As the world continues its slow shift to renewable energy, it would be great to limit the carbon dioxide produced from the fossil fuels we will burn in the meantime. Some researchers are working on capturing that CO2 from chimneys with as little energy as possible. Others are working on places to put it.
Deep, saline aquifers are an obvious choice. The concern there is the risk of leakage. Once we get the CO2 deep down in the earth we want it to stay there. Ultimately, the KO2 dissolved in those brines can precipitate out as carbonate minerals (which don’t go anywhere), but that takes quite a long time.
Brine is not the only option for storing captured CO2, although. There are also volcanic rocks that readily react with CO2, possibly speeding things up. In 2012, a pilot project to inject CO was started in Iceland2 in basalt – something the island nation has in abundance. An impressive result of this pilot is reported in a new paper published in Science.
A team led by researcher Juerg Matter from the University of Southampton was involved in the CarbFix project, located next to a geothermal power station outside Reykjavik. This plant actually taps a steam source above Iceland’s shallow magma chambers, but some volcanic CO2 and sulfur gas are added. The goal is to capture that gas and put it back underground. That was done with an injection well drilled into basalt rock, a material that was born as erupted lava.
Carbon dioxide is often compressed into a supercritical fluid for these injections, but in this project it was chosen to dissolve it in water instead. That requires a lot of water – about 25 tons of water for every ton of CO2– but the solution is not floating, so there is little risk of CO2 escape upwards. The researchers also added some chemical tags so they could track the fluid after it was injected between 400 and 800 meters below the surface. Those tags contain some CO2 high in carbon-14, and a few gases like sulfur hexafluoride that would just come along for the ride.
Monitoring wells at various depths and distances were regularly tested after the injections for signs of these chemical tags. The unreactive sulfur hexafluoride appeared on schedule, diluted as the injected water dispersed and mixed with groundwater. However, the carbon in those samples told a different story.
Very quickly, the amount of carbon (and carbon-14) dropped to less than 5 percent of what would be expected if all CO2 they injected was still there. This suggests that it must have been knocked down on the way. In fact, a pump in one of the wells stopped working because it was encased in precipitated calcium carbonate. The carbon-14 in that calcium carbonate showed that it indeed came from the injected CO2.
The surprise here is how quickly it all happened. The monitoring reported here covers less than two years. Lab experiments have shown that it should last decades for CO2 injected into basalt to mineralize. The researchers write: “The results of this study show that almost complete in situ CO22 mineralization in basaltic rocks can occur in less than 2 years. Once stored in carbonate minerals, the risk of leakage is eliminated and any storage site monitoring program can be significantly reduced, improving storage security and potentially public acceptance.
Following this early success, Reykjavik Energy, which operates the geothermal power plant, has ramped up injection in recent years. According to a press release from Columbia University, they will soon be injecting a quarter of the CO2 released by the plant. Because they don’t have to purify the gas they capture, the cost of the project is relatively small: about $30 per ton of CO2.
So is this a groundbreaking demonstration of carbon storage that can be replicated around the world? Not necessary. It’s not entirely clear what it is about the CarbFix site that enabled such rapid mineralization. It may be a combination of features of geology and groundwater chemistry, although the researchers believe their approach to solving the CO2 in water for injection played a role.
Dr. Charlotte Sullivan studies CO2 storage in basalt at the Pacific Northwest National Laboratory. She told Ars that slower mineralization rates are expected at other sites studied. Sullivan also described the trade-offs of the large volume of water used in the CarbFix project with the need to control fluid pressure underground. Just inject supercritical CO2 (to depths where it remains compressed) is not without advantages.
Even if CarbFix is an anomaly, getting these results is more encouraging than letting the trial program falter. But there are some major challenges to scaling this up: If the CarbFix site stored CO2 emissions from a single 300-megawatt coal-fired power plant, the injection rate should increase by a factor of 80. Reducing our greenhouse gas emissions with this technology takes some effort.
Science2016. DOI: 10.1126/science.aad8132 (About DOIs).