Converting CO2 emissions into rock

SCIENTISTS have developed a method for converting waste CO₂ emissions into environmentally benign rock.

Measures to prevent CO₂ emissions from entering the atmosphere and sequestering the gas underground in voids, such as depleted oil and gas reservoirs, via CCS have fallen short due to the possibility of leakage.

Now an international team involving the University of Southampton, UK and funded by the European commission and the US Department of Energy has focussed on mineralising CO₂ in order to permanently dispose of it.

The team conducted experimented at a pilot deep well site in Iceland, where the volcanic rock is comprised of 90% basalt, which is rich in calcium, magnesium and iron – elements required for carbon mineralisation.

The CO₂ was dissolved in water to form carbonic acid and was carried down the well. When the solution came into contact with the target basalt rocks at a depth of 400–800 m, it reacted quickly with calcium and magnesium deposits to form carbonate minerals in the pores of the rock.

Until now, it was thought that this process would take several hundreds to thousands of years and would be impractical to implement. However the team demonstrated that the process can take place in as little as two years.

Juerg Matter, associate professor in geoengineering at the University of Southampton, said: “Our results show that between 95–98% of the injected CO₂ was mineralised over the period of less than two years, which is amazingly fast.”

The team monitored the experiment underground by tagging the CO₂ solution with a radioactive carbon marker. These compounds traced the path and reactivity of the CO₂, allowing the team to detect any signs of leakage back to the surface and through local waterways. No leaks were detected.

“Storing CO₂ as carbonate minerals significantly enhances storage security which should improve public acceptance of CCS as a climate change mitigation technology,” added Matter.

The next step for the team will be to scale up the so-called Carbfix project. Matter told the BBC that for this pilot, 220 t of CO₂ was injected underground at the Hellisheidi geothermal power plant, operated by Reykjavik Energy, where 5,000 t/y of captured CO₂ is stored in a basaltic reservoir.

CCS has been deemed a necessary process by climate change scientists, including Lord Oxburgh, if countries are to meet the 2°C warming target set out in the COP21 Paris agreement. However, it is too expensive to implement according to carbon producing industries, calling it “uneconomical”, without receiving subsidies.

Matter hopes this technique will help eventually cut the cost of CO₂ storage, as basalt rock is very common, but still dependant on location. Even in regions with high concentrations of basalt, the infrastructure required to pump the CO₂ into the rock would require significant capital.

Science, DOI: 10.1126/science.aad8132