Carbon dioxide tucked into basalt converts to rock
Pacific Northwest National Laboratory scientists have turned carbon dioxide to rock deep underground at Wallula, demonstrating a way to permanently keep the greenhouse gas out of the atmosphere.
Conventional wisdom said it would take thousands of years for the carbon dioxide — the predominant gas implicated in climate change — to transform into a solid rock similar to limestone.
In fact, it took just two years.
PNNL laboratory fellow Pete McGrail was not surprised.
More than a decade ago, McGrail and fellow scientist Todd Schaef took small samples of Mid-Columbia basalt and exposed it to carbon dioxide and water under pressure in the lab.
They were interested in seeing if the carbon dioxide would react with the minerals in the basalt to form limestone-like crystals over long periods of time.
When they cracked open the first pressure vessel after a matter of weeks, they were surprised to find substantial crystallization already forming.
But what works on the laboratory bench does not always translate to a larger scale in real world conditions.
With $12 million from DOE and private industry, McGrail led a pilot test at the Boise Inc. paper mill southeast of the Tri-Cities at Wallula.
The mill sits above basalt formed from lava flows millions of years ago.
Nearly 1,000 tons of carbon dioxide dissolved in water was injected half a mile deep into the ground at the mill in 2013. Dozens of lava flows over the years had left layers of basalt, some of it pockmarked with holes like a sponge.
The carbon dioxide made its way through the holes, absorbing water and reacting with the elements in the basalt — calcium, iron, magnesium and manganese.
“What happens when you inject CO2 in that system with a little bit of water, the system becomes slightly acidic and those minerals don’t like that,” McGrail said.
The minerals dissolve and react with carbon dioxide to turn to rock.
Early results were promising. Fluid samples collected from deep underground after the injection showed increasing concentrations of the dissolved minerals that would lead to rock formation, McGrail said.
After two years, core samples of the rocks were drilled, showing a limestone-like carbonate mineral formed from the carbon dioxide was filling in holes in the basalt.
“We proved it. We did it. It was exactly what we expected from the lab experiments,” McGrail said.
Meeting the targets of the Paris Climate Agreement to reduce global carbon emissions is expected to require carbon capture and storage, according to the American Chemical Society. Results of the pilot test were reported in the society’s journal, Environmental Science & Technology Letters.
During the pilot study, a relatively small amount of carbon dioxide was injected. It was equal to the amount a typical coal-fired power plant emits in a few hours, according to the Big Sky Carbon Sequestration Partnerships.
But the United States and portions of Canada have enough potential capacity in geologic formations that they would not run out of space for carbon dioxide storage for 5,700 years, according to recent estimates by the Department of Energy. India and China, two countries with increasing energy use, also have basalt formations
The core samples were analyzed atom-by-atom with scientific instruments at the Environmental Molecular Science Laboratory on the PNNL campus in Richland to confirm results before they were announced.
Research is continuing.
Scientists at the DOE lab are interested in better understanding the volume of rock formed, which has been difficult to assess from the limited area where core samples were collected.
Questions also remain to be answered about scaling up the sequestration of carbon dioxide in underground basalt to the commercial level.