From: Andy Soos, ENN
Published January 20, 2011 04:54 PM

CO2 Ocean Sequestration

Carbon sequestration is "The process of removing carbon from the atmosphere and depositing it in a reservoir." When carried out deliberately, this may also be referred to as carbon dioxide removal, which is a form of geoengineering. The term carbon sequestration may also be used to refer to the process of carbon capture and storage, where CO2 is removed from flue gases, such as on power stations, before being stored in underground reservoirs. The term may also refer to natural biogeochemical cycling of carbon between the atmosphere and reservoirs, such as by chemical weathering of rocks. Using seawater and calcium to remove carbon dioxide (CO2) in a natural gas power plant's flue stream, and then pumping the resulting calcium bicarbonate in the sea, could be beneficial to the oceans' marine life or states a new research report.

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The oceans contain around 36,000 gigatons of carbon, mostly in the form of bicarbonate ion (over 90%, with most of the remainder being carbonate).

Inorganic carbon, that is carbon compounds with no carbon-carbon or carbon-hydrogen bonds, is important in its reactions within water. This carbon exchange becomes important in controlling pH in the ocean and can also vary as a source or sink for carbon. Carbon is readily exchanged between the atmosphere and ocean. In regions of oceanic upwelling, carbon is released to the atmosphere. Conversely, regions of downwelling transfer carbon (CO2) from the atmosphere to the ocean.

Greg Rau, a senior scientist with the Institute of Marine Sciences at the University of California Santa Cruz and who also works in the Carbon Management Program at Lawrence Livermore National Laboratory, conducted a series of lab-scale experiments to find out if a seawater/mineral carbonate (limestone) gas scrubber would remove enough CO2 to be effective, and whether the resulting substance -- dissolved calcium bicarbonate -- could then be stored in the ocean where it might also benefit marine life.

In addition to global warming effects, when carbon dioxide is released into the atmosphere, a significant fraction is passively taken up by the ocean in a form that makes the ocean more acidic. This acidification has been shown to be harmful to marine life, especially corals and shellfish.

In his experiments, Rau found that the scrubber removed up to 97 percent of CO2 in a simulated flue gas stream, with a large fraction of the carbon ultimately converted to dissolved calcium bicarbonate.

"The experiment in effect mimics and speeds up nature's own process," said Rau. "Given enough time, carbonate mineral (limestone) weathering will naturally consume most anthropogenic CO2. Why not speed this up where it's cost effective to do so?"

If the carbon dioxide reacted with crushed limestone and seawater, and the resulting solution was released to the ocean, this would not only sequester carbon from the atmosphere, but also would add ocean alkalinity that would help buffer and offset the effects of ongoing marine acidification. Again, this speeds up the natural CO2 consumption and buffering process offered by carbonate weathering.

"This approach not only mitigates CO2, but also potentially treats the effects of ocean acidification," Rau said. "Further research at larger scales and in more realistic settings is needed to prove these dual benefits."

Rau said the process would be most applicable for CO2 mitigation at coastal, natural gas-fired power plants. Such plants frequently already use massive quantities of seawater for cooling, which could be cheaply reused for at least some of the CO2 mitigation process.

There are many potential techniques to control or reduce CO2 air emissions such as growing new forests, underground injection, and even a newly developed cement type that can absorb CO2 from ambient air during hardening.

For further information: https://www.llnl.gov/news/newsreleases/2011/Jan/NR-11-01-03.html

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