture Technology

Humans can capture and release CO2 efficiently, so why can't power plants?
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© David Biello

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WASHINGTON—Every second, our bodies capture carbon dioxide in our tissues, transport it via the blood, and dump it in the lungs from where it is exhaled. This unconscious process is yet another way humans contribute to the accumulation of the greenhouse gas in the atmosphere—albeit in a minuscule volume compared with burning fossil fuels. The key to this metabolic process is an enzyme called carbonic anhydrase and it's efficiency at capturing and releasing CO2 is what human engineers want to mimic at the power plant scale.

Research engineer Harry Cordatos and his colleagues at United Technologies Corp. (UTC) are working on just such a system—and have garnered funding from the U.S. Department of Energy's new ARPA-E program. After all, UTC subsidiary Hamilton Sundstrand has been making CO2 capture units for the space program since the 1960s with different technology. But carbonic anhydrase "captures 600 molecules every second," Cordatos said at the ARPA-E summit in Washington, D.C., last week. "To take this enzyme out of the body is challenging. Our bodies continuously regenerate the enzyme because it degrades."

So Cordatos and UTC's idea is not to use the enzyme itself, but to master its chemistry and "use it in the unnatural environment of power plant flue gas," Cordatos said. The key appears to be a single zinc atom that sits at the core of the enzyme, which resembles a pyramid in structure. That structure allows the carbonic anhydrase to grip the CO2 "not too loose and not too tight," Cordatos explained, which is critical for efficiently capturing and then releasing the greenhouse gas.

UTC is not alone in this pursuit. In the ARPA-E program alone, four of the 37 funded developing technologies concerned researching more energy-efficient ways to capture the CO2 in a fossil fuel–fired power plant's flue gas. Chemist David Moore at Lehigh University in Bethlehem, Pa., employs electricity itself to charge absorbent materials for CO2 capture. Nalco Co., is developing an "electrochemical platform" to do the job. And physicist Olgica Bakajin of Hayward, Calif.–based Porifera, Inc., plans to use membranes composed of carbon nanotubes to separate CO2 from the other gases—using carbon to capture carbon.

"We need to develop the technologies that enable us to use our fossil fuels in a clean way," Secretary of Energy Steven Chu told ScientificAmerican.com at the conference. "This is something you don't solve in five years, 10 years. It will take a half century to get our carbon emissions down to where we need to go to protect the climate." At the same time, the U.S. will require a steady supply of electricity which, today, means coal burning or nuclear power, Chu said.

Some companies, however, are working on turning a fossil fuel–fired power plant's CO2 emissions into something useful, like cement. Calera bubbles the flue gas from the Moss Landing power plant on the California coast through seawater to produce an aggregate from carbonate, the same mineral sea creatures use to build their shells, perhaps more familiar as chalk. "They are producing hundreds if not thousands of tons today," said venture capitalist Vinod Khosla at the ARPA-E summit, an early investor in the technology. "It's a way to capture carbon at zero price because it's a product you can sell. It turns CO2 from a problem to a feed stock."

UTC, for its part, is developing a membrane system based on the chemistry of carbonic anhydrase that, although not zero cost, saves 30 percent of the energy that would be needed to capture carbon with amines or other known chemistries. "Liquid amines react rather slowly with CO2—sluggish kinetics," Cordatos noted.

Perhaps that's why Southern Co., has announced it will no longer fund a proposed project to use such amines to capture CO2 at a coal-fired power plant in Alabama, despite securing $295 million from the federal government to do so (or roughly 100 times what UTC received). "It is expensive," Doug May, vice president of energy and climate change at The Dow Chemical Co., which develops such amine-based technology, told ScientificAmerican.com at the conference. "It's not where it needs to be."

Adds Timothy Brown, a spokesman for France-based Alstom Power, which developed this amine-based capture technology for power plants. "We believe Congress should pass comprehensive climate legislation to establish the certainty needed to maintain the growing momentum for this technology, given its proven ability to mitigate CO2 emissions and create jobs."

Nor has it proved easy to find storage for the CO2 once captured. Although injection into deep rock formations has proceeded without difficulty at a pilot project at the Mountaineer coal-fired power plant in West Virginia—3,000 metric tons have been stored so far, says Gary Spitznogle of American Electric Power, which owns the facility—public opposition halted a similar government-funded project in Ohio.

Nevertheless, either carbon capture and storage will have to be developed and deployed or low-emission electricity sources, such as wind, solar and nuclear power, will have to grow significantly. "Eighty percent [of human greenhouse gas emissions] come from coal, oil and natural gas, which provide 80 percent of the world's energy supply," observed White House Office of Science and Technology Policy advisor John Holdren during remarks at the ARPA-E summit. "That tells you about how much of the energy system needs to be transformed."

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