Seawater Plus Calcium Could Cut Carbon, Aid Sea Life

Seawater and calcium can be used to remove
carbon dioxide from flue gas,
and the resulting calcium bicarbonate
 can be pumped back into the sea
to help marine life.Image: NSF/USAP photo by Steve Clabuesch

Limestone scrubbers deployed at natural gas power plants could help reduce carbon emissions as well as lower ocean acidification by pumping a byproduct of the scrubbing process back into the water, according to an experiment conducted by the Energy Department's Lawrence Livermore National Laboratory.

Greg Rau, a scientist at LLNL and the University of California, Santa Cruz, conducted a series of small-scale lab experiments that found seawater and calcium can be used to remove carbon dioxide from a gas-fired plant. When combined into a limestone scrubber, the mix can be used to remove CO2 in a plant's flue stream. The resulting calcium bicarbonate can be pumped back into the sea to help marine life, Rau said.

The dissolved calcium bicarbonate can be stored in the ocean. There it benefits marine organisms by helping to reduce the acidification caused by carbon concentrations that has been found to harm corals and shellfish. 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.

At scale, the process would hydrate the CO2 in power plant flue gas with water to produce a carbonic acid solution. This solution would then be reacted with limestone, neutralizing the CO2 by converting it to calcium bicarbonate, after which it would be released into the ocean.

Rau said that while this process occurs naturally (it's called carbonate weathering), his experiments might point to a much faster process.

"The experiment in effect mimics and speeds up nature's own process," he said. "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?"

Two benefits for the price of one
Rau's research, which appeared in the journal Environmental Science and Technology, goes on to suggest that the resulting solution would not only sequester carbon from the atmosphere, but would also "add to ocean alkalinity that would help buffer and offset the effects of ongoing marine acidification."

"This approach not only mitigates CO2, but also potentially treats the effects of ocean acidification," said Rau, adding that he believes more research at a larger scale is warranted.

The process would be most applicable at seaside gas-fired power plants that already use a large amount of seawater for cooling. Rau said the water could be cheaply reused to help CO2 mitigation.

Rau's work was funded by an grant program jointly overseen by LLNL and the California Energy Commission. The national lab is based in the Bay Area, in Berkeley, Calif.

Sullivan reported from San Francisco.