When it comes to making fuel from plants, the first step has always been the hardest — breaking down the plant matter in order to efficiently and responsibly process it into biodiesel.
New University of California (UC) research has determined that, by introducing a simple, renewable chemical to the pre-treatment step can finally make next-generation biofuel production both cost-effective and carbon neutral.
For biofuels to compete with petroleum, biorefinery operations must be designed to better utilise lignin, which 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 utilise from the plant matter, also known as biomass.
“Lignin utilisation is the gateway to making what you want out of biomass in the most economical and environmentally friendly way possible,” says UC Riverside associate research professor Charles Cai.
“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.”
To overcome the lignin hurdle, Cai invented a process called CELF, which stands for co-solvent enhanced lignocellulosic fractionation. It is an innovative biomass pre-treatment technology.
“CELF uses tetrahydrofuran or THF to supplement water and dilute acid during biomass pre-treatment. It improves overall efficiency and adds lignin extraction capabilities,” Cai says.
“Best of all, THF itself can be made from biomass sugars.”
A landmark Energy & Environmental Science journal paper details the degree to which a CELF biorefinery offers economic and environmental benefits over both petroleum-based fuels and earlier biofuel production methods.
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.
This has been a major issue in the EU where there is a requirement that diesel fuel bought at the pumps has a 3.5% bio-derived content. That has been tough and costly to deliver and led to environmental and food crop competition problems in countries around the globe where energy cropping is practised.
However, 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 by-products of forestry and agricultural operations.
This is where Cai’s invention comes into play.
Because a CELF biorefinery can more fully utilise 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.
And there’s a potential benefit for aviation too.
Using poplar wood 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 US is $5.96.
“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 says.
The paper also illustrates how lignin utilisation 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,” says Cai.
In addition to better lignin utilisation, the CELF biorefinery model also proposes to produce renewable chemicals.
These chemicals could be used as building blocks for bioplastics and food and drink flavouring 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 pre-treatment 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,” adds Cai.