5.1.1.3 Physico-Chemical Pretreatment
• AFEX pretreatment: AFEX is a kind of pretreatment method in which volatile
NH 4 is used for treating cellulose by the expansion of ammonium fibers. This
method was developed by Bruce Dale to minimize the challenges to processing of
lignocelluloses and increase the rate of destruction of biomass into simpler sugars
that can be easily fermented. In contrast to other pretreatment methods that are
aqueous in nature, this one is dry in nature process, and results in no significant
rise of biomass composition, and no washing is needed leading to no significant
waste and expenditure (Chundawat et al. 2020).
This technique of pretreatment has been applied to different biomasses including:
perennial grass, switchgrass, corn stover, bagasse, wheat straw, and alfalfa.
Alizadeh et al. (2005) performed switchgrass pretreatment by AFEX using the
best possible conditions of pretreatment. This study showed that the pretreatment
with AFEX results in ethanol yield which is much higher than that obtained
without the treatment with AFEX. Uppugundla et al. (2014) have also reported
similar results by this technique. The limitation of this pretreatment strategy is
that it cannot be effective with the LC biomasses having very high lignin
contents, e.g., nutshells and wood (Kumar et al. 2009).
• Hydrothermal pretreatment: When one moves toward the use of thermal properties of LC materials, the stabilities of each component should be kept in mind as
there is a wide range of thermal stabilities for each biomass. Hemicellulose is
easily decomposed as compared to lignin and cellulose. So it is removed from the
process earlier to the disruption of cellulose through hydrothermal techniques.
This pretreatment strategy is beneficial as it makes the enzymatic digestibility of
LC biomasses better. When the temperature of hydrothermal pretreatment is
higher than 240
C, then cellulose is adversely disrupted, so the range of
temperature of this process should be in the limit of 160–240
C (Cao et al.
2014a; Sun et al. 2014b). Xiao et al. (2014) in their study observed the effects of
different temperature ranges (i.e., 140–200
C) on the composition changes of
bamboo chemical nature; they also examined the hydrolysis characteristics at the
varied conditions. By introducing the extreme temperature ranges, small amounts
of lignin and celluloses were denatured and most of hemicelluloses were eliminated from the process. Pretreatment at 200
C resulted in about 76% conversion
into glucose but only 16% conversion was seen in the non-treated biomass (Xiao
et al. 2014). The liquids remaining after hydrothermal pretreatment can be further
treated to convert them to some other chemicals. When beech wood was treated
by this pretreatment strategy its enzymatic digestibility increased from 7% by
weight to 70% by weight, also its surface area and pore size was increased (Nitsos
et al. 2013). This pretreatment strategy is inviting due to the conveniences
including: low cost, lesser inhibitors production, no need of catalysts, and the
most important one is that fresh or wet LC biomasses can be used.
As hydrothermal pretreatment does not needcatalysts usually, but such
methods are also developed that make use of catalysts for efficient elimination
5 Challenges in Bioethanol Production: Effect of Inhibitory Compounds
129
• AFEX pretreatment: AFEX is a kind of pretreatment method in which volatile
NH 4 is used for treating cellulose by the expansion of ammonium fibers. This
method was developed by Bruce Dale to minimize the challenges to processing of
lignocelluloses and increase the rate of destruction of biomass into simpler sugars
that can be easily fermented. In contrast to other pretreatment methods that are
aqueous in nature, this one is dry in nature process, and results in no significant
rise of biomass composition, and no washing is needed leading to no significant
waste and expenditure (Chundawat et al. 2020).
This technique of pretreatment has been applied to different biomasses including:
perennial grass, switchgrass, corn stover, bagasse, wheat straw, and alfalfa.
Alizadeh et al. (2005) performed switchgrass pretreatment by AFEX using the
best possible conditions of pretreatment. This study showed that the pretreatment
with AFEX results in ethanol yield which is much higher than that obtained
without the treatment with AFEX. Uppugundla et al. (2014) have also reported
similar results by this technique. The limitation of this pretreatment strategy is
that it cannot be effective with the LC biomasses having very high lignin
contents, e.g., nutshells and wood (Kumar et al. 2009).
• Hydrothermal pretreatment: When one moves toward the use of thermal properties of LC materials, the stabilities of each component should be kept in mind as
there is a wide range of thermal stabilities for each biomass. Hemicellulose is
easily decomposed as compared to lignin and cellulose. So it is removed from the
process earlier to the disruption of cellulose through hydrothermal techniques.
This pretreatment strategy is beneficial as it makes the enzymatic digestibility of
LC biomasses better. When the temperature of hydrothermal pretreatment is
higher than 240
C, then cellulose is adversely disrupted, so the range of
temperature of this process should be in the limit of 160–240
C (Cao et al.
2014a; Sun et al. 2014b). Xiao et al. (2014) in their study observed the effects of
different temperature ranges (i.e., 140–200
C) on the composition changes of
bamboo chemical nature; they also examined the hydrolysis characteristics at the
varied conditions. By introducing the extreme temperature ranges, small amounts
of lignin and celluloses were denatured and most of hemicelluloses were eliminated from the process. Pretreatment at 200
C resulted in about 76% conversion
into glucose but only 16% conversion was seen in the non-treated biomass (Xiao
et al. 2014). The liquids remaining after hydrothermal pretreatment can be further
treated to convert them to some other chemicals. When beech wood was treated
by this pretreatment strategy its enzymatic digestibility increased from 7% by
weight to 70% by weight, also its surface area and pore size was increased (Nitsos
et al. 2013). This pretreatment strategy is inviting due to the conveniences
including: low cost, lesser inhibitors production, no need of catalysts, and the
most important one is that fresh or wet LC biomasses can be used.
As hydrothermal pretreatment does not needcatalysts usually, but such
methods are also developed that make use of catalysts for efficient elimination
5 Challenges in Bioethanol Production: Effect of Inhibitory Compounds
129