University of Kentucky
Feasibility of Capture & Utilization of C02 from
Kentucky Power Plants by Algae Systems
Technical Review of the Literature Related to the Cultivation and Harvesting of
Algae for CO2 Fixation and the Co-Production of Fuels and Chemicals
GOEP Seed Grant: P02 855 0700006898 1
Rodney Andrews, Kunlei Liu, Mark Crocker, Czarena Crofcheck and Aubrey Shea
Technical Review of the Literature Related to the Cultivation and Harvesting
of Algae for CO2 Fixation and the Co-Production of Fuels and Chemicals
On average, the United States consumes 7x109 barrels of oil energy per year (Huber et al., 2006). With
the gas prices continuing to rise, biofuels are a renewable energy source worth looking into. The US
Department of Agriculture (USDA) and Oak Ridge National Laboratory estimated that the US has the
sustainability to produce biomass with an energy content of 3.8x109 boe (barrels of oil energy equivalent)
without sacrificing its food, feed, and export demands (Huber et al., 2006). However, the competing
prices of food versus energy crops have created much debate and hesitation about biodiesel in the
agricultural industry. These debates have influenced researchers to pursue alternative technologies for
biodiesel production. One promising technology employs a microalgae feedstock because it is a non-food
oil-based biomass. Algae produce a significant amount of polyunsaturated hydrocarbons that have lower
melting points than monosaturated and saturated hydrocarbons, thus biodiesel derived from algal oil
promises have better cold temperature stability than from other renewable feedstocks. Algae and
microalgae provide several renewable energy possibilities, such as methane production by anaerobic
digestion of the algal biomass, biodiesel derived from algal oil, and biohydrogen produced
The Potential for Algae
Most living species can be divided into two categories: autotrophs and heterotrophs. An autotroph is an
organism that makes its own food and does not require organic materials from the environment.
Autotrophs synthesize all their organic molecules from the environment in the form of sunlight or
inorganic materials such as carbon dioxide and salts. They are the providers in the food chain in the
ecosystem. Animals, fungi, and microorganisms (heterotrophs) depend on autotrophs for energy and raw
material to survive. Heterotroph is an organism that requires biomass (organic substrates) for carbon
source to obtain energy and nutrition. Unlike autotrophs, heterotrophs cannot synthesize energy from the
light or inorganic compounds and are fully depended on autotrophs for food. All animals are heterotrophs
except microalgae and blue-green algae (Bergman, 2006). Due to their ability to fix CO2, microalgae can
be used for CO2 bio-mitigation (Wang et al., 2008).
Microalgae, microscopic photosynthetic organisms that grow in salt or fresh water, are fast growing
autotrophic plants that could produce approximately 5,000-15,000 gallons of oils per acre per year (Table
1). In addition, microalgae have the ability to capture solar energy with efficiencies 10 to 50 times greater
than other land based crops (Lin et al, 2008; Wang et al, 2008). They can grow under conditions that are
not suitable for conventional crops such as arid or semiarid land with poor soil quality or even areas that
would use service or saline water for irrigation. The growing cycle for microalgae can be completed in a
few days. Average doubling times of less than 24 hours have been reported (Chisti, 2007). The oil
content in most microalgae is between 20-50% by dry weight (Table 2) (Becker, 1994). Triglyceride
production rates in algae are 45-220 times higher than terrestrial plants (Huber et al., 2006). The oil has
similar characteristics as those from fish or vegetables and thus a potential alternative to fossil oil (FAO
Agricultural Service Bulletins, 1997). Microalgae contain fats, carbohydrates, and proteins of which 70%
of the total fat content can be directly harvested by press. The remaining biomass could be used as animal
feed or fermented for ethanol (Riesing, 2006). Some microalgae produce up to 60% of their cellular mass
as lipids and could yield 30 times more oil per unit growth area than land plants (Baum, 1994). Diesel
fuel and gasoline could be produced by transesterification and catalyst cracking of algal oil (FAO
Agricultural Service Bulletins, 1997). Cultivating microalgae on 1-3% of the total US cropping area
would be sufficient to fulfill 50% of the US transportation fuel demand (Chisti, 2007). In 2005, Michael
Briggs from the University of New Hampshire (UNH) Biodiesel Group estimated that it would take $46.2
billion to produce biodiesel from algae to satisfy 64% of the US oil consumption compared to the $100
trillion the US has spent on foreign crude oil (Riesing, 2006). However, these estimates may be overly
optimistic depending on the productivity and recovery costs of the algal systems.
Table 1. Comparison of oil production for various biomass (Riesing, 2006)
Crop Gallons of oil per acre per year
Oil Palm 635
Table 2. Chemical composition of algae in dry basis (%) (Becker, 1994)
Strain Protein Carbohydrates Lipids Nucleic acid
Scenedesmus obliquus 50-56 10-17 12-14 3-6
Scenedesmus quadricauda 47 - 1.9 -
Scenedesmus dimorphus 8-18 21-52 16-40 -
Chlamydomonas rheinhardii 48 17 21 -
Chlorella vulgaris 51-58 12-17 14-22 4-5
Chlorella pyrenoidosa 57 26 2 -
Spirogyra sp. 6-20 33-64 11-21 -
Dunaliella bioculata 49 4 8 -
Dunaliella salina 57 32 6 -
Euglena gracilis 39-61 14-18 14-20 -
Prymnesium parvum 28-45 25-33 22-38 1-2
Tetraselmis maculata 52 15 3 -
Porphyridium cruentum 28-39 40-57 9-14 -
Spirulina platensis 46-63 8-14 4--9 2-5
Spirulina maxima 60-71 13-16 6-7 3-4.5
Synechoccus sp. 63 15 11 5
Anabaena cylindrica 43-56 25-30 4-7 -
Since microalgae contain significant amounts of proteins, carbohydrates, other nutrients, and have a
balanced nitrogen: phosphorous ratio, residual biomass could be used as feedstock for ethanol production,
animal feed, or as organic fertilizer. They are capable of reducing carbon dioxide and nitrogen oxide
emission from power plants. Microalgae are grown photosynthetically so no carbon source other than
atmospheric CO2 is required for growth. Carbon dioxide and sunlight are the two key components
required by microalgae to grow. Microalgae do not require high purity carbon dioxide. While the
atmosphere contains 0.03-0.06% CO2, elevated CO2 levels result in an increase in yield. This makes flue
gas, with CO2 levels of 5-15%, obtained from power plants a favorable substrate to use in microalgae
processes (Doucha et al., 2005). Nitrogen oxide and sulfur oxide present in flue gas can also be used as
nutrients for microalgae, and can be applied to the scrubbing of combustible components in the flue gas.
In utilizing flue gas, microalgae have higher productivity due to the continuous supply of nutrients, and
the harvest rate is adjustable by controlling flue gas feed rate that keeps the algae at its optimum stage.
Residues of microalgal extraction could be used to produce methane or ethanol. The remaining water,
nutrients, carbon dioxide and wastes could be recycled and used for the next batch of cultivation
(Benemann, 1997). Research has shown that low concentration of flue gas from coal-fired energy plants
contain sulfur oxide, nitrogen oxide, and other trace compounds and do not affect microalgal growth in
either open or closed system (Olaizola, 2003; Pedroni et al., 2004). It also demonstrates a variety of
species of microalgae could grow successfully with flue gas and service water (Olaizola, 2003).
A variety of algal species accumulate large amount of oils that made up of triacylglycerols consisting of
three fatty acids bound to glycerol, excellent as a feedstock for biodiesel production. The fatty acids
consist of different lengths of saturated or unsaturated carbon chains, where the unsaturation of oil can be
reduced by partial catalytic hydrogenation of the oil (Carlsson et al., 2007). A selection of microalgae
with high accumulation of oils and tolerance to flue gas that are suitable for biodiesel production is
presented in Table 3 and Table 4 respectively. Strain selection is dictated by high productivity, ready
harvestability, and desirable co-products. Besides lipid content, selection of algal strain can also
determine by its ability to be mass-cultured, hardiness in extreme environments, relative growth rate
(versus potential contaminants), and competitiveness in dense mass culture (Benemann, 2003).
Table 3. Oil content in various microalgal species (Carlsson et al., 2007)
Species Oil content (% dw)
Ankistrodesmus TR-87 28-40
Botryococcus braunii 29-75
Chlorella sp. 29
Cyclotella DI-35 42
Dunaliella tertiolecta 36-42
Hantzschia DI-160 66
Isochrysis sp. 7-33
Nannochloris 31 (6-63)
Nannochloropsis 46 (31-68)
Nitzschia TR-114 28-50
Phaeodactylum tricornutum 31
Scenedesmus TR-84 45
Stichococcus 33 (9-59)
Tetraselmis suecica 15-32
Thalassiosira pseudonana (21-31)
Table 4. Microalgae that tolerate high CO2 content and moderate levels of SOx and NOx (up to 150 ppm)
in flue gas (Wang et al., 2008)
Microalgae CO2 % T °C P g/L/day PCO2 g/L/day Note
40 30 N/A 1.0
6 - 0.085 Maximum growth rate of 0.267/day
12 - 0.087 -
18 30 0.087 0.163a
15 35 N/A >1
Chlorella vularis 15 - N/A 0.624 Artificial wastewater
air 25 0.040 0.075a Watanabe’s medium
air 25 0.024 0.045a Low nitrogen medium
Chlorella sp. 40 42 N/A 1.0
Dunaliella 3 27 0.17 0.313a High salinity, β-carotene
16-34 20 0.076 0.143 Commercial scale, outdoor
air - 0.009 0.016 Wastewater, outdoor, winter
air - 0.016 0.031 Wastewater, outdoor, summer
Three-stage serial tubular
6 & 12 30 0.14 - photobioreactor, maxiumum growth
rate of 0.22/day
18 30 0.14 0.26
- 25-30 1.1 >1.0 Accumulating hydrocarbon
- - - - Accumulating hydrocarbon
Maximum growth rate of 0.44/day,
Spirulina sp. 6 & 12 30 0.22 0.413 maximum cell concentration of 3.5 g dry
En-dash not specified or not controlled
Calculated from the biomass productivity according to equation, CO2 fixation rat (PCO2)=1.88xbiomass
productivity (P), which is derived from the typical molecular formula of microalgal biomass,
CO0.48H1.83N0.11P0.01 (Chisti, 2007).
All species except Spirulina sp., which is a prokaryotic cyanobacteria (Cyanophyceae) species, are
eukaryotic green algae (Chlorophyta) species (Bold and Wynne, 1985).
In the study of microalgae cultivated with CO2 bio-mitigation, Wang and coworkers (2008) recommended
Chlorella species and Botryococcus braunii as good candidates for biodiesel production. A production
rate of 3200 GJ/ha/yr was achieved with the Chlorella species. This production rate is projected as a
potential to replace our reliance on fossil fuel by 300 EJ/yr and eliminate CO2 emission by 6.5 Gt/yr by
the year 2050. While cultivation of Haematococcus pluvialis yielded a maximum production rate of 1014
GJ/ha/yr and an average oil production rate of 420 GJ/ha/yr.
Some of the algal strains commonly found in Kentucky streams include chlorophyta (green algae),
cyanophyta (bluegreen algae), chrysophyta (diatoms, yellow-green or golden-brown algae), rhodophyta
(red algae), euglenophyta (euglenoids), and pyrrophyta (dinoflagellates). Common genera of chlorophyta
include Hydrodictyon, Spirogyra, and Chlamydomonas. They are found in lakes and streams. Bluegreen
algae, such as Anabaena, Aphanizomenon, and Microcystis, can be found in ponds, lakes, and reservoirs.
They can fix nitrogen and store phosphorus such that the algae can thrive in unfavorable environmental
conditions (nutrient depleted or water condition from pristine to severely polluted). Diatoms are found in
oceans, freshwater, soil, and damp surfaces. They can survive in a wide range of environmental
conditions. Common genera of diatoms include Navicula, Nitzschia, and Cymbella. Rhodophyta such as
Lemanea and Batrachospermum (freshwater red algae) are abundant in Kentucky and can be found in
cool shady streams with good water quality. Euglenophyta are commonly found in ponds and nutrient
enriched lakes and reservoirs. Examples of euglenophyta include Trachylomonas and Phycus.
Dinoflagellates are common in freshwater habitat such as ponds, lakes, and reservoirs (Algae, 2008).
Microalgae require light, carbon dioxide, water, and inorganic salts to grow. Temperature also has a
major effect in the production of algal fatty acids. The optimum temperature for growth averages
between 20-30°C (Chisti, 2007). Most cultured microalgae tolerate temperate between 16-27°C (FAO
Corporate Document Repository, 1996). If needed, cooling could be achieved by placing a heat
exchanger at various positions in the photobioreactor, a flow of cold water over the surface of the
bioreactor, or evaporative cooling from spraying water (Laing, 1991). At the lower temperature ranges,
an increase in fatty acid unsaturation has been observed (Roessler, 1990). Macronutrients for algal
growth include a continuous supply of carbon dioxide, nitrogen, phosphorous, iron, and silicon on
occasion. In which phosphorus is not readily bioavailable and may form complexes with some metal
ions, it should be supplied in excess (Wang et al., 2008). Micronutrients consist of various trace metals
and the vitamins thiamin (B1), cyanocobalamin (B12), and sometimes biotin. Two extensively used algal
growth media are the Walne medium and the Guillard’s F/2 medium (FAO Corporate Document
Repository, 1996). Carbon dioxide supply can be controlled by measuring pH, and must be supplied
continuously during the daylight hours. The pH range for most microalgae is between 7 and 9 (FAO
Corporate Document Repository, 1996). A pH tolerant study conducted by Olaizola (2003) reviewed that
at the lower and higher end of the pH levels tested (6.5, 7.5, and 8.5) growth was slow due to the acidity
or the limitation of carbon dioxide available in the medium (Olaizola, 2003). In addition, there is no
direct effect of high concentration of carbon dioxide on algae productivity if no excess of carbon dioxide
is added to the culture during the growth phase (Olaizola, 2003; Chisti, 2007).
Lighting is crucial for algae culture growth. Light is absorbed rapidly by the cells at the surface of the
photobioreactor. The light intensity drops exponentially as it approaches the center of the reactor
(Ogbonna and Tanaka, 1997; Hankamer et al., 2007). Initially, when the cell concentration is low, a low
light intensity is sufficient for growth. As the concentration of algae increases, light intensity should be
increased such that the light coefficient does not change. Light may be natural or artificially supplied
(fluorescent lighting). Fluorescent tubes emitting blue or red light spectrum are preferred as these
spectrums are most active for photosynthesis (FAO Corporate Document Repository, 1996). The optimal
light:dark zone is 16:8 hours respectively. Increasing light intensity in general increases the
triacylglycerol levels in algae, but it can also results in a 50-80% energy loss in the form of heat, causing
algal production rate to drop from photoinhibition (Roessler, 1990; Benemann, 1997; Ogbonna and
Tanaka, 1997). At low light intensity, a 20-24% photosynthetic visible light conversion efficiency
(conversion of light energy into biomass higher heating value) was achieved (Benemann, 1997). As cells
expose to low intensity, algae reportedly have a higher lipid conversion but a low biomass concentration;
with a high intensity, algae have a lower efficiency and a higher biomass conversion rate (Chisti, 2007).
At night, the cells metabolize their intracellular stored carbohydrates for cell maintenance, resulting in
decreased productivity (Ogbonna and Tanaka, 1997). Thus for maximum productivity, combination of
sunlight and artificial lighting at nights or on cloudy days should be used.
Nutrient deficiencies such as nitrogen and silicon can enhance algal lipid accumulation and calorific
values (Roessler, 1990; Wang et al., 2008). Under stressful growth conditions the cellular oil content
reportedly increases from 20% to 44% of dry weight in green algae and from 23% to 38% of dry weight
in diatoms (Hu et al., 2006). The cultivation of Nannocholis in low nitrogen medium (0.9 mM) has an
increase of lipid content from 32.9 g/L (with 9.9 mM nitrogen media) to 42.4 g/L (FAO Corporate
Document Repository, 1997). Whereas the calorific value of Chlorella vulgaris increased from 18 kJ/g to
23 kJ/g (Wang et al., 2008). As reported by Coomls and coworkers, deficiency in silicon could increase
cellular lipid content by 60% during a 14 hour starvation period (FAO Corporate Document Repository,
1997). Glycerol content is reportedly affected by sodium concentration.
For marine microalgae, salinity is another important parameter in regulating growth rate. For flagellate
culture, the optimal salinity is around 25 and 30 psu; for diatom culture, they grow best at 20 and 25 psu
salinity (Laing, 1991). Al-Hasan and coworkers (1987) proposed that under salt-stressed environment,
there is a substantial increase in the cells’ ability to produce glycerol via photosynthesis for osmotic
adjustment (Roessler, 1990). Low sodium concentration induces higher lipid content (up to 58%
accumulate in Chlorella species) and cell concentration in microalgae, while high sodium concentration
could inhibit cell growth (Scragg et al., 2002; FAO Corporate Document Repository, 1997; Takagi et al.,
2006). Addition of sodium to low medium salt concentration during the algal growth phase was found to
enhance cellular lipid content, but with a slight decrease in cell mass (6-17%) at the end of cultivation
(Takagi et al., 2006).
In general, cells with higher lipid content and lower carbohydrates and proteins have higher calorific
values, producing higher yields of oil. In stress environment such as nitrogen limiting medium, cells
accumulate more oil and increase their calorific values but it also leads to reduced cell division, resulting
in lower biomass yield and overall lipid/energy productivity. Thus, optimization of nutrient in medium is
needed to balance between producing high lipid content and high calorific values cells while maintaining
high biomass productivity (Wang et al., 2008).
Table 5. Optimum and tolerance ranges of algal growth conditions (FAO Corporate Document
Tolerable range Optimum range
Temperature 16-27°C (61-81°F) 18-24 °C (65-75 °F)
Salinity (g/L) 12-40 20-24
Light intensity 1000-10,000 2500-5000
Photoperiod (light: dark, hour) 16:8 (min) 24:0 (max)
pH 7-9 8.2-8.7
When inoculating a batch of microalgae, subculture is usually used to ensure continuous growth and
division of cells. Water should be filtered to remove any particles greater than 2 µm to reduce
contamination risk. Both air and carbon dioxide stream should be filtered through a 0.3-0.5 μm in-line
filter (Laing, 1991). Two types of reactors have been developed to cultivate algae: open system (such as
raceway ponds) and closed system (such as photobioreactors).
Cultivation can be conducted in batch, semi-batch, and continuous systems. Batch culture consists of a
single inoculation of cells in container of media over several days of growth period until the cell density
reaches a maximum/desirable level. Upon which the culture is ready to be transferred to larger culture
volumes to continue growth before reaching the stationary phase (FAO Corporate Document Repository,
1996). The semi-batch system allows a portion of the culture to be harvested and replenished with fresh
medium. After a few days, the culture matures and the harvesting process is repeated. In a continuous
system, two types can be used: turbidostat and chemostat culture. In a turbidostat culture, an automatic
system is used to monitor and maintain algae density. When the density reaches a pre-set level, fresh
medium is added to the culture as the cells continue to divide and grow. In the chemostat culture, a slow
but steady flow of fresh medium is continually introduced into the culture while excess culture overflows
and collected. An example of a semi-batch system is the use of polyethylene bag (Laing, 1991). The bag
is sealed at both ends displaying a tubular shape and suspended on a framework. Rods are inserted at
either ends into the bag for aeration and mixing purposes. The bag is suitable to use as both indoor
(fluorescent light) and outdoor (natural sunlight) settings.
Open Systems – Open Ponds
Raceway ponds are the most economical and simple cultivation systems. Raceway ponds are open,
outdoor ponds that are made of circulating loop channels and are typically shallow (less than 0.3 m deep)
and unlined. Open pond has moderate surface-to-volume ratio of 3-10/m (Wang et al., 2008).
Paddlewheels are used to circulate the suspended algae throughout the raceway channels. Cooling is
mostly done by evaporation, and the pond is illuminated solely by sunlight. The raceway pond can be run
continuously with growth medium and carbon dioxide feed continuously added to the pond while algal
broth is harvested at the end of the circulation loop. Production in the pond usually takes 6-8 weeks to
mature and typically yields only 0.1-0.2 g/L algae (Chaumont, 1993). Open ponds are dependent on
weather because temperature and light intensity vary throughout the day and year. Low temperatures (<
17°C) reduce algal growth rate while high temperatures (> 27°C) kill algal cells. If cultivation is a
success, high biomass yields are mostly seasonal. Tank size also influences algal growth. Israel and
coworkers (2006) reported that a smaller outdoor pond (40 L) produced higher algal yield than a larger
pond (24,000 L). The open pond system can be converted to an indoor system by covering the pond with
a layer of plastic or glass. The limitation with raceway pond includes high evaporative losses, diffusion
of carbon dioxide to the atmosphere, contamination risk, poor mixing and mass transfer rate, temperature
fluctuation, the inability to sustain an optically dark zone, and requirement of large land area. The
biomass productivity remains low and the algal strain of interest is difficult to cultivate. The highest
operating cost for an open system is the harvesting cost since the biomass concentration is usually low
Closed Systems - Photobioreactors
Closed systems like photobioreactors have higher efficiency and biomass concentration (2-5 g/L), shorter
harvest time (2-4 weeks), and higher surface-to-volume ratio (25-125/m) than open ponds (Lee, 2001;
Wang et al., 2008). Closed systems consist of numerous designs: tubular, flat-plated, rectangular,
continued stirred reactors, etc. Photobioreactors in general provide better control of cultivation
conditions, yield higher productivity and reproducibility, reduce contamination risk, and allow greater
selection of algal species used for cultivation. Light source is usually a combination of natural light and
artificial lighting. Light can be radiated inside the bioreactor with optical fibers or submerged lamps, or
provided externally by fluorescent lights or the sun). The bioreactor has a photolimited central dark zone
and a better lit peripheral zone close to the surface (Chisti, 2007). Increasing light intensity at short
intervals has been shown to promote algal growth and faster cell division and thus increases the algal
production (Laing, 1991). Carbon dioxide enriched air is sparged into the reactor creating a turbulent
flow. Turbulent flow simultaneously circulates cells between the light and dark zones and assists the
mass transfer of carbon dioxide and oxygen gases. The frequency of light and dark zone cycling is
depended on the intensity of turbulence, cell concentration, optical properties of culture, diameter of tube,
and the external irradiance level (Chisti, 2007). Regulation of carbon dioxide and dissolved oxygen
levels in the bioreactor is another key element to algal growth. Challenges with closed system
photobioreactors include reduced light penetration into the center of reactor, cell growth on the walls,
scale-up, and cost. The scale-up of bioreactors increases the percentage of dark zone and reduces algal
growth. The highest cost for closed system is the energy cost associated with the mixing mechanism
(Wijffels, 2008).Tubular photobioreactors have a large surface-to-volume ratio, occupy small ground
space, and require simple temperature control methods. A small scale bioreactor can be easily
incorporated into a pilot plant as an indoor or outdoor system (Riesing, 2006).
Tubular photobioreactors consist of transparent tubes that are made of flexible plastic or glass. Tubes
can be arranged vertically, horizontally, inclined, helical, or in a horizontal thin-panel design. Tubes are
generally placed in parallel to each other or flat above the ground to maximize the illumination surface-to-
volume ratio of the reactor. The diameter of tubes is usually small and limited (0.2 m diameter or less) to
allow light penetration to the center of the tube where the light coefficient and linear growth rate of
culture decrease with increasing unit diameter (Ogbonna and Tanaka, 1997; Riesing, 2006). Growth
medium circulates from a reservoir to the reactor and back to the reservoir. A turbulent flow is
maintained in the reactor to ensure distribution of nutrients, improve gas exchange, minimize cell
sedimentation, and circulate biomass for equal illumination between the light and dark zones.
Most of the algal cells are fragile and can only withstand a very low shear stress. Airlift pumps could be
used to provide the turbulent flow without exerting too much shear stress on algal cells. These
photobioreactors are usually referred to as airlift photobioreactors. A dissolved oxygen level that is
much greater than the saturated air value (> 400% of air saturation value) could potentially inhibit
photosynthesis. To prevent this, a degassing zone is used to lower the gas concentration and remove gas
bubbles from the growth medium; it could also serve as an entry point for carbon dioxide addition into the
culture. The pH level generally increases as the microalgae consume carbon dioxide. Addition of carbon
dioxide along the reactor would sustain microalgal growth by preventing carbon limitation and the excess
rise in pH. However, tubular photobioreactors do not work well in large scale production because the
surface-to-volume ratio is lower causing poor light absorption. Length of tubes is another concern of
tubular photobioreactors. As the length of the tubes gets larger, the time for microalgae exposure to light
increases, hence increasing the absorption of available carbon dioxide and increasing photosynthesis rate.
However, the dissolved oxygen level also increases which can easily lead to oxygen poisoning, and
photoinhibition can result from the excess light exposure (Ogbonna and Tanaka, 1997). In addition, the
increasing tube length could increase liquid friction inside the tube and the head pressure of the pump,
requiring a larger pump and more power consumption. If the system is built with manifolds, it would
reduce the size of the pump needed and extend the path length the microalgae would take, therefore
lowering the dissolved oxygen concentration and reducing the potential of cell damage (Ogbonna and
Flat-plated photobioreactors are usually made of transparent material. The large illumination surface
area allows high photosynthetic efficiency, low accumulation of dissolved oxygen concentration, and
immobilization of algae (Ugwu et al., 2007). The reactors are inexpensive and easy to construct and
maintain. However, the large surface area presents scale-up problems, including difficulties in controlling
culture temperature and carbon dioxide diffusion rate, and the tendency for algae adhering to the walls.
An inclined triangular tubular photobioreactor was designed to install adjacent to a power plant
utilizing flue gas as the feed gas. Flue gas entered the reactor from the bottom of the inclined tube. Gas
bubbles traveled along the inner surface of the tube generating eddies for mixing and preventing fouling.
The upper surface of the inclined tube absorbed natural light. The mixing to the algal culture and the flow
rate of flue gas influences the growth rate of algae. The system worked, and 15-30% of algae were
harvested each day. The setup was able to remove 82% carbon dioxide on a sunny day and 50% carbon
dioxide on a cloudy day. Nitrogen oxide was also lowered by 86% (Riesing, 2006).
Rectangular tanks are another example of photobioreactors. Unlike the circular tank design, rectangular
tanks do not require a stirring device when a sufficiently high gas velocity is used. Drain pipes and gas
spargers are located at the bottom of the tank. The void space between the top of the tank and medium
serves as a venting space for gases. The tank can be tilted in an angle to maximize light exposure. The
design is cost-effective and easy to harvest, maintain, and manufacture. Light attenuation in the medium
is one dimensional as the bioreactor illuminated on one side or both sides (Zijffers et al., 2008).
Continuous stirred tank reactors (CSTR) consists of a wide, hollow, capped cylindrical pipe that
operates both indoor and outdoor with low contamination risk. Mechanical stirrer and light source are
inserted from the top of the reactor. Drain channels and gas injectors position at the bottom (and
midsection) of the reactor. The uniform turbulent flow established within the reactor promotes algal
growth and prevents fouling. Algal growth can be modeled via kinetic models such as the Monod
equation. The disadvantages of CSTR include the small surface-to-volume ratio and high energy
requirement for stirring and internal illumination (Algae Blog Directory, 2008).
Another photobioreactor uses helical coils made of plastic tubing placed across a column-like structure.
A group of helical coils make up one unit of photobioreactor. Each helical coil runs independently with
its own gas injector, pump, and gas removal system. The helical coils operate both indoor (fluorescent
light) and outdoor (sunlight). They are space-efficient and can be installed on the rooftop of any power
plant. The system is easy to build, is low maintenance, and has little energy requirements (pump and
artificial lighting when needed). Since each unit operates separately, harvesting can be done on a timely
schedule and the risk of cross-contamination is eliminated.
Similar to the helical coils, square tubular reactors consist of plastic tubing arranged in a series of
squares. One pump is used to provide algal flow through the series of squares and back. Compared with
the helical coils, the square tubing is longer that holds more algal volume. The unit is also intended to be
installed on the rooftop of a power plant and can operate both outdoor and indoor. However, light is a
limitation where only one side of the square is exposed to the sun at a time. To maximize light
penetration, square tubular reactors cannot be packed as closely as the inclined triangular tubular reactor
or the helical coils.
Photobioreactor Design Considerations and Performance
When designing a photobioreactor, design parameters such as reactor dimension, flowrate, light
requirement, culture condition, algae species, reproducibility, and economic value need to be taken into
consideration. Depending on the reactor dimension, site location, and local climate these parameters can
determine the type of cultivation system needed (open versus closed). Light requirement (natural versus
artificial) and controllable light intensity affects the illumination surface-to-volume ratio of reactor and
algal growth for either open or closed systems. Algae species and growth index also influence the type
and the production scale of the photobioreactor. An efficient gas removal and gas injection system is
essential for optimal culture growth. Reactor design should have good mixing properties, efficiency,
reproducibility, and be easy to maintain and sterilize. Other design aspects to consider include
construction material, pump selection, reactor durability and life cycle, construction and operating costs,
and safety. An efficient photobioreactor not only improves productivity, but also is used to cultivate
multiple strains of algae. The scale up of photobioreactors can be done by increasing the length,
diameter, height, or the number of compartments; with difficulties in maintain the light, temperature,
mixing, and mass transfer (Ugwu et al., 2008).
Performance of photobioreactor is measured by volumetric productivity, areal productivity, and
productivity per unit of illuminated surface (Riesing, 2006). Volumetric productivity is a function of
biomass concentration per unit volume of bioreactor per time. Areal productivity is defined as biomass
concentration per unit of occupied land per time. Productivity per unit of illuminated surface is measured
as biomass concentration per area per time.
Algae for Flue Gas Scrubbing
A practical example of a current microalgae production process is Spirulina, a microalga already
produced commercially in open ponds in many countries around the world. In these production systems,
the algae are cultivated in large (typically 0.2 –0.4 hectares), raceway-type open ponds mixed by paddle
wheels. Nutrients, most importantly CO2, are added to the ponds and these filamentous algae are then
harvested by fine mesh screens, spray dried and sold as specialty human foods and animal feeds. The CO2
is typically purchased from commercial sources, although in some cases it is also derived from the flue
gas emitted by the drying operation.
At the Cyanotech Corp. algal production facility in Kona, Hawaii, a small power plant was built to
produce power and allow the capture of the CO2 required for algal production ponds. Two 180 kW
generators (with one spare) produce the electricity required to operate the paddle wheels on the 67 algal
production ponds (avg. 0.3 ha in size) and other process power needs. The stack gas comes out at
approximately 485 oC at 20 scm/min and contains 8% CO2. The flue gas is transferred to the bottom of a
CO2 absorption tower, 2.4 m diameters and with some 6.4 m high packing material. The spent culture
medium (after harvesting the Spirulina) enters at the top and is collected in the bottom. The
countercurrent absorption system is 75% efficient and provides some 67 t CO2/month, supporting 36 t/mo
of Spirulina production. The system generates an annual net income (credit) of almost $300,000 from
power and CO2 savings.
At present, the major limitations are technological and economic. The main technical obstacles to
increasing algal productivities are light saturation and respiration (both night-time dark respiration and
day-time photorespiration). Light saturation is the largest single factor limiting the productivity of algal
mass cultures. However, long sunlight hours and small temperature differential between day and night in
Kentucky may overcome this obstacle to some degree, as well as the availability of artifical lighting at
nights when the power plant is under (sub) base-load demand. The major economic obstacles are land
required and capital cost for the scale which can meet the required amount of power plant CO2 reduction.
Harvesting and Processing Algae
The typical cell density achieved in the industrial application is between 0.3-0.5 g dry cell/L or 5 g dry
cell/L at best, which makes harvest difficult and expensive (Wang et al., 2008). Two processes are
involved in harvesting, bulk harvesting and thickening. Bulk harvesting is a large scale operation
separating biomass from bulk culture. It has a concentration factor of 100-800 times, depending on the
culture and harvesting method. Bulk harvesting can be categorized into flocculation and floatation.
Flocculation reduces/neutralizes the negative surface charge of microalgal cells, allowing them to
aggregate into larger lumps with an efficiency of >80%. Flotation utilizes a foaming method to collect
microalgal cells. Gas is bubbled through the algal suspension in a bubble column, creating a froth where
the algal cells would stick to the bubbles and collected on the side near the top of the column. The final
cell concentration is dependent on the solution and column parameters such as the feed concentration,
solution pH, filter porosity, gas flow rate, and foam height (Guelcher and Kanel, 1998 and 1999;
Borodyanski and Konstantinov, 2003). However, flotation could be time-consuming and inefficient to
handle large algal volume especially in a continuous system.
Thickening process consists of either centrifugation or filtration. Centrifugation, a semi-continuous or
continuous process, utilizes centrifugal force generated by the spinning of a suspension to separate and
harvest algal cells. The magnitude of centrifugal force in most centrifugation system is adjustable to
allow separation of different sizes of particulates. However, centrifugation is not energy efficient and
readily scale up to large applications, with the potential of damaging the algal cell walls in the high shear
process (Chaplin, 2004). Filtration is preformed using a cellulose membrane and vacuum to draw the
liquid through the filter (Clark and Sigler, 1963). It is a preferred method for filamentous algae such as
Spirulina platensis (Wang et al., 2008). The final algal product is condensed into a dough consistency.
Filtrate (nutrient rich water) can be recycled back to the reservoir. For small microalgal cells,
microfiltration and ultrafiltration are more appropriate. Microfiltration is found to be more cost-effective
Algae processing can be broken down into three steps: isolation of the algae: algae dewatering/drying, oil
extraction, and recovery/utilization of the algae cake. Dewatering/drying process reduces the water
content of the algae prior to oil extraction process. The algae paste obtained from filtration/centrifugation
contains as much as ca. 90% water content. Drying algae to ca. 50% water content is necessary to
produce a solid material that can be easily handled. Solar drying, a popular and inexpensive method, is
used commercially in grains and timber drying (Kadam, 2001). However, it requires a considerable area
of land. A more efficient method would make use of the low grade waste heat from the power plant to
dry the algae contained in a vessel.
Oil extraction methods include expeller/press, hexane solvent method, and supercritical fluid extraction.
The expeller/press, the simplest oil extraction method, typically recovers ca. 70-75% algal oil from dried
algae. The solvent method, complex but efficient (>95% oil recovery), dissolves oil in cyclo-hexane, the
mixture is then separated by distillation (Oilgae Blog Directory, 2008). If wet algae are used, a water
miscible co-solvent is used in place to lyse the algal cells (open cells to expose their content). Other
available method includes sonication or acidification. Supercritical fluid extraction can extract almost
100% of the oil. Supercritical (liquefied and pressure heated) carbon dioxide is used as an extraction
solvent, oil can be recovered by depressurization (supercritical liquid returns to its gaseous state).
Optimization of the process depends on a numbers of variables, such as algae water content and the ease
with which the cells can be lysed (a function of the algal species).
Other available oil extraction techniques are enzyme extraction, osmotic shock, and ultrasonic-assisted
extraction. With water as a solvent, enzymes break down algal cell walls and extract the oils. This
method may be effective, but the cost of extraction is far more expensive than the hexane extraction
(Oilgae Blog Directory, 2008). The osmotic shock method takes advantage of the sudden change in
osmotic pressure by drawing water across the cell membrane, rupturing cell wall and releasing the oil
content. Ultrasonic-assisted extraction applies ultrasonic waves to create cavitation bubbles. Coalescence
of these cavitation bubbles near algal cells transmits a shockwave and promotes liquid to enter and disrupt
the cell walls to release the algal oils. Ultrasonic extraction can operate at lower temperatures than some
of the common extraction methods.
The recovered solid after the extraction step is often referred as the algae cake. The co-product can be
used as animal feed (with sufficient nutritional content and free of heavy metals), fuel, or feedstock for
ethanol fermentation (with sufficient carbohydrate content). Nutrient content can be affected by the oil
extraction process: if the cells are ruptured in the presence of water, significant fraction of nutrients could
be leached and lost into the aqueous phase.
For mass cultivation of algae (i.e., integration with power plant), optimization of algae harvesting and
processing is needed. Algae properties such as algae size, cell wall sensitivity to shear force, ease of
flocculation, and oil content need to be taken into consideration in the design process.
Economics of Algal Biodiesel Production
Currently, microalgae biofuel has not been deemed economically feasible compared to the conventional
agricultural biomass (Carlsson et al., 2007). Harvesting costs contribute 20-30% to the total cost of algal
cultivation (FAO Corporate Document Repository, 1996) with majority of the cost contribute to
cultivation expenses. Photobioreactors require 10 times capital investment than open pond systems. The
estimated algal production cost for open pond systems ($10/kg) and photobioreactors ($30-$70/kg) is two
order magnitudes higher and almost three order magnitudes higher than conventional agricultural biomass
respectively (Carlsson et al., 2007). Assuming that biomass contains 30% oil by weight and carbon
dioxide available at no cost (flue gas), Chisti (2007) estimated production cost for photobioreactors and
raceway ponds to be $1.40 and $1.81 per liter of oil respectively. However, for microalgal biodiesel to be
competitive with petrodiesel, algal oil price should be less than $0.48/L (excluding tax) (Chisti, 2007).
Potential for Improving Algae Economics
By combining fuel production with co-process such as wastewater treatment and co-production of higher-
value products, cost-effective algal fuel production is attainable (Benemann, 2003, Wang et al., 2008).
Municipal/agricultural wastewater treatment with CO2 mitigation is a co-process that could greatly
improve the overall economics of algal biodiesel production. Microalgae cultivated in
municipal/agricultural wastewater produce dissolved oxygen that is required by the bacteria to break
down toxic organic wastes. In the process, microalgae remove nutrients (mostly N and P) from
wastewater, a process that costs much less than the conventional secondary wastewater treatment (such as
activated sludge) and minimize the use of chemicals such as sodium nitrate and potassium phosphorus as
exogenous nutrients. The harvested algae next undergo anaerobic digestion producing methane which
could be used to produce electricity. The removal of nutrients avoids eutrophication of receiving waters
and recovers fertilizer values such as N and P from wastewater. The production of nitrogen-fixing algae
could be used as feedstock for biofuels or biofertilizers. Utilizing wastewater to cultivate algae also leads
to savings from using freshwater resources. Such wastewater treatment facilities utilizing microalgae
have demonstrated to be viable in Melbourne, Australia and Sunnyvale, California. Co-products, such as
bioplastics of PHA (polyhydroxyalcanoate)-based polymers and polysaccharides used in the food and
industrial applications, have large market value (valued at over $1000/ton) to make algal production
feasible (Benemann, 2003).
Genetic engineering, development of low cost harvesting processes, improvement on photobioreactor, and
integration of co-production of higher-value products/processes are other alternatives in reducing algal oil
production cost (Chisti, 2007; Benemann, 2003). Genetic technologies could improve microalgal
photosynthetic efficiency, growth rate, higher lipid content in biomass, and temperature and light
saturation tolerance. Techniques could be developed to maximize growth of selective dominant algal
strains in open pond mass culture (Benemann, 2003). Other co-products include ethanol and methanol
fermentation from residual biomass, generation of electricity from methane, or production of fertilizers or
animal feeds from residual biomass.
The wastewater effluent from algae cultivation contains valuable nutrients that may be used for energy
crops irrigation. The required nutrient content in the wastewater is highly dependent the type of crop
being irrigated. Influencing parameters in wastewater include pH, EC conductivity (salt level), pathogen
level, and nitrate, phosphate, potassium, and metals concentration. Other factors such as daily
productivity of algal wastewater, the amount of water plants require, evaporation rate of plants, the
distance between the cultivation and the watering site, transportation cost, and the versatility of irrigating
different types of plant also affect the irrigation potential.
Biodiesel produced from microalgal oil is comparable to conventional diesel fuel (Xu et al., 2006). It
burns cleaner (compared to petrodiesel) and has the potential of reducing greenhouse gas/power plant
emissions. Microalgae are a fast growing aqua species that yield a high percentage of lipids (by weight)
photosynthetically and produce polyunsaturated hydrocarbons that are capable of producing other
renewable biofuels such as methane, ethanol, and biohydrogen. The biodiesel derived from the
microalgal oil via acidic transesterification has a heating value of 41 MJ/kg, a density of 0.864 kg/L, and
a viscosity of 5.2x10-4 Pas at 40°C (Xu et al., 2006) that has the potential to substitute diesel fuel. Co-
processes (i.e., wastewater treatment) and co-products (i.e., bioplastics) greatly improve the economics of
algal oil production, proving conversion of microalgae products into an alternative fuel is a feasible
The major hurdles to be overcome before algal systems could effectively be used in scrubbing flue gas are
those relating to the scalability and economics of the process. Currently, for a 500 MW generation unit,
an open pond system would require 5,000-6,000 acres of ponds (See Appendix). Assuming intensive
culture in a closed system, an algal system could reduce the need for CO2 scrubbing by an order of
magnitude, but would still be large (~500 acres). A critical issue for further work is to determine how
compact a system can be realized by exploring novel reactor geometries.
Similarly, issues remain in the economic viability and technical feasibility of algae harvest. Focus must
be placed on scalable systems for dewatering and drying the algae cake. Subsequent to those steps, it
needs to be determined if the algal cake is suitable for animal feed purposes or if it is too high in heavy
metals due to absorption from the gas stream. The extraction of the oil from the cake also needs to be
explored; here options include using the oil (or entire raw cake) directly for combustion, extraction of oil
from the cake for subsequent upgrading to fuels or chemicals, or liquefaction of the complete cake into
crude bio-oil for refining or upgrading. All of these options are possible, although the economics of the
process may be determined by future carbon taxes or credit systems.
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undertaken. Within the study, various reactor configurations should be explored with a goal of decreasing
the footprint and the energy demand for any such system. Also, a process simulation looking at the use of
the system to replace various scrubbing components on existing generation units for process integration
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APPENDIX: Scalability Calculations for Algal Systems
ID Name Unit Amount Comments
1 Power Plant Information
Power Output MWe (net) 100
Coal Comsumption (approximately) lb/hr 78000 plant efficiency=36%, and HHV 12000 Btu/lb
Carbon Conetnt in the Coal % 72
Flue Gas Temperature F 130 use water scrubber for cooling
Flue Gas Mass Flowrate lb/hr 1137705 caculation using Aspen
CO2 Concentration in Flue Gas Stream % by wt 18.1 caculation using Aspen
Flue Gas Density (Standard) lb/SCF 0.08 caculation using Aspen
2 Alga Solution Information
Chemistry Reaction: CO2 -- CH1.8N0.17O0.56
Alga Concentration in the Blowdown Aqueous Solution % by wt 3 manipulated from availabe information
Blowdown Ratio % 3.5 appeared in website of www.greefuelonline.com
Alga Growth Rate %/hr 20 based on 3~7 hours for double weight
3 CO2 Capture Information
Target CO2 Capture Efficiency % 90
4 Lagoon or Closed Bioreactor Deminsion
For GreenFul Approach -- the depth of solution ft 0.5 assumption, based on pressure drop of 30"WC
For Closed Bioreactor
The height of support substrate ft 30 assumption
Space between two rows ft 5 assumption, with 4 ft clearance
Flue Gas Residual Time in the Reactor hr 0.5
SIZING ON SYSTEMS BASIS