Assessment of alkaline pretreatment for the production of bioethanol from eucalyptus, sugarcane bagasse and sugarcane straw

• Published in: Industrial crops and products Vol. 94; pp. 932 - 941
• Main Authors: Carvalho, Danila Morais de, Queiroz, José Humberto de, Colodette, Jorge Luiz
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.12.2016
• Subjects: Alkaline charge; Eucalyptus wood; Semi-simultaneous saccharification and fermentation (SSSF); Sugarcane bagasse; Sugarcane straw; Sugarcane straw; Semi-simultaneous saccharification and fermentation (SSSF); Alkaline charge; Eucalyptus wood; Sugarcane bagasse
• ISSN: 0926-6690; 1872-633X
• DOI: 10.1016/j.indcrop.2016.09.069

•Formation of pseudo-extractives in eucalyptus during alkaline pretreatments.•Higher delignification for bagasse and straw during alkaline pretreatments.•Effect of residual cell wall components on glucose releasing by presaccharification.•Higher ethanol production for bagasse pretreated using alkaline process. The impact of alkaline pretreatment at different alkaline charges (5%, 10% and 15% NaOH w/w, on dry basis) on the chemical composition of eucalyptus, sugarcane bagasse and straw were compared with the subsequent bioconversion into ethanol, using semi-simultaneous saccharification and fermentation (SSSF). By increasing alkaline charge in pretreatments, substantial amount of lignin, hemicelluloses and cellulose were fractionated. The chemical composition of biomasses was expressed based on the complete mass balance. The chemical transformation of biomasses during pretreatments was assessed by comparing the chemical composition of pretreated biomasses and their untreated counterparts. Pretreatments promoted delignification in the range of 11%–51% for eucalyptus, 22%–90% for bagasse, and 60%–99% for straw for the alkaline charges in the range of 5%–15% (NaOH w/w). The removal of lignin from bagasse and straw was higher than that from eucalyptus, which was due to the combined effect of higher frequency of both, the free phenolic groups and the ester bonds in grass lignin that made the lignin solubility escalate in alkaline conditions. It was also observed that bagasse had a removal by 37%–45% hemicelluloses and 0.8%–11% cellulose. For straw, higher amount of carbohydrates was removed, in the range of 55%–66% hemicelluloses and 19%–36% cellulose. Fragments of lignin and carbohydrates were converted into new structures called “pseudo-extractives” in eucalyptus during pretreatments. Pseudo-extractives and native extractives were quantified together, thus increasing the total extractives contents by 3.3 (5% NaOH), 3.5 (10% NaOH) and 2.9 (15% NaOH) times, in line with the original raw materials. Maximum ethanol yield and maximum volumetric productivity of ethanol were achieved for eucalyptus pretreated using 10% NaOH, bagasse pretreated using 15% NaOH and straw pretreated using 5% NaOH. At the optimal pretreatment condition, just about 51% glucose was released from pretreated bagasse. Sugarcane bagasse presented the highest values for the respective parameters, namely: 8.8gL−1 ethanol concentration, 0.101gethanol/gbiomass ethanol yield and 0.88gL−1h−1 volumetric productivity of ethanol. Alkaline charge proved to be an important control variable for alkaline pretreatments, with determinant effect on chemical transformations of biomasses and result on ethanol production.