Combined production of biogas and ethanol at high solids loading from steam-pretreated wheat straw impregnated with organic acids

Obtaining high ethanol concentration in lignocellulose-based ethanol production is of great importance in order to minimize the energy need in the downstream processes. In addition, high product yields, maximal utilization of the raw material and reduction of production costs are some of the main challenges in cellulosic ethanol production.

In the present study, a combined process for the co-production of ethanol and biogas from steam-pretreated wheat straw was investigated. Organic acids were used for the impregnation of wheat straw with the intention of providing milder pretreatment conditions and obtaining lower inhibitor concentrations after pretreatment, compared to when stronger acids are used.

In order to increase the ethanol concentration at high solids loadings without increasing the amount of inhibitors in the fermentation, and at the same time, improve the utilization of C5 sugars, the liquid fraction (C5 stream) was separated from the solid fraction (C6 stream) after steam pretreatment. The C6 stream was used at high solids concentrations to produce ethanol in simultaneous saccharification and fermentation (SSF), while the C5 stream and the filtered residue after distillation of ethanol (thin stillage) were evaluated for biogas production. The lignin-rich solid residue obtained after SSF was not utilized in this study; however, it is well suited for production of pellets, or for generation of steam and/or electricity. In order to evaluate the energy efficiency of the process, the total energy content of the products, i.e. ethanol, methane and the lignin-rich solid residue, was compared with the energy content in the dry wheat straw.