Biodiesel from microalgae production methods - a review
- 2. What are microalgae? Microalgae are prokaryotic or eukaryotic photosynthetic microorganisms that can grow rapidly and live in harsh conditions due to their unicellular or simple multicellular structure. Examples: Prokaryotic microorganisms: Cyanobacteria (Cyanophyceae) Eukaryotic microalgae: Green algae (chlorophyta) and diatoms (Bacillariophyta)
- 3. Biology of microalgae Algae are recognised as one of the oldest life-forms. They are primitive plants (thallophytes), i.e. lacking roots, stems and leaves, have no sterile covering of cells around the reproductive cells and have chlorophyll a as their primary photosynthetic pigment. Algae structures are primarily for energy conversion without any development beyond cells, and their simple development allows them to adapt to prevailing environmental conditions and prosper in the long term.
- 4. Biofuels cosmetics Pharmaceuticals Nutrition Aquaculture Food additives Pollution prevention Biotechnology areas of microalgae
- 5. Biodiesel from microalgae why? Energy is of vital importance to society and human. Biomass energy, as a green and renewable resource, has been considered to be one of the best ways to solve the global energy crisis. Microalgae is an economical and potential raw material of biomass energy, because it does not require a large area of land for cultivation, exhibits short growth period, possesses a high growth rate and contains more high-lipid materials than food crops.
- 6. Biodiesel from microalgae why? Recent studies have demonstrated that microalgae has been widely regarded as one of the most promising raw materials of biofuels. However, lack of an economical, efficient and convenient method to harvest microalgae is a bottleneck to boost their full- scale application.
- 7. Technologies for the production of microalgal biomass production Under natural growth conditions phototrophic algae absorb sunlight, and assimilate carbon dioxide from the air and nutrients from the aquatic habitats. Therefore, as far as possible, artificial production should attempt to replicate and enhance the optimum natural growth conditions. The use of natural conditions for commercial algae production has the advantage of using sunlight as a free natural resource . However, this may be limited by available sunlight due to diurnal cycles and the seasonal variations; thereby limiting the viability of commercial production to areas with high solar radiation. For outdoor algae production systems, light is generally the limiting factor .
- 8. Open ponds Open ponds are the most widely used system for large- scale outdoor microalgae cultivation. Commercially economical. Easy to build and operate. Depending on their size, shape, type of agitation and inclination, the open pond systems can be classified into (a) raceway pond, (b) circular pond, and (c) sloped pond
- 9. Figure 1 Three different designs of open pond systems
- 10. Enclosed PBR Two major types of enclosed PBR are tubular and plate types. Due to enclosed structure and relative controllable environment, enclosed PBR can reach high cell density and easy to maintain monoculture.
- 11. Hybrid systems Other types of systems are an internally illuminated photo bioreactor (Helix PBR) developed by Origin oil company. The light array rotates vertically that allows algae growth in deep media and provides agitation. The light array consists of blue, red and white lights, which are the wavelengths the algae prefer.
- 12. Harvesting and drying of algal biomass There are currently several harvesting methods, including mechanical, electrical, biological and chemical based. In mechanical based methods, microalgal cells are harvested by mechanical external forces, such as centrifugation, filtration, sedimentation, dissolved air flotation and usage of attached algae biofilms and ultrafiltration membranes.
- 13. Harvesting and drying of algal biomass (1) Bulk harvesting—aimed at separation of biomass from the bulk suspension. The concentration factors for this operation are generally 100–800 times to reach 2–7% total solid matter. This will depend on the initial biomass concentration and technologies employed, including flocculation, flotation or gravity sedimentation. (2) Thickening—the aim is to concentrate the slurry through techniques such as centrifugation, filtration and ultrasonic aggregation, hence, are generally a more energy intensive step than bulk harvesting.
- 14. Flocculation and ultrasonic aggregation Microalgae cells carry a negative charge that prevents natural aggregation of cells in suspension, addition of flocculants such as multivalent cations and cationic polymers neutralises or reduces the negative charge. It may also physically link one or more particles through a process called bridging, to facilitate the aggregation. Multivalent metal salts like ferric chloride (FeCl3), aluminium sulphate (Al2 (SO4)3) and ferric sulphate (Fe2 (SO4)3) are suitable flocculants.
- 15. Harvesting by flotation Flotation methods are based on the trapping of algae cells using dispersed micro-air bubbles and therefore, unlike flocculation, do not require any addition of chemicals. Some strains naturally float at the surface of the water as the microalgal lipid content increase.
- 16. Gravity and centrifugal sedimentation Gravity and centrifugation sedimentation methods are based on Stoke’s Law, i.e. settling characteristics of suspended solids is determined by density and radius of algae cells (Stoke’s radius) and sedimentation velocity. Gravity sedimentation is the most common harvesting technique for algae biomass in wastewater treatment because of the large volumes treated and the low value of the biomass generated.
- 17. Biomass filtration A conventional filtration process is most appropriate for harvesting of relatively large (>70 mm) microalgae such as Coelastrum and Spirulina. It cannot be used to harvest algae species approaching bacterial dimensions (<30 mm) like Scenedesmus, Dunaliella and Chlorella. Conventional filtration operates under pressure or suction, filtration aids such as diatomaceous earth or cellulose can be used to improve efficiency . For recovery of smaller algae cells (<30 mm), membrane microfiltration and ultra-filtration (a form of membrane filtration using hydrostatic pressure) are technically viable alternatives to conventional filtration.
- 18. Drying Harvested algae contain 97%-99% water. Removal of most of the water is necessary for long term storage of the algae feedstock and is required for many downstream processes. To keep algae from prolonged microbial growth, the moisture level of the harvested algae should be kept below 7%. Drying is an energy intensive process and can account for up to 30% of the total production costs. Natural drying (solar and wind) is the most economical way; however, its weather dependent nature could easily put the operation at risk of spoilage. It also requires a large space.
- 19. Processing Extracting the oil and converting the oil from algae to biodiesel are the primary driving force for algae to fuels technology development. The oil extracted can be converted to biodiesel via transesterification reaction. Nevertheless, the whole algae or the residues from oil extraction are excellent feedstock for making other fuels and products via different processes. For example, the starch and cellulose components are suitable for ethanol fermentation.
- 20. Oil extraction Extraction of oil from algal biomass has proven to be difficult and expensive. Organic solvent extraction is a widely used method for lipid extraction from traditional oilseed plants, and different extraction systems have also been tested with algae cultures . In order to maximize the lipid extraction efficiency, the organic solvent used has to match the lipid polarity profile in the cells.
- 21. In situ transesterification Conventional transesterification process requires costly oil extraction and separation. If fatty acid containing lipids are simultaneously extracted and transesterified, it would eliminate the need to extract and separate the lipids and fatty acids contained in the algae. Direct or in situ transesterification has been proven in a number of feed stocks including marine tissues, yeast and fungi, bacteria, microheterotrophs, algae fatty acid, and municipal primary and secondary sludge.
- 22. Ethanol fermentation of starch and cellulose in Algae Algae contain substantial amounts of starch and cellulose which, in theory, can be fermented to ethanol using existing technologies. The concept appears to be straightforward and has attracted business attention.
- 23. Thermochemical conversion Algal biomass, either whole algal cells or extraction residues, are suitable feedstock for thermochemical conversion such as gasification, pyrolysis, and hydrothermal liquefaction and gasification to produce syngas, bio-oil, biopolyols, and biochar.
- 24. Gasification In gasification, biomass is converted to a combustible gas mixture called “synthesis gas (syngas)” or “producer gas” through partial oxidation reactions at high temperature typically ranging from 700 to 1 100°C. Syngas may vary in composition with type and moisture content of feedstock, type of gasifiers, gasification conditions, etc. Syngas can be burned to produce heat or used in gas engines or gas turbines to produce electricity.
- 25. Pyrolysis Pyrolysis is another important thermochemical conversion process in which biomass is degraded to bio-oil, syngas, and biochars at medium high temperature (300-600°C) in the absence of oxygen. Biomass is usually heated through heated surface or sands. The pyrolysis products generally include bio-oil, gas and char, and nowadays bio-oils are preferred because they have the potential to be upgraded to liquid transportation fuels.
- 26. Hydrothermal liquefaction and gasification Hydrothermal liquefaction refers to decomposition reactions taking place in water media at high temperature and high pressure. The high temperature and high pressure create a unique condition termed “supercritical water” in which chemical reaction rates are significantly enhanced. At lower temperature range (200–400°C) the reactions produce more liquid products (bio-oil or bio-crude), and therefore termed hydrothermal liquefaction. Hydrothermal gasification processes generally take place at higher temperatures (400–700°C) and produce methane or hydrogen gases.
- 27. Anaerobic digestion The harvested algae biomass, under anaerobic condition and inoculation of certain groups of bacteria, i.e. hydrolytic and fermentative bacteria, acetogenic bacteria, methanogenic bacteria, can be fermented to methane, which is another way to produce renewable energy.
- 28. Value added co products of biodiesel Biodiesel production will generate about 10% (w/w) glycerol as the main byproduct. Omega 3 fatty acids are polyunsaturated fatty acids (PUFAs) and essential components for the growth of higher eukaryotes. Nutritionally, eicosapentaenoic acid (EPA, 20:5) and docosahexaenoic acid (DHA, 20:6) are the most important fatty acids belonging to this group of bioactive compounds. These long chain omega 3 fatty acids provide significant health benefits to the human population, particularly in reducing cardiac diseases such as arrhythmia, stroke and high blood pressure
- 29. Other uses of microalgae Microalgae are used as human nutrition, animal feed, aquaculture etc. Algal biomass contains 20%-40% protein, 30%-50% lipid, 20% carbohydrate, and 10% other compounds. Depending on the conversion processes, a range of products can be obtained from algal biomass.
- 30. Biodiesel future!!! Biodiesel has become more attractive recently because of its environmental benefits and the fact that it is made from renewable resources. With the increase in global human population, more land may be needed to produce food for human consumption (indirectly via animal feed).
Editor's Notes
- #9: ]. Although open pond systems have advantages of low construction and operation cost, there are many limitations that were widely discussed. They are (1) low productivity, (2) high harvesting cost, (3) water loss through evaporation. (4) Temperature fluctuation, (5) contamination by predators, and (6) lower carbon dioxide use efficiency [12, 14, 13, 17]. have evolved in the last 50 years.
- #11: Tubular PBR, constructed with transparent glass or plastic, is one of the popular outdoor systems for mass algae cultivations. By shape, it can be horizontal, vertical, conical, and inclined. By mixing, it can be airlift or pump system [18]. Plate type of PBR can be vertical, horizontal and inclined. The advantages of tubular and plate types of PBR are narrow light path (1.2-12.3 cm) that allows much higher cells concentration than open pond system, large illuminating area, and less contamination issues. The disadvantages are gradients of pH, dissolved oxygen and CO2 along the tubes, wall growth, fouling, hydrodynamic stress, and expensive to scale up [17, 18].
- #15: The main advantages of ultrasonic harvesting are that it can be operated continuously without inducing shear stress on the biomass, which could destroy potentially valuable metabolites, and it is a non-fouling technique
- #16: Although flotation has been mentioned as a potential harvesting method, there is very limited evidence of its technical or economic viability.
- #17: However, the method is only suitable for large (ca. >70 mm) microalgae such as Spirulina [37]. Centrifugation recovery (CR) is preferred for harvesting of high value metabolites and extended shelf-life concentrates for hatcheries and nurseries in aquaculture [38].