Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 35

Fig. 7 Process diagram of the NREL projected dilute-acid pretreatment of corn stover [ 28 ]

acid is added at the discharge point at a concentration of 18 mg/g dry biomass
before feeding into the horizontal reactor, which is operated at 158C (0.55 MPa),
with a residence time of 5 min. The feedstock from the horizontal reactor is
discharged into a blowdown tank operated at 130C (0.28 MPa). The slurry from
the blowdown tank goes into the oligomer conversion tank, where an additional
4.1 mg acid/g feedstock is added, making the total acid loading 22.1 mg/g dry
biomass. The oligomer conversion tank is also maintained at 130C, with a resi-
dence time of 20-30 min. Subsequently, the feedstock is discharged into a flash tank
operated at atmospheric pressure. At this stage, the hydrolysate containing 30% total
solids and 16.6% insoluble solids is pumped into the conditioning tank, in which the
slurry is diluted to slightly higher than 20% total solids for enzymatic hydrolysis and
cooled to 75C. Ammonia is sparged into the dilution water to adjust the hydrolysate
pH to 5 as well as to provide a nitrogen source for subsequent microbial growth and
ethanol fermentation. All volatile components from the blowdown tank, oligomer
conversion tank and flash tank are condensed and collected [ 29 ].
Although dilute acid pretreatment seems more economically competitive, some
disadvantages like corrosion, which requires expensive acid-resistant stainless
steel or coatings, and inhibitors produced during the pretreatment under high
temperatures, have led to the exploration of alternatives, one of them being
alkaline pretreatment. Various alkalis including sodium hydroxide, lime and
aqueous ammonia have been studied [ 30 - 32 ]. Basically, alkaline pretreatment is a
delignification process, and the underlying mechanism is the saponification of
intermolecular ester bonds crosslinking xylan hemicelluloses and lignin [ 33 ].
In addition, alkaline pretreatment also removes acetyl and other acidic substitu-
tions on hemicelluloses that protect cellulose from attack by cellulase [ 34 ].
Moreover, alkaline pretreatment causes swelling of the lignocellulosic biomass,
leading to the decrease of DP and crystallinity of cellulose and increase of the
surface area to facilitate the enzymatic hydrolysis of cellulose. The effectiveness of
alkaline pretreatment depends on the characteristics of lignocellulosic biomass
and reaction conditions. In general, alkaline pretreatment is more efficient with
herbaceous crops and agricultural residues with relatively low lignin content.