Research makes plant waste a viable option in ethanol production

By University of California-Riverside | November 09, 2017

University of California-Riverside researchers have developed a streamlined process that could finally make the ethanol production cost from abundant second-generation plant wastes competitive with first-generation ethanol made from sugars.

“We introduced a way to improve the production of fuel ethanol from non-food plant wastes by closing the performance gap between first and second generation feedstocks,” said Charles M. Cai, an assistant research engineer at UCR’s Center for Environmental Research and Technology, or CE-CERT, and an adjunct professor of chemical and environmental engineering.

The research was published in October in the leading journal Proceedings of the National Academy of Sciences, or PNAS.

Non-edible plant wastes such as corn waste (or corn stover), grasses, rice straw, wood chips, and other agricultural or forestry waste products, also called lignocellulosic biomass, are abundant all over the world and they contain plenty of energy to support current and future energy needs.

But breaking this biomass down to its sugars is difficult and process-intensive, typically requiring independent steps: first pretreating the biomass material to make it easier to break down, then using expensive enzymes that help digest the biomass to release sugars, and then performing fermentation. Sugars from biomass are the important food source that microbial yeast consumes during fermentation, producing ethanol.

Sugars from first generation food crops such as corn starch and sugarcane juice can be extracted with little effort. But the supply and production of these raw materials, or feedstocks, has raised concerns about their competition with our food supply and environmental impact. Second generation feedstocks, conversely, are usually byproducts, and cheaper than food crops. For example, a ton of corn stover costs about $50 a ton, whereas refined sugar from sugarcane costs about $350 a ton.

The problem: the high processing cost associated with producing biofuels from second generation biomass.

Now, UCR researchers have identified a way to improve the yield of ethanol from biomass. It involves direct fermentation of solid biomass, moving straight from pretreatment to a single process that both releases sugars and ferments them into ethanol. This streamlined strategy, called simultaneous saccharification and fermentation, or SSF, makes for a simpler process, and potentially reduces the enzymes needed to digest the solid material.

Ethanol yields from SSF strategies in the past have been too low, with a limited concentration of ethanol. Enter the new pretreatment method invented by the UCR researchers, called CELF, short for co-solvent enhanced lignocellulosic fractionation. Using CELF, researchers can pretreat biomass such as corn stover and produce a sugar-rich and highly digestible biomass that—using the SSF strategy—can be converted to ethanol while also maintaining high ethanol yields.

In fact, the UCR team achieved maximum ethanol concentrations similar to those produced from the expensive refined sugar of food crops, while saving more than 50 percent in enzyme costs than other SSF strategies.

“We were first to demonstrate an SSF-based strategy that was no longer limited by the process,” Cai said. The burden of further improving ethanol yields now depends on genetically modifying the yeast to tolerate higher concentrations of ethanol. The yeast dies from the high ethanol concentrations in this system.

This research was funded by the National Science Foundation and the BioEnergy Science Center through the Department of Energy’s Office of Science.

Cai’s co-authors in this paper, “Overcoming Factors Limiting High Solids Fermentation of Lignocellulosic Biomass to Ethanol,” include Thanh Yen Nguyen (Co-first author), Rajeev Kumar, and Charles E. Wyman, the Ford Motor Company chair in environmental engineering, all of whom are UCR colleagues.