Rock solid: Trapping carbon dioxide

The amount of research into global warming and how to reduce the increasing carbon dioxide (CO₂) has grown exponentially over the last 15 years. Carbon capture and storage (CCS) is one such growing field. Carbon capture cannot work without a method to safely store the CO₂ that is acceptable to the general public, A team in Iceland may have developed a method that safely stores 95% of captured CO₂ as environmentally benign carbonate minerals in basaltic rocks and importantly does not allow leaching back into the air. This method is a new form of mineral carbonatization (i.e., the conversion of CO₂ to carbonate minerals) via CO₂-fluid-rock reactions.

Previously mineral carbonatization has been limited by the reservoirs used to store the CO₂ in. Current reservoirs like deep saline aquifers and depleted oil and gas reservoirs are limited by the lack of calcium-, magnesium-, and iron-rich silicate minerals required to form carbonate minerals. The Icelandic team use basaltic rock which is rich in magnesium- and iron-silicates and is highly reactive. The CarbFix project in Iceland injected CO₂ into the basaltic rock in the form of a CO₂-H₂S gas mixture. They pumped 73 tonnes of the mixture in of which 55 tonnes was CO₂. They used a novel injection system to ensure that the CO₂ being pumped did not degas and escape. The system mixed the CO₂-H₂S gas with down running water. Once the CO₂ becomes soluble it stops being buoyant and reacts with the Ca-Mg-Fe rock.

Geological cross-section of the CarbFix injection site. CO₂ and H₂S are injected fully dissolved in water in injection well HN02 at a depth between 400 and 540 m. (SOURCE: Science 10 Jun 2016:
 Vol. 352, Issue 6291, pp. 1312-1314
DOI: 10.1126/science.aad8132)

The reservoir used by the team was shallow and located 400m to 800m below the surface 25 km east of Reykjavik near the Hellisheidi geothermal power plant. To discover what had happened to the injected CO₂, the team labelled the gas with carbon-14 (14C). They found that over 95% of the CO₂ injected into the CarbFix site in Iceland was mineralized to carbonate minerals in less than 2 years. The fact the CO₂ had essentially turned to stone means safety concerns of the CO₂ escaping had been alleviated.

Turning CO₂ into rock isn’t the cheapest method of storing the greenhouse gas and it requires a lot of water. However H₂S is abundant in geothermal gases and CO₂ rich sour gas, hence allowing a possible method of capture. Current methods of capture are expensive and unlikely to become common place, unless governments subside the processes. This method may provide a solution to decreasing anthropogenic CO₂ emissions that addresses the major concerns of escape post storage.

Reference(s)

1. Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions
   BY JUERG M. MATTER, MARTIN STUTE, SANDRA Ó. SNÆBJÖRNSDOTTIR, ERIC H. OELKERS, SIGURDUR R. GISLASON, EDDA S. ARADOTTIR, BERGUR SIGFUSSON, INGVI GUNNARSSON, HOLMFRIDUR SIGURDARDOTTIR, EINAR GUNNLAUGSSON, GUDNI AXELSSON, HELGI A. ALFREDSSON, DOMENIK WOLFF-BOENISCH, KIFLOM MESFIN, DIANA FERNANDEZ DE LA REGUERA TAYA, JENNIFER HALL, KNUD DIDERIKSEN, WALLACE S. BROECKER
   SCIENCE10 JUN 2016 : 1312-1314 DOI: 10.1126/science.aad8132