Tucking away vast amounts of atmospheric carbon dioxide (carbon capture and storage or CCS), or at least that emitted by fossil-fuel power stations, is a widely suggested and well supported approach to slowing down global warming. It has two main downsides: if successful it helps maintain the dominance of fossil fuels and vast amounts of buried greenhouse gas might simply leak out some time. Ideally, the storage part of CCS would involve CO2 being taken up by an inert solid. Carbonates may be stable enough but arranging the chemical reactions to make them seem difficult, the most widely considered being by encouraging weathering of ultramafic rocks to form magnesium carbonates as a by-product: huge areas would have be coated with finely-ground peridotite. A less satisfactory approach would to dissolve the gas in water held at great depths in sedimentary aquifers, but if that water doesn’t move and doesn’t get warmed it might do the trick.
Unsurprisingly, a lot of funds are available to research CCS and ideas are pouring forth, a recent, sober assessment focussing on the solubility option (Steele-MacInnis, M. et al. 2012. Volumetrics of CO2 storage in deep saline formations. Environmental Science and Technology (August 2012 online) DOI: 10.1021/es301598t). The team from Virginia Tech and the US Department of Energy conclude that solution in brines trapped in deep aquifers may help, although solution is an equilibrium between gas and dissolved CO2, so that a gas layer in the aquifer is always likely to be present, even at high pressures. The only way of avoiding that is if the dissolved gas reacted with carbonate in the aquifer so that calcium and hydrogen-carbonate (HCO3–) ions entered solution. That ‘enhanced’ solution is not so easy since, although it mimics the calcite-weathering effect by acid rain that naturally takes CO2 from the atmosphere, calcite dissolves very sluggishly. But solution adds to the density of already dense brine so that it is less likely to leak upwards into more shallow aquifers. Their preferred technology is to liquefy the gas under pressure and pump that to deep aquifers where eventually the supercritical CO2 liquid will dissolve. The problem is this: while experiment and theory suggest the approach will work, nobody knows how long CO2 solution in brine will take. There needs to be a sizeable pilot study…
- China’s first CCS project captures 40,000 tonnes of CO2 (eco-business.com)
- Shell To Build World’s First Oil Sands Carbon Capture And Storage (CCS) Project (eurasiareview.com)
- Carbon capture and fracking – the lowdown (environmentalresearchweb.org)