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Wednesday, 30 October 2019

A new device captures CO2 from the air, stores it and returns it on demand

With the growth of human industrial activities, the rate of atmospheric carbon dioxide has increased considerably. In recent years, various methods for capturing CO2 in the air and transforming it into useful products have been developed with more or less efficiency. Recently, a team of MIT researchers developed a device to capture CO2 in the air at any concentration, store it, and redistribute it for practical uses such as CO2 injection for agriculture. or the gasification of beverages.

A new way to remove carbon dioxide from the air could be an important tool in the fight against climate change. The system can operate at virtually any level of concentration, even at the roughly 400 parts per million currently in the atmosphere.

Most methods of removing carbon dioxide from a gas stream require higher concentrations, such as those present in flue gas emissions from fossil fuel plants. Some variants have been developed that can work with the low concentrations found in the air, but the new method consumes much less energy and costs less.

A "battery" to capture and release CO2 on demand

The technique, based on the passage of air through a stack of charged electrochemical plates, is described in the journal Energy and Environmental Science . The device is essentially a large specialized battery that absorbs carbon dioxide from the air (or other gas stream) passing on its electrodes during its charging, then releases the gas during its discharge.

In operation, the device would simply alternate between loading and unloading, with fresh air or feed gas being blown into the system during the loading cycle, and then the pure and concentrated carbon dioxide being expelled during unloading.

As the battery charges, an electrochemical reaction takes place on the surface of each electrode stack. These are covered with a compound called polyanthraquinone, composed of carbon nanotubes.

In this scheme of the new system, the air entering the top right passes into one of the two chambers (gray rectangular structures) containing battery electrodes that attract carbon dioxide. Then, the air flow passes into the other chamber, while the carbon dioxide accumulated in the first chamber is sent to a separate storage tank (right). These alternative flows allow a continuous operation of the process in two stages. Credits: Sahag Voskian / T. Alan Hatton

Electrodes have a natural affinity for carbon dioxide and readily react with its molecules in the airflow or feed gas, even when present at very low concentrations. The reverse reaction occurs when the battery is discharged - at this point, the device can provide some of the power needed for the entire system - and thus ejects a pure carbon dioxide stream. The entire system operates at ambient temperature and at normal atmospheric pressure.

Electrodes optimized to capture CO2 at any concentration

" The biggest advantage of this technology over most other carbon capture or absorption technologies is the binary nature of the adsorbent's affinity for carbon dioxide, " explains Voskian. In other words, the electrode material, by its nature, " has a high affinity or no affinity ", depending on the state of charge or discharge of the battery. Other reactions used for carbon capture require intermediate stages of chemical treatment or the provision of substantial energy such as heat or pressure differences.

Diagram of a unique electro-swing adsorption electrochemical cell, with porous electrodes and electrolyte separators. The outer electrodes, coated with a poly-1,4-anthraquinone composite, can capture CO2 when applying a reducing potential via the carboxylation of the quinone and release the CO2 during the polarity reversal. . Credits: Sahag Voskian / T. Alan Hatton

This binary affinity captures carbon dioxide in any concentration, including 400 parts per million, and releases it into any carrier stream, including 100% CO2, " says Voskian. That is, with any gas passing through the stack of these flat electrochemical cells, the captured carbon dioxide will also be ejected during the discharge. For example, if the desired end product is pure carbon dioxide for use in the carbonation of beverages, a stream of pure gas may be blown through the plates. The captured gas is then released from the plates and joins the stream.

A replacement for fossil fuels currently used to generate CO2

In some non-alcoholic beverage bottling plants, fossil fuels are burned to generate the carbon dioxide needed to make beverages. Similarly, some farmers burn natural gas to produce carbon dioxide to grow their crops in greenhouses. The new system could eliminate this need for fossil fuels in these applications and, at the same time, eliminate greenhouse gases from the air.

Alternatively, the flow of pure carbon dioxide could be compressed and injected underground for long-term disposal, or even converted into fuel through a series of chemical and electrochemical processes. " All of this is done under ambient conditions - no thermal, chemical or pressurized input is needed. It's just these very thin sheets, with both active surfaces, that can be stacked in a box and connected to a power source . "

The new device can be used in many fields requiring the generation and injection of carbon dioxide, replacing fossil fuels. Credits: Sahag Voskian / T. Alan Hatton

We have been striving to develop new technologies to solve a range of environmental problems, avoiding the use of thermal energy sources, changing the system pressure, or adding chemicals to complete the separation cycles. of liberation. This carbon dioxide capture technology is a clear demonstration of the power of electrochemical approaches that require only small voltage variations to drive separations, "says Hatton.

In a working installation, for example in a power plant producing continuous exhaust gas, two sets of cells of this type of electrochemical cells could be mounted side by side to operate in parallel, the combustion gases being directed from first to a first set for carbon capture, then diverted to the second set while the first set enters its discharge cycle.

An efficient, inexpensive method with low energy consumption

By alternating, the system can still capture and evacuate the gas. In the laboratory, the team proved that the system could withstand at least 7000 charge-discharge cycles, with a loss of efficiency of 30% during this period. Researchers estimate that they can easily improve this figure between 20,000 and 50,000 cycles.

The electrodes themselves can be made with standard chemical treatment methods. Although this is done today in a laboratory, they can be adapted so that they can finally be manufactured in large quantities through a roll-to-roll process, similar to a newspaper printing press. " We've developed very cost-effective techniques, " Voskian says, arguing that they could be produced for tens of dollars per square meter of electrode.

Compared to other existing carbon capture technologies, this system consumes little, as it only requires about one gigajoule of energy per tonne of carbon dioxide captured. Other existing methods have energy consumption ranging from 1 to 10 gigajoules per tonne, depending on the input carbon dioxide concentration.


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