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INTERNATIONAL JOURNAL OF ENERGY RESEARCH
Int. J. Energy Res. (2010)
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/er.1695
REVIEW
A review on microalgae, a versatile source for sustainable
energy and materials
K. G. Satyanarayana1, A. B. Mariano2 and J. V. C. Vargas2,,y
1PIPE & Department of Chemistry, Federal University of Parana, Centro Politecnico, CP 19081, Jardim das Americas, CEP:81531-980,
Curitiba, Parana, Brazil2Nucleo de Pesquisa e Desenvolvimento de Energia Auto-Sustentavel-DEMEC-UFPR, Centro Politecnico, CP 19011, Jardim das
Americas, CEP:81531-980, Curitiba, Parana, Brazil
SUMMARY
Increasing energy demands, predicted fossil fuels shortage in the near future, and environmental concerns due tothe production of greenhouse gas carbon dioxide on their combustion have motivated the search for alternativeclean energy sources. Among many resources for this, microalgae have been found to be most promising due totheir high production capacity of vegetable oils. They possess a high growth rate, need abundantly available solarlight and CO2, and thus are more photosynthetically efficient than oil crops. Also, they tolerate high concentrationof salts allowing the use of any type of water for the agriculture and the possibility of production using innovativecompact photobioreactors. In addition, microalgae are a potential source of biomass, which may have greatbiodiversity and consequent variability in their biochemical composition. This paper presents an overview onmicroalgae with particular emphasis as a source for energy (biofuel/electricity) and new materials. Criticalissues involved in production of microalgae and their use, future R & D to overcome these, including the workinitiated by the authors at Federal University of Parana , UFPR, in Brazil are discussed. Copyright r 2010 JohnWiley & Sons, Ltd.
KEY WORDS
biodiesel; biogas; biomass; microalgae; photobioreactor; biodigester; sustainable energy
Correspondence*J. V. C. Vargas, Nucleo de Pesquisa e Desenvolvimento de Energia Auto-Sustentavel-DEMEC-UFPR, Centro Politecnico, CP 19011,
Jardim das Americas, CEP:81531-980, Curitiba, Parana, Brazil.yE-mail: [email protected]
Contract/grant sponsor: CNPq; contract/grant number: 552867/2007-1,574759/2008-5
Contract/grant sponsor: Araucaria Foundation of Parana; contract/grant number: 13470
Received 16 August 2009; Revised 8 January 2010; Accepted 11 January 2010
1. INTRODUCTION
Continuously growing population has led to increasingenergy demands all over the world. The reported
current consumption of petroleum is at 105 times faster
than nature can create [1]. These facts, along with the
limited resources of oil reserves (stocks of fossil fuels)
and its use contributing to the increase of atmospheric
CO2 resulting in global warming [2] are currently
recognized as great threats to mankind. Hence, both
the demanding energy requirement and the ecological
considerations have led to finding substitutes for the
fossil fuels by other resources including renewable
sources derived from biologically based fuels such as
biomass and biofuels (biocombustible, methane and
ethane) [3], which have attracted increased attention asevident from the growing literature [270]. Biofuels have
become increasingly necessary for the global fuel market
[29] with the reported annual estimated world raw
biomass energy potential in 2050 to be 150450 EJ
(E51018) leading to higher net farm incomes [19]. Also,
biomass energy meets the increasing demands in various
countries with Brazil between 23 and 30%, Finland
20.4% and Sweden 17.5% [19]. Such a fuel should also
be biodegradable and non-toxic [16]. Biofuel is a fossil
fuel replacement that is produced from vegetable oils,
Copyrightr 2010 John Wiley & Sons, Ltd.
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recycled cooking fats or waste oils, animal fats, or
microalgal lipids [29] and is known to mankind even
from ancient days [37]. Table I lists some of the
renewable sources of biodiesel including microalgae.
Those oils are renewable since plants and microalgae
produce oils from sunlight and air, and can do so year
after year on cropland [29]. Substitution of the diesel
used in the transport sector by the biodiesel producedstarting from cultivated plants would need the use of
massive lands that are presently used to produce food
[27]. A plant with the largest oil production for culti-
vated area is the Palm [2,38]. Even with the use of palm
oil to produce sufficient amount of biodiesel just to
meet the half of the demand of fuel for transport in the
United States would require about 24% cultivable area
in that country [2]. Currently, the fiscal incentives of-
fered by several governments all over the world for the
production of biofuels from renewable resources are
contributing for the decrease of the land for the pro-
duction of food, which result in higher land costs.
Also, biodiesel derived from oil crops (e.g. soybean,palm) cannot realistically meet existing costs of higher
fraction of the raw materials and competitive demand
of the soil for their growth [29]. This is due to the cost
of raw material accounting 5085% for the total
production cost in the current technique of the
preparation of biodiesel from these sources [9].
Therefore, the cost of material is the dominant factor
in fixing the price of biodiesel. Hence, there are nu-
merous criticisms for such promotion of lands for re-
newable source of energy [23] and also arguments for
and against the biofuels from microalgae and plant
resources [2,3,30,40,41].
One possibility to overcome the problem is the cul-
tivation of microalgae, which is biological fuel source[3] and seems to be a promising source for the pro-
duction of biofuels since they use carbon dioxide for
their energy in addition to sun light and carbon supply.
Also, they have higher photosynthetic efficiency than
terrestrial plants and are efficient carbon dioxide fixers
[3]. Therefore, higher biomass productions along with
faster growth rate over energy crops [15,16,29] are
observed. Additionally, high production of biomass
and some metabolites are achieved by their hetero-
trophic growth [4244]. Microalgae (Dunaliella tertio-
lecta), which was grown under highly saline conditions
produced about 36% oil [17]. Further, depending on its
capability of higher photosynthetic efficiency and other
characteristics mentioned above, microalgae wouldhave cost advantage. One reason for this could be that
the oil content of several microalgae species might
reach up to 80% of its dry weight and their pro-
ductivity can be enhanced by genetic manipulations
making microalgal biodiesel economically competitive
with petrodiesel through large-scale production of ge-
netic microalgal biomass [29].
The utilization of microalgae as a renewable source
for obtaining fuel was an old concept proposed in fif-
ties with follow up in sixties and seventies particularly
for producing biogas [32] and later reported for liquid
fuel in eighties and nineties [2,3], but received increased
attention in recent times due to the reasons mentionedearlier (increasing petroleum and ecological con-
siderations) [12] Also, this seems to be the ideal solu-
tion for total substitution of the diesel used in the
transport [2].
Considering the above facts, this paper gives an
overview on microalgae with particular emphasis as a
source for energy (electricity) and new materials. Pro-
duction of microalgae, their characteristics and appli-
cations in various areas are presented. Perspectives for
microalgae including the work initiated by the authors
at Federal University of Parana (UFPR) are also given.
Therefore, in order to complete this review study, the
application of the ideas collected in the literature review
is also included with a brief description and status of anongoing project by the authors. The main objective is to
provide the reader with an assessment of the feasibility
of innovative microalgae biomass-based projects.
2. MICROALGAE
The microalgae (Figure 1) [71], one of the oldest living
organisms, are the unicellular algae that exist indivi-
dually, or in chains or groups [29] that form the base of
the alimentary chain in the seas and rivers and they are
known as plankton. There are more than 105 types of
microalgae used to produce biodiesel only. Further,
they are known as essential components of coral reefs.
It is reported [32] that in addition to being exception-
ally diverse, they represent highly specialized group of
organisms, which can adapt to various ecological
habits. The current market for microalgae is in the
cosmetics, food industries and also in aquaculture [45].
In the cosmetic industry, the algae are marketed in
frozen condition and they supply the matter, which is
necessary for the preparation of anti-wrinkle cream
Table I. Oil yield of sources of biodiesel (adapted from
Chisti [2]).
So urce Yi eld o f oil (L h a1) Required land area (Mha)
Corn 172 1540Soyabean 446 594
Canola 1190 223
Jatropha 1892 140
Coconut 2689 99
Oil palm 5950 45
Microalgaey 70 405 7.6
Microalgaez 35 202 15.2
To meet 50% of all transport fuel needs of U.S.A.y40% oil (% dry wt) in biomass.z20% oil (% dry wt) in biomass.
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due to its great concentration in long chain fatty acids
with great regenerative capacities of the skin. That
differentiated composition of long chain fatty acids,
mainly the unsaturated ones such as Omega-3 and
Omega-6, besides the high concentration of proteins
and carbohydrates makes the microalgae as ideal
sources of nutrients for the preparation of functional
foods, food additive or even in nutraceuticals [46].
Microalgae, which are the microscopic organisms,
present possibility for higher oil production capacity
reaching up to 77% of their dry weight [2]. Table II
lists oil content of some of the microalgae.
Besides, microalgae possess a high growth rate with
duplicating the number of cells several times in a singleday [32,47]. The microalgae have advantages over the
cultivated plants such as faster growth, yield of high
amount of oils and possibility of the use of any kind of
water for their culture. They can also generate biomass
in suitable reactors [32,47] many more times per unit
area of land than growing agricultural crops that
double in size over several days or weeks, or trees that
grow on a timescale of years throughout the year. The
microalgae present a very simple cellular structure in
relation to the oil crops that have been supplying
transport systems. Depending on the species, their sizes
can range from a few micrometers (`m) to a few hun-
dreds of micrometers [29]. The accumulated chemical
energy after the photosynthesis process is not diverted
for the construction of complex structures allowing
this way, the best use for the production of new cells.
In addition, they are also potential source of biomass
or specific products (e.g. lipids, pigments, antioxidants)
[18]. Other advantages of microalgae for their becom-
ing as feedstock for biofuels and materials include [32]:
ability to synthesize and accumulate about 2050%
dry cell weight of neutral lipids/oil; use of waste land
(desert, arid and semi arid), which is unsuitable for
agriculture and use of N2 and P as nutrients from
different kinds of waste water sources.
2.1. Market and cost for microalgae and
different products produced by it
Estimated annual world production of biomass is
about 50007500 t [24,48] generating annual turnover
of about US$ 1.25 billion. Meanwhile the market for
microalgae to produce the biomass to be used mostlyin health food, animal feed and aquaculture is fast
growing with an estimated retail value of US$
30004000 million [49]. With an assumption of free
availability of CO2, estimated cost [2] for producing
microalgae by two different methods (raceway ponds
and photobioreactors with identical production capa-
cities of 100 000 kg) are US$ 3.80 and US$ 2.95 per kg,
respectively. These costs could be reduced by increas-
ing the production to 10 000 t. Based on this, cost of 1 l
of biofuel produced by the photobioreactor produced
biomass is estimated to be US$ 2.8 assuming 30% oil
content in the biomass, which is higher than that
produced by vegetable oil such as palm oil (US$ 0.66),
which in turn is 35% higher than the petrodiesel (US$
0.49), both of which are free of tax and transportation
charges and prices as existed in 2006 in USA. Other
cost comparisons of 1 l of petrodiesel and biodiesel-
based waste cooking oil reported are US$ 0.35 and
US$ 0.50, respectively [7]. The world sale of one of the
algae (Chlorella) used in human food, animal feed and
as food additive was higher than US$ 38 billion per
annum, while annual estimated market for docosahex-
aenoic acid, another nutritional supplement or used in
Figure 1. Scanning electron micrograph of a microalgae
(Chlorella) (Zhang et al. [[71] Reproduced with the kind
permission of the Springer Publishers]).
Table II. Oil content in some microalgae.
Species
Oil content
(% dry wt) Referen ce
Botryococcus braunii 2575 [2,36]
Chlorella sp. 2832 [2]
Chlorella emersonii 63 [34]
Chlorella minutissima 57 [34]
Chlorella protothecoides 23 [34]
Chlorella sorokiniana 22 [34]
Chlorella vulgaris 40, 56.6 [34]
Cylindrotheca 1637 [36]
Crypthecodinium cohnii 20 [36]
Dunaliella primolecta 23 [2]
Isochrysis sp. 2533 [2]
M. Subterraneus 39.3 [34]
Monallanthus salina 420 [2]
N. laevis 69.1 [34]
Nannochloris sp. 2035 [2]
Nitzchia sp. 4547 [2,36]
P.incisa 62 [34]
Phaeodactylum tricornutum 2030 [2]Schizochytrium sp. 5077 [2,36]
Tetraselmis sueica 1523 [2]
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aquaculture, produced by microalgae (Crypthecodi-
nium or Schizochytrium) is about US$ 10 billion [48].
Similarly, cost per kg of pigment carotenoids derived
from a microalgae (D. salina) used for human food and
animal feeds is reported to be US$ 3003000 while that
of another pigment, Astaxanthin (used in aquaculture)
is US$ 2500 with an estimated world market of US$
200 million [18,24], which was expected to increaseto US$ 257 million in 2008 [50]. The market for
b-carotene was expected to reach US$ 253 million in
2008, while market for lutein, which is the most
important carotenoid used in human foods and serum
was expected to reach US$187 million by the same time
[50]. Comparison of market and cost of various types
of high molecules derived from microalgae and their
producers are available [48]. Cost of biomass also
depends on the type and region where it is produced
[19]. For example, in USA, the cost of plant biomass is
US$ 515 per barrel of oil energy equivalent compared
with US$ 1139 for solid industrial residues and energy
crops (e.g. soybean, rapeseed), respectively.
2.2. Production of microalgae and photo-
bioreactor design
Microalgae exist in different atmospheres and a lot of
species tolerate high concentration of salts allowing the
use of any type of water for the cultivation medium
[17]. A traditional cultivation of microalgae generating
the biomass for human consumption and aquaculture is
the use of tanks or ponds [51]. This type of reactors
called Raceway ponds are normally open shallow
ponds or channel type systems [52]. A schematic view of
this type of reactor is shown in Figure 2(A). Also,
production through ponds requires large areas despitebeing cheap since it uses very low amount of CO2 of the
air and thus contaminates other organisms such as
mushrooms, bacteria and protozoa. They also show
low photosynthetic efficiency [52], due to low CO2 and
sunlight available only at the pond surface. Hence,
closed type photobioreactors have been proposed,
which not only possess higher photosynthetic efficiency,
but also temperature control of the culture medium,
since temperature normally increases with the exposure
to the sunlight [52], and allow for the use of external
contamination control. A schematic view of this type of
reactor is shown in Figure 2(B). Further, despite several
research efforts for the design and operation of manyphotobioreactors, devising and developing suitable
apparatus, cultivation procedures and algal strains
susceptible of undergoing substantial increases in
efficiency for the use of solar energy and carbon
dioxide is major challenge for the industrial microalgal
culturing. Accordingly, there is no best reactor system
to achieve maximum productivity with minimum
operation costs, irrespective of the available biological
and chemical systems [14]. Accordingly, choice of the
most suitable system is situation-dependent, dictated by
both the available species of algae and the final
intended purpose. The need of accurate control impairs
the use of open-system configurations, so focus hasshifted mostly on closed systems.
Design and operation of the microalgal biomass
production systems have been discussed extensively
[24,7,14,15,18,28,43,44,47,5155,58] with a recent re-
view comprehensively presenting several types of
closed bioreactors for the production of microalgae
based on transport phenomena and process engineer-
ing methodological approaches [14].
Photobioreactors are closed systems that allow the
cultivation of single-species culture of microalgae [2].
The photobioreactors have minimum contamination
while having the advantage of using the solar light and
higher amount of CO2 [2,35,58]. It should be noted that
the objective of the photobioreactor photosyntheticproduction process of microalgal biomass is to obtain
simultaneously the reduction of input energy and the
achievement of high photosynthetic production [52].
Also, these closed photobioreactors may be located
indoors or outdoors, although outdoor location is more
common due to the ease of using free sunlight. Compa-
rison of performance of bioreactors can be done for
fixed time [3] by their volumetric productivity (biomass/
volume) or areal productivity (biomass/occupied area)
or productivity/unit illuminated (biomass) surface.
These productivities vary with type of the system.
For example, the productivity (mg L1 d1) values of
370, 400700 and 900 have been recorded for tubular,
shallow and coiled outdoor tubular ponds, respectively,
compared with 510mg L1 d1 obtained for the indoor
reactor. Table III compares some of the variables/
parameters of two types of bioreactors [2,50]. More
details on these can be seen in the references given in
Table III.
Although microalgae production efficiency is often
mentioned in the literature [272], no consensus was
observed on how to calculate it. Therefore, a definition
for microalgae production efficiency based on theFigure 2. Diagram of raceway ponds (A) and tubular photo-
bioreactors (B).
A review on microalgaeK. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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products/conversion efficiency definition for fluidized
catalytic cracking reactors in the oil refining industry
[73,74] is suggested as follows:
Yi _mdb;out
_mT;in1
where subscript i accounts for the microalgae growth
process type (control volume) through which the mi-
croalgae biomass was produced (e.g. open pond,
photobioreactor); _mdb;out is the microalgae dry biomass
output mass flow rate [kg d1], and _mT;in is the total
mass flow rate of all substances that enter the defined
control volume (process), including CO2, feedback
water, nutrients and others [kg d1].
Generally, in any photobioreactor design, the system
productivity in continuous operating mode is obtained
by multiplying the steady-state biomass concentrationby the dilution rate used. These are related to the
average irradiance inside the photobioreactor, which in
turn is a function of the irradiance on the reactor
surface, operational variables such as fluid-dynamics
and dilution rate along with the pigment content
[32,47,53,57,58].
Of several geometries of photobioreactors (helical,
vertical and horizontal), the most efficient one is re-
ported [2] to be tubular type, which should maximize
the use of solar light, to avoid large areas of shade and
facilitate the diffusion of CO2 along with the control of
temperature. The microalgae are maintained in circu-
lation with turbulent flow to avoid the sedimentation
and to reduce deposit in the walls of the tubes [2].
Further, time-dependent changes in the culture
medium temperature in every season have been pre-
dicted [52] using a heat balance model of the conical
helical tubular photobioreactor previously established
[4]. Using these results, the energy required to maintain
the temperature of culture medium within an appro-
priate range and the maximum and minimum culture
medium temperatures have been predicted for several
sites with different climate characteristics. This helps to
examine the possibilities for the combinations of the
microalgae used for practically higher photosynthetic
production of microalgal biomass, with less operating
energy consumption throughout the year at various
sites. A large difference in photosynthetic productivity
was caused by the difference in ambient temperature in
each site, if temperature control of the culture medium
was not maintained. This helped to get practically
higher photosynthetic production with less operating
energy consumption throughout the year, using a
combination of various strains that had different
characteristics relative to temperature. Figure 3 shows
the effect of high sunlight intensity on specific growth
rate of microalgae [2].
Studies have also been carried out to find the influ-
ence of various reactor operating conditions such as
temperature, solar irradiance and air flow rate on the
yield of the culture. In one such study, biomass pro-
ductivities up to 1.5 g L1 per day are reported with
photosynthetic efficiency up to 14% by maintaining
the cultures below 30.81C, dissolved oxygen levels less
Table III. Comparison of some parameters of two types of bioreactors (adapted from Chisti [2] and Del Campo et al. [50]).
Parameter
Open system
raceway pond
Closed system
photobioreactor Reference
Area needed (m2) 7828 5681 [2]
Annual biomass production(kg) 100 000 100 000 [2]
Volumetric Productivity (kg m3 d1) 0.117 1.535 [2]
Oil Yield (m3ha1) 56.8 78.2 [2]
Contamination control Difficult Easy [50]
Operation regime Batch or
semi-continuous
Continuous [50]
Area/volume ratio Low High [50]
Light utilization efficiency Poor Excellent [50]
Process control Difficult Easy [50]
Scale up Difficult Easy [50]
Based on 40 % dry wt oil in biomass.
Figure 3. Microalgae specific growth rate as a function of
sunlight intensity [[2] Reproduced with the kind permission of
the Elsevier Publishers].
A review on microalgae K. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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than 400% saturation (with respect to air saturated
culture) while controlling the cell density. This has led
to achieve an average irradiance within the culture less
than 250mE m2 s
1 [58].
Production of microalgal biomass may be improved
by specific cultivated conditions such as mixotrophic
and heterotrophic cultivation [58,69]. It is also de-
monstrated that [22,33,56,59] variations in the cultiva-tion conditions such as temperature, concentration of
salts, nitrogen and CO2, for instance, interfere directly
in the biochemical composition of the microalgae.
A modular concept of photobioreactors is expected to
allow verification of these different parameters to ob-
tain high amount of lipid and biomass production.
For using the microalgal fuel to produce electricity
(using stationary diesel engines), microalgae of lipid
content should be grown in large quantities with high
productivity (1.5g L1 d1) [2]. There are a large num-
ber of systems available for the mass cultivation of algae
[42,43] with outdoor systems (open ponds and raceway
types) [3] and closed bioreactors consisting of thin pa-nels or tubes laid horizontally [3]. Growth rate of mi-
croalgae depends on type of pumps used to circulate the
culture. For example, centrifugal and rolling pumps
damaged the algae, while a diaphragm pump showed
very little effect on the growth rate of S. platentis [3].
Advantages of a closed system particularly with he-
lical type are (i) increase in incidence of light energy per
unit volume and reduction of self shading due to the
large surface area to volume of the bioreactor, (ii) easy
control of temperature and contaminants and (iii) ex-
tensive pathways for CO2 absorption leading to better
CO2 transfer from gaseous stage to liquid stage [58].
Further, in recent times, based on increasing focus on
biotechnological potential of microalgae due mainly tothe identification of several substances synthesized by
these organisms, commercial scale production of mi-
croalgae has been drawing the attention of the scientific
community [38]. In fact, the great biodiversity and
consequent variability in the biochemical composition
of the biomass obtained from various microalgal cul-
tures, which can be subjected to genetic improvement
and for massive production possibilities have allowed
various species to be commercially cultivated. Thus, the
biomass production not only for use in the food ela-
boration but also for obtaining natural compounds
with high value in the world market have aimed at
developing microalgae cultivations on large scale [38].
Figure 4 illustrates an innovative integrated multi-
disciplinary process. The first step consists of CO2capturing by the photobioreactors followed by algae
growth in the presence of sunlight, biomass production
and possibilities to produce various useful products,
including CO2 fixation by microalgae and production
of biohydrogen [24].
The microalgae are then transferred to a separate
photobioreactor to produce H2 using energy by a
biophotolytic process without the use of sulfur. Then,
the nutrient-rich algal biomass is collected, which may
be used for different purposes such as health food for
human consumption, as animal feed or in aquaculture.
When the nutrient level goes below the limit for such
applications, the algal biomass may contain large
amounts of valuable biomolecules, which may be of
small percent of the biomass. They can be extracted for
pharmaceutical or industrial retail. The remainingbiomass will still contain good amount of the fixed
CO2. Hence, the residual algal biomass from different
process stages can be used as a fertilizer for agriculture,
wherein retention of fixed carbon for some years is
possible. Otherwise, the fixed CO2 may be stored by
industrial applications like production of plastics.
There is a possibility either to extract biodiesel from
the residual biomass (energy carrier) or its direct con-
version into other energy carriers using biological or
thermo-chemical methods.
2.3. Biodiesel production
The biodiesel consists of a biodegradable fuel pro-
duced from renewable sources. The synthesis of this
fuel can be accomplished by methodologies such as
cracking, esterification or transesterification using
animal fat or vegetable oils. Table IV shows a
comparison of characteristics of biofuels and petro-
diesel along with ASTM biodiesel standard [15,29].
The methodology mostly used for biodiesel pro-
duction is based on the transesterification reaction, as
follows:
(2)
The transesterification reaction, as stated by
Equation (2), takes place in the presence of either
homogeneous or heterogeneous catalysts (traditional
method). Those alternatives can be compared in search
for the most efficient method of biodiesel production
from microalgae lipids.
2.4. Chemical composition of microalgae
As the microalgae do not possess specialized structures,
except for the presence of the pigments and photo-
synthetizers, their composition basically consists of
carbohydrates, proteins and lipids. They are also
sources for almost all types of essential vitamins
(e.g. A, B1, B2, B6, C, E) although environmental
factors, harvesting treatment and cell drying method
determine their quantity [49]. Table V lists the chemical
composition of some microalgae, which are used to
produce food, cosmetics, big molecules and biofuels.
While Table V(A) compares the composition of some
microalgae with those of other food sources, Table V(B)
A review on microalgaeK. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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lists some microalgae products used in cosmetics.
Table V(C) summarizes the composition of some
microalgae that are of interest for biofuel production.
Therefore, Table V shows that most of the microalgae
have high protein content, particularly those ones that
are used as food sources. Also, carbohydrates are found
mainly as starch, glucose and other polysaccharides
whose digestibility being high, can be used in dry form
without any limitation [49]. Further, lipid content for use
in food varies between 1 and 35% while that for biofuels
lies between 20 and 80% [29,49] compared with 1530%
in vegetable oils, all on dry weight basis.
In order to use microalgae as a fuel, the algae should
be of high calorific value and must be capable of
growing in large volumes. Main contribution to the
calorific value of cells is from their carbohydrate,
protein and lipid content [3]. Microalgae grown under
normal conditions possess calorific values in the range
Table IV. Comparison of properties of biodiesel from microalgal oil, biodiesel fuel and ASTM biodiesel standard (adapted from Miao
and Wu [15]).
Properties
Biodiesel from
microalgal oil Biodiesel fuel
ASTM biodiesel
standard
Density (kgL1) 0.864 0.838 0.860.9
Viscosity (mm2 s1, cSt at 401C) 5.2 1.94.1 3.55.0
Flash point (1C) 115 75 Min 100
Solidifying point (1C) 12 50 to 10
Cold filter plugging point (1C) 11 3.0 (Max 6.7) Summer max 0;
winter maxp15
Acid value (mgKOH g1) 0.374 Max 0.5 Max 0.5
Heating value (MJ kg1) 41 4045
H/C ratio 1.81 1.81
Figure 4. Schematics of an innovative integrated multidisciplinary process [[24] Reproduced with the kind permission of the Elsevier
Publishers].
A review on microalgae K. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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of 1821 kJg1, while the value for petrodiesel is
42kJg1. Some microalgae such as Chlorella vulgaris
and C. emersonii have been shown to grow in a 230 L
pumped tubular photobioreactor in Watanabes med-
ium and a low nitrogen medium. While Chlorella species
can accumulate up to 58% lipid under low nitrogen
conditions [3], the Chlorella emersonii accumulates 63%
lipids in small (2 L) stirred-tank bioreactors, which re-
sulted in 29kJg1 of calorific value [4] although the
growth, productivity and lipid accumulation are yet to
be determined at a larger scale. It is found [3] that this
low nitrogen medium induces higher lipid accumulation
in both algae, which increased their calorific value; the
highest calorific value of 28 kJ g1 was obtained with C.
vulgaris with the biomass productivity of 24 mg dry
wt L1 d1 which was lower than that obtained with
Watanabes medium (40 mg dry wt L1 d1).
3. CHARACTERISTICS OFMICROALGAE
In this section, some characteristics of the microalgae
that have made them to be most promising source
Table V. (A) Chemical composition of some food source microalgae compared with other human food sources (% of dry matter)
(adapted from Miao and Wu [15]), (B) Some of microalgae products with applications in cosmetics (adapted from Derner et al. [38])
and (C) Chemical composition of biofuel source microalgae.
Source Carbohydrates (%) Proteins (%) Lipids (%)
(A)
Anabaena cylindrica 2530 4356 47
Chalmydomonas rheinhardii 17 48 21
Chlorella vulgaris 1217 5158 1422
Dunaliella salina 32 57 6
Porphyidium Cruentum 4057 2839 914
Spirulina maxima 1316 6071 67
Bakers yeast 38 39 1
Meat 1 43 34
Milk 38 26 28
Rice 77 8 2
Soya bean 30 37 20
(B)
Products
b-carotene
Vitamin C and E
Arachidonic acidARA
Eicosapentaenoic acidEPA
Starch
Poly-b-hydroxylbutyric acidPHB
Peptides
(C)
Microalgae species Carbohydrates (%) Proteins (%) Lipids (%) Reference
Chaetoceros muelleri 1119 4465 2244 [33]
[59]
Chaetoceros calcitrans 10% 58% 30 [59]
Isochrysis galbana 725 3045 2330 [61]
[62]Chlorella sp. 3840 1218 2832 [2]
Chlorella protothecoides 10.6215.43 10.2852.64 14.5755.20 [19]
Nannochloropsis sp. n.a. n.d. 3168 [2]
Neochloris oleoabundans n.a. n.d. 3554 [2]
Schizochytrium sp. n.a. n.d. 5077 [2]
Scenedesmus obliquus 1017 5056 1214 [y]
Quadricauda de Scenedesmus 47 1.9 [y]
n.a.not available.Values are for two types of reactors used and not the range.yHomero E Ban ados. Biodiesel de microalgas: part 1. (2007Unpublished).
A review on microalgaeK. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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compared with other resources of biofuels are hereby
recalled: (i) high production capacity of oils; (ii) high
growth rate and (iii) requirement of abundantly
available solar light and CO2, that make them more
photosynthetically efficient than oil crops. They are
tolerant to high concentration of salts allowing the use
of any type of water (fresh, brackish, highly saline and
marine) for the agriculture and possibility to producethem by photobioreactors [24,18,24,35,52,53,55,56,58].
In addition microalgae are a potential source of
biomass, which may have great biodiversity and
consequent variability in their biochemical composition
[38]. Their size (mean diameter in mm) such as of
Chlorella is about 510mm, similar to that of powdered
coal and cellulose to a few hundreds of micrpmetre.
Quantity of the oil produced by these depends mainly
on their lipid content. Higher lipid content gives higher
calorific value of the fuel produced by the microalgae
[4]. Some microalgae such as cyanobacteria (also called
blue algae) may be cultivated without nutrients from
N2 or C and hence cost-effective as well as manageable.In view of microalgae having a greater capacity for
photosynthesis than plants they are capable of syn-
thesizing a number of valuable substances (e.g. health
foods, food supplement, food color, food for livestock,
feeds for bivalves) [15,52].
4. APPLICATIONS OF MICROALGAE
Since the first use of microalgae in China about 2000
years back and the first concept of microalgae for use
in the production of biogas in fifties, and later proposal
as a source of different types of fuel, namely liquid fuel(from Botyrycoccus sp.) [4], and ethanol and methanol
after degradation of the algae [26], also converting into
a gaseous fuel (methane) [28] as well as to produce
hydrogen [75], a number of application areas have been
identified [24,24,37,38,4850,54,5761,71,72,7679].
They include human and animal nutrition, cosmetics,
high-value molecules such as fatty acids and pigments
as well as natural dyes. In fact, interest in the
development of active biomolecules from microalgae
is rapidly growing [55]. Microalgae have been used to
fix CO2, and hence its growth can be linked to the
removal of carbon dioxide from industrial waste gases
(stack and exhaust gasses) [4], for wastewater treat-
ments, as animal food as human food or to produce
numerous high-value bioactives [57,58].The photosynthetic product (microalgal biomass)
can be used as livestock fodder and as forage crops
substitute [4], while attempts have been made to
develop composite materials using a microalgae
(Chlorella vulgaris) as filler in various polymers such as
polypropylene, PVC, polystyrene and polyethylene
[2,3,71,72,7779]. As these microalgae may putrefy and
decompose, resulting in the release of CO2 to the en-
vironment, they have been used by incorporating them
in polymer matrices up to 50 wt.% and the resulting
composites exhibited interesting tensile properties. For
example, PVC-Chlorella composite prepared by hot
molding process exhibited tensile strength (TS) be-tween 30 and 41 MPa and % elongation of 1.86 for
average particle sizes of 5110mm. With microalgae
content waso20 wt.%, while it was 415 MPa when its
content waso50 wt.%. These values were lower than
that of PVC matrix (TS: 50.4 and % elongation: 180).
Similarly, its composite with polyethylene showed
good thermal plasticity whereby it could be shaped
into plates and dishes [79].
Figure 5 shows scanning electron micrographs of
two polymer composites containing Chlorella micro-
algae [71,72]. Figure 5(A) is the surface of the PVC
composite revealing the reinforcements lying in the
matrix surrounded by air gaps, but without any
changes in their shape due to the processing whileFigure 5(B) is the fractograph of this composite
showing fracture of microalgae. One can see the
bonding between the matrix and the reinforcement
does not exist and hence no increase in the tensile
properties was observed. However, about 22% in-
crease in volume of the composite over the matrix was
observed suggesting the microalgae could be good filler
Figure 5. Scanning electron micrographs PVCChlorellacomposite: (A) Surface; (B) Fracture surface; and (C) Fracture of PEChlorella
composite [[71,72] Reproduced with the kind permission of the Springer and American Chemical Society Publishers].
A review on microalgae K. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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for polymers. On the other hand, composite of PE with
the same reinforcement showed improved TS and
modulus when the matrix was modified.
Figure 5(C) is the fractograph of maleic anhydride
modified polyethylene composite containing 40 wt.% of
the same reinforcement. Here also good bonding (che-
mical bonds formed between Chlorella grains and the
PE matrix) between the matrix and the reinforcement isevident from the fracture of the reinforcement itself and
also non-existence of any gap between the matrix and
the reinforcement, which exhibited TS twice that of a
composite with unmodified PE. Some of these proper-
ties suggest these composites can be used as substitutes
for rigid and plasticized PVC and similar products.
Table VI summarizes the products and applications
of microalgae as reported by Rozas and Belli [46]
(Table VI(A)), and effective applications of microalgae
studied by Usui and Ikenouchi [24] (Table VI(B)).
5. CRITICAL ISSUES INVOLVED INTHE PRODUCTION AND USE OFMICROALGAE AND FUTURE R & D
Owing to increasing applications of microalgae parti-
cularly for meeting the energy demands, despite its cost
disadvantage, there is a growing interest to develop
cost-effective processes and to enlarge their application
areas based on various advantages mentioned earlier.
These include bulk biological chemicals and rapidly
growing biofuel industries. There are also limited
reviews in recent years on the perspectives on different
aspects of microalgae including critical issues and
possible remedies [2,24,29,32,34,36,50,6567]. Some of
these are summarized below:
Isolation, culturing and characterization of microalgae:
While isolation and characterization from any uniqueenvironment have been ongoing processes, culturing
still remains a niche area needing continued R&D
efforts towards cost-effective technologies [2,32].
Research efforts towards additional organisms which
may possess unique mechanism for efficient production
of lipid/oil should continue, while innovative develop-
ment of large-scale culture systems through proper se-
lection of algal strains that lead to high and sustained
growth rates of oil-rich biomass should be looked into
[32]. Production of higher biomass yield through the use
of genetic engineering to increase the photosynthetic
efficiency or to produce higher yields of oil, stability of
such strains, identification of new strains capable offaster growth at high cell densities, increasing the
growth rate of biomass and its oil content, reduction of
photooxidation susceptibility which damages cells,
identification of factors including biochemical triggers
and environmental that enhances the oil content are
some of the issues needing greater attention [2,32,50].
Design Aspects of Photobioreactors: This aspect is
an important issue to achieve cost-effective
Table VI. (A) Various products from microalgae with their applications [38] and (B) Microalgae applications considered effective
[Adopted from [55]].
Product Application
(A)
Biomass Biomass Natural health food, Functional food, Food
Additives, Aquaculture
Carotenes and antioxidants Xantophils, lutein, b-carotene, vitamin C and E,
arachidonic acid, eicosapentaenoic acid
Food additives, cosmetics
Fatty acids Docosahexaenoic acid, g-linolenic acid, L dismutase
superoxide linolenic acid
Food additives
En zyme s Phos phog lyc erate qui nas e, l ucip herase and luc iph eri n,
restrictive enzymes, polysaccharides
Natural food, research and medicine
Polymers Starch, polyhydroxybutyrate (PHB), peptides, toxins Natural food, cosmetics and medicine
Special products Istotopes, aminoacids, steroids Research and medicine
(B)
Item Method and application
Fuel Extraction of carbohydrate
Direct liquefaction using coal liquefaction technology
Manure Compost
Animal feed Fodder or feed for domestic animals or fish cultivation
Building materials Plastic filler
Concrete additives for high efficiency concrete
Biodegradable plastic Plastic forming processes, biodegradable polymer
products including biodegradable composites
Physiologically active material Reformation of carbohydrates
A review on microalgaeK. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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photobioreactors with high efficiency, keeping in mind
that there is no such thing as the best reactor system
to achieve maximum productivity with minimum
operation costs. Efficiency of present day photo-
bioreactors lies between 5 and 10% [65], so that de-
pendence of efficiency with high irradiance intensity
and reactor size is one of the major issues. Particularly
the latter with the retention of high photosyntheticefficiency even at large sizes and at high light intensities
at longer periods should be looked into. Probably,
optimization of photobioreactor for various variables
through separation of reactor and collection system for
light may be one way. Then, performance of micro-
algae culture should be tested. As the efficiency of
photobioreactors depends on irradiance of light,
proper design for motionless mixers inside the reactors
should be thought off to obtain better mixing between
properly lit zone and dark zone in the reactor [2].
Further, process strategy may be changed to get in-
creasing yields at lower costs as reported in the efficient
production of astaxanthin-rich biomass using con-tinuous photo-autotrophic cultures [50].
Downstream processing: This is one of the major
issues in microalgal biotechnology, which includes se-
paration of biomass and concentration of microalgae
culture. Attempts should be made to develop cheaper
and energy conserving processing methods [32,53,58]
including genetic modification and engineering of algal
strains from dilute cultures with optimum photosynth-
esis and product formation [50,65]. Also, development
of economical, quick and efficient processes for har-
vesting and de-watering of biomass depending on the
end use is another area of interest for R&D [32,50,65].
Cost: High cost to produce microalgae, which leads
to high cost of microalgal fuel (biodiesel), is anotherissue. Some of the methods to offset that problem may
be by (i) resorting to production strategy to integrated
biorefinery, where useful products are produced using
every component of biomass [2], (ii) identifying high-
value products particularly big molecules type based on
specific microalgae used [2] or broadening commer-
cially viable product range such as nutraceuticals based
on highly productive heterotrophic type cultures [32,65]
and (iii) sale of generated excess power [2]. Also, pro-
cessing of biomass for oil, wherein lipid extraction is an
important step [32], is another issue to be looked into.
Environmental aspects: Although biofuel is con-
sidered environmentally less harmful than diesel fuel,
its eco-compatibility depends on method of its pro-
duction, use and trade [80]. These in turn determine its
economic, environmental and social aspects since not
much is reported on its eco-toxicological information
of non-regulated emissions, effluents generated during
its production and on its water-soluble fractions
(WSF). These factors have to be considered as they are
important to follow the precautionary principle pre-
scribed by law in many countries that have plans to
increase the production of biofuel, which may lead to
environmental risks by its use. Hence, there is need for
studies including modeling to look into toxic effects
caused by this fuel particularly the WSF of biofuels
although some attempts towards these have recently
initiated [80].
6. WORK AT UFPR
Increasing global demand for fuels from renewable
energy sources, with motivation by tax exemptions of
biofuels has triggered many initiatives in the federal
and private sectors aimed at producing biofuels,
particularly in Brazil, USA and Europe [23]. For
example, European production of biodiesel was
reported to have increased from about 1.9 billion liters
in 2004 to about 4.9 billion liters in 2006 [66], while
estimated annual production of biodiesel in Brazil is
about 176 million liters, which advocates the first use
of diesel with 2% biodiesel in the current year and then
of 5% by 20122013 [see: http://www.biodiesel.gov.br].It is also interesting to note other advantages accruing
from this, such as increased employment avenues and
useful co-products obtained during the processing of
this new fuel such as about 110 kg of crude glycerin
from 1 t of biodiesel [31]. These require new develop-
ments in technology of biofuels.
As a first example, in a recent work, Gravalos et al.
[81] showed that vegetable oils are one of the alter-
natives utilized by farmers, which can be used as fuel in
diesel engines either in the form of straight vegetable
oil or in the form of biodiesel. The study presented
experimental data by utilization of home and industrial
biodiesel as fuel in an agricultural tractor diesel engine.
The home biodiesel production was made from dif-ferent vegetable oils (crude rapeseed, edible sunflower
and waste oil) with the process of one-stage-based
catalyzed transesterification. According to the results,
agricultural tractor diesel engine operating on home
biodiesel fuels had better performance characteristics
related to industrially produced biodiesel and similar
to conventional diesel fuel.
Physical properties of biodiesel are another im-
portant issue, playing an important role in the injec-
tion, atomization and combustion performance.
A recent work investigated the spray properties of
biodiesel [82]. In sum, the results indicated that, on the
macroscopically view, the shape of biodiesel spray is
similar to that of diesel.
Next, in order to complete this review study, the
application of the ideas collected in the literature re-
view is illustrated in this section with a brief descrip-
tion of an ongoing project by the authors of this study
under implementation at Federal University of Parana,
Curitiba, Brazil along with status of each of the items.
The main objective is to provide the reader with an
assessment of the feasibility of innovative microalgae
biomass-based projects.
A review on microalgae K. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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Keeping in sight the current needs of technology
development in the area of biofuels elsewhere and va-
lue addition to renewable resources, the Center for
Research and Development of Sustainable Energy
(NPDEAS) was created at UFPR in Curitiba, Parana,
Brazil. The main objective of this center is to demon-
strate the concept of energy sustainable plants powered
by renewable fuels in a possible future scenario ofdistributed power generation. Specific objectives in-
clude: (i) design and fabrication of compact photo-
bioreactors for microalgae cultivation; (ii) obtaining
biodiesel and other co-products and possible uses of
byproducts particularly residues; (iii) mathematical
modeling, experimental validation and thermodynamic
optimization of the components and processes, as well
as of the entire system, and finally (iv) divulgation of
the results of the project, improvement of the current
technology of microalgae cultivation, evaluation of
system functionality and the possibility of general re-
plication.
6.1. Photobioreactor development
An important step of this project is the development
and improvement of compact photobioreactors for
microalgae cultivation. Aiming to reach compatible
efficiency with the target plant needs and taking into
account the various parameters that affect the effi-
ciency of the bioreactor, the photobioreactor design
has an innovative geometric conception. The objective
is to achieve high biomass productivity through the
best use of the solar light under a fixed volume
constraint. The volumetric productivity (kg m3 d1)
and surface (kg m2 d1) will be analyzed and com-
pared with data available in the literature for tradi-tional methods of microalgae cultivation in tubular/
helical photobioreactors and ponds.
With a view to have freedom to vary several para-
meters allowing for appropriate flexibility of the sys-
tem, the proposed photobioreactors will be built in a
modular way. The modular construction will make it
possible to study different types of microalgae in par-
allel, as well as comparison of different ways for their
cultivation. The possibility to alter several parameters
in parallel allows the determination of the best condi-
tions for the cultivation with the proposed specific
objective of obtaining great biomass production and
high amount of fat. Considering the effect of the use of
inoculum, the Integrated Group of Aquaculture and
Environmental Studies at UFPR will produce in-
oculum and supply the required amount of microalgae
for this project taking into account the minimum time
to get the maximum productivity in the photo-
bioreactor. The processes of cultivation of unicellular
organisms usually begin with the addition of a stan-
dardized amount of cells called inoculum. As it could
be presumed, the quality of the growth process is di-
rectly dependent on the quality of the inoculums.
Sometimes, smaller amounts of additions may result in
slow growths and consequently low productivity.
A new design conception for compact photo-bior-
eactors for the cultivation of microalgae has been de-
veloped. The photo-bioreactor design has the
innovation of the maximization of the cultivation and
sun exposed area in a given volume, by utilizing cir-
cular transparent polymeric staggered tubes (crystalPVC). The geometric conception is based on the
compact heat exchangers technology [83].
Although attempts have been made to achieve cost-
effective photo-bioreactors with high efficiency, not
much success has been reported and photo-bioreactor
technology is still in its early steps as discussed earlier
in the text. In that direction, Prakash et al. [84] de-
veloped a transient thermal analysis and estimated the
incident solar energy for two designs of tubular photo-
bioreactor installed outdoors arranged in one and two
planes, respectively. The model was validated by
comparing the experimental data and predicted values
of culture temperature. The performance of the twophotobioreactors for mass culture of Spirulina was also
studied with respect to their design and culture tem-
perature. Among several important conclusions, it is
interesting to note that the average biomass yield ob-
tained in one-plane and two-plane photobioreactors
were (dry weight) 23.7g m2 day1 and 27.8g
m2 day1, respectively, giving a clear indication that
superimposing planes could increase density produc-
tion, therefore exploring design compactness.
Figure 6 shows the flowchart of the proposed sus-
tainable energy plant with the details of the integration
of all engineering subsystems. The main components
are described in the next subsections.
6.1.1. Gasser/degasser system. During the photosynth-
esis process, conversion of CO2 and H2O in sugar
(glucose) will take place along with oxygen (O2)
release. Therefore, there is no limitation for the growth
of the microalgae provided that CO2 is injected during
the growth process through a gasser/degasser system.
Injected CO2 may be originating from the atmospheric
air or from external gas sources (exhaust gases from
thermal plants, motors or industrial processes) [63].
The larger the concentration of CO2 in the injected air,
the better will be for the microalgae. In this project,
exhaust gases from a biodiesel powered motogenerator
as source of CO2.
The biodiesel produced from the microalgae-ex-
tracted oil as well as gases generated in biodigester will
feed a motogenerator, a multifuel (biodiesel/biogas)
internal combustion engine. The hot exhaust gases
coming from the engine will be the heat source for an
absorption refrigerator in order to produce cooling for
utilization by the plant processes and climatization. In
this way, the motogenerator will have three functions,
i.e. as supplier of electrical energy, heating and cooling,
namely a trigeneration system. Additionally, the
A review on microalgaeK. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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DOI: 10.1002/er
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system will supply CO2 that will be directed to the
photobioreactors, acting as substrate for the micro-
algae in the photosynthesis process.Another function of the gasser/degasser system
is the removal of O2 generated by the photosynthesis.
The excess oxygen is poisonous to the algae because
it promotes oxidative stress leading to death of cells.
On the other hand, small amount of O2 leads to in-
hibition of the photosynthesis, therefore impeding
the microalgae growth. The modular concept of
gasser/degasser system allows the adaptation of the
photobioreactors to operate with: (a) any type of
microalgae, (b) appropriate growth rate and (c) avail-
ability of solar light during the year. Alteration of
any one of these parameters alters the use of CO2and consequently, the production of oxygen. To in-
crease the solubility of CO2 during microalgae growth,
the medium will be artificially cooled. The cold
water used in the process will be supplied by the
absorption refrigerator powered by the tri-generation
system.
6.1.2. Composition of the medium. The composition of
the medium of culture is defined by the microalgae and
the cultivation conditions. As mentioned earlier in the
text, microalgae are capable of growing in different
ways including waters with high concentration of
pollutants such as in ponds and in treated industrial
wastewater. Hence, the project predicts the use ofdifferent types of water.
The utilization of sea and fresh water in the photo-
bioreactors are discussed separately, as follows:
6.1.2.1. Sea water. The use of algae that grow in
seawater allows for a biodiesel production that does
not compete with food-oriented agriculture since it
uses water that is not used for irrigation. The use of
such a system is best suited for coastal areas where
seawater is cheaply available and the production of a
residue containing high amounts of salt does not
matter. The seawater possesses mainly sodium chloride
with an average mass concentration of approximately
3 5 g l1. Besides CO2 and the sunlight, these organisms
need several other ions that are present in the sea. With
the objective of obtaining the best microalgae growth
conditions, besides seawater, other low-cost additives
are needed such as those used in agriculture, i.e. urea
(source of nitrogen) and superphosphate (source of
phosphate).
6.1.2.2. Fresh water. At places distant from the coast,
use of salty water becomes somewhat unfeasible for the
Figure 6. Flowchart of the sustainable energy plant under construction at UFPR, Curitiba, Brazil.
A review on microalgae K. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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DOI: 10.1002/er
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cultivation of sea microalgae in view of the cost
involved for the transport of seawater or sea salt.
Additionally, a problem would be created with the
production of residues with high amount of salt.
The cultivation of algae with fresh water consumes
higher amounts of water than with seawater initially.
After the establishment of the culture, the amount of
water eliminated by evaporation in the gasser/degasser
system has to be replaced. This water can be recycled in
the system with the need for addition of the elements
consumed by the algae. The cultivation medium in
fresh water also needs the addition of nitrogen, phos-
phate and some of the ions used by the metabolism of
the microalgae.
6.2. Biodiesel production
The project will use oil extracted from microalgae
biomass cultivated in compact tubular photobioreac-
tors to produce biodiesel and possibly other valuable
products, as discussed earlier in the text. Next, thesubcomponents utilized for biodiesel production ac-
cording to the flowchart shown in Figure 6 are
discussed.
6.2.1. Unit operations. A fundamental point in the
biodiesel production from microalgae consists of
the choice of the methodologies to be used in the
separation and drying of the microalgae biomass and
also in the oil extraction process. Methodologies that
are high energy consuming make the biodiesel produc-
tion process commercially unattractive. Further, since
the project is aimed at developing sustainable electric
power plants, it is mandatory to minimize energy
consumption at every stage of the biodiesel productionprocess, as follows.
6.2.1.1. Separation and drying of the microalgae. Dif-
ferent methodologies such as flocculation, centrifuga-
tion or filtration can be used for the separation of the
microalgae biomass from the culture medium.
After the separation, drying of the material takes
place through sun drying, liofilization, spray-drying or
even competitive flow, since the possibility of free
heating exists by burning natural gas produced in the
biodigester, as shown in Figure 6 which uses residues
generated by the system.
The choice of the methodologies to be used in the
microalgae biomass separation and drying will be
based on efficiency and cost, whichever produces the
most favorable results in operation.
The culture medium can be recycled after the re-
moval of the algae taking into account the necessary
corrections for consumed nutrients and also elimina-
tion of possible chemical or biological pollutants as it
has been reported by Hu et al. [32].
A study was developed by the UFPR team [85]
with the purpose of demonstrating the efficiency of the
increase in pH of a Nannochloropsis oculata microalgae
solution, by sodium hydroxide (NaOH) addition, to
the flocculation, making possible a simple harvesting
of the cells. The results suggest that increase of pH of
the culture broth is a suitable methodology for mi-
croalgae separation from the growth solution. Similar
results were obtained with cultures of Phaeodactylum
tricornutum.Another work by the UFPR team [86] was con-
ducted to produce dry Nannochloropsis microalgae
biomass with a spray dryer system before oil separa-
tion and biodiesel production. With this process it was
obtained, in the end, powdered N. oculata biomass,
which was submitted to lipid extraction with solvent,
and thereafter to biodiesel synthesis. The experimental
results suggest that the proposed process is a suitable
and low energy consumption methodology for micro-
algae drying.
6.2.1.2. Oil extraction and biodiesel. Separation meth-
ods used in plant oils can be used for microalgae. Thetechniques commonly used include: pressing, solvent
extraction and supercritical extraction [68]. Each one
of these processes requires energy and gives different
yield. With a view to get almost 100% yield, a
combination of two techniques can be used such as
pressing and solvent extraction. However, new ap-
proaches in the extraction of microalgae lipids are
necessary so that the total cost of biodiesel production
will become commercially competitive. Accordingly, it
is planned to have new approaches in this work for the
separation of microalgae lipids. These studies will be
taken up after the start of the photobioreactor
operation and consequently after the production of
biomass.It is proposed to synthesize the biodiesel using the
microalgae produced by the photobioreactors by em-
ploying cracking, esterification or transesterification
normally used with animal fat and vegetable oil. Also,
innovative chemical in situ transesterification process
will be proposed and tested in two different ways. In
the first, simultaneous extraction and transesterifica-
tion is being carried out with new catalysts by the
UFPR team [87]. The work consists of the develop-
ment of a new heterogeneous catalyst for the ester-
ification of free fatty acids and the transesterification
of vegetable oils. The layered compound zinc hydro-
xide nitrate (Zn5(OH)8(NO3)2
2H2O) was very effec-
tive in the alcoholysis of palm oil and the esterification
of lauric acid with m(ethanol), even when hydrated
ethanol was used. Over the range of 1001401C, the
ester yield was the highest at 1401C, while the catalyst
concentration had a much greater effect on ester yields
than the molar ratio of alcohol to acid did. Total ester
contents above 95 wt.% were obtained in both reac-
tions and 93.2 wt.% glycerin streams were recovered as
a result of methanolysis. Such process which has not
been reported till now with microalgae, although 98%
A review on microalgaeK. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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oil yield has been reported with sulfuric acid or hy-
drochloric acid as catalyst [21]. On the other hand, in
the second, alkaline catalysis is being substituted by
enzymatic catalysis, which has yielded conversion of
98% of oil in 12h in low temperatures with im-
mobilized lipase of Candida sp. [22]. Recently, a tech-
nique to minimize the cost of production of the
enzymatic catalyst was developed. It was demonstratedfor the first time that it is possible to produce a lipase
in fermentation in the solid state and directly apply the
dried and fermented solid in an organic reaction to
catalyze the transesterification reaction. In that way,
necessary extraction and immobilization of the enzyme
is avoided [60].
In a recent development by the UFPR team [88],
three methods for the extraction of lipids from two
different microalgal species were evaluated: N. oculata
and P. tricornutum. The methods were adaptations of
the Folch and the Bligh and Dyer methods [89,90],
both of which are based on the use of a monophasic
mixture of chloroform, methanol and water(CHCl3:CH3OH:H2O). The extraction mixture was
varied showing the best results with Method 1 which
consisted of CHCl3:CH3OH (2:1, v:v).
An evaluation of the potential performance of mi-
croalgae as a raw material for the production of bio-
diesel was conducted experimentally by Carvalho et al.
[91]. For that, organic solvent extraction of lipids from
microalgae that does not have dependency on petro-
leum was investigated as an alternative technique. The
microalgae-extracted oil had its chemical structure
checked via the Fourier Transform Infrared spectro-
scopy method, confirming that indeed triacylglycerol
was obtained. The main conclusion was that the pro-
duction of biodiesel through the proposed system, inview of the high production of ethanol in Brazil, has
the potential to be independent of exports such as
solvents of fossil origin, therefore allowing for a sus-
tainable microalgae derived biodiesel production. As a
sequence of this study, the in situ transesterification
process is currently being investigated, i.e. the direct
conversion of microalgae biomass into biodiesel, i.e.
without having to convert or extract oil from biomass,
to avoid unnecessary steps in the process, therefore
increasing energetic and productivity efficiency.
Additionally, biofuels present new challenges con-
cerning the engine adaptation and the pollutant emis-
sions. In this context, Torrens et al. [92] developed a
study in an attempt to clarify the relation between fuel
properties of microalgae biodiesel and pollutant emis-
sions, studying which properties are desirable in these
new fuels to guarantee engine operation without de-
gradation of performance in comparison to conven-
tional diesel. The methods used were accurate enough,
for a first estimative. Viscosity estimation should be
refined. As the method applied is very complex, the
deviation may be affected by an error. Other method,
based on experimental data might perform better in
such estimations. The simulation of microalgae bio-
diesel allowed a better understanding of the potential
of this feedstock. As the synthetic algae oil had high
amounts of oleic acid and palmitic acid, their esters
presented properties that were intermediary between
Soy/Canola and Palm oil ethyl esters, and might per-
form very well. Microalgae cultivation also allows a
better control on the fatty acid profile, which is espe-cially important for fuel optimization. Many fuel
properties affect directly the engine performance and
pollutant emissions, what makes the importance of
knowing and optimizing the fuel composition clear.
Even if many plants or microorganisms may produce
oils, not necessarily the biodiesel produced from it
have the quality necessary for the operation of modern
engines. With the improvement in biodiesel produc-
tion, it may be possible to produce high-quality fuel
that complies with international standards and helps
limiting the pollutant emission and has the potential to
replace conventional diesel.
6.3. Biodigester
During the process of biodiesel production from
microalgae, biomass residues will be generated at
different stages, which may contain some commercially
valuable substances [70]. The remaining substances will
be sent to a biodigester for anaerobic decomposition to
produce biogas (methane) in a modular biodigester
developed by the authors [93]. The equipment for
biogas production consists of a cylindrical and
hermetic reactor where the residues will ferment
producing the biogas that is captured at the top. The
process has approximately a 40 days retention time.
The gases generated in the biodigester are meant to beused by the tri-generation system or other heat-
demanding processes in the plant.
Recently, Ferna ndez et al. [28] reported the pro-
duction of about 180 mL g1 of dry microalgae d1 of
biogas, with 65% methane concentration, which de-
monstrates the energy potential of microalgae biomass
as substrate in biodigesters.
6.4. Biomass and residues
Residues are produced during microalgae derived
biofuel production as discussed throughout this study.
Currently, in the project, they are being analyzed to
evaluate their characteristics such as chemical compo-
sition, morphology and thermal stability. For example,
the morphologies of microalgae produced by the
authors in different conditions (triglyceride extracted,
sun and air dried, spray dried and P. tricornutum salt
water medium) have been observed in a scanning
electron microscope. Figure 7(A)(D) shows the
micrographs of two types of microalgae samples with
different drying processes indicating the morphological
details of these samples. This suggests that their
A review on microalgae K. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
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DOI: 10.1002/er
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morphology such as shape, size, porosity, etc. depends
on the way they are obtained and dried. Further
studies on their chemical composition and thermal
properties are being carried out. Based on this,
possibility for their use as fillers in polymers to develop
composite materials, as fertilizers or possibly for other
uses including as additives to soil after carbonizing will
be explored.
6.5. Software
A complete mathematical model of all components
shown in Figure 6 is currently under development,
with the objective of producing a software for the
simulation of the entire sustainable energy plant. The
idea is to produce a low computational time-demand-
ing graphical application through a thermodynamics-
based simplified mathematical model to analyze the
transient and spatial behavior of the plant using a
volume element methodology [94]. After experimental
validation of the numerical results obtained with the
mathematical model, the software will be available for
the thermodynamic optimization of design (geometric)
and operating parameters for maximum system global
performance.
Initial modeling attempts have been started by the
authors in two studies [83,95]. In the first study, a
simplified physical model was introduced [81] for one
pipe photo-bioreactor operating in a closed circular
mode, which combines fundamental and empirical
correlations, and principles of classical thermodynamics,
biochemistry, mass and heat transfer. The model is
expected to be a useful tool for simulation, design and
optimization of compact photo-bioreactors. In the
second study [95] the microalgae growth was modeled
based upon a mathematical relationship with the lightintensity. The numerical solution of this computational
model allows for the prediction of photobioreactor
biomass concentration and production per unit volume.
7. CONCLUDING REMARKS
The main objective of this overview is to provide the
reader with an assessment of the feasibility of
Figure 7. Scanning Electron micrographs showing the morphology of two different microalgae: (A) Nannochloropsis oculata
triglyceride extracted; (B) Nannochloropsis oculatasun and air dried; (C) Nannochloropsis oculataspray dried; and (D) Phaeodactylum
tricornutum spray dried.
A review on microalgaeK. G. Satyanarayana, A. B. Mariano and J. V. C. Vargas
Int. J. Energy Res. (2010) r 2010 John Wiley & Sons, Ltd.
DOI: 10.1002/er
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8/8/2019 A Review On Micro Algae, A Versatile Source for Sustainable
17/21
innovative microalgae-based projects including the one
being initiated in authors University, which address
important aspects of energy (biofuel) and materials. It
is now known that only renewable biofuel can
potentially replace non-renewable and limited sources
of petroleum-derived liquid fuels. Although microalgae
are known to be one such alternate source for biofuel
since fifties, its technical feasibility is underlined by thestudies carried out thereafter by many including the
authors, with increased attention being given in recent
times due to increasing problems posed by the
conventional petroleum-based fuels and ecological
considerations. It is also clear that microalgae are the
only biofuel source that could be grown without
competing with agricultural land, due to the possibility
of using photobioreactors that can grow vertically and
that can also be made compact whereby biomass
production per unit volume can be maximized.
Accordingly, design of suitable photobioreactors to
produce microalgae, their characteristics and applica-
tions in various areas are well documented. Whilecomparison of market with its fast growth and cost of
various types of high molecules derived from micro-
algae and their production are available, design and
operation of the microalgal biomass production
systems have been extensively discussed. Reported
characteristics including chemical composition of
microalgae indicate their high growth rate with high
production capacity of oils along with them being
more photosynthetically efficient than oil crops in
addition to being a potential source of biomass.
Successful attempts have been reported on their
application to develop new materials. However, to
make them more attractive source for energy and
materials, some critical issues such as their isolation,culturing and characterization, design of cost-effective
photobioreactors with high efficiency, downstream
processing for the separation of biomass and concen-
tration of microalgae culture along with modeling
studies to look into toxic effects of the fuel produced
need to be addressed. Also, genetic and metabolic
engineering should be associated to the scientific effort
for obtaining increased algae growth rates and lipid
content.
A brief description of an ongoing project carried out
by the authors along with its current status addresses
some of those issues. In the authors opinion, eco-
nomically this fuel may not be competitive to petro-
leum-based fuels, but other microalgae products such
as food for human and animal consumption and pro-
duction of various high-value bioactive molecules may
make it economically competitive. This may be feasible
by utilizing the remaining amount of biomass rather
than not being either discarded or overlooked since
only 40% (of the dry biomass) lipid content would be
used in the fuel production. An innovative cogenera-
tion biorefinery concept is suggested, which along with
advances in compact photobioreactor engineering are
expected to lower the cost of production. Indeed,
economically attractive biofuel associated with new
materials production have the potential to turn mi-
croalgae-based industry from a future possibility into a
present reality.
ACKNOWLEDGEMENTS
The authors sincerely acknowledge the publishersCopyright Clearance Centers Rightslinks service,Elsevier, Springer and American Chemical Society whohave given permission to reproduce figures and tables.The authors thank the Brazilian funding agencies,CNPq (projects 552867/2007-1 and 574759/2008-5)and Araucaria Foundation of Parana (project 13470)for the Fellowships and funds provided to carry outthis work. They also thank Director of AdvancedMaterials and Processes Research Institute (AMPRI),Bhopal (M.P. India) for permitting to use theirscanning electron microscope (SEM) and Mr TSV
Chakradhar Rao, Technical officer, who helped inobtaining the SEM photographs of our samples.
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