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Boosting biocrude: Gaining twice the fuel from corn stover

In the United States (US), researchers at the Pacific Northwest National Laboratory (PNNL) have demonstrated a way to double the amount of biofuel that can be extracted from the same quantity of corn stover by combining two promising techniques. First oleaginous yeast on grow on sugars extracted from the plant material and then the residual lignin from the corn stover is mixed into the broth before hydrothermal liquefaction (HTL) is used to convert the mixture into biocrude.

PNNL’s hydrothermal liquefaction system turns wet biomass—such as algae, sewage, and plant matter—into biocrude in a matter of minutes (photo courtesy PNNL).

Pacific Northwest National Laboratory (PNNL) hydrothermal liquefaction
(HTL) system turns wet biomass—such as algae, sewage, and plant matter
—into biocrude in a matter of minutes (photo courtesy PNNL).

PNNL’s hydrothermal liquefaction (HTL) system turns wet biomass—such as algae, sewage sludge, and plant matter—into biocrude in a matter of minutes. Corn stover, a plentiful farm byproduct can be turned into fuel, but it poses stubborn challenges for scientists aiming to make the process efficient and affordable. By combining two promising techniques, PNNL scientists have demonstrated a way to double the fuel that can be extracted from the same quantity of corn stover.

In the first phase, they grow oleaginous yeast on sugars extracted from the plant material, making a thick, lipid-rich broth. In the second phase, the broth is mixed with residual lignin from the corn stover and then converted into a biocrude oil via a high temperature and pressure process called hydrothermal liquefaction (HTL). The biocrude, which is rich with hydrocarbons, can then be used to make renewable diesel, biojet, and marine fuel.

The work, supported by the US Department of Energy (DOE) Bioenergy Technologies Office (BETO), was recently published as a paper titled “Renewable diesel via hydrothermal liquefaction of oleaginous yeast and residual lignin from bioconversion of corn stover” in the journal Applied Energy.

Biocrude from yeast and lignin

Brewer’s yeast is commonly used in biomass refineries to ferment the sugars in plant matter and turn them into ethanol. In PNNL’s new study, a different type, oleaginous yeast, was used because it is particularly good at making lipids that can be upgraded to diesel fuel.

However, when these yeasts are grown in a bioreactor using unrefined sugars from corn stover, the resulting lipids are not easy to extract from the broth.

In our paper, we show a way to sidestep the problem of separating out the fuel compound. We just take the whole bioreactor broth to HTL, explained Jim Collett, a bioprocess scientist at PNNL and lead author.

With HTL, the corn stover broth goes into a reactor where it meets heat up to 625 Kelvin (665 °F) and pressure up to 22 megapascal (nearly 3,200 pounds per square inch). What emerges is a biocrude liquid and some ash. After some chemical upgrading, the biocrude can be blended into renewable diesel or fuel for jets and ships.

This process solves a long-standing problem with corn residue – how to make use of lignin. Lignin separates from the stover during the initial phase, when it is mixed with water and pretreated with caustic chemicals. But the yeasts cannot break down the lignin within the bioreactor, so it typically becomes a residue often used as fuel at the biorefinery to produce process steam and electricity.

Hybrid process

In their new approach, PNNL researchers combined the leftover lignin with the oily yeast grown on the corn stover sugars, using HTL to derive biocrude from both. Previous research has tested HTL directly on biomass, and other work at PNNL has used HTL to derive biocrude from wastewater treatment sludge.

But the idea of combining the oily yeast broth, lignin, and HTL in one process to make biocrude from corn stover is new, the researchers write. That said, it does not require a biorefinery to be rebuilt from scratch.

We can take an existing bioconversion plant design and add the HTL operation. Instead of sending the lignin to be burnt for heat and power, we can direct it toward fuel, said Collett.

Produce more fuel at less cost

The researchers estimate that a biorefinery based on their new hybrid design could produce about 51 million gallons of gasoline equivalent (gge) fuel from about 800 000 tons of corn stover per year, up from a range of 24 to 32 million gge per year for a similar plant without HTL.

Still, the researchers’ preliminary economic analysis found that the estimated cost of making this fuel—around US$5 per gge—would need to come down further in order to be viable. About a fifth of the corn stover’s carbon gets stranded in the liquid byproduct from HTL, which also contains a mix of nitrogen, sulfur, chloride, and other chemicals. Future research will focus on finding ways to harvest that liquid for energy and valuable chemicals.

We’re trying to see how we can extract more value out of anything else that was formerly considered waste, and turn that into fuel as well ended Jim Collett.

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