Researchers at Idaho National Laboratory, working with Michigan Technological University and Convergen Energy, a company based in Green Bay, Wisconsin, have pioneered a technique for combining non-recyclable plastics and paper fibre that would otherwise end up in landfills to form pellets with an energy content like bituminous coal.
The key is torrefaction, a thermal process INL has used before, to convert biomass into a coal-like material. Torrefaction uses the mild application of heat, typically between 200°C and 320°C, to make biomass materials more homogenized and consistent while also removing chlorine and other compounds that can turn into harmful gases during combustion.
“We’re trying to find a solution for keeping waste plastics out of the environment,” said Ted Hansen, Convergen’s CEO. While plastic soda bottles and milk containers can be recycled, most of what you walk past on the shelves of your local supermarket — laminated pouches, labels, wax-coated paper — is nonrecyclable and goes to landfills. These plastics take hundreds of years to decompose. (While biodegradable plastics are being developed, in 2021, they represented less than 1% of the plastics made worldwide.)
On the other side of the equation, paper products, while biodegradable, decompose in landfills to create methane, a greenhouse gas 25 times more potent than carbon dioxide. Combining paper and plastic to form stable feedstocks that can substitute for coal and reduce landfill mass would be a green energy win-win. “Those are our target products,” Hansen said. “Instead of sending them to the landfill, we want to turn them into fuel or upcycled products.”
Compacting nonrecyclable plastics into pellets
Whether they come from non-recyclable materials or biomass residue, before torrefied fuels can be widely accepted as an inexpensive source of green energy generation, they must be chemically and physically uniform, weather resistant, and stable enough to transport and store. Product uniformity is essential for compatibility with fuel sale agreements like those required by fuel boilers, where consistent heating content is certified along with contaminants and inorganics levels.
With any biomass, pellet storage is a challenge if not managed properly. Whether it’s hay in a barn or wood pellets in the hold of a cargo ship, self-heating and spontaneous combustion become a serious and potentially catastrophic problem if biomass gets wet. There can also be environmental and health and safety problems related to off-gas emissions.
Being energy dense and hydrophobic in nature, torrefied biomass has been identified as a safer replacement. Through their collaboration, Dr. Jordan Klinger and Eric Fillerup of INL, Dr. Ezra Bar-Ziv of MTU, and Hansen discovered that nonrecyclable plastics can help increase pellet density while improving safety. Due to its softness at elevated temperatures and pressures during compaction, plastic provides a skeletal structure that encapsulates the fibre and forms a barrier to keep oxygen and water from reacting with torrefied fibres. This reduces the self-heating that can lead to spontaneous combustion.