AUTONEWS

Food waste can become jet fuel through simpler refining and 50-50 blending

The aviation industry accounts for a large portion of global greenhouse gas emissions. Biobased, sustainable aviation fuel (SAF) can mitigate climate impacts, but transitioning to SAF faces critical supply chain constraints. A research team at the University of Illinois Urbana-Champaign has developed a method to produce jet-grade fuel from food waste, contributing to a circular bioeconomy. In a paper published in Nature Sustainability, they focus on technical and economic considerations.

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.

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.

In a previous study, the researchers outlined the process of developing SAF to meet aviation standards. This study follows the same general approach, converting food waste to crude oil through hydrothermal liquefaction (HTL), a process that mimics the natural formation of crude oil in a fraction of the time, and refining it with a catalyst.

"However, here we use a simpler approach with less catalytic intensity and greater focus on distillation, which is commonly used for industrial purposes. This is a more economical and environmentally friendly method. But the quality of the fuel is not as good, and it needs to be mixed with regular jet fuel," said corresponding author Yuanhui Zhang, Founder Professor in the Department of Agricultural and Biological Engineering (ABE), part of the College of Agricultural, Consumer and Environmental Sciences and The Grainger College of Engineering at the U. of I.

Zhang compared this to the use of ethanol for cars; it must be blended with fossil fuel to work in car engines.

"It would be very difficult to produce enough SAF to meet industry needs, so it makes sense to take a biodiesel approach with a percentage blend," he explained. "Our tests are based on a 50–50 blend, so it will certainly be feasible to use a 10% or 20% blend of SAF with regular fuel."

A practical case for blending...The researchers conducted tests on key parameters to ensure their SAF product meets jet fuel standards set by the American Society for Testing and Materials (ASTM) and the Federal Aviation Administration.

"We are still doing this work on a very small scale. But my lab is now set up to produce several liters of upgraded fuel, which is enough for diesel engine tests. After that, the next step will be jet engine tests," Zhang said.

The biggest bottleneck in SAF production is getting the waste from disposal to reclamation and recovery, Zhang noted. Most food waste ends up either in a landfill or a wastewater treatment plant, where it is separated and converted into sludge. Collecting and reusing food waste presents logistical challenges, but the HTL process enables the use of treated wastewater as feedstock.

While HTL offers a promising approach to creating SAF from wet waste, it leaves a toxic, nutrient-rich byproduct called HTL aqueous phase, or HTL-AP. Zhang and his team explored ways to recover acid and nutrients from HTL-AP through electrochemical (EC) treatment.

The cost of handling byproducts...The researchers also conducted techno-economic and lifecycle analyses for the integrated process of upgrading the biocrude oil and treating the HTL-AP byproduct. They developed three scenarios for the analysis: a baseline where HTL-AP was sent to a centralized wastewater treatment plant; treatment with EC technology to recover and valorize HTL-AP; and a future scenario based on improved EC technology.

Compared with the baseline scenario, using EC technology nearly tripled the cost per gallon due to higher capital and operating costs. However, technological advances are expected to lower the EC costs, so they become equivalent to the baseline in the future.

The team also evaluated global warming potential (GWP), which indicates how much global warming is affected by CO2 emissions. They estimated that both the baseline and the improved EC treatment would be able to achieve negative carbon emissions, leading to lower GWP.

The study outlines a technically feasible and environmentally beneficial pathway for turning urban organic waste into SAF and promoting a circular bioeconomy, the researchers concluded. 

University of Illinois Urbana-Champaign