Rocks to soak up carbon dioxide

Research on a type of rock found mainly in Oman suggests it could be used to mop up billions of tons of carbon dioxide each year without having to be mined, according to US scientists.

Peter Kelemen and J?rg Matter, at Columbia University, US, say that peridotite rock (made mostly of the silicate minerals olivine and pyroxene) reacts naturally with CO₂ to lock it away in the form of carbonate much faster than expected, based on ¹⁴C dating studies. By artificially accelerating this reaction with heat and forcing CO₂ down into the rock through drilled holes, the researchers calculate that Oman’s peridotite alone could lock away over a billion tons of CO₂ a year - a significant slice of the 30 billion tons of CO₂ emitted annually worldwide by human activity. Peridotite is also found in the Pacific islands of Papua New Guinea and Caledonia, as well as in California.

The idea of capturing CO₂ in the form of carbonates inside rock is by no means new. But natural CO₂ capture is not very fast, and most schemes suffer from needing energy to mine the rock and spread it over a surface, or bring it to a power plant.   Kelemen and Matter suggest that with a little extra heat and some preparation, the peridotite rocks can be left where they are and the CO₂ taken to them.

One idea is to pre-heat the peridotite and inject pure CO₂ or a CO₂-rich fluid mixture. Since the reaction between silicates and CO₂ to form carbonates is exothermic, this would keep the rock’s temperature close to the optimum 200?C, maximising the rate of reaction. But CO₂ would have to be rapidly pumped into the rock to keep pace with the enhanced reaction rate. This could be a problem, since purifying CO₂ from power plant flue gas is extremely energy-intensive, points out Mercedes Maroto-Valer, who works on carbon sequestration at Nottingham University’s centre of excellence for carbon capture and storage. 

A second approach avoids this issue by using techniques from the oil industry to drill two holes deep into rock formations below shallow ocean waters, and fracture a path between them. Rock temperatures increase with depth and the temperature at the bottom of a 5km bore hole should be around 100^oC.

Cool ocean water containing CO₂ could be pumped down one of the holes, and once it reached the bottom, the exothermic reaction would then sustain the high temperatures needed to drive the process. The heated water would eventually find its way through the fractures to the second bore hole and rise to the surface via convection.

While this approach would be limited by the supply of dissolved CO₂ to a rate of around 10,000 tons of CO₂ per km³ of rock, the costs would be much lower, as circulating water would do the job of transporting the CO₂. 

The researchers are cautious about the proposals. More elaborate models and field tests would be required to evaluate them, they say.

Matt Wilkinson

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