Carbon capture and storage Turning air into stone

How to keep waste carbon dioxide in the ground

This year the world’s power stations, farms, cars and the like will generate the equivalent of nearly 37 billion tonnes of waste carbon dioxide. All of it will be dumped into the atmosphere, where it will trap infra-red radiation and warm the planet. Earth is already about 0.85°C warmer than last century’s average temperature. Thanks to the combined influence of greenhouse-gas emissions and El Nino, a heat-releasing oceanic phenomenon, 2016 looks set to be the warmest year on record, and by a long way.

It would be better, then, to find some method of disposing of CO₂. One idea, carbon capture and storage (CCS), involves collecting the gas from power stations and factories and burying it underground where it can do no harm. But CCS is expensive and mostly untried. One worry is whether the buried gas will stay put. Even small fissures in the rocks that confine it could let it leak out over the course of time, undoing much of the benefit. And even if cracks are not there to begin with, the very drilling necessary to bury the gas might create them.

A paper just published in Science offers a possible solution. By burying CO₂ in the right sort of rock, a team of alchemists led by Juerg Matter, a geologist at Southampton University, in Britain, was able to transmute it into stone. Specifically, the researchers turned it into carbonate minerals such as calcite and magnesite. Since these minerals are stable, the carbon they contain should stay locked away indefinitely.

Dr Matter’s project, called CarbFix, is based in Iceland, a country well-endowed with both environmentalism and basalt. That last, a volcanic rock, is vital to the process, for it is full of elements which will readily react with carbon dioxide. Indeed, this is just what happens in nature. Over geological timescales (ie, millions of years) carbon dioxide is removed from the air by exactly this sort of weathering. Dr Matter’s scheme, which has been running since 2009, simply speeds things up.

Between January and March 2012 he and his team worked at the Hellisheidi geothermal power station, near Reykjavik. Despite its green reputation, geothermal energy which uses hot groundwater to drive steam turbines is not entirely emissions-free. Underground gases, especially CO₂ and hydrogen sulphide (H2S), often hitch a ride to the surface, too. The H₂S, a noxious pollutant, must be scrubbed from the power-station exhaust before it is released, and the researchers worked with remainder, almost pure carbon dioxide.

They collected 175 tonnes of it, mixed it with a mildly radioactive tracker chemical, dissolved the mixture in water and pumped it into a layer of basalt half a kilometre below the surface. They then kept an eye on what was happening via a series of monitoring wells. In the event, it took a bit less than two years for 95% of the injected CO₂ to be mineralised.

They followed this success by burying unscrubbed exhaust gas. After a few teething troubles, that worked too. The H₂S reacted with iron in the basalt to make pyrites, so if exhaust gas were sequestered routinely, scrubbing might not be needed. This was enough to persuade Reykjavik Energy, the power station’s owners, to run a larger test that is going on at the moment and is burying nearly 10,000 tonnes of CO₂ and around 7,300 tonnes of H₂S.

Whether CarbFix-like schemes will work at the scale required for fossil-fuel power stations remains to be seen. In these, the main additional pollutant is sulphur dioxide, which has different chemical characteristics from hydrogen sulphide. Scrubbing may therefore still be needed. Another constraint is the supply of basalt. Though this rock is common, it is found predominantly on the ocean floor. Indeed, geologically speaking, Iceland itself is a piece of ocean floor; it just happens to be above sea level. There are some large basaltic regions on dry land, but they are not necessarily in convenient places.

Nevertheless, if the will were there, pipelines from industrial areas could be built to carry exhaust gases to this basalt. It has not, after all, proved hard to do the reverse-carrying natural gas by pipeline whence it is found to where it is used. It is just a question of devising suitable sticks and carrots to assist the process. How much those sticks and carrots would cost is crucial. But Dr Matter’s proof of the principle of chemical sequestration in rock suggests it would be worth finding out.