Mild Steam Explosion of Wood and Forest Residues in the Perspective of a Materials Biorefinery

Abstract: The main objective of this work was to explore the prospects of using mild steam explosion as a pretreatment step in a forest based material biorefinery. During the steam explosion saturated steam is applied to the biomass at elevated pressure leading to an autohydrolysis of the lignocellulosic tissue, which is followed by a rapid pressure discharge, disintegrating and opening up the structure. As a consequence, the pretreatment enables the extraction of the most sensitive hemicelluloses and facilitates further processing, e.g. enzymatic treatment and chemical pulping. To investigate the effects rendered by the pretreatment, it was performed on two different types of forest biomass: Norway spruce wood chips and forest residues of mixed origin. The focus was on investigating the effects on the chemical structure of the material. In order to gain improved understanding of the fundamental mechanisms behind the pretreatment, the local effects on the composition of the wood tissue pretreated using steam explosion were investigated and compared with those accomplished by hot water extraction. Furthermore, cooking experiments were performed on pretreated forest residues to evaluate how the changes rendered by the pretreatment affect further processing of the material. The effects of the steam explosion were evaluated in terms of compositional analysis, molecular weight distribution and structural changes of extracted material (lignin and hemicelluloses). The results show that, due to the advective mass transport during the explosion step, steam explosion accomplishes a more even removal of hemicelluloses from the pretreated wood chips compared to the hot water extraction. Moreover, the impact of the steam explosion was found to be limited when material of a smaller size, namely refined forest residues, was pretreated.