Advances and perspectives in using microalgae to produce biodiesel

Carbon-neutral renewable liquid biofuels are needed to displace petroleum-derived transport fuels in the near future – which contribute to global warming and are of a limited availability. A promising alternative is conveyed by microalgae, the oil content of which may exceed 80% (w/w_DW) – as compared with 5% of the best agricultural oil crops. However, current implementation of microalga-based systems has been economically constrained by their still poor volumetric efficiencies – which lead to excessively high costs, as compared with petrofuel prices. Technological improvements of such processes are thus critical – and this will require a multiple approach, both on the biocatalyst and bioreactor levels. Several bottlenecks indeed exist at present that preclude the full industrial exploitation of microalgal cells: the number of species that have been subjected to successful genetic transformation is scarce, which hampers a global understanding (and thus a rational design) of novel blue-biotechnological processes; the mechanisms that control regulation of gene expression are not fully elucidated, as required before effective bioprocesses based on microalgae can be scaled-up; and new molecular biology tools are needed to standardize genetic modifications in microalgae – including efficient nuclear transformation, availability of promoter or selectable marker genes, and stable expression of transgenes. On the other hand, a number of pending technological issues are also present: the relatively low microalga intrinsic lipid productivity; the maximum cell concentration attainable; the efficiency of harvest and sequential recovery of bulk lipids; and the possibility of by-product upgrade. This review briefly covers the state of the art regarding microalgae toward production of biofuels, both from the point of view of the microalgal cell itself and of the supporting bioreactor; and discusses, in a critical manner, current limitations and promising perspectives in this field.     

Screening of marine microalgae for biodiesel feedstock

Biodiesel production from microalgae lipids is increasingly regarded as a more sustainable and feasible alternative to conventional biodiesel feedstocks derived from terrestrial bioenergy crops. A total of ninety-six strains of marine microalgae, with an elevated biomass productivity and intracellular lipid content, were isolated from the coastal waters of Singapore using an automated flow cytometric cell-sorting technique. Cell sorting was based on the two-dimensional distribution of algal cells for red fluorescence (representing chlorophyll auto-fluorescence) against forward-light scatter (representing cell size) and red vs. green fluorescence. Twenty-one of the strains were further characterized with respect to cell growth rate, biomass concentration, lipid content (total and neutral lipid) and fatty acid profile. The growth rates of Skeletonema costatum, Chaetoceros and Thalassiosira species were greatest among the entire strains, but in terms of absolute lipid yield Nannochloropsis strains predominated. Nannochloropsis strains had a lipid content ranging from 39.4% to 44.9% of dry weight biomass. Transesterification of the lipids yielded 25-51% of fatty acid methyl ester (FAME) i.e. biodiesel, where total FAME content ranged between 11 and 21% of dry weight biomass. This study describes the microalgae screening process and demonstrates that Nannochloropsis is a promising species for biodiesel feedstock.     

Dual role of microalgae: Phycoremediation of domestic wastewater and biomass production for sustainable biofuels production

Global threats of fuel shortages in the near future and climate change due to green-house gas emissions are posing serious challenges and hence and it is imperative to explore means for sustainable ways of averting the consequences. The dual application of microalgae for phycoremediation and biomass production for sustainable biofuels production is a feasible option. The use of high rate algal ponds (HRAPs) for nutrient removal has been in existence for some decades though the technology has not been fully harnessed for wastewater treatment. Therefore this paper discusses current knowledge regarding wastewater treatment using HRAPs and microalgal biomass production techniques using wastewater streams. The biomass harvesting methods and lipid extraction protocols are discussed in detail. Finally the paper discusses biodiesel production via transesterification of the lipids and other biofuels such as biomethane and bioethanol which are described using the biorefinery approach.     

Technoeconomic analysis of an integrated microalgae photobioreactor, biodiesel and biogas production facility

As fossil fuel prices increase and environmental concerns gain prominence, the development of alternative fuels from biomass has become more important. Biodiesel produced from microalgae is becoming an attractive alternative to share the role of petroleum. Currently it appears that the production of microalgal biodiesel is not economically viable in current environment because it costs more than conventional fuels. Therefore, a new concept is introduced in this article as an option to reduce the total production cost of microalgal biodiesel. The integration of biodiesel production system with methane production via anaerobic digestion is proved in improving the economics and sustainability of overall biodiesel stages. Anaerobic digestion of microalgae produces methane and further be converted to generate electricity. The generated electricity can surrogate the consumption of energy that require in microalgal cultivation, dewatering, extraction and transesterification process. From theoretical calculations, the electricity generated from methane is able to power all of the biodiesel production stages and will substantially reduce the cost of biodiesel production (33% reduction). The carbon emissions of biodiesel production systems are also reduced by approximately 75% when utilizing biogas electricity compared to when the electricity is otherwise purchased from the Victorian grid. The overall findings from this study indicate that the approach of digesting microalgal waste to produce biogas will make the production of biodiesel from algae more viable by reducing the overall cost of production per unit of biodiesel and hence enable biodiesel to be more competitive with existing fuels.     

Anaerobic digestion of microalgae residues resulting from the biodiesel production process

The recovery of methane from post transesterified microalgae residues has the potential to improve the renewability of the ‘microalgae biomass to biodiesel’ conversion process as well as reduce its cost and environmental impact. This paper deals with the anaerobic digestion of microalgae biomass residues (post transesterification) using semi-continuously fed reactors. The influence of substrate loading concentrations and hydraulic retention times on the specific methane yield of the anaerobically digested microalgae residues was investigated. The co-digestion of the microalgae residues with glycerol as well as the influence of temperature was also examined. It was found that the hydraulic retention period was the most significant variable affecting methane production from the residues, with periods (>5 days) corresponding to higher energy recovery. The methane yield was also improved by a reduction in the substrate loading rates, with an optimum substrate carbon to nitrogen ratio of 12.44 seen to be required for the digestion process.     

Search for new strains of microalgae-producers of lipids from natural sources for biodiesel production

Biomass of high-yielding strains of phototrophic microorganisms actively accumulating lipids is a promising non-traditional raw material for bioenergy including the production of biodiesel. In this study, we present results of searching for new strains of microalgae-producers of lipids from hot springs. Within the framework of research, the primary screening of water for the presence of lipid - accumulative microalgae was carried out with the help of qualitative reaction with lipid-specific dyes, as well as 5 axenic isolates of microalgae with stable growth were identified in the laboratory and their productivity and fatty acid composition were studied. The isolated strains were identified as Chlorella vulgaris sp-1, Ankistrodesmus sp-21, Scеnеdеsmus obliquus sp-21, Chlorella pyrenoidosa sp-13 and Chlamydomonas sp-22. The obtained data showed that the isolated strains determined by biomass in the range 1.3 g/l to 1.81 g/l. As a result of the research, it was established that the highest content of lipids was observed in the strains Chlorella vulgaris sp-1 and Scеnеdеsmus obliquus sp-21, which is 28.7 and 29.8% of the cell dry weight, respectively. The analysis of the fatty acid composition of the cells showed that the largest mass fraction of saturated and monounsaturated fatty acids was found in strain Scеnеdеsmus obliquus sp-21 - 61.9%. In the result, Scеnеdеsmus obliquus sp-21 strain isolated from thermal sources was selected as a promising candidate for biodiesel production.     

Effective extraction of microalgae lipids from wet biomass for biodiesel production

Producing biodiesel from lipid extracted from microalgae is a promising approach for sustainable fuel production. However, this approach is not yet commercialized due to the high costs of upstream processes that are associated with the time consuming and/or energy intensive drying, and lipid extraction processes. In this study, the possibility of avoiding the drying process, and extracting the lipid directly from the wet concentrated cells, using enzymatic disruption to enhance the extraction, has been tested. Results showed that lysozyme and cellulase were both efficient in disrupting cell walls and enhancing lipid extraction from wet samples, with highest lipid extraction yield of 16.6% achieved using lysozyme. The applicability of using supercritical CO₂ (SC-CO₂) in extracting lipid from wet biomass was also tested and the highest yield of 12.5% was achieved using lysozyme. In addition, a two-step culturing process was applied, using Scenedesmus sp., to combine both high biomass growth and lipid content. The strain was able to increase its biomass productivity in the first stage, reaching 174 mg l⁻¹ d⁻¹, with almost constant lipid content. In the second stage, the lipid content was enhanced by six-fold after three weeks of nitrogen starvation, but with lower biomass productivity.     

Characterization of microalgal species isolated from fresh water bodies as a potential source for biodiesel production

Due to increasing oil prices and climate change concerns, biodiesel has gained attention as an alternative energy source. Biodiesel derived from microalgae is a potentially renewable and carbon–neutral alternative to petroleum fuels. One of the most important decisions in obtaining oil from microalgae is the choice of algal species to use. Eight microalgae from a total of 33 isolated cultures were selected based on their morphology and ease of cultivation. Five cultures were isolated from river and identified as strains of Scenedesmus obliquus YSR01, Nitzschia cf. pusilla YSR02, Chlorella ellipsoidea YSR03, S. obliquus YSR04, and S. obliquus YSR05, and three were isolated from wastewater and identified as S. obliquus YSW06, Micractinium pusillum YSW07, and Ourococcus multisporus YSW08, based on LSU rDNA (D1-D2) and ITS sequence analyses. S. obliquus YSR01 reached a growth rate of 1.68 ± 0.28 day⁻¹ at 680_nm and a biomass concentration of 1.57 ± 0.67 g dwt L⁻¹, with a high lipid content of 58 ± 1.5%. Under similar environmental conditions, M. pusillum reached a growth rate of 2.3 ± 0.55 day⁻¹ and a biomass concentration of 2.28 ± 0.16 g dwt L⁻¹, with a relatively low lipid content of 24 ± 0.5% w/w. The fatty acid compositions of the studied species were mainly myristic, palmitic, palmitoleic, oleic, linoleic, g-linolenic, and linolenic acids. Our results suggest that S. obliquus YSR01 can be a possible candidate species for producing oils for biodiesel, based on its high lipid and oleic acid contents.     

Microalgal biodiesel in China: Opportunities and challenges

With rapid economic development, energy consumption in China has tripled in the past 20 years, exceeding 2.8 billion tons of standard coal in 2008. The search for new green energy as substitutes for nonrenewable energy resources has become an urgent task. Biodiesel is one of the most important bioenergy sources. According to the Mid- and Long-term Development Plan for Renewable Energy in China, the consumption of biodiesel in China will reach 0.2 million tons in 2010 and 2.0 million tons in 2020. However, large-scale production of biodiesel is restricted by the limited sources of raw materials. Microalgal oil is a prospective raw material for biodiesel production. Development of technology for the production and commercialization of biodiesel from microalgae has become a hot topic in the field of bioenergy and CO₂ emission mitigation. Biodiesel from microalgae can be produced at laboratory-scale, but the cost is too high. Few studies on the commercialization of the technology of producing biodiesel from microalgae have been reported. In this review, recent progress on the research and development of biodiesel from microalgae that have resulted in scientific breakthroughs and innovation in engineering in China are introduced. The existing challenges are also discussed. Based on a detailed analysis, several novel strategies on commercial biodiesel production from microalgae are proposed.     

Net energy analysis of the production of biodiesel and biogas from the microalgae: Haematococcus pluvialis and Nannochloropsis

Microalgae have been proposed as possible alternative feedstocks for the production of biodiesel because of their high photosynthetic efficiency. The high energy input required for microalgal culture and oil extraction may negate this advantage, however. There is a need to determine whether microalgal biodiesel can deliver more energy than is required to produce it. In this work, net energy analysis was done on systems to produce biodiesel and biogas from two microalgae: Haematococcus pluvialis and Nannochloropsis. Even with very optimistic assumptions regarding the performance of processing units, the results show a large energy deficit for both systems, due mainly to the energy required to culture and dry the microalgae or to disrupt the cell. Some energy savings may be realized from eliminating the fertilizer by the use of wastewater or, in the case of H. pluvialis, recycling some of the algal biomass to eliminate the need for a photobioreactor, but these are insufficient to completely eliminate the deficit. Recommendations are made to develop wet extraction and transesterification technology to make microalgal biodiesel systems viable from an energy standpoint.     

Microalgae for biodiesel production and other applications: A review

Sustainable production of renewable energy is being hotly debated globally since it is increasingly understood that first generation biofuels, primarily produced from food crops and mostly oil seeds are limited in their ability to achieve targets for biofuel production, climate change mitigation and economic growth. These concerns have increased the interest in developing second generation biofuels produced from non-food feedstocks such as microalgae, which potentially offer greatest opportunities in the longer term. This paper reviews the current status of microalgae use for biodiesel production, including their cultivation, harvesting, and processing. The microalgae species most used for biodiesel production are presented and their main advantages described in comparison with other available biodiesel feedstocks. The various aspects associated with the design of microalgae production units are described, giving an overview of the current state of development of algae cultivation systems (photo-bioreactors and open ponds). Other potential applications and products from microalgae are also presented such as for biological sequestration of CO₂, wastewater treatment, in human health, as food additive, and for aquaculture.     

Prospects of using microalgae for biofuels production: Results of a Delphi study

Advanced biofuels, such as those obtained from microalgae, are widely accepted as better choices for achieving goals of incorporating renewables and non-food fuel sources into the transportation sector, and for overcoming land use issues due to biofuel crops. Main challenges are currently the feasibility of large-scale commercialization of microalgae biofuels, since there are still some technical problems to overcome (e.g. the high energy consumption associated with biomass processing) and the majority of economic and financial analyses are based on pilot-scale projects. Therefore, this article presents the results of a Delphi study aiming to identify the main obstacles and most critical issues affecting the potential of large-scale commercialization of microalgae biodiesel and its incorporation into the fuel market. According to the authors' knowledge, this is the first Delphi study with this objective. The respondents are worldwide market specialists in the survey themes that ranged from biofuels economics to their environmental sustainability. One of the key findings is that most of the experts believe that production of microalgae biofuels will achieve its full commercial scale until 2020, and that from 2021 till 2030 it could represent from 1% to 5% of the worldwide fuel consumption. The study results also showed that environmental issues are where expert opinion differs more.     

Microbial oil production from thermophile cyanobacteria for biodiesel production

Compared to lipid extraction from algae, little work has been performed for cyanobacteria. In this article it is aimed to show high lipid accumulation potential of Synechococcus sp., Cyanobacterium aponinum and Phormidium sp. cells in BG-11 medium. Four different pH values (6–9) and NaNO₃ (0.25, 0.5, 1.0, 1.5 g/L) concentrations were examined at different incubation days to discover the highest lipid accumulation. The maximum lipid content could be achieved in the medium containing 0.25 g/L NaNO₃ at pH 7 for Synechococcus sp., pH 8 for C. aponinum and pH 9 for Phormidium sp. after 15 days. The maximum lipid contents and C16 and C18 methyl ester yields were measured as 42.8% and 46.9% for Synechococcus sp., 45.0% and 67.7% for C. aponinum, 38.2% and 90.6% for Phormidium sp. The saturated compounds were 74.5%, 77.9%, 84.7% for Synechococcus sp., C. aponinum and Phormidium sp., respectively. These crude lipids could be promising feedstock for biodiesel production.     

Role of hydrogen in improving performance and emission characteristics of homogeneous charge compression ignition engine fueled with graphite oxide nanoparticle-added microalgae biodiesel/diesel blends

The development of low-temperature combustion models combined with the use of biofuels has been considered as an efficient strategy to reduce pollutant emissions like CO, HC. NO_x, and smoke. Indeed, Homogeneous Charge Compression Ignition (HCCI) is the new approach to drastically minimize NO_x emissions and smoke owing to the lower cylinder temperature and a higher rate of homogeneous A/F mixture as compared to compression ignition (CI) engines. The present research deal with the behavior analysis of a CI engine powered by diesel, Euglena Sanguinea (ES), and their blends (ES20D80, ES40D60, ES60D40, ES80D20). The experimental results revealed the highest brake thermal efficiency for ES20D80 although it decreased by 4.1% compared to diesel at normal mode. The average drop in HC, CO, and smoke was 2.1, 2.3, and 5.7% for ES20D80 as opposed to diesel fuel. Therefore, in the next stage, ES20D80 with various concentrations of graphite oxide (GO) nanoparticle (20, 40, 60, and 80 ppm) was chosen to carry out experiments in the HCCI mode, in which hydrogen gas was induced along with air through the intake pipe at a fixed flow rate of 3 lpm for the enrichment of the air-fuel mixture. As a result, the combination of hydrogen-enriched gas and GO-added ES20D80 in the HCCI mode showed similar performance to the CI engine but registered a major reduction of NO_x and smoke emissions, corresponding to 75.24% and 53.07% respectively, as compared to diesel fuel at normal mode.     

Microalgal biomass production and carbon dioxide sequestration from an integrated ethanol biorefinery in Iowa: A technical appraisal and economic feasibility evaluation

Microalgae present some advantageous qualities for reducing carbon dioxide (CO₂) emissions from ethanol biorefineries. As photosynthetic organisms, microalgae utilize sunlight and CO₂ to generate biomass. By integrating large-scale microalgal cultivation with ethanol biorefineries, CO₂ sequestration can be coupled with the growth of algae, which can then be used as feedstock for biodiesel production. In this case study, a 50-mgy ethanol biorefinery in Iowa was evaluated as a candidate for this process. Theoretical projections for the amount of land needed to grow algae in raceway ponds and the oil yields of this operation were based on the amount of CO₂ from the ethanol plant. A practical algal productivity of 20 g m⁻² d⁻¹ would require over 2,000 acres of ponds for complete CO₂ abatement, but with an aggressive productivity of 40–60 g m⁻² d⁻¹, a significant portion of the CO₂ could be consumed using less than 1,000 acres. Due to the cold temperatures in Iowa, a greenhouse covering and a method to recover waste heat from the biorefinery were devised. While an algal strain, such as Chlorella vulgaris, would be able to withstand some temperature fluctuations, it was concluded that this process is limited by the amount of available heat, which could maintain only 41 acres at 73 °F. Additional heating requirements result in a cost of 10–40 USD per gallon of algal oil, which is prohibitively expensive for biodiesel production, but could be profitable with the incorporation of high-value algal coproducts.     

The production of acetic acid from microalgae under hydrothermal conditions

Microalgae for carbon dioxide mitigation was applied to the production of acetic acid under hydrothermal conditions with H₂O₂ oxidant. Results showed that acetic acid was obtained with a good yield of 14.9% based on a carbon base at 300 °C for 80 s with 100% H₂O₂ supply. This result should be helpful to facilitate studies for developing a new green and sustainable process in order to produce acetic acid from microalgae, which are the fastest growing sunlight-driven cell factories. These results show that it is possible to develop a process for conversion of microalgae biomass into acetic acid.     

Biomass and lipid productivities of marine microalgae isolated from the Persian Gulf and the Qeshm Island

In this work, the screening of 147 microalgal strains from the Persian Gulf and the Qeshm Island (Iran) were done in order to choose the best ones, in terms of growth (biomass) rate and lipid content for biodiesel production. A methodology, combining experiments in lab-scale and pilot plant (open pond) used to produce and evaluate biomass and lipid productivity is presented for the systematic investigation of the potential of different microalgae species. The culture conditions, including photo flux (180 ??E m⁻² s⁻¹), photoperiod (12 h light/dark), temperature (25 °C), pH (≈8), air (carbon dioxide) and growth medium, were kept constant for all experiments. Microalgae were screened in two stages using optical density (for evaluation of biomass concentration) and Nile red and gas chromatography (for determination of lipid content and fatty acid fractions). In general, maximum specific growth rate and the maximum biomass productivity were obtained after 8-12-day culture. Nannochloropsis sp. and Neochloris sp. were selected from the marine microalgal culture collection, due to their high biomass (50 and 21.7 g L⁻¹, respectively) and oil content (52% and 46%, respectively). If the purpose is to produce biodiesel only from one species, Nannochloropsis sp. presented the most adequate fatty acid profile, namely linolenic and other polyunsaturated fatty acids. However, the microalgae Chlorella sp. can also be used if associated with other microalgal oils. In addition, selected strains could be potent candidates for commercial production in the open pond culture.     

Characterization and identification of lipid-producing microalgae species isolated from a freshwater lake

Microalgal lipids are the oils of the future for sustainable biodiesel production. One of the most important decisions in obtaining oil from microalgae is the choice of species. A total of 45 algal cultures were isolated from a freshwater lake at Wonju, South Korea. Five microalgal isolates were selected based on their morphology and ease of cultivation under our test conditions. These cultures were identified as strains of Scenedesmus obliquus YSL02, Chlamydomonas pitschmannii YSL03, Chlorella vulgaris YSL04, S. obliquus YSL05, and Chlamydomonas mexicana YSL07 based on microscopic examination and LSU rDNA (D1-D2) sequence analysis. S. obliquus YSL02 reached a higher biomass concentration (1.84 ± 0.30 g L⁻¹) with a lower lipid content (29% w/w), than did Chla. pitschmannii YSL03 (maximum biomass concentration of 1.04 ± 0.09 with a 51% lipid content). Our results suggest that Chla. pitschmannii YSL03 is appropriate for producing biodiesel based on its high lipid content and oleic acid proportion.     

Free lipase-catalyzed biodiesel production from phospholipids-containing oils

Free lipase-catalyzed biodiesel has drawn more and more attentions in recent years because of its advantages of lower cost and faster reaction rate. Utilizing free lipase to convert low quality oils such as crude vegetable oils and microbial oils is beneficial to further reduce the cost of biodiesel production. However, these oils typically contain some amount of phospholipids. Phospholipids were found to affect the lipase-catalyzed process and further influence the enzyme's thermal stability in biodiesel production process. In this work, free lipase NS81006-mediated biodiesel production from oils containing phospholipids at varied temperature was investigated systematically. It was found that the presence of phospholipids at high temperature led to a decreased fatty acid methyl esters (FAME) yield and poor reuse stability of the lipase during NS81006-catalyzed biodiesel production process. The higher the temperature was, the greater negative effect was observed. This inhibitory effect was found to be mainly caused by the coexistence of phospholipids and methanol in the system. Based on this finding, a novel two-step enzyme-mediated process was further developed, with which the above-mentioned inhibitory effect was eliminated, and a FAME yield of 95.1% could be obtained with oils containing 10% phospholipids even at high temperature of 55 °C.