Introducing a simple renewable chemical to the pretreatment step can finally make next-generation biofuel production both cost-effective and carbon neutral, a new study finds.

The Department of Energy’s Bioenergy Technology Office has awarded researchers a $2 million grant to build a small-scale pilot plant at the University of California – Riverside. lt could lead to larger-scale investment in the technology, as harnessing energy from fossil fuels adds to global warming and hurts the planet.

The secret: using lignin from plant cell walls

For biofuels to compete with petroleum, biorefinery operations must be designed to better utilize lignin, one of the main components of plant cell walls. It provides plants with greater structural integrity and resiliency from microbial attacks. However, these natural properties of lignin also make it difficult to extract and utilize from the plant matter, also known as biomass.

A billion tons per year of biomass could replace 30% of our petroleum consumption

To overcome the lignin hurdle, UC Riverside Associate Research Professor Charles Cai invented CELF (co-solvent enhanced lignocellulosic fractionation), an innovative biomass pretreatment technology. “CELF uses tetrahydrofuran (THF) to supplement water and dilute acid during biomass pretreatment. It improves overall efficiency and adds lignin extraction capabilities,” Cai said. “Best of all, THF itself can be made from biomass sugars.”

A Energy & Environmental Science paper details the degree to which a CELF biorefinery offers economic and environmental benefits over both petroleum-based fuels and earlier biofuel production methods.

The paper is a collaboration between Cai’s research team at UCR, the Center for Bioenergy Innovation managed by Oak Ridge National Laboratories, and the National Renewable Energy Laboratory, with funding provided by the U.S. Department of Energy’s Office of Science.

Non-edible plant biomass as feedstocks

First-generation biofuel operations use food crops like corn, soy, and sugarcane as raw materials, or feedstocks. Because these feedstocks divert land and water away from food production, using them for biofuels is not ideal. 

Instead, second-generation operations use non-edible plant biomass as feedstocks, such as wood residues from milling operations, sugarcane bagasse, or corn stover, all of which are abundant low-cost byproducts of forestry and agricultural operations. 

According to the Department of Energy, up to a billion tons per year of biomass could be made available for the manufacture of biofuels and bioproducts in the US alone, capable of displacing 30% of our petroleum consumption while also creating new domestic jobs. 

Because a CELF biorefinery can more fully utilize plant matter than earlier second-generation methods, the researchers found that a heavier, denser feedstock like hardwood poplar is preferable over less carbon-dense corn stover for yielding greater economic and environmental benefits. 

A break-even $3.15 per gallon of gasoline equivalent

Using poplar in a CELF biorefinery, the researchers demonstrate that sustainable aviation fuel could be made at a break-even price as low as $3.15 per gallon of gasoline equivalent. The current average cost for a gallon of jet fuel in the U.S. is $5.96. 

The U.S. government issues credits for biofuel production in the form of renewable identification number credits, a subsidy meant to bolster domestic biofuel production. The tier of these credits issued for second-generation biofuels, the D3 tier, is typically traded at $1 per gallon or higher. At this price per credit, the paper demonstrates that one can expect a rate of return of over 20% from the operation. 

“Spending a little more for a more carbon-rich feedstock like poplar still yields more economic benefits than a cheaper feedstock like corn stover, because you can make more fuel and chemicals from it,” Cai said.

The paper also illustrates how lignin utilization can positively contribute to overall biorefinery economics while keeping the carbon footprint as low as possible. In older biorefinery models, where biomass is cooked in water and acid, the lignin is mostly unusable for more than its heating value. 

“The older models would elect to burn the lignin to supplement heat and energy for these biorefineries because they could mostly only leverage the sugars in the biomass—a costly proposition that leaves a lot of value off the table,” said Cai. 

Renewable chemicals

In addition to better lignin utilization, the CELF biorefinery model proposes to produce renewable chemicals. These chemicals could be used as building blocks for bioplastics and food and drink flavoring compounds. These chemicals take up some of the carbon in the plant biomass that would not get released back into the atmosphere as CO2.

“Adding THF helps reduce the energy cost of pretreatment and helps isolate lignin, so you wouldn’t have to burn it anymore. On top of that, we can make renewable chemicals that help us achieve a near-zero global warming potential,” Cai said. “I think this moves the needle from Gen 2 biofuels to Gen 2+.”

Citation: Bruno Colling Klein,  ^ab   Brent Scheidemantle,^bcd   Rebecca J. Hanes,  ^be   Andrew W. Bartling,^ab   Nicholas J. Grundl,^ab   Robin J. Clark,^af   Mary J. Biddy,^ab   Ling Tao,  ^ab   Cong T. Trinh,^bg   Adam M. Guss,^bh   Charles E. Wyman,  ^bcd   Arthur J. Ragauskas,  ^bgi   Erin G. Webb,^bf   Brian H. Davison  ^bh  and  Charles M. Cai  *^bcd.  07 February 2024, Economics and global warming potential of a commercial-scale delignifying biorefinery based on co-solvent enhanced lignocellulosic fractionation to produce alcohols, sustainable aviation fuels, and co-products from biomass. Energy & Environmental Science Issue 3, Page 827 to 1296. https://pubs.rsc.org/en/content/articlelanding/2024/ee/d3ee02532b (open-access)

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