Environmental Engineering Reference
In-Depth Information
These radicals are very reactive and can undergo secondary reactions such as
cracking and carbon deposition. They can however be stabilized by the addition of
hydrogen (quenching). There are a range of chemicals that can be produced from
bio-oil including food flavouring agents and phenols. These compounds are
obtained by extraction or by performing subsequent reactions. Many examples of
power generation from biomass liquids produced by fast pyrolysis processes have
been reported [48].
Several parameters affect the yield and the composition of the volatile material
generated during biomass pyrolysis, including the origin of the biomass, its chem-
ical and structural composition, the pyrolysis temperature, and the particle size, to
name but a few [49].
Overall, fast/flash pyrolysis is a promising technique for the production of liquid
fuels from biomass in high yields. Such materials could potentially serve as renew-
able alternatives to fossil fuel precursors for various chemicals and fuels [44].
7.4.4
Liquefaction
Liquefaction is a thermochemical conversion process in which liquid fuels are
generated from biomass. It is sometimes called the biomass to liquid (BTL) pro-
cess. It involves the treatment of biomass at low temperatures (300-350°C) and
high pressures (5-20 MPa) with a residence time of about 30 min, and is often
performed using a catalyst in the presence of hydrogen (in which case the pro-
cess is called catalytic liquefaction). However, non-catalytic aqueous liquefac-
tion of biomass is also possible; this process is known as direct liquefaction.
Under these conditions, water remains in the liquid state and has a range of
interesting properties, including low viscosity and high capacity for dissolving
inorganic compounds. In liquefaction processes, biomass is directly converted
into products without drying. This is important because drying is relatively
energy intensive [50]. The yields and composition of the resulting bio-oil and
char differ depending on whether the catalytic or direct liquefaction process is
used. Liquefaction and pyrolysis are often confused, but there are several differ-
ences between them. For instance, pyrolysis usually occurs at higher tempera-
tures and lower pressures than liquefaction and there is no need to dry the
biomass before liquefaction; on the other hand, pyrolysis often requires a pre-
drying step. There is less interest in liquefaction than pyrolysis because the latter
requires more expensive reactors and fuel feeding systems. Liquefaction gener-
ates a mixture of gas, liquid (bio-oil), and solid products in varying proportions
depending on the reaction conditions [51, 52].
In addition to hydrothermal liquefaction, biomass conversion can be performed
by means of supercritical water oxidation (SCWO) and supercritical water gasifi-
cation (SCWG). SCWO processes are primarily used to degrade industrial waste
at temperatures above the critical temperature of water (374°C). This causes the
oxidation of biomass to produce thermal energy and a gaseous stream of CO 2 .
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