Characterization of Pretreated Fractions and Cellulosic Ethanol Production from Steam-Exploded Eucalyptus urograndis

Abstract

Biomass conversion processes have become increasingly important to mitigate fossil fuel consumption and to increase the contribution of renewable fuels into the world energy matrix. In this study, Eucalyptus urograndis wood chips were pretreated by autocatalytic steam explosion to produce cellulosic ethanol after enzymatic hydrolysis and fermentation. These experiments were organized in a central composite rotatable design using temperatures and reaction times ranging from 174 to 216 °C and from 4 to 11 min, respectively. Mass yields, cellulose degree of polymerization, and glucose yields after enzymatic hydrolysis showed a linear correlation with pretreatment severity. The best condition was set at 210 °C for 5 min due to its higher glucose yield after pretreatment and enzymatic hydrolysis. Pectins were almost completely solubilized after pretreatment while galactoglucomannans were more resistant to acid hydrolysis than arabinoglucuronoxylans. Alkaline delignification led to 90.3% lignin removal from steam-exploded materials, but its effect on enzymatic hydrolysis was almost negligible. For the best pretreatment condition, analyses by confocal laser scanning microscopy and solid-state nuclear magnetic resonance revealed important changes in fiber morphology and chemical composition, respectively. Enzymatic hydrolysis at 4 wt % total solids with Cellic CTec2 and Cellic CTec3 (Novozymes) led to 22.4 and 27.8 g L–1 glucose equivalents in 96 h, respectively, whose fermentation with a commercial strain of Saccharomyces cerevisiae led to ethanol productivities greater than 3.5 g L–1 h–1 in 6 h.

Steam Explosion of Eucalyptus grandis Sawdust for Ethanol Production within a Biorefinery Approach

“…Table 2 shows the final glucose concentration, enzymatic hydrolysis efficiency, and glucan conversion of unwashed exploded solids after incubation with a low enzyme loading of Cellic CTec2 for 72 h. Some glucan conversion efficiencies were higher than the corresponding hydrolysis efficiencies because the glucose present in the unwashed solids (therefore, in the retained pretreatment liquor) was considered in the former but not in the latter calculations. Enzymatic hydrolysis using only 9 FPU/g glucan released 3 to 13 g/L glucose in the substrate hydrolysate, while the glucan conversion varied from 17 to 67% with maximum values achieved for the pretreatments carried out at 205 • C and 10 min (EV7A and EV9A), values similar to those obtained by Martín-Sampedro et al [15] and Chiarello et al [49] for eucalyptus working at low concentrations of solids (4 and 9%). An improvement in the hydrolysis efficiency could be observed by increasing both pretreatment temperature and retention time of the eucalypt sawdust inside the reactor vessel.…”

“…López-Linares et al [51] and Martín-Davison et al [52] obtained ethanol conversion efficiencies of 70 and 78% by SSF after pretreatment of rapeseed straw and poplar hybrid at 215 and 220 • C, respectively (Table 6). Chiarello et al [6,49] achieved 83 and 88% ethanol conversion efficiencies from steam-exploded E. urograndis wood chips that were pretreated at 210 • C for 5 and 10 min, respectively, using the same equipment applied in this work. Despite using different saccharification and fermentation strategies, all of these values are close to what was achieved from the water-washed E. grandis sawdust (76%).…”