A simple, cheap pre-treatment method has the potential to radically lower the price of sustainable aviation fuel (SAF) based on waste wood biomass, making it cost-competitive with fossil-based jet fuel while reducing emissions by up to 80%.
SAF is not a perfect green aviation solution - in fact, no perfect solution exists yet. Using SAF instead of jet fuel still produces carbon dioxide, but it is a purer fuel that produces 3% more energy while burning cleaner and with significantly reduced sulfur and particulate emissions. It eliminates the entire cost of emissions from the oil extraction and refining process and, depending on the feedstock used, can even be net carbon negative—all without the need to modify jet engines.
Unlike biofuels, second-generation SAF does not use corn, sugar cane, soybeans or other food crops. This is an appalling waste of land and water. Instead, raw materials such as wood residue, sugarcane bagasse, corn stover, and other cheap, abundant waste products from milling operations are used.
Researchers at the University of California, Riverside, believe the problem lies in lignin, a key structural component in plant cells. Lignin is tough and stretchy, giving trees their strength, but also makes extracting carbon from biomass for fuel difficult, especially when working with harder woods. In fact, many companies choose to directly burn lignin to obtain heat and electricity. Although this process makes economic sense, it is absolutely undesirable from an environmental perspective.
However, a research team at the University of California, Riverside, developed a preprocessing method that changes the equation significantly. By adding tetrahydrofuran (THF) to water and dilute acid during biomass pretreatment, the team found that they could significantly increase overall efficiency while utilizing the lignin and sugars in the biomass to generate fuel.
The result: waste biomass can bring more aviation benefits. Using corn stover can increase fuel yield by 18%, and with THF pretreatment, the gasoline equivalent per ton of dry feedstock increases from 44 gallons (167 liters) to 51.8 gallons (196 liters). As for the tough poplar wood with high lignin content, the gasoline equivalent yield per ton of dry raw material is as high as 75.9 gallons (287 liters), which is almost twice the gasoline equivalent extracted from corn stalks by traditional processes.
Best of all, THF pretreatment chemical is low-cost and particularly easy to obtain because it can be made from biomass sugars already being processed at SAF plants.
GlobalAir cited average U.S. Jet-A prices at $6.45 per gallon and SAF average prices at $9.28 per gallon at the time of writing this report. The UC Riverside team calculated that its CELF (cosolvent enhanced lignocellulosic fractionation) production process could cost as little as $3.15 per gallon.
Does this mean 80% clean jet fuel costs half the price of regular jet fuel? No; production costs do not take into account transportation, logistics, business costs or profits, and fossil fuel prices still benefit from huge economies of scale. Back in November 2021, the International Energy Agency (IEA) estimated the cost of producing fossil fuels at $1.14 to $3.03 per gallon, while the highly volatile commodity was selling for $2.19 per gallon at the time, according to IndexMundi. Not to mention, there are factors like various biofuel credits to consider, so it’s difficult to determine the ultimate impact on price.
But if it does what it says, this development clearly makes better use of waste wood and should significantly reduce the price of SAF. Since price is a major barrier to the adoption of this technology, this would be a huge leap forward in sustainable transportation.
"I started this work more than a decade ago because I wanted to make an impact," Charles Kai, a research associate and leader of the Riverside team, said in a press release. "I wanted to find viable alternatives to fossil fuels, and my colleagues and I have done just that. Using CELF, we have shown that it is possible to make cost-effective fuels from biomass and lignin and help curb our carbon emissions into the atmosphere. Lignin utilization is key to extracting what you need from biomass in the most economical and environmentally friendly way. Designing a process that can better utilize the lignin and sugars in biomass is one of the most exciting technical challenges in this field."
The team's research paper was published in the journal Energy and Environmental Science.