Optimisation of steam pretreatment of SO2-impregnated mixed softwoods for ethanol production

Pretreatment of mixed softwoods with SO2 impregnation and steam for production of ethanol has been investigated. The optimal conditions, both for sugar yield and ethanol yield, to assess the effect of inhibitors formed in the pretreatment, have been determined. The parameters investigated were: SO2 concentration (1–6% (w/w) dry matter), temperature (190–230°C) and residence time (2–15 min). After pretreatment, the material was separated into a solid residue and a filtrate. The solid residue was enzymatically hydrolysed with 2% dry matter (w/w). To investigate fermentability, the hydrolysed filtrate was fermented using Saccharomyces cerevisiae. The effects of the different parameters are described by response-surface modelling. The highest experimental sugar yield of 42·1 g per 100 g dry matter was obtained at 210°C and 5·5 min residence time. Although the fermentabilities were good for all filtrates with yields greater than 90% of the yield obtained in a pure glucose reference solution, the pretreatment has a clear influence on the ethanol production rate. © 1998 SCI