Rau used a hollow, cylindrical Plexiglas reactor modified to allow seawater and gas streams to enter, longitudinally flow through, and exit the reactor chamber that had been filled with 2.2-2.5 kg of mineral carbonate aggregate of a specific size class. Two types of mineral carbonate aggregate were tested, one being aragonitic coral fragments and the other calcitic limestone from a local quarry.

After full equilibration with air, he found that up to 85% of the captured carbon was retained in solution—i.e., it did not degas or precipitate. He also calculated that the system could result in a cost of <$38/tonne CO₂ converted and stored as Ca(HCO₃)_2(aq). This compares to >$75/tonne in conventional plants.

Thus, above-ground CO₂ hydration and mineral carbonate scrubbing may provide a relatively simple point-source CO₂ capture and storage scheme at coastal locations. Such low-tech CO₂ mitigation could be especially relevant for retrofitting to existing power plants and for deployment in the developing world, the primary source of future CO₂ emissions.

Addition of the resulting alkaline solution to the ocean may benefit marine ecosystems that are currently threatened by acidification, while also allowing the utilization of the vast potential of the sea to safely sequester anthropogenic carbon. This approach in essence hastens Nature’s own very effective but slow CO₂ mitigation process; carbonate mineral weathering is a major consumer of excess atmospheric CO₂ and ocean acidity on geologic times scales.

...The relative technological simplicity of reaction 1 means that it could potentially be rapidly employed in the developing world where the need for CO₂ emissions reduction is greatest; analogous to widely used wet limestone or seawater flue gas scrubbing for SO₂ mitigation. However, the method’s large demand for carbonate mineral and water will likely limit its application to coastal sites. Further evaluation is needed of the economics, potential scale, permanence, environmental cost/bene?t, and societal acceptability of this and other approaches to CO₂ emissions reduction and ocean acidity mitigation.

—Greg Rau

Resources

• Greg H. Rau; CO₂ Mitigation via Capture and Chemical Conversion in Seawater. Environ. Sci. Technol., Article ASAP doi: 10.1021/es102671x