Steam Explosion of Wood

Licentiatavhandling, 2014

The rising price of petroleum and environmental concerns regarding CO2 emissions has increased interest in alternative renewable resources. Biomass can be considered as an excellent alternative raw material. A biorefinery uses biomass and produces fuel, energy and value-added chemicals. The biorefinery is an emerging field and requires much development to compete with already established petroleum-based industries. One of the greatest challenges to the biorefinery is that the raw material; biomass, has a complex chemical composition and physical structure. A pretreatment process is necessary to induce physico-chemical changes in the biomass and transform it into easily digestible material. The main factor limiting enzymatic digestion of biomass is accessibility to chemical constituents. Steam Explosion (SE) pretreatment is a promising process that has many potential benefits compared to the alternatives, e.g. it has less hazardous process chemicals and conditions, less environmental impact, fewer energy requirements and lower capital investment. Existing literature on the SE process mainly concerns products obtained after the process. Knowledge about the physical processes that take place during the SE pretreatment is limited. This licentiate thesis is based on experimental and modelling studies performed with the aim of gaining knowledge of the basic mechanisms of this process. The SE is a three-step process that involves; (i) treatment of wood with pressurized steam for a specific period of time, (ii) explosion of wood chips through the rapid release of pressure, and (iii) impact of softened wood chips with other chips and vessel walls. In the experimental part these steps have been carefully isolated and the effects of these steps on internal and external structures of single spruce wood pieces have been studied. The effect of vapour expansion and the creation of stresses during the explosion step on a single cell of spruce wood (with four layers; P, S1, S2 and S3) at high temperature and moisture content have been modelled using the Finite Element Method. The study reveals that all the steps of the SE process contribute to structural changes in the wood material and increase pore size which increases the accessibility of chemical reagents and enzymes. A wood piece disintegrates into smaller pieces during the impact step. The vapour expansion inside cells during the explosion step causes each cell to expand in all directions and creates high stress and strain fields perpendicular to the cell direction. In general, cell wall damage is more likely to occur in cells with thin walls, i.e. earlywood; damaged P, S1 and S3 layers; low MFAs; irregular cross-sections and sharp corners.