Figure 1. Team is using carbon dioxide to simplify and cut cost of producing ethanol. Source: Arthur H. Panganiban, JBEI and Berkeley Lab.
Using CO2 to control pH eliminates the need for washing or separation steps before the stream goes to the fermenter, explain the researchers. A preliminary economic analysis indicates the CO2-enhanced process could lower production costs by 50–65% compared to conventional ionic-liquid-based pretreatment methods. Retrofitting the approach into existing ethanol processes is easy, they say.
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“Pretreatment is the most expensive part of the biofuels production process,” notes Seema Singh, director of biomass pretreatment efforts at JBEI. “If you count the whole production cycle, pretreatment is second only to the cost of growing and obtaining the feedstock itself.”
“Incorporating CO2 in this process means there’s no need for a neutralization step, and the pH can be switched on a dime by the addition or release of CO2,” adds Blake Simmons, chief science and technology officer at JBEI. “When the pH adjustment is reversible, it makes the overall process more efficient because you can repeat the pretreatment cycle several times. And it costs less because now you can do everything in one reactor instead of three.”
The fermentation step produces CO2 that can be used in the process. Even in a worst-case scenario, when fermentation doesn’t generate enough CO2, that gas can serve as makeup for any losses of purchased CO2, he notes.
Regenerating the ionic liquid liberates the CO2, which is recovered and reused.
“The major remaining challenges are (1) scale-up of the process to prove marketplace relevance, and (2) efficient and affordable recycle and reuse of the ionic liquid,” Simmons says.
More details on the approach appear in a recent article in Energy & Environmental Science.
While the approach enables integrating pretreatment, saccharification and fermentation in one pot, ethanol producers — to take advantage of their existing infrastructure — likely would run two unit operations, pretreatment followed by simultaneous saccharification and fermentation, believes Simmons. “Most cellulosic ethanol producers already have what is needed in terms of reactors to carry out this process,” he notes.
“We are already talking with multiple companies about trying out the process, so the hope is that we could have it demonstrated… in the next two-to-four years,” Simmons says. This would involve retrofitting a[n] existing plant, he adds.
The researchers now are working to extend the CO2-based approach to the production of more-advanced biofuels such as isopentenol, bisabolene and methyl ketones, Simmons notes. “Basically this entails using an organism that has already been engineered to produce these biofuels and determining if they can perform in this environment… The initial results are very promising.”