Bioenergy systems for the future 14 integration of microalgae into an existing biofuel industry

Integration of microalgae into
an existing biofuel industry

14

M.R. Rahimpour, P. Biniaz, M.A. Makarem
Shiraz University, Shiraz, Iran

Abbreviations
MTOE
TAG
DO
DOC
UAE
MAE
IL
FFA
AD
PEM
SHE
ABE
HTL

14.1

million tonnes oil equivalent
triacylglycerol
dissolved oxygen
dissolved oxygen concentration
ultrasound-assisted extraction
microwave-assisted extraction

ionic liquid
free fatty acid
anaerobic digestion
polymer electrolyte membrane
sonication, heat, and enzymatic hydrolysis
acetone-butanol-ethanol
hydrothermal liquefaction

Introduction

Energy has always been one of the most challenging issues facing humanity. In the
year 2014, the total world energy consumption was equal to 12,928.4 million tonnes
oil equivalent (MTOE), which was increased by about 1% compared with the previous
year (plc, 2015). Generally, 70% of the needed energy is supplied by the fuels, especially for transportation, manufacturing, and domestic usages (Gouveia and
Oliveira, 2009).
Fossil fuels are one of the most common sources of energy due to their abundance.
However, they lead to serious environmental problems such as global warming
(Cheng and Timilsina, 2011). Moreover, about 98% of carbon dioxide emissions
are caused by fossil fuels (Najafi et al., 2011). On the other hand, these resources
are considered nonrenewable and will end gradually. Therefore, their supply and
increasing cost can cause serious economic, political, and even social problems
(Ribeiro et al., 2015). As a result, finding alternative sources of energy based on

Bioenergy Systems for the Future. />© 2017 Elsevier Ltd. All rights reserved.


482

Bioenergy Systems for the Future

renewable resources are considered vital (Mueller et al., 2011; Yusuf et al., 2011).
This alternative fuels must be technically feasible, be easily available, and be able
to compete economically with fossil fuels (Ayhan, 2009).
In general, these conditions can be found in various sources of energy such as wind,
solar, geothermal, hydroelectric, and biomass (Suganya et al., 2016). Among them,
biomass, that is, vegetable materials, is the largest source of renewable energy that
is produced by photosynthesis phenomenon (Wen et al., 2009). Biomass can be
obtained from various sources such as waste materials (agricultural wastes, crop
residues, urban wastes, and wood wastes), forest products (wood, logging residues,
shrubs, and trees), energy crops (starch crops such as corn, wheat, barley, sugar crops,
woody crops, grasses, vegetable oils, and hydrocarbon plants), or aquatic biomass
(algae, water hyacinth, and water weed) (Huber and Corma, 2007). These resources
can be turned into oil and used directly or with upgrading as fuel, which is named
biofuel.
In the 1930s and 1940s, the vegetable oil was used as fuel in emergencies (Shay,
1993). Later, biomass was utilized as feedstock for the production of different
chemicals and fuels including biomethanol (Chisti, 2007), bioethanol (Lee et al.,
2015), biobutanol (Hansen and Kyritsis, 2010), biohydrogen (Hallenbeck and
Benemann, 2002), bio-oil (Demirbas, 2011), and biogas (Harun et al., 2011). Generally, there are three main generations in developing biofuels (Adenle et al., 2013). At
first, they were produced from food crops and oil seeds (Naik et al., 2010a). Limitations on food resources and its impact on food economy led to food-for-fuel debate
(Gomez et al., 2008). Therefore, the second generation of biofuels was developed
from nonfood biomass such as lignocellulosic feedstock materials (Sims et al.,
2010). The low conversion rates of plant biomass to produce biofuels caused that
researchers began to think about the third generation of biofuels such as microalgae
(Milano et al., 2016). Microalgae, that is, microscopic algae, store solar energy in the
form of carbon products, which leads to the accumulation of lipids such as
triacylglycerol (TAG). TAGs then can be converted into biofuels (Maity et al.,
2014). This causes microalgae to have the highest potential to produce biofuels
(Suali and Sarbatly, 2012).
Many developing countries have potentials to grow algae. Therefore, oil imports

can be reduced, and the rural economy can be improved (Adenle et al., 2013). The
rapid growth of microalgae satisfies a great need for biofuels without shortage of
biomass resources and makes it more cost-effective. Most common algae
(Chlorella, Crypthecodinium, Cylindrotheca, Dunaliella, etc.) have oil content in
the range of 20%–50%; however, higher productivities can be reached (Mata et al.,
2010). In addition, greenhouse gas emissions are reduced using microalgae as a
biofuel source (Li et al., 2008c). Researchers believe that the producing biodiesel,
plant- or animal-fat-based diesel fuel, from microalgae is the most effective method
for making this kind of biofuels (Banerjee et al., 2002; Chisti, 2008; Demirbas and
Demirbas, 2011) and can generally supersede the fossil fuels (Ghadiryanfar et al.,
2016). Table 14.1 compares the advantages and disadvantages of different generations
of biofuels with fossil fuels


Table 14.1

Advantages and disadvantages of different generations of biofuels and fossil fuels

Fuel

Advantages

Fossil fuels

l

Provided 88% of the primary energy consumption, potential
reserves, and low cost (Brennan and Owende, 2010)

Disadvantages

l

l

Food crops

l

l

Decreased CO2 emission
Biodegradable, nontoxic, and essentially free of sulfur
and aromatics (Ayhan, 2009)

l

l

l

l

Nonfood
biomass

l

l

l

l

l

Microalgae

l

l

l

l

l

l

Decreased CO2 emission
Do not need arable land (Chisti, 2007)
Require less fertilizer, water, and pesticide inputs to low
environmental impact (Carriquiry et al., 2011)
Do not compete with food production (Havlı´k et al., 2011)
Form of recycling wood ash (Stupak et al., 2007), residual
agricultural biomass, and wastes (Nigam and Singh, 2011)
Decreased nitrous oxide and CO2 emission, high growth rate,
using limited land resources, less water (Li et al., 2008c)
Large quantities of lipids adequate for biodiesel production
(Demirbas and Demirbas, 2011)

Wastewater treatment (Hammouda et al., 1995)
Higher yield than most oil plants (Cheng and
Timilsina, 2011).
Potential to completely displaced liquid transport fuels
derived from petroleum (Chisti, 2008)
Coal power plants and hydrogen production plants can
supply large amounts of CO2 for microalgal culture at a low
cost (Takeshita, 2011)

l

l

l

l

l

l

Changes in global climate because of greenhouse gas emission
(Ugarte, 2000)
Increased atmospheric CO2 And environmental degradation
(M€
ollersten et al., 2003)
Competition between food and fuel and use of agricultural land
(Rathmann et al., 2010)
High production and processing costs (Sims et al., 2010)
More fertilizers, more irrigation, and more pesticides

(Odling-Smee, 2007)
Increase demand for food and related commodity prices
(Mueller et al., 2011)
Low conversion rates (Milano et al., 2016)
Lower energy density compared with coal (McKendry, 2002b)
Increase net deforestation drastically (Havlı´k et al., 2011)
Technical hurdles(Gomez et al., 2008)
Increased leaching after stump harvesting due to increased
decomposition and high costs of waste deposits if waste cannot
be recycled (Stupak et al., 2007)
Low biomass concentration, energy-consuming process, higher
capital costs, large-scale production necessary to be economical
(Li et al., 2008c)


484

14.2

Bioenergy Systems for the Future

An introduction to microalgae

Microalgae (algae with 1–50 μ m in diameter) are alternative resource for producing
energy. They are group of microorganisms, such as prokaryotic and eukaryotic photosynthetic microorganisms (Singh et al., 2011), with great variety, which are adapted to
different weather conditions (Hu et al., 2008). They are one of the fastest growing plants
in the universe (Demirbas, 2010) that can also be grown on seawater, freshwater, or even
on wastewater or sewage and do not require food crops and fertile land (Wang et al.,
2008; Demirbas and Demirbas, 2011). Although microalgae generally live in the waters,
they can grow on the surface of different types of soils (Richmond, 2008).

14.2.1 Various types of microalgae
Generally, alga (consist of microalgae and macroalgae) can be divided into various
groups based on their color, such as green (Chlorophyceae), blue-green
(Cyanobacteria or Cyanophyceae), diatoms (Bacillariophyceae), golden-brown
(Chrysophyceae), yellow-green (Xanthophyceae), brown (Phaeophyceae), red
(Rhodophyceae), dinoflagellates (Dinophyceae), and picoplankton (Prasinophyceae
and Eustigmatophyceae) (Hu et al., 2008; Ghasemi et al., 2012). Some properties
of different algae are summarized in Table 14.2. According to Maity’s investigation
(Maity et al., 2014), the lipid content order for various algae (percent in dry mass)
based on their colors are green > yellow-green > red > blood-red > blue-green.
Macroalgae, as a renewable resource of energy, can be used for biofuel production in
an economically impressive and environmentally sustainable manner (Li et al., 2008c).
However, many researchers reported that microalgae might be better for producing
higher biofuels (Hossain et al., 2008; Li et al., 2008c). Based on the cell, microalgae
can be divided into three general categories as shown in Fig. 14.1: colonial, unicellular,
and filamentous (Richmond, 2008). The unicellular microalgae are made from only one
cell and most of them are nonmotile; however, motile cells sometimes can take place
(Stanier et al., 1971). By comparison with most vascular plants, unicellular microalgae,
because of the behavioral, structural, physiological, and biochemical reasons, have ability to survive at low flux densities of photons (Richardson et al., 1983).
Colonial microalgae are made from one to several cell clusters unified by a
hydrocarbon-rich colonial matrix. When the light is sufficient for photosynthesis,
the colony size is increased with light intensity (Banerjee et al., 2002). They appear
as a green to orange, brown, or red floating scum on the calm water surface (Wolf,
1983). The green colonial microalgae have very high level of hydrocarbons
(70%–90% of the dry mass), making it potentially suitable for biofuel production
(Tsukahara and Sawayama, 2005; Tran et al., 2010).
Filamentous microalgae have single cells that form long chains or filaments at
microscopic dimension (Olson, 1950). They can be considered as a potential raw
material for producing biodiesel; however, it is rarely investigated (Wang et al.,

2013). One of the most advantages of filamentous species is that they can be cultivated
in wastewaters. They use organic and inorganic load of wastes for their growth and
reduce the harmful substances contained in them (Markou and Georgakakis, 2011).


Integration of microalgae into an existing biofuel industry

Table 14.2

485

Properties of different algae

Algae

Properties

Blue-green
(Cyanobacteria)

l

l

l

l

l

Diatoms
(Bacillariophyceae)

l

l

l

l

Green
(Chlorophyceae)

l

l

l

They are prokaryotes without
membrane-bond organelles
Could be unicellular, colonial,
and filamentous forms with or
without branching or
differentiation of specialized
cells
Transformed with
autonomously replicating
plasmids

High extraction rate
Having low lipid content
(% dry weight biomass)
They are ubiquitous in marine,
freshwater, and terrestrial
environments and include the
greatest number of extant
species (up to 10 million) of
any group of microalgae
Diatoms are mostly
unicellular, although
filamentous species are
abundant
Cell walls (frustules)
composed of silicon made of
two identical halves that fit
together
Use the triacylglycerol lipid
molecules (TAGs) as energy
storage molecules that can be
easily transesterified to
biodiesel
One of the largest number of
species, most widely
distributed morphologically
diverse groups unicellular,
colonial, filamentous, and
pseudoparenchymatous. Uni or
multinucleate forms
Having high lipid content

(% dry weight biomass)
Having eukaryotes
characterized by chlorophylls a
and b as the major
photosynthetic pigment

Reference
Metting Jr. (1996), Kanda
and Li (2011), Maity et al.
(2014), Wehr et al. (2015),
and Suganya et al. (2016)

Round et al. (1990),
Metting Jr (1996), Singh
et al. (2011), and Wehr
et al. (2015)

Metting Jr (1996), Maity
et al. (2014), and Wehr
et al. (2015)

Continued


486

Bioenergy Systems for the Future

Table 14.2

Continued

Algae

Properties

Yellow-green
(Xanthophyceae)

l

l

Golden-brown
(Chrysophyceae)

l

l

l

Red
(Rhodophyceae)

l

l

Reference

Living on fresh water and
moist soil
Their color is not always easy
to distinguish from true green
microalgae taxa
Macroalgae living in marine
and freshwater and have not
been reported on soil
Having a diverse class of taxa
as brown one
And are from filamentous
Including seaweeds and
microalgae and lacking of any
flagellate stages, red
microalgae occur in fresh
water and on soil

Metting Jr (1996) and
Wehr et al. (2015)

Metting Jr (1996) and
Wehr et al. (2015)

Metting Jr (1996), and
Wehr et al. (2015)

Having eukaryotic cells

Microalgae

Unicellular

Nonflagellate

Motile

Flagellate

Colonial

Flagellate

Nonflagellate

Filamentous

Branched

Unbranched

Nonmotile

Fig. 14.1 Different types of cell organization of microalgae (Richmond, 2008).

14.2.2 Microalgae potential for biofuel production
Green microalgae, with high content of lipid, are used to produce different types of
biofuels such as biodiesel, hydrogen, ethanol, and methanol as shown in
Table 14.3. Researchers also use blue-green microalgae for producing biogas purification (Converti et al., 2009) and methane production (Costa and De Morais, 2011;
Maity et al., 2014). In the case of biodiesel production, red marine microalgae can

be used as well (Wu and Merchuk, 2004).
One important factor for the production of biofuel from microalgae is the amount of
oil exists inside it. Table 14.4 compares oil content of different microalgae. Based on


Integration of microalgae into an existing biofuel industry

487

Various products derived from different green
microalgae

Table 14.3

Green microalgae

Biofuel

Reference

Arthrospira maxima
Chlorella biomass
Chlorella
minutissima
Chlorella
protothecoides
Chlorococcum sp
Chlorella fusca
Chlorella
protothecoides

Chlorella reinhardtii
Chlorella regularis
Chlorella vulgaris
Chlorococcum
humicola
Haematococcus
pluvialis
Neochloris
oleoabundans
Scenedesmus
obliquus
Spirulina platensis
Spirogyra

Hydrogen, biodiesel
Ethanol
Methanol

Dismukes et al. (2008)
Zhou et al. (2011)
Kotzabasis et al. (1999)

Biodiesel
Bioethanol
Hydrogen
Bio-oil

Li et al. (2007) and Chen
and Walker (2011)
Harun et al. (2010)

Ghirardi et al. (2000)
Miao and Wu (2004)

Hydrogen
Ethanol
Ethanol
Bioethanol

Ghirardi et al. (2000)
Endo et al. (1977)
Hirano et al. (1997)
Harun and Danquah (2011)

Biodiesel

Damiani et al. (2010)

Biodiesel

Gouveia and Oliveira (2009)

Hydrogen

Ghirardi et al. (2000)

Hydrogen gas, ethanol
Ethanol

Aoyama et al. (1997)
Sulfahri et al. (2011)

the reported data, oil content of microalgae can exceed up to 80% by weight of dry
biomass (Metting Jr, 1996).
Table 14.5 compares oil content and biofuel productivity of microalgae with other
biofuel feedstocks. As illustrated in this table, microalgae with high oil content and
with the least usage of land (0.1 m2 year/kg biodiesel) can produce the largest amount
of biodiesel (121,104 kg biodiesel/ha year)

14.2.3 Effects of nutrients on the growth rate
The main factors that affect the growth and oil content of microalgae are CO2 supply,
pH, light intensity, temperature, nutrients (carbon, nitrogen, sulfur iron, phosphate,
and in some cases silicon), salinity, and dissolved oxygen (DO) (Hu et al., 2008;
Kumar et al., 2010a). High dissolved oxygen concentration (DOC) levels can cause
photooxidative damage on microalgal cells (Suh and Lee, 2003). Moreover, some
toxic element compounds, such as synthetic organics or heavy metals, and some


488

Bioenergy Systems for the Future

Table 14.4 Oil content of various microalgae (Becker, 1994;
Chisti, 2007; Li et al., 2008a,b; Deng et al., 2009; Mata et al., 2010;
Verma et al., 2010; Ghasemi et al., 2012)
Microalgae
Anabaena
cylindrica
Ankistrodesmus
species
Botryococcus

braunii
Chaetoceros
calcitrans
Chaetoceros
muelleri
Chlamydomonas
reinhardtii
Chlamydomonas
species
Chlorella
Chlorella emersonii
Chlorella
minutissima
Chlorella
protothecoides
Chlorella
pyrenoidosa
Chlorella
sorokiniana
Chlorella species
Chlorella vulgaris
Chlorococcum
species.
Crypthecodinium
cohnii
Cyclotella species
Cylindrotheca
species
Dunaliella
Dunaliella

bioculata
Dunaliella
primolecta
Dunaliella salina

Oil content
(% dry wt.)

Microalgae

Oil content
(% dry wt.)
22–29.7

33

Nannochloropsis
oculata
Nannochloropsis
species
Neochloris
oleoabundans
Nitzschia
closterium
Nitzschia frustulum

21

Nitzschia species

16–47

23

Nitzschia laevis

69.1

18–57
25–63
57

Oocystis pusilla
Pavlova salina
Pavlova lutheri

10.5
30
35

14–57.8

Parietochloris
incisa
Phaeodactylum
tricornutum
Porphyridium
cruentum
Prostanthera incisa
Prymnesium

parvum
Pyrrosia laevis

62

4–7
24–40
25–86
14.6–39.8

2
19–22
10–48
5–58
19.3

23

Schizochytrium
species
Scenedesmus
obliquus
Scenedesmus
quadricauda
Selenastrum species
Skeletonema
costatum
Skeletonema sp

6–28.1

Spirulina maxima

20–51
42
16–37
67
8

12–68
29–65
27.8
25.9

18–57
9–18.8/60.7
62
22–39
69.1
50–77
11–55
1.9–19.9
19.6–21.7
13–51
13.3–31.8
4–9


Integration of microalgae into an existing biofuel industry

Table 14.4

489

Continued

Microalgae

Oil content
(% dry wt.)

Dunaliella species
Dunaliella
tertiolecta
Ellipsoidion sp.

17–67
16–71

Euglena gracilis
Haematococcus
pluvialis
Hantzschia species

14–20
25

Isochrysis galbana
Isochrysis species
Monallantus salina

Monodus
subterraneus
Nannochloris
species

7–40
7–33
20–72
16–39.3

27.4

66

Microalgae

Oil content
(% dry wt.)

Spirulina platensis
Stichococcus
species
Tetraselmis
maculata
Tetraselmis sp.
Tetraselmis suecica

16.6
33

Thalassiosira
pseudonana
Nitzschia sp

20.6

3
12.6–14.7
8.5–23

4547

20–56

biological factors such as viruses, predation, competition, and growth of epiphytes
may confine microalgae growth rates (Carlsson and Bowles, 2007).
Approximate molecular formula of the microalgal biomass should be
CO0.48H1.83N0.11P0.01 (Grobbelaar, 2004). The main sources of carbon dioxide
required for microalgae growth are atmospheric CO2, industrial exhaust gases (e.g.,
flue gas and flaring gas), and CO2 produced from soluble carbonates (e.g., NaHCO3
and Na2CO3) (Becker, 1994). Since the atmospheric CO2 level (0.0387% (v/v)) is not
sufficient for high microalgal growth rates (Kumar et al., 2010a), coal power plants
and hydrogen production plants can supply large amounts of CO2 for this purpose at a
low cost (Takeshita, 2011).
Carbon (generally derived from carbon dioxide) and nitrogen are the most important nutrients required for growing microalgae (Becker, 1994). Ammonium and
nitrates, which are primary nitrogen sources, are suitable for fast and medium growing
rates (Green and Durnford, 1996; Jin et al., 2006). After carbon and nitrogen, phosphor
is the third most important nutrient, which can be obtained from wastewater and
seawater (Green and Durnford, 1996; Kumar et al., 2010b). On the other hand, microalgae, by adsorbing and accumulating organic nutrients and heavy metals, can
enhance purifying process of domestic wastewater and changes the adsorbed species

to interesting raw materials for producing biofuels (Munoz and Guieysse, 2006).
However, it should be noted that microalgae are sensitive to toxic pollutants such
as phenolic compounds (e.g., chlorophenols) and volatile organic component
(Mun˜oz et al., 2003; Chen and Lin, 2006).


490

Bioenergy Systems for the Future

Comparison of microalgae with other biofuel feedstocks
(Mata et al., 2010)

Table 14.5

Plant source
Corn/maize (Zea mays L.)
Hemp
(Cannabis sativa L.)
Soybean (Glycine max L.)
Jatropha (Jatropha
curcas L.)
Camelina (Camelina
sativa L.)
Canola/rapeseed
(Brassica napus L.)
Sunflower (Helianthus
annuus L.)
Castor (Ricinus
communis)

Palm oil (Elaeis
guineensis)
Microalgae
(low oil content)
Microalgae
(medium oil content)
Microalgae
(high oil content)

Seed oil
content
(% oil
by wt. in
biomass)

Oil
yield
(L oil/
ha year)

Land use
(m2 year/kg
biofuel)

Biofuel
productivity
(kg biofuel/
ha year)

44

33

172
363

66
31

152
321

18
28

636
741

18
15

562
656

42

915

12

809

41

974

12

862

40

1070

11

946

48

1307

9

1156

36

5366

2

4747

30

58,700

0.2

51,927

50

97,800

0.1

86,515

70

136,900

0.1

121,104

14.2.4 Effects of environmental conditions on the growth rate
Microalgae usually utilize light energy for growing, and sunlight is the most common
source. A phototrophic organism uses the energy of light to perform various cellular

metabolic processes, while heterotrophic ones uses organic carbon for the plant growth.
Many microalgae species are generally mixotrophic, that is, they can switch from
the phototrophic to the heterotrophic growth. They can use photosynthesis for energy production and, alternatively, carbon compounds for biosynthesis (Carlsson and Bowles,
2007; Kumar et al., 2010a). Such a mixotrophic structure leads to higher biomass concentration and growth rate (Wang et al., 2014). In the existence of light, microalgae
convert CO2 and nutrients to biomass; by increasing the light density, microalgae
photosynthesis is increased up to an optimum point (i.e., 200–400 μEmÀ2sÀ1). By further
increasing the intensity, photosynthesis rate will be decreased (Sorokin and Krauss, 1958;
Ogbonna and Tanaka, 2000). On the other hand, low light intensity causes the formation


Integration of microalgae into an existing biofuel industry

491

of polar lipids, whereas high light intensity increases the amount of neutral storage lipids
(mainly TAGs) (Brown et al., 1996). Furthermore, higher light intensity with longer lighting duration promotes biomass accumulation (Li et al., 2011).
In the case of temperature, optimal condition (15°C–26°C) depends on both microalgal species and environmental parameters such as light intensity (Tamiya, 1957;
Ono and Cuello, 2003). The metabolic efficiency of microalgae is normally enhanced
by rising the temperature. This is while low temperatures prohibit the microalgal
growth (Abeliovich, 1986). The total lipid content of microalgae is increased with
increasing the temperature, as well (Hu et al., 2008).
Acidity is another important factor, which affects the microalgae growth rate. Different microalgae species grow in different pH ranges. However, most of them prefer
neutral pH (Kodama et al., 1993; Qiang et al., 1998). Addition of inorganic nitrogen,
such as nitrates or ammonium ion, increases the pH up to 8.5, and this value would
remain almost constant ( Jin et al., 2006). Moreover, adsorption of CO2 by microalgae
increases water pH up to 10–11 during the photosynthesis process, due to the change
in the carbon dioxide equilibrium of water. Although increased pH is useful for disinfection of pathogens, it decreases the efficiency of microalgae pollutant removal
phenomenon (Oswald, 1988; Schumacher et al., 2003).

14.3

From biomass to extracted oil sequence

Generally, the production of biofuel from microalgae needs many downstream
processing steps. Before producing the fuel, it is necessary to extract the oil content
of microalgae. These steps are microalgae cultivation, biomass harvesting, and
processing (dehydration, cell disruptions, and oil extraction), followed by biofuel production (Lee et al., 2015). Schematic representation of the microalgae-to-biofuel
chain stages is shown in Fig. 14.2. These steps are discussed in the following sections.

14.3.1 Cultivation
Biofuel production from microalgae requires an ability to produce economically large
amounts of oil-rich microalgal biomass. Raceway ponds (open systems) and tubular
photobioreactors (closed systems) are two suitable and applicable methods to cultivate
microalgae (Chisti, 2008). Besides, growing microalgae in salt, gray, and wastewaters, which are rich in minerals, are the other choices for this purpose with many benefits (Hammouda et al., 1995; Alley, 2003). In addition, researchers recently
investigated novel cultivation strategies such as biofilm systems; however, the technology is new and still underdevelopment (Gross et al., 2015; Heimann, 2016). On the
other hand, it is necessary to design facilities to produce biomass such as evaporation
control, water recycling, and efficient water conservation system. Additionally, it is
urgent to protect biomass from bacteria and other microbial flora, by setting up extensive water treatment equipment (Subhadra, 2011). (Kumar et al., 2010b) indicated that
the combination of CO2 sequestration, wastewater treatment, and biofuel production
in a fiber membrane photobioreactor is a strong potential way for producing


Cultivation
Photobioreactor
Hybrid production

Harvesting

Biofilm cultivation

Gravity
Centrifugsedimentation
ation

Flotation

Ultrasound

Flocculation

Dehydration

Filtration

Acid
treatment

492

• Oven drying
• Freeze drying
• Spray drying
• Solar drying
• Flashing drying
• Vacuum shelf drying
• Drum drying
• Low pressure shelf
drying
• Convective dryer
• Rotary drying

• Microwave drying
• Cross-flow drying
• Refractance Window TM
technology drying

Raceways ponds

Osmotic
shock

Bead milling

Celldisruption

Ultrasonication

Enzyme

Homogenization
Autoclave
Solvent
extraction

Mechanical
techniques

Extraction

Microwave

Biofuel
production

Bio
metanol

Bio
syngas

Bio
char

Fig. 14.2 Microalgae biofuels chain stages (Lee et al., 2015).

Bio
oil

Bio
diesel

Bio
butanol

Bio
ethanol

Bioenergy Systems for the Future

Supercritical fluid

Integration of microalgae into an existing biofuel industry

Harvest

493

Feed

Paddlewheel

Baffle
Flow

Fig. 14.3 Aerial view of a raceway pond (Chisti, 2007).

microalgal biomass while it is a hopeful alternative for greenhouse gas mitigation.
This is because discharging the pollutants into the environment is decreased in such
systems.
The open raceway pond is a closed-loop recirculation channel with a depth of
30 cm. It includes a paddle wheel, which mixes and circulates the stream, as illustrated
in Fig. 14.3. Open raceways are more economical than closed systems in microalgae
tillage (Huntley et al., 2015) and have been used since the 1950s (Borowitzka, 1999).
However, configurations must be very carefully controlled for these systems. Controlling the temperature, evaporation rate, and lighting within a diurnal cycle is very
difficult in open ponds, and this would affect the cooling process (Chisti, 2007).
Generally, open ponds have low yields and are not usually satisfactory, due to some
problems such as losing water by evaporation, more energy requirement, unstable
microalgal populations, and the difficulty of distributing nutrients (Terry and
Raymond, 1985).
On the other hand, closed systems generally offer higher biomass productivity and

better process control ability. Comparing with open ponds, though manufacturing of
photobioreactors is more expensive and they have complex performance and maintenance, they offer many advantages as illustrated in Table 14.6.
Photobioreactors are capable to minimize the required space, based on their
construction (Pulz, 2001; Munoz and Guieysse, 2006). Besides, controlling oxygen,
temperature, and contamination is more efficient. Photobioreactors have various types
including horizontal tubular, vertical tubular, helical tubular, fermenter-type,
α-shaped, flat-plate, and hollow-fiber membrane reactors (Carvalho et al., 2006).
Table 14.7 compares different types of photobioreactor. It should be noted that the
most effective parameter for designing photobioreactors is the source of light supply
(sunlight or artificial light). Therefore, based on the information given in Table 14.7,
flat-plate photobioreactors, with excellent and efficient use of sunlight, have high
capability to produce microalgae biomass in a large scale. Additionally, tubular
reactors are another popular choice for utilizing in large-scale productions
(Carvalho et al., 2006).


Comparison between open raceway pond and enclosed
photobioreactor (Carvalho et al., 2006; Genin et al., 2016)

Table 14.6

Parameter

Open systems

Closed systems

Area-to-volume ratio

Large (4–10 times higher than

closed counterpart)
Restricted
Growth competition

Small
Flexible
Shear resistance

Low
Low
Limited
Possible
Possible

High
High
Extended
Unlikely
Prevented

Poor/fair

Fair/excellent

Poor
None
Mixing

Fair/high
Excellent

Oxygen control,
temperature control
High
High
Excellent
Large

Microalgal species
Main criteria for species
selection
Population density
Harvesting efficiency
Cultivation period
Contamination
Water loss through
evaporation
Light utilization
efficiency
Gas transfer
Temperature control
Most costly parameters
Capital investment
Capital costs
Process control
Land required

Small
Low
Poor
Small

A comparison between different types of photobioreactors
(Carvalho et al., 2006)

Table 14.7

Reactor
type

Lightharvesting
efficiencya

Degree of
control

Land
area
required

Scale-up

References

Medium

Medium

Medium

Possible

Good

Medium

Poor

Possible

Brindley Alı´as et al.
(2004)
Carvalho et al. (2006)

Vertical
tubular
Horizontal
tubular
Helical
tubular
α-Shaped

Medium

Good

Excellent

Easy

M€arkl (1977)

Excellent

Good

Poor

Very
difficult

Flat-plate

Excellent

Medium

Good

Possible

Fermentertype

Poor

Excellent

Excellent

Difficult

Chrismadha and
Borowitzka (1994)
and Lee et al. (1995)
Tredici and Zittelli
(1998) and Morita
et al. (2000)
Tredici (2003)

a

Light-harvesting unit employs small-diameter tubing to provide a high area-to-volume ratio that favors high
photosynthetic activity (Carvalho et al., 2006).


Integration of microalgae into an existing biofuel industry

495

Combination of photobioreactor and open pond raceway is called hybrid two-stage
cultivation. At first, the conditions are controlled by photobioreactor, and then, the
pond exposes the cells to nutrient stresses, which enhances the synthesis of the desired
lipid products (Huntley and Redalje, 2007; Rodolfi et al., 2009).
One another way for microalgae cultivation is using microalgal film photobioreactors, which are capable to produce large biomass amount (Genin et al.,
2016) and do not consume large energy for mixing, dewatering, and harvesting.
This kind of cultivation will play an important role in the future of industrial
photosynthetic biomass production. The biomass is scraped from the cultivation surface by centrifugal force and separated from the air just by a thin water layer (Berner
et al., 2015).

14.3.2 Harvesting
Harvesting is the next step right after the cultivation. It is a difficult and expensive

process due to the small microalgae cell size. Nearly 20%–30% of microalgae total
production cost is assigned to harvesting process. There are several ways to this process including filtration, sedimentation, flocculation (biological, chemical, and electroflocculation), ultrasound flotation, and centrifugation (Heasman et al., 2000; Lee
et al., 2015). The proper technique for harvesting depends on features of the microalgae (size and density), salinity, water composition, and the value of the objective
biofuel (Olaizola, 2003; Barrut et al., 2013). In general, the biomass is separated from
the slurry with flotation, flocculation, or gravity sedimentation at first. Then, it is
followed by the downstream processes (i.e., centrifugation, filtration, or ultrasound
flotation) for more thickening of the biomass (Brennan and Owende, 2010;
Christenson and Sims, 2011).
Flocculation is an aggregation of the microalgae cells to enhance the separation
process with organic or inorganic materials, which are named flocculant (Chen
et al., 2003; Gregory, 2005). Since microalgae are negative charged that prevents natural aggregation, flocculants with cationic characteristic are added to the suspension.
Synthetic or natural polymeric flocculants, with higher molecular weights and the
ability to adsorb several particles at once, are more effective in the harvesting process
(Shih et al., 2001; Sharma et al., 2006). In the case of pH, though researchers reported
the range of 5–8 for the flocculation process (Wang et al., 2011), Ummalyma et al.,
2016 obtained 94% efficiency by changing in medium pH from 8.5 to 12.0.
After the flocculation process, it is needed to separate microalgae cells from the
slurry. It is then followed by filtration, centrifugation, or sedimentation processes
before further drying. Lananan et al., 2016 investigated a new flocculation harvesting
method based on biotechnology using microalgae-microalgae flocculants. This
method simplifies downstream processes, saves resources, and reduces production
costs. Besides, it provides a sustainable and low-cost wastewater treatment approach.
In another research, Das et al., 2016 studied coagulation-flocculation technique that
can be considered as one of the least energy-consuming processes for microalgae biomass harvesting. According to (Chatsungnoen and Chisti, 2016b) study, the efficiency
of microalgae sedimentation in this method depends on the type of the flocculant,


496

Bioenergy Systems for the Future

species and cell diameter of microalgae, biomass concentration in slurry, and the ionic
strength of the suspending fluid.
In flotation method, microalgae cells are trapped by dispersed microair bubbles
(Wang et al., 2008). Flotation contains three different types including dissolved air
flotation, dispersed air flotation, and microflotation (Hanotu et al., 2012). For solving
technical and economic problems of flotation, Barrut et al., 2013 investigated a lowenergy and low-cost separation method by using a vacuum gas lift. It is utilized before
complete harvesting using centrifugation with a potential to reduce costs from 10- to
over 100-fold. Laamanen et al., 2016 emphasized that, though flotation is still at laboratory scale, it can offer better harvesting characteristics than other methods.
High-speed centrifugation is one of the most appropriate microalgae harvesting
methods and can be used in large scales based on Stoke’s Law (Heasman et al.,
2000). This method almost does not depend on microalgal species, and all types of
microalgae can be separated easily (Mohn, 1988). On the other hand, filtration is
one of the cheapest harvesting techniques. Wide variety of filters and membranes
are available worldwide. In the membrane filtration methods, which are classified
by the pore or membrane size, only microalgae are allowed to pass through it
(Suali and Sarbatly, 2012; Milledge and Heaven, 2013). Permeation flux of microalgae is enhanced by utilizing membranes with greater pore density (Kanchanatip
et al., 2016).
The ultrasound technique was first introduced by Bosma et al. (2003). According to
their investigation, microalgae under an ultrasonic field are continuously pumped into
a resonator chamber. The cells are aggregated because of acoustic forces; however, by
switching off the wave, the aggregated cells would be separated and settled rapidly
because of the gravity force. Advantages and disadvantages of various harvesting
techniques are illustrated in Table 14.8.

14.3.3 Dehydration
After harvesting, the next step is dehydration or drying process. Unlike macroalgae,
dewatering is essential for microalgae species (Ghadiryanfar et al., 2016). Generally,
up to 84.9% of total energy consumed for biofuel production is assigned to the drying
processes (Lardon et al., 2009). Different techniques have been reported for this

purpose, including freeze-drying, oven drying, spray drying (Lee et al., 2015),
low-pressure shelf drying, solar drying, drum drying (Prakash et al., 1997), fluidizedbed drying (Leach et al., 1998), rotary drying, convective drying, cross flow drying,
flashing drying, vacuum shelf drying, microwave drying (Al Rey et al., 2016), and
Refractance Window technology drying (Nindo and Tang, 2007).
In the 1990s, drum drying was preferred for dehydrating microalgae, because of its
simplicity and convenience (Prakash et al., 1997). However, oven drying and freezedrying are the most common methods (Chatsungnoen and Chisti, 2016a). Freezedrying is an expensive method and has been widely utilized in laboratory scale. It
is much more productive to extract oil from freezing microalgae than wet ones
(Grima et al., 2003). Convective spray drying is another expensive method used
for high-value products. However, it leads to deterioration of some microalgae


Integration of microalgae into an existing biofuel industry

497

Advantages and disadvantages of different harvesting
techniques (Heasman et al., 2000; Brennan and Owende,
2010; Suali and Sarbatly, 2012; Milledge and Heaven, 2013)

Table 14.8

Harvesting
method
Centrifuge

Advantages
l

l

l

Filtration

l

l

Possible in large industrial
scale
Do not need excess chemicals
Can handle most algal types
with rapid and efficient cell
harvesting
Wide variety of filter and
membrane types available
Cheapest technique

Disadvantages
l

l

l

l

l

l

l

Flocculation

l

l

l

Ultrasound

l

l

l

l

l

Can be used in commercial
scale
Wide ranges of flocculants are
available
Organic flocculants (chitosan)
are capable for harvesting up
to 98% of the microalgae

Nonfouling
No shear stress
Absence of mechanical
failures
Possibility of continuous
operation
Small occupation space
Does not require any addition
of chemicals

Flotation

l

Sedimentation

l

Low cost

l

Potential for use as a first
stage to reduce energy input
and cost of subsequent stages

l

l

l

l

l

l

l

l

l

l

l

l

l

l

High capital and operation
costs
Needs large amount of
electricity
Mechanical problems can
occur due to the moving parts

Highly depends on microalgal
species
Time-consuming
Requires backwashing
Fouling and clogging can
occur due to the small size of
the microalgae
Uses toxic chemicals
Requires sedimentation units
High costs for excess
operations
Long processing period

Not for industrial scale
Power consumption is
very high
High capital and operation
costs
Need large amount of
electricity
Technical and economic
problem
Does not possible in large scale
Only for specific species of
microalgae
Best suited to dense nonmotile
cells
Separation can be slow
Low final concentration

498

Bioenergy Systems for the Future

components (Desmorieux and Decaen, 2005). On the other hand, Refractance
Window drying method is a new technique based on thermal energy, and the energy
is supplied from hot water. It must be considered that this approach is more expensive
than freeze-drying (Nindo and Tang, 2007).
Open sun drying is not a suitable method (Molina Grima et al., 2003) because of the
low quality of its target product, low drying rate, and the biomass degradation risk. In
one research, Gouveia et al. (2016) used a kind of efficient solar heater with solar
collectors, airflow, and electric fan for this process that just consumes 20 W energy.
It was operated faster than an oven or freeze-drying and has ability to take 80% of the
moisture content. When a rapid and effective method is needed, microwave drying is
an option, which obtains high lipid content even at low specific energy (the amount of
energy required to remove a unit mass of moisture ((Al Rey et al., 2016).

14.3.4 Cell disruption
Cell disruption is energy-intensive and costly process, which is required to prepare
microalgae for extracting its lipid content (G€
unther et al., 2016). Different cell disruption processes were used to facilitate the release of products inside the cells (Chisti and
Moo-Young, 1986; Mendes-Pinto et al., 2001). The disruption process depends on the
microalgae specifications (Kurokawa et al., 2016), and there are several methods for
this purpose including the following (Lee et al., 2015):
l

l

Physical and mechanical techniques (such as ultrasonication, bead milling, autoclave,

homogenization, and microwave)
Chemical and biological techniques (such as enzyme (Zheng et al., 2016), resin (Farooq
et al., 2016), cationic surfactant with nanoparticles (Seo et al., 2016), acid treatment, and
osmotic shock)

14.3.5 Oil extraction
To extract microalgae oil content, various techniques can be utilized including physical techniques (e.g., expeller or pressing, microwave extraction, and mechanical
milling), solvent extraction (e.g., hexane, alcohol, chloroform, water, acetone, and
ionic liquids), and supercritical fluid (Zinnai et al., 2016b), as illustrated in
Fig. 14.4 (Halim et al., 2012; Kim et al., 2012; Pragya et al., 2013).
The choice of solvent depends on the microalgae specie, and it should be inexpensive, nonpolar, and nontoxic solvent with poor extraction capability of other nonlipid
components. In some cases, a combination of physical (e.g., pressing and milling) and
solvent extraction methods is used to enhance the process yield (Cheng et al., 2011). In
pressing and milling methods, pressures and grinding media are used, respectively, to
disrupt cell walls (Mercer and Armenta, 2011). On the other hand, to facilitate the
hydrolysis of microalgae cell walls in the solvent extraction or physical disruption
methods, some kind of enzymes are used; this process is named enzymatic extraction
(Gong and Jiang, 2011).


Integration of microalgae into an existing biofuel industry

499

Oil extraction

Solvent extraction

Mechanical techniques

Supercritical fluid

Hexane
(Soxhelt extractio)

Microwave extraction

Supercritical CO2

Chloroform and alcohol
(Bligh and Dyer’s method)

Microwave extraction

Supercritical methanol

Ionic liquid

Electro mechanical
methods

Enzymatic
(with suitable solvent)

Ultrasonic extraction (or
combine with enzymatic)

Osmotic shock

Mechanical milling

Fig. 14.4 Different techniques for lipid extraction from microalgae (Halim et al., 2012;
Kim et al., 2012; Pragya et al., 2013).

Ultrasound-assisted extraction (UAE) and microwave-assisted extraction (MAE)
techniques can be utilized as complementary techniques to improve extractions of
microalgae, (Cravotto et al., 2008). Generally, they prohibit hydrophobic interactions
between nonpolar/neutral lipids or hydrogen bonding between polar lipids (Rawat
et al., 2011). Furthermore, ionic liquids (ILs) can be used as green solvents due to their
unique properties. They are nonvolatile, thermally stable, and nonflammable and exist
in liquid phase at ambient temperature (Young et al., 2010). Chiappe et al., 2016
investigated the efficiency of some “low-cost” protic ILs for the extraction of microalgae lipid. They used wet microalgae (85% water) and obtained high extraction yields
(up to 88%).
A combination of chloroform, water, and methanol is proved highly effective for
quantitative extraction of the lipids. Furthermore, there is no need for a prior freezedrying step before extraction (Chatsungnoen and Chisti, 2016a). On another technique, Chen et al., 2016 used nontoxic and cost-effective aqueous surfactant solutions
(anionic, nonionic, and a mixture of them) for extraction process. They achieved direct
lipid extraction with 88.3% efficiency from microalgae with 96.0% moisture content.
Other researchers mentioned that supercritical fluid extraction is the most efficient
technique with 100% extraction yield (Balaban et al., 1996; Reverchon, 1997;
Demirbaş, 2008; Zinnai et al., 2016a). Supercritical carbon dioxide (CO2), with both
liquid and gas properties at its above critical temperature and pressure, is utilized in


500

Bioenergy Systems for the Future

microalgae oil extraction (Demirbaş, 2008). In such a high-yield method, there is no
need for extra unit operations, and it is completely solvent-free technique. Finally,
some thermochemical, physical, and biological pretreatments can be used to enhance

lipid extraction efficiency from microalgal biomass (Neves et al., 2016).

14.4

Biofuel production

There exist usually some impurities such as free phospholipids, fatty acids, sterols, and
water with the extracted oil that causing the oil cannot be used directly as fuel. Therefore, it is necessary to perform some chemical modifications or upgrading processes on
it. Some examples of such processes are transesterification, fermentation, pyrolysis,
liquefaction, and anaerobic digestion, which are employed to produce various products
introduced in the following (Lee et al., 2015). It should be noted that some biofuels are
obtained directly from biomass or in some intermediate stages.

14.4.1 Biodiesel
Biodiesel, a diesel from biosources, is produced by transesterification or alcoholysis
process after the lipid extraction procedure. Transesterification is a three-step chemical reaction between triglycerides and a second alcohol (e.g., methanol). This reaction forms an ester of the second alcohol (i.e., methyl ester) of fatty acids, named
biodiesel, and glycerol as shown in Fig. 14.5 (Meher et al., 2006). Initially, triglyceride is converted to diglyceride followed by monoglyceride. Finally, glycerol and fatty
acid esters are formed (Ma and Hanna, 1999). This reaction typically occurs at the
temperature range of 60°C–80°C with a heterogeneous catalyst such as alkali (e.g.,
sodium hydroxide, potassium hydroxide, sodium methoxide, and potassium
methoxide), nonionic base, acid (e.g., HCl and H2SO4), enzyme, or lipase in 3 h period
(Schuchardt et al., 1998).
The transesterification process is affected by various factors including free fatty
acid, catalyst type, moisture content, molar ratio of alcohol to oil, type of alcohol, temperature, reaction time, mixing intensity, and the organic cosolvent. The accepted
molar ratio of alcohol to oil is 6:1, and lower than 3% of free fatty acid (FFA) is
needed. Basic catalysts (e.g., NaOH/KOH) with minimum amount of 1000 g are more
CH2

COOR1

CH

COOR2

CH2

COOR3

CH3

COOR1

CH3

COOR2

CH3

COOR3

CH2

OH

CH

OH

CH2

OH

Catalyst

(Triglyceride)

+3 CH3

OH

Methylesters
(Biodiesel)

+

(Glycerol)

Fig. 14.5 Transesterification reaction of triglycerides to form biodiesel (Meher et al., 2006).


Integration of microalgae into an existing biofuel industry

501

Biodiesel

CSTR or plug flow
60°C, RT. 1h

Acid

Methyl
ester

Methanol
Oil

Reactor

Settling
tank

Base Glycerol (50 wt%),
(catalyst) methanol, catalyst,
soap

Acid
Crude glycerol
(85%)

Neutralize
step

Stripping

Vacuum
flash
Water
washing

Distillation of MeOH

and Me-esters

Neutralize
step
Vacuum
flash

Water

Free fatty acids

Wash
water

Methanol

Distillation

Water
Methanol

Fig. 14.6 Block flow diagram for biodiesel production by transesterification of vegetable oils
(Knothe et al., 2010).

effective than acidic ones and enzymes. Although the reaction can occur at various
temperatures, higher ones speed up the reaction. Furthermore, the conversion rate
increases with increasing the reaction time and stirring speed (Meher et al., 2006).
A block flow diagram for the production of biodiesel from microalgae oils is shown
in Fig. 14.6.
Song et al., 2016 evaluated the techno-economic feasibility of hydrolysisesterification for producing biodiesel from wet microalgae. As mentioned by them,

the conventional biodiesel production process (including drying, oil extraction,
pretreatment, triolein transesterification, and biodiesel purification) is omitted in this
method. This is while the total energy consumption can be reduced to 1.81 MJ LÀ1 of
biodiesel, which leads to approximately 3.61 MJ LÀ1 energy saving. To avoid using
harmful solvents and catalysts during in situ transesterification reaction, it is possible
to use supercritical methanol for extracting the oil content of wet biomass. The supercritical methanol acts both as a catalyst and as a transesterification material. Moreover, such a reaction reduces the energy consumption level, by omitting the
product purification processes (Patil et al., 2012; Gunawan et al., 2014). According
to (Skorupskaite et al., 2016) study, ethanol dissolves triglycerides better than methanol, because it is less polar.

14.4.2 Bio-syngas
Syngas is widely used to produce fuels and chemicals and has many industrial applications. It can be produced during gasification, anaerobic digestion, or alcoholic fermentation processes of various biomasses including microalgae (Naik et al., 2010b; Milano
et al., 2016). Gasification is a process that converts solid- or liquid-based carbonaceous
materials such as biomass, organic, or fossil fuels into syngas. The product contains CO,
H2, CO2, CH4, and N2 in various proportions. Biomass gasification is a combination of
pyrolysis and oxidation of three major reactions (i.e., primary, secondary, and tertiary)


502

Bioenergy Systems for the Future

Pyrolysis and gasification severity (temperature, time)
Primary process (500°C–700°C)

Secondary process (700°C–850°C) Tertiary process (850°C–1000°C)
Lighthydrocarbons,
aromatics,
oxygenates

Primary

vapors

Olefins,
aromatics,
CO2, CO,
H2O, H2

PAN’s
H2
CO2,
CO,
H2O,
CH4

Biomass

H2
CO2,
CO,
H2O,

Primary
High pressure

CO, H2O,
CO2

Low pressure

Low pressure

High pressure
Condensed oils
(phenols,
aromatics)

Charcoal

Coke

Soot

Fig. 14.7 Gasification and pyrolysis reaction pathways (Milne et al., 1998).

in condensed and vapor phases, as shown in Fig. 14.7 (Milne et al., 1998). It is achieved
by reaching the materials at high temperatures (without combustion) with a controlled
amount of air, oxygen, and/or steam (Klass, 1998; Babu, 2005). The products can be
used as a fuel for gas engines and turbines or as a feedstock (syngas) for producing
chemicals (Amin, 2009).
On the other hand, anaerobic digestion (AD) is a technique to produce syngas from
wet microalgae feedstock. Thus, the cost associated with harvesting and drying steps
is omitted (Kwietniewska and Tys, 2014). In one research, Caporgno et al., 2016 studied a novel pretreatment for producing methane from microalgae by using industrial
solvents, before the anaerobic digestion process. The results indicated that the
pretreatment causes the cell wall to become considerably more susceptible to the
microorganisms attack during anaerobic digestion. In addition (He et al., 2016),
enhanced methane production from microalgal biomass is achieved by breaking up
the tough and rigid cell walls in the anaerobic biopretreatment step.

14.4.3 Bio-hydrogen
Hydrogen is a superior fuel for producing electricity (Gimpel et al., 2013) or used as a

fuel directly in polymer electrolyte membrane (PEM) fuel cells (Huber et al., 2004)
under sulfur starvation. The hydrogen gas is generated in the green microalgae cells
through photosynthetic metabolism (Hemschemeier et al., 2009). Generally, it is produced by two different methods, that is, biophotolysis and catabolism. Biophotolysis


Integration of microalgae into an existing biofuel industry

503

occurs by shedding light to the microalgae, which leads to dissociation of water into
molecular hydrogen and oxygen. This is while in catabolism, electrons are derived
from endogenous substrates (Pandey et al., 2013). The mentioned biological processes, among other conventional (chemical or physical) hydrogen production
methods, operate at ambient pressure and temperature, without having to use precious
metals as catalyst (Eroglu and Melis, 2016).

14.4.4 Bio-ethanol and bio-butanol
Bioethanol, which is one of the biofuels, is generally produced by the fermentation
of carbohydrates (Saı¨dane-Bchir et al., 2016). It can be used directly as a fuel or
blended with gasoline (10% ethanol and 90% gasoline) or water (i.e., hydrous
ethanol with 95.5% ethanol and 4.5% water) (Huber et al., 2006). Continuous
bioethanol production is enhanced by combining sonication, heat, and enzymatic
hydrolysis (SHE) processes of mixed microalgal biomass in a fermentor (Hwang
et al., 2016).
Biobutanol is another economically feasible biofuel that contains 22% oxygen. It
is a cleaner burning fuel than ethanol and has a high potential to get the place of
ethanol as a gasoline additive ( Jones and Woods, 1986; Qureshi et al., 2010).
Moreover, it is less corrosive and hydroscopic and produced by acetone-butanolethanol (ABE) fermentation (D€
urre, 1998). Efremenko et al., 2012 produced
biobutanol by immobilized biocatalyst based on one species of microalgae cells
(i.e., Clostridium acetobutylicum) for the first time, which was more efficient than

glucose biomass. Furthermore, Wang et al., 2016 successfully produced biobutanol
from carbohydrate-rich microalgae via a novel sequential alkali and acidic
pretreatment method.

14.4.5 Bio-oil
Bio-oil is a dark-brown, free-flowing, and viscous liquid with a smoky odor. Bio-oil
is distinctly different from similar petroleum products and requires upgrading for
fuel application because it contains very high oxygen levels (Saber et al., 2016).
Pyrolysis and hydrothermal liquefaction are two major processes for bio-oil production from microalgae. Pyrolysis requires a dry biomass, while hydrothermal
liquefaction is suitable for a wet feedstock. Pyrolysis is a thermochemical decomposition of biomass to produce solid, liquid, and gaseous species at temperature
range of about 350°C–700°C in the absence of oxygen. Based on the operating
conditions like the temperature, residence time, and heating rate, pyrolysis can be
categorized into slow pyrolysis, fast pyrolysis, and flash pyrolysis (Marcilla
et al., 2013). High-yield bio-oil (up to 80 wt% of dry mass) is caused by fast and
flash pyrolysis at about 500°C and few seconds of residence time (Huber et al.,
2006) in the presence of different catalysts such as HZSM-5, ZSM-5, Co/Al2O3,
Ni/Al2O3, γ-Al2O3, and nickel phosphide (Le et al., 2014; Liu et al., 2014).
Fig. 14.8 shows a typical fast pyrolysis system.


504

Bioenergy Systems for the Future

Gas
outlet

Biomass
Dryer
Heat for drying

Gas outlet

Cyclone
Gas-liquid
separator and
condenser

Pyrolysis
reactor
Grinder
Char

Bio-oil
Heat for
pyrolysis
Fluidizing gas

Fig. 14.8 Fast pyrolysis process principles (Bridgwater and Peacocke, 2000).

Hydrothermal liquefaction (HTL) is a reaction of biomass with water at
200°C–400°C in high pressures (5–20 MPa) (Xiu and Shahbazi, 2012). Using catalysts like Na2CO3, CH3COOH, KOH, HCOOH, NiO, Ca3 (PO4)2, H2SO4, and zeolite
usually increases the bio-oil yield (Zou et al., 2009; Ross et al., 2010; Jena et al.,
2012). As energetic point of view, HTL seems to be the most favorable process, because
of using wet microalgae as a feedstock. Additionally, experimental results reported by
Huang et al., 2016 showed that two-step sequential HTL process improves bio-oil quality and optimizes this process. Due to some significant problems such as poor volatility,
coking, high viscosity, corrosiveness, and cold flow problems, bio-oil is not usually
suitable for use directly as fuel (Czernik and Bridgwater, 2004). However, by upgrading,
it gains some industrial applications like combustion in boiler, burner, and furnace
systems and in diesel engines and turbines for heat and power generation, respectively

(Gust, 1997; Strenziok et al., 2001). Furthermore, it can be used as transportation fuel
(Wright et al., 2010), liquid smoke, and wood flavors or used for producing agrochemicals, fertilizers, emission control agents, acids, adhesives, and asphalt biobinder
(Czernik and Bridgwater, 2004; Mohan et al., 2006). Besides, bio-oil can directly be
used in a generator to produce electricity (Silva et al., 2016).

14.4.6 Bio-char
Biochar is produced via pyrolysis process (Bird et al., 2011) during bio-oil production and can be converted into H2 or syngas by steam reforming or gasification
(Chaudhari et al., 2001, 2003). Moreover, it can be burned as a solid fuel


Integration of microalgae into an existing biofuel industry

505

(Huber et al., 2006). Chaiwong et al., 2013 found that the suitable temperature to
obtain biochar is $500°C. Generally, slow pyrolysis produces larger amount of char
than fast and flash pyrolysis.
Different operational conditions (e.g., temperature, pressure, type of reactor, and
catalysts) and the type of feedstock for producing various biofuels including bioethanol, biomethanol, biobutanol, biohydrogen, bio-oil, biochar, and biogas (hydrogen and methane) via various upgrading methods such as transesterification,
fermentation, liquefaction, pyrolysis, and anaerobic digestion are shown in
Table 14.9.

14.5

Conclusion

Microalgae (algae with 1–50 μ m in diameter) are considered as sustainable alternative energy source of fossil fuels. They are technically feasible, easily available, and
able to compete economically with fossil fuels. Being one of the fastest growing plants
in the world with great variety of colors (green, yellow-green, blue-green, goldenbrown, red, and diatoms), microalgae are well adapted to different habitats such as
marine, freshwater, moist soil, terrestrial environments, or even on wastewater and

sewage.
Microalgae, with high content of lipid and the least land usage, are used to produce
different types of biofuels such as biodiesel, biosyngas, biohydrogen, bioethanol, biobutanol, biomethanol, bio-oil, and biochar. Based on the reported data, their oil content can exceed up to 80% by weight of dry biomass. Sunlight, CO2 supply, pH, light
intensity, temperature, nutrients (carbon, nitrogen, sulfur iron, phosphate, and in some
cases silicon), salinity, and dissolved oxygen are the main factors affecting the growth
and oil content of microalgae.
Before producing some kind of biofuels, it is necessary to extract the oil content.
At first, two applicable methods (open systems and closed systems) are used to cultivate microalgae. Then, several processes including filtration, sedimentation, flocculation, ultrasound flotation, and centrifugation are applied to harvest the
microalgae. After performing dehydration process, microalgae cells are disrupted
by physical, mechanical, chemical, and biological techniques. Finally, to extract
the oil content, physical techniques, solvent extraction, or supercritical fluids are
utilized. Finally, by transesterification or alcoholysis processes, biodiesel is produced. Besides, syngas can be produced during gasification, anaerobic digestion,
or fermentation processes.
Pyrolysis and hydrothermal liquefaction are two major processes for bio-oil production from microalgae biomass. Pyrolysis requires a dry biomass, while hydrothermal liquefaction is suitable for a wet feedstock. Moreover, biochar can be produced
via pyrolysis process during bio-oil production and converted into H2 or syngas by
steam reforming or gasification. Producing hydrogen gas is also achievable in the
green microalgae cells through photosynthetic metabolism. With this huge amount
of products gained from microalgae, it seems that they will play an important role
in future biofuel industries.