7.4 Chemical Pre-treatment Methods
189
dissolved in the alkaline solution to form “black liquor”, which is concentrated to 65–
85% solids by using a direct-contact evaporator or by an indirect-contact evaporator.
Then the concentrated black liquor is burned in a recovery boiler to recover heat
and pulping chemicals. For instance, in the recovery boiler, the sodium sulfate is
converted to sodium sulfide and the heat is used to produce high pressure steam that
is used in the pulping process [28–30]. The produced pulps are treated by purification
and bleaching processes to remove the impurities and alter some of its components
by using chemical agents such as H 2 O 2 in one or several stages.
Kraft pulping and other alkali pre-treatments require less severe operating conditions (lower temperatures and pressures) compared to other pre-treatment methods.
They can be performed even at ambient operating conditions. However, longer retention times in the order of hours or days instead of minutes are expected. Compared
with the acid pre-treatments, the alkali pre-treatments result in less sugar degradation
[17, 27]. The main reaction during alkali pre-treatment is the saponification, leading
to the cleavage of the intermolecular ether linkages between hemicellulose and lignin.
As a result of this reaction, lignin and hemicellulose dissolve and are removed from
the biomass and hence the accessibility of enzymes to cellulose increases [18]. The
most commonly used alkaline reagents are sodium, potassium, calcium and ammonium hydroxides. Many studies showed that sodium hydroxide is the most effective
alkaline pre-treatment reagent [17, 18, 26, 27]. Pre-treatment with lime has also been
demonstrated as a less expensive reagent compared with other reagents, and it is also
possible to recover lime with CO 2 precipitation followed by solid–liquid separation
[17, 26].
The main advantages of alkaline pre-treatment are the inexpensive chemicals,
mild reaction conditions, effective removal of lignin and xylan, and the possibility
of biomass fractionation. However, this method requires a long processing time,
expensive process for recovery of added alkali, and formation of inhibitors such as
salts, phenolic acids, furfurals and aldehydes in the process. Another disadvantage
of the Kraft pulping process is the Kraft lignin (if precipitated from “black liquor”)
contains 1–3 wt% of sulfur covalently bound to the Kraft lignin in the form of thiols,
which make it unsuitable for some value-added applications such as biofuels [30].
It should be noted that the Kraft pulping and alkaline pre-treatment method is more
favorable for low lignin content biomass such as herbaceous crops and agricultural
residues but less effective for hardwoods [18, 20, 26, 27]. Examples of the Kraft
pulping process for crop residues and woody biomass are summarized in Table 7.2.
7.4.3 Oxidative Pre-treatment
The oxidative pre-treatment is based on the use of oxidizing agents such as ozone,
hydrogen peroxide, oxygen or air. The main goal of the process is to remove lignin and
increase the accessibility of cellulose. The efficiency of the oxidative pre-treatment
depends on the moisture content of biomass, reaction time and concentration of the
oxidative agent [20].
189
dissolved in the alkaline solution to form “black liquor”, which is concentrated to 65–
85% solids by using a direct-contact evaporator or by an indirect-contact evaporator.
Then the concentrated black liquor is burned in a recovery boiler to recover heat
and pulping chemicals. For instance, in the recovery boiler, the sodium sulfate is
converted to sodium sulfide and the heat is used to produce high pressure steam that
is used in the pulping process [28–30]. The produced pulps are treated by purification
and bleaching processes to remove the impurities and alter some of its components
by using chemical agents such as H 2 O 2 in one or several stages.
Kraft pulping and other alkali pre-treatments require less severe operating conditions (lower temperatures and pressures) compared to other pre-treatment methods.
They can be performed even at ambient operating conditions. However, longer retention times in the order of hours or days instead of minutes are expected. Compared
with the acid pre-treatments, the alkali pre-treatments result in less sugar degradation
[17, 27]. The main reaction during alkali pre-treatment is the saponification, leading
to the cleavage of the intermolecular ether linkages between hemicellulose and lignin.
As a result of this reaction, lignin and hemicellulose dissolve and are removed from
the biomass and hence the accessibility of enzymes to cellulose increases [18]. The
most commonly used alkaline reagents are sodium, potassium, calcium and ammonium hydroxides. Many studies showed that sodium hydroxide is the most effective
alkaline pre-treatment reagent [17, 18, 26, 27]. Pre-treatment with lime has also been
demonstrated as a less expensive reagent compared with other reagents, and it is also
possible to recover lime with CO 2 precipitation followed by solid–liquid separation
[17, 26].
The main advantages of alkaline pre-treatment are the inexpensive chemicals,
mild reaction conditions, effective removal of lignin and xylan, and the possibility
of biomass fractionation. However, this method requires a long processing time,
expensive process for recovery of added alkali, and formation of inhibitors such as
salts, phenolic acids, furfurals and aldehydes in the process. Another disadvantage
of the Kraft pulping process is the Kraft lignin (if precipitated from “black liquor”)
contains 1–3 wt% of sulfur covalently bound to the Kraft lignin in the form of thiols,
which make it unsuitable for some value-added applications such as biofuels [30].
It should be noted that the Kraft pulping and alkaline pre-treatment method is more
favorable for low lignin content biomass such as herbaceous crops and agricultural
residues but less effective for hardwoods [18, 20, 26, 27]. Examples of the Kraft
pulping process for crop residues and woody biomass are summarized in Table 7.2.
7.4.3 Oxidative Pre-treatment
The oxidative pre-treatment is based on the use of oxidizing agents such as ozone,
hydrogen peroxide, oxygen or air. The main goal of the process is to remove lignin and
increase the accessibility of cellulose. The efficiency of the oxidative pre-treatment
depends on the moisture content of biomass, reaction time and concentration of the
oxidative agent [20].