Since
most of the world’s governments have not yet enacted regulations to curb
emissions of greenhouse gases, some experts have advocated the development of
technologies to remove carbon dioxide directly from the air. But a new Massachusetts
Institute of Technology (MIT) study shows that, at least for the foreseeable
future, such proposals are not realistic because their costs would vastly
exceed those of blocking emissions right at the source, such as at the powerplants
that burn fossil fuels.
Some
purveyors of various new technologies for scrubbing carbon dioxide out of the
air are reminiscent of “snake-oil salesmen,” says Howard Herzog, a senior
research engineer at the MIT Energy Initiative and coauthor of the new analysis
published in the Proceedings of the National Academy of Sciences. The
study was co-authored by MIT civil and environmental engineering postdoc Kurt
Zenz House, along with researchers at C12 Energy in Berkeley,
Calif., and at Stanford University.
Herzog
and his coauthors are not alone in criticizing these proposals. An analysis
earlier this year by the American Physical Society came to similar conclusions,
although Herzog, a peer reviewer of that study, says that report “didn’t go far
enough” in its criticism of air-capture systems. In that analysis, the best open-air
carbon-capture systems proposed were found to cost at least eight times as
much, per ton of carbon avoided, as those installed at the powerplant.
It’s
not surprising that those promoting these concepts find an eager audience,
Herzog says. “It’s so enticing—you don’t have to change anything about your
lifestyle” to reduce greenhouse gases and slow the global climate change that
virtually all the world’s climate scientists agree is underway. “It’d be such a
great solution—if it were real.”
Unfortunately,
when examined closely, it turns out that “many of those advocating air-capture
deployment and research are really low balling the cost,” Herzog says. When the
underlying chemistry and mechanics are analyzed, their numbers don’t hold up,
he says. Compared with removing carbon dioxide from the emissions at a
powerplant—technology that exists and can be measured—removing it from the
outside air means processing about 300 times more air per ton of carbon dioxide
removed, because that’s the difference in carbon dioxide concentration.
Numerous
studies have shown that the cost of removing one substance from a mixture
depends on its initial concentration, so the much lower concentration of carbon
dioxide in outside air makes its removal from air much more costly than from
exhaust gases. After a detailed comparison, the MIT-led team concluded that the
cost of such removal is likely to be more than $1,000 per ton of carbon dioxide
avoided, compared to $50 to $100 per ton for current powerplant scrubbers.
Jennifer
Wilcox, assistant professor of energy resources engineering at Stanford University and a co-author of the study,
says, “direct air capture sounds great in theory. In reality, though, using
fossil-based energy sources to capture and regenerate the carbon dioxide would
result in net-positive carbon dioxide emissions.”
“If
you look at the ideal equations,” Herzog says, it’s possible to come up with
air-scrubbing systems that appear 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. The burden is on the inventors to show that their
proposed systems really could work, he says: “It can be done, that’s not the
question. The question is what is the cost.”
Some
of the air-capture proposals are based on small-scale laboratory experiments,
but don’t address key questions of how they would process the huge volumes of
air required. “They don’t have answers, yet they give cost estimates,” Herzog
says. “It’s irresponsible of them to give cost estimates and hold out hope that
this could be a good alternative to addressing climate change.”
While
the study found that such technologies are unlikely to have a place over the
next few decades, it did see one possible area where a particular variation of
such systems might make sense, at least to a limited extent: planting trees or
other plants to extract carbon dioxide from air, then burning them to produce
electricity while scrubbing the carbon dioxide at that powerplant. That type of
process would take advantage of plants’ natural ability to carry out the
initial extraction from the air, and would be renewable because the plants
could be harvested and then replaced. The total cost of such a system could be
a few hundred dollars per ton of carbon removed—which is not competitive at today’s
prices, but might be in the future.
Even
then, it would be more expensive than cleaning up fossil-fuel emissions right
at the powerplants, Herzog says: “What makes us think, if we’re not willing to
do these cheaper alternatives today, that future generations will do these much
more expensive things?”
