Scrape the plates from a city’s worth of dinners, slurry it into a continuous reactor, and hold it in water at 280 degrees and pressures that would crush a submarine. Wait half an hour. What pours out the other end is not soup but crude oil, the same black, energy-dense liquid that took the planet tens of millions of years to brew underground. The trick is geological alchemy without the geology, and it sits at the heart of a refinery that a team at the University of Illinois Urbana-Champaign has now run from peelings all the way to a fuel that passed the bench tests for a jet engine.
The process is called hydrothermal liquefaction, or HTL, and it is well suited to the wet, messy stuff most fuel chemists would rather avoid. Food waste is roughly two-thirds water, a heterogeneous mush of fats, proteins and carbohydrates, and HTL converts all of those fractions at once rather than fussily extracting the oily bits.
From Soup to Spec
Aviation is the hard case in the decarbonisation story. Batteries are far too heavy for the energy a wing needs, so planes will keep burning hydrocarbons for the foreseeable future, and that makes a drop-in liquid fuel from waste one of the few credible routes to lower-carbon flight. The snag has always been quality. Raw HTL biocrude is sour, sulphurous and clogged with nitrogen and oxygen, and the upgraded oil tends to be dominated by straight-chain alkanes that freeze before a plane reaches cruising altitude. Strip those problems out and you usually end up with a thin, alcohol-derived fuel that has to be blended with conventional kerosene to fly at all.
The Illinois refinery, described in Nature Sustainability, takes a different tack. After distillation, the biocrude goes through hydrocracking over an iron-zeolite catalyst and then hydrotreating over palladium, and what emerges is unusually rich in cycloalkanes, the ringed hydrocarbons that give jet fuel its density and keep rubber seals from shrinking.
The numbers are the surprising part. The team’s candidate ran to 70.5 per cent cyclic hydrocarbons, a higher fraction than conventional Jet A, with a freezing point of minus 60 degrees C and an energy density that edges out the fossil benchmark. Deoxygenation reached 98 per cent and the sulphur was gone entirely. Crucially, the fuel cleared the aviation specifications as a 100 per cent drop-in, no blending required, which is the property that the usual waste-to-jet pathways cannot manage.
That last point comes with a wrinkle worth airing. In describing the work, corresponding author Yuanhui Zhang of Illinois has talked about a simpler, distillation-led variant of the process that produces a lower-grade fuel needing to be cut with kerosene, much as ethanol must be blended for car engines. The published refinery is the fuller version, hydrocracked and hydrotreated to the drop-in standard; the blend talk is really a comment on practicality at scale, where, as Zhang puts it, “it will certainly be feasible” to run modest blends of 10 or 20 per cent if making enough pure SAF proves too tall an order.
The Bit Everyone Ignores
Here is where the project gets clever, and a little unglamorous. HTL does not just yield biocrude; it leaves behind a foul, watery stream called the aqueous phase, laced with pyridines and other toxic, nitrogen-bearing compounds, and carrying off something like a third of the feedstock’s carbon and three-quarters of its nitrogen. Most studies wave this byproduct away towards a wastewater plant and move on. The Illinois group instead ran it through an electrochemical cell, partial electrooxidation followed by electrodialysis, which detoxified the organics, pulled out high-purity acetic acid and recovered the nitrogen, phosphorus and potassium as fertiliser feedstock. And the cell threw off hydrogen as it worked, enough, it turns out, to cover the entire hydrogen demand of the upgrading step. The loop closes on itself: the dirty water powers the clean fuel.
None of which makes the economics easy yet. At today’s costs, the electrochemical kit nearly triples the price of the fuel, dragged up by laboratory-grade platinum electrodes that run to forty thousand dollars a square metre.
The team’s modelling suggests that with cheaper electrodes, lower cell voltages and a clean grid, the minimum selling price falls to about $3.82 per gallon-equivalent, roughly competitive with fossil jet fuel once tax credits are counted, and the whole system tips to net-negative carbon at around minus 8.5 kilograms of CO2 per gallon-equivalent.
The honesty of the paper is in its caveats. That carbon credit depends almost entirely on diverting food waste that would otherwise rot in a landfill, the fate of about 60 per cent of America’s scraps; route the same waste through composting or anaerobic digestion, which are already low-emission, and the sums no longer beat fossil fuel. Scale is the other wall. A food-waste-only plant big enough to make sense would need a feedstock catchment of two to three million people, and barely a handful of US cities clear that bar, which pushes the researchers towards co-processing with sewage sludge and other wet wastes, of which there is no shortage. For now the work runs, in Zhang’s words, “on a very small scale”, though the lab can already brew several litres at a time.
Diesel engine trials come next, and then the real test. The next step, Zhang says, will be jet engine tests, the point at which a vial of upgraded dinner scraps either earns its wings or doesn’t.
DOI / Source: 10.1038/s41893-026-01848-1
Frequently Asked Questions
Why does jet fuel from food waste matter when we already have electric cars?
Aircraft cannot run on batteries any time soon, because the energy a plane needs would weigh far more in cells than in kerosene. That leaves liquid hydrocarbons as the only near-term option for flight, so a high-quality fuel made from waste is one of the few routes to cutting aviation’s carbon footprint. Aviation already accounts for around 9 per cent of US transport emissions, and that share is expected to grow.
How can food scraps turn into crude oil so quickly?
A process called hydrothermal liquefaction holds wet waste in water at roughly 280 degrees C under high pressure for about half an hour. Those conditions mimic the heat and pressure that slowly cook buried biomass into petroleum, but they compress a geological timescale into minutes. Because the method works on soggy material, it suits food waste, which is mostly water.
Is it true that this fuel can fly without being blended with regular kerosene?
In bench testing, yes. The refined fuel is dominated by ringed hydrocarbons called cycloalkanes, which give it the density and cold-weather behaviour that aviation standards demand, allowing it to clear the specifications as a 100 per cent drop-in. That is unusual, since most waste-derived jet fuels are too thin and must be mixed with conventional fuel to qualify.
What’s stopping this from scaling up tomorrow?
Cost and feedstock. The electrochemical step that cleans the wastewater currently relies on expensive platinum electrodes that nearly triple the fuel price, though the team expects cheaper materials and clean electricity to bring it down to roughly $3.82 per gallon-equivalent. Supply is the other limit: a food-waste-only plant would need a catchment of millions of people, so the realistic path involves mixing in sewage sludge and other wet wastes.
Is making fuel always better than composting the waste?
No, and the researchers are careful to say so. The carbon benefit comes mainly from rescuing scraps that would otherwise rot in a landfill and release methane. If the same waste would have been composted or anaerobically digested, both already low-emission, then turning it into fuel does not come out ahead, so this route is meant for landfill-bound waste rather than as a replacement for good recycling.
