fermentation inhibitors at elevated temperatures and improper digestion of the lignin-carbohydrate matrix limit the
process. Further, hydrolysate washing can lead to a drop in
overall sugar yield. Also, the method is not competent
enough to treat the woody substrate. All of these factors
make the process less desirable (Kumar and Sharma 2017;
Ren et al. 2016). Datar et al. reported the hydrogen molar
yield of 2.84 and 3 for neutral (water) pretreated steam
explosion (at 220 °C for 3 min) and acid (1.2% sulfuric acid
(H 2 SO 4 ) for 120 min) pretreated steam explosion (at 190 °C
for 2 min), respectively (2007). Ratti et al. reported the use
of a steam explosion (15.3 kg cm
−2 at 200 °C for 7 min)
followed by alkaline treatment (1% at 121 °C for 30 min) of
sugarcane bagasse, obtaining a yield of 1.2 mol H 2 /g substrate (2015).
Liquid Hot Water Treatment (LHW)
LHW pretreatment is analogous to the steam explosion
pretreatment except that pressurized water is used here to
increase its boiling point (170–230 °C). After the pretreatment (hydrolysis of hemicellulose and removal of lignin),
the cellulose can be made available for further processing
(Kumar and Sharma 2017; Jiang et al. 2015). The advantage
of the method is that there is no fermentation inhibitors
formation occurs throughout the process (Ren et al. 2016).
However, the process may be energy-intensive in order to
compress the water to such high pressure. Ko et al. reported
the use of liquid hot water at 180–210 °C for pretreating
hardwood for 5–15 min obtaining an 80–90% lignin
recovery (Ko et al. 2015). The efficiency of this process is
generally increased by using a small quantity of chemical
reagents. Li et al. have reported 96.4% degradation of
hemicellulose from the corn stover when pretreated by a hot
water method accompanied by a small quantity of NaOH
(2014).
Ammonia Fiber Explosion Treatment (AFEX)
In this method, the biomass is exposed to liquid ammonia at
high pressure (250–300 psi) and a moderate temperature
range (60–100 °C) for a few minutes, followed by sudden
de-pressurization (Kumar and Sharma 2017). The sudden
release in the pressure leads to the ammonia vaporization
that can be further collected and reused for the next cycle
(Kumar and Sharma 2017). The effect of the pretreatment is
the bulging of lignocellulose and breakdown of cellulosic
crystals (Salakkam et al. 2019). The method requires no
additional steps and can be carried out at mild temperatures
and forms a lower quantity of inhibitors. Also, ammonia
recovered in the process can be easily recycled (Ren et al.
2016; Salakkam et al. 2019; Kumar and Sharma 2017).
However, the method involves a toxic compound, i.e.,
ammonia. Nevertheless, Cao et al. have reported a 67.8%
enhanced yield of hydrogen from wheat straw after AFEX
pretreatment (2013).
Sulfite Pretreatment to Overcome Recalcitrance of Lignocellulose (SPORL) Treatment
SPORL is an effective two-step procedure for the pretreatment of various lignocellulosic biomass (Xu et al. 2016).
The first step involves removing lignin and hemicellulose
fraction by treating the lignocellulosic biomass with calcium
or magnesium sulfite. In contrast, the second step involves
the use of a mechanical disk miller to get the desired fine
particle from pretreated biomass (Salakkam et al. 2019;
Kumar and Sharma 2017). The method is energy efficient
that offers ease of operations. Further, it maximizes the
cellulose to glucose conversion rate while having the
potential to process a range of biomass. However, the
method requires a high amount of water for post-treatment
(washing). Also, the chemical recovery cost is significantly
high, which ultimately makes the process expensive (Kumar
and Sharma 2017). Idrees et al. have studied the effect of
sodium sulfite and sodium sulfide, together with sodium
hydroxide, on the pretreatment of bagasse, rice husk, corncob, and water hyacinth (Idrees et al. 2013). The pretreatment was found to be very effective in the removal of
hemicellulose and lignin from lignocellulosic biomass.
2.4 Biological Pretreatments
Conventional methods, such as physical and chemical pretreatments, require a piece of specific equipment, costly
reagents, and high energy input. The biological method
could overcome these limitations since it is an
environment-friendly method that requires less energy
(Kumar and Sharma 2017). The biological pretreatment
comprises the use of microorganisms (soft rot, white, and
brown fungi), and enzymes (manganese peroxidase, cellulase, xylanase, lignin peroxidase, and laccase) to degrade the
lignocellulosic biomass (Yadav et al. 2019; Patil and
Thombre 1978). The type of biomass that has to be degraded
is highly influenced by the microorganisms employed in the
process. Soft and white-rot fungi mostly degrade hemicellulose and lignin, whereas the brown rot fungi degrade
cellulose (Salakkam et al. 2019; Patil and Yadav 2019).
Biological treatments have several advantages over conventional methods since it is an environment-friendly
method that requires less energy and can be carried out at
mild reaction conditions. However, the microbial pretreatments, especially with fungi, take a longer time to process
due to slower metabolism (Ren et al. 2016). For degradation
of various lignocellulosic biomass, several basidiomycetes
64
P. D. Patil et al.
process. Further, hydrolysate washing can lead to a drop in
overall sugar yield. Also, the method is not competent
enough to treat the woody substrate. All of these factors
make the process less desirable (Kumar and Sharma 2017;
Ren et al. 2016). Datar et al. reported the hydrogen molar
yield of 2.84 and 3 for neutral (water) pretreated steam
explosion (at 220 °C for 3 min) and acid (1.2% sulfuric acid
(H 2 SO 4 ) for 120 min) pretreated steam explosion (at 190 °C
for 2 min), respectively (2007). Ratti et al. reported the use
of a steam explosion (15.3 kg cm
−2 at 200 °C for 7 min)
followed by alkaline treatment (1% at 121 °C for 30 min) of
sugarcane bagasse, obtaining a yield of 1.2 mol H 2 /g substrate (2015).
Liquid Hot Water Treatment (LHW)
LHW pretreatment is analogous to the steam explosion
pretreatment except that pressurized water is used here to
increase its boiling point (170–230 °C). After the pretreatment (hydrolysis of hemicellulose and removal of lignin),
the cellulose can be made available for further processing
(Kumar and Sharma 2017; Jiang et al. 2015). The advantage
of the method is that there is no fermentation inhibitors
formation occurs throughout the process (Ren et al. 2016).
However, the process may be energy-intensive in order to
compress the water to such high pressure. Ko et al. reported
the use of liquid hot water at 180–210 °C for pretreating
hardwood for 5–15 min obtaining an 80–90% lignin
recovery (Ko et al. 2015). The efficiency of this process is
generally increased by using a small quantity of chemical
reagents. Li et al. have reported 96.4% degradation of
hemicellulose from the corn stover when pretreated by a hot
water method accompanied by a small quantity of NaOH
(2014).
Ammonia Fiber Explosion Treatment (AFEX)
In this method, the biomass is exposed to liquid ammonia at
high pressure (250–300 psi) and a moderate temperature
range (60–100 °C) for a few minutes, followed by sudden
de-pressurization (Kumar and Sharma 2017). The sudden
release in the pressure leads to the ammonia vaporization
that can be further collected and reused for the next cycle
(Kumar and Sharma 2017). The effect of the pretreatment is
the bulging of lignocellulose and breakdown of cellulosic
crystals (Salakkam et al. 2019). The method requires no
additional steps and can be carried out at mild temperatures
and forms a lower quantity of inhibitors. Also, ammonia
recovered in the process can be easily recycled (Ren et al.
2016; Salakkam et al. 2019; Kumar and Sharma 2017).
However, the method involves a toxic compound, i.e.,
ammonia. Nevertheless, Cao et al. have reported a 67.8%
enhanced yield of hydrogen from wheat straw after AFEX
pretreatment (2013).
Sulfite Pretreatment to Overcome Recalcitrance of Lignocellulose (SPORL) Treatment
SPORL is an effective two-step procedure for the pretreatment of various lignocellulosic biomass (Xu et al. 2016).
The first step involves removing lignin and hemicellulose
fraction by treating the lignocellulosic biomass with calcium
or magnesium sulfite. In contrast, the second step involves
the use of a mechanical disk miller to get the desired fine
particle from pretreated biomass (Salakkam et al. 2019;
Kumar and Sharma 2017). The method is energy efficient
that offers ease of operations. Further, it maximizes the
cellulose to glucose conversion rate while having the
potential to process a range of biomass. However, the
method requires a high amount of water for post-treatment
(washing). Also, the chemical recovery cost is significantly
high, which ultimately makes the process expensive (Kumar
and Sharma 2017). Idrees et al. have studied the effect of
sodium sulfite and sodium sulfide, together with sodium
hydroxide, on the pretreatment of bagasse, rice husk, corncob, and water hyacinth (Idrees et al. 2013). The pretreatment was found to be very effective in the removal of
hemicellulose and lignin from lignocellulosic biomass.
2.4 Biological Pretreatments
Conventional methods, such as physical and chemical pretreatments, require a piece of specific equipment, costly
reagents, and high energy input. The biological method
could overcome these limitations since it is an
environment-friendly method that requires less energy
(Kumar and Sharma 2017). The biological pretreatment
comprises the use of microorganisms (soft rot, white, and
brown fungi), and enzymes (manganese peroxidase, cellulase, xylanase, lignin peroxidase, and laccase) to degrade the
lignocellulosic biomass (Yadav et al. 2019; Patil and
Thombre 1978). The type of biomass that has to be degraded
is highly influenced by the microorganisms employed in the
process. Soft and white-rot fungi mostly degrade hemicellulose and lignin, whereas the brown rot fungi degrade
cellulose (Salakkam et al. 2019; Patil and Yadav 2019).
Biological treatments have several advantages over conventional methods since it is an environment-friendly
method that requires less energy and can be carried out at
mild reaction conditions. However, the microbial pretreatments, especially with fungi, take a longer time to process
due to slower metabolism (Ren et al. 2016). For degradation
of various lignocellulosic biomass, several basidiomycetes
64
P. D. Patil et al.