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.