Sugars from biomass: High cellulose hydrolysability of oxygen alkali treated spruce, beech and wheat straw

Norway spruce (Picea abies (L.) Karst), common beech (Fagus sylvatica L.) and wheat straw (Triticum L.) were fractionated by VTT’s modified oxygen/carbonate process in varying conditions. Main focus of the study was to obtain a solid fraction in high yield having high hydrolysability of carbohydrates. In addition to evaluating the cellulosic fraction in enzyme hydrolysis, some characteristics of the lignin fractions in spent liquors were determined as well.
The process parameters (reaction temperature and time) were adjusted for the different raw materials used according to their chemical characteristics. Beech proved to require somewhat milder process conditions for efficient treatment than spruce. A yield of about 60% was obtained for the fibre fraction (calculated on the original o.d. raw material) at a lignin content of 9% for beech and at a lignin content of 15% for spruce. Wheat straw was processed at clearly milder conditions giving a fibre fraction yield of 56% at lignin content of 3%.
All the samples showed high hydrolysability by enzymes. A very high degree of hydrolysis (up to 95% in 24 h) could be reached in laboratory conditions with moderate enzyme dosages. Longer hydrolysis times enabled significant reduction in enzyme dosage, as compared to e.g. typical fibre fractions from steam explosion.
The main component in the spent liquors was oxidized sulphur-free lignin with a molecular weight (Mw) range < 10000. Part of the original hemicelluloses was also dissolved and found in the oligo/polysaccharides form. Substantial amounts of low molecular weight carboxylic acids, most likely derived from hemicelluloses and lignin were detected as well.