The process could work with the gas at all concentrations, from power plant emissions to open air.
A new method of removing carbon dioxide from an airstream could be an important tool in the fight against climate change. The new system can work with the gas in virtually any concentration, down to the current 400 ppm available in the atmosphere.
Most processes for removing carbon dioxide from a gas stream require higher concentrations, such as found in the flue gas emissions of fossil fuel-fired power plants. Some variations have been developed that can work with the low concentrations in the air, but the new method is significantly less energy intensive and expensive, the researchers say.
The technique is based on the passage of air through a stack of charged electrochemicals. This article was published in the journal Energy and Environmental Science by postdoc Sahag Voskian, who did the work during his Promotion, and described by T. Alan Hatton, the Ralph Landau Professor of Chemical Engineering.
The device is essentially a large specialized battery, the carbon dioxide from the air (or other gas stream) absorbs electrodes when it is charged, and then releases the gas when it is discharged. In operation, the device simply alternates between charging and discharging, during which fresh air or feed gas is blown through the system during the charging cycle and then the pure, concentrated carbon dioxide is blown out during unloading.
As battery charges, an electrochemical reaction takes place on the surface of each electrode stack. These are coated with a compound called polyanthraquinone which consists of carbon nanotubes. The electrodes have a natural affinity for carbon dioxide and easily react with its molecules in the air stream or feed gas, even when present in very low concentrations. The reverse reaction occurs when the battery is being discharged – during which the device can provide some of the power needed for the system as a whole – emitting a stream of pure carbon dioxide. The entire system operates at room temperature and normal air pressure.
"The biggest advantage of this technology over most other carbon capture or absorption technologies lies in the binary nature of the adsorbent's affinity for carbon dioxide," explains Voskian. In other words, the electrode material inherently has "either a high affinity or no affinity at all," depending on the charge or discharge state of the battery. Other reactions used to deposit carbon require intermediate chemical processing steps or the delivery of significant energy such as heat or pressure differences.
"This binary affinity allows the capture of carbon dioxide from any concentration, including 400 ppm, and allows its deposition release into any carrier stream, including 100 percent CO 2 ," says Voskian. That is, when any gas flows through the stack of these flat electrochemical cells, the trapped carbon dioxide is carried during the release step. For example, if the desired end product is pure carbon dioxide to be used in the carbonation of beverages, a stream of clean gas may be blown through the plates. The collected gas is then drained from the plates and flows into the stream.
In some soft drink bottling plants, fossil fuel is burned to produce the carbon dioxide needed to carbonize the drinks. Similarly, some farmers burn natural gas to produce carbon dioxide to feed their plants in greenhouses. The new system could eliminate the need for fossil fuels in these applications, removing the greenhouse gas directly from the air, says Voskian. Alternatively, the pure carbon dioxide stream could be compressed and injected underground for long-term disposal, or even fueled by a series of chemical and electrochemical processes.
The carbon dioxide capture and release process is revolutionary, "he says." All of this is possible under ambient conditions – no heat, pressure or chemical inputs are required. It is only these very thin foils where both surfaces are active and which can be stacked in a carton and connected to a power source. "
" In my labs, we've been trying to develop new technologies for a range of environmental issues that avoid the need for heat sources, changes in system pressure, or the addition of chemicals to complete the separation and release cycles, "says Hatton. "This carbon dioxide capture technology is a clear demonstration of the power of electrochemical approaches that require little voltage fluctuation to drive the deposits."
In a work shop – for example, in a power plant where exhaust gas is generated continuously – two sets of such stacks Electrochemical cells could be placed side-by-side to operate in parallel with the flue gas first directed to one set for carbon capture and then diverted to the second set as the first set goes into its discharge cycle. By switching back and forth, the system could always catch and release the gas. In the lab, the team has proven that the system survives at least 7,000 charge and discharge cycles with a 30 percent loss of efficiency. The researchers estimate that they can easily improve this value to 20,000 to 50,000 cycles.
The electrodes themselves can be prepared by standard chemical processing techniques. While this is done in a lab today, it can be adapted to ultimately be produced in bulk through a roll-to-roll manufacturing process similar to a newspaper press, Voskian says. "We've developed very cost-effective techniques," he says, estimating that they could be manufactured for about ten dollars per square meter of electrode.
Compared to other existing carbon capture technologies, this system is quite energy efficient. consistently with about one gigajoule of energy per tonne of carbon dioxide. Other existing methods have an energy consumption that varies between 1 and 10 gigajoules per ton depending on the carbon dioxide concentration, says Voskian.
Researchers have founded a company called Verdox to commercialize the process, and hope to develop a pilot plant over the next few years, he says. And the system scales very easily: "If you need more capacity, you just have to make more electrodes."
Reference: "Faraday Electro-swing Reactive Adsorption for CO 2 capture" by Sahag Voskian and T. Alan Hatton, October 1, 2019, Energy and Environmental Sciences .
DOI: 10.1039 / C9EE02412C