By Jon Luoma
Typical olivine-rich peridotite, cut by a centimeter-thick layer of greenish-black pyroxenite.
The problem of what to do with carbon dioxide once it's been captured has long perplexed scientists pursuing carbon sequestration. Do we store it in the ocean? In salt formations underground? And how long will those solutions work? A new study suggests that huge volumes of the key greenhouse gas could be converted into inert rock—and stored safely forever.
As PM reported in July, Columbia University geologist Peter Kelemen has been studying peridotite, a highly-reactive rock that covers about half the landscape of Oman, and appears at scattered locations worldwide. The rock naturally reacts with carbon dioxide (CO2), removing it from the air to form limestone and other carbonates.
In a study published November 11 in the Proceedings of the National Academy of Sciences, Kelemen and Columbia geochemist Jurg Matter suggest the natural process of removing CO2 from the air could be accelerated 100,000 fold, enough to make a significant dent in global warming. They calculate that Oman's peridotite alone could sequester 4 billion tons of CO2 per year, one-eighth of the 30 billion tons of CO2 humans emit annually. The researchers suggest that CO2 captured from power plants and other sources could be pumped down boreholes into peridotite. Using fracturing technology borrowed from the petroleum industry to shatter the rock and expose more of its surface area, CO2 would seep into the peridotite hundreds of feet below the ground. Heat would be added initially to accelerate chemical reactions. But as new carbonate rock begins forming, the process could start feeding on itself, with new carbonate rock continually fracturing the host rock further, and the heat from the reaction supplementing the deep-Earth's heat.
"It's a little like setting a coal seam on fire," Kelemen says.
The two scientists also offered a second scenario that Kelemen calls "even more intriguing." The alternative method would remove CO2 directly from the air and transfer it to boreholes drilled into peridotite formations in shallow water just off the Oman coast. Surface seawater naturally sponges up carbon dioxide until it reaches chemical equilibrium; a saturation point. In this scenario, seawater would be pumped deep into one borehole. Heated naturally by the Earth to about 185 C, it would release its CO2 again to form carbonate rocks. Rising to the surface via a second, paired hole, the seawater could then sop up more CO2, continuing a cycle that, once started, might be self-sustained by simple convection.
If it worked, the second method would require far more extensive fields of boreholes because of the limited ability of seawater to take up CO2. But it would also eliminate both the complexity and cost of capturing pure CO2 at the source, and of transporting it. "The air," says Kelemen, "transports CO2 for free."
There are other major advantages to what Keleman terms air capture." "Not only don't we have to capture the CO2 at places like power plants," he says, "there's a substantial portion of CO2 that comes from places where we wouldn't have any hope of capturing it—CO2 emitted by cars, for example." Kelemen notes that there appear to be few ways to accelerate the rate of carbon transformation in this second option. Even if it were possible to pump more seawater through the boreholes, doing so would be self-defeating. "If you pumped at an intensified rate, you'll just cool the rocks down," he says.
Yet he notes that "with enough holes" this approach alone might still be able to capture a large portion of the atmosphere's excess greenhouse gas. Effective air-capture technology could also mean that peridotite formations in shallow seas elsewhere, including remote New Caledonia and Papua New Guinea, could come into play.
Kelemen cautions, however, that the team has only begun its work on the seawater option, and that data are far more preliminary than for the more developed land-based scenario. In the short term, a land-based system in Oman could be fed pure CO2 captured from power plants and refineries across the Middle East and fed down a pipeline, one that might eventually be extended to the Balkan states or beyond.
Although some peridotite formations lie off the coast of California, options for using the technology to help directly control CO2 emissions in the U.S. are limited. No matter, says Kelemen. "The problem is global. We'll need lots of approaches. I don't think it's wise to even be looking for one, big golden fix."
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Typical olivine-rich peridotite, cut by a centimeter-thick layer of greenish-black pyroxenite.The problem of what to do with carbon dioxide once it's been captured has long perplexed scientists pursuing carbon sequestration. Do we store it in the ocean? In salt formations underground? And how long will those solutions work? A new study suggests that huge volumes of the key greenhouse gas could be converted into inert rock—and stored safely forever.
As PM reported in July, Columbia University geologist Peter Kelemen has been studying peridotite, a highly-reactive rock that covers about half the landscape of Oman, and appears at scattered locations worldwide. The rock naturally reacts with carbon dioxide (CO2), removing it from the air to form limestone and other carbonates.
In a study published November 11 in the Proceedings of the National Academy of Sciences, Kelemen and Columbia geochemist Jurg Matter suggest the natural process of removing CO2 from the air could be accelerated 100,000 fold, enough to make a significant dent in global warming. They calculate that Oman's peridotite alone could sequester 4 billion tons of CO2 per year, one-eighth of the 30 billion tons of CO2 humans emit annually. The researchers suggest that CO2 captured from power plants and other sources could be pumped down boreholes into peridotite. Using fracturing technology borrowed from the petroleum industry to shatter the rock and expose more of its surface area, CO2 would seep into the peridotite hundreds of feet below the ground. Heat would be added initially to accelerate chemical reactions. But as new carbonate rock begins forming, the process could start feeding on itself, with new carbonate rock continually fracturing the host rock further, and the heat from the reaction supplementing the deep-Earth's heat.
"It's a little like setting a coal seam on fire," Kelemen says.
The two scientists also offered a second scenario that Kelemen calls "even more intriguing." The alternative method would remove CO2 directly from the air and transfer it to boreholes drilled into peridotite formations in shallow water just off the Oman coast. Surface seawater naturally sponges up carbon dioxide until it reaches chemical equilibrium; a saturation point. In this scenario, seawater would be pumped deep into one borehole. Heated naturally by the Earth to about 185 C, it would release its CO2 again to form carbonate rocks. Rising to the surface via a second, paired hole, the seawater could then sop up more CO2, continuing a cycle that, once started, might be self-sustained by simple convection.
If it worked, the second method would require far more extensive fields of boreholes because of the limited ability of seawater to take up CO2. But it would also eliminate both the complexity and cost of capturing pure CO2 at the source, and of transporting it. "The air," says Kelemen, "transports CO2 for free."
There are other major advantages to what Keleman terms air capture." "Not only don't we have to capture the CO2 at places like power plants," he says, "there's a substantial portion of CO2 that comes from places where we wouldn't have any hope of capturing it—CO2 emitted by cars, for example." Kelemen notes that there appear to be few ways to accelerate the rate of carbon transformation in this second option. Even if it were possible to pump more seawater through the boreholes, doing so would be self-defeating. "If you pumped at an intensified rate, you'll just cool the rocks down," he says.
Yet he notes that "with enough holes" this approach alone might still be able to capture a large portion of the atmosphere's excess greenhouse gas. Effective air-capture technology could also mean that peridotite formations in shallow seas elsewhere, including remote New Caledonia and Papua New Guinea, could come into play.
Kelemen cautions, however, that the team has only begun its work on the seawater option, and that data are far more preliminary than for the more developed land-based scenario. In the short term, a land-based system in Oman could be fed pure CO2 captured from power plants and refineries across the Middle East and fed down a pipeline, one that might eventually be extended to the Balkan states or beyond.
Although some peridotite formations lie off the coast of California, options for using the technology to help directly control CO2 emissions in the U.S. are limited. No matter, says Kelemen. "The problem is global. We'll need lots of approaches. I don't think it's wise to even be looking for one, big golden fix."
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