Photo: Courtesy of National Oceanic and Atmospheric Administration |
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.
Greg
Rau, a senior scientist with the Institute of Marine Sciences at UC 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.
At
scale, the process would hydrate the carbon dioxide in power plant flue gas
with water to produce a carbonic acid solution. This solution would react with
limestone, neutralizing the carbon dioxide by converting it to calcium
bicarbonate — and then would be released into the ocean. While this process
occurs naturally (carbonate weathering), it is much less efficient, and is too
slow paced to be effective.
“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.
Earlier
research has shown that ocean acidification can cause exoskeletal components to
decay, retard growth and reproduction, reduce activity, and even kill marine
life including coral reefs.
“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.
“This
method allows a power plant to continue burning fossil fuel, but eliminates at
least some of the carbon dioxide that is emitted, and in a way that in some
locations should be less expensive and more environmentally friendly than other
carbon dioxide sequestration methods,” he said.
The
work, funded by the Energy Innovations Small Grant Program of the California
Energy Commission and LLNL, appears in the journal Environmental Science
& Technology.
