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Full-scale On-farm Pretreatment of Perennial Grasses with Dilute Acid for Fuel Ethanol Production

Digman, Matthew Francis, Shinners, Kevin J., Muck, Richard E., Dien, Bruce S.
Bioenergy research 2010 pp. 335
Panicum virgatum, Phalaris arundinacea, Saccharomyces cerevisiae, acetates, acid treatment, ambient temperature, bioethanol, biomass, biorefining, cell walls, cellulose, ethanol, ethanol production, feedstocks, fermentation, fuel production, grasses, lactic acid, lignocellulose, microbial activity, oxygen, perennials, saccharification, sulfuric acid
Biorefineries that rely on lignocellulosic feedstocks require dependable and safe methods for storing biomass. Storing biomass wet in the presence of sulfuric acid and the absence of oxygen has been shown to preserve carbohydrates and enhance cellulose conversion but has not been demonstrated at farm-scale. To that end, switchgrass (Panicum virgatum L.) and reed canarygrass (Phalaris arundinacea L.) were pretreated with 18 N sulfuric acid with two methods: during bagging (on-line) and thoroughly mixed in a commercial feed mixer (mixed) and both stored for 90 days. The two methods, applied at rates from 28 to 54 g(kg DM)-1 not only helped to preserve biomass substrates under on-farm conditions (anaerobic, ambient temperature and pressure) through inhibition of microbial activity but also enhanced conversion of cellulose to ethanol by simultaneous saccharification and fermentation (SSF) using Saccharomyces cerevisiae. Acid-pretreated substrate yielded 19 and 7 percentage points higher ethanol conversion efficiencies than fresh reed canarygrass and switchgrass, respectively. The on-line method of pretreatment out-yielded the mixed method both as a preservative and as an agent for enhanced cell wall degradation. This result was thought to be an outcome of more uniform acid application as indicated by the on-line method's more consistent pH profile and decreased fermentation products, as compared to the mixed method. Although significant levels of acetate and lactate were present in the biomass following storage, concentrations were not sufficient to inhibit S. cerevisiae in SSFs with a 10% solids loading.