Deep in the solidified lava beneath Iceland, scientists have managed an unprecedented feat: They've taken carbon dioxide released by a power plant and turned it into rock at a rate much faster than laboratory tests predicted.
The findings, described in the journal Science, demonstrate a powerful method of carbon storage that could reduce some of the human-caused greenhouse gas emissions contributing to climate change.
"These are really exciting results," said Roger Aines, a geochemist at Lawrence Livermore National Laboratory who was not involved in the study. "Nobody had ever actually done a large-scale experiment like they've done, under the conditions that they did it."
The pilot program, performed at Reykjavik Energy's geothermal power plant under a European-U.S. program called CarbFix, was able to turn more than 95% of carbon dioxide injected into the earth into chalky rock within just two years.
"We were surprised," said study co-author Martin Stute, a hydrologist at Columbia University in New York. "We didn't expect this. We thought this would be a project that would go on for decades. Maybe 20 years from now, we'd have an answer to the question. But that it happened so fast, and in such a brief period of time, that just blew us away."
When fossil fuels like coal or gas are burned, the carbon stored within them is released into the air in the form of carbon dioxide. This greenhouse gas traps heat in the atmosphere, triggering a spike in global temperatures that threatens polar ice reserves and contributes to rising sea levels. It also increases the acidity of the ocean, hastening the decline of corals and other marine life.
Researchers have tried for years to figure out how to get that carbon back into the ground. Carbon dioxide can be pulled out of emissions and injected underground into briny waters or emptied oil and gas reservoirs, but there's a risk that the gas eventually would seep back into the air or that the injection process itself might crack open a reservoir and allow its contents to escape.
Researchers have been looking to get that carbon back into the ground in solid form — something that nature's been doing for a while, although on a far longer timescale. For humans trying to quickly undo the damage of greenhouse gas emissions, that's easier said than done. Sandstone does not react much with carbon dioxide. Some lab tests showed that basaltic rock, laid down by volcanic activity, might be more effective but on a scale of centuries, if not longer.
An opportunity for a field test arose when the president of Iceland, Olafur Ragnar Grimsson, met researchers at Columbia and expressed his interest in cutting back the country's carbon dioxide emissions.
"This is really the start of this, at the highest level, which is sort of unusual for research projects," Stute said.
Together with Reykjavik Energy, the research team designed an experiment around the Hellisheidi geothermal power plant. In March 2012, they injected 175 tons of pure carbon dioxide into an injection well. A few months later, they followed with 73 tons of a mix of carbon dioxide and hydrogen sulfide. (The team wanted to see whether the process worked even if there were other gases present; if it did, it would save the time and money of having to separate the carbon dioxide out.)
The researchers separate the carbon dioxide from the steam produced by the plant and send it to an injection well. The carbon dioxide gets pumped down a pipe that's actually inside another pipe filled with water from a nearby lake. Hundreds of feet below the ground, the carbon dioxide is released into the water, where the pressure is so high that it quickly dissolves, instead of bubbling up and out.
That mix of water and dissolved carbon dioxide, which becomes very acidic, gets sent deeper into a layer of basaltic rock, where it starts leaching out minerals like calcium, magnesium and iron. The components in the mixture eventually recombine and begin to mineralize into carbonate rocks.
The basaltic rock is key, the scientists said: Sandstone would not react with carbon dioxide this way. So is the presence of water; if the mix had been pure gas instead of gas dissolved in water, it's unlikely the basalt would have helped form carbonate rocks — at least, not with such speed.
The scientists also injected chemical tracers into the mix, including a type of carbon dioxide made with the heavier, rarer isotope known as carbon-14. They also injected other trace gases such as sulfur hexafluoride, which is inert and does not react much with its surroundings.
When the researchers checked the water at monitoring wells later in the experiment, they found that the trace gases were still there (a sign that the water had gotten through) but that the proportion of carbon-14 molecules had significantly declined. As the water had continued to flow through the basaltic layers, the carbon dioxide had been left behind, in the rock.
While much of this happened underground, the researchers also saw fine crystals of carbonate sticking to the surface of the pump and pipes at the monitoring well.
"They look like salt from a salt shaker ... on the surface of this gray or black basaltic rock," Stute said.
Based on other laboratory results, the scientists had expected the process to take centuries, if not longer. But the field test showed that this process, under the right conditions, happens at remarkable speed.
There are some limitations to this method. It requires basaltic rock, which, while it can be found in abundance in places like the United States' Pacific Northwest region, can't be found everywhere on land. Under the ocean, there's plenty — but then there's the question of whether salty water will be as effective as the freshwater used in this study.
Storage is an issue. Pulling carbon dioxide out of emissions, let alone the atmosphere, is also a difficult challenge, the researchers pointed out.
Still, Aines said, "These results are so encouraging that it's worth figuring out some of the places where that could be done, and trying that out on a larger and longer scale."
In the meantime, at the Icelandic power plant, operators are reportedly looking to scale this process up.
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