Feb
29
Research Says Biomass Pretreatment Makes Biofuel Practical
February 29, 2024 | 1 Comment
A new University of California – Riverside (UCR) study found that introducing a simple, renewable chemical to the pretreatment step can finally make next-generation biofuel production both cost-effective and carbon neutral. The first step, breaking down the plant matter has always been the hardest when it comes to making fuel from plants.
For biofuels to compete with petroleum, biorefinery operations must be designed to better utilize lignin. Lignin is 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.
UC Riverside Associate Research Professor Charles Cai started the explanation with, “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.”
Simplified diagram for the proposed CELF-based biorefineries outlining the mass integration strategy. CELF: co-solvent enhanced lignocellulosic fractionation. Image Credit: University of California – Riverside. Click the study paper link for the open access research paper.
To overcome the lignin hurdle, Cai invented CELF, which stands for Co-solvent Enhanced Lignocellulosic Fractionation. It is an innovative biomass pretreatment technology.
“CELF uses tetrahydrofuran or 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.”
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. In it, the researchers consider two main variables: what kind of biomass is most ideal and what to do with the lignin once it’s been extracted.
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.
Second-generation operations use non-edible plant biomass as feedstocks. An example of biomass feedstocks includes 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.
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.
In addition to better lignin utilization, the CELF biorefinery model also 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+.”
Because of the team’s recent successes, the Department of Energy’s Bioenergy Technology Office has awarded the researchers a $2 million grant to build a small-scale CELF pilot plant at UCR. Cai hopes that demonstrating the pilot plant will lead to larger-scale investment in the technology, as harnessing energy from fossil fuels adds to global warming and hurts the planet.
“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,” Cai said. “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.”
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This is pretty encouraging work. Biomass will have an ever-increasing role in providing fuels in the future. The baseline fossil fuel cost is simply going to have to increase for biomass to become practical. It’s happening – at a pace so slow it drives the green enthusiasts a bit crazy.
What’s left out or perhaps misleading, is that the land and water resources are going to get used once per growing season. Whether for food or fuel the competition is going to be there. The choices are going to be economic based from producers through to the consumers.
Not to fret, corn and sugarcane ethanol were going to drive food prices into unaffordability for many. Yet today, as in some crop years since the market took off, the price paid to producers isn’t looking to cover the cost of production – and today there are mountains of corn to use up.
One more consideration that’s been left out – that will need funded during fuel production. The food crop folks have decades of experience in soil fertility, with professional plant science, agronomy, biology and other advanced degrees to keep the soil in good working order although in some locales the nutrient content of the foods is suffering.
The biofuel folks are missing some expertise and practical experience. We’re seeing fuel crops harvested to bare dirt taking the entirety of the soil nutrients used by the plants being hauled away. Replacing them is not a low-cost proposition. Nor do we understand them to the level the food producers have achieved.
Progress is being made, but the distance to go is much further than most people realize.
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Part of the reason for poplar is that if used as a feedstock for biofuel, it is coppiced to harvest it–leaving the stump and roots to re-grow. This page and video explain: https://hardwoodbiofuels.org/project-teams/feedstock/growing-and-production/#:~:text=Coppicing%20is%20a%20silvicultural%20method,trees%20for%20sawn%20wood%20products.