Humans produce more than 6 billion tons of plastic every year, and its accumulation in landfills and oceans is considered by many to be an environmental crisis. Only 9% of plastics are recycled in the U.S., and while bans on single-use plastic bags and straws have some benefits, we’re nowhere near addressing the volume of tossed plastic bottles and containers, toys, furniture and other items.
Researchers at Washington State University have developed one approach to recycling discarded plastics: turning them back into the petroleum products from which most of them came by using a process called hydrogenolysis.
The scientists take polyethylene — which is better known as number 1 plastic — and apply heat, a chemical catalyst and solvents to break the material into jet fuel components or other hydrocarbon products. The process takes roughly an hour and a temperature of 220 degrees Celsius, or 428 degrees Fahrenheit, and converts 90% of the plastic into fuel. Researchers from the University of Washington and Pacific Northwest National Laboratory collaborated on the project.
“Converting waste plastics for fuel could significantly reduce the accumulated solid waste in landfills,” said Hongfei Lin, associate professor in the Gene and Linda Voiland School of Chemical Engineering and Bioengineering at WSU, who led the study. “If the waste plastics are mismanaged, they will end up in oceans and slowly decompose to numerous microplastic particles, carrying contaminants and entering the food chain.”
Researchers have been using chemical recycling to turn plastics into fuel for many years, but the process can require temperatures of 300 degrees Celsius and much higher. The scientists from Washington employed a catalyst made from a rare metal called ruthenium that’s turned into nanoparticles and applied to a carbon substrate. The process also uses a common organic solvent.
Other chemists have conducted similar experiments, but the WSU-led team believes theirs is the first to turn the plastic into liquid fuels under these conditions. Their research was recently published in the journal Chem Catalysis. A group from the University of Delaware in April reported on a process called hydrocracking using different catalysts to turn plastic into fuels.
As the glut of trashed plastics has grown, researchers have been exploring a variety of approaches to chemical recycling to get rid of the waste. Other plastic reuse strategies fall under mechanical recycling — melting and reusing plastics for consumer goods including containers, clothes, furniture and other items. Waste plastics are also incinerated, and most wind up in landfills.
Chemical recycling has been touted by some as a smart solution to the plastics problem, but others point out numerous environmental downsides and put an emphasis on curbing the use of plastics. Depending on the method of chemical recycling, it can use large amounts of energy, emit greenhouse gases and produce and release highly toxic chemicals.
The approach used by Lin and his colleagues is more energy efficient and produces fewer dangerous byproducts. The fuel it creates, however, is still primarily a fossil-based fuel and carries with it climate impacts at a time when the world is seeking low-carbon energy.
“Low-carbon fuels are made from either renewable carbon sources such as biomass or CO2 captured from the air,” Lin explained. If you recycled plastics made from biomass or CO2 captured from the air as the source, “then fuels made from that portion of plastics will qualify for low-carbon fuels.”
The next step for the research is to test the process in a pilot project; Lin said the work has generated international interest and that he is talking to potential partners for scaling up their work.
One of the biggest difficulties in commercializing the technology is how to handle the mix of different kinds of plastics that get combined when we toss them into the recycling bin. Manually sorting the plastics is laborious and costly. The entire sector is wrestling with the problem.
“The challenge is not just for us,” Lin said. “It’s for the whole scientific community.”
WSU graduate student Chuhua Jia helped Lin lead the study. Other authors of the report were WSU’s Shaoqu Xie and Wanli Zhang; Nadia Intan of the UW’s Department of Chemical Engineering; Janani Sampath of PNNL; and Jim Pfaendtner of PNNL and the UW.
