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Fuel Options from 

Microalgae with Representative 

Chemical Compositions 

Juty 1984 

Frepomd urrdsr tcrrlr No. 4625.20 

FTP No. 386 

Solar En- Research Institute 

F fl v144r1 of l!irjaet?5? P~~PR~cCI ln¶t *IJ~P 

Pr~ared for ?fit? 

U.S. Department of Energy 

Co-tract N3. 3E-AC02-P3CH lC093 
PREFACE 

The Solar Fuels Research Division of the Solar Energy Research 

Institute, under the Aquatic Species Program, is carrying out a 

broad range of research activities to develop microalgae as a 

potential source of renewable liquid fuels. This study examines a 

variety of microalgal species with representative chemical compositions, ranking each for its potential as a renewable source of 

ethanol, methane, and lipid-derived fuels such as synthetic diesel 

fuel (methyl ester). This assessment is based solely on chemical 

composition and does not take into account the yield factor. The 

work was performed as a follow-up to "Fuel from Microalgae Lipid 

Products," by D. Feinberg and A. Hill, which was published in 

April 1984 as SERI/TP-231-2348. 

Daniel Feinberg 

Approved for 

SOLAR ENERGY RESEARCH INSTITUTE 

S. H. Browne, Group Manager 

Technical Evaluation and Planning Group 

S. dull. Dividon Director 

Solar Fiels ~Gearch Division 
Objective 

To examine various species of microalgae with representative chemical compositions and to evaluate a variety of renewable liquid (and gaseous) fuel 

options. 

Discussion 

Each of the three biochemical fractions of microalgae (lipids, carbohydrates, 

and proteins) can be converted into fuels. Lipids have the highest energy 

content of the three. The lipids of some species are hydrocarbons, similar to 

those found in petroleum, while those of other species resemble seed oils, 

which can be converted to a synthetic diesel fuel (ester fuel) by the process 

known as transesterification. Carbohydratees are commonly converted to 

ethanol by fermentation. Alternatively, all three fractions can be converted 

to methane gas by anaerobic digestion. A total of eleven different cases 

(nine different species) are examined in this report, including four species 

identified as high-lipid producers, three high-carbohydrate producers, three 

high-protein producers, and one high-glycerol producer. 

Based on the chemical compositions reported for the various species, an estimate is first made of the gross energy content available from a unit mass of 

each species. Then, options are considered that convert each fraction into 

the desired products. For example, the entire mass might be converted to 

methane, the carbohydrate fermented to ethanol, the lipid converted to ester 

fuel, or any combintaion of these. 

Conclusions 

Among the high-lipid producing species, Botryococcus braunii is unique in that 

it can produce hydrocarbons at 40% or higher of its total lipids. Most of 

these hydrocarbons are benzene-extractable, aromatic-type compounds, which 

might be directly usable as liquid fuel. A critical research question is 

whether sufficiently high growth rates could be achieved to permit econow 

ically competitive fuel production from this organism. Another high-lipid 

producer, Nannochloropsis~ salina, has shown the ability to produce up to 34% 

of its total lipid as fatty acids, which can in turn be esterified to produce 

ester fuel. Calculations indicate that 53.1% of the total energy content of 

this organism can be converted into liquid fuel. It is clear from a chemical 

composition standpoint that lipid producers offer the most potential for 

renewable production of high-energy liquid fuel. 

All three of the high-carbohydrate producing species examined offer potential 

as renewable sources of ethanol. The diatom Cyclotella cryptica, with a 

reported 67% carbohydrate content, and Chlamydomonas sp., with 59% carbohydrates, would produce exclusively ethanol, converting approximately 30% of 

their total energy contents into a single liquid fuel. Dunaliella salina, on 

the other hand, contains approximately 3% fatty acid and 1% hydrocarbon as 
well as 55.5% carbohydrate; up to 47% of its total energy can therefore be 

converted to liquid fuels. 

Although liquid fuels, especially hydrocarbons, are more valuable due to their 

ease of storage and handling (especially for the vast transportation fuel 

markets), all the species examined offer great potential as sources for the 

production of methane via anaerobic digestion. Due to the inherently lower 

energy requirements and higher conversions for this process, energy utilization efficiencies range from about 68% (for high-protein producers) to 75% 

(for high-lipid producers). The high-protein producing species, as well as 

another strain of D. salina that produces about 28% glycerol, might have an 

economic advantage due to their coproduction of higher valued products. 

Detailed examinations of algal productivity data, as well as economic studies, 

are not required to further quantify the potential of these organisms as 

renewable fuel producers. 
S=?I . @I I TR-2427 

\=4 

TABLE OF CONTENTS 

Page 

1.0 Introduction .................................................. . 1 

2.0 Approach ....................................................... 3 

3.0 Fuel Options as Determined by Chemical Composition .................. 6 

3.1 Botryococcus braunii ........................................... 6 

3.2 Ankistrodesmus falcatus ........................................ 8 

3.3 Isochrysis sp .................................................. 8 

3.4 Nannochloropsis salina ......................................... 10 

3.5 Dunaliella salina .............................................. 10 

3.6 Chlamydomonas sp ............................................... 10 

3.7 Cyclotella cryptica ............................................ 10 

3.8 Spirulina platensis ........................................... 13 

3.9 Chlorella sp ................................................... 13 

3.10 Nannochloropsis salina ......................................... 13 

3.11 Dunaliella salina .............................................. 15 

4.0 Discussion .......................................................... 18 

5.0 Conclusions ......................................................... 19 

6.0 References .......................................................... 23 
TR-2427 Si?I 4&1 

LIST OF FIGURES 

Page 

1-1 Microalgal Production and Products .................................. 2 

2-1 Ethanol Yield from Microalgae Biomass ............................... 3 

2-2 Hydrocarbon/Methyl Ester Yield from Microalgae Biomass .............. 4 

2-3 Methane Yield from Microalgal Biomass ............................... 5 

5-1 Gas vs . Fuel Production from Representative High-Lipid Producers .... 19 

5-2 Gas vs . Fuel Production from Representative High-Carbohydrate 

Producers ........................................................... 20 

5-3 Gas vs . Fuel Production from Representative High-Protein Producers .. 21 

LIST OF TAJSLES 

3-1 Chemical Composition of Various Microalgae .......................... 7 

3-2 Fuel Production Options for Botryoccus braunii ...................... 9 

3-3 Fuel Production Options for Ankistrodesmus falcatus ................. 9 

3-4 Fuel Production Options for Isochrysis sp ........................... 11 

3-5 Fuel Production Options for Nannochloropsis salina .................. 1'1 

3-6 Fuel Production Options for Dunaliella salina ....................... 12 

3-7 Fuel Production . Options for Chlamydomonas sp ........................ 12 

3-8 Fuel Production Options for Cyclotella cryptica ..................... 14 

3-9 Fuel Production Options for Spirulina platensis ..................... 14 

3-10 Fuel Production Options for Chlorella ............................... 15 

3-11 Fuel Production Options for Nannochloropsia salina .................. 15 

3-12 Fuel Production Options for Dunaliella salina ....................... 17 

SECTION 1.0 

INTRODUCTION 

Photosynthetic organisms such as microalgae "fix" atmospheric carbon into the 

energy-storage components of their cell mass; i.e. protein, carbohydrate, and 

lipid. Under the limitations of current technology, algae can convert up to 

15% of the photosynthetically available solar radiation (PAR), or roughly 6% 

of the total incident radiation, into new cell mass [I]. 

The conversion of solar energy into renewable liquid fuels and other products 

could become economically competitive with petroleum if research progress continues. The historical emphasis on high-energy lipids as the primary fuel 

products from microalgae was based on some species' ability to accumulate 

large quantities of these compounds, especially under stressful growth conditions [2]. The objective of this report is to examine the chemical makeup of 

a variety of algal species under different growth conditions and the conversion processes possible for each class of compounds (i.e., lipids, carbohydrates, and proteins). Based dn the results of this analysis, some projections are made about the optimum fuel product conversion route(s) for several 

representative species. 

As part of the Solar Energy Research Institute (SE~I)/United States Department 

of Energy (DOE) Aquatic Species Program's major study of fuel products from 

microalgae, this report provides an important link between cultivation and 

harvesting of a variety of algal species and conversion of specific biochemical fractions into the desired products. 

The recent work of Tornabene et al. and Ben-Amotz in characterizing different 

algae with respect to chemical composition, especially possible for each class 

of compounds i, lipids, carbohydrates, and proteins). Based on the 

results of this analysis, some projections are made about the optimum fuel 

product conversion route(s) for several representative species. 

As part of the Solar Energy Research Institute (SERI)/United States Department 

of Energy (DOE) Aquatic Species Program's major study of fuel products from 

microalgae, this -report provides an ,important link between cultivation and 

harvesting of a variety of algal species and conversion of specific biochemical fractions into the desired products. 

The recent work of Tornabene et al. and Ben-Amotz in characterizing different 

algae with respect to chemical composition, especially the lipid fractions, 

has been extremely valuable. Their reports [3, 4, 51 discuss in detail materials and methods for these characterizations. Other sources of biochemical 

data may be found in Burlew [2]. The first step is to examine the total chemical composition and gross energy content of several representative species. 

The next level of consideration is to examine the crude lipid and carbohydrate 

components, followed by any subfractions for which data are available; e.g., 

the neutral lipid fraction extracted by hexane or the benzene-soluble fraction 

containing aromatics and carotenoids. 
Resource Feedstock Process 

COz Fermentation 

Water Algal mass Anaerobic 

digestion 

N-source 

Catalytic 

Products 

Distillate 

Protein 

Fatty acids 

Glycerol 

SCP ethanol 

Mannitol 

Organic acids 

Solvents 

Methane 

Hydrogen 

Paraffins 

Olefins 

Aromatics 

Figure 1-1. Microalgal Production and Products 

It is beyond the scope of this report to consider all the potential conversion 

processes in detail with respect to process design and economics; however, a 

few process routes are considered, and each specie is evaluated to determine 

whether it might be a suitable feedstock. The process options considered are 

aerobic and anaerobic fermentation for carbohydrates, anaerobic fermentation 

only for proteins, and extraction and conversion processes in general and 

transesterification in particular for lipids. Figure 1-1 shows schematically 

how different conversion processes could produce a variety of fuel products 

from a microalgal feedstock. 
SECTION 2.0 

APPROACE 

Based on the reported algal composition, a gross energy content is calculated. 

"Typical" algal proteins are assumed to contain 23.86 MJ/kg (10,260 ~tu/lb); 

carbohydrates 15.92 MJ/kg (6840 ~tu/lb); lipids 38.93 MJ/kg (16,740 Btu/lb); 

and glycerol 18.05 MJ/kg (7760 ~tu/lb) [2]. 

In today 's conventional ethanol fermentation, yeasts such as -Saccharomyces 

cerevisiae can convert up to 95% of the "available" carbohydrate, i.e., glucose, into a 1:l (weight) ratio of ethanol and carbon dioxide. The main 

requirement here is to estimate the available carbohydrate. Algal carbohydrates typically are complex mixtures of mono-, poly-, and oligosaccharides, 

with pentoses and hexoses having been identified [2]. A reasonable approach 

is to assume that about two-thirds of the carbohydrate can be hydrolyzed to 

fermentable hexose monomer with the remainder essentially all pentose; i.e., 

not fermentable under current commercial practice. A combined hydrolysisfermentation yield of 80% is assumed for this carbohydrate fraction, which is 

much less homogeneous but more accessible to hydrolysis than, say, lignocellulose. These assumptions result in a net alcohol production of 0.329 L/kg 

(0.04 gal/lb) and a total energy content of 7.74 MJ/kg (3330 Btu/lb) of total 

carbohydrate. Figure 2-1 shows the ethanol yield and energy production as 

functions of the carbohydrate content A fermentation by-product, carbon 3- dioxide (approximately 39 L/L or 5.2 ft /gal of ethanol), ds also produced. 

Figure 2-1. Ethanol Yield from Microalgae Biolaass 

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This screening study examines two lipid utilization routes: transesterification (conversion of triacylglycerols to monoesters of either methanol or 

ethanol) and a "simple" extraction of hydrocarbons. Based on data from 

Tornabene et al. [3], the hydrocarbon extraction calculations assume that the 

neutral lipids directly usable as fuel would be equal to the hexane extract 

plus one-half the benzene extract. In fact, the hydrocarbons of many species, 

at 28+ carbons, may require cracking or other processing to insure their 

suitability. These fractions could be recovered with processing losses of only 

2%, yielding 1.15 L of hydrocarbon liquids per kg of lipid (0.65 galllb). 

Based on the energy content being equal to diesel fuel at 39 W/L 

(129,500 Btulgal), 44.96 MJ (19,370 Btu) of energy per kg (lb) of lipid in the 

form of hydrocarbon could be recovered (Figure 2-2). 

Transesterification, which is discussed in more detail elsewhere [61, produces 

a mixture of fatty acid esters, which have been shown to be suitable substitutes for diesel fuel. The assumed conversion of 95% of the triglycerides to 

fatty esters is routinely achieved in commercial processes. The critical 

parameter is the fraction of total lipids available for conversion; i.e., the 

triglyceridelfatty acid fraction. This fraction is estimated at 89% of the 

chloroform extract (exclusively triglyceride, converted to fatty acids) plus 

65% of the methanol extract (phospholipids converted to fatty acids) [7]. 

Alternatively, in some cases the triglyceride/fatty acid contents have been 

reported directly. The ester fuel produced has an energy content of 35 MJ/L 

(116,200 Btulgal) or about 10% below that of diesel fuel [81. These assumptions result in an ester fuel production of 1.25 L/kg (0.73 gal/lb) and an 

energy content of 43.8 MJ/kg (18,850 Btu/lb) of triglyceride (Figure 2-2). 

1.1 Hydrocarbon = 1.0 I% hexane) \ - - ""-I 

1 .o + 0.5 (96 benzene) fraction 445.0 

I I I I I I I I I I I 

0 10 20 30 40 50 60 70 80 90 100 

Percent of dry weight 

Figure 2-2. Eydrocarbon/Methyl Ester Yield from Microalgae Biorrass 
Anaerobic digestion is the most flexible process in terms of available feedstocks. Basically, the entire organic weight, which was not used by one of 

the previously mentioned pro esses, is digested. The theoretical methane production rates 191 are 0.49 n? of methane per kg of protein converted, 0.37 m3 

per kg of carbohydrate, and 1.04 m3 per kg of lipid, and it is assumed that 

80% of the volatile solids are converted to products (80% COD removal). The 

gaseous product is assumed to contain the typical biogas composition of 607 

methane and 40% carbon dioxide. With an energy content of 37.2 MJ/m 3 

(1000 ~tu/ft~) of methane, energy production per kilogram of volatile solids 

converted is 30.95 W/kg (9960 Btu/lb) lipid, 14.58 M.J/kg (4740 Btu/lb) protein, and 11.01 MJ/kg (3600 Btullb) carbohydrate (and glycerol).- This information is shown graphically in Figure 2-3. 

Figure 2-3. Methane Yield from Microalgal Bionaass 
SECTION 3.0 

FUEL OPTIONS BS DETERMINED BY CHEMICAL COMPOSITION 

Table 3-1 summarizes the chemical composition of representative algal species 

grown under a variety of nutrient and salinity conditions. The different 

species are grouped according to the primary products: lipids, carbohydrates, 

protein, and glycerol. The glycerol fraction could either be digested for gas 

or recovered as a by-product. For each species the amount of energy that can 

be converted to fuels and the fraction of the gross energy content are calculated in the following ways: 

Methane only: anaerobic digestion of the entire (ash-free) cell mass, 

including glycerol 

Methane-glycerol: anaerobic digestion of the cell mass, excluding glycerol 

Ester-methane: digestion of the protein and carbohydrate fractions only, 

with the lipids being converted to ester fuel and hydrocarbon 

Ethanol-methane: digestion of the protein and lipid fractions, with the 

carbohydrate being converted to ethanol 

Ester-ethanol-methane: digestion of the protein fraction only, with ester 

fuel, hydrocarbon, and ethanol production. 

These are identified in Tables 3-2 through 3-12 as Options 1 through 5, 

respectively. All energy values are reported in MJ/kg ash-free dry weight 

(multiply by 430 to convert to Btullb). 

3-1. Botryococcus braunii 

B. braunii, a slow-growing organism, is interesting because it accumulates a 

large fraction of its biomass as hydrocarbon/lipid. Table 3-1 (Example 1) 

shows that under nitrogen-deficient growth conditions in fresh water, this 

organism accumulates lipids at 54.2% of ash-free dry weight. A sizeable protein-carbohydrate fraction, about 34%, is also present. More than 53% of the 

total lipid content is benzene extractable and consists of aromatic hydrocarbons, which could be directly usable as fuels. Another 14.9% of total lipids 

are hexane-extractable, straight-chain alkanes (saturated hydrocarbons) primarily in the 30-32 carbon range. Coupled with this extremely high hydrocarbon 

content is a low fatty acid content estimated at 7.8% of the total lipid. 

Examination of this fraction shows more than 55% to be saturated and singly 

unsaturated, indicating an ester fuel product relatively stable toward autooxidation during storage. 

There is little question that' this lipid content would be high enough to use 

the organism as a conversion feedstock if it could be grown at sufficiently 

high rates (Table 3-2). In general, the options that convert lipids to liquid 

fuels would be preferred, since more of the cell's energy is then recovered in 

the premium liquid form. An overall energy utilization rate of 75% can be 

achieved by anaerobic digestion of the lipid and protein fractions, and either 

digestion or fermentation of the carbohydrate. Conversion of the lipids to 

liquid fuels (ester fuel and hydrocarbon) would recover 42% of the gross 

energy content in liquid form; fermentation would contribute another 5%, for a 

liquid fuel utilization rate of 47% and an overall utilization rate of 58%. 

One of the key issues to be addressed by those wishing to cultivate this 

organism for fuel production would be: what are the limitations on the direct 

use of the hexane and benzene fractions as fuels? The predominance of saturated hydrocarbons with 30-32 carbons mentioned earlier may be too long for 

diesel fuel and possibly even for heavy fuel oils. An additional processing 

step might be required for conversion of part (or all) of the lipid fraction 

into usable liquid fuels. With this arguement the most critical question is 

whether a high enough growth rate can be maintained to enable these fuel 

options to be economically pursued. 

3-2. Ankistrodes~ms falcatus 

The next high lipid-producing species considered is Ankistrodesmus falcatus, 

grown in nitrogen-deficient media. As shown in Table 3-1, more than 42% of 

the ash-free dry weight is lipid, with another 34% consisting of carbohydrate 

and protein. Unknown components represent a substantial 23%. The hexane 

fraction is negligible and the benzene extract constitutes only 3.3% of total 

lipids. The chloroform (13.5%) and methanol (12.1%) are also small, resulting 

in a total fatty acid content of 19.9% of total lipids. Maximum overall fuel 

utilizations of approximately 75% are again achieved by anaerobic digestion, 

with or without fermentation. Conversion of triglycerides into ester fuel 

(without ethanol fermentation) produces both an overall energy utilzation rate 

(58.3%) and a liquid fuel utilization (17.3%) much lower than Botryococcus; 

inclusion of fermentation further reduces overall utilization to 34.9% but 

increases liquid fuel utilization to 25.5% (Table 3-3). These calculations 

show that high lipid content alone may not be indicative of a promising algal 

species if the right kind of lipids is not prevalent. 

3-3. Isochrysis sp. 

Example 3 shows the composition of an Isochrysis cultured under moderately 

saline (0.5 M NaCl), nitrogen-deficient conditions. Although the lipid content is a moderate 26%, this organism is included among the high-lipid producers; lipid contents as high as 45% have been reported recently. Protein 

(23%) and carbohydrate (21%) are both substantial components. The hexane 

fraction represents 2.2% of total lipids, the benzene fraction 28.4%, the 

chloroform fraction 18%, and the methanol fraction 25.3% (total fatty acids 

32.5%). Tornabene has tentatively identified the major constituent of the 

benzene fraction as an oxygenated cyclic C-37 isoprenoid chain, which could be 

significant either for fuels or by-products. Table 3-4 shows that the total 

energy content under these conditions is 18.9 MJ/kg, considerably lower than 

the previous species. However, both the overall and liquid utilization efficiencies compare well with those of Botryococcus: the highest overall utilization is found in the methane-only option at 72.3%, with the highest liquid - - 

fuel utilization found in the ester-ethanol-methane- option with 40.3%. he 

overall utilization rate of 58.2% is slightly higher than the corresponding 

utilization rate for Botryococcus. 

3-4. Nannochloropsis salina (high lipid) 

This organism when cultured in nitrogen-deficient seawater by Ben-Amotz 

developed high levels of total lipids (54%). In particular the benzene 

(40.2%) and chloroform (35.5%) fractions were high [5]. Fatty acids are estimated at 34.4% of total lipids. A substantial carbohydrate fraction (23%) is 

also present. The gross energy content at 27.6 MJ/kg is only slightly lower 

than that of Botryococcus. Once again, the anaerobic digestion options 

recover 75% of the gross energy content, with the ester-ethanol option having 

both a very good overall utilization rate (62.3%) and a moderate liquid fuel 

utilization rate (53.1%) (Table 3-5). The ester-ethanol-methane option could 

produce a greater amount of energy than could Botryococcus (18.0 vs. 

16.3 MJ/kg) . 

3-5. Dunaliella salina (high carbohydrate) 

When subjected to both osmotic stress and nitrogen deficiency, Dunaliella 

salina produced large quantities of carbohydrate. Listed as Example 5, this 

organism accumulated 55.5% of ash-f ree dry weight as carbohydrate. Both protein and carbohydrate were present at approximately lo%, and glycerol (4.7%) 

was also present, which will be discussed later (Example 11). The large carbohydrate content and small lipid content contribute to the gross energy content being considerably lower than the lipid-producers (16.2 MJ/kg, including 

glycerol). The energy utilization rates (Table 3-6) are also somewhat lower, 

especially in the methane options; less lipid and more carbohydrate to be 

digested means a lower net energy production. Liquid fuel utilization in the 

ester-ethanol-methane option for this organism (47.4%) is comparable to the 

lipid producers; the difference is that 74% of the liquid fuel energy comes 

from ethanol. The methane-ethanol option, with an overall utilization rate of 

65.7% and a liquid fuel utilization rate of 35.4%, might also be acceptable as 

a lower-cost alternative that still offers moderate liquid fuel production. 

3-6. Chlamydomonas sp. 

This organism was cultured by Ben-Amotz in seawater with. no environmental 

stresses and is characterized by a high carbohydrate content (59%) 

(Example 6). Lipid (23%) and protein (17%) contents were moderate, resulting 

in a gross energy content (22.4 ~~/kg) that compares favorably with the lipid 

producers (Table 3-7). The ethanol-methane option has a high overall 

utilization (68.7%) and a moderate liquid fuel utilization (25.8%), while the 

ester-ethanol-methane option has the lowest overall utilization (41.1%) and a 

liquid fuel utilization (30%) that does not compare favorably with other 

species. The overall utilization of 71.9% in the methane-only option is good, 

however. 

3-7. Cyclotella Cryptica 

This diatom, grown by Werner [lo] in nitrogen-deficient media, produced exceptional carbohydrate content (67%). Lipid content is a moderate 18% 

(Example 7), which contributes to a gross energy content of 20.77 MJ/kg. No 


data is available on lipid subfractions, so digestion is essentially the only 

lipid utilization option considered. As Table 3-8 shows, the overall energy 

utilization rate is 71.5% in the methane-only option, and drops only to 67.6% 

in the ethanol-methane option. Liquid fuel utilization efficiency is 31.6%, 

which is close to that of D. salina (Example 5) for this option. Each of the 

three representative carbohydrate-producing species has a particular characteristic to recommend it: Dunaliella has the highest liquid fuel utilization 

rate (35.4% in the ester-ethanol-methane option); Chlamydomonas has the highest gross energy content (22.4 MJ/kg), resulting in the highest energy production values (up to 16.1 MJ/kg); and Cyclotella has extremely high carbohydrate 

content, resulting in the highest ethanol production (6.6 kg ash-free dry 

weight). 

3-8. Spirulina platensis 

Spirulina sp. has been cultivated for commercial protein production. Cultivated by Tornabene et al. in an unstressed environment [3], the organism produced 50% of its cell weight as protein (Example 8), with a moderate (16.6%) 

lipid content, of which 5.4% was in the hexane and benzene fractions and 38% 

in the chloroform and methanol fractions (fatty acids estimated at 25%). With 

the slightly higher energy content of protein versus carbohydrate, a gross 

energy content of 19.8 MJ/kg of dry biomass is calculated (Table 3-9). As 

discussed earlier, the only method considered here for using the protein fraction for fuel production is anaerobic digestion. Examination of all the fuelproducing options shows the methane-only option to have the highest overall 

utilization rate (67.7%), with both ester-producing options (with and without 

ethanol production) having overall utilization rates at approximately 52% and 

liquid fuel utilization rates at 10%-15%. 

3-9. Chlorella sp. 

Example 9 of Table 3-1 shows a Chlorella that was isolated from desert saline 

waters [4]. It was high in protein (45.9%), moderate in lipid (20.7%), and 

low in carbohydrate (9.7%). Neutral lipids totaled only 2.3% of total lipids, 

and fatty acids were estimated at 15.9%. Gross energy content was calculated 

at 20.8 MJ/kg (Table 3-40), and energy utilization ranged from 51.8% for the 

ester-ethanolmethane option (with a liquid fuel utilization rate of 11.7%) to 

68.8% for the methane-only option. 

3-10. Nannochloropsis salina (high protein) 

This organism has been shown to be a potential high-lipid producer when subjected to environmental stress (Example 4); however, Nannochloropsis produces 

high protein content in the absence of stresses (Example 10). With a protein 

content of 55.8%, lipid content of 28.6%, and a carbohydrate content of 15.6%, 

Nannochloropsis has a gross energy content of 26.9 MJ/kg, the highest of the 

protein-producing species (Table 3-11). The methane-only option has an energy 

utilization rate of 69.5%, and both ester-methane options (with and without 

fermentation) have overall utilization rates of approximately 50% and liquid 


fuel utilization rates of 15%-20%. All of these results are highest of the 

protein producers examined. The ester fuel (3.25 MJ/kg) and total lipid 

(3.83 MJ/kg) components of the net energy output are both quite high for what 

are essentially secondary products. 

3-11. Dunaliella salina (high glycerol) 

Example 11 in Table 3-1 shows the composition of D. salina grown in a saltstressed environment. Instead of principally carbohydrate as in optimal culture conditions (Example 5), the organism now produces a substantial quantity 

(27.7%) of glycerol. The major product by weight is protein (35%), with 

lesser amounts of lipid (18.5%) and carbohydrate (12.5%). Without going into 

great detail, a few important properties of glycerol should be noted. Glycerol is a commodity chemical (approximately 175,000 tons/yr) used in the food 

processing, cosmetic, and pharmaceutical industries, among others. A clear, 

high-boiling, viscous liquid, it is a member of the "generally regarded as 

safe" list, which makes its use common in the above applications. Another 

important property is that it is primarily a natural product; petroleum-based 

glycerol has always been more expensive than natural glycerol produced as a 

by-product of soap manufacture. It is thought that the ability of this 

organism to accumulate large quantities of glycerol could be put to economic 

advantage. This assertion would need to be tested in some detail by analysis 

that is beyond the present scope. 

Table 3-12 summarizes the energy recovery potential from this organism. Of 

the gross energy content of 22.8 MJ/kg, 5 MT or 22% is due to the glycerol 

content. Extraction of the glycerol for use of its potential by-product value 

actually increases the overall utilization efficiency from 67.6% in the 

methane-only option (which includes digestion of the glycerol to methane) to 

69.5% in the methane-glycerol option. Without the fuel value of the glycerol, 

the gross energy content is a somewhat low 17.8 MJ/kg, but overall utilization 

rates are approximately 53% in the ester fuel-methane and ester-methaneethanol options. Liquid fuel utilization ranges from 16% without fermentation 

to 23% with it. A final note of emphasis should be added concerning the 

potential economic benefit of glycerol as a by-product. 

SECTION 4.0 

DISCUSSION 

It is clear even from this limited-scope review that a number of microalgae 

represent potential sources of renewable energy. Many of the significant compounds found in a barrel of crude oil may also be found in or be produced from 

algal lipids. Biochemical characterizations have indicated a wide range of 

potential products from microalgae, this slate being dependent on the particular strain of algae and the conditions under which that strain was grown. 

What needs to be immediately addressed is whether the product slate of a particular species can be maintained (or even achieved) in outdoor mass culture 

for production of large quantities of renewable fuel products. 

The petroleum industry over a period of decades has developed a series of 

flexible downstream processing options that produce maximum amounts of gasoline during certain seasons of the year (or eertain economic conditions) or 

fuel oils at other times. The achievement of a similar state of affairs with 

microalgae, though a long way off, does appear feasible. If the kind of cor 

positional variability observed in many species when culture conditions change 

is the rule rather than the exception, then that kind of flexibility will be a 

necessity. When lipids or carbohydrates predominate, they would be economically recovered; when not, anaerobic digestion offers an alternative way to 

produce energy. A culture system where algal composition is continuously 

monitored for this kind of variation, followed by changes to the production 

schemes and the product slates, is easy to imagine. 
SECTION 5.0 

CONCLUSIONS 

Many of the species considered here have potential to become superior biomass 

feedstocks. Figure 5-1 shows the high lipid-producing organisms. Botryococcus braunii with its high hydrocarbon production (10.1 MJ/kg) and Nannochloropsis salina with high ester fuel production (8.1 MT/kg) are the most 

promising. Ankistrodesms and Isochrysis appear less promising based on these 

Botryococcus 

braunii 

(Example 1) 

Ankistrodesmus sp. 

(Example 2) 

:.:.:.:.:.:.:.: .=.:.:.:.:. Methane 

Ethanol NannochlorHydrocarbons opsis sp. 

Ester fuel (Example 4) 

lsochrysis sp. 

(Example 3) 

Gas Liquid Gas Liquid Gas Liquid Gas Liquid 

Figure El. Gas vs. Fuel Production from Representative Eigh-Lipid Producers 
data, but continued research could result in improvements to productivity, 

lipid content, or other factors. The results of this analysis seem to favor 

neutral lipids, whose energy is recoverable with minimal processing losses. 

Further analysis is required to determine which species could be economically 

produced, then where the economic trade-offs lie between anaerobic digestion 

and the liquid fuel options such as transesterification. 

Figure 5-2 summarizes the results for the carbohydrate producers. Data for 

Dunaliella salina are shown for the high-carbohydrate and high-glycerol producing strains. The former has the lowest gross energy content and overall 

- Dunaliella 

salina 

(Example 5) 

Chlamydomonas sp. 

(Example 6) 

Ethanol 

Hydrocarbons 

Ester fuel 

Cyclotella 

cryptica 

(Example 7) 

Dunaliella 

salina 

(Example 1 1 ) 

Gas Liquid Gas Liquid Gas Liquid Gas Liquid 

Figure 5-2. 'Gas vs. Liquid Fuel Production from Representative HighCarbohydrate Producers 
SIN TR-2427 

energy utilization but the highest liquid fuel production and utilization data 

due to its relatively high proportion of fatty acids. The diatom Cyclotella 

cryptica has the highest ethanol production and highest carbohydrate content, 

and Chlamydomonas, with the highest lipid content, has the highest gross 

energy content and highest overall utilization efficiency. It should be 

recalled that the standard set of assumptions made about carbohydrates leads 

each species equally from carbohydrate to ethanol; characterizations of each 

species' carbohydrates are required to differentiate them. 

Figure 5-3 shows graphically the energy data for the high protein-producing 

species. Nannochloropsis salina, with the highest lipid content; achieves the 

Spirulina 

platensis 

(Example 8) 

$$$@$ Methane R Ethanol 

Hvdrocarbons ~ ~ 

~iter fuel Nannochloropsis sp. 

(Example 10) 

Chlorella sp. 

(Example 9) 

Gas Liquid Gas Liquid Gas Liquid 

Figure 5-3. Gas vs. Liquid Fuel Production from Representative High-Protein 

Producers 
highest gross energy content and the highest individual production values for 

all the products examined (methane, ester fuel, hydrocarbons, and ethanol). 

This organism also has the highest overall utilization efficiencies in its 

methane-only option. Spirulina platensis has higher overall utilization efficiencies in integrated liquid-gaseous fuel processing options, while the 

Chlorella strain examined here shows slightly less promise than the others. 

A few quick points should be made in summary. The only cases we have seen 

where lipid conversion routes are clearly the best choice are in the cases of 

extremely high lipid content coupled with very high fractions of either neutral lipids or fatty acids. Species with more moderate lipid content await 

the results of more detailed examination of the processing options, e.g., 

transesterification. Similarly, more detailed process design information will 

be required for comparison of fermentation with anaerobic digestion with 

regard to the carbohydrate fraction. It is also clear that the actual process 

conversion efficiencies that can be achieved will be critical to the economic 

success of the fuel production schemes. 
SECTION 6.0 

REFERENCES 

1. Benemann, J.R., B.L. Koopman, D.C. Baker, R.P. Goebel, and W.J. Oswald, in 

"The Photosynthesis Energy Factory: Analysis, Synthesis, and 

Demonstration," Final Report for DOE Contract No. EX-76-C-01-2548: 

Intertechnology/Solar Corporation, 1978. 

2. Milner, H.W., "The Chemical Composition of Algae," in J.S. Burlew, Algal 

Culture from Laboratory to Pilot Plant, Washington, DC: Carnegie 

Institution, 1976. 

3. Tornabene, T.G., A. Ben-Amotz, S. Raziuddin, and J. Hubbard, "Chemical 

Profiles of Microalgae with Emphasis on Lipids," Final Report for SERI 

Subcontract No. XK-2-02149-1 (portions also published in "Screening for 

Lipid Yielding Microalgae," Final Subcontract Report SERI/STR-231-2207), 

Golden, CO: Solar Energy Research Institute, 1983. 

4. Tornabene, T.G., "Chemical Profiles of Microalgae with Emphasis on 

Lipids," in Aquatic Species Program Review, Proceedings of the April 1984 

Principal Investigators Meeting, SERI/CP-231-2341, Golden, CO : Solar 

Energy Research Institute, 1984. 

5. Ben-Amotz, A., "Development of Outdoor Raceway Capable of Yielding Oilrich Halotolerant Microalgae. Identification of Oil-rich Strains," in 

Aquatic Species Program Review, Proceedings of the April 1984 principal 

Investigators Meeting, SERI/CP-231-2341, Golden, CO: Solar Energy 

Research Institute, 1984. 

6. Hill, A.M., and D.A. Feinberg, "Fuel From Microalgae Lipid Products," 

SERI/TR-231-2348, Golden, CO: Solar Energy Research Institute, 1984. 

7. Tornabene, T.G., personal communication, 1984. 

8. Tahir, A.B., H.M. Lapp, and L.C. Buchanan, "Sunflower Oil as a Fuel for 

Compression Ignition Engines, " in Proceedings of the International 

Conference on Plant and Vegetable Oils as Fuels, American Society of 

Agricultural Engineers, 1982, p. 82. 

9. Cowley, I.D., and D.A.J. Wase, "Anaerobic Digestion of Farm Wastes; a 

Review (part I)," Process Biochemistry, August/September 1981, pp. 28-33. 

10. Werner, D., "Productivity Studies on Diatom Cultures," Helgolander wiss 

Meersunters, Vol. 20, 1970, p. 97. 
Page I SERI/TR-231-2427 I 

4. Tjtle and Subt~tle 5. Publication Date 

Fuel Options from Microalgae with Representative 

Chemical Compositions 

Daniel Feinbera 

9. Performing Organ~zation Name and Address 10. ProlecVTask/Work Unit No. 

Solar Energy Research Institute a676 7n 

161 7 Cole Boulevard 11. Contract (C) or Grant (G) No. 

Go1 den, Col orado 80401 (C) 

12. Sponsoring Organization Name and Address 13. Type of Report 8 Period Covered 

I I Technical Report 

15. Supplementary Notes 

16 Abstract (Llmtt 200 words) 

Representative species of microalgae are examined with respect to their reported 

chemical compositions. Each species is analyzed under a variety of culture 

conditions, with the objective being to characterize an optimum mixture of fuel 

products (e.g., methane, ethanol, methylester) which should be produced by the 

particular species. Historically the emphasis has been on the entire algal cell 

mass. Using the reported chemical composition for the representative species 

under specific sets of growth conditions, some conclusions can be drawn about 

the preferred fuel product conversion routes that could be employed. 

17. Document Analys~s 

a. Descriptors Algae ; Chlomydomonas ; Chlorella ; Fuels ; Unicellular Algae 

b. IdentifierS'Open-Ende* Terms Botryococcus Braunii ; Ankistrodesrnus Falcatus ; Isochrysis 

Nannochloropsis Salina ; Dunaliella Salina ; Cyclotella Cryptica ; 

Spirulina Platensis ; Nannochloropsis Salina 

c. UC Categortes 

61 a 

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