Integration of a hemicelluloses extraction step into a forest biorefinery for production of green chemicals

Abstract: Sustainable use of forest and agricultural resources will play an important role for solving urgent global challenges such as the enhanced green house effect and increasing demand for fossil fuels. The development of processes where lignocellulosic biomass can be refined to several different end-products in the same plant, i.e. a biorefinary, will be important in the development towards a more sustainable society where fossil fuels are replaced. To be able to compete with fossil resources, an efficient production of biomass based products is required in order to maximize overall process economics and to minimize negative environmental impact. One solution to increase profitability for forest biomass based plants can be production of value added derivatives produced through fermentation of sugars from hemicelluloses, extracted from lignocellulosic material. The first part of this thesis investigate the impact of hemicellulose pre-extraction on birch Kraft pulp properties. White liquor and water extractions of hemicelluloses from birch wood chips were performed under conditions compatible with Kraft pulping. The chips from select extractions were subject to subsequent Kraft pulping and the refined pulps were made into hand sheets. Several metrics for hand sheet strength properties were compared with a reference pulp made without an extraction step. This work also includes a demonstration of enzymatic hydrolysis and biological conversion of extracted xylan to succinic acid, a metabolite with the potential of a platform chemical. The study demonstrated that white liquor can be utilized to extract xylan from birch wood chips prior to Kraft cooking without decreasing the pulp yield and paper strength properties, while simultaneously impregnating cooking alkali into the wood chips. Alkaline conditions tested above pH 10 significantly degraded xylan and very low concentrations of xylose were obtained using any of the alkaline extractions. Water extractions resulted in the highest final concentration of xylose, 29.1 g/L; yielding fermentable liquor, but were found to negatively impact some pulp properties including decreases in compression strength, bursting strength, tensile strength and tensile stiffness while exhibiting minimal impact on elongation and slight improvement in tearing strength index. Since hot water extractions gave fermentable liquors, the next study was to integrate the production of green chemicals via hot water hemicellulose extraction of birch wood into a small-scale combined heat and power plant, in this case an externally fired gas turbine. The results show that the extracted wood chips would serve very well as a fuel for combustion and gasification processes due to the relatively high heating value. Most important, the extracted wood chips had low ash content and significantly lower concentrations of alkali metals. In addition a fermentable stream with a xylose concentration of 65 g/L was produced.The second part of this thesis was to optimise the production of the dicarboxylic acid, succinic acid, which can be produced via bioconversion as a renewable building block molecule for production of biodegradable solvents and polyesters. In this study the E. coli strain AFP184, which can ferment both five and six carbon sugars with a limited production of other organic acids was used. Earlier work using a high initial sugar concentration resulted in volumetric productivities of almost 3 g/L h, which is above estimated values for economically feasible production, and final succinic acid concentration was around 40 g/L. To further increase succinic acid concentrations, fermentations using NH4OH, NaOH, KOH, K2CO3, and Na2CO3 as neutralising agents were performed and compared. It was shown that substantial improvements could be made by using alkali bases to neutralise the fermentations. The highest concentrations and productivities were achieved when Na2CO3 was used, 77 g/L and 3 g/L h, respectively. A gradual decrease in succinate productivity was observed during the fermentations, which was shown to be due to succinate accumulation in the broth and not as a result of the addition of neutralising agent or the subsequent increase in osmolarity.