Researchers make hydrogen as cellulosic ethanol coproduct
Rather than focusing on a single microbe capable of converting cellulose to ethanol in a fermenter, researchers at Michigan State University are mimicking nature, culturing two bacteria that grow synergistically. The process creates two fuels, ethanol and hydrogen, the latter being produced from electricity generated by one of the bacteria.
Microbiologist Gemma Reguera explained her team of microbiologists at MSU are known for their work with the bacterium, Geobacter sulfurreducens, used for developing bioeletrochemical systems known as microbial electrolysis cells. The work done by MSU chemical engineer Bruce Dale treating corn stover and other waste biomass with the ammonia fiber expansion (AFEX) process was intriguing to them because it mimics what fungi do in in nature to remove lignin and facilitate the hydrolysis of cellulose for symbiotic microorganisms. They then looked for natural organisms that would degrade AFEX pretreated cellulose into ethanol. “They always produce ethanol and something else,” she explained, “and that something is usually acids that bring the pH in the fermentation broth down and back-inhibit the bacterial activity. This really slows growth or kills the fermentation organism.” Mimicking nature again, they looked for a bacterium that would produce ethanol and compounds that would support the electron-producing Geobacter, thus removing ethanol-production inhibitors from the fermentation broth.
In the lab, the electrons produced by Geobacter are moved via a wire from the fermentation vessel into another chamber where pure hydrogen is produced. The hydrogen gas can then be stored to power a fuel cell to produce electricity. The synergistic activities of the two bacteria increase the energy produced from the ethanol alone, which is about 56 percent of the total energy available in the substrate, to about 73 percent when the cogeneration of cathodic hydrogen is added.
A paper, “Consolidated Bioprocessing of AFEX-Pretreated Corn Stover to Ethanol and Hydrogen in a Microbial Electrolysis Cell,” is published in the current issue of Environmental Science and Technology. Since that paper was written, Reguera said, the researchers have continued to work on improved strains of the two bacteria and process optimization. The work has progressed to the point where if funding were available, the process could begin scaling up. “It’s ready now,” she said.
While she can envision the new ethanol/hydrogen system could be scaled up in tandem with Dale’s AFEX pretreatment process to industrial scale, she also sees a small-scale application. “Microbial fuel cell technology shows promise for waste water treatment,” she said. She believes a versatile platform can be developed for multiple scales where a farm could convert agricultural wastes and waste waters to produce ethanol to power vehicles and hydrogen that can be stored for fuel cells to take care of household and farmstead electrical needs.