Microalgae can be used to produce a number of valuable products (Belarbi et al., 2000; Del Campo et al., 2000; Li et al., 2001; Banerjee et al., 2002), animal food (Knauer and Southgate, 1999), human health food (Becker, 2007) and as a wastewater treatment (Travieso et al., 2002; Kebede-Westhead et al., 2006). In addition to these options, microalgae have been proposed as systems for the sequestration of CO2
Table 7.4. Applications of bio-oil. (Adapted from Brammer et al., 2006.)
Application Product
Boiler Heat
Duel fuel IC diesel engine Electricity
Duel fuel IC diesel engine Combined heat and power (CHP) Gas turbine Electricity
Gas turbine Combined heat and power (CHP) Gas turbine combined cycle Electricity
Gas turbine combined cycle Combined heat and power (CHP) Boiler, Rankine cycle Combined heat and power Diesel engine Emulsion use for transport
(Sawayamaet al., 1995; Zeiler et al., 1995; de Morais and Costa, 2007a,b) and the production of biofuels (Chisti, 2007). The biofuels include biogas (CH4) by anaerobic digestion of the biomass (Spolaore et al., 2006), biodiesel from microalgal oils (Nagle and Lemke, 1990; Sawayama et al., 1995; Minowa et al., 1995; Miao and Wu, 2006;
Xuet al., 2006; Chisti, 2007), hydrogen (Fedorov et al., 2005) and the direct use of algae in emulsion fuels (Scragg et al., 2003).
Biodiesel is one of the sustainable fuels which can replace diesel as a transport fuel and is usually made by the transesterification of plant-derived oils, waste cooking oils and animal fats. However, microalgae should be considered as another source of biodiesel because of the following:
They have higher photosynthetic efficiency than terrestrial plants.
●
They have rapid growth rate, with doubling times of 8–24 h.
●
They have high lipid content of 20–70%.
●
They facilitate direct capture of CO
● 2, 100 t algae fix ∼183 t CO2.
They can be grown on a large scale.
●
They will not compete with terrestrial plants in food production.
●
They produce valuable products.
●
They include freshwater and marine species.
●
They have a much better yield of oil per hectare, oil palm 5000 t/ha, algae 58,700 t/
●
ha (Table 7.5; Chisti, 2007).
The use of microalgal oil to produce biodiesel is very much in the developmental stage, and so it should be regarded as a third-generation biofuel.
To use microalgae for the production of biodiesel a number of processes must be carried out and these are outlined in Fig. 7.7 and consist of strain selection, large- scale cultivation, harvesting, extraction of the oil, production of biodiesel from the oil, and the economics of the process.
Strain selection
Not all microalgae accumulate high concentration of oil, but there are a number of freshwater and marine species that do. Some examples of the oil levels accumulated
Table 7.5. Comparison of an open raceway and closed photobioreactor.
(Adapted from Chisti, 2007.)
Parameter Raceway Photobioreactor
Biomass production per year (kg) 100,000 100,000 Volumetric productivity (kg/m3/day) 0.117 1.535 Areal productivity (kg/m2/day) 0.035 0.072 Biomass concentration (kg/m3) 0.14 4.00
Dilution rate (day−1) 0.25 0.384
Area needed (m2) 7,828 5,681
Oil yield (m3/ha)a 42.6 (37.5)b 58.7 (51.6) Carbon dioxide consumption per year (kg) 183,333 183,333
aBased on 30% oil in biomass.
b( ) values in tonnes per hectare; compare this with rapeseed at ∼1 t/ha.
are given in Chisti (2007) and Scragg (2005). However, not all microalgal oils are suitable for biodiesel production, as some contain high levels of unsaturated fatty acids which reduce the oxidative stability of the biodiesel. In many cases, high oil accumulation is only found under some form of stress such as nitrogen limitation (Illmanet al., 2002), and so growth may have to be in two stages in order to obtain high levels of oil. In contrast, heterotrophic growth in glucose stimulated oil accu- mulation in Chlorella protothecoides (Miao and Wu, 2006; Xu et al., 2006). Strain selection will also be important depending on the type of cultivation system used.
Large-scale cultivation
There is a considerable body of information on the large-scale cultivation of micro- algae in bioreactors of various designs (Molina Grima et al., 2001; Scragg et al., 2002; Acien Fernandez et al., 2003; Chisti, 2007; de Morais and Costa, 2007a,b).
The designs of photobioreactors can be divided into two types – open and closed – and the advantages and disadvantages of these types are outlined below:
Harvest (filter, centrifugation flocculation, settling)
Light
Cells
Oil
Biodiesel Glycerol
Cell debris
Biogas Combustion
(CHP)
Anaerobic digestion
Solvent extraction or cell breakage
Protein
Exhaust carbon dioxide Wastes
Lagoon or biocoil
Fig. 7.7. Various ways of utilizing microalgae for the production of energy including biodiesel.
Open bioreactors:
Natural water, raceway ponds, inclined surfaces. These can suffer from: water and
●
CO2 loss, contamination and pollution, requirement of large area, limitation on the number of species that can be grown, no pro cess control, dependency on weather, poor mixing and low biomass (0.1–0.2 g/l).
Closed bioreactors:
Stirred vessels, tubular bioreactor, laminar bioreactor, plastic bag vessels. Best for
●
high-value products, process control, continuous culture possible, all types of algae grown, flexible production, not affected by weather, high biomass (2–8 g/l).
It would appear that the two best bioreactor designs are the raceway and tubular designs. The raceway is considerably simpler but mixing is limited, temperatures vary and the biomass concentration is low. The tubular bioreactors are enclosed and with good mixing and circulation a high biomass concentration can be achieved without contamination. They are, however, more expensive to operate and require cooling during daylight. A wide range of bioreactor designs have been used to culture micro- algae and some examples of the various designs are given in Table 7.6.
Table 7.6. Alternative photobioreactor designs.
Bioreactor design Algal species Reference Closed
Vertical tube Chlorella sp. de Morais and Costa (2007b) Scendesmus obliquus
Spirulina sp.
Parabola, pipe, diamond Chaetoceros calcitrans Satoet al. (2006) Tubular horizontal Spirulina platensis Richmondet al. (1993);
Acien Fernandez et al.
(2003); Molina Grima et al. (2001)
Horizontal tubular Phaeodactylum tricornutum Mironet al. (1999) L-shaped Euglena gracilis Chaeet al. (2006) Internally illuminated stirred tank Chlorella pyrenoidosa Ogbonnaet al. (1996) 5 l stirred tank bioreactor Isochrysis galbana Molina Grima et al. (1993) Plexiglas annular Nannochloropsis sp. Zittelli et al. (2003) Tubular/flat Spirulina sp. Tredici and Zittelli (1998) Dual sparging column Rhodamonas sp. Eriksen et al. (1998) Cone-shaped helical tubular S. platensis Watanabe and Hall (1996) Helical tubular Chlorella sp. Scragg et al. (2003) Vertical flat plate Synechocystis aquatilis Zhanget al. (2002)
Light supplied by optical fibres Porphyridium purpureum Fleck-Schnieder et al. (2007) Stirred draft tube
Open
Inclined Chlorella sp. Doucha and Livansky (2006) Open tanks 220 l Chaetoceros sp. Csordas and Wang (2004) 0.7 m deep polyethylene tanks Chaetoceros sp. Elias et al. (2003)
Raceways Spirulina sp. Olquin et al. (1997)
Whatever design is used, microalgal growth in bioreactors is influenced by four parameters: the supply of light, the supply of CO2, mixing (turbulence) and the build-up of oxygen (Grobbelaar, 1994) (Fig. 7.8). The maximum amount of light is about 30% saturation, and values above this can cause photobleaching, the loss of chlorophyll. The CO2 levels of 0.03% are below the optimum for growth, concentra- tion of 0.1% is more suitable, and there have been cases where 10% CO2 did not inhibit growth. In the light, microalgae can produce oxygen rapidly and a build-up of oxygen can inhibit growth. Mixing in the form of turbulence is essential to keep the cells in suspension and for gaseous exchange.
Commercial production of microalgae has been used to produce pigments, food supplements and shellfish food. The designs that have been used to produce micro- algae commercially are given in Table 7.7.
Harvesting
The two critical stages in product development are the harvesting of the microorgan- isms and the extraction of the product as these unit operations can add considerable costs to the process. For microalgae, there are a number of methods for harvesting
Table 7.7. Commercial microalgae cultivation. (Adapted from Borowitzka, 1999.)
System Algae Max. volume Location
Tanks Many species 1 × 104 Worldwide Extensive open ponds Dunaliella salina 1 × 109 Australia Circular ponds with
rotating arm
Chlorella sp. 1.5 × 104 Taiwan, Japan Raceways Chlorella sp.,
Spirulina sp., D. salina
3 × 104 Japan, Taiwan, USA, Thailand, China, India, Vietman, Chile, Israel Large bags Many species 1 × 103 Worldwide
Bioreactors heterotrophic
Chlorella sp., Crypthecodinum cohnii
>103 Japan, Taiwan, Indonesia, USA
Two-stage (indoors and then outdoors in a paddlewheel pond)
Haematococcus pluvialis
– USA
Light
Maximum 30% saturation 1700–2000μE/m2/s
CO2 maximum 0.1%, 0.03%
suboptimal Cells can
produce 28–120 mg O2/g dry wt/h
Mixing, turbulence
Fig. 7.8. Outline of the parameters which affect the growth of algae in bioreactors.
the cells including centrifugation, filtration, flocculation and settling. Flocculation can be used to improve the other methods of harvesting. Flocculation can be carried out using multivalent metal salts (Molina Grima et al., 2003) or cationic polymers.
Centrifugal recovery is rapid and expensive but has been used for many microalgae.
Filtration using filter presses has proved unsuccessful with the smaller microalgae, and membrane filtration has not been extensively used.
Extraction
Intracellular oils are difficult to extract from wet biomass (Belarbi et al., 2000), but can be more easily extracted from freeze-dried cells or cell paste. Oil has been extracted from Phaeodactylum tricormutum (diatom) and Monodus subterraneus (green alga) with solvents under pressure (Belarbi et al., 2000). In the case of C. protothecoides, the cells were freeze-dried before solvent extraction as were the oils from Isochrysis galbana (Molina Grima et al., 1994). Microalgal cells may be disrupted to extract the oils using a number of microbial cell disruption methods, but these methods can be expensive.
Free fatty acids have been extracted from wet biomass using a potassium hydroxide- ethanol mixture (Molina Grima et al., 2003). Whole cells of Dunaliella tertiolecta have been liquefied at 300°C and 10 MPa to form oil compar able to fuel oil. Both supercriti- cal CO2 (Mendes et al., 2003; Gouveia et al., 2007) and thermochemical liquefaction have also been used to produce biodiesel from macroalgae (Aresta et al., 2005). In addi- tion, whole microalgal cells containing high levels of oil have been used directly in diesel and biodiesel in emulsion fuels (Scragg et al., 2003). In general, all methods both mechanical and solvent based are expensive and will affect the cost of the biofuel.
Production of microalgal biodiesel
Microalgal biodiesel will need to comply with the standard EN 14214 in the EU and ASTM D 6751 in the USA before it can be universally accepted. Microalgal oils tend to contain more polyunsaturated fatty acids than plant oils, and those with four or five double bonds are more susceptible to oxidative degradation. However, the biodiesel produced from oil extracted from C. protothecoides had characteristics which were simi- lar to diesel (Miao and Wu, 2006) apart from a slightly higher viscosity (Table 7.8).
Table 7.8. Comparison of the properties of microalgal biodiesel and diesel. (From Miao and Wu, 2006; Xu et al., 2006; Stanhope-Seta, 2007.)
Properties Microalgal biodiesel Diesel En 14214 specifications
Density (kg/l) 0.864 0.838 0.86–0.90
Viscosity (mm2 s−1 cSt at 40°C) 5.2 1.9–4.1 3.5–5.0
Flash point (°C) 115 75 >101
Pour point (°C) −12 −50–10 –
Cold filter plugging point (°C) −11 −3–6.7 Summer – 0 Winter –15 Acid value (mg KOH/g) 0.374 Max 0.5 0.5
Heating value (MJ/kg) 41 40–45 –