Cultivation of Microalgae for Biofuel Production: Coupling With Sugar Factories
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Date
2021-06
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Addis Ababa University
Abstract
Due to many important characteristics of microalgae including high oil content, ability to overcome harsh conditions and high photosynthetic efficiency the production of biofuels from algal biomass has gained interest. However, large scale production of biofuel from microalgae is still uncertain primarily due to a lack of feasibility of the process and that it proves to be capital and energy-intensive. Therefore, an integration of microalgal cultivation with other processes for achieving an inexpensive nutrient and a reduced energy use has paramount importance. Coupling of microalgal biofuel production with other process such as wastewater treatment, industrial and power plants has been found helpful in increasing feasibility of the process. The main objective of the present study was to evaluate the potential of sugar factories’ wastes to support the growth of microalgae for biofuel production. A case study approach was followed considering a sugar factory, Metahara sugar factory, with annexed distillery plant. Two scenarios were considered. In the first scenario the wastewater from the sugar mill was used as the only nutrient source for the growth of the microalgae. In the second scenario the vinasse from the ethanol production plant was used as an additional nutrient source along with the wastewater from the sugar mill. Economic feasibility was also performed for the second scenario. The first scenario of the study shows that 12 mg of total nitrogen (TN) and 7.4 mg of total phosphorus (TP) per liter of wastewater could be transferred to algal growth ponds, and approximately 121 tons/year algal biomass would be produced from the integrated process. By applying all the underlined assumptions reductions of COD (mg O2/L) from 2200 to 447, BOD5 (mg O2/L) from 1200 to 207, total nitrogen, TN (mg/L) from 15 to 0.6 and total phosphorus, TP (mg/L) from 10 to 1.5 were found from the coupled process. The results from second scenario of the study showed three products: biodiesel production of 188 ton/year, biogas production of 2011000 m3/year, and bio-fertilizer production of 42 tons/year. Economic evaluation of the coupled process for the biofuel production showed that in order for the biodiesel from microalgae to be competitive with the current petroleum its minimum selling price (MBSP) needs to be reduced at least by half. Sensitivity analysis on the MBSP shows that oil content of the microalgae and nitrogen content in the waste effluents are the two dominant factors which significantly affect the feasibility of the process. This study investigated the potential of a future possible bio-refinery and environmental pollution reduction concept by coupling microalgae biomass production with sugarcane-processing factory wastes and by-products. It was found that the factory wastes and by-products have a significant potential for a viable biofuel production from microalgae.
Study on the biology of the microalgae to get a robust strain with high oil content, the development of energy-efficient and cost-effective harvesting technology, and study on the development of selective, sensitive and inexpensive control methods, etc. are essential future research works for boosting viability of biofuel production from microalgae.
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wastewater, flue gas, vinasse, process coupling, process evaluation, TEA, MBSP, sensitivity analysis