A simple, cheap pretreatment promises to radically cut the price of sustainable aviation fuel (SAF) made from waste wood biomass – potentially making it cost-competitive with fossil-based jet fuel, while cutting down emissions by up to 80%.
SAF isn’t a perfect green solution to aviation – indeed, no perfect solution exists at this point. Running SAF instead of jet fuel still produces carbon dioxide – but it’s a higher-purity fuel, producing up to 3% more energy while burning cleaner, with heavily reduced sulfur and particulate emissions. It eliminates the entire emissions cost of the oil extraction and refining processes, and depending on the feedstock you use, you can even get it to be net carbon negative – all without modifying your jet engines.
It’s distinct from biofuel, in that second-generation SAF doesn’t use corn, sugar cane, soy or other food crops. That’s a heinous waste of land and water. Instead, feedstocks like wood residues from milling operations, sugarcane bagasse, corn stover and other cheap, abundant waste products are used.
The problem, according to UC Riverside researchers, has been lignin, a key structural component in plant cells. Tough and resilient, lignin gives trees their strength – and also makes it hard to extract carbon for fuel from biomass, particularly when you’re dealing with the harder woods. Indeed, many operations choose to just burn the lignin for process heat and power, a process which makes economic sense, but looks absolutely bananas from an environmental perspective.
The UC Riverside team, however, has developed a pretreatment that changes the equation significantly. Adding tetrahydrofuran (THF) to water and dilute acid during biomass pre-treatment, the team found it’s possible to significantly boost overall efficiency, while generating fuel from the lignin in the biomass as well as the sugars.
The result: lots more aviation bang for your waste biomass buck. Using corn stover, you get up to 18% more fuel, going from around 44 gallons (167 L) of gasoline equivalent per dry ton of feedstock up to 51.8 gallons (196 L) per ton using the THF pre-treatment. Going over to tougher poplar wood, with its higher lignin content, the yield is a remarkable 75.9 gallons (287 L) of gasoline equivalent per ton of dry feedstock – nearly twice as much as the conventional process squeezes out of corn stover.
And as a nice cherry on top, the THF pretreatment chemical is cheap and particularly easy to get hold of, since it can be made from the same biomass sugars that SAF operations are already processing.
So what’s the bottom line? Well, GlobalAir quotes the average price of Jet-A in the USA at US$6.45 per gallon, and the average price of SAF at $9.28 per gallon at the time of writing. The UC Riverside team calculates a production cost as low as $3.15 per gallon for its CELF (co-solvent enhanced lignocellulosic fractionation) production process.
Does that mean 80% cleaner jet fuel at half the price of regular jet fuel? No; production cost doesn’t take transport, logistics, business costs or profit into account, and fossil fuel prices still benefit from massive economies of scale. The IEA quoted the production cost for a gallon of fossil-based jet fuel at between $1.14 and $3.03 back in November 2021, when this highly volatile commodity was selling for $2.19 per gallon according to Index Mundi. Not to mention, there’s various biofuel credits and whatnot to factor in, so it’s tough to figure out what the final effect on price might be.
But if it does what it says on the tin, this development clearly makes better use of waste wood, and should substantially reduce the price of SAF. And since price is the main barrier to adoption, this could be a huge leap forward for sustainable transport.
“I began this work more than a decade ago because I wanted to make an impact,” said Associate Research Professor Charles Kai, leader of the Riverside team, in a press release. “I wanted to find a viable alternative to fossil fuels and my colleagues and I have done that. Using CELF, we have shown it is possible to create cost-effective fuels from biomass and lignin and help curb our contribution of carbon emissions into the atmosphere.”
“Lignin utilization is the gateway to making what you want out of biomass in the most economical and environmentally friendly way possible,” he continues. “Designing a process that can better utilize both the lignin and sugars found in biomass is one of the most exciting technical challenges in this field.”
The team’s research paper is open access at the journal Energy & Environmental Science.
