.. .MODELING ON ECONOMIC FEASIBILITY ANALYSIS OF BIODIESEL PRODUCTION FROM MICROALGAE IN CHINA JIA ZONGCHAO (M.Sc., Peking University) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE... extracted from the microalgae biomass, the next step is the conversion of lipid to biodiesel The common methods used for biodiesel production from microalgae consist of transesterification, either in. .. selection The selection of microalgae strains plays a crucial role for the success of microalgae based biodiesel production [84,85,86] The ideal microalgae strain for biodiesel production should:
MODELING ON ECONOMIC FEASIBILITY ANALYSIS OF BIODIESEL PRODUCTION FROM MICROALGAE IN CHINA JIA ZONGCHAO NATIONAL UNIVERSITY OF SINGAPORE 2014 MODELING ON ECONOMIC FEASIBILITY ANALYSIS OF BIODIESEL PRODUCTION FROM MICROALGAE IN CHINA JIA ZONGCHAO (M.Sc., Peking University) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2014 Declaration I hereby declare that this thesis is my original work and it has been written by me in its entirety, under the supervision of A/P Stephan Jaenicke, Chemistry Department, National University of Singapore, between 04/08/2013 and 04/08/2014. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. Name Signature I Date Acknowledgements I would like to extend my sincere thanks to my supervisor, Associate Professor Stephan Jaenicke, who has put great effort into my project during my study in NUS. I am also very grateful for the many valuable suggestions provided by Associate Professor Chuah Gaik Khuan. In addition, I have to thank Miss Toy Xiu Yi, Miss Han Aijuan, Mr. Sun Jiulong, Mr. Wang Jie, Miss Gao Yanxiu and all the other members of our catalytic research group, for their sincere assistance and encouragement during this very precious and memorable one-year stay in Singapore. In addition, I especially acknowledge the SPORE committee which provided the scholarship for my study in Peking University and National University of Singapore. II Table of Contents DECLARATION I ACKNOWLEDGEMENTS II TABLE OF CONTENTS III SUMMARY V LIST OF TABLES VI LIST OF FIGURES VII CHAPTER 1 : LITERATURE REVIEW 1 1.1. 1 Introduction 1.1.1. Background 1 1.1.2. Biodiesel 3 1.1.3. Feedstock of biodiesel 4 1.2. Biological characteristics of microalgae 10 1.3. Process of microalgal biodiesel production 11 1.3.1. Technologies for microalgal cultivation 12 1.3.2. Microalgal harvesting technologies 20 1.3.3. Dehydration techniques 25 1.3.4. Lipid extraction 26 1.3.5. Biodiesel production 31 1.3.6. Other techniques of producing energy from algae 36 1.3.7. Other applications of microalgae extracts 40 CHAPTER 2 : MODEL CONSTRUCTION 43 2.1. System description 43 2.2. Model description 43 2.2.1. Cultivation 44 2.2.2. Harvesting and dewatering 47 2.2.3. Lipid extraction 48 2.2.4. Lipid conversion 48 2.2.5. Co-products production 48 III CHAPTER 3 : MODEL ANALYSIS 49 3.1. Mass Balances 49 3.2. Net energy ratio 49 3.2.1. Energy estimations for each step 49 3.2.2. NER calculation 53 Production cost estimation 53 3.3. 3.3.1. Capital cost 54 3.3.2. Operating cost 57 3.3.3. Production cost calculation 57 3.4. Greenhouse gas emission rate 58 3.5. Sensitivity analysis 59 3.6. Conclusion 63 REFERENCES 66 IV Summary Although microalgae based biodiesel production has been studied for many years ever since microalgae have been recognized as the third generation biodiesel feedstock, there exists still a big gap when considering performing the whole process at an industrial scale to replace the conventional petroleum-based diesel. Therefore, this project presents an economic feasibility assessment for a facility that grows algae and transforms the algal biomass into transportation fuel. In addition to economic aspects, environmental impact assessment and an analysis of the carbon foot print are also covered. The whole system takes all the processes from microalgae cultivation to biodiesel production into account. The results obtained confirm that with the current technology, microalgal biodiesel production will not be competitive with the conventional diesel if an industrial scale facility were to be built today. However, the whole production is carbon neutral, or even carbon-negative, so that credits for greenhouse gas reduction, which have not been considered in this study, may impact the economic assessment. V List of Tables TABLES PAGE Table 1.1 Common feedstocks for biodiesel production worldwide 5 Table 1.2 Comparison of microalgae with other biodiesel feedstocks 10 Table 2.1 Design specification for PBRs and open pond 45 Table 2.2 Daily seawater consumption for the cultivation process 47 Table 3.1 Model parameters for energy consumed and produced 52 Table 3.2 Fixed parameters for energy consumption calculations 52 Table 3.3 Method and values of the parameters used for the 54 estimation of biodiesel production costs Table 3.4 Raceway and PBRs capital cost found in literatures 55 Table 3.5 Parameter values for capital cost calculations 56 Table 3.6 Parameters used for the calculation of GHG emission rate 58 Table 3.7 Basic parameters for the three cases. 59 VI List of Figures FIGURES PAGE Fig. 1.1 World marketed energy consumption, 1990-2040 2 Fig. 1.2 The chemical formula of a triacylglyceride (TAG) and of 3 biodiesel (fatty acid methylester, FAME). R is a long, linear alkyl with 11 to 21 carbons and possibly one or more not conjugated double bonds. Fig. 1.3 General cost breakdown for the biodiesel production 5 Fig. 1.4 A schematic of biofuels production from microalgae 12 Fig. 1.5 View of a raceway pond 15 Fig. 1.6 View of horizontal tubular photobioreactor 16 Fig. 1.7 Transesterification of triacylglycerols and alcohol in the 32 presence of catalysts to yield esters and glycerol Fig. 2.1 Schematic of microalgal biofuels production process 44 Fig. 3.1 NER of the three cases 60 Fig. 3.2 Required selling price of the biodiesel produced in these 62 three cases in order to achieve a 10% rate of return Fig. 3.3 GHG emission rate of three cases VII 63 Chapter 1 : Literature review 1.1. Introduction 1.1.1. Background Climate change has been recognized as the perhaps most urgent global environmental issue today, which requires international collaboration across countries, sectors and disciplines [1]. As global temperatures increase, all countries will have no choice but to adapt to limit the human, economic and social impacts of climate change [1]. It is estimated that if the average global temperature increases by more than 2 oC, hundreds of millions of people could lose their lives and over one million species could become extinct [2]. Among the total primary energy consumption of the world, fossil fuel accounts for 86.7%, while nuclear energy, hydroelectricity and renewable energy account for about 4.4%, 6.7% and 2.2%, respectively [3]. Considering the current technological feasibility, potential reserves, and increased exploitation of newer unconventional resources, such as natural gas and shale oil, it is highly likely that fossil fuels will continue to be used as the primary energy source at low cost for a considerable period of time. However, even if the depletion of fossil energy reserves is not the driving force towards renewable energy, attention has to be paid to global warming caused by continuing CO2 emissions. Targeting the problem of the atmospheric greenhouse gases (GHGs) could be an appropriate stabilization strategy as a starting point for a global deal [1]. The use of fossil fuels on a large scale and the concomitant emissions 1 of CO2 and other greenhouse gases has caused global warming; therefore, renewable and environmental-friendly energy sources should be utilized to replace fossil fuels [4]. Global warming will result in detrimental effects, such as the increase in sea level and the flooding of lowlands, as well as a transformation of the weather patterns [5]. It is widely accepted that continuing the use of fossil fuels as the major source of energy is unsustainable because of the environmental issues caused by the carbon emissions [6]. World energy consumption, 1990-2040 900.0 819.6 Quadrillion Btu 800.0 729.2 700.0 629.8 600.0 523.9 500.0 400.0 354.8 406.0 300.0 200.0 100.0 0.0 1990 2000 2010 2020 2030 2040 Year Fig. 1.1. World marketed energy consumption, 1990-2040. Source: Energy Information Administration (EIA). Fig. 1.1 shows the world energy consumption from 1990 to the present and projected through 2040. According to the EIA report, over 50% more energy will be needed in 2040 than today to satisfy the world demand [7]. Clearly, this additional energy demand cannot be accompanied by a similar increase in carbon dioxide 2 emissions. Accordingly, replacement of fossil fuels with renewable energy should be advocated and developed in order to tackle these critical issues. 1.1.2. Biodiesel Biodiesel, the first alternative biofuel known to the public and the main alternative to fossil fuels, has received much attention recently. Any diesel-equivalent biofuel made from renewable feedstocks can be accounted as biodiesel, if it can be produced through a special process from renewable feedstocks. More specifically, biodiesel refers to the monoalkyl esters of long-chain fatty acids ( Fig. 1.2) derived by chemical reaction, e.g. transesterification of feedstocks, such as the vegetable oil or animal fats. Since the vegetable oil is much more viscous than conventional diesel fuel or biodiesel, it cannot work in the present engines, thus the plant oil cannot be directly used as fuel. Fig. 1.2. The chemical formula of a triacylglyceride (TAG) and of biodiesel (fatty acid methylester, FAME). R is a long, linear alkyl with 11 to 21 carbons and possibly one or more not conjugated double bonds. 3 Biodiesel is an attractive alternative energy for the following several reasons: (a) it is a renewable biofuel that can be provided sustainably; (b) it is highly biodegradable and has hardly any toxicity; (c) it is eco-friendly, resulting in no net increased release of carbon dioxide, aromatic compounds or other chemical substances that are detrimental to the environment [6,8,9]; (d) it has a lower combustion emission profile than the petroleum-based diesel, and there is no contribution to the global warming due to the closed carbon cycle; (e) its use can decrease the dependence on imported crude oil, although the calorific value of biodiesel is less than the fossil fuel; (f) there is little or no need to modify the existing engines [10] where it can be used with better engine performance; (g) it can be blended with traditional petroleum-based diesel fuel in any ratio; (h) it can improve the lubricating properties when added to regular diesel fuel in an amount of 1-2% [11]. 1.1.3. Feedstock of biodiesel Because the cost of raw feedstocks accounts for about 75% of the total cost of biodiesel production (Fig. 1.3), choosing an appropriate feedstock is of vital importance to lower the biodiesel production cost, and then to make the whole process of biodiesel production feasible, which means that the biodiesel could substitute diesel at an industrial scale. The primary biodiesel feedstocks for several regions of the world are shown in Table 1.1. 4 Energy, 2% General overhead, 1% Direct labour, 3% Depreciation, 7% Chemical feelstocks, 12% Oil feedstocks, 75% Fig. 1.3. General cost breakdown for biodiesel production. Source: Ref. [12] Table 1.1. Common feedstocks for biodiesel production worldwide. Country Feedstock Reference USA Soybeans [13] Europe Rapeseed [13] Canada Canola oil [14] Africa Jatropha [15] China Waste cooking oil [16] Spain Linseed oil [15] 1.1.3.1. First generation biodiesel feedstocks First generation biodiesel was derived from edible oil feedstock, e.g., rapeseed [17], soybeans [18,19,20], palm oil [21,22,23,24] and sunflower [17,25,26], etc. Because more than 95% of the first generation biodiesel was made from edible 5 feedstocks, there was a big impact on the global food market and food security [27]. For instance, soy and rapeseed oil play a vital part in human food. Transforming these food crops to produce biodiesel on a large scale caused turbulence to the global food market [28], and consequently, the world encountered a “food versus fuel” crisis which no one had expected. Moreover, using the crops to produce biodiesel may incur competition with the edible oil market, which would increase the cost of both the edible oils and the biodiesel [29]. Producing biodiesel from edible food crops also has a negative impact on the environment because of the large areas of arable land required to cultivate enough of this type of feedstock. Therefore, serious ecological imbalances started to become apparent as countries began cutting down forests to make more land available for the cultivation of the feedstocks for biodiesel production. Thus, tropical countries such as Malaysia or Indonesia, which account for about 80% of the world’s palm oil supply, could face a serious deforestation problem. This could then have a large impact on the carbon balance because the additional CO2 from decomposing biomass and the reduced natural CO2 fixation by the forests as well as the long-term carbon storage in the soil would aggravate the situation of increasing global warming. Large scale deforestation has already been caused by the expansion of biodiesel production from food crops. Consequently, biodiesel produced from the first generation biodiesel feedstocks as a substitute biofuel for petroleum-based diesel fuel could cause enormous damage to the food market and also the environment around the world. 6 1.1.3.2. Second generation biodiesel feedstocks Alternative biodiesel feedstocks, such as non-food materials, have been developed to reduce the dependency on the food crops. The second generation biodiesel feedstocks include energy crops such as jatropha [25,30,31], tobacco seed [32], salmon oil [33], waste cooking oil, etc. Biodiesel production from these second generation biodiesel feedstocks has been widely investigated over the past several years. The following advantages are the main reasons why these feedstocks are popular: (a) the “food versus fuel” crisis has been eliminated. Non-edible feedstocks are not suitable for human consumption owing to the toxic substances in them [34]; (b) they are more eco-friendly and efficient than the first generation biodiesel feedstocks [35]; (c) they need less farmland to cultivate. Some of the non-edible feedstocks can be grown in wastelands that are not suitable for food crops [34]; (d) they can also produce some other useful by-products, which can be used in certain chemical processes or burned for power and heat, besides of the biodiesel; (e) animal fat methyl esters have some advantages compared to the first generation biodiesel feedstocks, such as a higher cetane number and non-corrosive qualities [36]. However, although the second generation biodiesel feedstocks do not compete with the human food sources and can be grown in wastelands, their production volume may not be large enough to fulfill the requirement of our total transportation fuels. Another disadvantage is that biodiesel derived from animal fats has relatively low performance in cold temperature. Animal fats usually contain a large number of saturated fatty acids, which makes the transesterification more difficult to proceed 7 [37]. For example, the saturated fatty acids, which account for about 50% of the total fatty acids in beef tallow, leads to a high melting point and high viscosity in the biodiesel [38]. In addition, using animal fats to produce biodiesel also presents a biosafety problem because they might be contaminated [39]. Accordingly, these second generation biodiesel feedstocks have not been used in biodiesel production at a significant scale. 1.1.3.3. Third generation biodiesel feedstocks The most important factor that interferes with the large scale commercial biodiesel production is the high cultivation cost of the feedstocks [17]. It had been shown that the first and second generation biodiesel feedstocks are not suitable for a sustainable energy economy [40]. Although an increasing amount of biodiesel has been produced from oil crops, its production in large quantities still cannot be considered as sustainability [41]. However, microalgae, as the third generation biodiesel feedstock, are a very promising alternative for biodiesel production because of their higher growth rates and productivity compared to the former biodiesel feedstocks [42]. Additionally, they are easier to cultivate than many other plants and can accumulate a higher yield of lipid for biodiesel production. As is shown in Table 1.2, compared to other biodiesel feedstocks, microalgae have the highest biomass productivity and oil content. Microalgae with high lipid content have the potential to produce up to 25 times more biodiesel per unit area than other biodiesel feedstocks, such as the palm. This very high production efficiency is 8 one reason that microalgae have been considered as a promising material for biodiesel production. The advantages of using microalgae as a source of biodiesel production are as follows: (a) reduction in cost and improved efficiencies. Compared to other biodiesel feedstocks such as non-food crops, the costs regarding to harvesting and transportation of microalgae are relatively low; (b) microalgae do not compete for land with food crops used for human food and other products [43], since they can be cultivated in places that are not suitable for growing other crops, such as brackish, salt water or non-arable lands [40]. As is shown in Table 1.2, microalgae require less land to grow compared to other feedstocks. They can also be grown in bioreactors [39]; (c) the typical oil content of microalgae is in the range of 20 to 50% by weight of dry biomass, but even higher productivity can be reached [44]; (d) microalgae can produce valuable co-products such as biopolymers, proteins and carbohydrates, etc. which could be used as fertilizer or animal food; (e) the photosynthetic efficiency of microalgae is higher than that of other green plants, which is considered a crucial advantage of algae to improve the biomass productivity [45]; (f) the lipid profiles in microalgae are mostly neutral lipids due to their high degree of saturation [46]. 9 Table 1.2. Comparison of microalgae with other biodiesel feedstocks. Feedstock Oil content (% oil by wt. in biomass) Oil yield (l oil/ha/year) Land use (m2 year/kg biodiesel) Corn 44 172 66 152 Soybean 18 636 31 321 Jatropha 28 741 15 656 Rapeseed 41 974 12 862 Sunflower 40 1070 11 946 Palm oil 36 5366 2 4747 Microalgae (low oil content) 30 58,700 0.2 51,927 Microalgae (medium oil content) 50 97,800 0.1 86,515 Microalgae (high oil content) 70 136,900 0.1 121,104 Biodiesel productivity (kg biodiesel/ha/year) Source: Ref.[44] 1.2. Biological characteristics of microalgae Microalgae are primitive plants, which lack roots, stems and leaves, and chlorophyll a acts as their primary photosynthetic pigment to absorb sunlight for their growth [47]. As prokaryotic cells, cyanobacteria lack membrane-bound organelles (plastids, nuclei and mitochondria) and are more similar to bacteria rather than algae. In contrast, eukaryotic cells, including many different microalgae species, have these organelles that regulate the normal functions of cells. Eukaryotic algae can be divided into a variety of classes mainly by their pigmentation, cell structure and life cycle 10 [48]. The following are the most important classes: red algae (Rhodophyta), green algae (Chlorophyta) and diatoms (Bacillatiophyta). Algae can be cultivated under either autotrophic or heterotrophic conditions. The autotrophic cultivation requires sunlight as the energy source, CO2 as the carbon source, and inorganic salts, while the heterotrophic one requires organic compounds as the carbon source and the energy source, as well as nutrients. In addition, some photosynthetic algae are mixotrophic, which means that they can be grown under either photosynthesis or exogenous organic substances [47]. 1.3. Process of microalgal biodiesel production The whole process from microalgae to biofuels is shown in Fig. 1.4. There are four main steps of this transformation process, namely cultivation, harvesting, extraction and conversion. Microalgae can be cultivated in either photobioreactor systems or open pond systems (e.g. raceway ponds). Then microalgae biomass can be harvested with either centrifugation or filtration with the assistance of flocculants. The harvested microalgae biomass will be extracted to obtain the desired lipid and then be converted to bio-oils with biochemical or thermochemical methods. 11 Fig. 1.4. A schematic of biofuels production from microalgae. At the end of the transformation route, there are the main products of each process [49]. 1.3.1. Technologies for microalgal cultivation Phototrophic microalgae absorb sunlight, and assimilate CO2 from the air and nutrients from the aquatic body to grow under natural environmental conditions. In contrast, in heterotrophic cultivation conditions, organic substances are utilized as carbon source (e.g. glucose) instead of CO2 for the growth of the microalgae. CO2 can be fixed by microalgae from three major different sources: directly from the atmosphere, from CO2-containing flue gases from industries such as the power plants, and from soluble carbonates [50]. Most microalgae can tolerate up to 12 150,000 ppmv levels of CO2 [51,52]. Therefore, in the microalgal biomass production systems, CO2 can be fed into the culture mediums either from external sources such as flue gases emitted from power plants or as soluble carbonates such as NaHCO3 and Na2CO3 [53,54]. Besides from sunlight and CO2, inorganic nutrients are required for microalgae production, primarily nitrogen, phosphorous and silicon [55]. Some microalgae can fix the nitrogen from the atmosphere [56,57], while most microalgae need it in a soluble form (e.g. urea) [58]. Phosphorous is another important nutrient, but its volume requirement is smaller than that for nitrogen during the production cycle [59]. However, because the phosphate ions can bond with metals ions, some excess of phosphorous must be added over the basic requirement [60]. Silicon plays a crucial part in the growth of certain microalgae such as diatoms [61]. There are altogether three different production mechanisms of microalgae, including the photoautotrophic production, heterotrophic production and mixotrophic production. They will be discussed in the following sections. 1.3.1.1. Photoautotrophic production Photoautotrophic production is the only way to make the large-scale microalgae biomass production technically and economically feasible for energy production [61]. In the following chapters, two photoautotrophic microalgae production systems are described, namely the open pond and closed photobioreactor technologies [62]. 13 1.3.1.1.1. Open pond production systems Open pond production systems have been used for microalgae cultivation since the 1950s [62]. The most commonly used system are the raceway ponds [63], which are made of a closed loop, oval shaped recirculation channel (Fig. 1.5) with a depth of 0.2-0.5m. In order to stabilize the microalgae growth and productivity, mixing and circulation are required during the cultivation. These ponds are usually built in concrete or from compacted earth lined with white plastic. The culture media and nutrients are added in front of a paddlewheel, which rotates continuously to prevent sedimentation during the whole production process. Submerged aerators are installed to enhance the CO2 concentration in the water [64]. Due to the potential threat from other algae species and algae-grazing protozoa, open pond systems require highly selective environments to make the microalgae well-cultivated [65]. Since the open ponds can be installed in marginal areas, there is no competition with agricultural crops [60] and the cleaning and regular maintenance are easier [66]. However, the biomass productivity of open pond systems is lower than that of closed photobioreactors [60], mainly because of the evaporation losses, temperature variation of the medium, inefficient mixing, less light and CO2 deficiencies [60,66,67]. 14 Fig. 1.5. View of a raceway pond. Culture medium is fed into the pond after the paddlewheel [60]. 1.3.1.1.2. Closed photobioreactor systems Recently, closed photobioreactors (PBRs), which constitute of an array of straight plastic or glass tubes as shown in Fig. 1.6 [66], have received major research attention. The tubes can be aligned horizontally [68], vertically [69], inclined [70], or as a helix [71], and the diameter of them is generally no more than 0.1m [60]. Microalgae cultures can be mixed and circulated either with a mechanical pump or with an airlift system, which allows the exchange of CO2 and O2 between the medium and aeration gas [72]. Most closed photobioreactor systems fall into one of the three categories: flat plate, tubular, and column photobioreactor systems. 15 Fig. 1.6. View of a horizontal tubular photobioreactor [73]. It constitutes of two parts, the airlift system and the solar receiver. The airlift system regulates the input of CO2 and output of O2 as well as the harvesting of the biomass. The solar receiver are responsible for the growth of the microalgae, and provide a high surface area to volume ratio. One of the earliest forms of closed PBRs systems is the flat-plate PBR [74], which has received much attention from researchers owing to the large surface area exposed to illumination [66] and high cells densities (> 80 g/l) observed [75]. Transparent materials are used for maximum sunlight absorption. The length of tubular PBRs is limited by the potential O2 accumulation, CO2 depletion, and pH change in the systems [72], which results in their finite scale-up. Large-scale production systems are generally based on the integration of many reactor units. Because the tubular PBRs systems can expose a larger surface area to 16 sunlight, they are generally considered to be more suitable for outdoor microalgae production. Column PBRs systems have the highest volumetric mass transfer rates, the best controllable cultivation conditions and the most efficient culture mixing [72]. The vertical reactors are aerated from the bottom, and illuminated across transparent walls [72], or internally [76]. Microalgae production with closed PBRs systems is designed to resolve some of the key issues related to the open pond production systems, such as the contamination by other algae species or protozoa and the low productivity in the open pond production systems. Owing to the higher biomass productivities obtained, harvesting costs can also be reduced significantly. Better process control and higher biomass productivity are the reasons that pilot-scale production of biodiesel and co-products is more frequently studied using closed PBRs rather than open ponds. However, the costs of closed PBRs systems are higher than open pond systems [77]. 1.3.1.1.3. Hybrid production systems The hybrid production systems combine the microalgae production in two distinct stages (photobioreactors and open ponds) together. The first stage is in the closed PBR system where the contamination from other organisms is reduced and the biomass productivity increased. In the second stage of production system, the microalgae are then exposed to the nutrient stresses conditions, aiming to accumulate and maximize the desired lipid content [78,79]. 17 1.3.1.2. Heterotrophic production Microalgae biomass and metabolites can also be successfully produced through heterotrophic cultivation of microalgae. In this process, microalgae are grown with organic carbon components as the carbon source, such as glucose, in stirred PBRs or fermenters. Due to the higher biomass productivity achieved, these systems provide a better growth control and also lower harvesting costs. Heterotrophic production systems consume more energy than the phototrophic production systems, because the whole process cycle includes the energy used for production of organic sources via the photosynthesis process [60]. 1.3.1.3. Mixotrophic production Many microalgae species can grow in either autotrophic or heterotrophic production systems, which we call mixotrophic production. They are capable of photosynthesis with sunlight and CO2 as well as heterotrophic cultivation with organic substances as the energy sources [80,81]. Therefore, sunlight is not an absolutely limiting factor for the cultivation of microalgae [82]. For example, the cyanobacteria Spirulina platensis, and the green alga Chlamydomonas reinhardtii have been cultured under mixotropic conditions [83]. 18 1.3.1.4. Impact factors of microalgal biomass production and biofuels productivity There are two major factors that affect the productivity of microalgal biomass production and biofuels production, the microalgae strains and their lipid content, which can be modified to improve the efficiency of the whole process. 1.3.1.4.1. Strain selection The selection of microalgae strains plays a crucial role for the success of microalgae based biodiesel production [84,85,86]. The ideal microalgae strain for biodiesel production should: (a) have high biomass and lipid productivity; (b) be robust enough to survive the shear stresses in a PBR; (c) dominate in the open pond production systems; (d) have high CO2 assimilation capability; (e) have limited nutrients requirements; (f) be tolerant to a wild range of temperatures owing to the diurnal cycle and seasonal variations; (g) produce valuable co-products; (h) have a fast growth cycle; (i) embody a self-flocculation ability. However, there is no known microalgae strain that can fulfill the above requirements concurrently. Genetic and metabolic engineering could be promising approaches to modify the microalgae strains for better performance of the biodiesel production [87]. Transgenic microalgae have increasingly attracted interest as they have the capabilities of producing both biofuels and valuable c-products, such as proteins and metabolites, however, this field has so far received little attention [88]. 19 1.3.1.4.2. Lipid productivity Microalgae strains usually have lipid contents ranging from 20% to 50% by dry weight. The concentration of lipid can be increased through optimizing certain key factors [89], such as the nitrogen content in the medium [90], temperature [90], salinity [90], CO2 concentration [52] and light intensity [90]. Microalgae with high lipid content that could also be cultivated in large-scale open ponds [91] have drawn the researchers’ extensive attention to conduct biodiesel production. It turns out that the most effective way to improve the lipid accumulation is nitrogen starvation, which not only results in increased lipid content within microalgal cells, but also in a gradual change of the lipid profile from free fatty acids to triacylglycerol (TAG) [92]. When nitrogen in the medium is completely consumed, cell proliferation is prevented, but cells still assimilate the carbon source, which could be subsequently converted to TAG to increase the lipid concentration within cells [91]. 1.3.2. Microalgal harvesting technologies It is essential to harvest the microalgae biomass with high efficiency in order to make the biodiesel production from microalgae economical. Currently, the primarily adopted technologies consist of centrifugation, flocculation, filtration and screening, sedimentation, flotation and electrophoresis [93]. Since the cell concentrations in the culture systems are generally low (in the range of 1g/L), the cost of harvesting microalgae can be very high [94]. 20 An appropriate harvesting approach can be developed according to some fundamental properties of microalgae, such as size and density [27]. The whole harvesting process can be divided into two steps: (a) Bulk harvesting. This step aims to separating the microalgal biomass from the bulk suspension. After concentration by flocculation, flotation, or gravity sedimentation, the total solid content can reach a level of 2-7% [27]. (b) Thickening. In this step, the microalgal slurry is further concentrated by filtration or centrifugation. This step requires higher energy consumption than the former step, and the final concentration is around 30% of dry weight of microalgae biomass [27]. 1.3.2.1. Centrifugation Centrifugation can recover most microalgae biomass from the culture systems, and it has been shown that about 80-90% microalgae biomass can be recovered within 2-5 min on pond effluent at 0.5-1.0 kg [95]. Centrifugation is a preferred method to harvest microalgae biomass, especially when one aims to produce extended shelf-like concentrates for aquaculture [96]. However, microalgal cells are exposed to high gravitational and shear forces which could damage the cell structure [97]. In addition, it is time-consuming and costly when a number of cultures are conducted with centrifugation [96]. 21 1.3.2.2. Flocculation Flocculation refers to a process that scattered particles are gathered together to form large particles for settling. In this process, colloids come out of suspension in the form of a floc or flake, either spontaneously or after adding chemical agents. Microalgal cells are negatively charged, thus they can adsorb ions originating from organic matter [93]. Microalgae can be harvested successfully by disrupting the stability of the system. 1.3.2.2.1. Autoflocculation An elevated pH in culture systems, carbonate ions will interact with certain microalgal cells, which will precipitate spontaneously. This process is called autoflocculation [98]. Previous studies have also shown that autoflocculation can be stimulated by adding NaOH to increase the pH value. 1.3.2.2.2. Chemical coagulation Flocculation can also be induced by adding certain chemicals to the microalgal culture system. Chemical coagulation is commonly used as a pre-treatment stage in many solid-liquid separation processes [99]. There are two major kinds of flocculants based on their chemical properties: (a) inorganic flocculants (such as iron-based or aluminum-based coagulants) and (b) organic polyelectrolytes (such as the chitosan or polyacrylamides). 22 1.3.2.2.3. Electrolytic process Electrocoagulation processes include three steps: (a) formation of coagulants by electrolytic oxidation of the sacrificial electrode; (b) breakage of the emulsion and destabilization of the particulate suspension; (c) flocculation formed by aggregation of the destabilized phases. The efficiency of the microalgal biomass flocculation is 80-95% when electrolytic flocculation is adopted in sweet water [100]. Electrolytic processes cannot be used in salt water because of the high conductivity of the medium. 1.3.2.3. Gravity sedimentation Gravity sedimentation is generally used for separating microalgae in waste-water treatment. Size and density of microalgae cells and the induced sedimentation velocity are factors that influence the settling time of the suspended solids [27]. However, because of their low density, most microalgal cells do not settle well and fail to separate successfully [101]. 1.3.2.4. Filtration and screening Filtration and screening refers to a process where the microalgal culture is passed through a screen with a particular pore size. There are two main screening devices that are commonly applied in microalgae harvesting, i.e., microstrainer and vibrating screen filters. Microstrainers are designed as rotating filters with fine mesh screens. They need frequent backwash. However, a high microalgal concentration 23 may block the screen, while a low microalgal concentration may cause inefficient capture [102]. Filters have the capability to recover relatively large microalgae; most filtration are performed under pressure or in a vacuum environment [96]. 1.3.2.5. Flotation Flotation refers to a process where air or gas bubbles attach to the microalgae biomass and the biomass is then carried to the liquid surface. Flotation can be more effective than sedimentation for the harvesting of microalgae [103]. There are three different applications based on bubble sizes utilized in the whole process, including dissolved air flotation, dispersed flotation and electrolytic flotation. 1.3.2.6. Electrophoresis techniques The electrophoresis is a potential method to harvest microalgae biomass without any addition of chemicals. The mechanism of this approach is that charged microalgae are driven out of the solution by an electric field, and then aggregate together [104]. There are several advantages of this method, including safety, cost effectiveness, environmental compatibility an energy efficiency [104]. However, it only works at low conductivity and is not applicable with algal cultures in salt water. 1.3.2.7. Comparison of the harvesting techniques In 1965, Golueke and Oswald compared the microalgae biomass harvesting efficiency using centrifugation, flotation, filtration, precipitation and ion exchange, 24 ultrasonic vibration and passage through a charged zone [105]. They concluded that centrifugation and chemical precipitation are the only two methods that could achieve economic feasibility. The optimal harvesting method of microalgae for biofuels production would be specific to species. In any case, it should require few or no chemicals and energy, and if possible, release the intracellular components for collection. If the cultivation of microalgae have a high overflow rate, flotation will have better harvesting efficiency than sedimentation since microalgae will move upward in flotation, while they will move downward in sedimentation [101]. Gravity sedimentation is better to be used for harvesting of large size microalgae, such as Spirulina. Furthermore, a flocculent can be added into the culture system in order to improve the sedimentation rate of microalgae. 1.3.3. Dehydration techniques The harvested microalgae biomass slurry (typical 5-15% dry solid content) must be processed quickly after harvesting due to their perishableness. According to the desired final products, dehydration is generally adopted to extend the viability. The dehydration techniques include sun drying [106], spray drying [107], drum drying [106], fluidized bed drying [108] and freeze drying. Sun drying is the cheapest dehydration methods among those mentioned above. However, this method has also some disadvantages, including long drying times, large drying areas required, and the risk of microalgae biomass loss [106]. Spry drying is commonly used for drying of microalgae biomass with high value products, 25 but it is relatively expensive and may induce significant deterioration of some microalgal pigments [107]. Freeze drying is even more expensive, especially for large scale dehydration, but it facilitates the subsequent lipid extraction from the microalgae cells. Lipids within cells are usually difficult to extract from the wet microalgae biomass with solvents without cell disruption, but are more easily extracted from freeze dried microalgae biomass [96,109]. 1.3.4. Lipid extraction Lipid extraction can be performed by physical and chemical methods, such as solvent extractions, or a combination of the two together. Methods used for lipid extraction should be effective, fast, easily scalable and should do no damage to the desired lipids [110]. Actually, not every lipid fraction is suitable for producing biodiesel. Moreover, some non-lipid components can be also extracted along with the lipid. Therefore, the extraction methods used should not only be lipid specific, but also be selective to desired lipid fractions [111]. As mentioned before, drying of the biomass is very energy-consuming. Thus, if the extraction can be performed for wet microalgae biomass, a lot of energy will be saved [112]. 1.3.4.1. Pre-treatment: cell disruption methods Pre-treatment of the microalgae biomass may be required before lipid extraction for certain types of biomass [110]. The purpose of this step is to break up the cells for 26 better extraction of the lipids within the cells. There are various cell disruption methods, including microwave, sonication, autoclaving, grinding, bead beating, homogenization, freeze drying, osmotic shock and 10% (w/v) NaCl addition [110,111]. Microwave was recently recognized as an efficient method to break up the microalgae cells, since it can generate high frequency waves, which can result in cells disruption via induction shock. Sonication can disrupt both the cell wall and membrane through the cavitation effect. The technique is successfully used for microbial cells. In bead-beading, mechanical disruption of cells can be achieved by high-speed spinning with fine beads [111]. After all, the efficiency of cell disruption for lipid extraction from microalgae biomass differs from species to species based on the extraction method utilized [111]. 1.3.4.2. Lipid extraction methods After the microalgae cells are disrupted, the intracellular components, including the desired lipids, can be easily extracted by established methods, primarily solvent extraction and supercritical carbon dioxide extraction. 1.3.4.2.1. Solvent extraction methods The lipids within the cell can be divided into polar lipids, which make up the cell membranes, and neutral lipids (triacylglycerides, TAG) for energy storage. To effectively extract the lipids, solvents or solvent mixtures with different polarity are 27 used. Non-polar organic solvents can be used to disrupt hydrophobic interactions between neutral/non-polar lipids, while polar solvents (e.g. alcohols) can disrupt hydrogen bonding between polar lipids. Strong ionic forces, if present, can be disrupted by increasing the pH towards more alkaline. Therefore, when choosing extraction solvents, the microalgae species is a key factor to consider. Moreover, solvents should be non-toxic, inexpensive, sufficiently volatile, and poor extractors for other non-lipid components within cells [110]. Soxhlet extraction and Bligh and Dyer’s method are the two classical approaches used for lipids extraction from microalgae biomass. Hexane is used in the Soxhlet method, while mixtures of chloroform and methanol are used in Bligh and Dyer’s method as solvents to extract lipids within cells [113]. Other solvents, such as benzene and ether, have also been used in the Soxhlet extraction, but hexane has earned more popularity as the extraction solvent and it is relatively inexpensive. Additionally, ionic liquids have also been studied successfully for lipid extraction in recent years. 1.3.4.2.1.1. Soxhelt extraction method The Soxhelt extraction can be carried out with hexane alone, or together with the oil press/expeller method. Lipids from the remaining pulp can also be extracted by mixing it with cyclo-hexane after the lipid extraction with expeller. The cyclo-hexane can dissolve lipids, and then the pulp is filtered out. After that, the cyclo-hexane can be separated via distillation. The extraction efficiency can achieve over 95% of the 28 total lipids content of the biomass when the two methods are combined together. However, using solvent extraction may lead to some potential dangers because of the chemicals used. Although hexane has been found to be less efficient than chloroform, it is less toxic, has higher selectivity for neutral lipids within cells and has lower affinity towards non-lipid components [112]. 1.3.4.2.1.2. Bligh and Dyer’s method Bligh and Dyer’s method was found to have the highest extraction efficiency (more than 95% of the total lipids) by Lam and Lee [114]. The advantage of this method is that it can be used for tissue containing over 80% water [115]. The critical ratio of methanol, chloroform and water is 2:1:1.8, while that of solvents to microalgae biomass is 3:1. After solvents and microalgae biomass are mixed according to the above given ratio, they are homogenized to form a monophasic system. After adding another similar quantity of chloroform, they are re-homogenized to form a biphasic system (lipid dissolved in chloroform and methanol dissolved in water). The overall ratio of methanol, chloroform and water will be 2:2:1.8 and that of solvent to microalgae biomass will be [(3+1):1] [115]. The biphasis layer can be separated by centrifuge, thus the lipids can be extracted from the microalgae cells [115]. 29 1.3.4.2.1.3. Ionic liquids Ionic liquids (ILs) are salts which consist of relatively large asymmetric organic cations and smaller organic or inorganic anions. Cations usually comprise of a nitrogen-containing ring structure (e.g. pyrimidine or imidazole) with a variety of functional side groups, which regulate the ILs’ polarity. As to the anions, they are vary from single ions (e.g. Cl-) to larger complex molecular ions like [N(SO2CF3)2][116]. ILs are recognized as an attractive alternative to volatile organic solvents, as they are essentially non-volatile and thermally stable, which make them known as green solvents [113]. In addition, they have relatively low toxicity, no vapor pressure and the capacity to be tailored for a specific polarity, electrical conductivity and solubility [116]. The efficiency of lipid extraction is highly dependent on the anion structure of ILs. ILs with hydrophobic nature like [Bmin][PF6] usually have a low extraction efficiency, whereas the hydrophilic ILs like [Bmin][CF3SO3] show a high extraction efficiency. The reason can be partially because of the solubility of lipids in ILs. Hydrophobic ILs with higher solubility can result in the separation of lipids to the methanol and IL mixture phase [113]. 1.3.4.2.2. Supercritical carbon dioxide (SC-CO2) extraction Supercritical carbon dioxide extraction is currently one of the promising green technologies to substitute the traditional lipid extraction with organic solvents. The 30 whole system comprises a heated micro-metering valve to depressurize the injected SC-CO2 and a feed pump used for compression and transportation of liquid CO2 to the extraction vessel inside an oven module. The compressed CO2 enters the oven when it’s heated to a supercritical state (above 35 oC) and the lipids will be extracted from the microalgae. After the CO2 is decompressed, it evaporates as gas to the ambient, and the extracted lipids will be forced to precipitate out to the adjoining glass vial [112]. SC-CO2 has high solvating power and low toxicity, however, the high installation cost and the operation cost are the main hurdles to prevent its utilization in large scale [112]. 1.3.5. Biodiesel production After the lipids are successfully extracted from the microalgae biomass, the next step is the conversion of lipid to biodiesel. The common methods used for biodiesel production from microalgae consist of transesterification, either in a separate process step or in-situ. 1.3.5.1. Transesterification Transesterification is the most commonly used method to convert lipid to biodiesel [117]. The biodiesel produced in this process are called fatty acid (m)ethyl esters (FAME or FAEE); their physical characteristics are very close to those of the petro diesel fuel. 31 The raw viscous lipids extracted from microalgae are converted to lower molecular weight fatty acid alkyl esters via transesterification [110]. The alkoxyl group of an ester compound is displaced by an ester (interesterification), alcohol (alcoholysis) or carboxylic acids (acidolysis). However, only interesterification and alcoholysis have earned importance of producing biodiesel [117]. Therefore, the reaction takes place between a short chain alcohol and the parent oil (triglyceride) with catalyst involved. Fatty acid methyl esters (FAME) and glycerol are the two products of the reaction [110]. Ethanol is less toxic and can be produced from biomass via fermentation, thus it is more renewable. However, methanol is cheaper, more reactive and produces more volatile FAMEs, which make it superior to ethanol [117]. With a suitable catalyst (acidic, basic or enzymatic), the reaction rate can be improved [110,118]. Fig. 1.7 shows the transesterificatiaon of triacylglycerols and alcohol to yield esters and glycerol in the presence of catalysts. The following are catalysts can be used for transesterification: (a) acid catalyst (sulfuric acid, sulfonic acid, phosphoric acid and hydrochloric) [117]; (b) alkaline catalyst (sodium hydroxide, potassium hydroxide and sodium methoxide); (3) enzymatic catalyst (lipase). 32 Fig. 1.7. Transesterification of triacylglycerols and alcohol in the presence of a catalyst to yield esters and glycerol [110]. 1.3.5.1.1. Acid catalysis Acid catalysts are the preferred options during transesterification. In an experiment carried out under the same condition except for the catalysts used, the yield of FAME obtained from base catalysis (sodium hydroxide) was only 1/3 of that obtained from acid catalysis (0.6N hydrochloric acid-methanol catalyst) [117]. The transesterification process catalyzed by chemicals require a large amount of energy and the separation of catalysts from the products [110]. 1.3.5.1.2. Base catalysis In-situ preparation of alkoxides from metallic sodium or potassium may cause problems during handling. Thus, metal alkoxides (e.g. sodium methoxide) are used. They are superior to metal hydroxides (e.g. NaOH). Alkaline metal alkoxides are highly active catalysts even in very small concentration of 0.5 mol%. Typically, a yield of about 98% is achieved within a short reaction time of about 30 min. However, the performance is better without water involved, which makes them inappropriate for industrial scale [119]. 33 Water in the presence of free fatty acids will result in product loss due to saponification. Even the recovery of glycerol is relatively difficult [110]. 1.3.5.1.3. Enzymatic transesterification Enzymatic technology has already been conducted on the industrial scale. These enzymatic catalysts consist of two types: extracellular lipase and intracellular lipase. They should be immobilized before use to eliminate downstream operations of separation and enzyme recovery [117]. Since methanol may deactivate the lipase, solvents like t-butanol have been proposed as suitable replacement for methanol for the enzymatic alcoholysis on an industrial scale. Lipase can be recycled without any loss of activity during the reaction with t-butanol. In addition, a stepwise addition of methanol could be another suitable way in the reaction [117]. However, glycerol produced during the process readily adheres to the surface of the lipase and deactivates it. Further removal of glycerol is so difficult and may prevent the feasibility of larger scale biodiesel production [110]. Moreover, large accumulation of glycerol may also inhibit the enzyme. The possible strategies for removing glycerol are in-situ removal by dialysis, or by extraction with isopropanol. Again, t-butanol can be a better choice to displace the methanol in the transesterification process since it can dissolve the glycerol [117]. 34 1.3.5.2. In-situ or direct transesterification In-situ transesterification is a one-step method that combines the processes of extraction and transesterification [120]. It reduces not only the number of operation units but also the biodiesel cost by cutting down the whole process cost [121]. Furthermore, the reaction time is reduced compared to the conventional two steps processes [122]. The dried microalgae has to be crushed into small solid particles in order to prevent soap formation during the direct transesterification. Methanol is used as the extractant and the reactant. During these two simultaneous processes extraction and transesterification, solvents with different polarities are required. Thus, methanol needs to be mixed with a non-polar solvent in an appropriate ratio. Previous experiments shown that mixture of methanol and methylene dichloride (v/v = 3:1) can enhance the extraction efficiency [121]. But methylene dichloride is very volatile and will lead to problems with VOC emissions. Also, the chlorinated hydrocarbons are all suspected carcinogens, and their use is therefore regulated. Results have shown that in-situ transesterification gave a higher FAME yield than the conventional two-step method. Moreover, it also contributed to the reduction of overall heat requirement and the cost of biodiesel production [121]. However, direct transesterification generally used homogeneous acid or alkali as catalyst, which inevitably causes environmental issues and complicates the product purification. In order to resolve the above problems, in-situ transesterification was 35 carried out with heterogeneous solid base catalyst (Mg-Zr solid base catalyst), which reduces the emission of the waste liquid and simplifis the product purification [121]. 1.3.6. Other techniques of producing energy from algae Techniques used for converting microalgae biomass to biofuels basically contain two types, namely thermo-chemical and biochemical conversion. Thermo-chemical conversion refers to the thermal decomposition of organic biomass to biofuels, and includes direct combustion, pyrolysis, thermo-chemical liquefaction and gasification [123]. Biochemical conversion technologies include alcoholic fermentation, anaerobic digestion and photo-biological hydrogen production [124]. 1.3.6.1. Thermo-chemical conversion Besides the biodiesel from the lipid fraction of the biomass, some other biofuels can be also produced from microalgae biomass via thermo-chemical conversion technologies [110]. Thermo-chemical conversion technologies mainly include gasification, liquefaction and pyrolysis [125]. 1.3.6.1.1. Gasification Gasification, also known as hydrothermal process [126], refers to a process of partial oxidation of microalgae biomass at high temperature (around 800-1000oC); the product is a syngas, a mixture of combustible gases, produced by the reaction of microalgae biomass, oxygen and steam. The syngas contains gases like methane, 36 carbon dioxide, nitrogen, and hydrogen in varying proportions. This syngas can be used as an energy source for heating, or as a fuel to run gas turbines or diesel engines [110]. During the gasification process, water is heated above its critical temperature and pressure. Under these conditions, certain physical properties of water like dielectric constant, viscosity and thermal conductivity decrease enormously. Water at high temperature is a favorable solvent and dissolves the organic compounds completely. Gasification is environmental friendly, and it does not require complete drying of the microalgae biomass, thus a lot of energy can be saved [126]. 1.3.6.1.2. Thermo-chemical liquefaction Thermo-chemical liquefaction is a process that can convert the microalgae biomass to liquid fuel at temperatures between 200-350 oC with a catalyst involved [127]. Microalgae biomass can be broken into small and reactive molecules at sub-critical condition of water, and then re-polymerizes to form a broad range of products. The catalysts needed can be alkali salts, such as potassium and sodium carbonate. The major advantage of liquefaction is that biofuels can be produced from wet microalgae biomass [110], which reduces the energy required for the dehydration of the microalgae biomass [125,128]. It was found that liquefaction was more effective in biofuels production from microalgae, compared to the supercritical carbon dioxide method [125]. However, the cost of the required equipment is very high [110]. 37 1.3.6.1.3. Pyrolysis Pyrolysis is a process that converts the microalgae into biofuels by high temperature in the absence of oxygen. This process produces less waste, thus is more environmental friendly. Microalgae biomass can be decomposed into methanol, acetic acid, acetone, charcoal, condensable organic liquids and non-condensable gaseous products [110]. Many studies showed that biofuels produced by fast pyrolysis are about 2-3 times cheaper than by the gasification process. However, the quality of the products is usually low, so that they cannot be used directly in conventional gasoline and diesel fuel engines. In order to make the product more suitable for the current gasoline and diesel fuel engines, it has to be deoxygenated. Some of the conversion methods include hydrotreatment, aqueous-phase processing and conversion over zeolite catalysts [129]. 1.3.6.2. Biochemical conversion 1.3.6.2.1. Anaerobic digestion Anaerobic digestion of whole microalgae or the residue after lipid extraction can produce biogas, a mixture of methane and carbon dioxide, without air involved [125]. This conversion process does not only convert the microalgae residues into energy-rich fuel molecules (i.e, methane), but also recycles the nitrogen and phosphorous as nutrient sources for further microalgae cultivation. It has been found 38 that in many cases, the energy content in the methane produced through this process is higher than that in the fuel produced from the lipids [130]. If methane is produced from the microalgae biomass residue after extraction of the lipids, the energy yield is increased over that from the biodiesel production. Furthermore, methane production via anaerobic digestion from microalgae biomass does not require drying of the biomass, and thus can decrease the overall production cost enormously by reducing the harvesting and drying cost. For microalgae with lipid content less than 40%, the energy input for recovering lipids is probably higher than the additional energy recovered in form of biodiesel [131]. Also, removal of the lipid fraction leads to an increased N/C ratio in the residue. However, high nitrogen content will inhibit the microbial digestion to methanol. Therefore, if the lipid content of the microalgae is lower than 40%, anaerobic digestion of the entire biomass will be a better choice regarding to the energy recovery and energy balance of the microalgae [130]. 1.3.6.2.2. Fermentation Fermentation, which has been commercially used on a large scale worldwide, refers to a process that produces ethanol from sugar and starch stored in biomass. For instance, corn, with a starch content at about 60-70%, has become the dominant feedstock for bioethanol production in many countries [128]. Similarly, microalgae can also be used to produce bio-ethanol. With the assistance of enzymes, the 39 microalgal starch, which includes many poly-saccharides with different sugars can be first converted to sugar, and then further to bio-ethanol by yeast. 1.3.6.2.3. Photo-biological hydrogen production Cyanobacteria and green algae can be used to produce bio-hydrogen via a process that can be described as “bio-photolysis of water”. There are mainly three ways to produce hydrogen, including direct photolysis, indirect photolysis and ATP-driven hydrogen-production. The hydrogen and oxygen produced in direct photolysis are continuously flushed out. The processes of photosynthesis and water splitting take place simultaneously and then hydrogen and oxygen are produced, which could be a major safety issue, and extra cost of separating hydrogen and oxygen will inevitably be caused. Moreover, the cost of the photobioreactor and hydrogen storage system will be another big problem [125]. 1.3.7. Other applications of microalgae extracts There are still many opportunities to exploit the full commercial potential for microalgae. In the early 1960s, commercial large-scale cultivation of microalgae for food additives was begun in Japan, and later in the 1970s and 1980s, the microalgae production expanded to other countries, such as America, India, Israel and Australia [62,65,132]. The productivity of the whole microalgae industry achieved 7,000 tonnes of dry biomass per annum in 2004 [133]. 40 1.3.7.1. Human nutrition Owing to the strict food safety regulations [133], market demand, specific preparation and commercial factors, microalgae biomass used for human nutrition is currently limited to very few species, including Chlorella, Spirulina and Dunaliella. Most microalgae biomass is used as food additives in the health food market, which is recognized as a stable market [132]. 1.3.7.2. Animal feed and aquaculture Microalgae species such as Chlorella, Scenedesmus and Spirulina are suitable for production of animal feed and aquaculture. There are many advantages claimed for the application of algae-based feed, including improved fertility, better weight control and improved immune response [133]. However, it could be detrimental if the feeding time is prolonged at high concentration [132], especially for cyanobacteria. It is well known that microalgae are the natural food source of lots of aquaculture species, such as molluscs, shrimps and fish [132]. Microalgae biomass is therefore mainly used for fish feed [134], primarily for ornamental fish, where it can enhance the immune system of the fish [133]. 1.3.7.3. Biofertiliser In some conversion technologies, specially in pyrolysis processes, a solid charcoal residue or “biochar” is one of the products, which can be used as a biofertiliser and for carbon sequestration besides from the biofuel commonly used 41 [135]. As for the carbon sequestration, a long-term sink is considered that the emission of carbon dioxide could be reduced up to 84% [136]. Thus Lehmann et al. [136] suggested that biochar could be a potential carbon-negative biofuel. However, the net reduction value of GHG emission is still uncertainty in the process of biochar as a biofertiliser [137]. 1.3.7.4. Source of poly-unsaturated fatty acids Polyunsaturated fatty acids (PUFAs) play a crucial role in human development and physiology [138]. It has already been proven that PUFAs are capable to reduce the risk of cardiovascular diseases [139]. Since higher plants and animals do not have the enzymes required to synthesize PUFAs [133], microalgae, as a major source of PUFAs, can provide these important components for them and humans along the whole food chains. In addition, PUFAs produced from microalgae can serve as additives for infant milk. Chicken can be fed with them for the production of omega-3 containing eggs [133]. However, decosahexaenoic acid (DHA) is currently the only microalgae PUFAs that is already commercially available, since the other PUFAs, such as eicosapentaenoic acid (EPA) and arachidonic acid (AA) made from microalgae are still not competitive against other sources [132]. 42 Chapter 2 : Model construction 2.1. System description In this chapter, we present an economic assessment for a facility that grows algae and transforms the algal biomass into transportation fuel. The assessment covers economic aspects as well as environmental impact assessment and an analysis of the carbon foot print. The whole system takes into account all the processes from microalgae cultivation to biodiesel production. A relatively small facility with a total area of 100 ha is modeled to perform the system analysis for an industrial scale microalgal biodiesel production. The facility of the model is presumed to be located in Shenzhen (South of China), which is a sunny region with adequate solar radiation. It is also assumed that the plant is near to a power plant which can provide the required CO2 via the flue gases, and also adjacent to the ocean with access to sea-water. These prerequisites will restrict the number of sites available for such installations. The cost for constructing the infrastructures is included in the economic estimations but is not considered for the energy balance and GHG emission rate. The Grobbelaar formula [140], CO0.48H1.83N0.11P0.01, is commonly considered as a general formula for microalgae, and is thus used for mass balance calculations for nutrients. 2.2. Model description The model can be divided into five main sections, including cultivation, harvesting & dewatering, lipid extraction, lipid conversion, and co-products production. The chosen process configuration is shown in Fig. 2.1. The selected 43 technology of each process may be not recognized as the most or best optimized process, but is one of the most feasible options currently used in industrial scale today. Fig. 2.1. Schematic of microalgal biofuels production process [141]. 2.2.1. Cultivation A hybrid growth system is modeled in this stage. In order to speed up the whole cultivation process and to minimize culture crashes, microalgae are initially cultivated in PBRs. These cultures can be utilized as a dense inoculum for subsequent cultivation in open raceway ponds. The cultivation in PBRs is continuous and contamination-free for biomass production, while that in open raceway pond is more suitable for efficient and large-scale production of lipid-enhanced microalgae. Based on currently achievable data reported in previous literatures [27,96,142,143,144,145], the microalgal productivity (in terms of dry cell weight) and lipid content in open raceway pond were assumed to be 25 g/m2/d and 58%, respectively. 44 2.2.1.1. PBRs and open ponds The production facility was modeled to have three sub-areas based on the 100 ha we mentioned above, including 10 ha for 400 horizontal tubular PBRs, 70 ha for 80 open raceway ponds, and the remaining 20 ha for other processing and access purpose. The design specifications of the PBRs and open raceway pond are shown in Table 2.1. The growth cycle of the hybrid cultivation system consists of 1 day of growth in 20 PBRs and 4 days of growth in an open raceway pond. The PBRs serve for biomass growth and the raceway ponds for lipid accumulation. Table 2.1. Design specification for PBRs and open pond. PBR Open Raceway Pond Area 250 m2 8,758m2 Unit configuration Length: 500 m Pond length: 350 m Diameter: 0.4 m Pond width: 25 m Culture volume: 50 m3 Pond depth: 0.2 m Culture volume: 1500 m3 The cultivation of the microalgae in the PBRs is quasi-continuous. After one-day growth, half of the volume of 20 PBRs was transferred into one open raceway pond via gravity-driven flow. After the transfer, the PBRs were refilled with sterilized sea-water, and the culture is allowed to grow to the original cell density. A batch mode with a time period of 4 days was performed for microalgae cultivation in open 45 raceway ponds. On the morning of day 1, an open pond was incubated with half the content from 20 PBRs, which amounts to 500 m3 in volume. Another 1000 m3 of seawater is required to fill up the pond. On day 1 and 2, nitrogen and phosphorous were supplied to provide optimal growth conditions, while in the last 2 days, the microalgae were cultured with no nutrients added to the ponds. Since the growth cycle of microalgae in the open ponds was 4 days, one PBR was responsible for four ponds every four days. Thus, 20 PBRs were devoted to provide culture for 4 ponds, which leaded to a PBR to pond ratio of 5. Assuming the whole system operates 360 days/year, 90 harvests per pond per year can be achieved, resulting in altogether 7200 open-raceway-pond harvests per year. 2.2.1.2. CO2 and nutrients CO2 was supplied in the form of flue gases produced by a nearby power plant as stated in the assumptions. It was treated as a “free” input with no extra energy required since it would otherwise be emitted to the air. In order to provide sufficient CO2 to the cultivation system, the flue gases should be compressed from 1 to 2 bar absolute. 100% assimilation of the nutrients N and P was assumed as microalgae were cultivated under nutrient depletion during the last two days in open ponds. 2.2.1.3. Water The sea water was used to refill all 400 PBRs, fill 20 open ponds, refill the water loss through evaporation, and clean the 20 ponds after harvesting. The evaporation 46 loss rate was set at 10% of pond volume per day and the water needed for cleaning up the harvested ponds was estimated to be 20% of pond volume. Table 2.2 shows the daily seawater consumption for the cultivation process. Table 2.2. Daily seawater consumption for the cultivation process. Usage Amount (m3) 400 PBRs refill 10,000 20 ponds fill 20,000 Compensation for evaporation loss 12,000 20 ponds clean up 6,000 Total daily need 48,000 2.2.2. Harvesting and dewatering 2.2.2.1. Harvesting Auto-flocculation was carried out to harvest the microalgae in the first step. About 1 hour’s settlement of the biomass was assumed without any flocculants involved (Mark Huntley, personal communication, October, 2011). Then the supernatant was removed out of the pond and transferred back into the ocean via gravity after the biomass settled. The settled algal slurry was assumed to have a biomass solids content of 2% total solids (TS). 47 2.2.2.2. Dewatering After the auto-flocculation, the microalgal slurry was pumped from the open ponds to the dewatering site. A centrifuge was used to dewater the biomass to a solids content of 20% TS. Thermal drying of microalgal biomass to 90% TS is required when the lipid extraction will be conducted with dry algae. A bed drying was assumed in this stage. 2.2.3. Lipid extraction A N-hexane extraction method was modeled to extract the lipids within microalgae cells in this process. Lipids within the biomass dissolve in the hexane, while the carbohydrates, protein and ash were left in the residues. 2.2.4. Lipid conversion After the lipids are successfully extracted from the microalgal cells, the next step is to convert them to biodiesel. Thus in the lipid conversion stage, transesterification is modeled to conduct the conversion of lipids. 2.2.5. Co-products production After the lipid extraction, there are still some other substances left in the residues, which should be utilized as much as possible to make the whole biofuels production process economically feasibility. Thus, the aim of the anaerobic digestion process is to recover energy, nutrients and carbon left in the microalgae residues after extraction. 48 Chapter 3 : Model analysis 3.1. Mass Balances According to the given parameters of microalgae cultivation, the productivity of microalgae can achieve about 6,305.76 metric tonnes (MT) per year. The amount of N and P that have to be recycled within the system are 416.18 and 81.97 MT per year. Thus, the amount of biodiesel will be about 3,657.34 MT per year. During the centrifugation process, the water removed is about 283,759.2 MT per year, whereas the water evaporated in the bed drying process is about 24,522.4 MT per year. 3.2. Net energy ratio NER refers to the ratio between the “Energy produced” and the “Primary energy input”. “Energy produced” includes both the energy contained in the diesel produced from transesterification and gas made from anaerobic digestion of the microalgal residues. “Primary energy input” contains the electricity and natural gas consumed by the whole process of biofuels production. 3.2.1. Energy estimations for each step Values for the energy consumption of cultivation given in the literature vary widely. Most of the values are predictions based on various models and assumptions, rather than values from actual production. The values are calculated for a common basis, namely 1 kg of dry weight biomass (DWB). For the culturing step, Lundquist et al. reported a value of 0.37 kWh/kg DWB[146], which is within the range of energy 49 consumption calculated by Lardon et al. [147]: [0.35-0.42] kWh/kg of DWB and Sturm et al. [148]: 0.27 kWh/kg of DWB. In contrast, Stephenson et al. [149] and Jorquera et al. [150] calculated a somewhat higher energy consumption of 0.76 and 1.05 kWh/kg of DWB, respectively. Based on these findings, the energy consumption of cultivation is assumed as [0.27-1.05] kWh/kg of DWB. In the harvest and dewatering process, the energy consumption of centrifugation was assumed at a range of [4-12] kWh/ ton of water evaporated (TWE) based on Molina-Grima et al.’s study [96], 8 kWh/TWE, Sturm et al.’s study [148], 0.42 kWh/kg of DWB, and Stephenson et al.’s study [149], 0.56 kWh/kg of biodiesel. For the bed drying, the energy demand is assumed at a range of [0.07-0.2] kWh/TWE according to the estimated value of 0.13 kWh/TWE from New York State Energy Research and Development Authority. The energy consumption of hexane extraction is assumed based on the values found in previous literatures: 0.19 kWh/kg of biodiesel for Stephenson et al. [149], 0.40 kWh/kg of DWB for Lardon et al. [147] and 0.57 kWh/kg of DWB for Batan et al. [151]. Thus the range of energy demand for hexane extraction is estimated at [0.32-0.57] kWh/kg of DWB. The amount of solvents needed during the extraction is 10 kg/TDWB estimated by the method from Chauvel et al.’s study [152]. The energy consumption for transesterification is assumed at a range of [0.25-0.83] kWh/kg of DWB, according to previous studies: 0.25 kWh/kg of DWB by Lardon et al. [147], 0.50 kWh/kg of biodiesel b Stephenson et al. [149] and 0.61 kWh/kg of DWB for Batan et al. [151]. The requirements for the chemicals: methanol, 50 NaOH and H3PO4, are 100 kg/ ton of convertible lipids (TCL), 8.6 kg/TCL and 7.0 kg/TCL estimated by the method from Chauvel et al.’s study [152]. The range of energy demand for anaerobic digestion can be estimated from the study of Couturier et al. [153], who reported a range of [0.15-0.5] kWh/kg of algal residue. The biogas yield in this process is estimated at a range of [0.87-4.35] kWh/kg of algal residue based on Sialve et al.’s study [130]. Table 3.1 shows the model parameters for energy consumed and produced. Table 3.2 shows the fixed parameters for energy consumption calculations. 51 Table 3.1. Model parameters for energy consumed and produced. Parameter Unit Min. Max. 0.27 1.05 kWh/TWE 4 12 (Consumed) 0.07 0.2 0.32 0.57 0.25 0.83 0.15 0.5 0.87 4.35 kWh/kg of DWB Raceway & PBRs Cultivation (Consumed) Harvesting & Centrifuge Drying Bed drying kWh/kg of DWB Lipid extraction Hexane extraction (Consumed) kWh/kg of DWB Lipid conversion Transesterification (Consumed) kWh/kg of algal residue (Consumed) Co-products Anaerobic digestion production kWh/kg of algal residue (Biogas produced) Table 3.2. Fixed parameters for energy consumption calculations. Parameter Unit Value Nitrogen nutrient kWh/kg of N nutrient 11.8 [154] Phosphorus nutrient kWh/kg of P nutrient 4.1 [154] Hexane kWh/kg 6.25 [155] Methanol kWh/kg 9.14 [155] NaOH kWh/kg 2.53 [155] H3PO4 kWh/kg 0.15 [156] 52 References 3.2.2. NER calculation Based on Table 3.1 and Table 3.2, the energy consumed during the whole process is at the range of [15.9−29.2] million kWh/year, while that of the energy produced is [40.1−49.4] million kWh/year. Therefore, the NER can be calculated ranging from 1.37 to 3.11, which means that for every 1 kWh energy consumed along the whole process, 1.37−3.11 kWh energy can be produced finally. If we only consider the feasibility of biodiesel production in an NER way, this result indicates that it would be advantageous for us to conduct this project. 3.3. Production cost estimation The method and values of the parameters shown in Table 3.3 were used for the estimation of the whole biodiesel production process. The operating cost consists of the utilities, labor and the other operating costs at 0.9% of the capital cost, while the other costs include general maintenance cost, as well as taxes and insurance at 6% of the capital cost per year. The plant life is assumed to be 20 years with a discount rate of 8%. 53 Table 3.3. Method and values of the parameters used for the estimation of biodiesel production costs [152]. Parameter Method of calculations Capital cost Estimated from previous literatures Utilities cost Prices of electricity, nutrients, solvents, chemicals etc. Operating cost 𝐶𝑎𝑝𝑖𝑡𝑎𝑙 𝑐𝑜𝑠𝑡 0.2 ) 10 6 ×500 Labor cost Labor cost = 106 ( Other operating cost 0.9% of the capital cost Maintenance cost 4% of the capital cost Taxes and insurances 2% of the capital cost per year Plant life 20 years Discount rate 8% per year Other costs 3.3.1. Capital cost The capital costs of the PBRs and raceway, including the land, infrastructure and CO2 delivery system costs) found in the literature are shown in Table 3.4. The cost range for PBRs and raceway pond can be set as [1.2-2.2] and [0.145-0.863] million ￥/ha, respectively, based on these data. 54 Table 3.4. Raceway and PBRs capital cost found in literatures. Reactor Cost (￥/ha) References 615,454 [146] 862,798 [157] 330,512 [158] 374,319 [159] 535,117 [160] 145,134 [161] 2,180,008 [158] 1,245,907 [162] Raceway pond PBR The capital cost of the centrifuges is assumed based on a study by Camp Dresser Mckee, an engineering consultancy with many years’ experience in environmental engineering [163], at the range of [4-12] ￥/ton of water removed (TWE)/year. While for the bed drying, based on study of Lundquist et al. and data from EPA [164], the range of the capital cost is estimated at [30-80] ￥/TWE/year. The capital cost of lipid extraction is estimated according to Chauvel’s study [152], which is around 698￥/ton of dry weight biomass (TDWB)/year. Thus the range of capital cost related to the hexane extraction is assumed at [350-1050] ￥/TWDB/year. For the cost of transesterification equipment, 2860￥/ TCL/year is estimated by Chauvel [152]. Thus, the range of the capital cost of the transesterification process is assumed at [1430-4290] ￥/TCL/year. 55 The cost estimated for the anaerobic digestion plant vary widely in the literature: from 546￥/tons of residue/year given by Davis et al. [158] to 3078￥/ tons of residue /year reported by Couturier et al. [153] and between 1564 and 3137￥/ tons of residue /year from Perron et al. [165]. A range of [546-1598] ￥/ ton of residue/year seems appropriate based on the findings mentioned above, since low cost agricultural engineering methods are adopted [146]. The parameter values for capital cost calculation are presented in Table 3.5. Table 3.5. Parameter values for capital cost calculations. Parameter Unit Raceway Max. 145,000 863,000 1,200,000 2,200,000 4 12 30 80 ￥/ha Cultivation PBR Harvesting Min. & Centrifuge ￥/TWE/year Drying Bed drying Lipid extraction Hexane extraction ￥/TDWB/year 350 1050 Lipid conversion Transesterification ￥/TCL/year 1430 4290 546 1598 ￥/tons of Co-products Anaerobic digestion residue/year production 56 3.3.2. Operating cost According to the data from National Agricultural Statistics Service 2011, prices of nitrogen and phosphorus nutrients are set as 6.14￥/kg of N and 8.6￥/kg of P, respectively. Price of electricity is fixed at 0.64￥/kWh (Shenzhen Power 2011). The cost of the hexane used in the lipid extraction is 1218￥/ ton (International Construction Information Society, ICIS), which can be converted into 12.18 ￥/TDWB in terms of 10 kg/TDWB hexane needed. The range of the hexane cost can be thus set at [6-18]￥/TDWB. Prices of methanol, NaOH and H3PO4 are 2.6￥/kg, 3 ￥/kg and 2.7￥/kg (ICIS). Then the total average chemicals cost can be estimated at 298￥/TCL when considering the chemical needs during this process. Thus the range of the chemicals cost is [150-450]￥/TCL. In addition, the maintenance cost and the taxes & insurances are assumed at 4% and 2% of the capital cost per year, respectively. 3.3.3. Production cost calculation According to the calculation methods mentioned above, the capital cost of the whole facility is at the range of [300.53−800.83] million ￥ in 20 year’s operation. The operation and other costs can also be estimated at the range of [15,554.54−27,106.5] million ￥/year. Considering the plant life of the whole facility, if we want to achieve a 10% rate of return, the required biodiesel selling price was estimated to be at a range of [9,198−20,196] ￥/ton (7.73−16.96￥/l). Given the current selling price of traditional 57 diesel is 7.24￥/l, these results, which do not yet include the cost for delivery and transportation, indicate that the process will not be competitive with traditional diesel in terms of the cost, if a large industrial scale facility were to operate at present. 3.4. Greenhouse gas emission rate All the GHG emissions of combustion of biodiesel and methane produced in the whole process are calculated for the GHG emission rate. The parameters used in the calculation of GHG emission rate are shown in Table 3.6. GHG emission from biodiesel combustion can be calculated via mass balance by using C17H31O2 as a general formula for biodiesel. The methane yield is assumed at the range of [0.09-0.45] m3/kg microalgal residue [130]. And the GHG emission rate can be calculated as CO2 emitted divided by CO2 consumed. Table 3.6. Parameters used for the calculation of GHG emission rate. Parameter Unit Value Reference Biodiesel kg CO2-eq/kg 2.8 Calculation Methane kg CO2-eq/m3 of CH4 1.815 Calculation Based on the calculations, the GHG emission can be obtained at a range of [10,673.17−12,403.65] ton CO2-eq per year. In contrast, during the cultivation of microalgae, the CO2 consumed can be calculated from the mass balance: 11,877.29 ton CO2-eq per year, which is around the average value of our estimated GHG emission. Thus GHG emission rate can be calculated at the range of 0.90 to 1.04. 58 3.5. Sensitivity analysis Sensitivity analysis was conducted for several alternative growth scenarios, to evaluate the effect of potential strain characteristics on the NER, production cost and the GHG emission rate. Among these scenarios, both the biomass productivity and oil content were examined for a “low” case and a “high” case based on the baseline values assumed before. With the sun radiation to the earth (about 1 kW/m2 during sunshine) and 5% photosynthetic efficiency, the maximum biomass productivity is about 71.6 g/m2/d. The values assumed for the three cases are shown in Table 3.7. These values are about 20, 35 and 55% of the maximum possible biomass yield. Based on these values, we can get the Net Energy Ratio, Biodiesel Production Cost and the GHG Emission Rate of these two scenarios. Table 3.7. Basic parameters for the three cases. Parameters Biomass productivity Oil content (g/m2/d) (%) “Low” Case 15 30 “Base” Case 25 58 “High” Case 40 80 Fig. 3.1 shows the NER of three cases. In the “low” case, the worst case assumptions for the process give a NER of only 0.82. This value below 1 indicates that the amount of energy produced is lower than the energy consumed. However, 59 with the optimum assumptions, the NER is 2.67 even in this case. For the other two cases, the energy produced is every case higher than the energy utilized for the whole process. Surprisingly, the 60% higher biomass productivity and the higher lipid content between the “base case” and the “high case” translates only into an improvement by about 30% (under the cautious assumptions) and 10% (for the most optimistic assumptions) in the NER. In other words, when we cultivate the microalgae with higher biomass productivity and lipid content, although they consume more energy than those with low biomass productivity and lipid content, but the energy produced via biodiesel and methane will be a lot more that the energy consumed. Therefore, if we want to get more energy return after the whole production process, we should perform this whole system with microalgae which have higher biomass productivity and lipid content. NER of three cases 4 3.4 3.5 3.11 3 2.67 2.5 1.79 2 1.37 1.5 1 0.82 0.5 0 "Low" Case "Base" Case Min. Fig. 3.1. NER of the three cases. 60 Max. "High" Case Fig. 3.2 shows the required selling price of the biodiesel produced in these three cases in order to achieve a 10% rate of return. According to this figure, with the increase of the microalgal biomass productivity and lipid content, the selling price tends to decrease from the “low” case to the “high” case. As can be seen, only in the “high” case is the minimum selling price lower than the current price of diesel fuel in the retail market. In contrast, the minimum selling price of the “base” case is close to the usual price, while in the “low” case, even the minimum selling price is two times more than the usual price. However, it is also worthwhile to comment on the maximum selling price in three cases. As for the “low” case, the maximum price is nearly 5 times higher than the usual price, and that in the “base” case is more than 2 times higher than the usual price. Even in the “high” case, the maximum price is about 1.5 times higher than the current market price. This huge disparity accounts for the differences of the biomass productivity and lipid content. The higher the biomass productivity and lipid content are, the lower can the selling price be in the market. If the price of biodiesel is higher than the usual price, then biodiesel will not be a viable substitute for the conventional diesel, and the production of algal biofuel will not be economically feasible. 61 Biodiesel prices of three cases "High" Case 12.59 5.67 7.24 "Base" Case 16.97 7.73 7.24 "Low" Case 7.24 0 5 32.68 15.18 10 15 20 25 30 35 Unit: ￥/L Max. Min. Usual Fig. 3.2. Required selling price of the biodiesel produced in these three cases in order to achieve a 10% rate of return. Fig. 3.3 shows the GHG emission rate of three cases. According to this figure, with the increase of the microalgal biomass productivity and lipid content, the productivity of biodiesel and methane will be improved. Therefore, the carbon dioxide emitted by combustion of biodiesel and methane will be increased. As can be seen, the carbon dioxide emitted in the “low” case is lower than the carbon dioxide consumed during the cultivation, while for the “high” case, the CO2 produced is a little higher than CO2 utilized. Based on the GHG emission rate of three cases shown in Fig. 3.3., we can associate them into three different carbon properties. The “Low” case is a carbon negative system because its GHG emission rate is less than 1; the “base” case can be recognized as carbon neutral system because the average of the maximum and minimum GHG emission rate is about 1, but the “high” case can be regarded as a carbon positive system because the GHG emission rate is a little over 1. 62 GHG emission rate of three cases 1.40 1.21 1.20 1.04 1.00 0.90 0.75 0.80 0.60 1.28 0.51 0.40 0.20 0.00 "Low" Case "Base" Case Min. "High" Case Max. Fig. 3.3. GHG emission rate of three cases. 3.6. Conclusion According to the estimated results from our “base” case, the required selling price of biodiesel should be set at the range of 7.73 to 16.69 ￥/L in order to achieve a 10% rate of return on investment, considering that the current selling price for conventional diesel in the market (Shenzhen, China) is 7.24 ￥ /L. Therefore, although the energy produced is more than the energy consumed, these results confirm that with the current technology, microalgal biodiesel production would not be competitive with petrol-derived diesel if an industrial scale facility were to be built today. However, the whole production is carbon neutral, or even carbon-negative, so that credits for greenhouse gas reduction, which have not been considered in this study, may impact the economic assessment. Our results are in agreement with previous reports [166,167], however, several important conclusions can be drawn based on these analysis: (a) there is still much room for reducing the cost significantly through advances in engineering and 63 biological improvement opportunities; (b) the NER can be improved and the biodiesel selling price can be reduced through maximizing the lipid content and biomass productivity, which could be the focus of future studies from an economic standpoint; (c) with the increase of the microalgal biomass productivity and lipid content, the carbon dioxide emitted via combustion of biodiesel and methane would exceed the carbon dioxide consumed during cultivation, which need further studies to achieve a balance between the NER, biodiesel production cost and GHG emission to prevent the occurrence of carbon positive. In the near future, the microalgal biofuel economics could be further improved by utilizing microalgae biomass for more valuable co-products (such as the docosahexaenoic acid, DHA) production in addition to the biogas for power generation. However, the market needs and supply of such co-products should be considered carefully when we try to bring the microalgae toward that way. Currently, with the increasingly energy crisis all over the world, it is a little difficult to discover a valuable products that can be comparable with the fuel market. But with the development of production quantities of microalgal biomass, the co-products production could be a potential solution to incentive the economics. Finally, it has to be noted that the processes selected for biodiesel production from microalgae may not be the best or most optimized methods, but they represent a feasibly approach if the whole system is to be built on an industrial scale at the present time. Furthermore, considering that no commercial scale system exists currently and most of the steps included in the model for the whole system have to be 64 assumed based on previous literatures, therefore, the results presented above may contain a relatively high degree of uncertainty in the calculations of NER, biodiesel production cost and GHG emission rate. Meanwhile, given that sea water was assumed in the whole facility, and most of the sea water was recycled except the water evaporated, we did not involve the water footprint along with the calculation. In further studies, the whole production system could be associated with wastewater treatment, which can provide both water and nutrient for microalgae cultivation, and thus to bring the whole production cost down. 65 References [1] D. Zenghelis, N. Stern, Principles for a Global Deal for Limiting the Risks from Climate Change, Environmental & Resource Economics 43 (2009) 307-311. [2] P. M., Million at risk, School of Environmental Science, University of East Anglia, Norwich. [3] BP, Statistical Review of World Energy 2014, (2014). [4] S. Amin, Review on biofuel oil and gas production processes from microalgae, Energy Conversion and Management 50 (2009) 1834-1840. [5] Y.S. Hassan MA, Ghani BA, Utilization of biomass in Malaysia-potential for CDM business, University Putra Malaysia, Faculty of Biotechnology (2005). [6] S.A. Khan, Rashmi, M.Z. Hussain, S. Prasad, U.C. Banerjee, Prospects of biodiesel production from microalgae in India, Renewable & Sustainable Energy Reviews 13 (2009) 2361-2372. [7] International Energy Outlook, International Energy Agency (2013). [8] G. Vicente, M. Martinez, J. Aracil, Integrated biodiesel production: a comparison of different homogeneous catalysts systems, Bioresource Technology 92 (2004) 297-305. [9] G. Antolin, F.V. Tinaut, Y. Briceno, V. Castano, C. Perez, A.I. Ramirez, Optimisation of biodiesel production by sunflower oil transesterification, Bioresource Technology 83 (2002) 111-114. [10] A. Demirbas, Diesel fuel from vegetable oil via transesterification and soap pyrolysis, Energy Sources 24 (2002) 835-841. [11] J. Van Gerpen, Biodiesel processing and production, Fuel Processing Technology 86 (2005) 1097-1107. [12] S. Lim, L.K. Teong, Recent trends, opportunities and challenges of biodiesel in Malaysia: An overview, Renewable and Sustainable Energy Reviews 14 (2010) 938-954. [13] P. Cao, M.A. Dubé, A.Y. Tremblay, High-purity fatty acid methyl ester production from canola, soybean, palm, and yellow grease lipids by means of a membrane reactor, Biomass and Bioenergy 32 (2008) 1028-1036. [14] A comprehensive analysis of biodiesel impacts on exhaust emissions. Draft technical report, US Environmental Protection Agency (EPA) (October 2002). [15] Alternative Profits Report. Profiting from the alternative energy revolution, Available at:http://www.altprofit.com (January 2010). [16] Biofuels for transport: global potential and implications for sustainable energy and agriculture, 1st ed, Worldwatch Institute London: Earthscan (2007). [17] X. Lang, A.K. Dalai, N.N. Bakhshi, M.J. Reaney, P.B. Hertz, Preparation and characterization of bio-diesels from various bio-oils, Bioresource Technology 80 (2001) 53-62. [18] İ. Çelikten, A. Koca, M. Ali Arslan, Comparison of performance and emissions of diesel fuel, rapeseed and soybean oil methyl esters injected at different pressures, Renewable Energy 35 (2010) 814-820. [19] R. Alcantara, J. Amores, L. Canoira, E. Fidalgo, M.J. Franco, A. Navarro, Catalytic production of biodiesel from soy-bean oil, used frying oil and tallow, Biomass and Bioenergy 18 (2000) 515-527. 66 [20] M.J. Haas, Improving the economics of biodiesel production through the use of low value lipids as feedstocks: vegetable oil soapstock, Fuel Processing Technology 86 (2005) 1087-1096. [21] S. Sumathi, S.P. Chai, A.R. Mohamed, Utilization of oil palm as a source of renewable energy in Malaysia, Renewable and Sustainable Energy Reviews 12 (2008) 2404-2421. [22] M.A. Kalam, H.H. Masjuki, Biodiesel from palmoil—an analysis of its properties and potential, Biomass and Bioenergy 23 (2002) 471-479. [23] M. Canakci, A.N. Ozsezen, E. Arcaklioglu, A. Erdil, Prediction of performance and exhaust emissions of a diesel engine fueled with biodiesel produced from waste frying palm oil, Expert Systems with Applications 36 (2009) 9268-9280. [24] J. Kansedo, K.T. Lee, S. Bhatia, Biodiesel production from palm oil via heterogeneous transesterification, Biomass and Bioenergy 33 (2009) 271-276. [25] S. Siler-Marinkovic, A. Tomasevic, Transesterification of sunflower oil in situ, Fuel 77 (1998) 1389-1391. [26] D. Rattanaphra, P. Srinophakun, Biodiesel Production from Crude Sunflower Oil and Crude Jatropha Oil Using Immobilized Lipase, JOURNAL OF CHEMICAL ENGINEERING OF JAPAN 43 (2010) 104-108. [27] L. Brennan, P. Owende, Biofuels from microalgae—a review of technologies for production, processing, and extractions of biofuels and co-products, Renewable and Sustainable Energy Reviews 14 (2010) 557-577. [28] M.M. Gui, K.T. Lee, S. Bhatia, Feasibility of edible oil vs. non-edible oil vs. waste edible oil as biodiesel feedstock, Energy 33 (2008) 1646-1653. [29] J. Kansedo, K.T. Lee, S. Bhatia, Cerbera odollam (sea mango) oil as a promising non-edible feedstock for biodiesel production, Fuel 88 (2009) 1148-1150. [30] N. Foidl, G. Foidl, M. Sanchez, M. Mittelbach, S. Hackel, Jatropha curcas L. as a source for the production of biofuel in Nicaragua, Bioresource Technology 58 (1996) 77-82. [31] K. Pramanik, Properties and use of jatropha curcas oil and diesel fuel blends in compression ignition engine, Renewable Energy 28 (2003) 239-248. [32] N. Usta, Use of tobacco seed oil methyl ester in a turbocharged indirect injection diesel engine, Biomass and Bioenergy 28 (2005) 77-86. [33] J.F. Reyes, M.A. Sepúlveda, PM-10 emissions and power of a Diesel engine fueled with crude and refined Biodiesel from salmon oil, Fuel 85 (2006) 1714-1719. [34] D.Y.C. Leung, X. Wu, M.K.H. Leung, A review on biodiesel production using catalyzed transesterification, Applied Energy 87 (2010) 1083-1095. [35] S. Pinzi, I.L. Garcia, F.J. Lopez-Gimenez, M.D. Luque de Castro, G. Dorado, M.P. Dorado, The Ideal Vegetable Oil-based Biodiesel Composition: A Review of Social, Economical and Technical Implications, Energy & Fuels 23 (2009) 2325-2341. [36] M. Gürü, B.D. Artukoğlu, A. Keskin, A. Koca, Biodiesel production from waste animal fat and improvement of its characteristics by synthesized nickel and magnesium additive, Energy Conversion and Management 50 (2009) 498-502. [37] S.P. Singh, D. Singh, Biodiesel production through the use of different sources and characterization of oils and their esters as the substitute of diesel: A review, Renewable and Sustainable Energy Reviews 14 (2010) 200-216. 67 [38] F. Ma, M.A. Hanna, Biodiesel production: a review, Bioresource Technology 70 (1999) 1-15. [39] J. Janaun, N. Ellis, Perspectives on biodiesel as a sustainable fuel, Renewable and Sustainable Energy Reviews 14 (2010) 1312-1320. [40] V. Patil, K.-Q. Tran, H.R. Giselrød, Towards sustainable production of biofuels from microalgae, International journal of molecular sciences 9 (2008) 1188-1195. [41] M. Balat, H. Balat, Progress in biodiesel processing, Applied Energy 87 (2010) 1815-1835. [42] T. Minowa, S.-y. Yokoyama, M. Kishimoto, T. Okakura, Oil production from algal cells of Dunaliella tertiolecta by direct thermochemical liquefaction, Fuel 74 (1995) 1735-1738. [43] G. Huang, F. Chen, D. Wei, X. Zhang, G. Chen, Biodiesel production by microalgal biotechnology, Applied Energy 87 (2010) 38-46. [44] T.M. Mata, A.A. Martins, N.S. Caetano, Microalgae for biodiesel production and other applications: A review, Renewable and Sustainable Energy Reviews 14 (2010) 217-232. [45] A. Vonshak, Recent advances in microalgal biotechnology, Biotechnology Advances 8 (1990) 709-727. [46] M.K. Danquah, B. Gladman, N. Moheimani, G.M. Forde, Microalgal growth characteristics and subsequent influence on dewatering efficiency, Chemical Engineering Journal 151 (2009) 73-78. [47] L. RE, Phycology, New York: Cambridge University Press (1980). [48] S.A. Khan, Rashmi, M.Z. Hussain, S. Prasad, U.C. Banerjee, Prospects of biodiesel production from microalgae in India, Renewable and Sustainable Energy Reviews 13 (2009) 2361-2372. [49] E. Suali, R. Sarbatly, Conversion of microalgae to biofuel, Renewable & Sustainable Energy Reviews 16 (2012) 4316-4342. [50] B. Wang, Y. Li, N. Wu, C. Lan, CO2 bio-mitigation using microalgae, Applied Microbiology and Biotechnology 79 (2008) 707-718. [51] D. Bilanovic, A. Andargatchew, T. Kroeger, G. Shelef, Freshwater and marine microalgae sequestering of CO2 at different C and N concentrations – Response surface methodology analysis, Energy Conversion and Management 50 (2009) 262-267. [52] S.-Y. Chiu, C.-Y. Kao, M.-T. Tsai, S.-C. Ong, C.-H. Chen, C.-S. Lin, Lipid accumulation and CO2 utilization of Nannochloropsis oculata in response to CO2 aeration, Bioresource Technology 100 (2009) 833-838. [53] I.E. Huertas, B. Colman, G.S. Espie, L.M. Lubian, Active transport of CO2 by three species of marine microalgae, Journal of Phycology 36 (2000) 314-320. [54] B. Colman, C. Rotatore, Photosynthetic inorganic carbon uptake and accumulation in two marine diatoms, Plant, Cell & Environment 18 (1995) 919-924. [55] I. Suh, C.-G. Lee, Photobioreactor engineering: Design and performance, Biotechnology and Bioprocess Engineering 8 (2003) 313-321. [56] D.T. Welsh, M. Bartoli, D. Nizzoli, G. Castaldelli, S.A. Riou, P. Viaroli, Denitrification, nitrogen fixation, community primary productivity and inorganic-N and oxygen fluxes in an intertidal Zostera noltii meadow, Marine Ecology Progress Series 208 (2000) 65-77. 68 [57] J. Moreno, M.Á. Vargas, Rodrı, amp, x, H. guez, J. Rivas, amp, x, M.G. Guerrero, Outdoor cultivation of a nitrogen-fixing marine cyanobacterium, Anabaena sp. ATCC 33047, Biomolecular Engineering 20 (2003) 191-197. [58] C.-H. Hsieh, W.-T. Wu, Cultivation of microalgae for oil production with a cultivation strategy of urea limitation, Bioresource Technology 100 (2009) 3921-3926. [59] A. Çelekli, M. Yavuzatmaca, H. Bozkurt, Modeling of biomass production by Spirulina platensis as function of phosphate concentrations and pH regimes, Bioresource Technology 100 (2009) 3625-3629. [60] Y. Chisti, Biodiesel from microalgae, Biotechnology Advances 25 (2007) 294-306. [61] V. Martin-Jézéquel, M. Hildebrand, M.A. Brzezinski, SILICON METABOLISM IN DIATOMS: IMPLICATIONS FOR GROWTH Journal of Phycology 36 (2000) 821-840. [62] M.A. Borowitzka, Commercial production of microalgae: ponds, tanks, and fermenters, in: J.T.J.G.B. R. Osinga, R.H. Wijffels (Eds.), Progress in Industrial Microbiology, Elsevier, 1999, pp. 313-321. [63] C. Jiménez, B.R. Cossı ́o, D. Labella, F. Xavier Niell, The Feasibility of industrial production of Spirulina (Arthrospira) in Southern Spain, Aquaculture 217 (2003) 179-190. [64] K.L. Terry, L.P. Raymond, System design for the autotrophic production of microalgae, Enzyme and Microbial Technology 7 (1985) 474-487. [65] O. Pulz, K. Scheibenbogen, Photobioreactors: Design and performance with respect to light energy input, Bioprocess and Algae Reactor Technology, Apoptosis, Springer Berlin Heidelberg, 1998, pp. 123-152. [66] C.U. Ugwu, H. Aoyagi, H. Uchiyama, Photobioreactors for mass cultivation of algae, Bioresource Technology 99 (2008) 4021-4028. [67] O. Pulz, Photobioreactors: production systems for phototrophic microorganisms, Applied Microbiology and Biotechnology 57 (2001) 287-293. [68] E. Molina, J. Fernández, F.G. Acién, Y. Chisti, Tubular photobioreactor design for algal cultures, Journal of Biotechnology 92 (2001) 113-131. [69] A. Sánchez Mirón, A. Contreras Gómez, F. Garcı ́a Camacho, E. Molina Grima, Y. Chisti, Comparative evaluation of compact photobioreactors for large-scale monoculture of microalgae, Journal of Biotechnology 70 (1999) 249-270. [70] C. Ugwu, J. Ogbonna, H. Tanaka, Improvement of mass transfer characteristics and productivities of inclined tubular photobioreactors by installation of internal static mixers, Applied Microbiology and Biotechnology 58 (2002) 600-607. [71] Y. Watanabe, H. Saiki, Development of a photobioreactor incorporating Chlorella sp. for removal of CO2 in stack gas, Energy Conversion and Management 38, Supplement (1997) S499-S503. [72] N. Eriksen, The technology of microalgal culturing, Biotechnology Letters 30 (2008) 1525-1536. [73] E.W. Becker, Microalgae, Cambridge: Cambridge University Press (1994). [74] R. Samson, A. Leduy, Multistage continuous cultivation of blue-green alga spirulina maxima in the flat tank photobioreactors with recycle, The Canadian Journal of Chemical Engineering 63 (1985) 105-112. 69 [75] Q. Hu, N. Kurano, M. Kawachi, I. Iwasaki, S. Miyachi, Ultrahigh-cell-density culture of a marine green alga Chlorococcum littorale in a flat-plate photobioreactor, Applied Microbiology and Biotechnology 49 (1998) 655-662. [76] I.S. Suh, S.B. Lee, A light distribution model for an internally radiating photobioreactor, Biotechnology and Bioengineering 82 (2003) 180-189. [77] A.P. Carvalho, L.A. Meireles, F.X. Malcata, Microalgal Reactors: A Review of Enclosed System Designs and Performances, Biotechnology Progress 22 (2006) 1490-1506. [78] L. Rodolfi, G. Chini Zittelli, N. Bassi, G. Padovani, N. Biondi, G. Bonini, M.R. Tredici, Microalgae for oil: Strain selection, induction of lipid synthesis and outdoor mass cultivation in a low-cost photobioreactor, Biotechnology and Bioengineering 102 (2009) 100-112. [79] M. Huntley, D. Redalje, CO2 Mitigation and Renewable Oil from Photosynthetic Microbes: A New Appraisal, Mitigation and Adaptation Strategies for Global Change 12 (2007) 573-608. [80] G.J. Graham LE, Wilcox LW, Algae, 2nd ed., San Francisco: Pearson Education, Inc. (2009). [81] X.W. Zhang, Y.M. Zhang, F. Chen, Application of mathematical models to the determination optimal glucose concentration and light intensity for mixotrophic culture of Spirulina platensis, Process Biochemistry 34 (1999) 477-481. [82] M.R. Andrade, J.A.V. Costa, Mixotrophic cultivation of microalga Spirulina platensis using molasses as organic substrate, Aquaculture 264 (2007) 130-134. [83] F. Chen, Y. Zhang, S. Guo, Growth and phycocyanin formation of Spirulina platensis in photoheterotrophic culture, Biotechnology Letters 18 (1996) 603-608. [84] J.N. Rosenberg, G.A. Oyler, L. Wilkinson, M.J. Betenbaugh, A green light for engineered algae: redirecting metabolism to fuel a biotechnology revolution, Current Opinion in Biotechnology 19 (2008) 430-436. [85] J. Sheehan, T. Dunahay, J. Benemann, P. Roessler, A look back at the US Department of Energy's Aquatic Species Program: Biodiesel from algae, National Renewable Energy Laboratory Golden, 1998. [86] T. Burton, H. Lyons, Y. Lerat, M. Stanley, M.B. Rasmussen, A review of the potential of marine algae as a source of biofuel in Ireland, Dublin: Sustainable Energy Ireland-SEI, 2009. [87] T.G. Dunahay, E.E. Jarvis, S.S. Dais, P.G. Roessler, Manipulation of microalgal lipid production using genetic engineering, Seventeenth Symposium on Biotechnology for Fuels and Chemicals, Springer, 1996, pp. 223-231. [88] R. León-Bañares, D. González-Ballester, A. Galván, E. Fernández, Transgenic microalgae as green cell-factories, TRENDS in Biotechnology 22 (2004) 45-52. [89] Q. Hu, M. Sommerfeld, E. Jarvis, M. Ghirardi, M. Posewitz, M. Seibert, A. Darzins, Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances, The Plant Journal 54 (2008) 621-639. [90] J. Qin, Bio-hydrocarbons from algae: impacts of temperature, light and salinity on algae growth, Rirdc, 2005. [91] E. Laws, S. Taguchi, J. Hirata, L. Pang, High algal production rates achieved in a shallow outdoor flume, Biotechnology and bioengineering 28 (1986) 191-197. 70 [92] A. Widjaja, C.-C. Chien, Y.-H. Ju, Study of increasing lipid production from fresh water microalgae Chlorella vulgaris, Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 13-20. [93] N. Uduman, Y. Qi, M.K. Danquah, G.M. Forde, A. Hoadley, Dewatering of microalgal cultures: a major bottleneck to algae-based fuels, Journal of renewable and sustainable energy 2 (2010) 012701. [94] M.K. Danquah, L. Ang, N. Uduman, N. Moheimani, G.M. Forde, Dewatering of microalgal culture for biodiesel production: exploring polymer flocculation and tangential flow filtration, Journal of Chemical Technology and Biotechnology 84 (2009) 1078-1083. [95] C.-Y. Chen, K.-L. Yeh, R. Aisyah, D.-J. Lee, J.-S. Chang, Cultivation, photobioreactor design and harvesting of microalgae for biodiesel production: A critical review, Bioresource Technology 102 (2011) 71-81. [96] E. Molina Grima, E.-H. Belarbi, F. Acién Fernández, A. Robles Medina, Y. Chisti, Recovery of microalgal biomass and metabolites: process options and economics, Biotechnology advances 20 (2003) 491-515. [97] R.M. Knuckey, M.R. Brown, R. Robert, D.M. Frampton, Production of microalgal concentrates by flocculation and their assessment as aquaculture feeds, Aquacultural Engineering 35 (2006) 300-313. [98] A. Sukenik, G. Shelef, Algal autoflocculation—verification and proposed mechanism, Biotechnology and bioengineering 26 (1984) 142-147. [99] S. Lee, S. Kim, J. Kim, G. Kwon, B. Yoon, H. Oh, Effects of harvesting method and growth stage on the flocculation of the green alga Botryococcus braunii, Letters in Applied Microbiology 27 (1998) 14-18. [100] E. Poelman, N. De Pauw, B. Jeurissen, Potential of electrolytic flocculation for recovery of micro-algae, Resources, conservation and recycling 19 (1997) 1-10. [101] J. Edzwald, Algae, bubbles, coagulants, and dissolved air flotation, Water Science & Technology 27 (1993) 67-81. [102] E.W. Wilde, J.R. Benemann, J.C. Weissman, D.M. Tillett, Cultivation of algae and nutrient removal in a waste heat utilization process, Journal of applied phycology 3 (1991) 159-167. [103] Y. Chen, J. Liu, Y.-H. Ju, Flotation removal of algae from water, Colloids and Surfaces B: Biointerfaces 12 (1998) 49-55. [104] M.Y. Mollah, P. Morkovsky, J.A. Gomes, M. Kesmez, J. Parga, D.L. Cocke, Fundamentals, present and future perspectives of electrocoagulation, Journal of Hazardous Materials 114 (2004) 199-210. [105] C.G. Golueke, W.J. Oswald, Harvesting and processing sewage-grown planktonic algae, Journal (Water Pollution Control Federation) 37 (1965) 471-498. [106] J. Prakash, B. Pushparaj, P. Carlozzi, G. Torzillo, E. Montaini, R. Materassi, Microalgal biomass drying by a simple solar device, International journal of solar energy 18 (1997) 303-311. [107] H. Desmorieux, N. Decaen, Convective drying of spirulina in thin layer, Journal of food engineering 66 (2005) 497-503. 71 [108] G. Leach, G. Oliveira, R. Morais, Spray-drying of Dunaliella salina to produce a β-carotene rich powder, Journal of industrial Microbiology and Biotechnology 20 (1998) 82-85. [109] E.M. Grima, A.R. Medina, A.G. Giménez, J.S. Pérez, F.G. Camacho, J.G. Sánchez, Comparison between extraction of lipids and fatty acids from microalgal biomass, Journal of the American Oil Chemists’ Society 71 (1994) 955-959. [110] I. Rawat, R. Ranjith Kumar, T. Mutanda, F. Bux, Dual role of microalgae: phycoremediation of domestic wastewater and biomass production for sustainable biofuels production, Applied Energy 88 (2011) 3411-3424. [111] H.M. Amaro, A. Guedes, F.X. Malcata, Advances and perspectives in using microalgae to produce biodiesel, Applied Energy 88 (2011) 3402-3410. [112] R. Halim, B. Gladman, M.K. Danquah, P.A. Webley, Oil extraction from microalgae for biodiesel production, Bioresource technology 102 (2011) 178-185. [113] Y.-H. Kim, Y.-K. Choi, J. Park, S. Lee, Y.-H. Yang, H.J. Kim, T.-J. Park, Y. Hwan Kim, S.H. Lee, Ionic liquid-mediated extraction of lipids from algal biomass, Bioresource Technology 109 (2012) 312-315. [114] M.K. Lam, K.T. Lee, Immobilization as a feasible method to simplify the separation of microalgae from water for biodiesel production, Chemical Engineering Journal 191 (2012) 263-268. [115] S.J. Iverson, S.L. Lang, M.H. Cooper, Comparison of the Bligh and Dyer and Folch methods for total lipid determination in a broad range of marine tissue, Lipids 36 (2001) 1283-1287. [116] G. Young, F. Nippgen, S. Titterbrandt, M.J. Cooney, Lipid extraction from biomass using co-solvent mixtures of ionic liquids and polar covalent molecules, Separation and Purification Technology 72 (2010) 118-121. [117] A. Robles-Medina, P. González-Moreno, L. Esteban-Cerdán, E. Molina-Grima, Biocatalysis: towards ever greener biodiesel production, Biotechnology advances 27 (2009) 398-408. [118] A. Demirbas, Progress and recent trends in biodiesel fuels, Energy Conversion and Management 50 (2009) 14-34. [119] U. Schuchardt, R. Sercheli, R.M. Vargas, Transesterification of vegetable oils: a review, Journal of the Brazilian Chemical Society 9 (1998) 199-210. [120] R. Maceiras, A. Cancela, S. Urréjola, A. Sánchez, Macroalgae: raw material for biodiesel production, Applied Energy 88 (2011) 3318-3323. [121] Y. Li, S. Lian, D. Tong, R. Song, W. Yang, Y. Fan, R. Qing, C. Hu, One-step production of biodiesel from< i> Nannochloropsis sp. on solid base Mg–Zr catalyst, Applied Energy 88 (2011) 3313-3317. [122] M.G.M. D’Oca, C.V. Viêgas, J.S. Lemões, E.K. Miyasaki, J.A. Morón-Villarreyes, E.G. Primel, P.C. Abreu, Production of FAMEs from several microalgal lipidic extracts and direct transesterification of the< i> Chlorella pyrenoidosa, Biomass and Bioenergy 35 (2011) 1533-1538. [123] A national vision of America's transition in a hydrogen economy—in 2030 and beyond, U.S. Department of Energy (2002). 72 [124] B. Grant, Biofuels made from algae are the next big thing on alternative energy horizon, Scientist (2009) 37-41. [125] A. Demirbas, Use of algae as biofuel sources, Energy Conversion and Management 51 (2010) 2738-2749. [126] Z. Shen, J. Zhou, X. Zhou, Y. Zhang, The production of acetic acid from microalgae under hydrothermal conditions, Applied Energy 88 (2011) 3444-3447. [127] D.R. Vardon, B. Sharma, J. Scott, G. Yu, Z. Wang, L. Schideman, Y. Zhang, T.J. Strathmann, Chemical properties of biocrude oil from the hydrothermal liquefaction of Spirulina algae, swine manure, and digested anaerobic sludge, Bioresource technology 102 (2011) 8295-8303. [128] M.F. Demirbas, Biofuels from algae for sustainable development, Applied Energy 88 (2011) 3473-3480. [129] T.P. Vispute, H. Zhang, A. Sanna, R. Xiao, G.W. Huber, Renewable chemical commodity feedstocks from integrated catalytic processing of pyrolysis oils, Science 330 (2010) 1222-1227. [130] B. Sialve, N. Bernet, O. Bernard, Anaerobic digestion of microalgae as a necessary step to make microalgal biodiesel sustainable, Biotechnology Advances 27 (2009) 409-416. [131] S. Chinnasamy, A. Bhatnagar, R.W. Hunt, K. Das, Microalgae cultivation in a wastewater dominated by carpet mill effluents for biofuel applications, Bioresource technology 101 (2010) 3097-3105. [132] P. Spolaore, C. Joannis-Cassan, E. Duran, A. Isambert, Commercial applications of microalgae, Journal of bioscience and bioengineering 101 (2006) 87-96. [133] O. Pulz, W. Gross, Valuable products from biotechnology of microalgae, Applied microbiology and biotechnology 65 (2004) 635-648. [134] M. Brown, S. Jeffrey, J. Volkman, G. Dunstan, Nutritional properties of microalgae for mariculture, Aquaculture 151 (1997) 315-331. [135] E. Marris, Putting the carbon back: black is the new green, Nature 442 (2006) 624-626. [136] J. Lehmann, A handful of carbon, Nature 447 (2007) 143-144. [137] L. Reijnders, Are forestation, bio-char and landfilled biomass adequate offsets for the climate effects of burning fossil fuels?, Energy Policy 37 (2009) 2839-2841. [138] C. Hu, M. Li, J. Li, Q. Zhu, Z. Liu, Variation of lipid and fatty acid compositions of the marine microalga Pavlova viridis (Prymnesiophyceae) under laboratory and outdoor culture conditions, World Journal of Microbiology and Biotechnology 24 (2008) 1209-1214. [139] C. Ruxton, S.C. Reed, M. Simpson, K. Millington, The health benefits of omega‐3 polyunsaturated fatty acids: a review of the evidence, Journal of Human Nutrition and Dietetics 17 (2004) 449-459. [140] A. Richmond, Handbook of microalgal culture: biotechnology and applied phycology, John Wiley & Sons, 2008. [141] F. Delrue, P.A. Seiter, C. Sahut, L. Cournac, A. Roubaud, G. Peltier, A.K. Froment, An economic, sustainability, and energetic model of biodiesel production from microalgae, Bioresource Technology 111 (2012) 191-200. 73 [142] M.J. Griffiths, S.T. Harrison, Lipid productivity as a key characteristic for choosing algal species for biodiesel production, Journal of Applied Phycology 21 (2009) 493-507. [143] N.T. Eriksen, The technology of microalgal culturing, Biotechnology Letters 30 (2008) 1525-1536. [144] L. Rodolfi, G. Chini Zittelli, N. Bassi, G. Padovani, N. Biondi, G. Bonini, M.R. Tredici, Microalgae for oil: Strain selection, induction of lipid synthesis and outdoor mass cultivation in a low‐cost photobioreactor, Biotechnology and bioengineering 102 (2009) 100-112. [145] A. Illman, A. Scragg, S. Shales, Increase in Chlorella strains calorific values when grown in low nitrogen medium, Enzyme and microbial technology 27 (2000) 631-635. [146] T.J. Lundquist, I.C. Woertz, N. Quinn, J.R. Benemann, A realistic technology and engineering assessment of algae biofuel production, Energy Biosciences Institute (2010) 1. [147] L. Lardon, A. Hélias, B. Sialve, J.-P. Steyer, O. Bernard, Life-cycle assessment of biodiesel production from microalgae, Environmental science & technology 43 (2009) 6475-6481. [148] B.S. Sturm, S.L. Lamer, An energy evaluation of coupling nutrient removal from wastewater with algal biomass production, Applied Energy 88 (2011) 3499-3506. [149] A.L. Stephenson, E. Kazamia, J.S. Dennis, C.J. Howe, S.A. Scott, A.G. Smith, Life-cycle assessment of potential algal biodiesel production in the United Kingdom: a comparison of raceways and air-lift tubular bioreactors, Energy & Fuels 24 (2010) 4062-4077. [150] O. Jorquera, A. Kiperstok, E.A. Sales, M. Embiruçu, M.L. Ghirardi, Comparative energy life-cycle analyses of microalgal biomass production in open ponds and photobioreactors, Bioresource technology 101 (2010) 1406-1413. [151] L. Batan, J. Quinn, B. Willson, T. Bradley, Net energy and greenhouse gas emission evaluation of biodiesel derived from microalgae, Environmental science & technology 44 (2010) 7975-7980. [152] A. Chauvel, G. Fournier, C. Raimbault, Manuel d'évaluation économique des procédés, Editions Technip, 2001. [153] C. Couturier, Berger, S., Meiffren, I., La digestion anaérobie des boues urbaines, état des lieux, état de l'art. Toulouse (France): Solagro, Sponsored by Agence de l’eau Adour-Garonne (2001). [154] UNEP, The Fertilizer Industry's Manufacturing Processes and Environmental Issues. Paris (France): United Nations Environment Programme Industry and the Environment. Report No.: 26, Part 1, (1998). [155] ADEME, Analyses de Cycle de Vie appliquées aux biocarburants de première génération consommés en France, (2010). [156] EFMA, Production of phosphoric acid. Belgium : European Fertilizer Manufacturers' Association, (2000). [157] E. Stephens, I.L. Ross, Z. King, J.H. Mussgnug, O. Kruse, C. Posten, M.A. Borowitzka, B. Hankamer, An economic and technical evaluation of microalgal biofuels, Nature biotechnology 28 (2010) 126-128. 74 [158] R. Davis, A. Aden, P.T. Pienkos, Techno-economic analysis of autotrophic microalgae for fuel production, Applied Energy 88 (2011) 3524-3531. [159] J.R. Benemann, W.J. Oswald, Systems and economic analysis of microalgae ponds for conversion of CO2 to biomass. Final report, California Univ., Berkeley, CA (United States). Dept. of Civil Engineering, 1996. [160] P.K. Campbell, T. Beer, D. Batten, Life cycle assessment of biodiesel production from microalgae in ponds, Bioresource technology 102 (2011) 50-56. [161] R. Putt, Algae as a biodiesel feedstock: A feasibility assessment, Center for Microfibrous Materials Manufacturing (CM 3) Department of Chemical Engineering Auburn University, Alabama (2007). [162] L.S. Carlson, M.Y. Lee, C.A. Oje, A. Xu, Algae to alkanes, (2010). [163] C.D. McKee, Wastewater Treatment Plant Sludge Dewatering Centrifuge, Schenectady (NY) (1995). [164] EPA, Innovation in sludge drying beds - A practical technology, Washington D.C.: Office of Water EPA 832/R-87-101 (1987). [165] F. Perron, Potentiel énergétique et gains environnementaux générés par la biométhanisation des matières organiques résiduelles au Québec, Sherbrooke (Canada): Centre Universitaire de Formation en Environnement (2010). [166] S. A, Techno-economic analysis of algae biofuel deployment. San Diego, CA, In: Proceedings to the 2009 algae biomass summit (October 2009). [167] E. Bibeau, Microalgae Technologies & Processes for Biofuels/Bioenergy Production in British Columbia, (2009). 75 [...]... This very high production efficiency is 8 one reason that microalgae have been considered as a promising material for biodiesel production The advantages of using microalgae as a source of biodiesel production are as follows: (a) reduction in cost and improved efficiencies Compared to other biodiesel feedstocks such as non-food crops, the costs regarding to harvesting and transportation of microalgae are... CO2 from the air and nutrients from the aquatic body to grow under natural environmental conditions In contrast, in heterotrophic cultivation conditions, organic substances are utilized as carbon source (e.g glucose) instead of CO2 for the growth of the microalgae CO2 can be fixed by microalgae from three major different sources: directly from the atmosphere, from CO2-containing flue gases from industries... compared the microalgae biomass harvesting efficiency using centrifugation, flotation, filtration, precipitation and ion exchange, 24 ultrasonic vibration and passage through a charged zone [105] They concluded that centrifugation and chemical precipitation are the only two methods that could achieve economic feasibility The optimal harvesting method of microalgae for biofuels production would be specific... excess of phosphorous must be added over the basic requirement [60] Silicon plays a crucial part in the growth of certain microalgae such as diatoms [61] There are altogether three different production mechanisms of microalgae, including the photoautotrophic production, heterotrophic production and mixotrophic production They will be discussed in the following sections 1.3.1.1 Photoautotrophic production. .. Photoautotrophic production is the only way to make the large-scale microalgae biomass production technically and economically feasible for energy production [61] In the following chapters, two photoautotrophic microalgae production systems are described, namely the open pond and closed photobioreactor technologies [62] 13 1.3.1.1.1 Open pond production systems Open pond production systems have been used for microalgae. .. of microalgal biodiesel production The whole process from microalgae to biofuels is shown in Fig 1.4 There are four main steps of this transformation process, namely cultivation, harvesting, extraction and conversion Microalgae can be cultivated in either photobioreactor systems or open pond systems (e.g raceway ponds) Then microalgae biomass can be harvested with either centrifugation or filtration... dehydration techniques include sun drying [106], spray drying [107], drum drying [106], fluidized bed drying [108] and freeze drying Sun drying is the cheapest dehydration methods among those mentioned above However, this method has also some disadvantages, including long drying times, large drying areas required, and the risk of microalgae biomass loss [106] Spry drying is commonly used for drying of microalgae. .. and the reduced natural CO2 fixation by the forests as well as the long-term carbon storage in the soil would aggravate the situation of increasing global warming Large scale deforestation has already been caused by the expansion of biodiesel production from food crops Consequently, biodiesel produced from the first generation biodiesel feedstocks as a substitute biofuel for petroleum-based diesel fuel... carbon source, which could be subsequently converted to TAG to increase the lipid concentration within cells [91] 1.3.2 Microalgal harvesting technologies It is essential to harvest the microalgae biomass with high efficiency in order to make the biodiesel production from microalgae economical Currently, the primarily adopted technologies consist of centrifugation, flocculation, filtration and screening,... sustainable energy economy [40] Although an increasing amount of biodiesel has been produced from oil crops, its production in large quantities still cannot be considered as sustainability [41] However, microalgae, as the third generation biodiesel feedstock, are a very promising alternative for biodiesel production because of their higher growth rates and productivity compared to the former biodiesel