Optimization of bioethanol fuel production from lignocellulosic biomass

Abstract

The aim of this research was to enhance the process of bioethanol fuel production from selected lignocellulosic residues – corn stover and wheat straw. In order to obtain a high conversion yield of sugars, dilute sulfuric acid/steam explosion pretreatment was applied, and the process conditions were optimized. After the optimization studies, the pre-treated residue was subjected to enzymatic hydrolysis using Cellic®CTec2 enzyme in order to decrease the optimum temperature of 50°C while still achieving a high glucan recovery. The sugars released by enzymatic saccharification using the selected enzyme loads of 30 and 45 FPU g/cellulose were fermented by recombinant S. cerevisiae ATCC® 20618™ and S. cerevisiae to increase the ethanol yield. The ethanol yields of different batch reactors were also examined, including “separate hydrolysis and co-fermentation (SHCF)” at 50°C for pre-hydrolysis and at 30°C for fermentation, as well as “simultaneous saccharification and co-fermentation (SSCF)” at 32°C. The results indicated that the selected feedstocks contain fermentable sugars at a level of approximately 46-48%. The optimum conditions for dilute sulfuric acid/steam explosion pretreatment of the selected feedstocks almost doubled the cellulose content, indicating efficient sugar hydrolysis. Xylan loss and concentrations of by-products from the pretreatment hydrolysate were found to be linear with pretreatment severity. The ethanol yield for SSCF was similar to SHCF (Pearson's r= 0.99), demonstrating the applicability of the optimum conditions found in this study. After the laboratory studies, the environmental and economic performance of bioethanol fuel compared to conventional gasoline (CG) was evaluated using Life Cycle Assessment (LCA) and Environmental Life Cycle Cost (ELCC) Analysis for a 1-km travel distance functional unit (FU). The selected ethanol blends were E10 (10% ethanol/90% gasoline by volume) and E85 (85% ethanol/15% gasoline by volume). The results showed that E10 and E85 fueled vehicles can reduce the global warming potential by 4.7% and 47.1% with respect to CG, respectively. According to the ELCC calculations, E85 provided a 23% lower driving cost compared to CG.