On-Farm Acidification and Anaerobic Storage for Preservation and Improved Conversion of Switchgrass into Ethanol

Abstract

Treating biomass on-farm with dilute acid following harvest has the advantages of preventing excess mass loss during storage and helping to prepare the biomass for biochemical processing to biofuel. As the biomass is stored wet, dilute acid treatment also dispenses with the need for field-drying of the biomass; as long as the biomass is kept anoxic, it can be safely stored for months in this manner. Previous work has demonstrated that on-farm acidification of reed canarygrass (Phalaris arundinacea) in such a manner was sufficient to obtain high ethanol yields that were competitive with alternative industrial pretreatments. However, this was not the case for switchgrass (Panicum virgatum), where even long-term storage at low pH values resulted in poor yields. In this study, integrating on-farm acidification of switchgrass with a second-stage treatment, which would be carried out at the biorefinery, was investigated for improving ethanol yields. Freshly harvested switchgrass was blended with various loadings of sulfuric acid (25, 50, 75, 100, and 125 g kg-1 DM) and stored anaerobically at ambient temperature and pressure for 30 days. We found that a second-stage thermal treatment (1 h, 120C) resulted in significantly higher conversion of cellulose to ethanol (increasing from 45.3% to 52.2%) as well as higher hydrolysate levels of xylose, which were measured to increase from 15.3% to 35.1%. A maximum yield of 63% was observed for samples treated at the highest acid loading. Therefore, on-farm acidification has value for crop preservation as well as lowering the required pretreatment severity at the biorefinery.