Analyzing Material Properties for Efficient Processing of Spruce to Ethanol

Abstract: Fuel production from renewable resources will likely be a prerequisite for reducing the negative anthropogenic impact on the environment and, at the same time, ensuring future energy security. In this thesis, the conversion of typical Swedish biomass, mainly Norway spruce, to ethanol was studied. The first part of the work focused on the degradation of biomass to fermentable sugars. More specifically, the relation between pretreatment conditions, surface characteristics and enzymatic degradability was investigated. Furthermore, factors determining the rheology of pretreated biomass and the effect of mixing on productivity in the enzymatic hydrolysis were studied. The second part of the work dealt with the fermentation of the hydrolyzate to ethanol. Novel strategies for improving xylose utilization in simultaneous saccharification and co-fermentation (SSCF) were developed based on enzyme feeding. By the use of several surface characterization methods, it was shown that the rate of enzymatic hydrolysis is improved by more severe pretreatment conditions due to increased cellulose pore sizes. The negative effect of increasing amount and hydrophobicity of surface lignin at higher severities is thus overcome by improving the cellulose accessibility. The enzymatic hydrolysis of pretreated spruce furthermore showed a strong dependence on the power input for mixing. The rheological characterization revealed that steam pretreated spruce exhibits non-Newtonian flow behavior and a yield value. The flow properties of pretreated spruce were strongly dependent on the WIS concentration and degree of hydrolysis. In addition, the variance of the fiber size distribution - and not only the average fiber size - was shown to influence the rheological properties. Controlling the glucose level at a low but non-zero concentration was shown to favor the xylose uptake during SSCF. Prefermentation, i.e. fermentation of initial glucose before addition of enzymes, resulted in a rapid consumption of the initial glucose and consequently improved xylose uptake. Favorable co-fermentation conditions in the SSCF were also obtained by using a controlled enzyme feeding strategy. In this way, a constant glucose release rate and a low glucose concentration in the broth were obtained, allowing efficient xylose uptake.