A review of microalgae harvesting technology for biofuel production

86 Nguyen Thi Thanh Xuan A REVIEW OF MICROALGAE HARVESTING TECHNOLOGY FOR BIOFUEL PRODUCTION TỔNG QUAN VỀ CÔNG NGHỆ THU HOẠCH VI TẢO NHẰM SẢN XUẤT NHIÊN LIỆU SINH HỌC Nguyen Thi Thanh Xuan The Univers[.]

86 Nguyen Thi Thanh Xuan A REVIEW OF MICROALGAE HARVESTING TECHNOLOGY FOR BIOFUEL PRODUCTION TỔNG QUAN VỀ CÔNG NGHỆ THU HOẠCH VI TẢO NHẰM SẢN XUẤT NHIÊN LIỆU SINH HỌC Nguyen Thi Thanh Xuan The University of Danang, University of Science and Technology; Email: nttxuan@dut.udn.vn Abstract - Microalgae are receiving increasing attention worldwide as an alternative and renewable source for energy production Through various conversion processes, microalgae can be used to produce many different kinds of biofuel, which include biodiesel, bio-syngas, bio-oil, bio-ethanol, and bio-hydrogen However, large scale production of microalgal biofuel, via many available conversion techniques, faces a number of technical challenges which have made the current growth and development of the algal biofuel industry economically nonviable Many researchers consider efficient harvesting is the major challenge for commercializing micro-algal biofuel Algae can be harvested by a number of methods; sedimentation, filtration flocculation, flotation, centrifugation and or a combination of any of these This review aims at collating and presenting an overview of current harvesting technologies from microalgae for the production of biofuel ElectroCoagulation Flocculation (ECF) is seen as a promising approach for reducing the cost and energy input of microalgae harvesting for biofuel However, choosing a harvesting method needed to be considered with a microalgal strain in combination with a considerable influence on the design and operation of both upstream and downstream processes is an overall microalgal biofuel production process Tóm tắt - Vi tảo ngày nhận quan tâm quy mơ tồn cầu cho mục tiêu sản xuất nhiên liệu sinh học lượng thay Các loại nhiên liệu điều chế từ vi tảo biodiesel, biogas, bioethanol bio-hydro Tuy nhiên, việc sản xuất loại nhiên liệu quy mô lớn đối mặt với nhiều thách thức kinh tế kỹ thuật ngăn sản tiến trình thương mại hóa Nhiều nhà nghiên cứu nhận định khâu thu hoạch thách thức lớn Các kỹ thuật thu hoạch vi tảo bao gồm lắng, lọc, keo tụ, tuyển nổi, ly tâm kết hợp nhiều phương pháp Nghiên cứu đánh giá tổng quan công nghệ thu hoạch vi tảo nhằm sản xuất nhiên liệu sinh học Phương pháp keo tụ tuyển điện phân dường công nghệ hứa hẹn chi phí lượng thấp Tuy nhiên việc lựa chọn công nghệ thu hoạch cần phù hợp với chủng vi tảo xem xét yếu tố kinh tế kỹ thuật trình sản xuất nhiên liệu từ vi tảo Key words - microalgae biofuels; microalgae harvesting; sedimentation; flocculation; centrifugation; flotation; electro coagulation Từ khóa - vi tảo; nhiên liệu sinh học; thu hoạch sinh khối; lắng; lọc; ly tâm; keo tụ; tuyển nổi; điện phân keo tụ Introduction An increasing energy demand and potential fossil fuel depletion have become major concerns for people around the world Furthermore, climatic changes and global warming possibly caused by the emission of greenhouse gases have become contemporary issues to be solved [1] Among the renewable energy sources, such as solar, geothermal, wind, hydropower, biomass is considered a renewable, biodegradable, and carbon dioxide-neutral energy source derived from plenty of resources, for example, agricultural residue and waste, forestry waste, municipal solid and industrial waste, terrestrial crops, and aquatic plants [2] However, alternate energy resources akin to first generation biofuels derived from terrestrial crops such as sugarcane, sugar beet, maize and rapeseed place an enormous strain on world food markets, contribute to water scarcity and deforestation [3] Second generation biofuels derived from lignocellulosic agriculture and forest residues and from nonfood crop feedstocks address some of the above problems; however there is concern over competing land use or required land use changes [3] Therefore, based on current knowledge and technology projections, third generation biofuels specifically derived from microalgae are considered to be a technically viable alternative energy resource that is devoid of the major drawbacks associated with first and second generation biofuels [4] With the outstanding advantages of microalgae: high biomass yield (their growth rates tend to doubling as little as hours) and oil yield per unit area for some species are more than 20x higher than e.g for oil seed rape [5]; as well as their high adaptability with a variety of culture media: freshwater, saltwater, brackish or nutrient-rich waste water in particularly; combined with their consumption ability of large amounts of CO2 (1.8 tones of CO2 could create tons of algae biomass [6]), they have been highlighted as a feedstock of biofuels Algae based biofuels are not tainted with the ethical dilemmas of current, food-based biofuels: they not require arable land; need not to compete for freshwater; not threaten food security [7] These fuels produced from microalgae biomass can be bio-methane (by anaerobic digestion process), bio-ethanol (by fermentation), biodiesel (by trans-esterification of fatty acids in biomass), bio oil (by pyrolysis of biomass), biohydrogen (by fermentation and electro-hydro-genesis process) and green diesel (by pyrolysis of biomass associated with HDO process) [8],… Despite its inherent potential as a biofuel resource, many challenges have impeded the development of algal biofuel technology to commercial viability that could allow for sustainable production and utilization Algae biofuels production process involves four major steps, including cultivation, harvesting, lipid extraction, and conversion THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 6(79).2014, VOL Many researchers consider efficient harvesting is the major challenge of commercializing micro-algal biofuels [9, 10, 11] The goal of this research is to identify and evaluate several potentially viable harvesting methods that could be incorporated into end-to-end algae biofuel production This review provides useful information to help future development of efficient and commercially viable technology for microalgae-based biofuels production Microalgae harvesting technologies Efficient harvesting of biomass from cultivation froth is essential for mass production of biodiesel from microalgae The major techniques presently applied in the harvesting of microalgae include centrifugation, flocculation, filtration and screening, gravity sedimentation, flotation, and electrophoresis techniques [12] The cost of algae harvesting can be high, since the mass fractions in culture broth are generally low, while the size of micro-algal cells is small (most algae are below 30 µm) and the negative surface charge on the algae that results in dispersed stable algal suspensions, especially during the growth phase [12] The selection of harvesting technique is crucial to economic production of microalgal biomass and dependent on the properties of microalgae, such as density, size, and the value of the desired products [11] 2.1 Gravity sedimentation Gravity sedimentation is a simple process of solidliquid separation under gravity This is a commonly applied for separating microalgae in water and waste-water treatment The major advantages of sedimentation are low power requirement, low design cost and low manpower requirement The sedimentation rates depend on cell density, cell size [13], cell motility and type of water flow Enhanced microalgal harvesting by sedimentation can be achieved through lamella separators and sedimentation tanks [9] In sedimentation the recoveries are too low for large scale harvesting however sedimentation can be used as one of the primary harvesting process to concentrate the microalgae 2.2 Filtration Filtration is a mechanical separation method, which usually uses a bed of granular media or a porous membrane Many types of filters have been used to harvest algae and filtration has been found satisfactory at recovering relatively large algal cells [14] but can be hampered by low throughput and rapid clogging [15] Although there is a wide a variety of filter designs, membrane filters can be simply classified by the pore or membrane size; macro filtration >10µm, micro-filtration 0.1–10µm, ultrafiltration 0.02–0.2µm and reverse osmosis