Since from the last few decades, there is an increased demand for energy consumption and according to the data in 2008 the annual world primary energy consumption was estimated at 11,295 million tonnes of oil equivalent (mtoe). Fossil fuels accounted for 88% of the primary energy consumption, with oil (35% share), coal (29%) and natural gas (24%) as the major fuels, while nuclear energy and hydroelectricity account for 5% and 6% of the total primary energy consumption. Due to these consumption rates there is rapid depletion of fossil fuels which in turn leads to search for alternative sources of energy. Biofuel such as biodiesel and bioethanol are viewed as attractive potential solutions to alleviate the existing dependence on petroleum-based fuels. In addition, burning of fossil fuels has raised numerous environmental concerns, including greenhouse gas (GHG) effects which significantly contribute towards global warming. With the increase of GHG emissions, mainly due to large scale use of fossil fuels for transport, electricity and thermal energy generation, it has become increasingly important to develop abatement techniques and adopt policies to minimize impacts of global warming. In order to cop up with these problems a selection of a wide range of effective technologies should be applied. The biofuels produced from the renewable resources could help to reduce both the world's dependence on oil and CO2 production. These biofuels have the potential to cut CO2 emission because algae have potential to absorb CO2 for the process of photosynthesis. Biofuels and bio products produced from algal biomass would mitigate global warming.
Biofuel are derived from organic matter and may be solid, liquid or gaseous. Biofuel are divided into primary and secondary fuels and secondary fuel is further divided into 1st generation, 2nd generation and 3rd generation fuels. A 'first generation' biofuel (i.e. biodiesel (bio-esters), bio-ethanol, and biogas) is characterized either by its ability to be blended with petroleum-based fuels, combusted in existing internal combustion engines, and distributed through existing infrastructure, or by the use in existing alternative vehicle technology like FFVs ('Flexible Fuel Vehicle') or natural gas vehicles.
The second generation biofuels is intended to produce fuels from plant biomass mainly lignocellulosic components. Converting the woody biomass into fermentable sugars requires costly technologies involving pre-treatment with special enzymes meaning that second generation biofuels cannot yet be produced economically on a large scale. (Dragone et, al., 2010)That's why third generation biofuels derived from microalgae are considered to be a viable alternative energy resource that is devoid of the major drawbacks associated with first and second generation biofuels. This article will focus on the potential source of the 3rd generation biofuel that is microalgae.
Biology of Microalgae
Microalgae are a diverse group of prokaryotic and eukaryotic photosynthetic microorganisms which live in harsh conditions due to their simple structure. Microalgae are thallophytes plant like but lacking roots, stems, and leaves that have chlorophyll a as their primary photosynthetic pigment plants. Prokaryotic cells lack membrane-bound organelles (plastids, mitochondria, nuclei etc) eg (cyanobacteria). Eukarkotic cells, which comprise of many different types of common algae and having well defined membrane -bound organelles. The most important classes are: green algae (Chlorophyta), red algae (Rhodophyta) and diatoms (Bacillariophyta). Microalgae have very high photosynthetic rates which allow them to translate sunlight into high rates of biomass growth that are unavailable from crops, e.g. at least ten times the yield per hectare compared to palm oil. Apart from a small amount of nutrients, their only requirements are a water supply and a CO2 feed, both of which can be taken from waste sources such as power station chimneys. Microalgae can be either autotrophic or heterotrophic. Autotrophs can be photoautotrophic, require inorganic compounds such as carbon dioxide, salts and light as a source of energy while the heterotrophic are non-photosynthetic therefore require an external source of organic compounds as well as nutrients as an energy source. Some photosynthetic algae are mixotrophic, i.e. they have the ability to both perform photosynthesis and acquire exogenous organic nutrients. For autotrophic microalgae, photosynthesis is a key component of their survival, as microalgae are able to fix CO2 whereby they convert solar radiation and CO2 absorbed by chloroplasts into adenosine triphosphate (ATP) and O2. Microalgae are able to fix CO2 efficiently from different sources, including the atmosphere, industrial exhaust gases, and soluble carbonate salt
Why Microalgae for Biofuel?
• Microalgae have ability to grow under harsher conditions and can be cultivated in saline/brackish water/coastal seawater on non-arable land.
• Microalgae can grow rapidly, 100 times faster than terrestrial plants and they can double their biomass in less than one day.
• Some stains of microalgae have ability to accumulate large quantity of lipid inside their cells from which the lipid can be converted to biodiesel. Microalgae cultivation consumes less water than land crops.
• Microalgae did not need fertilizers, herbicides or pesticides for their rapid growth.
• The tolerance of microalgae to high CO2 content in gas streams allows high-efficiency CO2 mitigation.
• Nitrous oxide release could be minimized when microalgae are used for biofuel production.
• Microalgae are also used to treat the waste water by removing the phosphorous and nitrogen by taking these elements as nutrient for growth.
• Microalgae is used to sequester CO2 from the atmosphere helps in mitigation of global warming
• microalgae species other compounds may also be extracted, with valuable applications and products such as polyunsaturated fatty acids, natural dyes, polysaccharides, pigments, antioxidants, high-value bioactive compounds, and proteins
• After oil extraction the resulting algae biomass can be processed into ethanol, methane, livestock feed, used as organic fertilizer due to its high N:P ratio, or simply burned for energy cogeneration (electricity and heat) .
• The biochemical composition of the algal biomass can be modulated by varying growth conditions; therefore, the oil yield may be significantly enhanced.
Thus from above reasons microalgae is widely used for biofuel production and most important step is the selection of appropriate algae strains. The ideal algal strain for biofuel production should have firstly high lipid productivity, be able to survive in harsher conditions and the shear stresses common in photo bioreactors, should have rapid growth rate, be able to dominate wild strains in open pond production, be able to have high photosynthesis rate and should have salinity tolerance. Many microalgae stain like Botryococcus braunii, Chlorella vulgaris, and Scenedesmus Sp, as shown in table 1 have high lipid content. Many Genetically modified strains of algae are being developed for algae biofuels, especially high lipid-content algae. Solazyme is commercializing technology that utilizes a proprietary strain of genetically modified algae that grows in the dark. Not only the algae stain but also the site selection and resource evaluation cultivation is important and must have proper water supply/ demand, its salinity and chemistry; the land topography, geology, and ownership; the climatic conditions, temperature, insulation, evaporation, precipitation; (iv) the easy access to nutrients and carbon supply sources. The isolation of local stain for biofuel production should be considered a basic step for the biofuel production.
About Author / Additional Info:
Research scholar at central university of Gujarat