Page 52 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 31

the low cost and ease of recovery. High boiling point alcohols, usually
poly-hydroxy alcohols, such as ethylene glycol and glycerol, present an
advantage of performing under atmospheric pressure [67]. However, the
relatively high price of solvents and high energy consumption for solvent
recovery significantly decrease the attractiveness of using organic solvent
for biomass pretreatment [68]. The acid catalysts used include organic
(e.g., formic acid, acetic acid, and oxalic acid) or inorganic acids (e.g., sul-
furic acid and hydrochloric acid) [69]. Zhao and Liu [70] have developed
an organosolv pretreatment based on formic acid delignification followed
by alkaline deformylation, namely, the Formiline process. The final etha-
nol concentrations obtained by simultaneous saccharification and fermen-
tation (SSF) of the Formiline-pretreated sugarcane bagasse reached 55.4
and 80.1 g/L, respectively, at initial solid consistencies of 15% and 20%.
Goh et al. [71] optimized the H 2 SO 4 -catalyzed ethanol pretreatment of
empty palm fruit bunch, and the optimal condition was found to be etha-
nol/water ratio of 65:35 (v/v), sulfuric acid concentration of 2.0% (w/w),
at 160°C for 78 min.
By acid-catalyzed organosolv pretreatment, fractionation of cellulose,
hemicellulose, and lignin can be achieved. Cellulose is recovered as a
solid while most lignin and hemicellulose are dissolved in organic sol-
vent [72]. High-quality lignin, sulfur-free with high purity and low
molecular weight, can be isolated from the organic solvents recovery
process as a solid material, while the sugars as a syrup [73].The removal
of lignin and hemicellulose can substantially deconstruct the biomass
recalcitrance, therefore improving the enzymatic digestibility and fer-
mentability of cellulose [74]. Nevertheless, there are inherent drawbacks
for acid-catalyzed organosolv pretreatment. It is necessary to wash the
pretreated solids with organic solvent previous water washing in order
to avoid the reprecipitation of dissolved lignin. Although the organic
solvents can be easily recovered by distillation or evaporation for reuse,
the cost of chemicals and energy consumption for solvent recovery is
still higher than those of other leading pretreatment processes.
Moreover, organosolv pretreatment must be carried out under extremely
tight and efficient control because of the volatility of organic solvents,
and thus no digester leaks can be tolerated due to inherent fire, explo-
sion hazards, and environmental and health and safety concerns [68].
Organic acid pretreatment also causes acylation of cellulose which
may significantly reduce the cellulose digestibility by decreasing the
molecular recognition of cellulose by cellulases.