Microbial electrolysis cells (MECs) that use two types of bacteria to treat lignocellulosic wastes such as corn stover yield up to 20 times more energy than current techniques for making biofuels, report researchers at Michigan State University, East Lansing, Mich. The cells produce both ethanol and hydrogen.
One bacterium breaks down and ferments the agricultural waste into ethanol. The other removes all fermentation byproducts and generates electricity that is used to produce hydrogen. The process achieves 35-40% energy recovery from the fermentation portion alone, compared to about 3.5% for other microbial cells handling corn stover. The generation of hydrogen about doubles the energy recovery — to 73%. "So, the potential is definitely there to make this platform attractive for processing agricultural wastes," says Gemma Reguera, a microbiologist who leads the research team.
The cells use corn stover treated by the ammonia fiber expansion (AFEX) process, a technique developed by another researcher at the university.
"We have begun to optimize the platform to process industrial solid loadings and to increase ethanol titers to make ethanol distillation cost-effective. As our platform 'cleans up' all the fermentation byproducts, we are also investigating whether we are, in fact, reducing the costs associated with ethanol distillation," notes Reguera. "…Our first goal is to adapt standard laboratory and then industrial reactors to function as MECs." The team is working to fine-tune the bacteria, the growth medium and operational parameters. "We have already developed improved strains [of bacteria] via adaptive evolution."
"One of the reasons I chose the AFEX pretreatment was because of its proven versatility with various substrates (other agricultural residues as well as bioenergy crops)… In principle, the same microorganisms we are using for the AFEX corn stover are expected to perform well with other AFEX-treated substrates. Mixing them is a very interesting possibility that we also want to explore," she adds.
"Scaling up is the top priority at the moment and it could be the most challenging. However, we are setting reasonable goals so as to develop small-scale bioreactor units first… If our small-scale bioreactors perform as expected, we would like to begin pilot-scale trials using industrial-scale reactors. This could happen within the next year," Reguera says.
"We are setting a goal of 40g/L of ethanol, which is the minimum that is considered to make ethanol distillation cost-effective. In parallel, we are also optimizing the electrochemical platform to improve H2 production from electricity… This number could be increased significantly. In the end, we may wind up with a platform that produces less ethanol but recovers all the energy of the fermentation byproducts as electricity or H2 — and that would be okay, too, because diversifying the fuel output may enable much higher energy recoveries than with ethanol alone."
More details on the process appear in a recent article in Environmental Science & Technology.