An Ocean Trap for Carbon Dioxide
In an attempt to address global warming, a handful of power plants are capturing carbon dioxide during the energy-generation process, liquefying the gas under high pressure and piping it to geologic storage sites miles away. But sequestering carbon dioxide underground is impractical in many areas, and it raises fears that the stored gas will escape.
Now a new plant in Linden, NJ, will test an ocean carbon-sequestration technology that could expand its potential dramatically. If permits are approved, the plant, operated by SCS Energy, based in Concord, MA, will pump its carbon dioxide pollution into sandstone located almost two miles beneath the floor of the Atlantic Ocean.
Previous storage efforts have focused on filling underground structures such as depleted oil reservoirs, but these structures don’t contain enough volume to accommodate the vast amounts of CO2 produced. On the other hand, undersea storage has raised concern that carbon dioxide could slowly leak into ocean water.
Harvard University professor of Earth and Planetary Sciences Daniel Schrag addressed some of these concerns in a 2006 PNAS paper, in which he suggested storing carbon dioxide in porous sediment hundreds of meters below the sea floor in deep parts of the ocean. Stored at this depth, under higher pressure and temperatures, the carbon dioxide should be less buoyant and remain trapped indefinitely.
The two injection sites being surveyed for the new carbon-sequestration project are under about 100 meters of water, and about 2,500 to 3,500 meters down in the rock. “We are going deeper overall under the floor, but we aren’t working in a deep region of the sea,” says Schrag, who serves as a consultant to the project.
Pressure-management systems should make the process possible, Schrag adds. “It turns out pressure management is the most important part of this, and it’s much easier under the ocean,” he says.
Both on land and offshore, pumping carbon dioxide into sandstone usually displaces water, causing pressure to build up. “If you inject vast amounts of CO2, you have to make space,” says Schrag. “You push the water to the side, but it can’t go anywhere.” Injecting the CO2 too quickly, or adding more than the rock can hold, risks fracturing the sandstone, allowing the CO2 to slowly leech out over time.
Schrag argues that drilling an escape route for water that has remained trapped in the porous rock for millions of years will help ease the pressure on the rock. “This ancient seawater is very similar to modern seawater, so there should be no ecological impact from letting it out,” he says.
The release of seawater from the rock will raise sea levels over time, but not by much, says Schrag. The four million tons of CO2 produced by the plant each year will only cause about a micrometer rise in seawater over 100 years. Even if 1,000 coal plants began sequestering carbon offshore, sea levels should only rise by a millimeter during this time frame, Schrag says.
Dave Goldberg, a research scientist at Columbia University’s Lamont-Doherty Earth Observatory, agrees that the idea is sound but says that any pilot project should be carefully shepherded to make sure that there is no harm to the ecosystem. The ocean is so vast that injecting CO2 shouldn’t raise water levels much, either by lifting the floor or displacing trapped water, he says, but the bacterial ecology might potentially be changed. “Water slowly percolates through the rock at the bottom of the ocean in many places,” Goldberg says. “The impact of speeding this up and introducing new opportunities for water movement is an open research question.”
Carbon sequestration remains a controversial issue, however, and many environmental groups worry that it could allow coal plants to earn approval ahead of cleaner energy technologies. Another concern is how sequestration will impact ocean wildlife.
Whether carbon sequestration will become a reality will also be an issue of politics. Schrag was recently appointed to President Obama’s Council of Advisors on Science and Technology. Schrag says that carbon sequestration should be one of several ways to combat climate change. “We need it all,” he says. “We need renewables, we need better energy efficiency, we need energy conservation, and we need carbon sequestration.”
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