7.4 Chemical Pre-treatment Methods
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variety of biomass such as wheat straw, corn stover, sugarcane bagasse, cassava,
peanuts, rye, canola, faba beans and reed [26]. This method can be used for biomass
fractionation as it solubilizes hemicellulose and removes lignin. During the process,
lignin decomposes to carbon dioxide, water, and carboxylic acids. It can be combined
with other treatment methods such as alkaline pre-treatment and steam explosion (wet
explosion) for improving sugar production in enzymatic hydrolysis.
The main drawback of the oxidative pre-treatments is that the removal of lignin
is often accompanied by the loss of hemicellulose and cellulose. It also has high
energy demand and a high cost, thus not a viable pre-treatment method for large
scale applications [20].
7.4.4 Organosolv Fractionation Processes
In an organosolv fractionation process, lignocellulosic biomass is treated with organic
solvents or their aqueous solution to break the internal bonds of lignin and hemicellulose and hydrolyze the ether and ester interpolymer bonds between them, resulting in
lignin removal and almost complete solubilization of hemicellulose, leaving a relatively pure cellulose residue [6, 17, 18, 20]. A wide variety of organic solvents such
as methanol, ethanol, acetone, ethylene glycol, triethylene glycol, tetrahydrofurfuryl
alcohol, and acetic acid have been used for this treatment. The process is usually
performed in the presence of a catalyst to either reduce the treatment temperature or
to enhance the delignification rate [17, 18]. The catalysts are usually mineral acids
such as hydrochloric, sulfuric and phosphoric acid [6], bases such as lime, sodium
hydroxide and ammonia, or some salts. The optimum temperature of the treatment
depends on type of the biomass and solvent used, ranging from 80 to 200 °C [18, 20].
Higher temperatures, higher acid concentrations and longer retention times could
lead to the formation of inhibitors of fermentation [27]. Some other types of the
organosolv process such as the Battelle and formasolv have been also used in pretreatment of lignocellulosic biomass. In the Battelle method, the biomass is treated
with phenol, HCl and water at 100 °C and 1 atm. Acid causes the depolymerization
of lignin and hydrolyses the hemicellulose fraction. The lignin is then dissolved in
phenol and the monosaccharides are dissolved in the aqueous phase. The formasolv
method involves formic acid, HCl and water, in which the formic acid dissolves the
lignin fraction, carried out at low temperatures and pressures [27].
The organosolv process was developed in response to the drawbacks of the
traditional pulping processes. Compared to the traditional methods, the organosolv
process is more environmentally friendly (as it uses sulfur-free reagents and operates at mild conditions), has lower investment costs and has higher profitability on
small and medium scale processes. A typical organosolv process for the fractionation
pre-treatment of lignocellulosic biomass (crop residue), as illustrated in Fig. 7.6, is
briefly as follows: The biomass is mixed with a catalyst and solvent (recycled organic
solvent and/or fresh solvent), and fed to a cooking reactor, where the cellulose fibers
are isolated from lignin by solvolysis reactions to degrade lignin and allow the release
191
variety of biomass such as wheat straw, corn stover, sugarcane bagasse, cassava,
peanuts, rye, canola, faba beans and reed [26]. This method can be used for biomass
fractionation as it solubilizes hemicellulose and removes lignin. During the process,
lignin decomposes to carbon dioxide, water, and carboxylic acids. It can be combined
with other treatment methods such as alkaline pre-treatment and steam explosion (wet
explosion) for improving sugar production in enzymatic hydrolysis.
The main drawback of the oxidative pre-treatments is that the removal of lignin
is often accompanied by the loss of hemicellulose and cellulose. It also has high
energy demand and a high cost, thus not a viable pre-treatment method for large
scale applications [20].
7.4.4 Organosolv Fractionation Processes
In an organosolv fractionation process, lignocellulosic biomass is treated with organic
solvents or their aqueous solution to break the internal bonds of lignin and hemicellulose and hydrolyze the ether and ester interpolymer bonds between them, resulting in
lignin removal and almost complete solubilization of hemicellulose, leaving a relatively pure cellulose residue [6, 17, 18, 20]. A wide variety of organic solvents such
as methanol, ethanol, acetone, ethylene glycol, triethylene glycol, tetrahydrofurfuryl
alcohol, and acetic acid have been used for this treatment. The process is usually
performed in the presence of a catalyst to either reduce the treatment temperature or
to enhance the delignification rate [17, 18]. The catalysts are usually mineral acids
such as hydrochloric, sulfuric and phosphoric acid [6], bases such as lime, sodium
hydroxide and ammonia, or some salts. The optimum temperature of the treatment
depends on type of the biomass and solvent used, ranging from 80 to 200 °C [18, 20].
Higher temperatures, higher acid concentrations and longer retention times could
lead to the formation of inhibitors of fermentation [27]. Some other types of the
organosolv process such as the Battelle and formasolv have been also used in pretreatment of lignocellulosic biomass. In the Battelle method, the biomass is treated
with phenol, HCl and water at 100 °C and 1 atm. Acid causes the depolymerization
of lignin and hydrolyses the hemicellulose fraction. The lignin is then dissolved in
phenol and the monosaccharides are dissolved in the aqueous phase. The formasolv
method involves formic acid, HCl and water, in which the formic acid dissolves the
lignin fraction, carried out at low temperatures and pressures [27].
The organosolv process was developed in response to the drawbacks of the
traditional pulping processes. Compared to the traditional methods, the organosolv
process is more environmentally friendly (as it uses sulfur-free reagents and operates at mild conditions), has lower investment costs and has higher profitability on
small and medium scale processes. A typical organosolv process for the fractionation
pre-treatment of lignocellulosic biomass (crop residue), as illustrated in Fig. 7.6, is
briefly as follows: The biomass is mixed with a catalyst and solvent (recycled organic
solvent and/or fresh solvent), and fed to a cooking reactor, where the cellulose fibers
are isolated from lignin by solvolysis reactions to degrade lignin and allow the release