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As reported by Virginia Cooperative Extention

With energy prices reaching historical highs, biodiesel

as an alternative fuel is increasingly attracting attention.

Currently, biodiesel is made from a variety of

feedstocks, including pure vegetable oils, waste cooking

oils, and animal fat; however, the limited supply

of these feedstocks impedes the further expansion of

biodiesel production. Microalgae have long been recognized

as potentially good sources for biofuel production

because of their high oil content and rapid biomass production.

In recent years, use of microalgae as an alternative

biodiesel feedstock has gained renewed interest

from researchers, entrepreneurs, and the general public.

The objective of this publication is to introduce the

basics of algal-biofuel production and the current status

of this emerging biodiesel source.

Current Feedstock for

Biodiesel Production

Biodiesel can be made from any oil/lipid source; the

major components of these sources are tricylglycerol

molecules (TAGs, figure 1). In general, biodiesel feed

can be categorized into three groups:

stock

1. Pure Vegetable Oil

The first group is pure oils derived from various crops

and plants such as soybean, canola (rapeseed), corn,

cottonseed, flax, sunflower, peanut, and palm. These

are the most widely used feedstocks by commercial

biodiesel producers. The oil composition from vegetable

crops is pure; this cuts down on preprocessing

steps and makes for a more consistent quality of

biodiesel product. However, there is an obvious disadvantage

for vegetable oils as the biodiesel feedstock:

Wide-scale production of crops for biodiesel feedstock

can cause an increase in worldwide food and commodity

prices. Such a “food vs. fuels” debate has reached

national attention when using vegetable oils for biodiesel

production.

CH

CHO–OCHCH

CH

2O–OCHCH2CH2 …….. CH2CH3 |2CH2 ……… CH2CH3 |2O–OCHCH2CH2 …….. CH2CH3

Figure 1. Molecular structure of tricylglycerols

2. Animal Fats

The second group of feedstock for biodiesel production

is fats and tallow derived from animals. Compared

to plant crops, these fats frequently offer an economic

advantage because they are often priced favorably for

conversion into biodiesel. Animal fat, however, has its

own disadvantage when used for producing biodiesel.

Because it contains high amounts of saturated fat, biodiesel

made from this feedstock tends to gel, limiting

widespread application of this type of fuel, particularly

for winter-time use (Wen et al. 2006).

3. Waste Cooking Oils

The third group of biodiesel feedstock is comprised

of recycled oil and grease from restaurants and food

processing plants. The use of recycled oil and grease

is often highlighted in the mainstream news because it

utilizes waste products that can otherwise be disposal

problems. However, recycled oils have many impurities

that require preprocessing to ensure a biodiesel

product of consistent quality. Preprocessing also makes

the biodiesel production process more complicated and

costly (Canakci and Van Gerpen 1999, 2001).

Microalgae as a Feedstock

for Biofuel Production

Michael B. Johnson, graduate student, Biological Systems Engineering, Virginia Tech

Zhiyou Wen, Virginia Cooperative Extension engineer, Biological Systems Engineering, Virginia Tech

2

Background of Algae

Macroalgae vs. Microalgae

Algae are organisms that grow in aquatic environments

and use light and carbon dioxide (CO

There are two classifications of algae: macroalgae

2) to create biomass.

and microalgae. Macroalgae are the large (measured

in inches), multi-cellular algae often seen growing in

ponds. These larger algae can grow in a variety of ways.

The largest multi-cellular algae are called seaweed; an

example is the giant kelp plant which can be more than

100 feet long. Microalgae, on the other hand, are tiny

(measured in micrometers), unicellular algae that nor

grow in suspension within a body of water.

mally

Macroalgae & Microalgae

Algae as a Bioenergy Source

Algae can also be used to generate energy in several

ways. One of the most efficient ways is through uti

of the algal oils to produce biodiesel. Some

algae can even produce hydrogen gas under specialized

growth conditions. The biomass from algae can

also be burned, similar to wood, to generate heat and

electricity.

Algal biomass contains three main components: carbohydrates,

proteins, and lipids/natural oils. Because

the bulk of the natural oil made by microalgae is in the

lization

form of TAGs (figure 1)—which is the right kind of oil

for producing biodiesel—microalgae are the exclusive

focus in the algae-to-biofuel arena. Microalgae grow

very quickly compared to terrestrial crops. They commonly

double in size every 24 hours. During the peak

growth phase, some microalgae can double every 3.5

hours (Chisti 2007). Oil content of microalgae is usu

between 20 percent and 50 percent (dry weight,

ally

table 1), while some strains can reach as high as 80 percent

(Metting 1996; Spolaore et al. 2006).

Table 1. Oil content of microalgae

Microalga

Oil content

(% dry weight)

Botryococcus braunii 25–75

Chlorella sp. 28–32

Crypthecodinium cohnii 20

Cylindrotheca sp. 16–37

Nitzschia sp. 45–47

Phaeodactylum

tricornutum

20–30

Schizochytrium sp. 50–77

Tetraselmis suecia 15–23

Source: Adapted from Chisti 2007

Compared with terrestrial crops—which take a sea

to grow and only contain a maximum of about 5

son

percent dry weight of oil—microalgae grow quickly

and contain high oil content (Chisti 2007). This is why

microalgae are the focus in the algae-to-biofuel arena.

Table 2 lists the potential yields of oil produced by various

crops and compares these values to oil yields from

an open pond growing microalgae.

Table 2. Oil yields based on crop type

Crop

Oil yield

(gallons/acre)

Corn 18

Soybeans 48

Canola 127

Jatropha 202

Coconut 287

Oil Palm 636

Microalgae

1 6283–14641

Source: Adapted from Chisti 2007

1

dry biomass

Oil content ranges from 30 percent to 70 percent of

Other Uses of Algae

In addition to producing biofuel, algae can also be

explored for a variety of other uses, such as fertilizer,

pollution control, and human nutrition. Certain species

of algae can be land-applied for use as an organic

fertilizer, either in its raw or semi-decomposed form

(Thomas 2002). Algae can be grown in ponds to collect

fertilizer runoff from farms; the nutrient-rich algae can

then be collected and reapplied as fertilizer, potentially

3

reducing crop-production costs. In wastewater-treatment

facilities, microalgae can be used to reduce the

amount of toxic chemicals needed to clean and purify

water. In addition, algae can also be used for reducing

the emissions of CO

2 from power plants.

Seaweeds are often used as food—for people and for

livestock. For example, it is often used in food preparation

in Asia. Seaweed is rich in many vitamins, includ

A, B1, B2, B6, C, and niacin. Algae are also rich

ing

in iodine, potassium, iron, magnesium, and calcium

(Mondragon and Mondragon 2003). Many types of

algae are also rich in omega-3 fatty acids, and as such,

are used as diet supplements and components of livestock

feed.

The Synergy of Coal and Algae

One advantage of using algae biomass for biodiesel

production is the potential mitigation of CO

from power plants. Coal is, by far, the largest fossilenergy

resource available in the world. About onefourth

of the world’s coal reserves reside in the United

States. Consumption of coal will continue to grow

over the coming decades, both in the United States

and the world. Through photosynthetic metabolism,

microalgae absorb CO

farm is built close to a power plant, CO

the power plant could be utilized as a carbon source

for algal growth, and the carbon emissions would be

reduced by recycling waste CO

clean-burning biodiesel.

2 emissions2 and release oxygen. If an algae2 produced by2 from power plants into

Algae Mass-Cultivation

Systems

Most microalgae are strictly photosynthetic, i.e., they

need light and carbon dioxide as energy and carbon

sources. This culture mode is usually called photoautotrophic.

Some algae species, however, are capable of

growing in darkness and of using organic carbons (such

as glucose or acetate) as energy and carbon sources.

This culture mode is termed heterotrophic. Due to high

capital and operational costs, heterotrophic-algal culture

is hard to justify for biodiesel production. In order

to minimize costs, algal-biofuel production usually

must rely on photoautotrophic-algal growth using sun

as a free source of light—even though it lowers

light

productivity due to daily and seasonal variations in the

amount of light available.

Photoautotrophic microalgae require several things to

grow. Because they are photosynthetic, they need a light

source, carbon dioxide, water, and inorganic salts. The

water temperature should be between 15°C and 30°C

(approximately 60°F to 80°F) for optimal growth. The

growth medium must contribute the inorganic elements

that help make up the algal cell, such as nitrogen, phos

iron, and sometimes silicon (Grobbelaar 2004).

phorus,

For large-scale production of microalgae, algal cells

are continuously mixed to prevent the algal biomass

from settling (Molina Grima et al. 1999), and nutrients

are provided during daylight hours when the algae are

reproducing. However, up to one-quarter of algal biomass

produced during the day can be lost through res

during the night (Chisti 2007).

piration

There are a variety of photoautotrophic-based, microalgal

culture systems. For example, the algae can be

grown in suspension or attached on solid surface. Each

system has its own advantages and disadvantages.

Currently, suspend-based open ponds and enclosed

photobioreactors are commonly used for algal-biofuel

production. In general, an open pond is simply a series

of outdoor “raceways,” while a photobioreactor is a

sophisticated reactor design that can be placed indoors

(greenhouse) or outdoors. The details of the two sys

are described below.

tems

Open Ponds

Open ponds are the oldest and simplest systems for

mass cultivation of microalgae. In this system, the

shallow pond is usually about one-foot deep, and algae

are cultured under conditions identical to their natural

environment. The pond is designed in a raceway con

in which a paddlewheel circulates and mixes

the algal cells and nutrients (figure 2). The raceways are

figuration,

typically made from poured concrete, or they are simply

dug into the earth and lined with a plastic liner to

prevent the ground from soaking up the liquid. Baffles

in the channel guide the flow around the bends in order

to minimize space.

The system is often operated in a continuous mode, i.e.,

the fresh feed (containing nutrients including nitrogen

phosphorus and inorganic salts) is added in front of the

paddlewheel, and algal broth is harvested behind the

paddlewheel after it has circulated through the loop.

Depending on the nutrients required by algal species,

several sources of wastewater—such as dairy/swine

lagoon effluent and municipal wastewater—can be

used for algal culture. For some marine-type microalgae,

seawater or water with high salinity can be used.

4

Open ponds

Harvest Feed Paddlewheel

Baffle Flow Baffle

Figure 2. Open pond system

Although open ponds cost less to build and operate

than enclosed photobioreactors, this culture system has

its intrinsic disadvantages. Because they are open-air

systems, they often experience a lot of water loss due to

evaporation. Thus, open ponds do not allow microalgae

to use carbon dioxide as efficiently, and biomass production

is limited (Chisti 2007). Biomass productivity

is also limited by contamination with unwanted algal

species as well as organisms that feed on algae. In addi

optimal culture conditions are difficult to maintain

tion,

in open ponds, and recovering the biomass from such a

dilute culture is expensive (Molina Grima et al. 1999).

Enclosed Photobioreactors

Enclosed photobioreactors have been employed to

overcome the contamination and evaporation problems

encountered in open ponds (Molina Grima et al. 1999).

These systems are made of transparent materials and

are generally placed outdoors for illumination by natural

light. The cultivation vessels have a large surface

area-to-volume ratio.

The most widely used photobioreactor is a tubular

design, which has a number of clear transparent tubes,

usually aligned with the sun’s rays (figure 3). The tubes

are generally less than 10 centimeters in diameter to

maximize sunlight penetration. The medium broth

is circulated through a pump to the tubes, where it is

exposed to light for photosynthesis, and then back to

a reservoir. A portion of the algae is usually harvested

after it passes through the solar collection tubes, making

continuous algal culture possible. In some photobioreactors,

the tubes are coiled spirals to form what is

known as a helical-tubular photobioreactor. These sys

sometimes require artificial illumination, which

tems

adds to production costs, so this technology is only

used for high-value products—not biodiesel feedstock.

Either a mechanical pump or an airlift pump maintain

a highly turbulent flow within the reactor, which prevents

the algal biomass from settling (Chisti 2007).

Photobioreactors

The photosynthesis process generates oxygen. In an

open raceway system, this is not a problem as the oxygen

is simply returned to the atmosphere. However, in

the closed photobioreactor, the oxygen levels will build

up until they inhibit and poison the algae. The culture

must periodically be returned to a degassing zone—an

area where the algal broth is bubbled with air to remove

the excess oxygen.

Also, the algae use carbon dioxide, which can cause carbon

starvation and an increase in pH. Therefore, carbon

dioxide must be fed into the system in order to successfully

cultivate the microalgae on a large scale. Photobioreactors

require cooling during daylight hours, and

the temperature must be regulated in night hours as well.

This may be done through heat exchangers located either

in the tubes themselves or in the degassing column.

5

Exhaust

Air

Degassing

column

Fresh

medium

Cooling

water

Pump

Solar array

Harvest

Figure 3. Schematic tubular photobioreactor

The advantages of enclosed photobioreactors are obvious.

They can overcome the problems of contamination

and evaporation encountered in open ponds (Molina

Grima et al. 1999). The biomass productivity of photobioreactors

can average 13 times more than that of

a traditional raceway pond. Harvest of biomass from

photobioreactors is less expensive than from raceway

ponds, because the typical algal biomass is about 30

times as concentrated as the biomass found in raceways

(Chisti 2007).

However, enclosed photobioreactors also have some

disadvantages. For example, the reactors are difficult to

scale up. Moreover, light limitation cannot be entirely

overcome because light penetration is inversely proportional

to the cell concentration. Attachment of cells

to the tubes’ walls may also prevent light penetration.

Although enclosed systems can enhance biomass concentration,

the growth of microalgae is still suboptimal

due to variations in temperature and light intensity.

After growing in open ponds or photobioreactors, the

microalgae biomass needs to be harvested for further

processing. The commonly used harvest method is

through gravity settlement or centrifuge. The oil from

the biomass is extracted through solvent and further

processed into biodiesel.

Research and Development

of Algal-Biofuel Production

Algal-biofuel research originated in 1979, when the

U.S. Department of Energy (DOE) initiated a research

program called the Aquatic Species Program (ASP).

The program was closed in 1995 due to a budget reduction.

Over the 16-year project period, ASP pursued

research in three major areas.

The first area was the study of the biological aspect of

microalgae. It included screening and collecting a variety

of algal species to access their potential for high oil production,

investigating the physiology and biochemistry

of the algae, and using molecular-biology and geneticengineering

techniques to enhance the oil yield.

The second research area was the development of algal

mass-production systems. Several demonstration culture

systems located in California, Hawaii, and New

Mexico were conducted during the project period.

However, in these outdoor systems, it was difficult

to maintain the algal-oil production capacity originally

obtained in the laboratory scale, and researchers

encountered a severe contamination of undesirable

native species. It should be noted that DOE suggested

open ponds as the major system for algal-biofuel production

because of their relative low cost. The cost of

enclosed photobioreactors was still prohibitive due to

capital and maintenance costs, particularly for production

of biofuels.

The third research area was analysis of the resource

availability, including land, water, and CO

2 resources.

DOE concluded that there were significant amounts of

land, water, and CO

In summary, after 16 years of research, DOE

2 to support the algal-biofuel technology.

concluded that algal-biofuel production was still too

expensive to be commercialized in the near future. In

its research, three factors limited commercial algal pro

the difficulty of maintaining desirable species

duction:

in the culture system, the low yield of algal oil, and the

high cost of harvesting the algal biomass.

In recent years, with energy prices reaching historic

highs, algal-biofuel production has gained renewed

interest. Both university research groups and start-up

businesses are researching and developing new meth

to improve algal-process efficiency, with a final

ods

goal of commercial algal-biofuel production. The

research and development efforts can be categorized

into several areas:

1. Increasing oil content of existing strains or selecting

new strains with high oil content;

2. Increasing the growth rate of algae;

3. Developing robust algal-growing systems in either

open-air or enclosed environments;

4. Developing co-products other than oil;

5. Using algae in bioremediation; and

6. Developing an efficient oil-extraction method.

One way to achieve these goals is to genetically and

metabolically alter algal species. The other way is to

develop new growth technologies or to improve existing

ones so that the same goals listed above are met.

6

However, it should be noted that this new wave of inter

has yet to result in a significant breakthrough.

est

Economics of Algal-

Biofuel Production

The production cost of algal oil depends on many factors,

such as yield of biomass from the culture system,

oil content, scale of production systems, and cost of

recovering oil from algal biomass. Currently, algal-oil

production is still far more expensive than petroleum

fuels. For example, Chisti (2007) estimated

diesel

the production cost of algae oil from a photobioreac

with an annual production capacity of 10,000 tons

tor

per year. Assuming the oil content of the algae to be

approximately 30 percent, the author determined a pro

cost of $2.80 per liter ($10.50 per gallon) of

duction

algal oil. This estimation did not include costs of converting

algal oil to biodiesel, distribution and marketing

costs for biodiesel, and taxes. At the same time, the

petroleum-diesel price in Virginia was $3.80 to $4.50

per gallon.

Whether algal oil can be an economic source for biofuel

in the future is still highly dependent on the petroleum

price. Chisti (2007) used the following equation to

oil

estimate the cost of algal oil where it can be a competitive

substitute for petroleum diesel:

C

algal oil = 25.9 x 10-3 Cpetroleum

where: C

microalgal oil in dollars per

gallon and

C

in dollars per barrel

algal oil is the price ofpetroleum is the price of crude oil

This equation assumes that algal oil has roughly 80

percent of the caloric energy value of crude petroleum.

For example, with petroleum priced at $100 per barrel,

algal oil should cost no more than $2.59 per gallon in

order to be competitive with petroleum diesel.

Algal Biofuel in

the Near Future

Algal biofuel is an ideal biofuel candidate which eventually

could replace petroleum-based fuel due to several

advantages, such as high oil content, high production,

less land, etc. Currently, algal-biofuel production is

still too expensive to be commercialized. Due to the

static cost associated with oil extraction and biodiesel

processing and the variability of algal-biomass production,

future cost-saving efforts for algal-oil production

should focus on the production method of the oilrich

algae itself. This needs to be approached through

enhancing algal biology (in terms of biomass yield and

oil content) and culture-system engineering. In addi

using all aspects of the microalgae for producing

tion,

value-added products besides algal fuel—such as in an

integrated biorefinery—is an appealing way to lower

the cost of algal-biofuel production. Indeed, microalgae

contain a large percentage of oil, with the remaining

parts consisting of large quantities of proteins, carbohy

and other nutrients (Spolaore et al. 2006). This

drates,

makes the residue after oil extraction attractive for use

as animal feed or in other value-added products.

References

Canakci, M., and J. Van Gerpen. 1999. Biodiesel pro

via acid catalysis.

ductionTransactions of the ASAE 42:

1203–10.

Canakci, M., and J. Van Gerpen. 2001. Biodiesel pro

from oils and fats with high free fatty acids.

duction

Transactions of the ASAE

44: 1429–36.

Chisti, Y. 2007. Biodiesel from microalgae.

Advances

Biotechnology25: 294–306.

Grobbelaar. J. U. 2004. Algal nutrition. In

Microalgal Culture: Biotechnology and Applied Phycology

ed. A. Richmond, 97–115. Ames, Iowa: Black

Publishing.

Handbook of.well

Metting, F. B. 1996. Biodiversity and application of

microalgae.

477–89.

Journal of Industrial Microbiology 17:

Molina Grima, E., F. Acien Fernandez, F. Garcia

Camacho, and Y. Chisti. 1999. Photobioreactors: Light

regime, mass transfer, and scale up.

Journal of Biotechnology

70: 231–47.

Mondragon, Jennifer and Jeff Mondragon. 2003.

of the Pacific Coast

ISBN 0 930118 29 4.

Seaweeds. Monterey, Calif.: Sea Challengers.

Spolaore, P., C. Joannis-Cassan, E. Duran, and A. Isambert.

2006. Commercial application of microalgae.

Journal of Bioscience and Bioengineering

101: 87–96.

7

Thomas, D. N. 2002.

Seaweeds. Washington, D.C.:

Smithsonian Books; London: Natural History Museum.

ISBN 0 565 09175 1.

Wen, Z., R. Grisso, J. Arogo, and D. Vaughan. 2006.

Biodiesel Fuel

, Virginia Cooperative Extension

publication 442-880.

http://pubs.ext.vt.edu/442-880/

(accessed November 4, 2008).

Acknowledgements

The authors express their appreciation for the review

and comments made by Bobby Clark, Virginia Coop

Extension agent, Shenandoah County Office;

erative

Bobby Grisso, Extension engineer, biological systems

engineering, Virginia Tech; John Ignosh, area specialist,

biological systems engineering, Virginia Tech; and

Wenqiao (Wayne) Yuan, assistant professor, Depart

of Biological and Agricultural Engineering, Kansas

State University.

ment

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