Aquatic biomass and carbon dioxide trapping

Energy Convers. Mgmt Vol. 34, No. 9-11, pp. 1005-1013,1993

0196-8904/93$6.00+ 0.00 Copyright © 1993PergamonPress Ltd

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Aquatic Biomass and Carbon Dioxide Trapping

Lewis M. Brown and Kathryn G. Zeiler National Renewable Energy Laboratory 1617 Cole Blvd. Golden, CO 80401 ABSTRACT

The technology for growing microalgae as a renewable biomass source can be applied to the production of a diesel fuel substitute Coiodiesel). Microalgae are of interest because of their high growth rates and tolerance to varying environmental conditions, and because the oils (lipids) they produce can be extracted and converted to substitute petroleum fuels. Projected global climate change provides a second important rationale for this approach. Climate change has been linked to the accumulation of excess carbon dioxide in the atmosphere. The buming of fossil fuels in power plants is the primary contributor to excess carbon dioxide. Inasmuch as the primary nutrient for microalgal growth is carbon dioxide, operation of microalgal biomass farms has emerged as a promising candidate in the search for alternative approaches to ameliorate global climate change. The production of diesel fuel by microalgae requires very large quantities of carbon dioxide as a nutrient. In areas where microalgae fuel farms operate in tandem with fossil fuel plants to scrub carbon dioxide from flue gases, the release of carbon dioxide could be significantly reduced.

If the microalgae are used to produce fuel, a mass culture facility

reduces by approximately 50% the carbon dioxide emissions from the power plant per million Btu delivered. For example, although coal is ordinarily considered to be the most polluting fossil fuel on the basis of carbon dioxide emilted per amount of energy produced, the incorporation of microalgal ponds with a coal-fired plant would make this fossil fuel less polluting than existing oil- and natural-gasfired plants. Similar advantages can be achieved for oil- and gas-fired plants. If commodity chemicals are produced from algae instead of fuels, the net carbon dioxide reduction is significantly greater. Commodity chemicals can be used to produce goods with long-term uses such as building materials. Such uses would result in the sequestering of carbon dioxide for long periods.

Of the photosynthetic organisms, microalgae are the most productive carbon dioxide users and can fix greater amounts of carbon dioxide per land area than higher plants. Also, maximum productivities of higher plants and trees are restricted to areas with prime soil, water, and climate (primarily the tropics). Plant leaves exist in an aerial environment and are subject to large evaporative moisture losses, which 1005

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BROWN and ZEILER: AQUATIC BIOMASS AND CO2 TRAPPING

directly inhibit the process of photosynthesis (carbon dioxide uptake). Microalgae in mass culture are not subject to such photosynthetic inhibition because the water content of the culture can be controlled by proper engineering, and saline water can be used if necessary. This difference is the basis for the several-fold higher carbon dioxide absorption capacity of microalgae compared to plants.

Initial application of this technology is envisioned for the Desert Southwest of the United States because this area provides high solar radiation and offers flat land that has few competing uses (hence low land costs). Also, there are large saline aquifers with few competing uses in the region. These could provide a suitable, low-cost culture medium for the growth of many species of microalgae.

BACKGROUND

One possible way to supply energy for CO2 trapping is using photon energy from the sun. Although man has yet to develop a way to chemically fix carbon with low energy requirements and high yields, photosynthetic microorganisms routinely fix large amounts of carbon dioxide with water by a series of low-temperature reactions through photosynthesis (capture of carbon dioxide in biomass).

Fig. 1. Outdoor pond technology that may be used for capturing flue gas from power plant flue gases.

Any scheme that involves trapping of carbon dioxide in biomass, or through a biomass-related process

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will result in a need to handle very large amounts of material, preferably in some way that will provide revenue to offset the costs of trapping carbon dioxide. One approach is to make useful products. Products derived from microbial photosynthetic biomass include extraction products such as hydrocarbons, fatty acids, glycerol, protein, pigments, and polysaccharides; bioconversion products such as alcohols, organic acids, and methane; and catalytic conversion products such as paraffins, olefins, and aromatics. Also, a number of chemical products could be produced that would fix carbon for extended periods, and even mineral products can be formed that fix carbon in the long-term geological cycle. A major potential application is the production of transportation fuel from microalgae such as biodiesel.

Biodiesel Fuel from Microalgae Biodiesel from microalgae could play a significant role in filling future domestic transpoCtation fuel needs. It is anticipated that gasoline production in the U.S. will be reduced as petroleum feedstock availability becomes more limited and alcohol-based fuel production increases. Since diesel fuel is a coproduct of the gasoline refining process, the future reduction in gasoline production requires the development of a substitute, renewable source of diesel fuel. Biodiesel is an extremely attractive candidate to fulfill this need for a diesel fuel substitute. Biodiesel is a cleaner fuel than petroleum diesel. It is virtually free of sulfur, thereby eliminating the production of sulfur oxides during combustion. Emissions of hydrocarbons, carbon monoxide, and particulates during combustion are also significantly reduced as compared to petroleum diesel. These properties make biodiesel an attractive candidate to facilitate compliance with the amended 1990 U. S. Clean Air Act. Additional benefits of biodiesel are that it is thought to be a biodegradable fuel that can be utilized in standard, unmodified diesel engines. Biodiesel provides essentially the same energy content and power output as petroleum-based diesel fuel while reducing emissions.

Biodiesel's low pollutant emissions can be extremely useful in U. S.

Environmental Protection Agency non-attainment areas; typically, cities with acute local air pollution problems. Buses and other fleet vehicles running on hiodiesel have the potential to make a major impact in these markets. Biodiesel is ordinarily considered to be derived from oilseeds, but an essentially identical biodiesel can be made from microalgae.

Productivities Of the photosynthetic organisms, microbial systems can be employed as the most productive carbon dioxide users and can fix greater amounts of carbon dioxide per land area than higher plants (e.g., trees and sugar cane). Also, maximum productivities of higher plants and trees are restricted to areas with prime soil, water, and climate (primarily the tropics). Plant leaves exist in an aerial environment and are subject to large evaporative moisture losses, which directly inhibit the process of photosynthesis (carbon dioxide uptake).

Photosynthetic microorganisms in mass culture are not subject to such

photosynthetic inhibition because the water content of the culture can be controlled by proper

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engineering, and saline water can be used if necessary. This difference is the basis for the several-fold higher carbon dioxide absorption capacity of microbial photosynthetic systems compared to plants. Furthermore, carbon dioxide can be trapped effectively in photobioreactors without covers, while higher plants would require expensive canopies to contain the carbon dioxide for efficient plant growth.

PHOTOSYNTHETIC MICROBIAL SYSTEMS AND CARBON DIOXIDE TRAPPING

This study considers the broadest possible approach to using photosynthetic microbial systems to trap carbon dioxide released from power plants. The analysis addresses the photosynthetic organisms that have been demonstrated to trap carbon dioxide, documents their characteristics and evaluates the potential of these organisms for use in a bioengineering system to trap carbon dioxide. The organisms that can fix carbon dioxide using light energy are the purple bacteria, the green bacteria, the cyanobacteria and the algae.

1) Purple and Green Bacteria. a) Taxonomy:

All purple and green bacteria have bacteriochlorophylls, but they lack chlorophyll a of plants.

Purple Bacteria: Rhodospirallaceae - non-sulfur purple bacteria Chromatiaceae - purple sulfur bacteria

Green Bacteria: Chlorobiaceae - non-motile green bacteria Chloroflexaceae - motile filamentous gliding green bacteria

b) Photosynthesis:

The green and purple bacteria perform a primitive type of photosynthesis with one photosystem which operates under strictly anaerobic conditions to fix carbon dioxide. The only group that can tolerate the presence of oxygen is the purple non-sulfur bacteria (Rhodospirallaceae).

However, the

Rhodospirallaceae cannot use light under aerobic conditions as their bacteriochlorophyll pigments become bleached (Stanier et al., 1970; Pfennig 1978). Inasmuch as flue gases typically contain 4-5 % oxygen, the green and purple bacteria cannot be used with flue gas as a carbon dioxide source unless oxygen is removed. This may make the process economically unattractive when compared to other

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microbial options.

In contrast to other photosynthetic microorganisms, the green and purple bacteria do not perform the photolysis of water (water-splitting), but instead derive their electrons from reduced materials. Such materials as hydrogen sulfide, thiosulfate, molecular hydrogen or organic compounds can be used. Some typical reactions are as follows (Truper, 1978):

light 4C02 + 2I~S + 4I-I~0 ~

4(CI-I~0) + 2I~SO,.

or

light

c02 + 2Na~S203 + I-t20 --> (CI-t20) +2S + 2Na2SO,

or

light 3CO~ + 2s + 5H20 ~

3(C~O) + 2H~SO,

c) Conclusions:

The key conclusion that can be drawn from examiningall of these reactions is that a reduced chemical is necessary in addition to light in order to fix carbon dioxide for green and purple bacteria. The quantity of reduced chemical required to act as an electron donor ranges from 0.5 to 1 molar quantity compared to carbon dioxide. This is a large amount of material when one considers carbon dioxide resulting from fossil fuel combustion. It is extremely unlikely that quad level amounts of these materials could be provided, and even if quad level amounts could be provided, the economics would be highly unfavorable.

This problem, along with the sensitivity of these bacteria to oxygen, make the

photosynthetic green and purple bacteria an unsuitable based on currently projected technology. Purple and green bacteria in nature are restricted to specialized environments with reduced substrates and no oxygen such as the lower depths of deep stagnant lakes.

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BROWN and ZEILER: AQUATIC BIOMASS AND CO2 TRAPPING

2) Cyanobacteria and Microalgae. a) Taxonomy: Cyanobacteria - bacteria which possess advanced photosynthetic capacity (two photosystems and the ability to split water) and chlorophyll a (plant chlorophyll) Microalgae- mostly single-celledplants which possess advancedphotosyntheticcapacity (two photosystems and the ability to split water). About 16 discrete groups exist with various discrete characteristics and pigmentation type. All have chlorophyll a (plant chlorophyll).

The Algal Classes: Common Name

Scientific Name Chlomphyceae

green algae

Charophyceae

stoneworts

Prasinophyceae

scaly-green monads

Rhodophyceae

red algae

Cryptophyceae

cryptophytes

Glancophyceae

glaucophytes diatoms brown algae

Baeillariophyceae Phaeophyceae Chrysophyceae

golden algae

Prymnesiophyceae(=Haptophyceae)

haptophytes/coccolithophorids

Tribophyceae (=Xanthophyceae)

yellow-green algae

Eustigmatophyceae

eustigmatophytes

Chloromonadophyceae(=Raphidophyceae)

chloromonads

Dinophyceae (=Pyrrbophyceae)

dinoflagellates

Euglenophyceae

euglenoids

Pmchlorophyceae

prochlorophytes

b) Photosynthesis: The purple and green bacteria evolved early in the primitive earth. Their photosynthesis depended on

BROWN and ZEILER: AQUATIC BIOMASS AND CO2 TRAPPING the presence of reduced materials such as hydrogen sulfide.

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In contrast, the cyanobacteria and

microalgae evolved later, with a more advanced type of photosynthesis.

This more advanced

photosynthesis has two photosystems. This new second photosystem performs the water splitting reaction:

light 21-120 ~

02 + 41-1"+4e

This reaction allowed the evolution of advanced life on earth including all plants and animals. The reason for this is that water is universally available on earth, thus allowing the production of greater amounts and diversity of biomass. The same principle applies when considering the potential for absorbing cation dioxide. The inexpensive reactant, water, is far more preferable to the low abundance reduced substrates.

The dark reactions of photosynthesis utilize the reducing power generated in the preceding (light) reactions for the reduction of carbon dioxide (equation is a simplified approximation):

CO2 + 2H ÷ + 2e --->(CH20)

The overall reaction for photosynthesis by cyanobacteria, microalgae and plants is

light CO2 + I-I20 ~

(CH20) + O2

c) Conclusions:

The cyanobacteria and the microalgae are the only groups of microbes with the potential to fix carbon dioxide from power plant emissions using only light energy and water as primary components.

ENGINEERING ANALYSIS

As discussed above, one possibility for supplying energy for CO2 trapping is photon energy from the sun. Man has yet to develop a way to chemically fLX carbon with low energy requirements and high yields. However, algae, cyanobacteria, and plants routinely fix large amounts of carbon dioxide with water by a series of low-temperature reactions through photosynthesis (capture of carbon dioxide in

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BROWN and ZEILER: AQUATIC BIOMASS AND CO2 TRAPPING

biomass) that employs water-splitting and two photosystems.

A more detailed account of the issues

relating to flue gas processing is in press (Brown and Chelf, 1993).

Microalgae biomass production can increase the CO2-trapping capacity of desert land almost 70-fold (Weissman and Tillett, 1989) to a level more than twice that of a typical tropical rainforest (Whittaker and Likens, 1975). We estimate that microalgal biomass production can increase the productivity of desert land 160-fold (6 times that of a tropical rainforest). Furthermore, in contrast to the limited available arable land for forestry, much desert land is available globally for microalgae farming. Microalgae require only 140-200 lb of water per pound of carbon fixed even in open ponds (based on average pond evaporation rates (Neenan et al., 1986), and this water can be low-quality, highly saline water. Thus, the biophysical and thermodynamic constraints favor microalgae, especially in arid and semi-arid regions. Microalgae are also unique among photosynthetic organisms in that they can achieve extremely high productivities at salinities as high as twice that of seawater (Brown and Hellebust, 1978; Brown, 1982; Brown, 1985), and thus use low-quality (saline) water that is unusable for agriculture or urban uses.

The use of microalgae in mass culture as a means to reduce the amount of carbon dioxide emitted affects process economics for fuel production. Instead of requiring that microalgae-derived fuel to be cost competitive with fossil fuels, the process economics must be compared with other technologies proposed to deal with the problem of carbon dioxide pollution. However, development of environmentally beneficial energy technologies will be of benefit whether or not global climate change occurs. Fuels from microalgae have great potential to contribute to world energy supplies and to provide a method control CO2 emissions as the demand for energy increases.

SUMMARY AND FUTURE WORK

All possible photosynthetic systems that could be used to trap carbon dioxide were considered. The analysis indicated that only those organisms that possess two photosystems and perform photolysis of water have any immediate potential for trapping carbon dioxide from flue gas. These microbes are the microalgae and cyanobacteria. The approach will be focussed in three major technology areas. The first, is the effect of simulated flue gas mixtures on the growth and survival of photosynthetic microbes in culture. This work will also deal with the effects of various nutritional, chemical and physical factors on growth, and the effects and metabolism of other flue gas components such as sulfur and nitrogen oxides. The second area will consist of engineering analysis focussed on topical areas which arise as a result of research on culture issues.

Finally, powerful tools will be derived from genetic

engineering/molecular biology research. These technological tools will allow new approaches to the definition of key factors in carbon dioxide capture, and lead to methods for better strain improvement or mass culture management. Early results from genetic engineering will provide opportunities for spinoff research through technology transfer to the private sector.

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REFERENCES

Brown, L. M. and P. Chelf. (1993). Aquatic Biomass Resources and Carbon Dioxide Trapping. Biomass Bioenerg. (in press). Brown, L.M. (1982). Photosynthetic and growth responses to salinity in a marine isolate of Nannochloris bacillaris (Chlorophyceae). J. Phycol. 18, 483-488. Brown, L.M. (1985). Stepwise adaptation to salinity in the green alga Nannochloris bacillaris. Can. J. Bot. 63, 327-332. Brown, L.M. and J.A. Hellebust. (1978). Sorbitol and praline as osmotic solutes in the green alga Stichococcus bacilIaris. Can. J. Bot. 56, 676-679. Neenan, B., D. Feinberg, A. Hill, R. Mdntosh, and K. Terry. (1986). Fuels from Microalga¢: Technology Status, Potential, and Research Requirements. SERI/SP-231-2550, Solar Energy Research Institute, Golden, CO. 149 pp. Pfennig, N. 1978. General Physiology and Ecology of the Photosynthetic Bacteria. pp. 3-18. in The Photosynthetic Bacteria. pp 3-18. Plenum Press, New York and London. Stanier, R. Y., M. Doudoroff, and E. A. Adelberg. 1970. The Microbial World. Prentice-Hall, Inc., Englewood Cliffs. Truper, H. G. 1978. Sulfur Metabolism. In: The Photosynthetic Bacteria. (R. K. Clayton, and W. R. Sistrom, ed.), pp. 677-690. Plenum Press, New York and London. Weissman, J.C., and D.M. Tillett. 1989. Unpublished. Whittaker, R. H. and G. E. Likens. 1975. In: Primary Productivity in the Biosphere. (H. Lieth and R. H. Whittaker, ed.). Springer-Verlag, NY. pp. 305-328.

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