Microalgae – The next generation biofuel

Effect of urbanization leads to increase in energy demand as the fossil fuel resources are limited which will further generate an energy crisis. Based on the international energy agency report (2014), there will be an increase in energy demand by 37% in 2040. On the other CO2 emissions the global threat as it reaches the alarming limits which generated more from electricity generation, natural gas production process, cement, iron, steel manufacturing and combustion of municipal solid waste areas contribute 75% of total global CO2 emission. On this context, Development of photosynthetic biomass based fuels is receiving a greater attention towards economic and environmental sustainability to fossil fuels.

Microalgae is an alternative feedstock for the production of biofuel. Comparing to cultivation of terrestrial crops, mass production of microalgae is highly efficient due its photosynthetic efficiency as it can assimilate 4-5% of solar energy, being the terrestrial crops is 1-2%. During the absorption of solar energy, microalgae have the capability to fix CO2 with the efficiency of 10% to 50% higher than terrestrial plants which pave the way to reduce anthropogenic carbon emissions. The distinctive composition of microalgae represents 5-23% of carbohydrate, 6-52% of proteins, and 7-23% of lipids. Production of renewable energy in the form of biogas using the microalgal biomass through anaerobic digestion can be promising alternate and sequestration of CO2 will be the better prospective for development of carbon neutral biofuel. This carbon neutral biofuels gradually decrease the greenhouse gas emissions when fossil fuels are compared and by the end, the nutrient rich anaerobic digestate can be used as manure in farming lands.

As large amounts of nutrients needed for the mass production of organic biomass for the production of biogas, which further ends in the limitations of economic feasibility. Here, Microalgae plays a major role in nutrient recycling where it requires only fewer resources when comparing to other photosynthetic organisms. They don’t require any fertile land area and clean water like terrestrial plants, where it can be cultivated in nutrient rich wastewater streams discharged from various industries. In addition the digestate from anaerobic digester can be fed into the photo-bioreactors or algal ponds for the biomass production, as it is rich in nitrogen and phosphorous. So based on the nutrient assimilation, environmental factors such as sunlight and temperature the organic fractions of microalgae i.e., carbohydrates, lipids and proteins varies from each species.

The concept biofuel production from microalgal biomass was illustrated here. There are different routes for the biomass conversion to biofuel. Among those first one direct conversion of microalgal biomass into biogas via anaerobic digestion process or after disruption of algal cells to enhance the biogas production. The next route for the production of biofuel is extraction of lipid via conventional method and residual biomass is converted into biogas through anaerobic digestion. Even though Lipids considered to be one of major substrate for anaerobic digestion but because of their low alkalinity and intermediate compounds like long chain fatty acids (LCFAs) and volatile fatty acids which are considered to be a inhibitory factors of anaerobic digestion when the substrate has high concentration of lipids.  The composition of biogas is mainly of CH4, CO2, and H2S. Microalgae also has ability to purify this biogas which is rich in methane as the CO2 from the biogas can be striped due to their high solubility which can be assimilated by microalgae as a nutrient and leaving the high concentration of low solubility methane at the end of the purification step shows the synergistic mechanism between both the process. As the resulting digestate is rich in nutrients will be an ultimate source for their growth and productivity that makes the closed loop assisting the mass production of microalgal biomass. So the concept biofuel with the better understating of microalgae nutrient uptake and organic fractions of microalgal biomass and their residues will be greater alternative for clean energy production with more economic sustainability for the future.

Work was carried out under Dr. S.V. Srinivasan - CLRI, Chennai

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