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Alkaline Pretreatment of Corn Stover: Bench-Scale Fractionation and Stream Characterization

Karp, Eric M., Donohoe, Bryon S., O’Brien, Marykate H., Ciesielski, Peter N., Mittal, Ashutosh, Biddy, Mary J., Beckham, Gregg T.
ACS sustainable chemistry 2014 v.2 no.6 pp. 1481-1491
alkali treatment, anthraquinones, bioethanol, biomass, cell walls, cellulose, corn stover, crystal structure, depolymerization, enzymes, ethanol, fractionation, hydroxyl radicals, lignin, peeling, pulping, sodium hydroxide, sugars, temperature, transmission electron microscopy
Biomass pretreatment generally aims to increase accessibility to plant cell wall polysaccharides for carbohydrate-active enzymes to produce sugars for biological or catalytic upgrading to ethanol or advanced biofuels. Significant research has been conducted on a suite of pretreatment processes for bioethanol processes. An alternative option, which has received less attention in the biofuels community, is the use of alkaline pretreatment for the partial depolymerization of lignin from intact biomass. A known issue with alkaline pretreatment is the loss of polysaccharides from peeling reactions, but this loss can be mitigated with anthraquinone, as commonly practiced in pulping. Here, we conduct a comprehensive bench-scale evaluation of alkaline pretreatment using corn stover at temperatures of 100, 130, and 160 °C and sodium hydroxide loadings from 35 to 660 mg NaOH/g dry biomass with anthraquinone. Compositional analysis is conducted on the starting material and residual solids after pretreatment, and mass balance is inferred in the liquor by difference. The residual solids after alkaline pretreatment are characterized for crystallinity and imaged by scanning and transmission electron microscopy to reveal the physical changes in the carbohydrate portions of the biomass remaining after pretreatment, which demonstrate dramatic modifications to biomass cell wall architecture with lignin removal but rather insignificant changes in cellulose crystallinity. Our results show that alkaline pretreatment at relatively mild conditions is able to remove substantial amounts of lignin from biomass. Going forward, to be an economically feasibile process, technologies will be required to upgrade the resulting lignin-rich liquor stream.