With mounting concern over climate change, scientists around the world are looking for ways to keep carbon dioxide out of the atmosphere.
A team of geologists and engineers at Washington University is testing ways to trap carbon dioxide beneath the Earth’s surface. The process, known as carbon sequestration, involves injecting carbon dioxide deep underground. Over time, the gas reacts with the surrounding rock and water and becomes rock itself.
It may sound like science fiction, but the technique isn’t new. For more than 20 years, researchers have experimented with pumping carbon dioxide into sandstone aquifers.
Since 2011, researchers have injected roughly 1 million tons of carbon dioxide into the Mount Simon Sandstone formation near Decatur, Illinois.
Sandstone is naturally porous, which allows carbon dioxide to permeate its crevices. The problem, said Daniel Giammar, is that the carbon dioxide can take thousands of years to turn into rock when injected into sandstone.
“In sandstone systems, the carbon dioxide stored there remains as essentially this bubble that's trapped beneath a roof,” said Giammar, a professor of environmental engineering at Washington University.
That big carbon dioxide bubble is buoyant, meaning it can slip through cracks in the surface rock, re-entering the atmosphere. In large quantities, the gas can also contaminate shallow groundwater by increasing the acidity of water supplies and causing lead and other heavy metals to leach from surrounding rock.
As an alternative to sandstone, Giammar and his colleagues at Wash U are experimenting with injecting carbon dioxide into a type of volcanic rock known as basalt.
Scientists have long known that basalt, which is rich in calcium and iron, reacts with carbon dioxide, spurring its transformation from a gas into limestone rock. But until now, it has been unclear how quickly that process happens or how much gas is actually converted to rock.
To help answer these questions, Giammar set up in his lab a series of environmental chambers meant to simulate conditions underground. The soda can-sized chambers reach temperatures topping 200 degrees Fahrenheit, with pressures of about 1,500 pounds per square inch.
Each pressurized chamber was an experiment in miniature, containing a smooth gray cylinder of basalt rock, water and carbon dioxide gas. Using 3-D imaging, the team tracked how quickly the carbon dioxide turned into rock.
As it turned out, the transformation was fairly quick. In six weeks, the gas began turning into limestone. The faster the carbon dioxide turns into rock, Giammar said, the sooner it’s trapped underground permanently.
“It’s not going to leak; it’s not going to pose any environmental hazards,” he said. “You could walk away from that site.”
Before you can inject carbon dioxide underground, however, you have to capture it. Because the gas is relatively diluted in the atmosphere, trying to suck it out of the air is a bit like pulling needles from a haystack. Most options focus on so-called “post-combustion” capture, which collects exhaust gas rich in carbon dioxide from coal-burning power plants.
Based on a number of large-scale experiments around the world, Giammar said carbon sequestration is ready to be implemented as a strategy for addressing climate change.
“The scientific questions about its effectiveness have been answered,” he said. “I think the questions are going to be ones of economics and policy.”
But he also pointed out that carbon sequestration is not a silver bullet.
“Most of the studies that look at how are we going to reduce CO2 emissions, it is really a multipronged approach,” he said. “It won't just be carbon sequestration; it won't just be renewables; it won't just be energy efficiency. We need to do a whole range of approaches.”
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