Plastic collected from the ocean could be converted to oil, and a study of the idea suggests conversion should be onboard, powering the ship and creating a circular economy. The alternative of bringing the plastic to land for conversion to fuel is less efficient, a new study has found, despite the economies of scale of land-based conversion plants.
The plastic dumped into the oceans represents not only a threat to wildlife, but stored energy. Hydrothermal liquefaction (HTL) converts it back to oil with less leftover waste than pyrolysis. In Proceedings of the National Academy of Sciences, researchers have modeled the best way to run an operation to clean up the Great Pacific Garbage Patch, an area of the ocean where currents concentrate plastic waste.
Some plastic retrieved from the ocean is recycled into consumer materials that carry an extra price to fund further operations. However, it’s unlikely there will be demand for more than a tiny fraction of the millions of tonnes floating around in the oceans' great gyres to be used in this way. Fuel is a different matter, however, making HTL an attractive option.
One obstacle, however, is that currently, “[a] ship that must store plastic on board until it returns to port, often thousands of kilometers away, to unload the plastic, refuel, and resupply,” the paper notes.
“Oil yields from HTL are typically >90 percent even in the absence of catalysts and, unlike pyrolysis, yields of solid by-products – which would need to be stored or burned in a special combustor – are less than 5 percent,” the authors add, while acknowledging HTL requires temperatures of 300-550 ºC (570-1020 ºF) and pressures 250-300 times sea-level atmospheric conditions.
There is considerable uncertainty about how much plastic a ship of a certain size could collect a year, and how much energy it would need to expend in doing so.
Nevertheless, the authors calculate a ship carrying an HTL converter could produce enough oil from the plastic it collects to operate permanently at sea even using pessimistic assumptions, with one caveat. Their calculations require the ship to be collecting plastic concentrated by a separate boom, such as those being trialed by The Ocean Cleanup, rather than trying to catch the plastic itself. It is likely there would be enough oil left over for trips to and from base when necessary. However, even in the most polluted parts of the ocean, a ship without a boom could not collect enough plastic to keep it moving.
Much as a fleet of ships scouring collector booms seems like a win-win, ocean clean-up may not be as great as it appears. Some analysis has shown efforts to clean the Great Pacific Garbage Patch will cause considerable harm to zooplankton and may do less to remove plastic than hoped. There is little point in removing mid-ocean plastic as long as we keep adding more.
Moreover, burning former-plastic releases all the carbon dioxide and most other pollutants of fresh oil, plus more for the conversion process. Recycled plastics may be environmentally attractive compared to sulfur-rich bunker fuel, but the balance may shift as electric-powered shipping becomes increasingly available.