The CO2 sponge
It may be possible to sop up some of the greenhouse gas problem directly, according to new research.
Capturing heat-trapping CO2 and storing it underground may help slow global warming. But plans so far have focused on scrubbing CO2 from power plant stacks, which account for only half of all emissions. Now, a few scientists hope to catch at least some of the other half—which puffs out of tailpipes, homes, and other nonpoint sources—directly from the air.
New research published in ES&T (DOI: 10.1021/es070874m) suggests that it may be feasible to suck CO2 out of the atmosphere using relatively straightforward technology, similar to that currently used by pulp and paper mills. Frank Zeman, an engineer at Columbia University, reports that industrial-scale scrubbers could pull CO2 from ambient air, and the gas then could be stored in the same ways proposed for CO2 from power plants (e.g., by pumping it deep underground or into the ocean). According to Zeman's calculations, the process would store more carbon than it burns, addressing a key challenge faced by air capture technology.
"Air capture of CO2 has come a long way in 5 years," Zeman says. Once purely theoretical, "it's moved to not only a design but a fully delineated technology," he adds. His process works by forcing air through a chamber filled with packing material that provides plenty of surface area for air to come into contact with sodium hydroxide solution, a highly alkaline liquid that absorbs CO2. The carbon-containing solution is then combined with lime to precipitate a powdery limestone. The limestone is heated in a kiln to release a pure stream of CO2, ready for storage.
The price tag: as much as $200 per metric ton of carbon captured, Zeman estimates. According to his collaborator, David Keith of the University of Calgary (Canada), the steep price could come down with technical improvements. The captured CO2 could also be used to make products such as transportation fuel, he adds. And the basic technology for air capture is used in "every pulp and paper mill in the world," he says. "If you wanted to, you could build a full-scale air capture system today and it would work."
"Nature does this every day, all the time," says earth scientist Greg Rau of Lawrence Livermore National Laboratory. "The ocean has consumed 30% of all industrial CO2 emissions ever made." Rau is developing his own methods for capturing CO2 from the air and from power plants.
For now, only a few groups are working seriously on air capture, but the numbers may grow. Billionaire Richard Branson recently announced the Virgin Earth Challenge, a $25 million prize for a practical design to reduce greenhouse gas levels. Zeman says he has no plans to enter the competition.
Zeman's former mentor, Klaus Lackner of Columbia University, has a technology that is possibly the closest to going commercial. Lackner did much of the early design work on air capture and now has a private company, Global Research Technologies, that has developed a prototype system. The ultimate plan involves carbon-sucking towers nearly 100 meters tall placed in remote locations where they won't be eyesores.
Air capture of CO2 is difficult at best, Zeman acknowledges. Ambient air contains only 380 parts per million of CO2, so removing it means moving a lot of air. A scrubber has to process about 250 times more ambient air than flue gas to capture the same amount of carbon. Detractors point to this problem as well as other practical limitations. "This is a patch solution in the sense that you would still have mercury from burning coal" and other issues related to mining and using fossil fuels, Zeman says.
"Does [air capture] condone the wasteful, SUV lifestyle? I don't know," Zeman admits. "The best solution is always to not make the problem in the first place." But cleaning up the mess that's been made requires a variety of tools, possibly including air capture, he contends.
