Locking Up CO2: Once carbon dioxide (CO2) has been stripped from smokestacks, engineers face a bigger problem: where to put it. A popular proposal is to pump it as a liquid into geologic formations underground (the depths would vary depending on location). Other ideas include adding it to a rock formation in Oman or spreading it onto the ocean floor. (Illustration by Headcase Design)
The 6-ft.-high steel wellhead jutting up from a weedy field next to the R.E. Burger coal-fired power plant in eastern Ohio doesn't look momentous. If anything, it resembles a supersize fire hydrant. Yet the wellhead and the borehole extending through a mile and a half of rock below it represent an ambitious goal: nothing short of entombing global warming.
Sponsored by the Department of Energy, the Electric Power Research Institute and a host of public and private partners, this experiment is one of several designed to bury carbon dioxide (CO2)—the climate-changing byproduct of burning fossil fuels—permanently, deep in the Earth. Called carbon sequestration, the process seems straightforward: Capture the gas, just as power plants today filter out pollutants such as soot or sulfur dioxide, then find places—underground, in the oceans or elsewhere—to dispose of it.
In practice, simply capturing CO2 is a daunting task, both energy-intensive and costly. But this pales beside the task of storing so much carbon. Bradley Jones, a vice president at the giant Texas utility TXU, has compared the dilemma to a small dog chasing a car. "Once he catches it, he's got to figure out what to do with it."
A single 1000-megawatt coal-fired power plant can send 6 million tons of CO2 up its stack annually—as much as two million cars. Hundreds of such plants around the world spew more than one-third of the 25 billion metric tons of CO2 humans pump into the atmosphere each year, with no sign of slowing. More than 100 new coal-burning power plants are on utility company drawing boards in the United States. China plans to commission about one new coal-burning plant every week for the next five years. In October, an international study based on a series of meetings of national academies of science concluded that coal emissions present the single greatest challenge in combating global warming.
Even compressed to a liquid, the amount of CO2 produced by a 1000-megawatt power plant over its 60-year lifetime is staggering: the equivalent of 3 billion barrels of oil. Underground storage for that much CO2 would be six times larger than what the oil industry calls a giant—a field with reserves of at least 500 million barrels. Multiply that by hundreds of power plants, and the sequestration challenge might seem overwhelming. According to Howard Herzog, a senior research engineer at the Massachusetts Institute of Technology, it would become a new global industry. "The amount of oil we consume in one day might be similar to the amount of CO2 we'll have to handle daily," he says. Scientists and engineers are laying the groundwork for carbon storage now, with proposals that range from proven technology to borderline fantastical.
Lock It in Saline VaultsLater this year, a prototype ammonia-based filtering device will begin capturing a fraction of the CO2 from the Burger power plant. The recovered gas, pressurized to a supercritical state, will flow 5000 ft. down the borehole into a vast formation of porous sandstone filled with brine.
In a small trailer a few feet from the wellhead, site manager Phil Jagucki, of Battelle Laboratories, points to a diagram of rock strata—and to a layer of dense and, project sponsors hope, impermeable rock that lies above the sandstone. "That's the cap rock, the containment layer that should keep the carbon where we put it," Jagucki says.
It should work: Similar formations entomb oil and natural gas for millions of years—or at least until drillers punch through. But are there enough geologic containers with tightly sealed lids to hold industry's CO2? A recent study estimated that deep saline formations in Pennsylvania could store 300 years' worth of emissions from the state's 79 coal-fired plants.
Since 1996, the Norwegian oil company Statoil has injected 10 million metric tons of CO2 into sandstone below the floor of the North Sea. Seismic time-lapse surveys show that, so far, a thick layer of shale has prevented the CO2 from migrating out. Statoil scientists say that this single saline formation is so large that, in theory, it could store all the carbon dioxide produced over the next several centuries by Europe's power plants.
Use It to Recover OilOn the windswept plains of North Dakota sits another example of carbon capture—and 205 miles to the north, a different version of carbon storage. Since 2002, the pure stream of carbon dioxide produced by the Great Plains Synfuels Plant—a byproduct of synthesizing natural gas from a soft brown coal called lignite—has been compressed by 20,000-hp engines and piped to the Canadian province of Saskatchewan. There, it is forced about a mile underground into a formerly depleted oil field, scouring out petroleum that otherwise wouldn't have been recovered and replacing it with the greenhouse gas. Over the next 20 years the energy company EnCana expects to increase the field's total yield by about half while storing some 20 million tons of carbon dioxide.
The process, known as enhanced oil recovery, isn't new: For years drillers have been injecting commercially produced carbon dioxide to recover oil. But this is the first project to use CO2 captured from waste.
To lock up harmful CO2, Norway’s Statoil injects the gas into an aquifer below the North Sea.
Use It to Recover GasNot every major emission source will be near a suitable oil field. So where else might the CO2 go? One promising answer: back from whence it came, into coal fields. Methane (natural gas) is tenaciously bound, or adsorbed, to the surface of coal. Some early experiments have shown that carbon dioxide gloms onto coal even more readily. When pumped into mines with unrecoverable seams, CO2 displaces the methane, which can then be brought to the surface and sold; the coal, meanwhile, locks up the carbon dioxide.
Turn It Into RockLate in 2008, scientists at the Pacific Northwest National Laboratory plan to begin experimentally injecting 1000 tons of CO2 into porous volcanic basalt in Washington state. Within two to three years, the lab's studies suggest, the carbon dioxide will begin a chemical transformation to a mineral.
First, some of the CO2 will react with water trapped in the basalt, forming weak carbonic acid. "Think of the acidity of orange juice," says laboratory fellow Peter McGrail. The acid should dissolve calcium in the basalt, which in turn will react with more CO2 to form calcium carbonate—in effect, limestone. According to McGrail, such basalt occurs worldwide, including under large expanses of India, with enormous storage potential.
Columbia University geologist Peter Kelemen has begun researching a way to use surface formations to seize CO2 straight from the atmosphere. About half of the Sultanate of Oman, a Kansas-size country on the Persian Gulf, is dominated by a rock typically found in the ocean crust. Called peridotite, it reacts with carbon dioxide and water to form carbonates.
Kelemen says that deposits of peridotite in Oman are so extensive—in fact, sufficient to store the excess CO2 now in the air many times over—it might be possible to accelerate this natural process and solidify significant amounts of the greenhouse gas.
Pipe It Into the OceanIn a more unlikely scenario, some scientists have proposed that large quantities of carbon dioxide could be stored on the bottom of the deepwater ocean, where high pressures would compress the gas into liquid form. Denser than seawater, the liquefied CO2 would pool on the seabed. But no one yet knows whether it would stay put or harm the marine environment.
In the nearer term, oil recovery and other established technologies offer the best odds. MIT's Herzog says he's concerned that policymakers are not moving more aggressively. Yet he remains confident that massive sequestration infrastructure can be built, whatever form it takes.
"Do you think," Herzog says, "when Henry Ford drove his first car down the road he envisioned the infrastructure that's arisen—millions of miles of highway, millions of gas stations, supertankers hauling oil around the world? We're not technologically ready to build all the infrastructure now, but we know enough to start."