Rising levels of atmospheric carbon dioxide affect nature in many ways, including warming temperatures and ocean acidification. New research points toward a solution that could kill two birds with one stone: Remove CO2 from a natural-gas-powered plant's waste gas stream using seawater and mineral calcium carbonate, and then pump the resulting calcium bicarbonate into the sea to neutralize it (Environ. Sci. Technol., DOI: 10.1021/es102671x).
Roughly one-third of anthropogenic CO2 emissions come from burning fossil fuel in electricity plants. Most of the research on mitigating CO2 emissions from these plants has focused on carbon capture and storage. Yet most of those projects are still in the pilot stage and the technology is costly. In the new paper, Greg Rau, a senior researcher with the Institute of Marine Sciences at the University of California, Santa Cruz, and the Lawrence Livermore National Laboratory, instead sequesters CO2 by building on a well-established technology known as wet limestone scrubbing, used by power plants.
Rau built a lab-scale scrubber that used seawater and mineral carbonate to remove CO2 from a simulated flue gas stream. The scrubber worked by pumping CO2 over or through a porous bed of limestone particles sprayed with a continuous flow of water. He found that the process removed up to 97% of the CO2 in the gas. Water hydrated the waste CO2 to produce carbonic acid, which then reacted with, and was neutralized by the limestone. As a result, the CO2 gas transformed into dissolved calcium bicarbonate.
Dumping the dissolved calcium bicarbonate into the ocean would provide a second benefit: The calcium bicarbonate can increase seawater alkalinity, Rau says, by speeding up a natural but very slow process known as carbonate weathering, which captures carbon in the ocean.
The world's oceans would benefit from increasing alkalinity because they absorb as much as one-third of man-made CO2 emissions and are becoming more acidic. Ocean acidity in turn threatens the health of coral reefs, calcareous plankton, and other sea life. "We might be able to safely modify ocean chemistry to help mitigate both CO2 and ocean acidification," Rau says.
This approach is best suited for power plants located on the coast, Rau adds. These facilities already use large amounts of seawater to cool their operations. Also, limestone sources often exist nearby. The technology could be particularly useful for gas-fired power plants in developing countries, where future CO2 emissions are expected to rise. Yet the process may be too costly for coal-fired plants, Rau cautions, because their gas streams are littered with mercury and other pollutants that would have to be removed before pumping the dissolved calcium bicarbonate into the ocean.
David Archer, professor of geophysical sciences at the University of Chicago, says that the key consideration for a technology like this is cost. The process that removes CO2 from flue gas will use some of the plant's energy, which could roughly double the cost of the plant's energy for consumers, Archer estimates. Nonetheless, he says, a doubling of the cost of energy would pale in comparison with the costs of climate change.