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Title: Sodium Hydroxide Pretreatment of Switchgrass for Ethanol Production

item XU, JIELE - North Carolina State University
item CHENG, JAY - North Carolina State University
item SHARMA-SHIVAPPA, RATNA - North Carolina State University
item Burns, Joseph

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/2010
Publication Date: 8/20/2011
Citation: Xu, J., Cheng, J.J., Sharma-Shivappa, R.R., Burns, J.C. 2011. Sodium Hydroxide Pretreatment of Switchgrass for Ethanol Production. Energy and Fuels. 24:2113-2119.

Interpretive Summary: The lignocellulosic structure in plant biomass is a major obstacle to the efficient conversion of biomass to ethanol. NaOH pretreatment, however, was found to be effective in improving the enzymatic digestibility of switchgrass regardless of the temperatures studied (121, 50, and 21 oC). At the best combinations of exposure time and NaOH concentration for each of three different temperatures (1.0% NaOH, 0.5 h at 121 oC; 1.0% NaOH, 12 h at 50 oC; and 2.0% NaOH, 6 h at 21 oC), total reducing sugar yields were, respectively, 425.4, 453.4, and 406.2 mg/g raw biomass. This was 3.55, 3.78 and 3.39 times, respectively, that of untreated biomass. Based on total reducing sugar yield, the best conditions for switchgrass pretreatment were 50 oC, 12 h, and 1.0% NaOH, giving the subsequent enzymatic conversion of switchgrass to sugars of 74.4% for glucan, 62.8% for xylan, and 70.8% for total available carbohydrates. NaOH pretreatment showed outstanding delignification capacity, and the removal of lignin barrier was closely related to the pretreatment severity applied. Lignin reduction, however, should not be regarded as an appropriate indicator for pretreatment effectiveness because of greater carbohydrate loss at more severe pretreatment conditions. NaOH is a strong base and has shown great potential to work at reduced temperatures. The extended residence times or greater chemical loadings required at reduced temperatures would offset the cost reduction from decreased heating requirement. These variables require that primary cost-benefit analysis be conducted before initiating any scale-up studies.

Technical Abstract: Lignocellulose-to-ethanol conversion is a promising technology to supplement corn-based ethanol production. However, the recalcitrant structure of lignocellulosic material is a major obstacle to the efficient conversion. To improve the enzymatic digestibility of switchgrass for the fermentable sugar production in hydrolysis, sodium hydroxide pretreatment of the biomass feedstock was investigated. At 121, 50, and 21 oC, raw switchgrass biomass at a solid : liquid ratio of 0.1 g/ml was pretreated respectively for 0.25-1, 1-48, and 1-96 h at different NaOH concentrations (0.5, 1.0, and 2.0%, w/v). Pretreatments were evaluated based on the yields of lignocellulose-derived sugars in the subsequent enzymatic hydrolysis. At the best pretreatment conditions (50 oC, 12 h, and 1.0% NaOH), the yield of total reducing sugars was 453.4 mg/g raw biomass, which was 3.78 times that of untreated biomass, and the glucan and xylan conversions reached 74.4 and 62.8% respectively. Lignin reduction was closely related to the degree of pretreatment. The maximum lignin reductions were 85.8% at 121 oC, 77.8% at 50 oC, and 62.9% at 21 oC, all of which were obtained at the combinations of the longest residence times and the greatest NaOH concentration. Cellulase and cellobiase loadings of 15 FPU/g dry biomass and 20 CBU/g dry biomass were sufficient to maximize sugar production.