SERIITP-231-3202

UC Category: 61

Liquid Fuels from

Microalgae

Donna A. Johnson

Sarah Sprague

August 1987

Prepared for the

Intersociety Energy Conversion

Engineering Conference

10-14 August 1987

Philadelphia, Pennsylvania

Prepared under Task No. 4750.100

FTP No. 656

Solar Energy Research Institute

A Division of Midwest Research Insti tute

1617 Cole Boulevard

Golden, Colorado 80401-3393

Prepared for the

U.S. Department of Energy

Contract No. DE-AC02-83CH10093
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· ,

LIQUID FUELS FROM MICROALGAE

Donna A. Johnson1 and Sarah Sprague2

ISolar Fuels Research Division

Solar Energy Research Institute (SERI)

1617 Cole Boulevard, Golden, Colo. 80401

2Department of Energy (DOE)

1000 Independence Avenue, S.W•

Washington, D.C. 20585

ABSTRACT

The goal of the DOE/SERI Aquatic Species Program is to develop the technology to

produce gasoline and diesel fuels from microalgae. Microalgae can accumulate large

quantities of lipids and can thrive in high-salinity water, which currently has no other

use. The best site for success was determined to be the U.S. desert Southwest with

applications to other warm areas. A technical and economic analysis, "Fuels from

Microalgae," demonstrates that liquid fuels can be produced from mass-cultured

microalgae at prices that will be competitive with those of conventional fuels by the

year 2010. Aggressive research is needed, but the improvements are attainable. Algae

are selected for three criteria; tolerance to environmental fluctuations, high growth

rates, and high lipid production, From 1982 to 1986, the program collected strains that

are twice as tolerant to temperature and salinity fluctuation.. Productivity has been

increased from 35096 to 50096 (outdoor culture versus indoor culture respectively) and

lipid content has also been increased from 100% to 20P%" The microalgae can be

harvested using polymers at a cost of O..5¢ - 1.5¢ kg- dry weight. New program

activities include the design and construction of an outdoor test facility in New Mexico

and research on converting microalgae lipids into liquid fuels.

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INTRODUCTION

The worldwide energy shortage and Arab oil embargo of the early 1970s encouraged many

nations to look for new sources of oil, electricity, and gas. Resources such as biomass

were often viewed as attractive solutions to the energy problem because of their nondepletable, renewable nature. While the first biomass sources considered were readily

available, such as wood or corn, it was apparent that new biomass sources should also be

developed, including aquatic species. The current U.S. emphasis is placed on technology

for our future energy supplies and not commodities (DOE 1987).

The emphasis of the DOE!SERI Aquatic Species Program is to develop the technology

base for large-scale production of lipid-yielding microalgae and conversion of the lipids

into liquid fuels (Johnson 1987). The site with the most promising chance of success was

determined to be the U.S. desert Southwest with technology spillover to other warm

areas of the United States (Figyre I). This technology has the potential of producing

between 150-400 barrels oil acre yr based on the growing season.

Microalgae are small unicellular plants that range in size from 1 to 200 urn, They are

unique organisms in that they can accumulate storage lipids in large quantities within

their bodies (Figure 2). Historically, microalgae have been grown in mass culture mainly

for food production and waste treatment (Benemann et ale 1987). Initial efforts at mass

culture of microalgae were directed toward food production, but the hope of producing

an abundant, low-eost source of protein has not been realized. However, the most

promising early results of mass algae culture have been in the field of sanitary

engineering, where microalgae are used to treat wastewater in oxidation ponds. This

wastewater technology has been expanded to include protein production and treatment of

irrigation water. Microalgae are being grown in Israel, Australia, Mexico, Taiwan, and

the United States for high-value products for the health food market; these microalgae

include the alga Spirulina (at SlO,OOO!dry ton) and the vitamin beta carotene (at

$60,OOO/dry ton). The cultivation of microalgae as a soil conditioner and a food source

for culturing fish and shellfish is increasing in importance. More recently, the possibility

of using algae as a source of energy received widespread attention after the energy crisis

of the 1970s.

The algae can be grown in large outdoor ponds, using the resources of sunlight, saline

water, nitrogen, phosphorus, and carbon dioxide. The algae can convert these raw

materials into proteins, carbohydrates, and lipids. In the process, they can double their

biomass three to five times a day. After a rapid growth phase, the algae are transferred

to induction ponds where nutrient limitation is allowed to occur. Under these conditions,

many algae stop growth and division and use all their energy to make lipids as storage

products to survive. Once the cells have accumulated lipids, they are harvested and the

water is recycled back into the growth ponds. The harvested cells then are subjected to

an extraction process to remove the lipids. Algal lipids are primarily triglycerides with

fractions of isoprenoids, phospholipids, glycolipids, and hydrocarbons. They contain more

oxygen and are more viscous than crude petroleum. The two most promising fuel conversion options are transesterification to produce fuels similar to diesel fuels and catalytic

conversion to produce gasoline. While microalgal lipids represent the premium energy

product, the energy trapped in the other biomass constituents can also be used; e.g., the

cell residue after lipid extraction can be digested anaerobically to produce methane and

carbon dioxide, which can be recycled for use in the algae production system.

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There are four components to large scale algal production and conversion into liquid

fuels: 1) microalgae growth and production; 2) engineering design; 3) harvesting; and 4)

conversion (Figure 3). The design of a microalgae mass culture system is a synergistic

process: the design must be tailored to the characteristics of the culture organism while

species must be selected that contribute to economic construction and operation of the

facility. Microalgae must be selected that are environmentally tolerant, have high

growth rates, and produce large quantities of lipids. In addition, the choice of a suitable

species affects harvesting ease. The types of lipids that the algae produce will determine the conversion methods. Thus, all four areas of development are highly interactive

with each other. Each of these four areas of research and technology development will

be discussed in detail in the remainder of the paper.

Engineering design

Growth and Extraction

production - Harvesting and

conversion

Figure 3. Four Stages Necessary to Produce Liquid Fuels from Microalgae

MICROALGAE GROWTH AND PRODUCTION

As mentioned in the previous section, for this technology to be successful it is necessary

to cultivate microalgae species that are tolerant to fluctuating temperatures and

salinity, have high growth rates and can produce large quantities of lipids. A past

research effort collected various algal strains from diverse geographical locales and

ecological niches. By 1986, over 3000 microalgal strains had been collected. A major

screening effort is now under way to reduce these to the best 10-25 strains that have the

characteristics described above.

The most promising strains collected are Bacillariophyceae (diatoms), Chlorophyceae

(green algae), and Eustigmatophyceae (Eustigmatophytes), The program has identified

several strains that tolerate severe environmental fluctuations in temperature and

salinity. Strains of microalgae used by the program in 1982 exhiyited temperature

tolerances of 15°-25°C and salinity tolerances of 20-40 mmho ern">. With intensive

collection efforts natiorwide, the program now has strains that can tolerate 10o-35°~

and 10-70 mmho ern" (for comparison, seawater is approximately 35 mmho em"

salinity).

Research efforts are continuing to increase the rates of productivity of microalgae to

enhance yields of energy products. We are conducting research in both laboratory and

outdoor cultures to identify species and to develop culture management strategies that

improve productivity rates. Rates of productivity of algae are higher than those of most

other plants. Table I shows the productivity rates of many other plants and indicates the

order of magnitude greater productivity we expect to obtain in the outdoor culture

ponds. This level of productivity has been met in the laboratory and in small outdoor test

ponds on a short-term basis. On a long-term basis we can sustain productivity rates at

7096 of the target.

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The goal of the third major research area in production is to increase the amount of lipid

in each algal cell. Significant increases have been made in quantity, from 2096 of cell

content in 1982 to 66% in the laboratory and 4-0% outdoors. Our target is 60% lipid in

outdoor cultures. We are able to induce lipid accumulation by removing nitrogen or silica

(major nutrients required for growth) from the media. During this period of stress, some

algae will begin to build up reserves of carbohydrates while others will accumulate

lipids. In addition to nutrients, temperature, pH, inorganic carbon and light can all

affect the cell's lipid quantity.

Table 1. Comparative Productivity Rates in Different Plant

Communities (adapted from Whittaker 1975)

Plant Community

Continental:

Tropical rain forest

Temperate deciduous forest

Woodland and scrubland

Desert and semidesert scrub

Cultivated land

Marine:

Open ocean

Continental shelf

Algal beds and reefs

Microalgae ponds

2200

1200

700

90

650

125

360

2500

12500

To improve lipid yields in microalgae, we must understand the physiological and biochemical basis for partitioning photosynthetically fixed CO2 into lipids. The rate of lipid

synthesis and final lipid yield will depend on the availability of carbon for lipid synthesis

and the actual levels and activities of the enzymes used for lipid synthesis. Conditions

such as nitrogen deficiency that induce the accumulation of lipid by algae often

drastically reduce the capacity of photosynthetic CO2 fixation. Low lipid yields could

result either from an absence of carbon skeletons or from low levels of enzymes.

Improvements in lipid yield can be achieved only when the limiting factors have been

determined.

Research efforts are continuing in order to determine the pathways of lipid biosynthesis

in algal cells, especially in the cytoplasm, chloroplast, and mitochondrion. Each pathway

possesses potential lipid triggers. Once the trigger is determined, biochemical and

genetic engineering techniques can be used to increase the lipid yield of promising algal

strains.

To date, no single microalgae strain has been found that exhibits environmental

tolerance, high productivity, and high lipid yield. All three characteristics are necessary

in one organism to meet program goals. For this reason, work has begun on developing

genetic engineering methods so that by 1990 when the program has reduced its strains to

the best 10 to 25, the methods to modify these organisms genetically will be available.

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We are working in three areas of genetic engineering research: 1) classical genetic

manipulation methods; 2) intraspecific genetic variability; and 3) vector and protoplast

fusion methodology. Each research area provides different parts of the total knowledge

that we will need to genetically engineer a better organism.

ENGINEERING DESIGN

Growth conditions in algae mass cultures can be divided into two categories: those

dictated by the location of the culture and those based on culture management

strategy. Location-related variables include insolation, evaporation, rainfall, temperature, and wind velocity. Variables that can be managed include salinity, nutrient concentration, carbon dioxide concentration, culture mixing, culture aeration, and residence

time of the population.

Growth conditions dictated by the facility's location will be among the prime considerations in siting the production facility. The DOE/SERI program has been based on the

assumption that it will be necessary to locate a production facility in an area that

receives large amounts of sunlight and has relatively warm temperatures. To be eco2omical1y feasible, the facility must be locat1d in an area that receives 5000 kcal m" d- 1

and has greater than 200 frost-free d yr">, This limits large scale production to the

southern United States (Figure 4).

Sunlight drives the production of biomass; therefore, ideal production systems will be

located in areas that receive high insolation. It is also necessary to consider the tradeoffs involved in achieving high insolations. If the costs of land, raw materials, or

operation are significantly increased at a location with high insolation, siting solely by

the solar input may be disadvantageous.

Many other production conditions in outdoor systems are dictated by the engineering

design and the management strategy. For example, if saline water is used, the salinity

can be maintained at any given range governed by the algal species selected. Nutrient

concentrations, carbon dioxide concentration, culture mixing, culture aeration and

culture turnover can also be selected.

Three designs were examined for the large scale production of microalgae, These

included open ponds, raceways and enclosed tubes. The proposed costs for construction

and operation of these three systems is shown in Table 2. Since all costs need to be kept

to a minimum for the feedstock to be produced inexpensively, thereby producing an

economic liquid fuel, open pond systems were chosen as the facility plan for outdoor

production (Figure 5).

Table 2. Capital Construction Costs for

Three Different Algal Production

Systems

Engineering Design

Open ponds

Raceways

Enclosed tubes

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Cost ($/ha)

76,000

161,000

348,000

Using the open pond design, a microalgae outdoor test facility (OTF) is currently being

constructed in Roswell, New Mex. for the Aquatics Program. Microbial Products, Inc.,

California, is designing, constructing and will operate the facility. The OTF will allow

the program to evaluate outdoor production performance of microalgae productivities

and to examine the problems and potential of scaling-up and operating large microalgae

production systems. This facility will allow the program to begin evaluations using salin

groundwater in the desert Southwest. The plan that is to operate calls for six 3 m"2

ponds for small scale screening of microalgae growth, two 0.1 ha ponds for large scale

replicate or control versus manipulated experiments, and one 0.5 ha pond to study scaleup problems. The facility will be built over a three-year period as funding is available.

Both biological and engineering assessments of microalgae production systems will be

performed at the OTF. In addition, after several years of operation, a detailed economic

analysis shall be done to provide state-of-the-art economics for large scale production

costs.

HARVESTING

After the cells are grown, they must be separated from the water efficiently and

economically. Techniques for harvesting microalgae include settling or flotation,

centrifugation, and filtration. These processes are aided by cell flocculation, either

through the addition of chemical flocculants or through culture autoflocculation.

Flocculation causes the cells to become aggregated into larger clumps which are more

easily filtered and/or settle more rapidly. The ease in harvesting the algae depends

primarily on the organism's size, which determines how easily the species can be settled

and filtered. The most rapidly growing algal species are frequently very small, and often

motile unicells, These are the most difficult to harvest. Thus, it is necessary to

maintain an effective interaction between the development of harvesting technologies

and the selection of algal species for mass culture.

With current techniques and instrumentation, all microalgae can be harvested with

polymers, although this is not economical. Polymer harvesting is technically feasible but

different algae need different polymers. The amount of polymer increases as the clarification requirement becomes more stringent, making it more cost effective not to require

greater than 8596 removal. With the most suitable polymers and appropriate rpplication

techniques, harvesting can be accomplished for polymer costs of O.5¢-1.5¢ kg- dry mass,

with removal efficiencies of 8596-9596. Polymers with higher rigid backbones are less

affected by the salt concentration and are recommended as flocculants of microalgae in

saline water.

Another method of harvesting that has been examined is chemical flocculation.

Flocculant dose was reduced 7596 by recyling the precipitant following flocculation back

into the mixing-flocculation chamber. Using three flocculation cycles reduced the

required chemicals and removed 9096 of the microalgae from the water.

CONVERSION

Analysis of fuel conversion options for microalgae biomass has demonstrated that the

promise of microalgae for fuel production is best realized through using conversion

processes based on cellular lipids. The two most promising fuel conversion options are

transesterification to produce fuels similar to diesel fuels and catalytic conversion to

produce gasoline. Although microalgae lipids represent the premium energy product, the

energy trapped in the other biomass constituents can also be used; e.g., the cell residue

after lipid extraction can be anaerobically digested for the production of methane and

carbon dioxide.

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We do not believe that the algal lipids can be used directly as a fuel either alone or

blended with crude petroleum. Algal lipids contain approximately 1096 oxygen, while

crude petroleum contains essentially no oxygen. The oxygenates would react at the high

temperatures used in crude distillation and cause polymerization or other undesirable

reactions.

Research is just beginning on the processes to extract and convert algal lipids into

gasoline and diesel fuels. It appears that bioconversion can occur through the enzymes

that occur naturally in the algae. If the algae are left for 16-24h at 20°C, more than

one-half of the triglycerides are converted to fatty acids. This is the first step required

in the conversion process to liquid fuels and may be an economical method, since no

chemicals are required.

A second and unusual method that is being explored is using pyrolysis over a zeolite

catalyst to convert the entire algae cell into aromatic hydrocarbons. Results are not yet

available for this technique. Future research efforts of the program will focus on

increasing the amount of research into lipid-to-fuel conversion.

SUMMARY

A technical and economic analysis, ''Fuels From Microalgae" (Neenan et ale 1986),

demonstrates that liquid fuels can be produced from mass-cultured microalgae at prices

that will be competitive with those of conventional fuels by the year 2010. Aggressive

research is needed in the areas of microalgae growth and production, engineering design,

harvesting, and conversion. We believe that the research improvements necessary to

produce an economic fuel are all attainable. Almost all the required improvements have

been successfully accomplished on an individual basis in small-scale controlled conditions.

BIBLIOGRAPHY

Benemann, J. R., D. M. Tillet, and J. C. Weissman, 1987, "Microalgae Biotechnology,"

Trends in Biotechnology, Vol. 5, pp. 47-53.

Department of Energy, 1987, Energy Security: A Report to the President of the United

States, Washington, D.C.: Department of Energy.

Johnson, D., 1987, FY 1986 Annual Report of the AquatiC Species Program, SERI/SP-231-

3071, Golden, Colo.i Solar Energy Research Institute.

Neenan, B., D. Feinberg, A. Hill, R., McIntosh, and K. Terry, 1986, Fuels from Microalgae: Technology Status, Potential and Research Requirements, SERI/SP-231-2550,

Golden, Colo.: Solar Energy Research Institute.

Whittaker, R. H., 1975, Communities and Ecosystems, 2nd edition, New York, N.Y.:

MacMillan Publishing Company.

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