Enhanced subcritical water pretreatment by water-ethanol system for bioethanol production from corn straw

Subcritical water was a promising, clean technique to efficiently pretreat the biomass and further promote the bioethanol conversion. However, the relatively lower removal rate of lignin from biomass after subcritical water pretreatment might be the main reason for the low conversion efficiency of bioethanol. For the purpose of resolving the proposed problem, ethanol was added into subcritical water as an additive to improve pretreatment efficiency in this present study. Response surface methodology based on glucose concentration after enzymatic hydrolysis was adopted to optimized pretreatment process, and the optimum conditions were obtained as 220 degrees C in temperature, 20 min in time, 29.19% in ethanol concentration, and 35.73 (v/w) in liquid-solid ratio. After water-ethanol system pretreatment, the glucose concentration was up to 19.80 +/- 0.38 g/L, which was much higher than that of 9.99 +/- 0.44 g/L of subcritical water pretreatment. Besides, the chemical components, surface morphology, and crystallinity of residues in vital steps were also conducted, indicating that water-ethanol system might be beneficial to improve the pretreatment efficiency. Moreover, the optimal enzymatic hydrolysis conditions of 30 FPU/g (CEL-01), 1 U/g (SPE-007AL), 16% (w/v) loading, and 72 h of time were determined. Meanwhile, based on optimal enzymatic hydrolysis conditions, the ethanol concentration generated by waterethanol system was up to 27.38 +/- 0.47 g/L (SHF) and 29.98 +/- 0.24 g/L (SSSF), which were all much higher than that of subcritical water pretreatment. Besides, ethanol recovery was also investigated, which could be reused in the next pretreatment process. In summary, water-ethanol system was a more effective method for biomass pretreatment to promote bioethanol conversion.