7.4 Pretreatment of Lignocellulose Material
To overcome the resistance of crystalline cellulose to degradation, many treatment
processes have been developed. These processes may include exposition to
physical, chemical, and thermal treatments (Martínez et al. 2005). Even when all
treatments are effective to degrade cellulose in some grade, the use of those
pretreatments may represent a major ecological inconvenience and can increase
degradation price to make it economically unsustainable (Kumar et al. 2008).
7.4.1 Physical Pretreatments
Physical pretreatments are based mainly on reducing particle size of lignocellulosic material to increase interchange surface area and availability of the fibers to
enzyme degradation; this can be accomplished using different technology levels
from milling, micromilling, or ball milling to steam or ammonia explosion. Steam
explosion in one of the most commonly used pretreatment techniques; in this
treatment, material is subjected to high pressure and temperature for a short period
of time after which it is rapidly depressurized causing an explosion of the fibers of
cellulose. The main disadvantage of this treatment is the need to use diluted acids
to increase yields, which causes the accumulation of by-products that can inhibit
the culture of many organisms; factors as pressure, residence time, and temperature have been studied and recent studies indicate that longer residence time at
lower temperatures are better for lignocellulose degradation (McMillan 1994; Duff
and Murray 1996; Wright 1998; Bhatia et al. 2012). Another interesting treatment
is ammonia fiber explosion where biomass is treated with liquid ammonia at high
pressure and after a short residence time it is rapidly depressurized. The temperature used for this treatment is about 60 °C lower than the temperature of near to
160 °C necessary for steam explosion; this results in a much lower energy
expenditure for the process. This process has advantages such as lower moisture
content, less sugar degradation, and there is no loss of biomass. The main disadvantage is higher cost compared to other treatments (Mosier et al. 2005;
Chundawat et al. 2007).
7.4.2 Chemical Treatment
Chemical treatment can be divided into alkaline or acid treatments; in alkaline
treatment, basic salts as sodium, potassium, or ammonium hydroxide are used to
cause the degradation of glycosidic side chains, cellulose swelling, partial decrystallization of cellulose, and partial solvation of hemicelluloses, an advantage
of this treatment is alkaline treatment can be performed at lower temperatures and
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239
To overcome the resistance of crystalline cellulose to degradation, many treatment
processes have been developed. These processes may include exposition to
physical, chemical, and thermal treatments (Martínez et al. 2005). Even when all
treatments are effective to degrade cellulose in some grade, the use of those
pretreatments may represent a major ecological inconvenience and can increase
degradation price to make it economically unsustainable (Kumar et al. 2008).
7.4.1 Physical Pretreatments
Physical pretreatments are based mainly on reducing particle size of lignocellulosic material to increase interchange surface area and availability of the fibers to
enzyme degradation; this can be accomplished using different technology levels
from milling, micromilling, or ball milling to steam or ammonia explosion. Steam
explosion in one of the most commonly used pretreatment techniques; in this
treatment, material is subjected to high pressure and temperature for a short period
of time after which it is rapidly depressurized causing an explosion of the fibers of
cellulose. The main disadvantage of this treatment is the need to use diluted acids
to increase yields, which causes the accumulation of by-products that can inhibit
the culture of many organisms; factors as pressure, residence time, and temperature have been studied and recent studies indicate that longer residence time at
lower temperatures are better for lignocellulose degradation (McMillan 1994; Duff
and Murray 1996; Wright 1998; Bhatia et al. 2012). Another interesting treatment
is ammonia fiber explosion where biomass is treated with liquid ammonia at high
pressure and after a short residence time it is rapidly depressurized. The temperature used for this treatment is about 60 °C lower than the temperature of near to
160 °C necessary for steam explosion; this results in a much lower energy
expenditure for the process. This process has advantages such as lower moisture
content, less sugar degradation, and there is no loss of biomass. The main disadvantage is higher cost compared to other treatments (Mosier et al. 2005;
Chundawat et al. 2007).
7.4.2 Chemical Treatment
Chemical treatment can be divided into alkaline or acid treatments; in alkaline
treatment, basic salts as sodium, potassium, or ammonium hydroxide are used to
cause the degradation of glycosidic side chains, cellulose swelling, partial decrystallization of cellulose, and partial solvation of hemicelluloses, an advantage
of this treatment is alkaline treatment can be performed at lower temperatures and
7 Integral Management of Lignocellulosic Biomass by Biorefining
239