Algae are generated from sunlight, water, CO₂, and nutrients as well as algal cultures. There are more than 30,000 species of algae. One of the major factors in the use of algae to generate fuels is choosing the best species for oil generation and developing methods for removing the oil and making it into a fuel. Fuels that can be made from algae oil are biodiesel, n-alkane hydrocarbons, ethanol, methane, and hydrogen. Algae can also be used for soil conditioners and agrochemicals such as fertilizers and proteins as well as fine chemicals and bioactive substances such as polysaccharides, antioxidants, omega-3 and-6 fatty acids, proteins, and enzymes.

There are currently several applications for algae including: 1) algin – a thickening agent for food processing (brown algae), 2) carrageenan – foods, puddings, ice cream, toothpaste (red algae), 3) iodine (brown algae), 4) agar – growth media in research (red algae), 5) as food (red and brown algae), 6) plant fertilizers, and 7) diatomaceous earth – used for filtering water, insulating, soundproofing. The table below shows some additional applications detailing the species, end product, origin, and main way to culture the algae.

The role of algae in the aqueous world is that they are the base of the aquatic food chain and are photosynthetic organisms. There is a symbiotic relationship between fungi and algae known as lichens. A lichen is a composite organism that emerges from algae or cyanobacteria living among filaments of a fungus in a mutually beneficial relationship. Their properties are plant-like, but lichens are not plants. Lichens help to cause the pigments of algae, reduce harmful amounts of sunlight, and kill bacteria. They can also serve as shelters, such as kelp forming underwater forests and red algae that form reefs.

Algae can have some negative impacts via eutrophication. Eutrophication is the ecosystem response to the addition of artificial or natural substances, mainly phosphates through detergents, fertilizers, or sewage to an aquatic system. It can also be caused by dense blooms of cyanobacteria or algae. These can cause impacts through 1) clogging of waterways, streams, and filters, 2) a decrease in water taste and quality, and 3) potential toxification. Red tide is one event that can be caused by dinoflagellates.

So why make biofuels from algae? There are several reasons. Algae have high lipid content (up to 70%); they grow rapidly and will produce more lipids per area than other terrestrial plants (10-100 times). To grow algae, non-arable land (this can be thought of as land that is not typically used for farming) can be used along with saline or brackish water. Algae don’t have the same competition with generating food or feed as other oil-producing plants. One of the most amazing features is the use of CO₂ in growing algae; it helps grow algae significantly. It also provides nutrient (N, P) removal in agricultural and municipal wastewater.

The table below shows a comparison of annual oil yield from a variety of plants and algae. Even microalgae with lower lipids content (30%) will generate 50.00 m³/ha, significantly higher than palm oil. (Mata et al., 2010)

Algae are eukaryotic organisms, which are organisms whose cells contain a nucleus and other structures (organelles) enclosed within membranes. They live in moist environments, mostly aquatic, and contain chlorophyll.

Algae are not terrestrial plants, which have 1) true roots, stems, and leaves, 2) vascular (conducting) tissues, such as xylem, and phloem, and 3) lack of non-reproductive cells in the reproductive structures. Algae are not cyanobacteria. Cyanobacteria are prokaryotes, which lack membrane-bound organelles and have a single circular chromosome. The figure below shows the cellular composition of blue-algae and the subsequent one shows a micrograph of the cells. The cell has a wall with a gelatinous coat. Just beneath the cell wall is a plasma membrane. Within the cell, there are layers of phycobilisomes, photosynthetic lamellae, ribosomes, protein granules, and circular DNA known as nucleoids. These are typical components of growing plants - however, the components we are interested in are lipid droplets, which are oils that can be extracted from the algae.

Algae is composed of ~ 50% carbon, 10% nitrogen, and 2% phosphorus. The table below shows the composition of various algae looking at the percentages of protein, carbohydrates, lipids, and nucleic acid.

So what are the characteristics of algae?

1. Eukaryotic organisms:

As mentioned above, algae are eukaryotic organisms. The structure of a eukaryote (a typical plant cell) is shown in the first figure below. The second figure below shows the cell structure of a prokaryote, a bacterium, one of two groups of prokaryotic life. Some do not consider the prokaryotes as true algae because they have a different structure, but most include these in the family of algae. There are labels for the different parts of the organisms, but I will not require you to know this information in detail - it is there so if you have a desire to look up more information, you can. The table shows a comparison of both these types of cells.

2. Live in moist environments

These organisms lack a waxy cuticle (the wax in terrestrial plants prevents water loss). There is a wide variety of growth environments for algae. The typical conditions for algae are moist, tropical regions and they can grow in marine and freshwater. Freshwater algae grow in animals, aquatic plants, farm dams, sewage, lakes, rivers, lagoons, snow, mud/sand, and soil.

3. Contain chlorophyll

Algae are mostly photosynthetic, like plants. They have five kinds of photosynthetic pigments (chlorophyll a, b, c, d, and f) and have many accessory pigments that are blue, red, brown, and gold. Chlorophyll is a green pigment found in almost all plant algae and cyanobacteria. It absorbs light and transfers light energy to ATP (adenosine triphosphate).

So how are algae classified?

Algae belong to the Protista kingdom. The figure below shows a schematic of where Protista fits with other classifications of Plantae, Animalia, Fungi, Eubacteria, and Archaebacteria.

Algae can also be classified based on chlorophyll content. The first type is Chromista. These types of algae contain chlorophylls a and c, and examples of the algae include brown algae (golden-brown algae), kelp, and diatoms. These materials are a division of Phaeophyta. These types have a habitat on rocky coasts in temperate zones or open seas (cold waters). The structure is multicellular and they can grow up to 50 m long.

Red algae are another type and contain chlorophyll a, such as marine algae (seaweed). These organisms are in the division of Rhodophyta, which has over 4000 species. These are some of the oldest eukaryotic organisms on Earth (there are 2 billion-year-old fossils). They are abundant in tropical, warm waters. They act as food and habitat for many marine species. The structure ranges from thin films to complex filamentous membranes. These algae have accessory pigments and the phycobilins (red) mask chlorophyll a. The second figure below shows various red algae. Dinoflagellates are unicellular protists, and these are associated with red tide and bioluminescence.

Green algae contain chlorophylls a and b. They are in the division Chlorophyta. This is the largest and most diverse group of algae. It is found mostly in freshwaters and also on land (rocks, trees, and soil). The structures are single cells (Micrasterias), filamentous algae, colonies (Volvox), and leaf-like shapes (Thalli). Terrestrial plants arose from a green algal ancestor. Both have the same photosynthetic pigments (chlorophyll a and b). Some green algae have a cell wall made of cellulose, similar to terrestrial plants.

There are two primary ways that algae reproduce. Some algae are unicellular and demonstrate the simplest possible life cycles. Note that there is a generative phase and a vegetative phase. During the generative phase, cysts are freed. The cysts open to form gametes and then form the zygote. From there, the vegetative phase occurs so the plant grows and new cysts can form. Most algae have two recognizable phases, sporophyte and gametophyte. The second figure below shows a schematic of the two phases. The main difference is a male and female type is required to form the zygote. I will not be expecting you to know the details in depth, but want you to recognize there are differences.

Algae have a particular path of growth, beginning with a lag phase, and continuing on to an exponential phase, a linear phase, and stationary phase, and a decline of death phase. The figure below shows a schematic of the algal growth rate in a batch culture.

There are several factors that influence the growth rate. The temperature will vary with algae species. The optimal temperature range for phytoplankton cultures is 20-30°C. If temperatures are higher than 35°C, it can be lethal for a number of algal species, especially green microalgae. Temperatures that are lower than 16°C will slow down the growth of algae.

Light also has an effect on the growth of algae: it must not be too strong or weak. In most algal growth cultivation, algae only need about 1/10 of direct sunlight. In most water systems, light only penetrates the top 7-10 cm of water. This is due to bulk algal biomass, which blocks light from reaching into deeper water.

Mixing is another factor that influences the growth of algae. Agitation or circulation is needed to mix algal cultures. An agitator is used for deep photo reactor systems. Paddle wheels are used for open pond systems. And pump circulation is used for a photo-tube system.

Of course, algae need nutrients and the proper pH to grow effectively. Autotrophic growth requires carbon, hydrogen, oxygen, nitrogen, phosphorous, sulfur, iron, and trace elements. The compositional formula of C O1.48 H1.83 N0.11 P0.01 can be used to calculate the minimum nutrient requirement. Under nutrient-limiting conditions, growth is reduced significantly and lipid accumulation is triggered. Algae prefer a pH from neutral to alkaline.

There are particular steps for algal biodiesel production. The figure below shows the processing steps for algae production in biodiesel production. The first step is the cultivation of algae, which includes site selection, algal culture selection, and process optimization. Process optimization includes the design of the bioreactor and necessary components for algal cell growth (nutrients, light, and mass transfer). Once the algae grow to the necessary level, the algae are harvested. The biomass must first be processed in order to dewater, thicken, and dry the algae in order to extract the oil that will then be processed into biodiesel. The biomass process differs depending on the method of oil extraction and biodiesel production. You primarily learned about transesterification to make biodiesel in Lesson 9, but there are other processes being researched and developed.

Site selection is an important area to investigate. The best areas to grow algae are areas with adequate sunlight year round, with tropical and subtropical climates. In the US, this includes the following states: Hawaii, California, Arizona, New Mexico, Texas, and Florida. This also means that the temperature will moderate year-round. There also has to be adequate land availability (for open-pond systems) and close proximity to CO₂ (i.e., near a power plant or gasifier). To keep costs at a minimum, water and nutrients must be available at lower costs and manpower kept at reasonable rates.

There are two main types of culturing technologies: open systems and closed systems. Open systems include tanks, circular ponds, and raceway ponds. Closed systems include three different types: flat-plate, tubular, and vertical-column enclosed systems. The figures below show several different examples of open and closed systems.

Open systems can be in natural waters, or specifically engineered to grow algae. Natural water systems include algae growth in lakes, lagoons, and ponds, while the engineered systems are those described in the previous paragraph: tanks, circular ponds, and raceway ponds. Of course, there are going to be advantages and disadvantages of open systems. The main advantages are that open systems are simple in design, require low capital and operating costs, and are easy to construct and operate. However, disadvantages include little control of culture conditions, significant evaporative losses, poor light utilization, expensive harvesting, use of a large land area, limited species of algae, problems with contamination, and low mass transfer rates. One of the more common designs is the raceway pond. It has existed since the 1950s and is a closed-loop recirculation channel for mass culture. The design includes a paddlewheel for mixing and recirculation, baffles to guide the flow at bends, and algal harvesting is done behind the paddlewheel. Cyanotech has a field of raceway ponds located in Kona, Hawaii, with a wide variety of algae.

So, what are some of the design features to keep in mind with algae systems? Algal systems are phototrophic, which means they need to obtain energy from sunlight to synthesize organic compounds. Therefore, the growth rate depends on light intensity, temperature, and substrate concentration, as well as pH and species type.

Some factors affect how specific systems are designed. Open pond design is affected by factors including the pond size, the mixing depth, the paddle wheel design, and the carbonator. The carbonator is how the carbon is added to the algae - it can be done in a number of ways, including carbonaceous seed materials, but utilizing CO₂ from power systems (generated from the combustion of carbon-based materials) is one of the more common for algae growth - it also mitigates generation of GHG. We will not go into ways to design these systems, as that is above the level of this course.

There are also a variety of closed systems. One type of system is the photobioreactor (PBR). Advantages of a system such as this include 1) compact design, 2) full control of environmental conditions, 3) minimal contamination, 4) high cell density, and 5) low evaporative losses. The disadvantages include: 1) high production costs, typically an order of magnitude higher than open ponds, 2) overheating, and 3) biofouling. A company that has systems such as this is Algatechnologies. They have a plant located in Kibbutz Ketura, Israel. The figure below shows a picture of the various algae they have growing in Israel.

There are three types of designs for the PBRs: flat plate, tubular, and vertical column. Advantages of the flat plate PBR include 1) large surface area, 2) good light path, 3) good biomass productivity, and low O₂ build-up. However, the drawbacks include 1) difficulty in scaling up, 2) difficulty in controlling temperature, and 3) algae wall growth. A flat plate PBR is shown in the third figure. For the tubular PBR, advantages include 1) good biomass productivity, 2) good mass transfer, 3) good mixing and low shear stress, and 4) reduced photoinhibition and photooxidation. Tubular PBRs also have disadvantages: 1) gradients of pH, dissolved O₂ and CO₂ along the tubes, 2) O₂ build-up, 3) algae wall growth, 4) requirement of large land area, and 5) a decrease in illumination surface area upon scale-up. The two previous figures are examples of tubular PBRs. Vertical column PBRs have different advantages and disadvantages. The positive features include 1) high mass transfer, 2) good mixing and low shear stress, 3) low energy consumption, 4) high potential for scalability, 5) easy sterilization, and 6) reduced photoinhibition and photooxidation. The negative features are 1) a small illumination surface area, 2) the need for sophisticated materials for construction, and 3) decrease in illumination surface area upon scale-up. The figure below shows a vertical column PBR.

We can compare open and closed systems by looking at various parameters and providing general comparisons of these systems. The table below provides a list of parameters to compare for each type of system. Open systems tend to cost less, but process control is difficult, and the growth rate is lower. Closed systems are a much higher cost, but control is much better and productivity is therefore higher.

The following video is produced by Los Alamos National Laboratory in New Mexico. The video provides a nice overview of how algae are generated, how to harvest them, and the areas of research LANL are focusing on to improve various aspects of the process to make it more economical (3:12).

Algae are typically in a dilute concentration in water, and biomass recovery from a dilute medium accounts for 20-30% of the total production cost. Algae can be harvested using: 1) sedimentation (gravity settling), 2) membrane separation (micro/ultrafiltration), 3) flocculation, 4) flotation, and 5) centrifugation.

Sedimentation is the initial phase of separating the algae from water. Once agitation is completed, the algae are allowed to settle and densify. However, other methods most likely will also be required to achieve complete separation.

Membrane separation is a form of filtration. In the lab, a funnel is attached to a vacuum flask. The contents are poured out onto the filter on the funnel and allowed to dry some on the filter as the vacuum continues to be pulled. This method can be used to collect microalgae with low density but is typically done on a small scale. But the main disadvantage is membrane fouling. There are three modifications: 1) reverse-flow vacuum, 2) direct vacuum with stirring blade above the filter, and 3) belt compression.

Flocculation is another technique. Flocculation is a method where something is added to the mixture of water and algae that causes the algae to “clump” together (or aggregate) and form colloids. Chemical flocculants include alum and ferric chloride. Chitosan is a biological flocculant, but has a fairly high cost. Autoflocculation is an introduction of CO₂ to an algal system to cause algae to flocculate on their own. Flocculation is often used in combination with a filter compressor, as described in the last paragraph.

Froth floatation is another method for harvesting and separating algae from water. This is a technique that has been used in coal and ore cleaning technology for many years. It is based on density differences in materials. Typically air bubbles are incorporated into the unit. Sometimes an additional organic chemical or adjustment of pH will enhance separation. For algal systems, the algae will accumulate with the froth of bubbles at the top, and there is some way to collect or scrape the froth and algae from the top to separate it from the water. It is an expensive technology that, at this point, it may be too expensive to use commercially. There is also the possibility of combining froth flotation with flocculation. For example, when alum is used as a flocculant for the algae, air is bubbled through to separate the flocculant by density. It can also be combined with a filter compressor.

One of the more commonly used machines is a continuous-flow centrifuge. It is efficient and collects both algae and other particles. However, it is more commonly used for the production of value-added products from algae and not for fuel generation.

Along with these separation techniques, moisture needs to be removed from algae to improve the shelf-life. Algae are concentrated from water through a series of processes, including the separation process. The concentration of algae in the pond starts at about 0.10-0.15% (v/v). After flocculation and settling, the concentration is increased to 0.7%. Using a belt filter process, the concentration increases to 2% (v/v). Drying algae from 2% to 50% v/v requires almost 60% of the energy content of the algae, which is a costly factor of algae use.

Lipid Separation Technologies

This is an important aspect of the use of algae to generate fuels. It is likely also an expensive option. The algae cells have to be subjected to cell disruption for the release of the desired products. Physical methods include 1) mechanical disruption (i.e., bead mills), 2) electric fields, 3) sonication, 4) osmotic shock, and 5) expeller press. There are also chemical and biological methods, including 1) solvent extraction (single solvent, co-solvent, and direct reaction by transesterification), 2) supercritical fluids, and 3) enzymatic extraction.

Single solvent extraction is one of the more common methods. A solvent that is chemically similar to the lipids is used, such as hexane or petroleum ether (this is just a light petroleum-based solvent). This is a commercial process. Extraction takes place at elevated temperatures and pressure. Advantages include an increased rate of mass transfer and solvent accessibility, and a reduced dielectric constant of immiscible solvents. The use of a co-solvent process is a little different. There are two criteria used to select a co-solvent. Selection should include: 1) a more polar co-solvent that disrupts the algae cell membrane, and 2) a second less polar co-solvent to better match the polarity of the lipids being extracted (alkanes can meet this criterion). There are several examples of co-solvent extraction. One method was developed by Bligh and Dyer in 1959. Alcohol and chloroform are the solvents, and the majority of the lipids dissolve into the chloroform phase. The interactions include water/methanol > methanol/chloroform > lipid/chloroform. Other combinations of co-solvents include 1) hexane/isopropanol, 2) dimethyl sulfoxide (DMSO)/petroleum ether, and 3) hexane/ethanol.

Supercritical extraction is similar to solvent extraction. The main difference is that the solvent is maintained until certain pressure and temperature conditions are met, which changes the solvent properties and helps extract the materials. It is often done on a smaller scale and may not be useful at an industrial level.

Enzymatic extraction is also similar to solvent extraction, except instead of a solvent, an enzyme is used to separate the materials.

As discussed in the biodiesel lesson , the reaction of transesterification is often used to convert lipids into fatty ester methyl esters (FAMEs) using alcohol and a catalyst. The advantages of using this method are the high recovery of volatile medium-chain triglycerides and the fact that antioxidants are not necessary to protect unsaturated lipids. There are other methods as well, as discussed near the end of the biodiesel lesson.

Direct Biofuel Production from Algae

Besides separating out the lipids to make diesel fuel, other fuels can be obtained from algae directly. These include alcohols such as ethanol and butanol, hydrogen, and methane.

Alcohols can be made from algae by heterotrophic (carbon nutrients from organic materials) fermentation of starch to alcohols, including ethanol and butanol. Marine algae used for this are Chlorella vulgaris and Chlamydomonas perigramulata. Procedures include starch accumulation via photosynthesis, subsequent anaerobic fermentation under dark conditions to produce alcohol, and alcohol extracted directly from the algal culture media. Hydrogen can also be produced directly from algae through photofermentation and dark fermentation. Methane can be produced by anaerobic conversion of algae. It can be coupled with other processes (using the residue after lipids are removed, for example). Challenges include the high protein content of biomass, which can result in NH₃ inhibition and can be overcome by co-digestion with high carbon co-substrates. The figure below shows a schematic of different processes to convert algae and the range of fuel products that can be made.

Fuel products from various processes of algae.

Click here for a text description of the figure.

Algae

• Gasification to syngas
  • Fischer Tropch to catalytic upgrading to liquid hydrocarbon fuels
  • Higher alcohol synthesis to MeOH, EtOH, etc
  • Hydrogen
• Pyrolysis to liquid or vapor fuel to catalytic upgrading to transportation fuels: liquid or gas
• Supercritical Fluids to liquid or vapor fuel to catalytic upgrading to transportation fuels: liquid or gas
• Anaerobic digestion to biogas

Credit: BEEMS Module A3

Summary

Algae are an excellent source of lipids for biodiesel production. They can grow under conditions that terrestrial plants cannot grow, in water, using CO₂ and waste materials. Pairing the proper species of algae with appropriate growing conditions will increase the amount of oil produced by the algae. Discussion on the economics of algae production will be included in another lesson, in order to compare it to other sources of biodiesel. For now, I will note that production under highly controlled conditions can be expensive.

Lesson Objectives

By the end of this lesson, you should be able to:

• explain how algae grow and the important factors in growth for fuels;
• describe the use of algae for various fuels, from making biodiesel to gases and alcohols;
• evaluate the efficacy of using algae for fuels.

References

Mata et al. (2010) Renew. Sust. Energy Rev. 14: 217–232.

Liao, W., Khanal, S., Park, S.Y., Li, Y., BEEMS Module A3, Algae, USDA Higher Education Challenger Program 2009-38411-19761, 2009.