Someday the
world may be in a position to lower the concentration of heat-trapping carbon
dioxide in the atmosphere by chemically removing it from the air.
But not soon;
the process is simply too expensive, say scientists from Stanford University
and the Massachusetts Institute of Technology (MIT).
A study published
in the Proceedings of the National
Academy of Sciences, co-authored by Stanford energy and
environmental researcher Jennifer Wilcox, concludes that if air-capture of carbon
dioxide with chemicals is ever used, it will be far in the future.
For now, it is
much more economically efficient to capture the carbon dioxide that enters the
atmosphere from the smokestacks of large centralized sources such as power
plants, cement plants, fertilizer plants, and refineries.
After a
detailed comparison, the research team concluded that the cost of removal from
air is likely to be on the order of $1,000 per ton of carbon dioxide, compared
with $50 to $100 per ton for current power-plant scrubbers.
Indeed, the
researchers say, the cost of removing carbon dioxide directly from the air
would be so large that paying for it would require the equivalent of a
$10-per-gallon tax on gasoline. The cost estimates are similar to those
presented earlier this year in an American Physical Society study, although the
APS study emphasized
that future costs might come down with new technology.
“Like the
PNAS article, the APS report
concludes that direct air capture must be powered by low-carbon energy,”
says Robert Socolow, a professor of mechanical and aerospace engineering at Princeton University who is a co-author, with
Wilcox, of the APS study.
“Direct
air capture sounds great in theory,” Wilcox says. “In reality,
though, a lot of energy is required, and using fossil-based energy sources to
capture and regenerate the carbon dioxide could readily result in more carbon
dioxide entering the atmosphere than is captured.
“For
direct air capture to be feasible, carbon-free energy, such as solar or wind,
is required. But that carbon-free energy would be used more cost effectively to
replace carbon dioxide-emitting power plants.”
Technology
exists today for removing carbon dioxide emissions from coal-fired power plants
and is much cheaper than removal from the atmosphere. “The concentration
of carbon dioxide in outside air is 300 times less than in the coal-fired flue
gases emitted from a power plant. The lower atmospheric concentration makes
removal from air much more expensive than removing carbon dioxide directly from
the flue gases at the source,” Wilcox says.
“Ultimately,”
Wilcox says, “society needs to move completely away from carbon-based
energy resources.”
Air capture systems are attractive because they are theoretically possible.
“If you look at the ideal equations,” says Howard Herzog, a senior
research engineer at the MIT Energy Initiative who worked with Wilcox on the
research, “it’s possible to come up with air-scrubbing systems that appear
to be feasible. But if you look at empirical data—how engineers look at this,
with real-world efficiencies—you don’t find many reasons to be hopeful.”
