In plant-based fuel production, the initial hurdle has long been the most daunting: breaking down plant matter. However, a recent study reveals that incorporating a basic, renewable chemical into the pretreatment process can finally render next-generation biofuel production economically viable and environmentally neutral.
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UCR associate research professor Charles Cai, who developed the CELF biomass pretreatment technology. Image Credit: Stan Lim/UCR.
To stand on equal footing with petroleum, biorefinery processes must be reconfigured to more effectively harness lignin, a crucial component in plant structure.
Lignin stands as a primary constituent of plant cell walls, endowing plants with heightened structural integrity and protection against microbial threats. Yet, while these inherent attributes of lignin are beneficial for plants, they pose challenges in terms of extraction and utilization from the plant matter, commonly referred to as biomass.
Lignin utilization is the gateway to making what you want out of biomass in the most economical and environmentally friendly way possible. 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.
Charles Cai, Associate Research Professor, University of California, Riverside
To surmount the lignin obstacle, Cai devised CELF, an acronym for co-solvent enhanced lignocellulosic fractionation, an inventive biomass pretreatment technology.
Cai adds, “CELF uses tetrahydrofuran or THF to supplement water and dilute acid during biomass pretreatment. It improves overall efficiency and adds lignin extraction capabilities. Best of all, THF itself can be made from biomass sugars.”
The study published in Energy & Environmental Science outlines the significant economic and environmental advantages offered by a CELF biorefinery compared to petroleum-based fuels and previous biofuel production techniques.
This collaborative effort involves Cai's research team at UCR, the Center for Bioenergy Innovation under Oak Ridge National Laboratories' management, and the National Renewable Energy Laboratory, supported by funding from the US Department of Energy's Office of Science. The researchers delve into two key considerations: determining the optimal biomass type and devising strategies for using extracted lignin.
First-generation biofuel processes rely on food crops such as corn, soy, and sugarcane as raw materials, or feedstocks. However, their utilization for biofuel production is less than ideal due to the diversion of land and water resources away from food production.
In contrast, second-generation operations utilize non-edible plant biomass as feedstocks. Examples of biomass feedstocks include wood residues from milling operations, sugarcane bagasse, or corn stover, all of which are plentiful and low-cost byproducts of forestry and agricultural activities.
As per the Department of Energy, the United States alone could potentially make available up to a billion tons per year of biomass for the production of biofuels and bioproducts. This quantity has the capacity to offset 30% of the nation's petroleum consumption while simultaneously generating new domestic employment opportunities.
Given that a CELF biorefinery can more effectively harness plant matter compared to previous second-generation techniques, researchers concluded that a denser, heavier feedstock like hardwood poplar offers superior economic and environmental advantages over less carbon-dense options such as corn stover, resulting in greater benefits.
Using poplar in a CELF biorefinery, the researchers illustrate that sustainable aviation fuel could be produced at a break-even price as low as $3.15 per gallon of gasoline equivalent. This is significantly lower than the current average cost of jet fuel in the US, which stands at $5.96 per gallon.
To further support domestic biofuel production, the US government issues credits known as renewable identification number (RIN) credits. These subsidies, aimed at bolstering the biofuel industry, include the D3 tier specifically designated for second-generation biofuels. Typically traded at $1 per gallon or higher, the paper demonstrates that at this price per credit, one can anticipate a rate of return exceeding 20% from the biorefinery 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.
Charles Cai, Associate Research Professor, University of California, Riverside
The study further elucidates how effective lignin utilization can significantly enhance overall biorefinery economics while minimizing carbon footprint. In older biorefinery models, where biomass undergoes cooking in water and acid, lignin is predominantly relegated to its heating value and remains largely unusable for other purposes.
“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,” added Cai.
In addition to optimizing lignin utilization, the CELF biorefinery model advocates for producing renewable chemicals. These chemicals have potential applications as foundational components for bioplastics, as well as flavoring compounds for food and beverages. Importantly, by incorporating these chemicals into the process, a portion of the carbon present in the plant biomass is captured, preventing its release 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. I think this moves the needle from Gen 2 biofuels to Gen 2+.
Charles Cai, Associate Research Professor, University of California, Riverside
Following the team's recent achievements, the Department of Energy's Bioenergy Technology Office has bestowed a $2 million grant upon the researchers to construct a small-scale CELF pilot plant at UCR.
Cai is optimistic that showcasing the pilot plant's capabilities will attract greater investment in the technology on a larger scale. He emphasizes the urgency of transitioning away from fossil fuels, citing their contribution to global warming and environmental harm.
Cai concludes, “I began this work more than a decade ago because I wanted to make an impact. 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.”
Journal Reference:
Klein, B. C., et al. (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. doi.org/10.1039/D3EE02532B.