Combined Supercritical/subcritical Pretreatment And Hydrolysis Of Stalks For Ethanol Production

Ethanol production from lignocellulosic waste, one of the most important sources of bio-energy, is receiving much attention due to its feasibility and valuable products. Pretreatment and hydrolysis are the key processes of ethanol production from plant stalks because of the special structure of lignocellulose. Due to its high dissolving and catalyzing capacity, supercritical water can separate lignin from cellulose and rapidly hydrolyze the cellulose into water-soluble oligosaccharides and then to hexoses, primarily glucose. However, glucose decomposes rapidly into unfermentable products in supercritical water. In contrast, glucose decomposition rate is much lower in subcritical water. Therefore, a combined supercritical and subcritical process was applied in this study, in which lignocelluloses are pretreated and hydrolyzed in supercritical water to remove the lignin and to produce oligosaccharides from cellulose. This is followed by a second step using subcritical water that further converts the oligosaccharides into fermentable hexoses. This may prove to be a promissing strategy for utilizing lignocellulosic waste.The investigation of cellulose hydrolysis showed that solid/liquid ratio, temperature, and reaction time were the key parameters in the supercritical process. A temperature slightly above the critical point of water and a relatively short reaction time favored oligosaccharide production. In the combined supercritical and subcritical experiments, an approximately 40% yield of hexoses was obtained in the combined process under the conditions of 380°C, 16s (supercritical) and 280°C, 44s (subcritical). Kinetics of oligosaccharide hydrolysis and hexose decomposition under subcritical conditions revealed that the smaller reaction rate constants for hexoses decomposition compared to those for oligosaccharides hydrolysis allowed hexoses to accumulate. Based on the kinetic analysis, the theoretical value for the maximum yield of hexoses and the corresponding optimum reaction time can be both calculated, which can then be used to provide key parameters for the combined process of lignocellulosic waste conversion. Combined supercritical and subcritical experiments with either corn stalks or wheat straw as a starting maerial were proven to be efficient for lignin separation and cellulose hydrolysis. Cellulose in these plant materials could be converted into oligosaccharides in supercritical water and then to fermentable hexoses in subcritical water. In the combined processes, the mixture of the supercritical hydrolyzate from corn stalks (obtained at 384°C, 17s) and the water extraction containing soluble sugars was treated at 280°C for 27s to obtain a high yield of fermentable hexoses (66.7% of the available compositions). In wheat straw, 30.7% of the available components were converted into fermentable hexoses when a supercritcal hydrolyzate from wheat straw (obtained at 384°C, 19s) was further hydrolyzed at 280°C for 54s. This research demonstrated the successful pretreatment and hydrolysis of lignocellulosic waste using a combined supercritical and subcritical technology.