An experiment in Iceland has shown how carbon dioxide emissions can be trapped deep underground and converted into a solid mineral faster than previously thought. Locking CO2 underground—by combining the pollutant with water and injecting it into volcanic rock—could help combat climate change by keeping the primary greenhouse gas out of the air, scientists say.
In the initial experiment near a geothermal power plant outside Reykjavík, researchers injected 175 tons of carbon dioxide dissolved in water into a basaltic rock formation. It didn't take long for the mix to interact chemically with surrounding rock, forming the mineral carbonate. In a second experiment, scientists dissolved 73 tons of CO2 and hydrogen sulfide in water and injected the solution into the same area, with the same effect.
Both carbon dioxide and hydrogen sulfide are unwelcome byproducts of the geothermal plant as it draws heat from deep below ground. In both experiments, about 95 percent of the gases transformed, or mineralized, into carbonate in about two years, according to a study published Thursday in the journal Science.
Carbon capture and storage (CCS) has long been hoped for as a technological tool in battling climate change. By returning the CO2 to the ground when fossil fuels are burned, it reduces the greenhouse effect in the atmosphere. Many experts view CCS as critical to reaching the international goal of keeping global warming well below 2 degrees Celsius by bringing net emissions to zero as rapidly as possible. But first CCS needs to be proven workable at large scale, both safely and cost-effectively.
This study confirmed what lab experiments and modeling have long predicted, and what the preliminary results from this project and a similar 2013 experiment in the U.S. had indicated: basalt rock, a type of volcanic rock abundant on land and under the ocean, can permanently store CO2. The question was how fast the process could happen.
"That was a big surprise," said lead study author Juerg Matter, who had expected the mineralization process to take years longer. Matter and others remain unsure exactly why the mineralization occurred so quickly, but they agree it's promising in terms of using this technology as a potential climate solution.
Matter is an associate professor in geoengineering at the University of Southampton in England. He collaborated with 17 other researchers from the energy industry, universities and other research institutions in the United States and Europe.
The inspiration for carbon storage came from nature, where the carbon mineralization process happens over thousands and hundreds of thousands of years. The Icelandic study aimed "to mimic and speed up" the process, said Matter.
The Iceland project, called the CarbFix pilot, marked the first time scientists tested the potential of storing CO2 in basaltic rock in the field at this scale. Going in, researchers did not know how much the mineralization process could be accelerated. Previous studies and modeling estimates varied widely, from hundreds to thousands of years on the long end to five to 10 years on the short end.
Based on previous experiments, the researchers predicted the mineralization process would be faster if CO2 was dissolved into water and then injected into the basalt, compared to injecting the CO2 alone. From January to March 2012, they injected water and CO2 down parallel pipes into the same well. Around 800 feet deep, the pipes converged and the result was like a soda stream: the CO2 bubbled and fizzed as it merged with the water. Hundreds of feet farther down, the CO2-and-water mix entered the basaltic formation.
Researchers conducted another round of injections from June to August 2013, this time using a mix of gases from the nearby power plant, CO2 and hydrogen sulfide, to see if the CO2 would mineralize at the same rate as when it was injected alone. It did. This was good news for the geothermal plant, which was planning to use this technology to manage its emissions and leaving the gases mixed is less expensive.
Researchers confirmed that mineralization was occurring by running chemical analyses on water samples collected from a monitoring well located downstream of the injection site. Freshly formed carbonate also clogged parts of the monitoring well intermittently during the experiment, providing further evidence the CO2 was transforming.
More than a year later and thousands of miles away, a similar project launched in July 2013 in Wallula, Washington involving the U.S. Department of Energy's National Energy Technology Lab. This experiment successfully stored about 1,000 tons of supercritical CO2—CO2 in a state in which it can act like both a gas and liquid—in basalt. Mineralization started within a short timeframe, but researchers did not track the exact rate, according to NETL researcher Grant Bromhal. The study results will likely be published this year.
The Iceland project's success prompted Reykjavik Energy to launch a slightly larger operation, injecting at least 5,000 tons of CO2 and hydrogen sulfide underground; Matter and his colleagues are tracking its progress. Eventually, the power company aims to handle all of its emissions (40,000 tons of CO2 annually) this way.
"It's still small but in the right direction," said Dennis Kent, a Rutgers University geology professor not involved in the project. Operations would need to be scaled up significantly to handle the much larger emissions coming from gas- and coal-fired power plants, he said, and they could only be applied to plants near basalt formations. "It's hard to fathom how it could be done," he said.
Matter is less concerned about scaling up than he is about the lack of policies surrounding carbon capture and storage technologies. "It's not really a technological engineering question anymore," he said. "The big push should come from policymakers...because the engineers are ready."