Alternative Green Biofuel from Microalgae: A Promising Renewable Resource 257
commercial use. Flotation has advantages of flexible operation and needs a small footprint as compared
with sedimentation and coagulation-flocculation (Liu et al. 1999). With added cationic surfactants,
dissolved air flotation (DAF) effectively separated microalgae from water (Liu et al. 1999). Although
flotation has been reported as a potential harvesting method, there is very limited evidence of its technical
or economic feasibility. Chlorella vulgaris has been harvested using a dispersed ozone flotation process
(Ya et al. 2010), whereas Microcystis cyanobacteria have been eliminated by using an ozoflotation method
(Benoufella et al. 1994). Pure oxygen aeration failed to yield algal flotation, while ozone produced
flotation efficiently. The ozone dose needed to harvest microalgae ranged between 0.005 and 0.03 mg/mg
biomass, making the ozone flotation a promising option for algal harvesting. However, ozone is a strong
oxidant that normally oxidizes unsaturated compounds and aromatic rings into carbonyls and carboxylic
acids, respectively (Von 2003; Beltran 2004; Li et al. 2008). Production of biofuel from microalgae
should minimize the addition of chemicals for minimizing contamination.
Conversion to biofuels
There are several ways to convert biofuels from microalgae biomass including: (i) direct combustion (ii)
thermochemical (iii) and chemical reactions, as well as (iv) biochemical conversions. Direct combustion
corresponds to burning biomass in the presence of air at temperatures above 800ºC to produce heat and
electricity. These produced gases cannot be stored and must be used immediately. The conversion efficiency
of biomass to energy is more favourable than that of direct combustion of coal. The main disadvantage of
biomass is that it contains more water; thus, prior to combustion, biomass needs to be dried, and undergo
other pre-treatments that may affect the energy balance (Brennan and Owende 2010). Gasification also
generates a syngas (i.e., carbon dioxide, hydrogen gas, nitrogen, and methane) and bio-oil that can be
combusted directly at high temperature (800–1000ºC) by partial oxidation, producing heat, or used in
turbines for electricity generation (Brennan and Owende 2010; Mutanda et al. 2011). Thermochemical
liquefaction of biomass is used for conversion of wet biomass to bio-crude oil and into small molecules
in the presence of a catalyst at 300–350ºC and high pressure (50–200 bar) (Ross et al. 2010). Pyrolysis
is another alternative method of bio-oil production, as it is more cost effective for microalgal biomass as
feedstock than lignocellulosic biomass. This is due to the fact that microalgae contain higher amounts of
cellular lipids, carbohydrates, and proteins, which are more easily pyrolysed and result in higher quality
bio-oil production (Huang et al. 2010). Both gasification and pyrolysis require dried biomass as feedstock,
and the processes operate at a temperature higher than 800°C. Several methods reported on the pyrolysis
of Chlorella protothecoides and Oscillatoria tenuis (Wu et al. 1996), Arthrospira platensis and Chlorella
protothecoides (Peng et al. 2001), Microcystis aeruginosa (Miao et al. 2004; Miao and Wu 2006),
Chlorella muelleri and Synechococcus (Grierson et al. 2009), and Dunaliella tertiolecta (Shuping et al.
2010) have exhibited competitive potential for biofuel production from microalgal biomass (Xiaoling et
al. 2004). Pyrolysis of microalgal biomass has yielded promising results and shown to produce higher
quality bio-oil than lignocellulosic compounds (Brennan and Owende 2010). Nonetheless, there are still
some problems in the process of producing biofuels from microalgae by pyrolysis (Miao et al. 2004)
due to longer residence time, which can cause secondary cracking of the primary products, reducing
yield, and adversely affecting bio-oil properties. Other investigations have been carried out regarding
the suitability of microalgal biomass for bio-oil production (Miao and Wu 2004; Miao et al. 2004). It
was shown that microalgal bio-oils are of higher quality than bio-oil from wood (Demirbas 2006). All of
these are not attractive for commercial application of liquid fuel production like biodiesel and bioethanol.
Biodiesel
Biodiesel is considered a promising alternative to fossil fuel. It is a monoalkyl ester of long-chain fatty
acids derived from first generation renewable feedstocks, such as vegetable oil or animal fats (Meher et
al. 2006), and waste cooking oils (Issariyakul and Dalai 2014). Microalgae are also identified as potential
sources for biodiesel production, as they have the ability to synthesize triacylglycerols (TAG) and other
neutral lipids that can be converted to biodiesel. The rapid growth potential and numerous species
commercial use. Flotation has advantages of flexible operation and needs a small footprint as compared
with sedimentation and coagulation-flocculation (Liu et al. 1999). With added cationic surfactants,
dissolved air flotation (DAF) effectively separated microalgae from water (Liu et al. 1999). Although
flotation has been reported as a potential harvesting method, there is very limited evidence of its technical
or economic feasibility. Chlorella vulgaris has been harvested using a dispersed ozone flotation process
(Ya et al. 2010), whereas Microcystis cyanobacteria have been eliminated by using an ozoflotation method
(Benoufella et al. 1994). Pure oxygen aeration failed to yield algal flotation, while ozone produced
flotation efficiently. The ozone dose needed to harvest microalgae ranged between 0.005 and 0.03 mg/mg
biomass, making the ozone flotation a promising option for algal harvesting. However, ozone is a strong
oxidant that normally oxidizes unsaturated compounds and aromatic rings into carbonyls and carboxylic
acids, respectively (Von 2003; Beltran 2004; Li et al. 2008). Production of biofuel from microalgae
should minimize the addition of chemicals for minimizing contamination.
Conversion to biofuels
There are several ways to convert biofuels from microalgae biomass including: (i) direct combustion (ii)
thermochemical (iii) and chemical reactions, as well as (iv) biochemical conversions. Direct combustion
corresponds to burning biomass in the presence of air at temperatures above 800ºC to produce heat and
electricity. These produced gases cannot be stored and must be used immediately. The conversion efficiency
of biomass to energy is more favourable than that of direct combustion of coal. The main disadvantage of
biomass is that it contains more water; thus, prior to combustion, biomass needs to be dried, and undergo
other pre-treatments that may affect the energy balance (Brennan and Owende 2010). Gasification also
generates a syngas (i.e., carbon dioxide, hydrogen gas, nitrogen, and methane) and bio-oil that can be
combusted directly at high temperature (800–1000ºC) by partial oxidation, producing heat, or used in
turbines for electricity generation (Brennan and Owende 2010; Mutanda et al. 2011). Thermochemical
liquefaction of biomass is used for conversion of wet biomass to bio-crude oil and into small molecules
in the presence of a catalyst at 300–350ºC and high pressure (50–200 bar) (Ross et al. 2010). Pyrolysis
is another alternative method of bio-oil production, as it is more cost effective for microalgal biomass as
feedstock than lignocellulosic biomass. This is due to the fact that microalgae contain higher amounts of
cellular lipids, carbohydrates, and proteins, which are more easily pyrolysed and result in higher quality
bio-oil production (Huang et al. 2010). Both gasification and pyrolysis require dried biomass as feedstock,
and the processes operate at a temperature higher than 800°C. Several methods reported on the pyrolysis
of Chlorella protothecoides and Oscillatoria tenuis (Wu et al. 1996), Arthrospira platensis and Chlorella
protothecoides (Peng et al. 2001), Microcystis aeruginosa (Miao et al. 2004; Miao and Wu 2006),
Chlorella muelleri and Synechococcus (Grierson et al. 2009), and Dunaliella tertiolecta (Shuping et al.
2010) have exhibited competitive potential for biofuel production from microalgal biomass (Xiaoling et
al. 2004). Pyrolysis of microalgal biomass has yielded promising results and shown to produce higher
quality bio-oil than lignocellulosic compounds (Brennan and Owende 2010). Nonetheless, there are still
some problems in the process of producing biofuels from microalgae by pyrolysis (Miao et al. 2004)
due to longer residence time, which can cause secondary cracking of the primary products, reducing
yield, and adversely affecting bio-oil properties. Other investigations have been carried out regarding
the suitability of microalgal biomass for bio-oil production (Miao and Wu 2004; Miao et al. 2004). It
was shown that microalgal bio-oils are of higher quality than bio-oil from wood (Demirbas 2006). All of
these are not attractive for commercial application of liquid fuel production like biodiesel and bioethanol.
Biodiesel
Biodiesel is considered a promising alternative to fossil fuel. It is a monoalkyl ester of long-chain fatty
acids derived from first generation renewable feedstocks, such as vegetable oil or animal fats (Meher et
al. 2006), and waste cooking oils (Issariyakul and Dalai 2014). Microalgae are also identified as potential
sources for biodiesel production, as they have the ability to synthesize triacylglycerols (TAG) and other
neutral lipids that can be converted to biodiesel. The rapid growth potential and numerous species