This paper investigates the production of ethanol from steam pretreated aspen. The optimal conditions for both the pretreatment and ethanol production were determined. In the former step the parameters investigated were temperature (180-220 °C) and residence time (2-6 min). The most effective combination was 214 °C and 6 min. The exploded substrates were detoxified in three ways, washing with water at 65 °C proving to be the most effective. The substrate was then converted into ethanol via simultaneous saccharification and fermentation. Influences of the reactor type (shaken flasks and stirred bioreactors) and process parameters (solid-to-liquid ratio, enzyme loading, and stirrer speed) have been investigated. The highest ethanol yield obtained from solid-to-liquid ratios of 0.20 g/g was 85% in shaken flasks and 79% in helical stirred bioreactors. In the former case, the ethanol concentration in the broth was 47 g/L. The fermentation unit returns a solid residue with a calorific value of 5612 kcal/kg. The chemical oxygen demand due to compounds dissolved in the stillage is 28 800 mg of O 2 /L. Test runs were carried out at bench and pilot scales.
The pretreatment step has a key role in the enzymatic conversion of lignocellulosic biomass to bioethanol. Steam pulping of biomass has long been recognized as being effective in producing high biomass fractionation. In this step, the preliminary impregnation of biomass with acid catalysts, including SO 2 , has been shown to further improve the hydrolytic effect and increase the digestibility of the fibers by enzymes. This work focused on developing an experimental setup for the SO 2 impregnation, enabling accurate control of the process variables. The final purpose was the assessment of a method to minimize the quantity of SO 2 used, and that could be applicable to continuous operations. Aspen chips were impregnated with the acid catalyst at room temperature in a stainless steel batch reactor, and the influence of the biomass humidity and contact time on SO 2 uptake was explored. After the catalyst adsorption, biomass was fed into the steam explosion batch reactor and steamed at 205°C for 3 and 10 min to obtain slurries. The addition of catalyst (∼0.9% w/w raw material dry matter) reduced the degree of polymerization of the cellulose by 50%, on average. The highest yield of xylose (10.3 g/100 dry chips) was obtained via water extraction following a steam pretreatment of 3 min. The amount of monomer xylose was 80% of the total extracted dry matter. Simultaneous saccharification and fermentation (SSF) of the washed fibers yielded 37 g of glucose/100 g dry chips, 96% of which fermented to ethanol.
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