Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 34

Fig. 6 Schematic diagram of
the LHW process
reactors are preferred, and residence time and temperatures can be optimized for
different types of feedstocks. Compared to SE with high solid uploading, the solid
concentration in the slurry for the LHW process is much lower.
During the LHW pretreatment, the pH of the biomass can drop below 4, which
results in the formation of inhibitors due to the degradation of sugars under acidic
conditions [ 21 ]. Thus, a pH control strategy can be applied to the system to
maintain the pH value above 4, preferably between 5 and 7, by adding a base as
needed [ 22 , 23 ]. Since the alkali is not a catalyst as in alkaline pretreatment to be
addressed below, this process is termed as pH-controlled hot water pretreatment.
3.2 Chemical Pretreatment
High temperatures applied during the hydrothermal pretreatments under SE and
LHW conditions dehydrate sugars and produce inhibitors such as furfural from
xylose and hydroxymethyfurfural from glucose. To address this problem, acids
can be supplemented to facilitate the deconstruction of LCCs under less severe
conditions, either lower temperature or shorter reaction time. Among various
acids, sulfuric acid is most commonly used. Although the temperatures in con-
centrated acid pretreatment are much lower, acid recovery presents a big challenge
for the economic viability of the process. Therefore, dilute acid with concentra-
tions less than 2% is preferred, which can be conveniently neutralized by lime or
ammonium during the conditioning process [ 24 ]. Dilute acid pretreatments have
been intensively studied over the years with various feedstocks and reactors at
different scales [ 25 - 28 ]. Recently, the National Renewable Energy Laboratory
(NREL) updated its technical report on Process Design and Economics for
Biochemical Conversion of Lignocellulosic Biomass to Ethanol, in which the
dilute-acid pretreatment was described in details (Fig. 7 ).
Milled corn stover is conveyed into a screw feeder and dilute acid is injected at
the discharge point. The feedstock is then fed into a mixing and heating screw, and
further conveyed into the vertical presteamer. Hot water is added to bring the
effluent to 30% total solids. The presteamer is operated at 100C, with a retention
time of 10 min. The feedstock is then discharged through the screw feeder, and