Common clay could curb methane emissions

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Researchers in the US have developed an approach to controlling methane emissions using a common, inexpensive type of clay called zeolite.

methane emissions
Image credit: MIT

The MIT team’s approach, described in ACS Environment Au, involves treating zeolite clays with a small amount of copper, making it effective at absorbing methane from the air even at very low concentrations.

The paper’s co-author Desiree Plata, associate professor Civil and Environmental Engineering, said that while many people associate atmospheric methane with drilling and fracking for oil and natural gas, those sources only account for around 18 per cent of global methane emissions. 

The majority comes from sources such as slash-and-burn agriculture, dairy farming, coal and ore mining, wetlands and melting permafrost.

Zeolite is so inexpensive that it is currently used to make cat litter. In lab tests, tiny particles of the copper-enhanced zeolite material were reportedly packed into a reaction tube, which was heated from the outside as the stream of gas — with methane levels ranging from just two parts per million to up to two per cent concentration — flowed through the tube. 

This range covers everything that might exist in the atmosphere, the team said, down to sub-flammable levels that cannot be burned or flared directly.

Plata said the process has several advantages over other approaches to removing methane from air. Other methods typically use expensive catalysts such as platinum or palladium, require high temperatures of at least 600°C, and complex cycling between methane-rich and oxygen-rich streams. This makes devices more complicated and risky, as methane and oxygen are highly combustible on their own and in combination.

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According to researchers, the new process has peak effectiveness at about 300°C. It can also work at concentrations of methane lower than other methods can address, even small fractions of one per cent, and it does so in air rather than pure oxygen.

The method converts the methane into carbon dioxide, which many people would view negatively due to worldwide efforts to reduce CO2 emissions, Plata said. But she pointed out that carbon dioxide is much less impactful in the atmosphere than methane, which is about 80 times stronger as a greenhouse gas over the first 20 years and about 25 times stronger for the first century.

This arises from the fact that methane turns into carbon dioxide over time in the atmosphere. By accelerating the process, researchers said the method would ‘drastically’ reduce the near-term climate impact, and that even converting half of the atmosphere’s methane to CO2 would increase levels of the latter by less than one part per million (about 0.2 per cent of today’s atmospheric CO2) whilst saving around 16 per cent of total radiative warming.

The systems’ ideal location, the team concluded, would be in places with a concentrated source of methane such as dairy barns and coal mines. These already tend to have air-handling systems in place since a buildup of methane can be a safety hazard.

Adapting the technology to specific sites should be relatively straightforward, Plata said, however large volumes of gas don’t flow easily throughout clay so the team’s next steps will focus on ways of structuring the clay material in a multi-scale, hierarchical configuration that will aid air flow.

Rob Jackson, a professor of earth science systems at Stanford University said: “We need new technologies for oxidising methane at concentrations below those used in flares and thermal oxidisers.

“There isn’t a cost-effective technology today for oxidising methane at concentrations below about 2,000 parts per million.

“Many questions remain for scaling this and all similar work: How quickly will the catalyst foul under field conditions? Can we get the required temperatures closer to ambient conditions? How scaleable will such technologies be when processing large volumes of air?”

One potential advantage of the new system is that the chemical process involved releases heat. By catalytically oxidising the methane, the process is effectively a flame-free form of combustion. If the methane concentration is above 0.5 per cent, the heat released is greater than the heat used to start the process, and this could be used to generate electricity.

The team’s calculations suggest that at coal mines enough heat could  generate electricity at 'the power plant scale', so the the device ‘pays for itself’. The team has been awarded a $2m grant from the US Department of Energy, which it will now use to demonstrate a proof of concept and ultimately make more devices that could be compatible with existing air-handling systems.