A novel carbon capture technique can scrub the gas out from the air even at relatively low concentrations, such as the roughly 400 parts per million (ppm) currently found in the atmosphere.
We have a climate problem: namely, we’re making the planet hotter and hotter. This change is caused by a build-up of greenhouse gases released by our various activities, and carbon dioxide (CO2) is the single most important such gas. Tackling climate heating hinges on our ability to reduce emissions or to find ways of scrubbing them from the air. Since the former would involve at least some economic contraction, neither industry nor politicians are very keen on it. So there’s quite a lot of interest in developing the latter approach.
Most of the methods available today need high concentrations of CO2 (such as the smoke emitted by fossil fuel-based power plants) to function. The methods that can work with low concentrations, on the other hand, are energy-intensive and expensive, so there's little economic incentive for their use. However, new research from MIT plans to change this state of affairs.
"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," explains MIT postdoc Sahag Voskian, who developed the work during his PhD.
"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."
The technique relies on passing air through a stack of electrochemical plates. The process Voskian describes is that the electrical charge state of the material -- charged or uncharged -- causes it to either have no affinity to CO2 whatsoever or a very high affinity for the compound. To capture CO2, all you need to do is hook the material up to a charged battery or another power source; to pump it out, you cut the power.
The team says this comes in stark contrast to carbon-capture technologies today, which rely on intermediate steps involving large energy expenditures (usually in the form of heat) or pressure differences.
Essentially, the system functions the same way a battery would, absorbing CO2 around its electrodes as it charges up, and releasing it as it discharges. The team envisions successive charge-discharge cycles as the device is in operation, with fresh air or feed gas being blown through the system during the charging cycle, and then pure, concentrated carbon dioxide being blown out during the discharge phase.
The electrochemical plates are coated with a polyanthraquinone and carbon nanotubes composite. This gives the plates a natural affinity for carbon dioxide and helps speed up the reaction even at low concentrations. During the discharge phase, these reactions take place in reverse, generating part of the power needed for the whole system during this time. The whole system operates at room temperature and normal air pressure, the team explains.
The authors hope the new approach can help reduce CO2 production and increase capture efforts. Some bottling plants burn fossil fuels to generate CO2 for fizzy drinks, and some farmers also burn fuels to generate CO2 for greenhouses. The team says the new device can help them get the carbon they need from thin air, while also cleaning the atmosphere. 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.
"All of this is at ambient conditions -- there's no need for thermal, pressure, or chemical input," says Voskian. "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."
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. Other existing methods use up to 10 gigajoules per ton, depending on the inlet carbon dioxide concentration, Voskian says.
The paper "Faradaic electro-swing reactive adsorption for CO2 capture" has been published in the journal Energy & Environmental Science.