The world’s oceans have a plastic problem. However, a bold new approach from researchers at several institutions says that the same plastic could also be the solution.

The team, with members from Worcester Polytechnic Institute, Woods Hole Oceanographic Institution, and Harvard University, believes that the plastic clogging up our oceans can be used as fuel for ships that work to clean the oceans of plastic. In a new study, they describe the process through which plastic can be converted to ship fuel in order to support such a scheme.

If applied, this approach would allow ships to operate continuously to clean the oceans.

Putting it to good use

“Plastic waste accumulating in the world’s oceans forms massive ‘plastic islands’ in the oceanic gyres. Removing [it] offers an opportunity to restore our oceans to a more pristine state,” the authors explain. “To clean the gyres, ships must collect and store the plastic before transporting it to port, often thousands of kilometers away. Instead, ocean plastic waste can be converted into fuel shipboard, for example, using hydrothermal liquefaction”.

Millions of tons of plastic find their way into the ocean year after year. The smaller fragments disperse, while larger pieces of plastic clump together forming plastic ‘islands’. These tend to end up in ocean gyres, large systems of ocean currents generated by winds and the rotation of the planet that ‘spin’ in place.

Plastic waste poses a very real threat to marine life. As such, efforts to clean up the seas have been repeatedly attempted over time. Ships are sent out to garbage patches where they collect as much plastic as they can hold and bring it back to port for processing. Although this approach works, it’s by no means ideal. Going back and forth between these patches and port areas takes time, fuel, and slows down the efforts overall.

The authors of this study propose using the plastic itself as fuel for the ships and machines used to process the waste. This could have a powerful dual benefit. It would dramatically improve the efficiency of clean-up efforts by slashing downtime, while also being a greener option overall, as it would reduce emissions associated with fuel use (and ships can be very polluting).

Plastic waste can be converted to a type of oil via a process known as hydrothermal liquefaction (HTL), the authors explain. During HTL, plastic is heated to around 300–550 degrees Celsius (572-1022 Fahrenheit) at high pressure — 250 to 300 times the standard atmospheric pressure.

According to their estimates, one ship equipped with an HTL converter could produce enough oil to be self-sustainable (i.e. to keep both the ship and the converter operational). They envision a system where permanent collection booms would be stationed at multiple sites around a large garbage patch and maintain a steady supply of plastic for the ships to convert.

Such an approach is not without its problems. The HTL process itself, as well as the burning of the oil it produces, would obviously release carbon dioxide. That being said, the authors explain that it would still be a lower quantity than what a ship burning conventional fuel would emit during a clean-up mission. There would still be practical constraints on how long a mission could carry on for; the HTL process would produce a relatively small quantity of solid waste that would eventually need to be returned to port, and there’s only so much time a crew’s supplies and sanity can last for on the open seas. However, it would reduce the need for round trips down to once every few months or so, which would also be fueled by the oil produced by the converters.

I personally like the idea of such an approach. It makes practical sense, and I’m sold on the idea of turning a problem into an opportunity or solution. So far the idea is still in its theoretical stage, but it definitely has promise. Fingers crossed that we’ll see it implemented in the not-so-distant future.

The paper “Thermodynamic feasibility of shipboard conversion of marine plastics to blue diesel for self-powered ocean cleanup” has been published in the journal PNAS.