lignin was eliminated by alkaline peroxide method, also the digestibility by enzymes
was also made better by this method as compared to the hydrothermal process used
alone. By using combined pretreatments the cost of the process is increased but it
helps in making the hydrolysis better. A better pretreatment method is the one that is
cost-effective and at the same time have the ability to make LC digestibility and their
usability better. Hemicelluloses are linked with lignin in LC biomasses and they
need to be removed in the initial step of combined pretreatment. The efficient
methods of removing hemicelluloses from the LC biomass are steam explosion,
hydrothermal, and alkaline pretreatment methods, removal of hemicelluloses must
be the first step. On the other hand, biological, oxidative, alkaline, and Organosolv
are better choices for removing lignin from the biomass. Oxidative and Organosolv
methods are costly than the alkaline method. Also, oxidative delignification helped
by alkaline methods is the best choice for making chemical pulp with lower lignin
percentage. Lignin is advantageously removed from LC material by biological
pretreatment but the efficiency of removal is lower so its use may be limited. Till
now the most effective and efficient lignin removal strategy is alkaline pretreatment.
Keeping in view this factor, hydrothermal, steam explosion, and dilute acid methods
combined with the alkaline method become the most advantageous ways to pretreat
LC biomasses. Lee et al. (2015) studied the role of alkaline (NaOH) and dilute
sulfuric acid methods combined to pretreat corn stover for improving its digestibility
by enzymes. Report showed that dilute sulfuric acid hydrolyzed xylan to
74.6–77.3%, and sodium hydroxide in the second step removed lignin up to
89.4%. This combined pretreatment increased the enzymatic digestibility of corn
stover. Enzymatic hydrolysis 97.9% and 75.9% glucose and xylose were obtained,
respectively. Sun et al. (2014b) used combination of NaOH and hydrothermal
pretreatment to pretreat fiber from Eucalyptus urophylla and also examined digestibility by enzymes. All the hemicellulose was converted to liquid in 30 min during
the initial step of pretreatment (temperature was higher than 180
). But lignin was
not converted at such high temperature and alkaline conditions. In this experiment,
enzymatic hydrolysis of Eucalyptus urophylla was increased because almost
50–60% lignin was dissolved into alkaline solution. Taking the energy consumption
and recovery of hemicelluloses and lignin into consideration, an optimum 66.3% of
cellulose was converted into glucose in the final enzymatic hydrolysis process. Sun
et al. (2014a) also investigated the feasibility of steam explosion combined with
alkali pretreatment. The material pretreated by this method had been seen to give a
lower percentage of xylan, i.e., 8.32–20.85, but the concentration of lignin was
reduced by the steam explosion method. In the alkaline pretreatment method, high
pressure resulted in the removal of lignin. Steam explosion when used alone results
in enzymatic hydrolysis by 7.9–33.1%; on the other hand, when alkaline
pretreatment was combined with steam explosion method, it resulted in
45.7–63.9%. When such a combined pretreatment method is used it results in
hydrolysate that can further be dehydrated to prepare furfural, this is due to the
high content of hemicellulose in the hydrolysate. Solutions of alkalis, having higher
ratios of lignin can be burned or concentrated for the provision of energy and
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FaizaKausar et al.
was also made better by this method as compared to the hydrothermal process used
alone. By using combined pretreatments the cost of the process is increased but it
helps in making the hydrolysis better. A better pretreatment method is the one that is
cost-effective and at the same time have the ability to make LC digestibility and their
usability better. Hemicelluloses are linked with lignin in LC biomasses and they
need to be removed in the initial step of combined pretreatment. The efficient
methods of removing hemicelluloses from the LC biomass are steam explosion,
hydrothermal, and alkaline pretreatment methods, removal of hemicelluloses must
be the first step. On the other hand, biological, oxidative, alkaline, and Organosolv
are better choices for removing lignin from the biomass. Oxidative and Organosolv
methods are costly than the alkaline method. Also, oxidative delignification helped
by alkaline methods is the best choice for making chemical pulp with lower lignin
percentage. Lignin is advantageously removed from LC material by biological
pretreatment but the efficiency of removal is lower so its use may be limited. Till
now the most effective and efficient lignin removal strategy is alkaline pretreatment.
Keeping in view this factor, hydrothermal, steam explosion, and dilute acid methods
combined with the alkaline method become the most advantageous ways to pretreat
LC biomasses. Lee et al. (2015) studied the role of alkaline (NaOH) and dilute
sulfuric acid methods combined to pretreat corn stover for improving its digestibility
by enzymes. Report showed that dilute sulfuric acid hydrolyzed xylan to
74.6–77.3%, and sodium hydroxide in the second step removed lignin up to
89.4%. This combined pretreatment increased the enzymatic digestibility of corn
stover. Enzymatic hydrolysis 97.9% and 75.9% glucose and xylose were obtained,
respectively. Sun et al. (2014b) used combination of NaOH and hydrothermal
pretreatment to pretreat fiber from Eucalyptus urophylla and also examined digestibility by enzymes. All the hemicellulose was converted to liquid in 30 min during
the initial step of pretreatment (temperature was higher than 180
). But lignin was
not converted at such high temperature and alkaline conditions. In this experiment,
enzymatic hydrolysis of Eucalyptus urophylla was increased because almost
50–60% lignin was dissolved into alkaline solution. Taking the energy consumption
and recovery of hemicelluloses and lignin into consideration, an optimum 66.3% of
cellulose was converted into glucose in the final enzymatic hydrolysis process. Sun
et al. (2014a) also investigated the feasibility of steam explosion combined with
alkali pretreatment. The material pretreated by this method had been seen to give a
lower percentage of xylan, i.e., 8.32–20.85, but the concentration of lignin was
reduced by the steam explosion method. In the alkaline pretreatment method, high
pressure resulted in the removal of lignin. Steam explosion when used alone results
in enzymatic hydrolysis by 7.9–33.1%; on the other hand, when alkaline
pretreatment was combined with steam explosion method, it resulted in
45.7–63.9%. When such a combined pretreatment method is used it results in
hydrolysate that can further be dehydrated to prepare furfural, this is due to the
high content of hemicellulose in the hydrolysate. Solutions of alkalis, having higher
ratios of lignin can be burned or concentrated for the provision of energy and
132
FaizaKausar et al.