204 Introduction to Renewable Biomaterials
at elevated temperatures (Mosieret al., 2005). Flow-through processes pass water
with high temperatures through lignocellulosic biomass. This type of pretreatment has
been named hydrothermolysis, aqueous or steam/aqueous fractionation, uncatalyzed
solvolysis, and aquasolv (Allenet al., 1996). The residence time for this process is usually
around 15 min at temperatures in the range between 200 and 230∘C. Approximately
40–60% of the total biomass is dissolved. In this process, 4–22% of the cellulose,
35–60% of the lignin, and all of the hemicelluloses are removed. There are three types
of liquid hot water reactor configurations, namely co-current, counter-current, and
flow-through reactor. In co-current pretreatment conditions, water and lignocellulosic
biomass move in the same direction. The biomass/water slurry is heated to a given
temperature and maintained at pretreatment for a controlled residence time before
being cooled. Counter-current pretreatment is designed to move water and lignin in
opposite directions through the pretreatment reactor. In a flow-through reactor, hot
water is passed over a stationary bed of lignocellulosic biomass. For liquid hot water
pretreatment, size reduction of the biomass is not needed because the lignocellulosic
biomass particles break apart when cooked in water (Weilet al., 1997).
Martinezet al. (1990) studied the pretreatment of few woods. The yield of sugar
production after 48 h enzymatic hydrolysis of steam-explosion-pretreated samples
at a temperature of 230∘C and residence time 1–2 min was higher than 90%. Others
researches also showed similar results. Nunes and Pourquie (1996), Moniruzzaman
(1996), and Daraei Garmakhanyet al. (2014a) reported the effect of steam-explosion
pretreatment on increasing the yield of saccharification of eucalyptus wood, rice straw,
and canola straw, respectively.
6.7.2 Hydrothermolysis
In this way, biomass is exposed to high-pressure hot water at high temperatures (about
200 ∘C) for 30 min. Hydrothermolysis has been used for a long term, but it is expensive
in large scale. Yuet al. (2010) investigated the effect of hot-compressed water (HCW)
pretreatment on rice straw as a cost-effective pretreatment before enzymatic hydrolysis
at a temperature ranging from 140 to 240∘C for 10 or 30 min. They evaluated the
different characteristics of HCW-pretreated rice straw including sugar and inhibitor
production as process yield. The maximum production of total glucose at 180∘Cwas
4.4–4.9% of glucan in raw material. Total maximum xylose production was obtained at
180 ∘C, accounting for 43.3% of xylan in raw material for 10 min pretreatment and 29.8%
for 30 min pretreatment. The production of acetic acid increased at higher temperatures
and longer pretreatment time that led to significant disruption of the lignocellulosic
biomass structure. Maximum furfural formation (2.8 mg ml−^1 ) was achieved at 200∘C
for both 10 and 30 min processing time. The glucose yield of enzymatic hydrolysis
of pretreated rice straw was not lower than 85% for 30 min pretreatment at 180∘C
were similar to 10 min pretreatment at 200∘C or higher temperature by considering
sugar recovery, inhibitors formation and process severity. It is recommended that
the temperature of 180∘Cfor30mincanbethemostefficientprocessforHCW
pretreatment of rice straw. Jakobsson pretreated the wheat straw with steam at different
temperatures (190, 200, and 210∘C) and residence times (2, 5, and 10 min). Sulfuric
acid was used as a process catalyst. The straw was impregnated with sulfuric acid
before pretreatment. For the evaluation of the pretreatment, enzymatic hydrolysis and
