Combined production of bioethanol and biogas from wheat straw

Abstract: Currently the production cost of lignocellulosic ethanol is considered to be relatively high. Large fractions of the production cost come from the raw material and the energy consumed in the distillation. Therefore it is desirable to have a process that yields high concentration of ethanol and maximizes the utilization of the biomass. Wheat straw is a promising raw material for lignocellulosic ethanol production. It contains large fractions of glucose (C-6 sugar), xylose (C-5 sugar) and lignin. Since the yeast (Ethanol red) is only able to convert C-6 sugars to ethanol, the rest has to be efficiently separated and turned into other value-added products. The approach in this study was to pretreat the wheat straw by steam explosion with addition of a catalyst, which recovers most of the glucose and lignin in the solid fraction and most of the xylose as well as degradation products in the liquid fraction. The solid fraction which contains most of the C-6 sugars is utilized as substrate in the fermentation and the liquid fraction can be utilized as substrate for biogas production. In order to achieve higher concentrations of ethanol, the water insoluble solids (WIS) content was increased in the fermenter vessel. WIS-contents investigated were 20%, 25% and 30%. But by doing so, there will be an increased resistance to stirring, mass transfer limitations and increased inhibitor concentration. This will most likely have a negative impact on the ethanol yield. The pretreatment was carried out by steam explosion at 190°C and 10 minutes residence time, using lactic acid as a catalyst. About 91% of the glucose was recovered in the solid fraction and about 54% of the xylose as well as most of the degradation products were recovered in the liquid fraction. The fermentations were carried out by simultaneous saccharification and fermentation (SSF) with or without a pre-hydrolysis step (PSSF). The experiments were performed in fed batch mode, which was used in order to avoid increased viscosity. The highest ethanol concentration (about 102 g/l, corresponding to 9.7% (w/w)) was achieved by the PSSF concept at 30% WIS. The ethanol yield was about 88% of the theoretical yield, based on the glucose available in the substrate. The liquid fraction was further separated from the solids and distilled, intended to be used as substrate for biogas production. The solid residues can be used for process heat. The highest methane yield obtained from the pretreatment hydrolysate was about 168 ml/g volatile solids (VS), corresponding to 2 g methane per 100 g wheat straw. The methane yield of the stillages from the SSF experiments was estimated by stoichiometric calculations to be about 1 – 3 g of methane per 100 g wheat straw.