Biodiesel from microalgae production methods - a review

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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 harvestingaimed at separation of biomass from the bulk suspension. The concentration factors for this operation are generally 100800 times to reach 27% total solid matter. This will depend on the initial biomass concentration and technologies employed, including flocculation, flotation or gravity sedimentation. (2) Thickeningthe 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 Stokes Law, i.e. settling characteristics of suspended solids is determined by density and radius of algae cells (Stokes 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 (