With special treatment, minerals called zeolites – commonly found in cat litter – can effectively remove greenhouse gases from the air, the researchers report.

By David L. Chandler

Methane is a much more potent greenhouse gas than carbon dioxide, and it has a pronounced effect during the first two decades after its presence in the atmosphere. At the recent international climate negotiations in Glasgow, reducing methane emissions was identified as a top priority in attempts to quickly curb global climate change.

Today, a team of researchers at MIT has come up with a promising approach to controlling methane emissions and removing it from the air, using an inexpensive and abundant type of clay called zeolite. The results are described in the journal ACS Environment Au, in an article by doctoral student Rebecca Brenneis, associate professor Desiree Plata and two others.

Although many people associate atmospheric methane with drilling and hydraulic fracturing for oil and natural gas, these sources only account for about 18% of global methane emissions, Plata says. The vast majority of methane emitted comes from sources such as slash-and-burn agriculture, dairy farming, coal and ore mining, wetlands and melting permafrost. “A lot of the methane that goes into the atmosphere comes from distributed and diffuse sources, so we started to think about how you could get it out of the atmosphere,” she says.

The answer the researchers found was something very cheap – in fact, a special type of “earth” or clay. They used zeolite clays, a material so inexpensive that it is currently used to make cat litter. The team found that treating the zeolite with a small amount of copper makes the material very effective at absorbing methane from the air, even at extremely low concentrations.

The concept of the system is straightforward, although much work remains on the engineering details. In their lab tests, tiny particles of copper-enriched zeolite, similar to cat litter, were packaged in a reaction tube, which was then heated from the outside as a gas stream, with Methane levels ranging from just 2 parts per million up to 2 percent concentration, flowed through the tube. This range covers everything that may exist in the atmosphere, down to flammable levels that cannot be burned or burned directly.

The process has several advantages over other approaches to removing methane from the air, says Plata. Other methods tend to use expensive catalysts such as platinum or palladium, require high temperatures of at least 600 degrees Celsius, and tend to require a complex cycle between methane and oxygen rich streams, resulting in makes the devices both more complicated and risky, since methane and oxygen are highly combustible either alone or in combination.

“The 600 degrees that they run these reactors make it almost dangerous to be around methane,” as well as pure oxygen, says Brenneis. “They solve the problem by simply creating a situation where there is going to be an explosion.” Other technical complications also result from the high operating temperatures. Not surprisingly, such systems have not found much use.

As for the new process, “I think we’re still surprised how well it works,” says Plata, Gilbert W. Winslow associate professor of civil and environmental engineering. The process appears to have its maximum efficiency at around 300 degrees Celsius, which requires much less energy for heating than other methane capture processes. It can also operate at lower methane concentrations than other methods can handle, even small 1% fractions that most methods cannot remove, and does so in the air rather than the air. pure oxygen, a major advantage for real-world deployment.

The method converts methane into carbon dioxide. This may seem like a bad thing, given the global efforts to tackle carbon dioxide emissions. “A lot of people hear ‘carbon dioxide’ and panic; they say ‘it’s bad,’ ”says Plata. But she points out that carbon dioxide has much less impact in the atmosphere than methane, which is about 80 times more potent as a greenhouse gas in the first 20 years, and about 25 times more potent. for the first century. This effect stems from the fact that methane naturally converts to carbon dioxide over time in the atmosphere. By speeding up this process, this method would significantly reduce the short-term climate impact, she said. And even converting half of the methane in the atmosphere to carbon dioxide would increase the latter’s levels by less than 1 part per million (about 0.2% of current atmospheric carbon dioxide) while saving about 16% of radiative warming. total.

The ideal location for such systems, the team concluded, would be in places where there is a relatively concentrated source of methane, such as dairy barns and coal mines. These sources already tend to have powerful air handling systems, as a build-up of methane can pose a fire, health and explosion hazard. To overcome the exceptional technical details, the team has just received a grant of $ 2 million from the United States Department of Energy to continue to develop specific equipment for the removal of methane in these types of locations.

“The main advantage of extracting air is that we move a lot of it,” she says. “Fresh air must be sucked in to allow miners to breathe and reduce the risk of the pockets of enriched methane exploding. Thus, the volumes of air that are moved in the mines are enormous. The methane concentration is too low to ignite, but it’s in the catalysts sweet spot, she says.

Adapting the technology to specific sites should be relatively straightforward. The lab setup the team used in their tests included “just a few components, and the technology you would put in a cow barn could also be pretty straightforward,” says Plata. However, large volumes of gas do not flow so easily through clay, so the next phase of research will focus on ways to structure the clay material into a hierarchical, multi-scale configuration that will facilitate circulation. air.

“We need new technologies to oxidize methane at lower concentrations than those used in flares and thermal oxidizers,” says Rob Jackson, professor of earth systems science at Stanford University, who was not involved. to this job. “There is no cost effective technology today to oxidize methane at concentrations below about 2000 parts per million.”

Jackson adds, “There are many questions that remain for scaling this and all similar work: How quickly does the catalyst foul under field conditions? Can we bring the required temperatures closer to the ambient conditions? How scalable will these technologies be when treating large volumes of air? “

A major potential benefit of the new system is that the chemical process involved releases heat. By catalytically oxidizing methane, the process is actually a form of flameless combustion. If the methane concentration is greater than 0.5%, the heat released is greater than the heat used to start the process, and this heat could be used to generate electricity.

The team’s calculations show that “in coal mines, you could potentially generate enough heat to generate power plant-wide electricity, which is remarkable because it means the device could s. ‘self-finance’, explains Plata. “Most air capture solutions cost a lot of money and would never pay off. Our technology could one day be a counterexample.

Using the new grant, she says, “over the next 18 months we aim to demonstrate proof of concept that it can work in the field,” where conditions can be more difficult than in the lab. Eventually, they hope to be able to make devices that are compatible with existing air handling systems and that could just be an extra component added in place. “The coal mining app is supposed to be at a stage that you could hand over to a builder or commercial user in three years,” Plata explains.

Along with Plata and Brenneis, the team included Eric Johnson, a doctoral student at Yale University, and Wenbo Shi, a former post-doctoral fellow at MIT. The work was supported by Gerstner Philanthropies, Vanguard Charitable Trust, the Betty Moore Inventor Fellows program, and the MIT Research Support Committee.

Originally published atMIT Press Office.

Photo by Litter Robot on Unsplash

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