Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 32

3 Pretreatment
The self-assembly architecture of plant cell walls, with crystalline cellulose
microfibrils interacting and entangling with hemicelluloses and lignin, creates ligin
carbohydrate complexes (LCCs) [ 14 ], which are inaccessible for cellulases to bind
onto surfaces of cellulose molecules. Therefore, after a preliminary size reduction to
10-30 mm through mechanical methods such as chopping, pretreatment is needed to
deconstruct LCCs for efficient enzymatic hydrolysis of cellulose [ 15 ]. The smaller
the size, the more efficient the mass and heat transfer will be for subsequent
pretreatment and enzymatic hydrolysis. However, power requirement increases
significantly with reduction in size. Therefore, a compromise between size reduction
and energy consumption is needed from the economic point of view. Pretreatment
technologies can be classified in general into four categories: physical pretreatment,
chemical pretreatment, solvent fractionation and biological decomposition [ 16 ].
An ideal pretreatment process should maximize sugar yield from cellulose and
hemicelluloses, and in the meantime minimize energy consumption and environ-
mental impact. Unfortunately, none of them alone can satisfy all of these criteria.
3.1 Physical Pretreatment
Physical pretreatments do not use any chemicals. Size reduction by mechanical
methods such as grinding or milling is one of them, through which the surface area
of biomass is increased, and the degree of polymerization (DP) and crystallinity of
cellulose is decreased to some extent, but the power requirement for reducing the
feedstock from millimeter size to fine particles of micrometers is extremely high,
which is unacceptable from the engineering point of view. Radiation such as
microwaves that can penetrate and heat the feedstock instantly has also been
studied [ 17 ]. However, it is problematic to process the feedstock in large quan-
tities, not to mention the power requirement to generate the radiation. Therefore,
more attention regarding physical pretreatment has been focused on the hydro-
thermal processes of steam explosion (SE) and liquid hot water (LHW) treatment.
SE involves heating the feedstock at elevated temperature and pressure for a
short duration, followed by depressurizing the system to disrupt the structure of
LCCs. Due to lower capital investment, less impact on the environment, and
simple process design and operation, the SE process has been tested at pilot scales
worldwide. The mechanism underlying the pretreatment is assumed to be the
partial degradation of LCCs catalyzed by acetic acid released from acetylated
hemicelluloses and other organic acids such as formic and levulinic acids, making
the process autohydrolytic in nature [ 18 ]. The major parameters of the SE process
are temperature or pressure and holding time, which should be optimized based on
the characteristics of feedstocks. In general, a temperature from 160 to 260C
(corresponding pressure of 0.69-4.83 MPa) is applied, with a holding time of a few
minutes [ 15 ].