The aerospace industry is responsible for a significant share of air pollution. 2.4% of the emissions in the US are caused by aviation. Left unchecked, this number is likely to increase as the population grows and more people travel by air. Fortunately, innovations in sustainable jet fuel are working to make aviation more sustainable—including technology to turn trash into usable fuel.
Traditional jet fuel is hydrocarbon-based. Conventional plastic is also hydrocarbon-based. Recently, researchers at Washington State University developed a way to turn plastics back into hydrocarbons—which could then be made into fuel.
The research used #1 plastic, also known as polyethylene. This is the most common type of plastic, and one that can be found polluting waterways and filling landfills worldwide. Using a ruthenium on carbon catalyst and a solvent, researchers were able to convert as much as 90% of the polyethylene to hydrocarbons, including jet fuel and lubricants. The reaction took only an hour, and was successful at a relatively low temperature of 220°C (428 °F). By adjusting these conditions, it's possible to obtain different end products. This means that the reaction can be fine-tuned to yield whatever hydrocarbon-based compounds are needed.
A similar discovery by researchers at the University of Delaware's Center for Plastics Innovation involved a process called “hydrocracking.” This uses zeolite and a mixture of metal oxides to break down plastics into smaller molecules, then adds hydrogen molecules to stabilize the resulting product. This turns them into branched molecules that may be made into a variety of end products. Like the process used by the Washington State University researchers, this reaction requires a fairly low temperature of about 250°C (482°F). Interestingly, this method works with a variety of plastics—even when they are mixed together. This gives it an advantage over other types of plastic recycling, where they must be separated.
Plastic trash is a global problem. Leaving it in the environment allows it to enter the food chain, where compounds in the plastics may aggregate and act as endocrine disruptors. Some types of plastic trash like shopping bags, mimic food sources for aquatic animals and cause gastric blockages. To many people, recycling looks like the solution to this issue. Unfortunately, it's not that simple.
The popular conception of recycling is that it keeps trash from ending up in landfills. While some waste (like aluminum cans) can be recycled relatively cheaply, easily, and indefinitely, plastic cannot. Recycling plastic typically means just melting it down and pouring it into a new mold. This can only be done a few times before the recycled plastic is no longer usable and must be thrown away. Plastics that are contaminated with food or other products can also ruin an entire batch of recycled plastic, and must be discarded. In the end, less than 10% of plastics get recycled every year.
The reaction studied by the Washington State University researchers uses a common solvent, a low temperature, and only a brief period of time. This makes it a very efficient, low-cost way to break down and recycle plastic.
The aerospace industry has been working to increase engine fuel efficiency and find new, greener sources of fuel. The ability to turn plastic waste into jet fuel is a valuable intermediate step in this process.
Some researchers and manufacturers are working on converting to use renewable fuels, like solar- or wind-sourced electricity or liquid hydrogen. The process of turning plastic into jet fuel yields a traditional, hydrocarbon-based fuel. This is still an important step, as much of the aerospace industry still relies on hydrocarbons as a fuel source. If it can decrease the demand for virgin hydrocarbon fuel, it will still have a beneficial impact on aerospace-related emissions overall.
Plastic recycling is an environmental bugbear. While popularly regarded as a way to keep trash from polluting the environment, in reality, it's a slow, inefficient process that can only temporarily remove about 9% of plastic from the environment. The chemical recycling methods discovered by the University of Delaware and Washington State University researchers present a potentially more viable alternative that can yield high-quality jet fuel. Until the aerospace industry has fully weaned itself from hydrocarbon-based fuels, finding ways to produce these fuels without crude oil will be an important step toward reducing aviation-based carbon emissions.
