3.3 Thermochemical Processing
Thermochemical processing is the decomposition of organic materials from biomass for conversion at elevated temperatures and pressures into fuels. It comprises:
pyrolysis, torrefaction, hydrothermal liquefaction, and gasification (Chen et al.
2015; Tan et al. 2014; Toledo-Cervantes and Morales 2014). Through these conversion technologies, solid, liquid, and gaseous biofuels are produced for heat and
power generation. Pyrolysis is the combustion taking place at high temperatures
(350–800 °C) in the absence of oxygen. It produces fuels with medium–low
calorific power (Brennan and Owende 2010), such as charcoal, gas, and biocrude.
Torrefaction is a mild pyrolysis at lower temperatures (200–300 °C) lasting minutes
to hours, whose main product is a solid biofuel. Biocrude is also produced by
hydrothermal liquefaction performed at 300–350 °C and pressures of 5–20 MPa to
convert wet microalgal biomass into liquid fuel without using hot compressed or
subcritical water (Chen et al. 2015). The refining of biocrude produces fuels and
lubricants, and some of the byproducts form materials, such as plastics, detergents,
solvents, elastomers, and fibers, such as nylon and polyesters, and asphalts
(Chailleux et al. 2012). On the other hand, the main product of gasification is
syngas (CO, CO 2 , H 2 ), which is obtained when dry microalgae react with an
oxidizer, such as air, oxygen, and water or steam, in a partial oxidation environment
at a temperature ranging between 800 and 1000 °C, within a pressure range of 1–
10 bar in an environment of insufficient oxidizer used for producing fuels and
chemical intermediates. Comprehensive reviews of recent progresses and development of thermochemical processing are found in the latest literature
(Toledo-Cervantes and Morales 2014; Chen et al. 2015; Chiaramonti et al. 2015).
3.3.1 Biochemical Processing
Biochemical conversion depends on the cell wall digestibility, which could be
enhanced by physical, chemical, or biological pre-treatment of either whole or
residual defatted biomass to reduce the processing time and increase the biomass.
Pre-treatments are classified into physical, chemical, or biological. They include
bead milling, ultrasound, alkaline, acidic or thermal hydrolysis, ionic liquid, pulsed
electric field, microwave or enzymatic pre-treatment, among others (Eldalatony
et al. 2016; Jankowska et al. 2017).
3.3.2 Anaerobic Digestion
Anaerobic digestion is the bacterial decomposition of organic biopolymers (i.e.,
carbohydrates, lipids, and proteins) into monomers in the absence of oxygen over a
temperature range of about 30–65 °C. These monomers are easier to convert into a
methane-rich gas via fermentation (typically 50–75% CH 4 ), and CO 2 is the second
5 Microalgae Biorefineries for Energy …
111
Thermochemical processing is the decomposition of organic materials from biomass for conversion at elevated temperatures and pressures into fuels. It comprises:
pyrolysis, torrefaction, hydrothermal liquefaction, and gasification (Chen et al.
2015; Tan et al. 2014; Toledo-Cervantes and Morales 2014). Through these conversion technologies, solid, liquid, and gaseous biofuels are produced for heat and
power generation. Pyrolysis is the combustion taking place at high temperatures
(350–800 °C) in the absence of oxygen. It produces fuels with medium–low
calorific power (Brennan and Owende 2010), such as charcoal, gas, and biocrude.
Torrefaction is a mild pyrolysis at lower temperatures (200–300 °C) lasting minutes
to hours, whose main product is a solid biofuel. Biocrude is also produced by
hydrothermal liquefaction performed at 300–350 °C and pressures of 5–20 MPa to
convert wet microalgal biomass into liquid fuel without using hot compressed or
subcritical water (Chen et al. 2015). The refining of biocrude produces fuels and
lubricants, and some of the byproducts form materials, such as plastics, detergents,
solvents, elastomers, and fibers, such as nylon and polyesters, and asphalts
(Chailleux et al. 2012). On the other hand, the main product of gasification is
syngas (CO, CO 2 , H 2 ), which is obtained when dry microalgae react with an
oxidizer, such as air, oxygen, and water or steam, in a partial oxidation environment
at a temperature ranging between 800 and 1000 °C, within a pressure range of 1–
10 bar in an environment of insufficient oxidizer used for producing fuels and
chemical intermediates. Comprehensive reviews of recent progresses and development of thermochemical processing are found in the latest literature
(Toledo-Cervantes and Morales 2014; Chen et al. 2015; Chiaramonti et al. 2015).
3.3.1 Biochemical Processing
Biochemical conversion depends on the cell wall digestibility, which could be
enhanced by physical, chemical, or biological pre-treatment of either whole or
residual defatted biomass to reduce the processing time and increase the biomass.
Pre-treatments are classified into physical, chemical, or biological. They include
bead milling, ultrasound, alkaline, acidic or thermal hydrolysis, ionic liquid, pulsed
electric field, microwave or enzymatic pre-treatment, among others (Eldalatony
et al. 2016; Jankowska et al. 2017).
3.3.2 Anaerobic Digestion
Anaerobic digestion is the bacterial decomposition of organic biopolymers (i.e.,
carbohydrates, lipids, and proteins) into monomers in the absence of oxygen over a
temperature range of about 30–65 °C. These monomers are easier to convert into a
methane-rich gas via fermentation (typically 50–75% CH 4 ), and CO 2 is the second
5 Microalgae Biorefineries for Energy …
111