Development of Pretreatment Technology and Enzymatic Hydrolysis for Biorefineries.

摘要

The growing demand for energy, materials and food, depletion of fossil raw material reservoirs and increasing environmental concerns have all increased interest in renewable resources. Lignocellulosic biomass is an alternative for replacing fossil raw materials in the production of fuels, materials and various chemicals.Lignocellulose present in plant cell walls consists mainly of polysaccharides, celluloseand hemicellulose, and aromatic lignin. These major components form a complex structure that is resistant to microbial and enzymatic activity. Due to there calcitrant structure of plant cell walls, lignocellulosic raw materials must be pretreated before their enzymatic hydrolysis to monosaccharides. Various pretreatment methods; chemical, physical, biological or their combinations, have been developed. After pretreatment, polysaccharides can be hydrolysed enzymatically to monosaccharides, which in turn can be fermented to different products such as ethanol. Currently the first commercial scale lignocellulosic ethanol plants have started production. A secure supply of biomass is one of the key factors for a feasible biorefinery, and new alternative feedstocks are still required especially in northern climates in order to fulfill the raw material demands of biorefineries in a sustainable way. In addition, development of new pretreatment technologies and more efficient enzymatic hydrolysis are needed.New lignocellulosic feedstocks and improved pretreatment methods were studied in the work described in this thesis. Reed canary grass and barley straw were found to be interesting carbohydrate-rich raw materials that could be pretreated by steam explosion and hydrolysed enzymatically with yields comparable to those obtained from wheat straw. Selection of the most favourable harvest time for reedcanary grass, autumn or spring, was studied in relation to pretreatment and hydrolysis yields. Spring harvested reed canary grass was found to be the more suitable raw material as it had a higher cellulose content and the pretreated fibre was hydrolysed more efficiently compared to autumn harvested material.A new pretreatment method using sodium carbonate and oxygen pressure was developed. The alkaline oxidation method fractionated biomass into a carbohydrate-rich fibre and a dissolved fraction containing most of the lignin. The produced carbohydrate-rich fibre could be efficiently hydrolysed by enzymes and the hydrolysis was also efficient at 12% dry matter content. Compared to the 52% total glucose yield obtained in enzyme hydrolysis of spruce after pretreatment by steam explosion, a significantly higher glucose yield of 84% was obtained in hydrolysis after alkaline oxidation. Different kinds of raw materials, such as spruce, birch and sugar cane bagasse, could be efficiently pretreated by alkaline oxidation.