Ocean cleanup vessels may be considering a move away from storing recovered plastics on board to burning it as fuel for the voyage.
Vessels must often store the plastic before transporting it to port, often thousands of kilometres away. Instead, researchers say ocean plastic waste can potentially be converted into fuel on board the ship using hydrothermal liquefaction, which depolymerizes plastics at 300°C to 550°C under high pressure and turns them into monomers and other small molecules.
The resulting fuel has been nicknamed “blue diesel”, according to a new research study published on self-powered ocean cleanup in the Proceedings of the National Academy of Sciences in the United States of America (PNAS).
“The time required for recovering plastics could be reduced if return trips to refuel and unload plastic were eliminated,” the study’s authors state. “Indeed, the harvested plastic has an energy density similar to hydrocarbon fuels; harnessing this energy to power the ship could thereby eliminate the need to refuel or unload plastic from the ship, reducing fossil fuel usage and potentially cleanup times,” the paper adds.
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The study provides a schematic of the proposed plastic conversion system, including components to collect and shred the plastic, remove salts and other impurities, and convert the plastic feed into blue diesel, “a marine plastic–derived fuel with energy density and volatility similar to marine diesel,” it states.
The research team, ranging from members of the Worcester Polytechnic Institute, to the Woods Hole Oceanographic Institution, and Harvard’s Department of Chemistry and Chemical Biology, states that a plastic conversion system can easily fit on most vessels currently involved in the removal of ocean plastic.
An additional benefit of the plastic conversion system could be the implementation of an automated collection system with a boom that could feed the debris for the conversion process through a conveyor. Plastic harvesting is typically performed manually to minimize fuel use, with workers manually scooping the plastic out of the booms and into small bags, the researchers said.
To test the thermodynamic performance of blue diesel, the researchers performed simulations for two individual pure plastics commonly found in the ocean, polypropylene and polyethylene, as well as a “mixed feed” based on the typical composition of marine plastics.
At least one of the simulations showed the probability of the ship’s plastic conversion system producing more fuel than the process and the ship itself could consume on the voyage, when the ship travels at full engine power, and optimized engine power. Surplus fuel generated could be stored for later use, researchers said.
Importantly, the research team said that the plastic conversion process may only be feasible when cleaning marine gyres, a kind of giant whirlpool with floating plastic waste sucked into the middle. The technology may not be feasible on the open ocean or within a gyre lacking booms. The largest accumulation of ocean plastics is in the Great Pacific Garbage Patch, located in the North Pacific Subtropical Gyre between Hawaii and Northern California. An estimated 79,000 tonnes of ocean plastic pollutes the 1.6 million square kilometre area.
Pyrolysis, or the thermal degradation of plastic, was also considered as a possible technology option for self-powered ocean cleanup.