Development of a two-stage steam explosion pretreatment process for beech wood

Beech wood is an attractive lignocellulosic feedstock in central Europe. However, the compact structure with a high lignin content demands harsh pretreatment conditions in order to achieve high glucose yields in enzymatic hydrolysis. In steam explosion pretreatment, these conditions lead to the degradation of the more sensitive hemicellulose fraction and consequently only moderate total sugar yields. Furthermore, more inhibitory compounds for hydrolytic enzymes and fermentation organisms are released under harsh conditions. Consequently, we are developing and investigating a two-stage steam pretreatment process that allows for the successive pretreatment of the biomass at two conditions with different severities optimized individually for maximal xylose and glucose yield. The constructed pretreatment plant allows to perform this two stage process without a decompression of the reactor between the steps by releasing the hemicellulosic sugar syrup through a bleed line.

To determine promising conditions for two-stage pretreatment, beech wood chips (Fagus sylvatica) are conventionally steam pretreated at a temperature range from 160°C to 230°C and durations corresponding to severities from 3 to 5.25. Solubilized compounds (sugars, acetic acid, formic acid, HMF, furfural, phenolics) in the hydrolysate were quantified and enzymatic hydrolysis of washed solids as well as of the whole pretreatment slurry was done. The two best conditions for maximum xylose and glucose yield serve as the basis for the development of an optimized two stage pretreatment process. Results are compared with respect to fractionation efficiency as well as total sugar yields and inhibitor formation.