Scientists convert household trash into jet fuel — cutting aviation emissions by up to 90%

A walk through any busy airport makes it clear that flying remains a central part of modern life. Yet every plane that lifts off depends on fuel that adds to a warming climate. Aviation is responsible for roughly 2.5 percent of the world’s carbon emissions, and demand for air travel is expected to keep rising.

Unlike cars or buses, long flights cannot easily switch to batteries or hydrogen because they need far more energy. That reality has pushed scientists and policymakers to search for fuels that can power jets without heating the planet.

One possible option is gaining traction. Sustainable aviation fuel, or SAF, is already compatible with the jet fuel formulated today and in use across the globe, and it could reduce aviation emissions by over 65 percent through mid-century. However, SAF currently accounts for less than 1 percent of global jet fuel use due to high costs, tight supplies, and limited experience.

A recent article published in Nature Sustainability highlights an unexpected way forward. The researchers investigated a new source of low-carbon jet fuel, not from crops, cooking oil, or forestry waste, but from everyday municipal solid waste. The results provided a potential pathway to reducing waste and emissions that also alleviates the pressure on landfills.

Municipal solid waste is the mix of food scraps, paper, plastics, metals, and other materials found in trash cans and on the curb each week (see image below). The considerable accumulation of waste is diverted to landfills nearing saturation or incinerators that must undergo costly pollution reduction procedures. Converting this waste into jet fuel could help cities handle rising trash while giving airlines a cleaner energy supply.

Researchers from Tsinghua University and the Harvard-China Program utilized real operational data from a commercial gasification facility and Fischer-Tropsch operation facility to further evaluate this concept at the industrial scale. Their study analyzed each part of the process from the waste collection point to the loading of the finished fuel product into the aircraft.

Trash is first sorted, crushed, dried, and shaped into pellets that combust more evenly. Gasification, gas cleaning, and reactor processes convert feedstock pellets into liquid hydrocarbons, which have been upgraded through further refining in order to meet jet fuel specifications.

With the full life-cycle evaluation, the biogenic fraction of municipal solid waste generated an intensity of emissions of 14.1 grams CO2 equivalent per MJ of fuel energy produced. Conventional jet fuel has an intensity of emissions in the range of 89 grams CO2 equivalent per MJ of fuel energy produced, indicative of at least an 84% emissions reduction that could be realized via waste-derived sustainable aviation fuel when all systems are operating properly.

Not all aspects of the system are the same. The most carbon-intensive step is the gas cleaning process, due in part to the water gas shift reaction that consumes carbon monoxide (CO) and produces CO2 as a product that is released (and not captured or used). Additionally, the drying process of shredding waste adds a large energy burden to the total emissions reduction assessment, as much of the waste in many countries has a high amount of moisture.

The most uncertain aspect of the overall gasification process is gasification performance. Municipal waste may contain varied sizes, moisture, or composition, making it difficult to gasify consistently. When the hydrogen-to-carbon monoxide ratio drifts far from the preferred amounts for reactors, additional energy is needed to maintain the fuel output. Even modestly reducing the effective gas composition can result in life cycle emissions increasing by more than half and the amount of fuel yield being reduced by nearly one-third.

The energy source also matters. Running the system on clean power can reduce carbon emissions by more than 75 percent. The waste composition plays an additional role. Plastics can support improved gasification by increasing the overall heating value, but as a fossil-based material, they can nearly double life cycle emissions if not carefully managed.

Improvements can take place through sorting or shifting to lower-energy drying processes, like biodrying, to reduce the environmental impact of gasification. Such improvements are most beneficial when food waste makes up a large portion of the trash stream, making heating for drying more energy-intensive.

Even gasification, with very low carbon emissions, operates at only about 1/3 of the carbon in the waste to fuel output conversion. This is the case because the fuel gas coming from the waste does not match the preferred reactant or gas mixture the reactor wants.

Researchers looked at two alternatives. In one example, nearly pure carbon dioxide is extracted from the gasifier during the cleaning process. This carbon dioxide can be stored underground, resulting in a net negative carbon intensity for the system runs. However, this means that some of the carbon that could become fuel is locked below ground.

The carbon dioxide can also be reacted with “green” hydrogen as a way to produce fuel. This method, referred to as a power-to-liquid route, has the potential to elevate carbon conversion efficiencies above 60 percent; however, it requires substantial use of renewable hydrogen and energy.

An alternative method can also add renewable hydrogen directly into the gas mix rather than converting the carbon monoxide. This has already had preliminary trials in industrial processes. This process increases the carbon conversion amount by 1.5 times the amount of emissions, reducing the emissions to around 7 grams per megajoule.

Emissions could also potentially go down below 4 grams per megajoule with an excellent gasification process and lower energy drying methods. For example, with one ton of municipal waste, this could displace over 170 kg of carbon dioxide while using a fraction of the hydrogen required by the power-to-liquid processes.

Researchers have mapped the potential production of jet fuel from municipal waste worldwide under different assumptions. Under one scenario, reflecting current practices, the potential was up to 50 million tons of fuel per year. In that scenario, we would have reduced the global amount of aviation emissions by about 16 percent in 2019.

If you add renewable hydrogen to the conversion, the potential jet fuel is estimated to be 80 million tons per year, which would satisfy greater than 25 percent of the global demand for jet fuel and reduce annual emissions by way of avoiding around 270 million tons of carbon dioxide.

Different geographic regions are more suited than others. China is estimated to produce over 11 million tons of waste-derived SAF, representing the potential for roughly 30 percent of its jet fuel. The European Union, the United States, and India are anticipated to contribute around 5 million tons each. In those regions where food- and feed-based fuels are banned, such as Europe, some type of waste-derived fuel is likely to be necessary in order to meet long-term fuel blending regulations.

Governments are providing impetus to this industry now. The United States anticipates getting to 3 billion gallons of sustainable aviation fuel (SAF) per year by 2030, and 35 billion gallons a year by 2050. European regulations require increasing SAF fractions to be implemented on all flights leaving EU airports starting at 2 percent next year. And, under the International Civil Aviation Organization framework, airlines are expected to offset growth in emissions, making waste-based SAF increasingly financially attractive as carbon prices increase.

Costs are still a factor. Initial commercial plants may produce waste-derived SAF at 0.89 to 1.70 dollars per liter. If plants ramp up production and improve technology, these costs may fall to between 0.60 and 1.08 dollars per liter. This is still above fossil jet fuel and almost consistent with current SAF prices for sale in today’s market.

The bulk of costs will be capital outlay. Gasification and Fischer-Tropsch units spend the most money for the total implementation. Any pretreatment adds to operating costs and significant investments as well. Even small charges for the waste itself could raise fuel prices quite a lot.

In the end, though, waste-derived SAF starts to become a better deal versus purchasing offsets under CORSIA prices around the year 2030. A more salient switch occurs in countries where offset prices rise quickly or public subsidies decrease the price of SAF production. In China, using waste-derived SAF instead of offsets from 2027 to 2035 could save over 2 billion dollars, and in the United States, the savings would be around 1 billion dollars. In high offset price scenarios, savings are even larger.

Flying will persist as part of everyday life, but the way the world powers those flights is beginning to change. While waste-derived SAF will never mitigate the entire climate impact of aviation, the thought that household garbage could help fuel global travel hints (at the very least) at a surprising new addition to both waste management and clean energy.

The sentiments of the study from Tsinghua University and Harvard University note that this pathway could significantly mitigate emissions through their findings, as long as the technology improves and policy persists and solidifies.

Jingran Zhang, the first author of the study, argues that the switch could occur quickly in time. "Turning everyday waste into jet fuel could be an innovative but significant near-term step toward cleaner aviation," Zhang argues. Michael B. McElroy, a senior researcher at Harvard and co-author, stated that broad cooperation across government, airlines, and manufacturers would be required to make this efficient on a larger scale.

The plane that carries you across the world one day could, in part, run on the waste you left at the curb. At first thought, the idea is surprising, but the associated science suggests it could make a significant difference if the world chooses to invest in it.

Research findings are available online in the journal Nature Sustainability.

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