Structural Modifications in Spruce Wood during Steam Explosion Pretreatment

Doctoral thesis, 2016

The rising price of petroleum and environmental concerns regarding CO2 emissions have 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 valueadded chemicals. The biorefinery is an emerging industry and requires much development to compete with 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. Enzymatic hydrolysis of native biomass for the production of biofuel yields very small amounts of product. A pretreatment process is necessary to induce physico-chemical changes in the biomass and transform it into easily hydrolysable material. The main factor limiting enzymatic digestion of biomass is accessibility to chemical constituents. Steam explosion pretreatment is a promising pretreatment that modifies both the physical and chemical structure of biomass and significantly enhances the subsequent enzymatic hydrolysis of the pretreated material. Steam explosion pretreatment can be studied as a three-step process that involves; (i) the treatment of wood with pressurized steam for a specific period of time, (ii) the rapid release of pressure which causes vapours inside the wood cells to expand, and (iii) the discharge of the wood chips into a blow tank which results in collisions between wood chips and impact with vessel walls. This thesis is based on experimental and modelling studies performed with the aim of gaining knowledge of the basic mechanisms of the steam explosion process. In the experimental part, the three steps of steam explosion pretreatment were carefully isolated, and the effect of these steps on the structural changes in spruce wood pieces was studied. The study revealed that all the steps of the steam explosion process contribute to structural changes in the wood material, which increases the enzymatic hydrolysis of the material. It was found that wood chips disintegrate into small fragments and that the microstructure of the wood is vigorously destroyed as a result of the collision between wood chips and vessel walls. The deformation in the cellular structure of softwood due to the rapid decompression of vapour inside the wood cells was modelled using the Finite Element Method. Simulations identified the regions where microcracks were likely to appear. These regions showed much resemblance to the experimentally obtained steam-exploded wood. Simulations of collisions between wood chips and impact with a steel wall were also performed. It was found that the wood chip that moves at high velocity and impacts with the steel wall in the radial direction acquires the most damage.