Scientists are seeking a patent for a technique that would use a form of plasma to remove trace contaminants from landfill gas, allowing the renewable energy source to be more easily harnessed.
The researchers use nonthermal plasma to separate compounds called siloxanes from the gases, and say the approach could be more efficient than traditional methods while also creating a useful byproduct. The patent application was published by the U.S. Patent and Trademark Office on April 1 and builds on work the researchers previously reported in the Journal of Physics D: Applied Physics.
"The idea was, can we remove siloxane but in the process transform it into something so it becomes the raw product for another industry?" said Shamia Hoque, an associate professor of civil and environmental engineering at the University of South Carolina and co-inventor of the technique. "That way we don't reintroduce it back into the waste cycle; it's truly recycled."
The U.S. has sent more than 140 million tons of solid waste annually to landfills in recent years. When the organic matter in all this garbage begins to rot, it emits greenhouse gases such as carbon dioxide and methane. These landfill gases can escape into the atmosphere and contribute to climate change; landfills are the third-largest human-related source of methane emissions in the U.S.
However, this gas can also be piped out of landfills and used to generate energy. About 500 landfills currently contribute to landfill gas energy projects in the U.S.
Siloxanes encompass a group of chemicals that are found in silicones and widely used in medical devices, plastics, cosmetics and many other products that wind up in landfills. When landfill gases are combusted, siloxanes in the gas break down into silica and silicates. These materials build up on and clog components of the engines operated using the landfill gas, and removing them is costly and time-consuming.
Although siloxanes make up less than 1% of the composition of landfill gases, Hoque said, "It's restricting the use of something which ... provides us with an alternative energy resource."
There are several ways to remove siloxanes from landfill gases, including capturing the contaminants in silica gel or activated carbon filters.
"All removal mechanisms take the siloxane and once again put it back into the waste stream," Hoque said. "You basically put it back into the landfill; you're not really removing it."
The alternative technique she and her collaborators developed relies on plasma. Generally this state of matter — in which electrons have been stripped from a gas — is found in lightning and space and is driven by heat. For their work, Hoque and her colleagues instead used an electric field to strip electrons from the gas molecules, creating so-called nonthermal plasma and allowing the bulk of the gas to stay at room temperature.
The researchers have so far tested their experimental setup with helium and carbon dioxide to mimic landfill gas. In the future, they plan to add other components of actual landfill gas such as methane, water vapor and contaminants such as hydrogen sulfide.
To test the technique, the researchers fed the gas into a conical flask where it picked up siloxane. The gas then passed through a plasma reactor, where it met the electric field. The resulting high-energy electrons then reacted with the siloxane molecules and broke them apart. Finally, these compounds recombined into a solid residue on the bottom of the reactor.
Meanwhile, "The gas stream that comes out of the reactor is much cleaner," said Malik M. Tahiyat, a Ph.D. research assistant in mechanical engineering at the University of South Carolina and co-inventor of the technique.
In the experiments he and his colleagues ran, the approach removed about 80% of the siloxanes from the gas in 20 minutes, while carbon filters and other traditional methods can take hours to days to achieve similar efficiency.
The researchers discovered that the residue was a material called polydimethylsiloxane, or PDMS, which is used to make microfluidic chips, contact lenses, sealants, baby bottle nipples and even Silly Putty.
"We're hoping that we can scale the system in a way that is tailored to the plant so it doesn't result in excess cost but it pays for itself through the products that are being produced," Hoque said.
She and her colleagues are planning to partner with landfills so they can test the technology under more realistic conditions. It will also be important to determine how the composition of landfill gas, which varies from one site to the next, might affect how the siloxanes interact with the plasma.
The team hopes to have the technology ready for use in landfills in about two years.
The application for the patent, "Siloxane removal off landfill gas using dielectric barrier discharge plasma," was filed Sept. 25, 2020 to the U.S. Patent and Trademark Office. It was published April 1 with the application number US17/032123. The earliest priority date was Sept. 26, 2019. The inventors of the pending patent are Shamia Hoque, Tanvir Farouk, Malik M. Tahiyat, University of South Carolina. The assignee is University of South Carolina.
Parola Analytics provided technical research for this story.