photoautotrophic algae: raceway pond systems and photobioreactors (PBRs). A
typical raceway pond comprises a closed loop oval channel, w0.25e0.4 m deep,
opens to the air, and mixed with a paddle wheel to circulate the water and prevent
sedimentation (Ponds are kept shallow as optical absorption and self-shading by the
algal cells limit light penetration through the algal broth). In PBRs the culture
medium is enclosed in a transparent array of tubes or plates and the microalgal
broth is circulated from a central reservoir. PBR systems allow for better control of
the algae culture environment but tend to be more expensive than raceway ponds.
Auxiliary energy demand may also be higher. The perceived potential of microalgae
as a source of environmentally sustainable transport fuel is a strong driver behind
their development and provides the underpinning rationale for much of the public
support directed towards microalgae R&D. It is important, therefore, that algae
biofuel systems are able to clearly demonstrate their environmental and
longer-term economic credentials. Researcher explained three aspects of microalgae
production that will ultimately determine the future economic viability and environmental sustainability: the energy and carbon balance, environmental impacts, and
production cost (Hannon et al. 2010a, b).
1.2
Algae: Promising Future Feedstock for Biofuels
Microalgae are currently being promoted as an ideal third generation biofuel feedstock because of their rapid growth rate, greenhouse gas fixation ability (net zero
emission balance), and high production capacity of lipids (fat). They also do not
compete with food or feed crops, and can be grown on non-arable land and saline
water. Biofuels are generally referred to solid, liquid, or gaseous fuels derived from
organic matter (Chisti 2007). The classification of biofuels is shown in Fig. 1.1.
The oil contents of various microalgae in relation to their dry weight are shown in
Table 1.1. It is clear that several species of microalgae can have oil contents up to
80% of their dry body weight. Some microalgae can double their biomasses within
24 h and the shortest doubling time during their growth is around 3.5 h which makes
microalgae an ideal renewable source for biofuel production.
Oil productivity, the mass of oil produced per unit volume of the microalgae broth
per day, depends on the algal growth rate and the oil content of the biomass. The
yield of the oil produced by algae is significantly higher (100,000 L/ha) in comparison to other crops, for example, soybean (446 L/ha), sunflower (952 L/ha), rapeseed
(1200 L/ha), castor (1413 L/ha), coconut (2689 L/ha), and palm (5950 L/ha). There
exist three distinct algae production mechanisms including photoautotrophic, heterotrophic, and mixotrophic production. Currently, photoautotrophic production is
the only method which is technically and economically feasible for large-scale
production of algae biomass for non-energy production. Two developed systems
are based on open pond and closed pond photobioreactor technologies. High algae
production rates are achievable with open pond systems. The recovery of microalgal
biomass which generally requires one or more solid–liquid separation steps is a
challenging phase of the algal biomass process, and accounts for 20–30% of the total
1 Algal Biofuel: A Sustainable Approach for Fuel of Future Generation
5
typical raceway pond comprises a closed loop oval channel, w0.25e0.4 m deep,
opens to the air, and mixed with a paddle wheel to circulate the water and prevent
sedimentation (Ponds are kept shallow as optical absorption and self-shading by the
algal cells limit light penetration through the algal broth). In PBRs the culture
medium is enclosed in a transparent array of tubes or plates and the microalgal
broth is circulated from a central reservoir. PBR systems allow for better control of
the algae culture environment but tend to be more expensive than raceway ponds.
Auxiliary energy demand may also be higher. The perceived potential of microalgae
as a source of environmentally sustainable transport fuel is a strong driver behind
their development and provides the underpinning rationale for much of the public
support directed towards microalgae R&D. It is important, therefore, that algae
biofuel systems are able to clearly demonstrate their environmental and
longer-term economic credentials. Researcher explained three aspects of microalgae
production that will ultimately determine the future economic viability and environmental sustainability: the energy and carbon balance, environmental impacts, and
production cost (Hannon et al. 2010a, b).
1.2
Algae: Promising Future Feedstock for Biofuels
Microalgae are currently being promoted as an ideal third generation biofuel feedstock because of their rapid growth rate, greenhouse gas fixation ability (net zero
emission balance), and high production capacity of lipids (fat). They also do not
compete with food or feed crops, and can be grown on non-arable land and saline
water. Biofuels are generally referred to solid, liquid, or gaseous fuels derived from
organic matter (Chisti 2007). The classification of biofuels is shown in Fig. 1.1.
The oil contents of various microalgae in relation to their dry weight are shown in
Table 1.1. It is clear that several species of microalgae can have oil contents up to
80% of their dry body weight. Some microalgae can double their biomasses within
24 h and the shortest doubling time during their growth is around 3.5 h which makes
microalgae an ideal renewable source for biofuel production.
Oil productivity, the mass of oil produced per unit volume of the microalgae broth
per day, depends on the algal growth rate and the oil content of the biomass. The
yield of the oil produced by algae is significantly higher (100,000 L/ha) in comparison to other crops, for example, soybean (446 L/ha), sunflower (952 L/ha), rapeseed
(1200 L/ha), castor (1413 L/ha), coconut (2689 L/ha), and palm (5950 L/ha). There
exist three distinct algae production mechanisms including photoautotrophic, heterotrophic, and mixotrophic production. Currently, photoautotrophic production is
the only method which is technically and economically feasible for large-scale
production of algae biomass for non-energy production. Two developed systems
are based on open pond and closed pond photobioreactor technologies. High algae
production rates are achievable with open pond systems. The recovery of microalgal
biomass which generally requires one or more solid–liquid separation steps is a
challenging phase of the algal biomass process, and accounts for 20–30% of the total
1 Algal Biofuel: A Sustainable Approach for Fuel of Future Generation
5