Two-stage acidic-alkaline pretreatment of Miscanthus for bioethanol production

Pretreatment is the rate-limiting step for bioethanol production from lignocellulosic biomass, and subsequently intensive studies have been undertaken to improve the pretreatment efficiency. However, so far most pretreatment methods failed to achieve desirable sugar recovery from both cellulose and hemicellulose in the biomass, which is essential to improve process economics and competitiveness of bioethanol. To address the issue, this research developed two innovative pretreatment methods successively. Miscanthus was used as the model feedstock. The effects of primary pretreatment conditions on the performance were examined. Process optimization was conducted to locate the best operational conditions. The pretreatment effectiveness was evaluated in terms of sugars yield, biomass structure alteration and ethanol yield. A two-stage acidic-alkaline pretreatment was proposed to obtain most intact monosaccharides from cellulose and hemicellulose. Dilute sulfuric acid pretreatment was performed in the first stage mainly for hemicellulose removal while the second stage carried out lime pretreatment primarily for delignification. The process was optimized by using Response Surface Methodology (RSM) analysis taking account of temperature, catalyst loading and residence time. It was demonstrated that the maximized sugars yield could be attained at medium severities in acid stage and higher severities in alkaline stage. The best pretreatment conditions were found at 0.73 wt% H2SO4, 150 ºC, 6 min in acid stage, and 0.024 g/g dry biomass of lime loading, 202 ºC in alkaline stage. In addition to the greatly improved sugars yield, the two-stage process also showed great promises in considerably reduced induction of primary degradation by-products, with proven significantly enhanced ethanol yield. To further improve hemicellulose hydrolysis in acid stage, a second pretreatment method, combined acid hydrolysis, was developed to replace the conventional dilute acid pretreatment. The applied combined acid catalysts included sulfuric acid and two biomimetic acids, trifluoroacetic acid (TFA) and maleic acid (MA), respectively. The influences of acid blending ratio, temperature, and acid dosage on pretreatment performance were investigated. Synergistic effects on hemicellulose decomposition were observed under all studied conditions. Further, combined TFA pretreatment could efficiently prevent xylose degradation. Combined acid hydrolysis was shown to be a favorable pretreatment method for its improved xylose yield, reduced catalyst costs and enhanced ethanol yield. Ultimately, further study indicated adoption of combined acid hydrolysis in the two-stage acidic-alkaline pretreatment could achieve higher sugars recovery