VALUE-ADDED PRODUCTS FROM FORAGES AND BIOMASS ENERGY CROPS
Location: Cell Wall Biology and Utilization Research
Title: Optimizing On-farm Pretreatment of Perennial Grasses for Fuel Ethanol Production
Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 8, 2010
Publication Date: April 21, 2010
Citation: Digman, M.F., Shinners, K.J., Casler, M.D., Dien, B.S., Hatfield, R.D., Jung, H.G., Muck, R.E., Weimer, P.J. 2010. Optimizing On-farm Pretreatment of Perennial Grasses for Fuel Ethanol Production. Bioresource Technology. 101:5305-5314.
Interpretive Summary: There is great interest in using switchgrass and other grasses as biomass for production of cellulosic ethanol. One of the main costs and barriers to cellulosic ethanol from these grasses is pretreatment at the bioprocessing plant. Pretreatment under high temperatures and pressures is required to make the cellulose in the grasses available to the enzymes and microorganisms used to produce ethanol. An alternative is to pretreat the grasses at the farm with dilute chemicals after harvest, taking advantage of the long storage period. We studied the use of sulfuric acid and lime to pretreat both switchgrass and reed canarygrass at various loading rates (up to 10%) and storage times (up to 180 days) in laboratory silos. At the end of storage, we measured the amount of ethanol that could be produced from the grasses using commercial enzymes and yeast. Sulfuric acid was more effective than lime in reed canarygrass, permitting up to 83% of the cellulose to be converted to ethanol. Switchgrass was more difficult to pretreat; both sulfuric acid and lime performed similarly, allowing just more than half of the cellulose to be converted to ethanol. Overall, these pretreatments show promise for reducing the cost of pretreating grasses for cellulosic ethanol production. This will be of interest to both producers and the developing bioprocessing industry.
Switchgrass (Panicum virgatum L.) and reed canarygrass (Phalaris arundinacea L.) were pretreated under ambient temperature and pressure with sulfuric acid and calcium hydroxide in separate experiments. Chemical loadings from 0 to 100 g/kg DM and durations of anaerobic storage from 0 to 180 days were investigated by way of a central composite design at two moisture contents (40 or 60% w.b.). 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). Xylose levels were also measured following fermentation because xylose is not metabolized by S. cerevisiae. After sulfuric acid pretreatment and anaerobic storage, conversion of cell wall glucose to ethanol for reed canarygrass ranged from 22 to 83% whereas switchgrass conversions ranged from 16 to 46%. Pretreatment duration had a positive effect on conversion, but was mitigated with increased chemical loadings. Conversions after calcium hydroxide pretreatment and anaerobic storage ranged from 21 to 55% and 18 to 54% for reed canarygrass and switchgrass, respectively. The efficacy of lime pretreatment was found to be highly dependent on moisture content. Moreover, pretreatment duration was only found to be significant for reed canarygrass. Although significant levels of acetate and lactate were observed in the biomass after storage, S. cerevisiae was not found to be inhibited at a 10% solids loading.