13-11: Production of fuel ethanol from softwood by high temperature prehydrolysis prior to high solid SSF

Bio-ethanol can be produced from cellulose-rich material such as softwood by enzymatic hydrolysis and fermentation. Simultaneous saccharification and fermentation (SSF) of steam pretreated SO₂-impregnated spruce has been shown to result in almost 100% conversion of fermentable sugars in the raw material to ethanol at moderate dry matter contents up to around 10% WIS (water insoluble solids).

SSF at higher dry matter contents, which is important  to achieve higher ethanol concentrations and lower the energy demand in the distillation step as well as to reduce the production cost, resulted in significantly decreased ethanol yield. This is mainly due to increased mixing difficulties and increased concentrations of compounds inhibitory to the enzymes and yeast used in the process.

 The use of high temperature prehydrolysis, prior to SSF, resulted in  an ethanol concentration of 48 g/L with an ethanol yield of 72% of the theoretical possible over the SSF step using 13.7% WIS steam-pretreated spruce . Even though this process configuration, i.e. prehydrolysis before SSF, has been shown not to be able to improve ethanol concentration or yield at moderate WIS contents (up to 10% WIS), high temperature prehydrolysis appears to more than double the ethanol yield and concentration at 13.7% WIS. The influence of different process parameters on high solid SSF with high temperature prehydrolysis has been investigated. The results will be presented at the conference.