ARS | Publication request: Pilot-scale On-farm Pretreatment of Perennial Grasses with Dilute Acid and Alkali for Fuel Ethanol Production

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 21, 2010
Publication Date: June 1, 2010
Citation: Digman, M.F., Shinners, K.J., Muck, R.E., Dien, B.S. 2010. Pilot-scale On-farm Pretreatment of Perennial Grasses with Dilute Acid and Alkali for Fuel Ethanol Production. Transactions of the ASABE. 53(3):1007-1014.

Interpretive Summary: Switchgrass and other grasses are potential biomass sources for the production of cellulosic ethanol. However, pretreatment at the bioprocessing plant is a major barrier to development of this industry. Earlier laboratory research suggested that sulfuric acid and lime could be used as a pretreatment at the farm, rather than at the bioprocessing plant, but these treatments need to be tested at a pilot-scale to determine their efficacy. Both switchgrass and reed canarygrass were chopped, treated with and without sulfuric acid or lime, and baled in a custom baler. The bales were wrapped with plastic film and stored for 60 or 180 days outside. For comparison, the same treatments were carried out on the same grasses in laboratory silos. At the end of storage, the amount of ethanol that could be produced from the grasses using commercial enzymes and yeast was measured. Interestingly, the amount of ethanol produced from the bales was either similar to or higher than that produced from the grasses stored in laboratory silos. This indicates that the sulfuric acid and lime pretreatments at the farm have potential to reduce pretreatment costs at the bioprocessing plant at a cost of approximately $5 per ton of dry grass. This interaction helps put a value on pretreatment at the farm and could help to bring to fruition a cellulosic ethanol industry for farmers and all of the associated enterprises.

Technical Abstract: Switchgrass (Panicum virgatum L.) and reed canarygrass (Phalaris arundinacea L.) were pretreated with sulfuric acid or calcium hydroxide 50 g/kg DM at both the laboratory (250 g DM) and pilot-scale (250 kg DM) and anaerobically stored for two durations, 60 and 180 days. Pretreated and untreated samples were fermented to ethanol by Saccharomyces cerevisiae D5A in the presence of a commercially available cellulase (Celluclast 1.5L) and ß-glucosidase (Novozyme 188). With acid pretreatment, conversion of cellulose to ethanol was 35 and 12 percentage units higher than for untreated controls for reed canarygrass and switchgrass, respectively. Similarly, lime-pretreated reed canarygrass and switchgrass out-yielded controls by 3 and 13 percentage units, respectively. Cellulose and lignin were largely unaffected by pretreatment and anaerobic storage, but hemicelluloses were lower for both pretreatments. Chemical costs for biomass pretreatment applied at a rate of 50 g/kg DM were estimated to be as low as $4.05 and $5.20 per Mg DM for calcium hydroxide and sulfuric acid, respectively.