Mild Steam Explosion of Norway Spruce

Doctoral thesis, 2014

The most common wood species in Sweden, and one of the most important renewable raw materials in Northern Europe, is Norway spruce (Picea abies). Today, it is utilized mainly for sawed timber and the production of pulp and paper. A modern kraft pulp mill that produces bleached pulp has a material efficiency of about 40-45%, and the final product contains mainly cellulose. The other components of the wood, e.g. hemicelluloses and lignin, are heavily degraded during the process and end up in the mill’s recovery boiler, where they are burned to recover the latent energy. The biorefinery concept is an approach where biomass is used for the production of a variety of products, e.g. new materials, chemicals and fuels. The aim of this work is to investigate a process called “mild steam explosion” as a pre-treatment step in a biorefinery. During steam explosion, saturated steam is applied to biomass at elevated pressure, which is followed by a fast pressure release. The treatment leads to both mechanical rupture and chemical reactions, such as acid hydrolysis. The conditions of the steam explosion treatment are kept mild (approx. 140-170°C) to ensure that the degradation of the wood components is kept at a minimum. The idea behind the treatment is to make the structure of wood more accessible and facilitate the extraction and isolation of the wood components, preferably those of high molecular weights. The most abundant hemicellulose component in spruce is (galacto)glucomannan and is of primary concern. This is a challenge since it is also the component that is the most sensitive to chemicals. Treatment with reducing agents, such as sodium borohydride and dithionite, are therefore used to stabilize the (galacto)glucomannan. The findings in this thesis showed that steam explosion, even at modest conditions, made the wood structure accessible for enzymatic reactions. It was also shown that wood components from hemicelluloses and wood extractives were released into the condensed steam. Mild steam explosion was also seen to increase the rates of both extraction and delignification during subsequent treatments. The mechanical effects of the steam explosion treatment originated from steam heating, expansion during pressure release and impact. The properties of pulps after kraft cooking and oxygen delignification of steam-exploded wood chips were comparable to reference pulps. It was also found that treatment with a reducing agent stabilizes the (galacto)glucomannan during both mild steam explosion and various chemical treatments.