Dec. 14, 2001

New possibilities unveiled in ocean storage of excess power plant emissions

Lab researchers have presented evidence that a new method for capturing carbon dioxide from power plants and placing it in the ocean has less impact on marine life than atmospheric carbon dioxide release or other global warming mitigation methods, such as direct injection and ocean fertilization.

LLNL earth scientists Greg Rau, Ken Caldeira and Kevin Knauss showcased the research, called carbonate dissolution, Wednesday at the 2001 fall meeting of the American Geophysical Union in San Francisco.

The proposal would hydrate the carbon dioxide in power plant flue gas with water to produce a carbonic acid solution. This solution would be mixed with limestone — which neutralizes the carbon dioxide by converting it to bicarbonate — and then would be released in the ocean. This process occurs naturally (carbonate weathering), but at a much slower pace.

"You are altering the chemistry of the carbon dioxide that causes a less drastic change to ocean pH (acidity) and is less biologically harmful than other methods such as direct injection ocean carbon sequestration and ocean fertilization," said Rau, an LLNL guest scientist who also works as a senior researcher with the Institute for Marine Sciences at UC Santa Cruz.

Whether carbon dioxide is released in the atmosphere or the ocean, eventually about 80 percent of the carbon dioxide will end up in the ocean in a form that will make the ocean more acidic. While the carbon dioxide is in the atmosphere, it could produce adverse climate change. When it enters the ocean, the acidification could be harmful to marine life.

"If the carbon dioxide were reacted with crushed limestone and seawater, and the resulting solution released to the ocean, the limestone would buffer the pH (acidity) of the ocean and prevent it from becoming more acidic," Caldeira said. "Furthermore, the dissolved limestone would tend to keep the carbon dioxide in the ocean and out of the atmosphere. This process would occur naturally anyway, but on about a 6,000-year time scale."

LLNL scientists are now engaged in both experimental and modeling work to study the feasibility of the proposed method of ocean carbon sequestration. Caldeira said initial results appear promising.

Researchers believe that the carbonate dissolution process would expand the capacity of the ocean to store carbon dioxide while minimizing the amount of carbon going back into the atmosphere, unlike some of the other forms of carbon dioxide sequestration.

Direct injection of carbon dioxide into the deep ocean will likely negatively impact marine organisms and their ecosystems, due to the increased acidity. Recent research shows that the acid-base imbalance can cause exoskeletal components to decay, retard growth and reproduction, reduce activity and cause loss of consciousness and even death to deep ocean marine life because of a disruption of oxygen-transport mechanisms (Science, Vol. 294, p. 319-320).

In addition, Caldeira showed in previous studies that unless carbon dioxide is converted to some other form before injection, it will degas back to the atmosphere when diffusion or ocean circulation returns it to the ocean surface.

In ocean fertilization, the biology of phytoplankton (which grows close to the ocean surface) is changed so that it increases the conversion of carbon dioxide to biomass. The conversion is likely to transport acidity from the surface ocean to the deep ocean.

Rau said private companies operating power plants would need incentives to start a carbonate dissolution program.

"They need motivation to sequester carbon dioxide, and they need methods that are effective and are economically and environmentally practical," he said.

"Carbonate dissolution allows a power plant to continue burning fossil fuel but eliminate at least some of the carbon dioxide that is emitted, and in a way that is probably less expensive and more environmentally friendly than other carbon dioxide sequestration methods."

LLNL researchers presented their findings Wednesday at the AGU meeting during a press conference titled "Global Warming Mitigation."