Abstract
Cellulosic ethanol is one of the most important biotechnological products to mitigate the consumption of fossil fuels and to increase the use of renewable resources for fuels and chemicals. By performing this process at high total solids (TS) and low enzyme loadings (EL), one can achieve significant improvements in the overall cellulosic ethanol production process. In this work, steam-exploded materials were obtained from Eucalyptus urograndis chips and sugarcane bagasse to be subsequently used for enzymatic hydrolysis at high TS (20 wt%) and relatively low EL (13.3 FPU g−1 TS of Cellic CTec3 from Novozymes). Also, the fermentability of their corresponding hydrolysates was tested using an industrial strain of Saccharomyces cerevisiae (Thermosacc Dry from Lallemand). Enzymatic hydrolysis of steam-treated E. urograndis reached 125 g L−1 of glucose in 72 h, while steam-treated bagasse gave yields 25 % lower. Both substrate hydrolysates were easily converted to ethanol, giving yields above 25 g L−1 and productivities of 2.3 g L−1 h−1 for eucalypt and 2.2 g L−1 h−1 for bagasse after only 12 h of fermentation. Under the conditions used in this study, sugarcane bagasse glucans showed the potential to boost the ethanol production from sugarcane culms by 31 %, from the 80 L t−1 of first generation to a total production of 105 L t−1. On the other hand, E. urograndis plantations are able to achieve cellulosic ethanol productivities of 2832.2 L ha−1 year−1, which was 57.8 % higher than the projected value of 1794.5 L ha−1 year−1 that was obtained for sugarcane bagasse.
Highlights
Cellulosic ethanol is currently produced by fermentation of carbohydrates that are released from plant polysaccharides by enzymatic hydrolysis
Chemical analysis and pretreatment by steam explosion As expected, differences were observed in the chemical composition of both native sugarcane bagasse and E. urograndis chips (Table 1) and these were in good agreement with the general knowledge about the compositional analysis of both grasses [16, 17, 25] and hardwoods [5, 26]
Sugarcane bagasse presented lower glucan and lignin contents while its total extractives, pentosan and ash contents were much higher than those of E. urograndis wood chips. Both eucalypt and sugarcane bagasse hemicelluloses were partially quantified as xylans because the High Performance Liquid Chromatography (HPLC) method used for analysis was not able to quantify uronic acids and to resolve xylose, galactose and mannose in biomass acid hydrolysates
Summary
Cellulosic ethanol is currently produced by fermentation of carbohydrates that are released from plant polysaccharides by enzymatic hydrolysis. Brazil produced 659 million tons of sugarcane in the 2014/2015 harvest season and this resulted in the accumulation of 92 million tons of sugarcane bagasse (14 % on dry basis) after its processing for sucrose and/or first generation ethanol [5, 6]. Most of this bagasse is currently used for energy purposes but there is a surplus that still represents a great opportunity for the development of sustainable biorefineries. Biomass upgrading to fuels, chemicals and materials offers no immediate risks to food security issues
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