A new technique of removing carbon dioxide from a stream of air could provide a unique tool in the fight against climate change. The new system can operate on the gas at almost any concentration level, even down to the roughly 400 parts per million presently found in the atmosphere.
Most means of removing carbon dioxide from a stream of gas require higher concentrations, such as those found in the fuel emissions from fossil fuel-based power plants. A few modifications have been developed that can work with the low levels found in air, but the new method is considerably less expensive and energy-intensive, the researchers say.
The technique, centered on passing air through a stack of charged electrochemical plates, is defined in a new research paper in the journal Energy and Environmental Science, by MIT postdoc Sahag Voskian, who established the work during his Ph.D., and T. Alan Hatton, the Ralph Landau Professor of Chemical Engineering.
The device is fundamentally a large, specialized battery that takes in carbon dioxide from the air (or another gas stream) passing above its electrodes as it is being charged up, and then emits the gas as it is being discharged. In process, the device would simply swap between charging and discharging, with feed gas or fresh air being blown through the system during the charging phase, and then the unadulterated, concentrated carbon dioxide being given out during the discharging.
As the battery charges, an electrochemical reaction happens at the exterior of each of a stack of electrodes. These are coated with a compound called polyanthraquinone, which is integrated with carbon nanotubes. The conductors have a natural attraction for carbon dioxide and willingly bond with its molecules in the airstream or feed gas, even when it is existent at very low concentrations. The opposite reaction takes place when the battery is discharged — during which the device can offer part of the power needed for the whole system — and in the method ejects a stream of pure carbon dioxide. The whole system functions at room temperature and normal air pressure.
“The greatest benefit of this technology over most other carbon-absorbing or carbon capture technologies is the dual nature of the adsorbent’s affinity to carbon dioxide,” describes Voskian.
In other word, the electrode material, by its character, “has either a no affinity or high affinity whatsoever,” depending on the battery’s condition of charging or discharging. Other reactions used for carbon capture need intermediate chemical processing steps or the input of significant energy such as pressure or heat differences.
“This binary affinity enables capture of carbon dioxide from any concentration, including 400 parts per million, and permits its release into any carrier stream, including 100 percent CO2,” Voskian says. That is, as any gas flows across the stack of these flat electrochemical cells, during the release phase, the captured carbon dioxide will be carried along with it. For example, if the chosen end-product is pure carbon dioxide to be used in the carbonation of beverages, then a jet of the pure gas can be blown through the plates. The taken gas is then released from the plates and merges with the stream.
In some fizzy-drink bottling plants, fossil fuel is burned to produce the carbon dioxide needed to give the drinks their fizz. Likewise, some farmers burn natural gas to produce carbon dioxide to feed their plants in greenhouses. The new system could remove that need for fossil fuels in these applications, and in the procedure actually be taking the greenhouse gas right out of the air, Voskian says. On the other hand, the pure carbon dioxide stream could be compressed and introduced underground for long-term disposal, or even turned into fuel through a sequence of chemical and electrochemical processes.
The procedure this system deploys for taking and releasing CO2 “is revolutionary,” he says. “All of this is at ambient settings – without need for pressure, thermal or chemical input. It’s just these extremely thin sheets, with both surfaces active, that can be stacked in a box and attached to a source of electricity.”
“In my laboratories, we have been struggling to form new technologies to tackle a range of environmental issues that sidestep the need for thermal energy sources, the addition of chemicals to complete the separation and release cycles or changes in system pressure,” Hatton says. “This carbon dioxide capture technology is an obvious display of the power of electrochemical approaches that require only small swings in voltage to make the separations.”
In a working plant — for example, in a power plant where exhaust gas is being formed continuously — two sets of such stacks of the electrochemical cells could be set up next to each other to operate in parallel, with flue gas being guided first at one set for carbon capture, then redirected to the second set while the first set goes into its discharge cycle. By switching back and forth, the system could always be both capturing and discharging the gas. In the lab, the team has demonstrated the system can endure at least 7,000 charging-discharging cycles, with a 30 percent loss in efficiency over that time. The researchers approximate that they can readily increase that to 20,000 to 50,000 cycles.
Standard chemical processing methods can build electrodes. While nowadays this is done in a laboratory setting, it can be modified so that eventually they could be made in huge quantities through a roll-to-roll manufacturing process similar to a newspaper printing press, Voskian says. “We have established very cost-effective techniques,” he says, approximating that it could be manufactured for something like tens of dollars per square meter of the electrode.
Matched against other prevailing carbon capture technologies, this system is relatively energy efficient, using about one gigajoule of energy per ton of carbon dioxide captured constantly. Other existing techniques have energy consumption, which varies between 1 to 10 gigajoules per ton, depending on the inlet carbon dioxide concentration, Voskian states.
The researchers have established a company called Verdox to commercialize the process, and hope to develop a pilot-scale plant within the following few years, he says. And the system is straightforward to scale up, he says: “If you want more capability, you just need to make more electrodes.”