The use of lignocellulosic biomass for fermentative butanol production in biorefining processes

The aim of this study was to efficiently use barley straw as a lignocellulosic feedstock for biobutanol production. Dilute sulfuric acid pretreatment was employed to solubilize hemicellulose in barley straw from cellulosic residues. The pretreated hydrolysate was cofermented with starch-based biomass in acetone–butanol–ethanol (ABE) fermentation. There were two co-fermentation processes: I) Barley straw was mixed with barley grain, and the mixture was pretreated with dilute acid pretreatment. The sugars mainly released from hemicellulose and starch into the pretreated hydrolysate of the mixture were fermented for biobutanol production; II) Pretreated barley straw hydrolysate and gelatinized barley grain slurry was mixed and fermented. The pretreated solid residues containing mostly the cellulosic biomass was used for efficient enzymatic hydrolysis by synergistic cooperation of cellulases with xylanase and surfactants to produce fermentable sugars, and followed by ABE fermentation. Furthermore, the use of fresh barley silage for biobutanol production as an example of the co-fermentation process was investigated. By pretreatment of a mixture of barley straw and grain (process I), optimal fermentable sugar yields were obtained under the pretreatment condition with 1.5% sulfuric acid in 60 min. However, the pretreatment with 1.0% sulfuric acid resulted in better ABE fermentability of the hydrolysate mixture (M1.0) than that with 1.5% sulfuric acid. The fermentation of M1.0 produced 11.3 g/L ABE, but only 19% of pentoses were consumed. In process II, fermentation of the mixture of pretreated straw hydrolysate and gelatinized grain slurry produced more ABE (13.5 g/L) than that in process I, and 95% of pentoses were utilized in the hemicellulosic hydrolysate pretreated with more severe condition (1.5% sulfuric acid). The use of pretreated hydrolysate defined as pretreatment liquor from green and yellowish barley silage supplemented with gelatinized barley grain slurry showed feasibility for ABE fermentation, and 9.0 g/L and 10.9 g/L total ABE was produced, respectively. The combined application of xylanase and PEG 4000 in the hydrolysis of pretreated solid residues by cellulase increased the glucose and xylose yields, which were considerably higher than that obtained with the application of either one of them. The enhanced sugar production increased ABE yield from 93.8 to 135.0 g/kg pretreated straw. The results suggest that it is feasible to ferment the hemicellulosic biomass with starchbased biomass, and improve the sugar production from cellulosic biomass in straw by combined application of xylanase and surfactants in enzymatic hydrolysis for biobutanol production. For the efficient utilization of hemicellulosic biomass, the process II is more favorable, particularly for sustainable biofuel production from variety of lignocellulosic feedstocks. The enzymatic hydrolysate obtained by additive xylanase and surfactants showed good fermentability in biobutanol production, and the efficiency of straw utilization was apparently increased. Moreover, the pretreatment liquor of fresh barley silage was efficiently used for butanol fermentation with the co-fermentation processes, indicating the feasibility of utilization of green field biomass preserving by “silage” technique in biorefining processes.