Page 47 - Lignocellulosic Biomass to Liquid Biofuels

26 Lignocellulosic Biomass to Liquid Biofuels

High-strength external electric field leads to a rapid electric potential
across the cell membrane with a rapid electrical breakdown of plant cell
wall [37]. The loss of membrane barrier functions is the primary impact of
PEF [38]. The advantages of PEF are of low energy and normal operation
temperatures and pressure [4]. PEF has been used in pretreating rapeseed
green biomass (stems) to enhance pressing and extractives recovery. For
the effects of pressure, when electric field strength and pulse number were
employed for juice expression, the juice yield increased from 34% to 81%
[38]. Besides, PEF can be also applied in nonthermal inactivation of
microbial cells, electroporation of cell membranes, softening tissues and
peeling, and extraction of thermal labile biocompounds [37]. However,
more investigation on the pretreatment of lignocellulosic biomass for
cellulose hydrolysis has to be further performed.

2.2.2 Energy and cost consideration on physical
pretreatment

Relatively high energy consumption is the main limitation of physical
pretreatment by grinding or milling. Compared with herbaceous biomass,
pretreatment of woody biomass consumes more energy [39]. It has been
reported that the energy consumption for milling wood chips into fibers
is about 500 800 W h/kg [39,40]. As found by Zhu et al., for ethanol
production from wood with a yield of 300 L/t dry wood, the conversion
efficiency from thermal energy (stored in ethanol) to electric-mechanical
energy is only about 30%, and the thermal energy in ethanol produced is
just sufficient for wood-size reduction [39]. The milling process is also
known as a pretreatment process with low efficiency [41]. In most cases, a
combination of chemical or other physical pretreatments in order to
improve the milling efficiency and overcome the energy barrier is needed.
Using a low liquid-to-solid ratio (L/W) is an advantageous way to reduce
thermal energy consumption in physical pretreatment process [42].
Postchemical pretreatment after size-reduction is another method, and
there are several advantages for this combination. For example, it can
avoid energy-intensive operation of mixing high-consistency pulp with
chemicals and improve the permeation of chemicals in the biomass parti-
cle thus increasing pretreatment efficiency [42].
For extrusion pretreatment the energy cost mainly comes from elec-
tricity consumption for providing thermal energy to reach and keep the
operation temperature, and mechanical energy of the motor to rotate the
screws for extrusion machines [32]. Therefore choosing appropriate