compounds (cellulose and hemicelluloses), to remove or modify lignin or
hemicelluloses, to diminish the cellulose crystallinity, to enlarge the lignocellulosic
biomass accessibility and porosity, to minimize the sugars loss, and to limit the
inhibitor formation (Kumar and Sharma 2017). The principal path to assess the
pretreatment effect on lignocellulosic materials is based on determination of mass
composition, x-ray crystallography, scanning electron microscope, highperformance liquid chromatography (HPLC), nuclear magnetic resonance (RMN),
and Fourier-transform infrared spectroscopy (FT-IR).
Among pretreatments, the steam explosion is a technique utilized on lignocellulosic biomass, which promotes chemical modifications in its composition and fracks
the cellular wall structure, guaranteeing more accessibility to cellulose and
hemicelluloses (Pielhop et al. 2016). Steam explosion is a practice where lignocellulosic biomass is submitted to conditions of temperature (160–240
C), pressure
(0.7–4.8 MPa) and superheated or saturated vapor (it can use water or prepared
solutions with acid or alkali) (Han et al. 2018; Silva et al. 2018). Hemicellulose
solubilization and lignin structure modifications are caused by a sudden decompression, which lets a higher accessibility to cellulose and higher digestion of lignocellulosic biomass (Kumar et al. 2018).
The method takes place in two steps, the first named autohydrolysis and the
second one known as decompression. At the first stage, the substance changes the
phase (from liquid to vapor) and gets into lignocellulosic structure, and it allows to
remove acetyl, carbonyl, and carboxyl groups present in hemicellulose chains and to
obtain acetic and uronic acid, which act in hemicellulose hydrolysis. In the second
stage, condensed steam goes from lignocellulosic biomass to outside, causing a
mechanical breaking (explosion) into lignocellulosic structure (Martino et al. 2017;
Asada et al. 2018; Wang et al. 2018). The steam explosion pretreatment can be
ameliorated by using acid catalysts, which help to reduce the operation time of the
whole process, because hemicellulose hydrolysis (the first stage) occurs faster than
without catalyst process (Silva et al. 2018).
The gains of using steam explosion as pretreatment are limited use of chemical
products (generally water or dilute solutions), low energy expense compared to
mechanical pretreatment, hemicelluloses that are partially hydrolyzed, and more
accessibility to cellulose and lignocellulosic biomass after process is more susceptible to enzymes action (Chen et al. 2015). The disadvantages of steam explosion as
pretreatment are obtaining by-products as furfural and hydroxymethylfurfural, partial solubilization of hemicelluloses, and the necessity to wash the lignocellulosic
biomass after process and not acting on lignin (Bhagwat et al. 2015).
On the other side, thermal pretreatment with dilute acid is another kind of process
that has been considered in the last years as a method to get unstructured lignocellulosic biomass. The substances generally employed are the hydrochloric and sulfuric
acid, which are prepared in low concentration solutions (1–3% w/v). The dilute acid
pretreatment is selected over the concentrated acid process, because it reduces the
inhibitor formation and minimizes the corrosion problems (Loow et al. 2016).
The temperature conditions of this pretreatment generally are between 90 and
140
C, and it is selected when it is wanted to act on hemicelluloses and low
2 Biofuels Generation Based on Technical Process and Biomass Quality
55
hemicelluloses, to diminish the cellulose crystallinity, to enlarge the lignocellulosic
biomass accessibility and porosity, to minimize the sugars loss, and to limit the
inhibitor formation (Kumar and Sharma 2017). The principal path to assess the
pretreatment effect on lignocellulosic materials is based on determination of mass
composition, x-ray crystallography, scanning electron microscope, highperformance liquid chromatography (HPLC), nuclear magnetic resonance (RMN),
and Fourier-transform infrared spectroscopy (FT-IR).
Among pretreatments, the steam explosion is a technique utilized on lignocellulosic biomass, which promotes chemical modifications in its composition and fracks
the cellular wall structure, guaranteeing more accessibility to cellulose and
hemicelluloses (Pielhop et al. 2016). Steam explosion is a practice where lignocellulosic biomass is submitted to conditions of temperature (160–240
C), pressure
(0.7–4.8 MPa) and superheated or saturated vapor (it can use water or prepared
solutions with acid or alkali) (Han et al. 2018; Silva et al. 2018). Hemicellulose
solubilization and lignin structure modifications are caused by a sudden decompression, which lets a higher accessibility to cellulose and higher digestion of lignocellulosic biomass (Kumar et al. 2018).
The method takes place in two steps, the first named autohydrolysis and the
second one known as decompression. At the first stage, the substance changes the
phase (from liquid to vapor) and gets into lignocellulosic structure, and it allows to
remove acetyl, carbonyl, and carboxyl groups present in hemicellulose chains and to
obtain acetic and uronic acid, which act in hemicellulose hydrolysis. In the second
stage, condensed steam goes from lignocellulosic biomass to outside, causing a
mechanical breaking (explosion) into lignocellulosic structure (Martino et al. 2017;
Asada et al. 2018; Wang et al. 2018). The steam explosion pretreatment can be
ameliorated by using acid catalysts, which help to reduce the operation time of the
whole process, because hemicellulose hydrolysis (the first stage) occurs faster than
without catalyst process (Silva et al. 2018).
The gains of using steam explosion as pretreatment are limited use of chemical
products (generally water or dilute solutions), low energy expense compared to
mechanical pretreatment, hemicelluloses that are partially hydrolyzed, and more
accessibility to cellulose and lignocellulosic biomass after process is more susceptible to enzymes action (Chen et al. 2015). The disadvantages of steam explosion as
pretreatment are obtaining by-products as furfural and hydroxymethylfurfural, partial solubilization of hemicelluloses, and the necessity to wash the lignocellulosic
biomass after process and not acting on lignin (Bhagwat et al. 2015).
On the other side, thermal pretreatment with dilute acid is another kind of process
that has been considered in the last years as a method to get unstructured lignocellulosic biomass. The substances generally employed are the hydrochloric and sulfuric
acid, which are prepared in low concentration solutions (1–3% w/v). The dilute acid
pretreatment is selected over the concentrated acid process, because it reduces the
inhibitor formation and minimizes the corrosion problems (Loow et al. 2016).
The temperature conditions of this pretreatment generally are between 90 and
140
C, and it is selected when it is wanted to act on hemicelluloses and low
2 Biofuels Generation Based on Technical Process and Biomass Quality
55