A team of researchers at the U.S. National Renewable Energy Laboratory (NREL) have developed a type of sustainable aviation fuel (SAF) derived from food waste that promises to result in net-zero or even negative net emissions from airliners. The novel process, on which NREL scientists have worked for about five years, avoids the organic waste fermenting into methane, which is harmful to the environment. Rather than allow that refuse to create greenhouse gas emissions in landfills, for example, the process essentially arrests the production of methane during fermentation and instead produces volatile fatty acids (VFA) that can be converted into SAF.
Life cycle analysis shows that by diverting methane-producing waste from landfills, VFA-SAF could cut greenhouse emissions by 165 percent and soot by 34 percent compared with fossil fuel, according to a paper recently published in the Proceedings of the National Academy of Sciences. The study evaluated the catalytic conversion of food waste-derived VFAs to produce a paraffin SAF to meet a 10-percent “fast track” ASTM specification blend for use by Southwest Airlines in 2023. It also details how researchers improved the VFA-SAF flash point and viscosity to allow for a 70 percent renewable blend limit.
“We kind of pushed the boundary on what the fuel can do in terms of going to higher blend limits by further changing the chemistry to see how much renewable content you could get in the fuel, and 70 percent could do it,” explained the study’s principal investigator, Derek Vardon. “When we looked at the overall lifecycle emissions, that's where we got excited, because if you're no longer taking the food waste and putting it in the landfill, the lifecycle analysis supported that you could actually get to a net-zero blend."
According to Vardon, food waste accounts for 6 percent of all greenhouse gas emissions. “In an ideal world, we’d want to make food waste [a non-issue]," he said. "But we wanted to highlight how this technology could address the wet waste problem as well as make energy-dense fuel that could be used for aviation.”
Vardon explained that one of the processes of making SAF involves the use of vegetable oils, fats, and greases, supplies of which are limited. However, if they become dependent on those mediums, refiners will soon “run into a wall” once they exhaust them, he added.
“This technology that we worked on for the paper is new and different because we try to open up the feedstocks you can use and hijack the fermentation process that is currently used to make methane by anaerobic digestion,” said Vardon. “So the novelty is, ‘How do you make more feedstock, specifically wet waste feedstock for SAF, and how can you do that with existing infrastructure—in this case, anaerobic digestion infrastructure.”
Vardon further explained that the ability to make VFA molecules bigger by increasing their carbon content allows not only for the production of hydrocarbons but for direct use in current airplane configurations.
Although the NREL team has worked on the technology for five years, it has spent the last year and a half adapting it for the creation of jet fuel. The team plans to start with a 10 percent blend in a Southwest Airlines 737, hopefully in 2023 if the study proves “wildly successful,” noted Vardon. Meanwhile, the researchers continue to work on a fuel that will allow for the 70 percent blend they have targeted as their ultimate goal within three to five years.