The process could work on the
gas at any concentrations, from power plant emissions to open air
October 25, 2019
Massachusetts Institute of
Technology
A new way of removing carbon
dioxide from a stream of air could provide a significant tool in the battle
against climate change. The new system can work on the gas at virtually any
concentration level, even down to the roughly 400 parts per million currently
found in the atmosphere.
A new way of removing carbon
dioxide from a stream of air could provide a significant tool in the battle
against climate change. The new system can work on the gas at virtually any
concentration level, even down to the roughly 400 parts per million currently
found in the atmosphere.
Most methods of removing
carbon dioxide from a stream of gas require higher concentrations, such as
those found in the flue emissions from fossil fuel-based power plants. A few
variations have been developed that can work with the low concentrations found in
air, but the new method is significantly less energy-intensive and expensive,
the researchers say.
The technique, based on
passing air through a stack of charged electrochemical plates, is described in
a new paper in the journal Energy and Environmental Science, by MIT
postdoc Sahag Voskian, who developed the work during his PhD, and T. Alan
Hatton, the Ralph Landau Professor of Chemical Engineering.
The device is essentially a
large, specialized battery that absorbs carbon dioxide from the air (or other
gas stream) passing over its electrodes as it is being charged up, and then
releases the gas as it is being discharged. In operation, the device would
simply alternate between charging and discharging, with fresh air or feed gas
being blown through the system during the charging cycle, and then the pure,
concentrated carbon dioxide being blown out during the discharging.
As the battery charges, an
electrochemical reaction takes place at the surface of each of a stack of
electrodes. These are coated with a compound called polyanthraquinone, which is
composited with carbon nanotubes. The electrodes have a natural affinity for
carbon dioxide and readily react with its molecules in the airstream or feed
gas, even when it is present at very low concentrations. The reverse reaction
takes place when the battery is discharged -- during which the device can
provide part of the power needed for the whole system -- and in the process
ejects a stream of pure carbon dioxide. The whole system operates at room
temperature and normal air pressure.
"The greatest advantage
of this technology over most other carbon capture or carbon absorbing
technologies is the binary nature of the adsorbent's affinity to carbon
dioxide," explains Voskian. In other words, the electrode material, by its
nature, "has either a high affinity or no affinity whatsoever,"
depending on the battery's state of charging or discharging. Other reactions
used for carbon capture require intermediate chemical processing steps or the
input of significant energy such as heat, or pressure differences.
"This binary affinity
allows capture of carbon dioxide from any concentration, including 400 parts
per million, and allows its release into any carrier stream, including 100
percent CO2," Voskian says. That is, as any gas flows through the stack of
these flat electrochemical cells, during the release step the captured carbon
dioxide will be carried along with it. For example, if the desired end-product
is pure carbon dioxide to be used in the carbonation of beverages, then a
stream of the pure gas can be blown through the plates. The captured gas is
then released from the plates and joins the stream.
In some soft-drink bottling
plants, fossil fuel is burned to generate the carbon dioxide needed to give the
drinks their fizz. Similarly, some farmers burn natural gas to produce carbon
dioxide to feed their plants in greenhouses. The new system could eliminate
that need for fossil fuels in these applications, and in the process actually
be taking the greenhouse gas right out of the air, Voskian says. Alternatively,
the pure carbon dioxide stream could be compressed and injected underground for
long-term disposal, or even made into fuel through a series of chemical and
electrochemical processes.
The process this system uses
for capturing and releasing carbon dioxide "is revolutionary" he
says. "All of this is at ambient conditions -- there's no need for
thermal, pressure, or chemical input. It's just these very thin sheets, with
both surfaces active, that can be stacked in a box and connected to a source of
electricity."
"In my laboratories, we
have been striving to develop new technologies to tackle a range of
environmental issues that avoid the need for thermal energy sources, changes in
system pressure, or addition of chemicals to complete the separation and
release cycles," Hatton says. "This carbon dioxide capture technology
is a clear demonstration of the power of electrochemical approaches that
require only small swings in voltage to drive the separations."
In a working plant -- for
example, in a power plant where exhaust gas is being produced continuously --
two sets of such stacks of the electrochemical cells could be set up side by
side to operate in parallel, with flue gas being directed first at one set for
carbon capture, then diverted to the second set while the first set goes into
its discharge cycle. By alternating back and forth, the system could always be
both capturing and discharging the gas. In the lab, the team has proven the
system can withstand at least 7,000 charging-discharging cycles, with a 30
percent loss in efficiency over that time. The researchers estimate that they
can readily improve that to 20,000 to 50,000 cycles.
The electrodes themselves can
be manufactured by standard chemical processing methods. While today this is
done in a laboratory setting, it can be adapted so that ultimately they could
be made in large quantities through a roll-to-roll manufacturing process
similar to a newspaper printing press, Voskian says. "We have developed
very cost-effective techniques," he says, estimating that it could be
produced for something like tens of dollars per square meter of electrode.
Compared to other existing
carbon capture technologies, this system is quite energy efficient, using about
one gigajoule of energy per ton of carbon dioxide captured, consistently. Other
existing methods have energy consumption which vary between 1 to 10 gigajoules
per ton, depending on the inlet carbon dioxide concentration, Voskian says.
The researchers have set up a
company called Verdox to commercialize the process, and hope to develop a
pilot-scale plant within the next few years, he says. And the system is very
easy to scale up, he says: "If you want more capacity, you just need to
make more electrodes."
Story Source:
Journal Reference:
Sahag Voskian, T. Alan
Hatton. Faradaic electro-swing reactive adsorption for CO2 capture. Energy
& Environmental Science, 2019; DOI: 10.1039/C9EE02412C
