Environmental Engineering Reference
In-Depth Information
under controlled conditions. Initial moisture content, temperature, and aeration
rate of the biomass are the three crucial factors that directly affect the fungal
growth and its ability to produce lignin-degrading enzymes. These factors play an
important role in delignification of biomass. For example, it has been reported
that when the moisture content of the biomass is too high, it may promote fungal
mycelium formation and inhibit the delignification process [48]. In contrast, low
moisture content may retard fungal growth and lead to poor delignification. The
optimal temperature for delignification depends greatly on the selection of the
fungal strains, which ranges from 20 to 30°C [49]. Good aeration should also be
ensured throughout the process.
This approach is however time consuming. Normally, several weeks are
required for significant delignification and thus use of white-rot fungi for deligni-
fication in industries has been limited to date [50].
3.2.6
Summary
Pretreatment is a critical step to increase biomass conversion efficiency. In general,
pretreatment can be divided into four major groups: mechanical, chemical,
physicochemical, and biological methods. Each pretreatment method has its own
advantages and limitations depending on the type of biomass used. There is no single
method that can be regarded as the best option in biomass pretreatment. In general,
an efficient pretreatment method should be able to eliminate the unwanted biomass
fraction selectively in a cost-effective, environmentally friendly, and time-efficient
manner. The process conditions, major effects, advantages, and disadvantages of the
pretreatment methods reported in this chapter are summarized in Table 3.1.
3.3
Thermochemical Processing of Biomass
3.3.1
Direct Liquefaction
Direct liquefaction is a high-pressure thermal decomposition and hydrogenation
process which converts biomass into hydrocarbon oil as fuel in solvent under ele-
vated pressure (up to 250 bars) and temperature (up to 500°C). Through this pro-
cess, biomass is depolymerized into unstable monomers and then re-polymerized
to a broad range of hydrocarbon compounds under an oxygen-deficient or depleted
environment [51]. Water and carbon dioxide are formed to remove the oxygen in
the biomass. Basically, direct liquefaction has a high feed flexibility but only a
single type of feedstock can be processed in a single batch under the specific condi-
tion with a suitable catalyst in order to obtain consistent products and yields. Since
direct liquefaction is performed in solvent, no pre-drying is required. The process
can therefore be applied to wet biomass with up to 78% moisture content [52],
while other processing technologies such as gasification and combustion are only
suitable for biomass with low moisture content [53].
