Clay curbs methane emissions for mining sector
US researchers have demonstrated how minerals zeolites can absorb and remove methane from the atmosphere even at low concentrations.
Associate Professor Desiree Plata, at Massachusetts Institute of Technology (MIT), notes that most global methane emissions are from slash-and-burn agriculture, dairy farming, coal and ore mining, wetlands, and melting permafrost – with only 18% from oil and gas exploration and extraction. ‘A lot of the methane that comes into the atmosphere is from distributed and diffuse sources, so we started to think about how you could take that out of the atmosphere,’ she says.
Having known bacteria could convert methane into oxygen at low temperatures, the team has pursued biomimetic methane catalysts for about five years. They identified zeolite after testing a range of materials.
Here, 1g of the catalyst made from zeolite enhanced with 1% of copper is packed into a heated reaction tube at 300°C. Through this, a stream of air is passed with methane at levels of 2% down to 2ppm – to replicate amounts found in the atmosphere.
The paper, Atmospheric- and low-level methane abatement via an earth-abundant catalyst published in ACS Environmental, describes how the ammonium mordenite zeolite powder (5 ± 0.1g Alpha Caesar) is stirred with 0.05M copper nitrate solution (500Ml) for 22-26h and then vacuum-filtered. The filtered solids are dried at 130°C for 10-14h, ‘without acidic or organic solvents, with low energy requirements, and without the need for exotic or complex multi-step syntheses’.
The catalyst is said to still work when methane levels are fractions of a per cent in air not pure oxygen, unlike many other methods, while being more affordable too. Common catalysts palladium and platinum cost about US$77/g and US$33/g respectively, note the researchers, compared to copper at around US$0.01/g. These catalysts often need heating to 600°C or more and cycle between methane-rich and oxygen-rich streams, which is both complex and increases the risk of combustion.
The new process also lasts for extended periods of time – 1g of catalyst in the lab worked for weeks.
The zeolites convert the methane into CO₂. Plata points out that CO₂ 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 effect arises because methane turns into CO₂ naturally over time in the atmosphere. By accelerating that process, she believes this method would drastically reducethe near-term climate impact, as converting half of the atmosphere’s methane to CO₂ would only increase it by 1ppm, or 0.2% currently, while saving about 16% of total radiative warming.
The catalyst is aimed at enriched methane sources that cannot be dealt with any other way, such as at cow barns and coal mines which have air systems to prevent methane becoming a health or fire hazard.
Plata explains that a mine is ideal because of the large amount of air being moved around and the methane concentration is too low to ignite but is in the catalyst’s sweet spot.
She adds that it requires ‘only a few components, and the technology you would put in a cow barn could be pretty simple’. However, the team will need to configure their catalyst in layers to aid air flow as large amounts of gas do not easily flow through the clay. They hope eventually it will be reusable, or simply regenerate in place.
The paper adds, ‘Hierarchical material construction, such as preserving the nanoconfinement of the copper-aluminosilicate active sites but supporting those on a substrate to promote better gas-catalyst contact, improved heat transfer properties by varying the material choice, and novel reactor design are all viable routes to enhance conversion rates.
‘This potential for improvement aside, for the first time, these results demonstrate that copper mordenite can convert methane at low-level concentrations, previously untested by either other zeolites, lean combustion, or ventilation air methane catalysts.’
The technique also releases heat and, as it oxidises the methane, it is effectively flame-free combustion. If the methane concentration is above 0.5%, the heat released is more than the amount needed to start the reactions, which could be used to pre-heat incoming air to make the process self-sustaining, with the excess used to generate electricity potentially at power plant scale.
The US Department of Energy has awarded US$2mln for specific equipment development for in situ demonstration.