Environmental Engineering Reference
In-Depth Information
13.3.1 Pretreatment
In order to make lignocellulosic feedstocks suitable for fermentation processes, a
series of steps is required aiming at transforming the lignocellulosic feedstock (mainly
containing cellulose, hemicellulose, and lignin) into a solution of fermentable sugars
(i.e., pentoses and hexoses):
Breakage of the bonds between cellulose, hemicellulose, and lignin, so that their
structure is exposed to the hydrolysis catalysts
Hydrolysis of hemicellulose and cellulose, which results in soluble fermentable
pentose and hexose sugars
Removal of the lignin fraction, a nonfermentable fraction that otherwise
interferes with subsequent processing steps
Depending on the feedstock, an initial mechanical size reduction step (e.g., chip-
ping, grinding, milling) might be required in order to facilitate handling and to
increase the surface area for subsequent steps. Many pretreatment methods have been
evaluated and implemented on a processing scale. Table 13.7 provides a summary of
the most common methods and their functionality. In the following, these methods are
shortly described.
13.3.1.1 Acid Hydrolysis
This method can be divided in two general approaches,
namely, the use of diluted (or weak) acids at high temperature or the use of concen-
trated (strong) acids at low temperature. In the former, dilute (mostly sulfuric) acid
is sprayed onto the raw material, and the mixture is held at high temperature
(160
220
C) for up to a few minutes. Hydrolysis of the hemicellulose occurs,
releasing soluble oligomers and monomers. As an alternative to inorganic acids,
organic acids such as maleic acid and fumaric acid are being used.
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TABLE 13.7 Most common pretreatment methods and their functionality
Functionality
Breakage
of bonds
Hydrolysis of
hemicellulose
Hydrolysis
of cellulose
Hydrolysis/
removal of lignin
Method
Dilute/weak acid
hydrolysis
X
X
Acid hydrolysis
X
X
X
Alkaline hydrolysis
X
Organosolv
X
X
X
Wet oxidation
X
X
X
Ozonolysis
X
X
Steam explosion
X
X
Ammonia fiber explosion
X
X
X
Biological pretreatment
X
X
X
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