Submitted to: Biological Engineering (ASABE)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2011
Publication Date: 4/16/2012
Publication URL: http://handle.nal.usda.gov/10113/54155
Citation: Digman, M.F., Dien, B.S., Hatfield, R.D. 2012. On-farm acidification and anaerobic storage for preservation and improved conversion of switchgrass into ethanol. Biological Engineering (ASABE). 5(1):47-58.
Interpretive Summary: If plant biomass is to be used to produce cellulosic ethanol, there needs to be a pretreatment stage to break down plant cell walls (unlike corn ethanol production in which the grain can be fermented without pretreatment). If this pretreatment could be done on the farms where the biomass is grown, it would add value at the farm. Previous work demonstrated that on-farm pretreatment of reed canarygrass with a dilute acid (to lower pH) following harvest, and then storing wet, is sufficient to obtain quite high ethanol yields that are competitive with alternative industrial pretreatments. This process also eliminates the need for field drying and gives the farmer the flexibility to harvest biomass over a range of moistures and to store it for long durations. However, with switchgrass, even long-term storage at low pHs results in poor ethanol yields. Integrating this on-farm pretreatment with a second-stage thermal treatment (which would be carried out at the biorefinery) was investigated for improving ethanol yields. We found that this second-stage thermal treatment of switchgrass resulted in significantly higher conversion of cellulose to ethanol (increasing from 48.7 to 56.2%). The maximum ethanol yield was observed for samples treated at the highest acid loading. We conclude that on-farm pretreatment still has value. This information will be useful to farmers, biorefinery operators, and others interested in cellulosic ethanol production.
Technical Abstract: Treating biomass on-farm with dilute acid following harvesting has the advantages of preventing excess mass losses during storage and helping to prepare it for biochemical processing to biofuels. As the biomass is stored wet it also dispenses with the need for field-drying of the biomass. As long as it is kept anoxic, the biomass can be safely stored in this manner for months. Previous work demonstrated that on-farm pretreatment of reed canarygrass (Phalaris arundinacea) in such a manner is sufficient to obtain quite high ethanol yields that are competitive with alternative industrial pretreatments. However, this was not the case with switchgrass (Panicum virgatum), where even long-term storage at low pHs results in poor yields. Integrating on-farm pretreatment 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 DM)) and stored anaerobically at ambient temperature and pressure for 30 days. We found that a second-stage thermal treatment (1h, 120 °C) resulted in significantly higher conversion of cellulose to ethanol (increasing from 48.7 to 56.2%), as well as higher hydrolysate levels of xylose that were measured to increase from 19.2 to 46.2%. The maximum yield was observed for samples treated at the highest acid loading. Therefore, on-farm pretreatment still has value for lowering the required severity at the biorefinery.