approach aims in efficient utilization of residues or co-products for further
applications.
34 kg of co-products (glycerol, lipids, defatted biomass) are produced for every
24 kg of algal biodiesel generated [74]. Glycerol and lipids (unsaponifiable) can act
as precursors in the manufacturing of industrial chemicals like 1,3-propanediol
which are used normally in paint and polymer factories [75]. The DAB could be
utilized for production of biogas via anaerobic digestion [76] or as an animal feed
(carbon-neutral) [9].
Dry algal biomass could potentially be cracked at very high temperatures of
nearly 500
C under anaerobic environment for a period of 30–120 min through
process called pyrolysis [77]. It contains three major steps such as dehydration
(intracellular components vaporized at 80–190
C), volatilization and condensation
(volatile components are produced in the temperature ranges of 190–500
C and
condensed to form stable liquid or gases), and decomposition at temperatures equal
to or above 500
C to generate solid biochar [77, 78]. The bio-oil yield of 20–45%
having an approximate energy value of 35 kJ g
À1 was attained via pyrolysis.
Pyrolysis results in yield 20–45% of bio-oil with an average energy content of
35 kJ g
À1 . However, the major issue involved with the usage of pyrolysis biocrude is
due to their acidic nature (2.5 and 3.7); it causes storage concerns [79].
Hydrothermal liquefaction (HTL) is another alternative technique for transformation of wet algae biomass to energy dense heavy oil. One of the key benefits of
this technique is that it can potentially transform high-moisture wet biomass (moisture content above 75%) by heating at subcritical temperatures (200–350
C) and by
employing high pressure (5–20 MPa) to oil. HTL can be able to produce biocrude oil
comprising of energy-dense hydrocarbons of chain length C17-C18 and PAHs. The
resultant biocrude oil has heating value of about 30–40 kJ g
À1 with an approximate
yield of 30–50% of initial wet biomass. It also generates various by-products like
mixture of gases like CO 2 , H 2 , CH 4 , C 2 H 4 , C 2 H 6 , and N 2 which may be recycled for
production of energy [77]. The residual solids generated through HTL have C
content >20%, which shows promise to be used for energy conversion either by
anaerobic digestion or by biomass gasification. The yield of biocrude oil is significantly affected by the biomass composition in both HTL and pyrolysis process,
where higher lipid content results in higher yield of biocrude.
In addition to HTL and pyrolysis, direct biomass gasification offers a third route
for generating bioenergy based on thermal decomposition strategy from dry algal
biomass. In gasification technique, the biomass is partially oxidized at high
temperatures varying between 800 and 1,000
C. Gasification produces a mixture
of flammable gases like C 2 H 4, CH 4 , CO 2 , and H 2 [77]. Gasification is a highly
energy-intensive process with low product calorific value (only 4–6 MJ kg
À1 ). The
mixture of gases can be directly used for burning, in engines, or as syngas, and as
feedstock for chemical production like methanol [80]. However, this method is very
much energy-consuming with low economic profitability.
Algal Biomass for Biofuels and Bioproducts
155
applications.
34 kg of co-products (glycerol, lipids, defatted biomass) are produced for every
24 kg of algal biodiesel generated [74]. Glycerol and lipids (unsaponifiable) can act
as precursors in the manufacturing of industrial chemicals like 1,3-propanediol
which are used normally in paint and polymer factories [75]. The DAB could be
utilized for production of biogas via anaerobic digestion [76] or as an animal feed
(carbon-neutral) [9].
Dry algal biomass could potentially be cracked at very high temperatures of
nearly 500
C under anaerobic environment for a period of 30–120 min through
process called pyrolysis [77]. It contains three major steps such as dehydration
(intracellular components vaporized at 80–190
C), volatilization and condensation
(volatile components are produced in the temperature ranges of 190–500
C and
condensed to form stable liquid or gases), and decomposition at temperatures equal
to or above 500
C to generate solid biochar [77, 78]. The bio-oil yield of 20–45%
having an approximate energy value of 35 kJ g
À1 was attained via pyrolysis.
Pyrolysis results in yield 20–45% of bio-oil with an average energy content of
35 kJ g
À1 . However, the major issue involved with the usage of pyrolysis biocrude is
due to their acidic nature (2.5 and 3.7); it causes storage concerns [79].
Hydrothermal liquefaction (HTL) is another alternative technique for transformation of wet algae biomass to energy dense heavy oil. One of the key benefits of
this technique is that it can potentially transform high-moisture wet biomass (moisture content above 75%) by heating at subcritical temperatures (200–350
C) and by
employing high pressure (5–20 MPa) to oil. HTL can be able to produce biocrude oil
comprising of energy-dense hydrocarbons of chain length C17-C18 and PAHs. The
resultant biocrude oil has heating value of about 30–40 kJ g
À1 with an approximate
yield of 30–50% of initial wet biomass. It also generates various by-products like
mixture of gases like CO 2 , H 2 , CH 4 , C 2 H 4 , C 2 H 6 , and N 2 which may be recycled for
production of energy [77]. The residual solids generated through HTL have C
content >20%, which shows promise to be used for energy conversion either by
anaerobic digestion or by biomass gasification. The yield of biocrude oil is significantly affected by the biomass composition in both HTL and pyrolysis process,
where higher lipid content results in higher yield of biocrude.
In addition to HTL and pyrolysis, direct biomass gasification offers a third route
for generating bioenergy based on thermal decomposition strategy from dry algal
biomass. In gasification technique, the biomass is partially oxidized at high
temperatures varying between 800 and 1,000
C. Gasification produces a mixture
of flammable gases like C 2 H 4, CH 4 , CO 2 , and H 2 [77]. Gasification is a highly
energy-intensive process with low product calorific value (only 4–6 MJ kg
À1 ). The
mixture of gases can be directly used for burning, in engines, or as syngas, and as
feedstock for chemical production like methanol [80]. However, this method is very
much energy-consuming with low economic profitability.
Algal Biomass for Biofuels and Bioproducts
155