Oct
29
Battery Design Sequesters CO2 For Recycling
October 29, 2019 | Leave a Comment
MIT researchers have devised a new way of removing carbon dioxide from a stream of air. The press release asserts it 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 flow of air or flue gas (blue) containing carbon dioxide (red) enters the system from the left. As it passes between the thin battery electrode plates, carbon dioxide attaches to the charged plates while the cleaned airstream passes on through and exits at right. Image Credit: Sahag Voskian and T. Alan Hatton, MIT. Click image for the largest view.
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 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,” explained 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 said. 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 said. 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 said. “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 said. “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 said. “We have developed very cost-effective techniques,” he said, 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 said.
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 said: “If you want more capacity, you just need to make more electrodes.”
Many readers will note that lots of media picked up the press release adding in some cases some editorial comments. The post above is the press release in full with minor edits for clarity and readability.
Regular readers will note this is an “in its infancy development”. There remains a very long way to go. Recycling CO2 and getting humanity current in the planetary carbon cycle is always an interesting and worthy idea.
The first question is cycling time. The paper at Figure 6 graphs suggests the device cycles the charge density in about 100 minutes. The graph implies that would be a full charge and discharge cycle, a very encouraging result. At 50,000 cycles the device lifetime (50,000 * 100 / 60) would exceed 83,300 hours or 3472 days, nearly 9 ½ years. But these numbers depend on the proportion of CO2 in the inlet’s gas mix.
The financial values depend on the creation of a large market for gaseous CO2, beyond fizzing sodas and dry ice. That implies fuels to get economic scale. So far, even with no cost CO2 those technologies are not ready just yet either.
Above all, the MIT team deserves congratulations and some sort of public recognition. Perhaps soon the CO2 back to fuel technology at real low cost will make the impetus for this technology into an imperative.
But remember, it will be about and made or broken by the money.
The author’s “Broader context” of “clear scientific consensus on the role of anthropogenic greenhouse gas (GHG) emissions” is already long known by credible science to be a dead end. It may get warmer a bit, a few times over the coming fewer centuries, but there is a certainly, its going to get colder, lots colder for hundreds of centuries.
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