Algae Biofuels 101: Chapter 2-The 8 best Algae Strains to Start With and Where To Get Them
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The biomass from algae can also be burned similar to wood or anaerobically digested to produce methane biogas to generate heat and electricity. Algal biomass can also be treated by pyrolysis to generate crude bio-oil. Microalgae grow quickly and contain high oil content compared with terrestrial crops, which take a season to grow and only contain a maximum of about 5 percent dry weight of oil, Chisti, They commonly double in size every 24 hours.
During the peak growth phase, some microalgae can double every three and one-half hours Chisti, Oil content of microalgae is usually between 20 percent and 50 percent dry weight, Table 1 , while some strains can reach as high as 80 percent Metting, ; Spolaore et al.
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This is why microalgae are the focus in the algae-to-biofuel arena. Most microalgae are strictly photosynthetic — that is, they need a light and carbon dioxide as energy and carbon sources. This culture mode is usually called photoautotrophic. Some algae species, however, are capable of growing in darkness and using organic carbons such as glucose or acetate as energy and carbon sources. This culture mode is termed heterotrophic.
Due to high capital and operational costs, heterotrophic algal culture is hard to justify for biodiesel production. In order to minimize costs, algal biofuel production usually relies on photoautotrophic culture that uses sunlight as a free source of light. Phototrophic microalgae require light, carbon dioxide, water, and inorganic salts to grow.
The growth medium must contribute the inorganic elements that help make up the algal cell, such as nitrogen, phosphorus, iron, and sometimes silicon Grobbelaar, For large-scale production of microalgae, algal cells are continuously mixed to prevent the algal biomass from settling Molina Grima et al.
However, up to one-quarter of algal biomass produced during the day can be lost through respiration during the night Chisti, A variety of photoautotrophic-based microalgal culture systems are available. For example, the algae can be grown in suspension or attached on solid surface. Each system has its own advantages and disadvantages.
Currently, the suspension-based open ponds and enclosed photobioreactors are commonly used for algal biofuel production. In general, an open pond is simply a series of raceways outside, while a photobioreactor is a sophisticated reactor design which can be placed indoors in a greenhouse, or outdoors. The details of the two systems are described below. Open ponds: Open ponds are the oldest and simplest systems for mass cultivation of microalgae.
In this system, the shallow pond is usually about 1 foot deep; algae are cultured under conditions identical to their natural environment. The pond is designed in a raceway configuration, in which a paddlewheel provides circulation and mixing of the algal cells and nutrients Figure 2.
Life Cycle Assessment for Microalgae Oil Production
The raceways are typically made from poured concrete, or they are simply dug into the earth and lined with plastic to prevent the ground from soaking up the liquid. Baffles in the channel guide the flow around bends in order to minimize space. The system is often operated in a continuous mode — that is, the fresh feed containing nutrients including nitrogen phosphorus and inorganic salts is added in front of the paddle wheel. Algal broth is harvested behind the paddle wheel after it has circulated through the loop Figure 2.
For some marine types of microalgae, seawater or water with high salinity can be used. Although open ponds cost less to build and operate than enclosed photobioreactors, this culture system has its intrinsic disadvantages. Since these are open-air systems, they often experience a lot of water loss due to evaporation. Thus, microalgae growing in an open pond do not uptake carbon dioxide efficiently, and algal biomass production is limited Chisti, Biomass productivity is also limited by contamination with unwanted algal species as well as other organisms from feed.
In addition, optimal culture conditions are difficult to maintain in open ponds, and recovering the biomass from such a dilute culture is expensive Molina Grima et al. Enclosed photobioreactors: Enclosed photobioreactors have been employed to overcome the contamination and evaporation problems encountered in open ponds Molina Grima et al.
These systems are made of transparent materials and generally placed outdoors for illumination by natural light. The cultivation vessels have a large surface area-to-volume ratio. The most widely used photobioreactor is a tubular design, which has a number of clear transparent tubes, usually aligned with the sun rays Figure 3.
Life Cycle Assessment for Microalgae Oil Production | Disruptive Science and Technology
The tubes are generally less than 10 centimeters in diameter to maximize sunlight penetration Chisti, The medium broth is circulated through a pump to the tubes, where it is exposed to light for photosynthesis, and then back to a reservoir. The algal biomass is prevented from settling by maintaining a highly turbulent flow within the reactor, using either a mechanical pump or an airlift pump Chisti, A portion of the algae is usually harvested after the solar collection tubes. In this way, continuous algal culture is possible Chisti, In some photobioreactors, the tubes are coiled spirals to form what is known as a helical tubular photobioreactor, but these sometimes require artificial illumination, which adds to the production cost.
Therefore, this technology is only used for high-value products, not biodiesel feedstock. The photosynthesis process generates oxygen. In an open-raceway system, this is not a problem as the oxygen is simply returned to the atmosphere. However, in the closed photobioreactor, the oxygen levels will build up until they inhibit and poison the algae. The culture must periodically be returned to a degassing zone, an area where the algal broth is bubbled with air to remove the excess oxygen. Also, the algae use carbon dioxide, which can cause carbon starvation and an increase in pH.
Therefore, carbon dioxide must be fed into the system in order to successfully cultivate the microalgae on a large scale. Photobioreactors may require cooling during daylight hours, and the temperature must be regulated at night hours as well. This may be done through heat exchangers, located either in the tubes themselves or in the degassing column.
The advantages of the enclosed photobioreactors are obvious. They can overcome the problems of contamination and evaporation encountered in open ponds Molina Grima et al. The biomass productivity of photobioreactors can be 13 times greater than that of a traditional raceway pond, on average Chisti, Harvesting of biomass from photobioreactors is less expensive than that from a raceway pond, since the typical algal biomass is about 30 times as concentrated as the biomass found in raceways Chisti, However, enclosed photobioreactors also have some disadvantages.
For example, the reactors are more expensive and difficult to scale up. Moreover, light limitation cannot be entirely overcome since light penetration is inversely proportional to the cell concentration. Attachment of cells to the tube walls may also prevent light penetration. Although enclosed systems can enhance the biomass concentration, the growth of microalgae is still suboptimal due to variations in temperature and light intensity. Harvesting: After growing in open ponds or photobioreactors, the microalgae biomass needs to be harvested for further processing.
The enzymes that can be targeted to enhance growth and carbon fixation can be determined from enzyme flux control coefficient data of Calvin cycle enzymes [ 42 ]. Likewise, the targets involved in lipid metabolism can also be found by 13C metabolic flux data and subsequent metabolic map derived from oleaginous algae. These flux data reveal which enzymes and the pathways they regulate are rate limiting and exert significant control over the larger metabolism [ 43 , 44 ].
These developments would facilitate to increase the yield, concurrent with an economical algal biodiesel production in the near future. In addition to biofuels, microalgae are feedstock to several other high-value products such as vitamins, pigments, proteins, carbohydrates, amino acids, antioxidants, high-value long-chain polyunsaturated fatty acids PUFAs and biofertilizers.
Natural microalgal pigments such as carotenoids, chlorophylls and phycobiliproteins serve as precursors of vitamins in food, pharmaceutical industries, cosmetics and coloring agents [ 45 ]. Several studies are focussing on microalgal genes encoding enzymes, which are involved in high-value carotenoid synthesis. Microalgae such as C. In another study, C. Microalgae also serve as potential expression systems for the synthesis of biopolymers such as polyhydroxybutyrate, which is a key precursor for the synthesis of biodegradable plastics.
In a recent study, an ATP hydrolysis-based driving force module was engineered into Synechococcus elongatus PCC to produce 3-hydroxybutyrate [ 48 ]. The strain which was engineered by having a provision for a reversible outlet for excessive carbon flux was capable of producing significantly high amounts of 3-hydroxybutyrate over the native strain. Several microalgae secrete extracellular polymeric substances in their immediate living environment as a hydrated biofilm protective matrix [ 49 ].
These substances are known for high-value applications such as anti-inflammatories, antivirals, antioxidants, anticoagulants, biolubricants and drag reducers. In a recent study, LEA has been used as a substrate for biomethanation through anaerobic processes [ 51 ]. This study showed that the rate of biogas production was comparatively higher in product-extracted algal samples lipid and protein extracted , whilst the cumulative methane production was higher for pretreated algae dried powdered algae and heat-treated algae.
LEA has also been used as raw material for butanol fermentation [ 53 ]. Bench-scale tests demonstrated that LEA could also be effectively converted to liquid fuel, mainly alkanes via hydrothermal liquefaction and upgrading processes such as via hydrotreating and hydrocracking. The overall energy efficiency on a higher heating value basis of this process was estimated to be A study conducted by Gu et al.
Photosynthesis-dependent accumulation of biomass takes place under nitrogen-rich environment, whereas accumulation of lipids in microalgae occurs under stress conditions such as limited nutrient conditions. This contradiction hugely offsets continuous lipid production, thus affecting the economics of the system. The fed batch process has often been reported as the most suitable method for microalgal cultivation, because it offers the flexibility of customization of nutrients provided during the process.
stepabtathe.tk A study conducted by Zheng et al. Moreover, studies proved that supply of light in phototrophic and mixotrophic processes during the fed batch culture increased the lipid productivity [ 57 ]. Light spectral quality also plays a key role in facilitating photosynthesis. Absorption of irradiation corresponding to the absorption band of the algal chlorophyll can lead to enhanced photosynthesis [ 58 ].
In a recent study, batch process was attempted in an open thin-layer cascade photobioreactor for high-cell density cultivation of a saline microalga N. Similarly, fed batch heterotrophic microalgae cultivation of Auxenochlorella protothecoides was employed to maximize lipid production. An optimal feeding strategy was determined by interior point optimization [ 60 ].
On the contrary, Tang et al. Furthermore, to maintain the steady state, the system is continuously being supplied with essential nutrients, which prevents the formation of inhibitory metabolites. Although continuous feeding is required for maintenance of growth rate, the accompanying non-limiting nutrient supply may hamper accumulation of lipid and carbohydrates.