Page 49 - Lignocellulosic Biomass to Liquid Biofuels

28 Lignocellulosic Biomass to Liquid Biofuels

pretreatment, the formation of monomeric sugars could be changed by
controlling the pH of the aqueous phase [4]. Thus the toxic inhibitors,
such as furfural and HMF formatted by sugar degradation reactions, can
be limited by maintaining pH from 4 to 7 [47]. Another advantage of
LHW pretreatment is the low cost of reaction medium (only water), and
thus the need for washing or neutralization of pretreated biomass and the
inhibitor concentration in hydrolyzates are low. The biomass feedstock
usually does not need preprocessing for size reduction, and the material
for reactor construction is not expensive. After LHW pretreatment, the
hemicellulose recovery is relatively high. However, LHW also shows
some disadvantages including relatively high energy consumption, low
concentration of hemicellulosic sugar in the pretreatment hydrolyzate, and
a large amount of waste water in downstream processing [4].
Researches on LHW pretreatment of various biomass feedstocks can
be found in literatures. When LHW was used to pretreat microalgae for
bioethanol production with solid-to-liquid ratio of 1:13 (w/v) at 147°C
for 40 min, the glucose concentration and recovery were 14.2 g/L and
89.32%, respectively, which were up to fivefold higher than that without
LHW pretreatment [48]. LHW was also used for bamboo pretreatment to
enhance the enzymatic hydrolyzability [49]. Weinwurm et al. combined
LHW with ethanol organosolv (EO) pretreatment, and the maximum
delignification and carbohydrate removal were achieved when EO treat-
ment at 200°C with 20% ethanol was used after LHW treatment [50].
Efficient improvement of cellulose digestibility by LHW pretreatment was
also found for other biomass feedstocks, including wheat straw [51], giant
reed [52], sugarcane bagasse [53], and so on.

2.3.2 Acid-catalyzed chemical pretreatment
2.3.2.1 Dilute acid prehydrolysis
Dilute acid prehydrolysis was reported as early as the 19th century, with
commercial applications from the beginning of the 20th century. It has
been used for pretreatment of a wide range of biomass including herba-
ceous crops, agricultural residues, hardwood, and softwoods. The effective
concentration of acid for pretreatment is usually below 4%. It can be per-
formed at high temperature (e.g., 180°C) for a short period of time or at
low temperature (e.g., 120°C) for a longer retention time (30 90 min)
[54]. The main objective of the dilute acid pretreatment is to solubilize
the hemicellulose fraction thereby improving the enzymatic accessibility
of cellulose [6]. Sulfuric acid, hydrochloric acid, phosphoric acid, and