Lignin, a multifunctional major biomass component, has a prominent potential as feedstock to be converted into high value-added products. Lignin is available in high amounts as side streams during cellulosic ethanol production, and within the biorefinery context, it is important to assess its structural characteristics in order to explore its potential to replace some petroleum-based reactants. In this study, some important features were evaluated for different lignins such as lignin purity and the amounts of syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units. Four alkaline lignins, generated from a pilot-scale pretreatment of sugar cane bagasse (NaOH 1.5%, 30 min), were evaluated according to the severity of the alkaline pretreatment (130 or 170 °C, with or without the addition of anthraquinone). The different pretreatments produced lignins with different chemical characteristics that can be used for different purposes in sugar cane biorefineries. As the severity of alkaline pretreatment increased, the recovered lignins presented higher amounts of H-and lower amounts of S-lignin units. In particular, the lignin obtained at 170 °C with the addition of anthraquinone presented the highest content of H-and the lowest content of S-lignin units, which would present higher reactivity toward formaldehyde in phenolic resins.
Primary sludge from a Portuguese pulp and paper mill, containing 60% of carbohydrates, and unbleached pulp (as reference material), with 93% of carbohydrates, were used to produce ethanol by simultaneous saccharification and fermentation (SSF). SSF was performed in batch or fed-batch conditions without the need of a pretreatment. Cellic CTec2 was the cellulolytic enzymatic complex used and Saccharomyces cerevisiae (baker's yeast or ATCC 26602 strain) or the thermotolerant yeast Kluyveromyces marxianus NCYC 1426 were employed. Primary sludge was successfully converted to ethanol and the best results in SSF efficiency were obtained with S. cerevisiae. An ethanol concentration of 22.7 g L was produced using a content of 50 g L of carbohydrates from primary sludge, in batch conditions, with a global conversion yield of 81% and a production rate of 0.94 g L h. Fed-batch operation enabled higher solids content (total carbohydrate concentration of 200 g L, equivalent to a consistency of 33%) and a reduction of three-quarters of cellulolytic enzyme load, leading to an ethanol concentration of 40.7 g L, although with lower yield and productivity. Xylitol with a concentration up to 7 g L was also identified as by-product in the primary sludge bioconversion process.
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