Giant carbon vacuums could cool Earth

Tall metal structures would scrub the greenhouse gas from the air.

For a decade, Columbia University physicist Klaus Lackner has written about a way to stave off – and even reverse – climate change from human-emitted carbon dioxide: Scrub it directly from the atmosphere. And now, after three years of R&D, a Tucson, Ariz., company has unveiled a working model of a device based on Professor Lackner's idea.

Nine-feet tall and able to remove 50 grams of CO2 from the atmosphere daily, the device is a far cry from Lackner's vision of a 300-foot-tall structure sucking 15,000 cars' worth of emissions from the atmosphere yearly. But it fulfills the basic criterion of removing more carbon than it emits.

"We've got the way," says Allen Wright, president of Global Research Technologies, LLC, the company that developed the contraption. "Now we have to get the will."

Important details such as where to store the captured CO2 have yet to be resolved. And the carbon-capping regulation that would make such a device profitable has yet to be implemented. But with predictions of a 5.7 degrees F. increase and changes in rainfall patterns by century's end, the potential benefits of a direct control over atmospheric CO2 are evident to all. Although far from cost-effective, the technology exists to capture CO2 at coal-fired electric plants. But nothing yet exists for mobile sources such as cars and planes that account for about one-third of emissions. Capturing carbon directly from the atmosphere, says Lackner, precludes the need for cumbersome – and impractical – storage devices on vehicles. In theory, this technology could both offset emissions from human activity and remove greenhouse gases accumulated since the industrial revolution.

And it could allow civilization to burn through the estimated 100 to 200 years' worth of coal reserves without disastrously changing the climate.

"Fossil fuels will run out not because of limited resources but because of the environmental impact," says Lackner. "If I can solve that impact, I have basically increased the resource base by a vast amount."

But Lackner's approach is not without its critics. Some surmise that the chemical processes involved in capturing CO2 directly from the air demand too much energy. Others think that, because it's likely decades away from functional deployment, the mere suggestion of such technology at this point is a distraction, and could divert resources away from more concrete steps. One scientist says that more can be done to harness photosynthesis, Mother Nature's carbon-capturing process, instead.

Although CO2 is relatively scarce in the atmosphere, only 380 molecules for every million, Lackner figures there's enough to make going after it feasible. He imagines it this way: Assuming a brisk breeze, one American's yearly share of emissions – 22 tons – would pass through a medium-size window. By extrapolating, he calculates that, in order to capture all of humanity's emissions, an area the size of Arizona would have to be planted with some 250,000 of his proposed devices. Each would capture 90,000 tons of CO2 yearly.

As for cost, in the long run, Lackner foresees a price of $30 per ton of CO2 captured, about 25 cents on the gallon at the pump. "There is actually something positive to be said with 'Look, you don't have to change anything; you just have to pay the bill,' " he says.

The major cost would be in the chemistry, says Lackner. He initially proposed sodium hydroxide, or lye, which, when wet, snatches CO2 from the atmosphere. (Global Research Technologies claims to have pioneered another method employing a compound much less caustic than lye, says company president Wright.) Separating the CO2 is the costly part, says Lackner, requiring temperatures of 900 degrees C. But this type of cost is not without precedent, he points out. For every unit of energy used at your home's electrical outlet, three have been spent at the plant, for example. The real issue, says Lackner, is not the energy consumed but the CO2 emitted. He estimates that for every ton of CO2 he captures, he'll generate another 0.4 ton. But because this process will take place at a plant, where emissions are concentrated relative to air, it will be easily captured.

Robert Williams, a research scientist at Princeton University in New Jersey, thinks Lackner's research is important for the long term, but short-term steps are still needed. And there are possibilities in using what is already available. Instead of burning only coal at electricity plants, for example, combust biomass as well. Fast-growing and easily cultivable plants like prairie or switch grasses could be used. They've already captured CO2 from the atmosphere, and by capturing the CO2 at the flue, you end up with net negative emissions. "Mother Nature knows how to take the carbon dioxide out of the atmosphere," he says. "We can put it underground."

But Gary Rochelle, professor of chemical engineering at the University of Texas, Austin, thinks projects like Lackner's are a distraction at a critical time. More effort should be made toward retrofitting older plants, he says. And more resources should be directed toward perfecting the technology of coal-gasification plants, which allows for easier CO2 capture. "There are other lower hanging fruit alternatives to deal with the carbon dioxide problem that we need to do now," he says.

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Where to stash the captured carbon?

Once removed from the atmosphere by mechanical "trees," the captured carbon dioxide would have to be stored. Suggestions include putting it into geologic formations or deep in the ocean. But C02 might leak out from a spent oil well through a fissure. Put into the ocean, it might acidify it.

Physicist Klaus Lackner favors storing the C02 as a mineral. Carbon dioxide reacts with certain rocks deep underground to form carbonates. "It's much more easily certified as safe and permanent," he says. But the natural process is far too slow. Technology may help speed it up someday.

That leaves one more option involving gravity and neutral buoyancy.

At about 3,000 meters depth in the ocean (nearly 10,000 feet), the intense pressure makes C02 denser than water, so it sinks. At certain places inthe ocean floor, magma is close enough to the surface that, within a few hundred meters, temperatures begin to rise. If injected in this deep ocean crust, C02 would be too dense to rise. And if it sank too far, it would expand from the magma's heat and rise, returning to its original place. "It can't go up, and it can't go down," says Professor Lackner. "We call that a gravitational trap."

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