CO2 capture by basalt, granite, silicates and other alkaline natural rocks
Renaud-KdeR
Speeding up earth's natural climate control
Could removing acid from seawater slow global warming?
As greenhouse gas levels rise faster, some scientists hope to drive
the trend in the other direction by speeding up the planet's natural
abilities to soak up CO2. A new study published in ES&T (DOI: 10.1021/
es0701816) proposes a novel way to accelerate the ocean chemistry that
absorbs CO2.
(See ES&T article on the FILES menu of this google group)
Kurt House / Rhonda Saunders
A new process would harness renewable energy such as geothermal power
to bump up the ocean's alkalinity, which pulls CO2 from the
atmosphere.
Oceans have absorbed about one-third of the CO2 that humans have
produced so far, and if emissions ceased, the oceans would eventually
take up all of it. But the process takes thousands of years, which is
much too slow to keep up with the current CO2 rise, says Kurt House, a
Harvard doctoral student and lead author of the new study.
"The more acidic the ocean is, the less CO2 it will hold," House
explains. On the other hand, alkaline or basic solutions have a strong
tendency to absorb CO2. Thus, a more alkaline ocean would pull more of
the gas from the air.
Other ideas for increasing ocean CO2 uptake have focused on
stimulating phytoplankton growth or increasing alkalinity directly.
While House was jogging along the Charles River near Harvard one day,
it struck him that he could instead remove acid to achieve the same
effect. Eventually, he developed an approach called electrochemical
weathering.
Weak acids in water normally dissolve rocks on land over time, forming
an alkaline solution that runs into rivers and then the sea.
Electrochemical weathering creates a stronger acid to drive much
faster reactions. Still at a theoretical stage, the method involves
passing an electric current through seawater to separate out chlorine
and hydrogen gas, similar to the industrial chloralkali process used
to make chlorine gas. The chlorine and hydrogen are then combined in
fuel cells to form strong hydrochloric acid. The fuel cells would be
housed in an industrial-scale plant that would collect and use the
acid to dissolve silicate rocks, which are common worldwide. This
would neutralize the acid and the resulting alkaline solution could
then be returned to the sea. Overall, the process would help stabilize
the oceans' pH, House says, and could benefit corals which are dying
from ocean acidification caused by rising CO2 levels.
For profit, plants could sell carbon reduction credits in a cap-and-
trade scheme. House says the process could potentially absorb 1
gigaton of CO2 annually. This would require building coastal
processing plants equivalent in capacity to about 100 large sewage
treatment plants, according to House and his coauthors, Michael Aziz
and Daniel Schrag of Harvard University and House's brother
Christopher House of Pennsylvania State University. That number of
plants is "not a lot," says David Archer, an ocean chemist at the
University of Chicago. Archer calls the approach "clever" and says
that compared with planting more trees to take up CO2, "with this
method you have dealt with CO2 in a more leakproof way."
The idea faces major hurdles. If put into practice today, it would
cost at least $100 per ton of CO2 removed; more efficient electrolysis
and fuel cells could reduce the cost. Electrochemical weathering also
uses a lot of electricity; if coal is burned to generate that power,
the whole process saves less CO2 than could have been reduced by
replacing one coal-burning power plant with a plant run on carbon-free
renewable energy. But in the best case, an electrochemical-weathering
plant running on renewable energy could offset nearly twice as much
CO2 compared with the reduction from replacing a coal plant. The team
suggests tapping into geothermal energy, which is underused because
geothermal supplies are often located far from cities with high
electricity demand.
Another problem is localized pollution. "Around the plant you would
get a very basic solution," which could contain chlorinated
byproducts, House says. These byproducts could harm sea life locally.
Researchers Greg Rau of the Lawrence Livermore National Laboratory and
Ken Caldeira of Stanford University's Carnegie Institution proposed
another weathering process. Called accelerated limestone weathering,
their method would capture CO2 from power plants to dissolve
limestone, creating carbon-rich brine for ocean storage. Caldeira says
the approach would use less energy than electrochemical weathering but
would not capture airborne emissions. House's process "is on sound
theoretical ground" but would likely be too expensive and inefficient
to be practical unless it was run on cheap renewable energy, he says.
Nevertheless, Caldeira calls for research funding for a broad array of
possible climate solutions in order "to get new ideas on the table." -
ERIKA ENGELHAUPT