Short-Term Lime Pretreatment And Enzymatic Conversion Of Sawdust Into Ethanol

ABSTRACT

The utilization of lignocellulosic biomass as feedstock for the production of fuel ethanol has attracted considerable interests in the last few decades. The emergence of new technologies has provided hope for fuel ethanol potential uses. Lignocellulose is a valuable alternative energy source. The enzymatic hydrolysis of lignocellulosic biomass is constrained due to its complex structural features, so pretreatment is important to enhance its enzymatic digestibility. In this study, the influence of process parameters – temperature, air addition, hydrogen peroxide addition, and time – on the pretreatment of sawdust (a wood residue) was investigated. The optimization of the pretreatment step was performed by using the full factorial and central composite designs of experiments. The study assessed the compositional changes by applying short-term oxidative pretreatments such as alkaline wet air oxidation, alkaline peroxide oxidation, and alkaline peroxide assisted wet air oxidation methodologies, and their effects on the yields of reducing sugar. The best pretreatment condition based on the yield of the reducing sugar was the alkaline peroxide-assisted wet air oxidation at 150 oC, 1%H2O2, 10 bar air pressure, 45 min. The optimal 4-day reducing sugar yield was 335.35 mg equivalent glucose/g dry biomass at 40 g/L substrate concentration, 25 FPU/g dry substrate of cellulase enzyme, and 5 IU/g dry substrate of β-glucosidase. Furthermore, when considering the fermentability of the treated solids, at 2% effective cellulose loading, 9.71 g/L ethanol (23.43% theoretical ethanol yield) was obtained for pretreatment at 150 oC, 1%H2O2, 10 bar air pressure, and 45 min. At the optimum pretreatment condition, 0.1 g Ca(OH)2/g dry biomass was enough to cause appreciable lignin removal. Lignin removal was largely dependent on temperature, and the prevailing oxidative conditions. Cellulose was highly preserved in the solid fraction, while more of the hemicellulose was solubilized/degraded. The high-lignin content of the raw material was a great obstacle to the digestibility of the treated material. The lignin remained largely undissolved in the solid fraction.