Sample the pungent air inside an animal barn, and it will be a muggy mix of emitted ammonia and especially methane, a potent greenhouse gas that accelerates climate change. Now, a team of researchers explain in a new study that they have built a contraption that can strip over 90% of this gas from barn air—using just chlorine and light.
Methane is a short-lived greenhouse gas compared to carbon dioxide, and yet its danger lies in its enormous warming potential: it traps 28 times more heat in the atmosphere than does CO2. While technologies already exist to trap emitted methane from industrial facilities, part of the challenge of this greenhouse gas is that much of it is released in low concentrations from thousands of point sources—like livestock barns—where the small amounts can’t really justify the high removal costs.
Yet collectively, those point emissions make up a sizable chunk of agriculture’s 40% contribution to global methane emissions. “It turns out that three-quarters of methane emissions are from low concentration sources, where methane is below 1000 parts per million,” says the new study’s lead author, Matthew Johnson. He and his colleagues wanted to tailor-make a removal device for these unique locations, one that would cost less and also require less energy than conventional removal methods do.
Their starting point was an unexpected one: chlorine, and UV light. Chlorine behaves in a very specific way when exposed to the high energy of UV, which causes the chlorine to break apart into two chlorine atoms, creating free radicals that naturally seek out a hydrogen to attach to.
Conveniently, abundant hydrogen is available in methane. So when the light-activated chlorine comes into contact with the methane, those chlorine atoms strip it of its hydrogens, a process that in turn breaks down the methane and dramatically accelerates the time it takes to decompose.
To facilitate this process, the researchers built a metal box housing a network of pipes that channeled methane, chlorine, and air together into a mixture that was then funneled into a reaction chamber where the magic occurred under exposure to UV light. At the other end of this chamber, sensors detected the amount of methane remaining in the air, once it was released.
After running multiple tests with their novel device, the research team showed that it could successfully remove 58% of ambient methane, breaking the gas down into carbon dioxide, monoxide, and hydrogen. The hydrogen byproduct can be recycled, while the carbon remnants can be diverted before they’re released, if the device is paired with capture technologies, explains Johnson, who is a professor of atmospheric chemistry at the University of Copenhagen.
Even more promisingly, in lab tests that he and his colleagues have carried out since the paper was published, Johnson says that they’ve managed to strip over 90% of the methane from ambient air.
Meanwhile through its hydrogen-pinching habits, the chlorine was converted into hydrochloric acid, a substance that can be recycled within the system. Furthermore, the researchers say that the chamber’s reactions work at ambient temperatures, which requires less energy than many conventional methane-harvesting technologies, where often large amounts of heat are needed to power catalysis.
With this relatively simple invention, the researchers have found a way to intercept potentially thousands of tons of methane before it leaches into the atmosphere from farms. Soon they’ll be developing new real-world prototypes: the reaction chamber is scalable, and they envision that it could be tailored to a range of other applications.
“Next we are going to test the system on exhaust air in the field, at an agricultural research station,” Johnson says. “At the same time we are setting up [the device] inside a shipping container that we can take to locations that emit lots of low concentration methane, like a pig barn, a biogas plant and a wastewater treatment plant.”
Johnson et. al. “A high efficiency gas phase photoreactor for eradication of methane from low-concentration sources.” Environmental Research Letters. 2023.