The process can run on gas at any concentration, from power plant emissions to open air.
A new way of removing carbon dioxide from airflow can provide a significant means of combating climate change. The new system can run gas at almost any concentration level, even up to the approximately 400 parts per million currently found in the atmosphere.
Most methods for removing carbon dioxide from a gas stream require higher concentrations, such as those found in flue gas emissions from fossil fuel-based power plants. Several variants have been developed that can handle the low concentrations found in the air, but the new method is significantly less energy-intensive and expensive, researchers say.
The technique based on the passage of air through a stack of charged electrochemistry plates is described in a new material in the journal Energy and Environmental Sciences by MIT postdoc Sahag Voskian, who is developing the work to his doctor, and T. Allen Hutton, professor of chemical engineering to Ralph Landau.
The device is essentially a large, specialized battery that absorbs carbon dioxide from the air (or other gas stream), passing electrodes through it while charging, and then releasing gas while charging. During operation, the device will simply alternate between charging and discharging, with fresh air or supply gas being blown through the system, and then pure concentrated carbon dioxide being purged during dilution.
As the battery charges, an electrochemical reaction takes place on the surface of each bunch of electrodes. They are coated with a compound called polyanthraquinone, which is made up of carbon nanotubes. The electrodes have a natural affinity for carbon dioxide and easily react with its molecules in the air stream or supply gas, even when present at very low concentrations. The feedback occurs when the battery is discharged – during which the device can provide some of the energy required for the whole system – and releases a stream of pure carbon dioxide in the process. The whole system operates at room temperature and normal air pressure.
"The biggest advantage of this technology over most other carbon capture or carbon sequestration technologies is the binary nature of the affinity of the adsorbent to carbon dioxide," explains Voskian. In other words, the electrode material, by its nature, "either has high affinity or no affinity", depending on the state of charge or discharge of the battery. Other reactions used for carbon sequestration require intermediate stages of chemical treatment or significant energy input such as heat or pressure differences.
"This binary affinity allows carbon dioxide to be captured at any concentration, including 400 parts per million, and allows flow into any carrier, including 100 percent CO 2 ," says Voskian. That is, as each gas flows through the stack of these flat electrochemical cells, the captured carbon dioxide will be carried along with it during the release phase. For example, if the desired end product is pure carbon dioxide to be used in carbonation of beverages, then a stream of pure gas can be blown through the plates. The trapped gas is then released from the dishes and connected to the stream.
In some soft drink bottling plants, fossil fuels are burned to generate the carbon dioxide required to drink the beverages. 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, and in fact is the removal of greenhouse gases from the air, Voskian says. Alternatively, the pure stream of carbon dioxide can be compressed and injected underground for long-term disposal or even converted into fuel through a series of chemical and electrochemical processes.
The process that this system uses to capture and release carbon dioxide is revolutionary, "he says." All of this is atmospheric – no heat, pressure or chemical input needed. Just these very thin sheets, with both active surfaces can be stacked and connected to an electricity source. "
" In my labs, we strive to develop new technologies to deal with a number of environmental issues that avoid the need for heat sources, pressure changes system or adding 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 only small fluctuations in voltage to drive the divisions. "
In a working installation – for example in a power plant where exhaust gas is continuously produced – two sets of such columns of electrochemical cells can be created side by side to work in parallel, such as the flue gases are first directed to one carbon capture kit, then diverted to the second set, while the first set goes into its ejection cycle. Alternating back and forth, the system can always catch and release gas. In the lab, the team has proven that the system can withstand at least 7,000 charge and dilution cycles, with a 30 percent loss of efficiency during this time. Researchers believe they can easily improve this to 20,000 to 50,000 cycles.
The electrodes themselves can be produced by standard chemical treatment methods. While it's being done in a lab environment today, it can be adapted so that they can ultimately be made in large quantities through the production of a roll of paper, similar to a print press, says Voskian. "We've developed a lot of cost-effective techniques," he says, calculating that it can be made 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 around one gigajoule of energy per tonne of carbon dioxide retained in succession. Other existing methods have an energy consumption that varies between 1 and 10 gigajoules per ton, depending on the concentration of carbon dioxide at the inlet, Voskian says.
Researchers have set up a company called Verdox to commercialize the process and hope to develop a pilot project. "A large-scale plant over the next few years," he says. And the system is very easy to scale, he says, "If you want more capacity, you just have to make more electrodes."
Reference: "Faraday Electro-Rocket Reactive Adsorption for CO 2 Capture" by Sahag Voskian and T. Alan Hatton, October 1, 2019, Science and Environmental Science .
doi: 10.1039 / C9EE02412C