Perspectives and advances of microalgal biodiesel production with supercritical fluid technology

Abstract

Biodiesel, a sustainable and clean energy source, has been greatly attracting interest to compete against serious challenges like energy crisis and environmental pollution. Microalgae are currently promoted as a biodiesel feedstock with the most potential and advantages of high lipid content and productivity. This paper provides an overview on a selection of microalgal strains, supercritical carbon dioxide (SCCO₂) extraction of microalgal lipids and the advances of microalgae oils transesterification for producing biodiesel with supercritical alcohols. In particular, a two-step process of microalgal biodiesel production using supercritical technology and the following SCCO₂ extraction are generalized in this study. Considering the commercialization of microalgal biodiesel in the future, the cost of microalgal biodiesel published in recent literature is analysed. Furthermore, feasible strategies for improvement are proposed. The overall economic efficiency of microalgal biodiesel industry can be improved by the multi-effect co-production coupling technology.

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Dan Zeng

Dan Zeng, education and positions: (1) Bachelor's degree, Xian Jiaotong University, Xi'an, Shanxi, P. R. China, 2007–2011. (2) Master's degree, Xi'an Jiaotong University, Xi'an, Shanxi, P. R. China, 2011–2012. (3) Ph.D. degree, Xi'an Jiaotong University, Xi'an, Shanxi, P. R. China, 2012–now. Interests and projects: (1) to study biodiesel production with supercritical fluid technology. (2) Fundamental research on thermodynamics of biodiesel.

Ruosong Li

Ruosong Li, education and positions: (1) Bachelor's degree, Xian Jiaotong University, Xi'an, Shanxi, P. R. China, 2007–2011. (2) Master's degree, Xi'an Jiaotong University, Xi'an, Shanxi, P. R. China, 2011–2012. (3) Ph.D. degree, Xi'an Jiaotong University, Xi'an, Shanxi, P. R. China, 2012–now. Interests and projects: (1) to study the supercritical fluids in polymer processes. (2) Physical properties of supercritical fluids and polymers.

Ting Yan

Ting Yan, education and positions: (1) Bachelor's degree, Northwest University, Xi'an, Shanxi, P. R. China, 2007–2011. (2) Master's degree, Xi'an Jiaotong University, Xi'an, Shanxi, P. R. China, 2012–2013. (3) Ph.D. degree, Xi'an Jiaotong University, Xi'an, Shanxi, P. R. China, 2013–now. Interests and projects: (1) to optimize conditions upgrading of coal tar using supercritical technology. (2) To study the mechanism of supercritical methanol related to upgrading coal tar.

Tao Fang

Tao Fang Positions: (1) Research Associate, Department of Applied Chemistry and Biochemistry, Kumamoto University, Japan, April, 2007–January 2008. (2) Alexander von Humboldtian (Guest Scientist), Institute of Separation Science and Technology, University of Erlangen-Nuremberg, Germany, March 2008–June 2009. (3) Professor, Department of Chemical Engineering, School of Chemical Engineering &Technology, Xi'an Jiaotong University, Xi'an, P. R. China, July 2009–now. Interests and Projects: (1) processing of natural materials. (2) Updating biomass technology. (3) Supercritical fluids in combination with high electric field. (4) Fundamental research on phase equilibrium. (5) Novel hydrogen storage technology with new organic liquids.

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1. Introduction

Recently, as the price of crude oil increases, the supply of fossil oil becomes limited, and the environmental consequences of exhaust gases from petroleum-fuelled engines persist, alternative renewable fuels research has been attracting global attention. Biodiesel, produced from renewable resources, has been found to have the most potential as a diesel oil substitute due to its great molecular similarities to paraffinic diesel fuel compounds.¹ This fuel is renewable, biodegradable, non-toxic, and it generates lower emissions in comparison to petroleum-based diesel.² According to the Mid- and Long-term Development Plan for Renewable Energy in China, the consumption of biodiesel in China will reach 2.0 million tons in 2020.³ Usage of biodiesel will allow a balance to be sought between agriculture, economic development and the environment.

It has been found that feedstock alone represents more than 75% of the overall biodiesel production cost. Therefore, selecting the appropriate feedstock is of significance to lower the production cost.⁴ The four main categories of raw materials for biodiesel are edible vegetable oil, non-edible vegetable oil, waste or recycled oil and animal fats.⁵ Edible vegetable oils that include soybeans, palm oil, sunflower, safflower, rapeseed, coconut and peanut are considered as the first generation of biodiesel feedstocks because they were the first crops used for biodiesel production.^6,7

Last decade, the majority of biodiesel production in the world used edible vegetable oils, such as in Germany, USA, Malaysia and Brazil.^8–10 However, the dramatically increasing prices of these oils raised a food versus fuel crisis, leading us turn attention to non-edible oils, which are more efficient and economical. Above all, the usage of non-edible oils eliminated the competition between food and fuels, revitalizing the long-term development of the biodiesel production industry. Waste or recycled oil and animal fats, which are like the non-edible oils have been reported as the second generation feedstock oils in literature.^11–16 While the industrialized application of these feedstocks still faces some challenges like the pre-treatment of the high amount of saturated fatty acids and the collection of decentralized waste oils. Microalgae, which holds great potential for carbon neutral biofuels production are regarded as the third generation sustainable feedstock for biodiesel production because of its high lipid content and high photosynthetic efficiency.¹⁷

Brentner et al.¹⁸ divided the algal biodiesel pathways for sustainable full-scale production into five distinct process steps: (1) microalgae cultivation, (2) harvesting and/or dewatering, (3) lipid extraction, (4) conversion (transesterification) into biodiesel, and (5) by-product management. The latter three steps can be performed using corresponding supercritical fluid technology. Microalgae lipids and high-value by-products can be extracted by SCCO₂ as the primary solvent due to its low toxicity, low flammability, and lack of reactivity. Its moderate critical pressure (72.9 bar) allows for a modest compression cost, while its low critical temperature (31.1 °C) enables a successful extraction of thermally sensitive lipid fractions without degradation.¹⁹

Transesterification is the most widely used method to produce biodiesel, in which the triglycerides present in the different types of oils reacts with an alcohol to produce alkyl esters and glycerol.²⁰ The conventional transesterification reaction often proceeds with an acid, alkali metal, or enzyme catalyst.^21–25 In the catalytic process, free fatty acids (FFAs) and water always have negative effects like soap formation and catalyst consumption, reducing the effectiveness of the catalyst and resulting in a low conversion.²⁶ To overcome these problems, Saka and Kusdiana¹ studied the transesterification of the rapeseed oils with supercritical methanol and found that this new process shortened reaction times and simplified the purification procedure. The transesterification of microalgae oils via supercritical alcohols has received attention from researchers, entrepreneurs, and governments in many countries. After many investigations on transesterification of various oils in supercritical alcohols, it has been proven to be the most promising method for biodiesel production.²⁷ Among the commonly used lower alcohols, methanol is the most suitable one for the transesterification based on a previous thermodynamic analysis.²⁸

The whole process involves genetic, biological, agricultural and chemical engineering. From a view of chemical engineering, how to improve the conversion technology of microalgae oils to biodiesel and reduce the total cost is important to the development of the biodiesel industry.

This review focuses on microalgae strain selection, microalgae lipids extraction in SCCO₂ and advances of the transesterification reaction with microalgae oils in supercritical alcohols for biodiesel production. Herein, the microalgal biodiesel production and separation process using SCF technology are summarized in this study. Also, an economic analysis regarding the producing cost of microalgal biodiesel is conducted to evaluate the viability of biodiesel production with microalgae is included. The multi-effect co-production coupling technology is also stated to connect high value added functional ingredients in microalgae. Finally, proposed strategies for using microalgae oils for biodiesel production are presented.

2. Selection of microalgal strain

Microalgae are recognized as one of the oldest living micro-organisms on earth.²⁹ They grow at an exceptionally fast rate: 100 times faster than terrestrial plants and they double their biomass in less than one day.³⁰ Many species of microalgae are known for their high content of lipids in the cells, the average fatty acid contents of the algal oils are 36% oleic (18 : 1), 15% palmitic (16 : 0), 11% stearic (18 : 0), 8.4% iso-17 : 0, and 7.4% linoleic (18 : 2).³¹ They can be used for biodiesel production; some microalgae are able to accumulate a large amount of hydrocarbon, which can be made into gasoline or diesel, and in some cases, they can produce hydrogen. With the advantages of easy cultivation, fast growth, high biomass production and enriched quantities of lipids and hydrocarbon, microalgae oils are novel and important bio-energy resources. Fig. 1 summarizes the potential conversional pathways for using microalgae to produce bio-fuels, hydrogen, hydrocarbon and lipids.

Fig. 1 Potential energy conversion processes from microalgae.

Selection of adequate microalgal strains is the basis of the development of an algae-based biodiesel industry. At the present time, the microalgae use as the feedstock for biodiesel production has been studied extensively at laboratory-scale due to the high lipid content of certain strains. Besides the lipid content, the biomass and lipid productivity seem to be other two adequate criteria for evaluating the potential of various microalgae species for producing biodiesel. Table 1 presents the experimental values for the three criteria of the 25 strains from the main groups of microalgae in different cultivating conditions. Compared with the specific data of each strain, Botryococcus braunii is a green microalga that produces hydrocarbons for up to 75% of its dry biomass and its lipid productivity is relatively higher than those of most other strains. More than half of the mentioned studies focused on the chlorella and scenedesmus strains. Chlorella vulgaris and Scenedesmus obliquus are the most appropriate strains for producing biodiesel. The lipid productivity is around 130 mg L⁻¹ per day with an average biomass productivity of 550 mg L⁻¹ per day. For other strains, such as Nannochloropsis oculata, although the maximum neutral lipid content can achieve 65%, the lipid productivity is much lower than 100 mg L⁻¹ per day, which is unacceptable for industrial cultivation. As microalgal strains are used to produce triacylglycerols,⁴⁷ species selection of microalgal for biodiesel production should give priority to the lipid productivity and the lipid content.⁴⁸

Table 1 Lipid content, biomass productivity and lipid productivity of selected microalgal strains used for biodiesel production

Microalgal group	Strain	Lipid content/dwt%	Biomass productivity/mg L^−1 per day	Lipid productivity/mg L^−1 per day	Ref.
Ankistrodesmus	A. falcatus	17	340	54–58	32
	A. fusiformis	19–23	240	43–56	32
Botryococcus	B. braunii	25–75	27–250	4–124	32–35
	B. terribilis	49	200	95–102	32
Chlorella	Ch. emersonii	19–34	36–290	54	36 and 37
	Ch. minutissima	50–57	320-400	48–70	38
	Ch. protothecoides	18	250	45	37
	Ch. saccharophila	28	220	60	38
	Ch. salina	11	170	18	37
	Ch. sorokiniana	19	230	45	39
	Ch. sp.	19	230	42	39
	Ch. vulgaris	5–58	104–482	0.5–127	33, 36, 37 and 39–41
Dunaliella	D. salina	24	150	37	37
	D. sp.	20–24	120	26	37
Nannochloropsis	N. oculata	11–65	30–380	41–65	42 and 43
	N. sp.	22–31	170–210	38–61	39
Pavlova	P. salina	31	160	49	39
	P. lutheri	36	140	50	39
Scenedesmus	S. acutus	13–28	74	13	44
	S. obliquus	22–60	250–626	41-140	34 and 45
	S. obtusus	18–26	179–193	36-43	46
	S. rubescens	15	298	50	38
	S. pectinatus	23	119	27	46
	S. quadricauda	18	190	35	39
	S. sp.	19–21	210–260	41-54	39

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3. SCCO₂ extraction of microalgal lipids

Before the production of microalgal biodiesel, efficient lipid extraction from microalgae is required before the following transesterification step.

The two main kinds of extraction methods are chemical solvents extraction and supercritical fluids extraction (SFE).⁴⁹ The fundamental difference between SFE and traditional organic solvent extraction method for lipid extraction from microalgae is that there is no requirement of catalyst with SFE.⁵⁰ In addition, the chemical solvent method has some drawbacks like inherent toxicity, poor selectivity, difficult separation of the contaminants and solvents from the product, energy consumption and pollution.⁵¹ SFE has several advantages of offering mild operating conditions, a negligible environmental impact, higher selectivity, shorter extraction times, favourable mass transfer and it produces a solvent-free extract.^51,52,53

SCCO₂ extraction is currently gaining considerable attention as the promising green technology method, which can possibly replace the conventional organic solvent lipid extraction method.⁵⁴ SCCO₂ has high solvation ability and no toxicity. Intermediate diffusion/viscosity properties lead to favourable mass transfer equilibrium and this process produces solvent-free crude lipids.⁵⁵ Solvent extraction using hexane was found to be remarkably less efficient than SCCO₂ extraction, which achieved a comparable lipid yield.⁵⁶

The methods and results from recent studies investigating SCCO₂ extraction of microalgal lipids are summarized in Table 2. The corresponding significant discussions are presented as well.

Table 2 Methods and results summary of recent studies investigating SCCO₂ extraction of microalgal lipids

Microalgal species	T (°C)	P (bar)	Flow rate (g min^−1)	Duration (min)	Optimum conditions and discussion	Optimum lipid yield (dwt%)	Ref.
Scenedesmus sp.	35–65	200–500	1.38–4.02	—	500 bar, 53 °C and 1.9 g min^−1	7.41	60
Chlorococcum sp.	60, 80	300	—	80	(i) Decreasing temperature and increasing pressure resulted in increased lipid yields; (ii) the rate of lipid extraction decreased with experimental time.	7.1	54
Scenedesmus dimorphus	50–100	100–500	3.33	60	414 bar and 100 °C	9	61
Pavlova sp.	60	306	—	360	The SFE method is effective and provides higher selectivity for triglyceride extraction.	10.4–17.9	62
Crypthecodinium cohnii	40 and 50	200, 250 and 300	—	180	300 bar and 50 °C. (i) At constant pressure, temperature increase has the effect of decreasing the density of the supercritical fluid and thus its solvation capacity; (ii) increasing the temperature increased the vapour pressure of the solutes; thus, it increased their solubility in the supercritical solvent.	9	63
Nannochloropsis sp.	40, 55	400, 550, 700	170	360	(i) At constant T, lipid extraction rate increased with P; (ii) at constant P, lipid extraction rate slightly increased with T.	25.0	64
Spirulina maxima	20–70	15–180	1.998	—	150 bar and 50 °C (i) both the temperature and the pressure affected the extraction rate; (ii) the effect of temperature prevailed over that of pressure.	16.2	65

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4. Uncatalyzed microalgal biodiesel production

The viscosities of vegetable oils and microalgal oils are usually higher than those of diesel oils.⁵⁷ Hence, they cannot be directly applied to engines. The transesterification of microalgal oils dramatically reduces the oil viscosity and increases the fluidity. The transesterification of microalgae oils for biodiesel produced with different kinds of catalysts (like acid, alkali and enzyme catalysts) or SCFs is being investigated by others as an alternative.

Huang et al.⁵⁸ summarized the advantages and disadvantages of the three types of catalytic transesterification reactions. The conventional catalysis process can obtain high conversion of biodiesel with low cost and well-controlled reaction conditions, which is appropriate for large-scale production. However, the post reaction disposal process is always complex due to the presence of the catalyst. The enzymatic catalysis process requires moderate reaction conditions with a small amount of alcohol present, and it does not pollute the environment. Nonetheless, the enzymes are easily poisoned and are restricted to the conversion of short-chain fatty acids.

However, direct (or in situ) transesterification with supercritical methanol has the advantages of the minimal usage of solvents, easy separation of products, and reduced reaction time.⁵⁹ Another positive aspect of the SCF technology is that the alcohol is not only a reactant but it also acts as an acid catalyst.

Furthermore, the SCF method requires no pre-treatment of microalgae because impurities in the feedstocks do not significantly affect the reaction. The three types of reactions (transesterification, hydrolysis of triglycerides, and alkyl esterification of fatty acids) simultaneously occur.⁶⁶

With the above-mentioned advantages, the catalyst-free biodiesel synthesis process using SCF technology is expected to replace catalytic production methods, especially for microalgae, which contains a proportion of water and FFAs. Bello et al.¹⁷ technically compared two transesterification processes for biodiesel production from microalgae (Chlorella protothecoides). The energy efficiency of the supercritical transesterification process was reported as 52.85% with most energy (75.55%) used in the separation step. The alkali-catalytic process had 49.67% energy efficiency with 35.25% used in the product purification step. The analysis data suggests that the supercritical transesterification method has higher energy efficiency and a slightly lower the unit price than that of biodiesel in comparison to the alkali-catalytic process.

Patil et al.^67–70 performed a series of transesterification reactions to convert wet or dry algae biomass to biodiesel using supercritical methanol or ethanol. The latest proposed integrated approach converts dry algae (Nannochloropsis sauna) into FAEEs via non-catalytic transesterification under microwave-mediated supercritical ethanol (MW-SCE) conditions.⁷⁰ Lipids extraction from algal biomass and transesterification of them simultaneously proceeds to convert into biodiesel in a relatively short reaction time. Energy consumption is reversed versus traditional processes by simplifying the separation and purification steps. High conversion rates are available when the extractive transesterification of the algal biomass is performed in subcritical or supercritical conditions. It was demonstrated that the direct transesterification with SCFs provides an energy-efficient and economical route for algal biodiesel production.

Cao et al.⁷¹ studied the biodiesel prepared with large-scale aquaculture microalgae and also analysed the oil quality. The results indicated that the output rate of the biodiesel prepared with Chlorella was evidently higher than that of other types of algae. The supercritical reaction condition almost doubled the output rate of biodiesel and the oil quality had nearly the same carbon/hydrogen mass ratio, density and calorific value with no. 0 diesel.

Interestingly, Tsigie et al.⁷² developed a direct process for biodiesel production from wet Chlorella vulgaris biomass (80% moisture content) using subcritical water as catalyst. The yield of FAMEs achieved 89.71% at the ratio of wet biomass to methanol 1/4 (g mL⁻¹), 175 °C and 4 h. The yield is 0.29 g FAMEs per g dry biomass. This is considerably higher than the yield of 0.20 g FAMEs per g dry biomass obtained when the neutral lipid of Chlorella vulgaris biomass was extracted and converted into FAMEs.

Compared with direct transesterification in a single step, the two-step process suggested by Kusdiana and Saka has more advantages.⁷³ The first step is the hydrolysis of triglyceride in subcritical water to produce FFAs. The second step is the subsequent alkyl esterification of FFAs in supercritical alcohol to produce biodiesel. The rate of alkyl esterification is higher than that of transesterification. Besides, the alkyl esterification ensures that all FFAs in microalgae, whether present originally or as products of hydrolysis, are completely transformed into FAMEs.⁷⁴ Furthermore, it can be accomplished at lower temperatures and pressures, which may reduce the cost of production and the ratio of alcohol to oil.⁷⁵ Before the second step, the sole by-product glycerol from the ester formation step is already removed. Therefore, the two-step process is cleaner than the direct transesterification one.⁷⁶

Levine et al.⁷⁷ developed a catalyst-free, two-step technique for biodiesel production from lipid-rich, wet algal biomass. In the first step, wet algal biomass (ca. 80% moisture) reacts in subcritical water to hydrolyze intracellular lipids and conglomerate cells into an easily filterable solid at 250 °C for 15 to 60 min. It retains the lipids and produces a sterile, nutrient-rich aqueous phase. In the second step, the wet fatty acid-rich solids undergo supercritical in situ transesterification (SC-IST/E) with ethanol to produce biodiesel in the form of fatty acid ethyl esters (FAEEs). On the basis of lipid in the hydrolysis of solids, longer time, higher temperature, and more ethanol increases the crude biodiesel yield that ranges from 56–100% and FAEE yields 34–66%. A considerable benefit of the process described herein is the ability of hydrolysis to create two sterile products: relatively low moisture (<50% water), FA-rich solid and a nutrient-rich aqueous phase. They both may be amenable to a variety of downstream processes. In the subsequent work, Levine et al.⁷⁸ focused on the production of biodiesel from wet, lipid-rich algal biomass using a two-step process that involves hydrothermal carbonization (HTC) and supercritical in situ transesterification (SC-IST). SC-IST has superiority of the reduced costs since it does not require catalysts and generally has a higher tolerance for feedstocks that contain water and FFAs. Reaction temperatures above 275 °C resulted in substantial thermal decomposition of unsaturated FAEE, thereby reducing yields. At 275 °C, time and ethanol facilitate the FAEE yield, while increasing reaction water content and pressure reduced yields. The results indicated that overall FAEE yields from partially dried hydrochars obtained 89% with 20 : 1 EtOH/FA molar ratio at 275 °C for 180 min. This work demonstrates that nearly all lipids within algal hydrochars are available to be converted into biodiesel through SC-IST with only a small excess of alcohol and water tolerance, at a small cost savings.

Savage et al.⁷⁹ proposed a method to produce biodiesel from a wet biomass that includes water and biomass solids. The method minimizes biodiesel synthesis time and minimizes prevalence of heteroatoms in the biodiesel by transesterifying the hydrolyzed lipid component to form biodiesel.

Research on transesterification of microalgae with SCFs has begun in recent years. There are still many topics that need to be explored. Because the reaction conditions for esterification are more moderate than those for transesterification with supercritical methanol. This new process is especially suitable for the low price materials (i.e. microalgae oil, waste oil and oil by-products) with high water or FFAs content, the two step process of combining hydrolysis and esterification is considered as a more promising technology for microalgal biodiesel production.

We conclude the production of microalgal biodiesel by a two-step SCF process and the following downstream step in Fig. 2. In the first step, the disrupted microalgae by high-pressure slush pump react in subcritical water to release the intracellular lipids and hydrolyze them into fatty acids, producing the by-product glycerol. The fatty acids rise to the top of the gravity clarifier for the next reaction step. The water dissolves the glycerol is in the middle and the cell debris settles to the bottom and can be discharged from the gravity clarifier. In the second step, the fatty acids are preheated and esterified with supercritical methanol to produce FAMEs. Then, after the products are washed with hot water, the unreacted methanol is rectified from the top of the rectification column and then recycled for the reaction step. The upper lipid phase materials containing FAMEs and other oil soluble fractions are mixed with the SCCO₂ to transfer into the extraction column. After the further step of fractionation with SCCO₂, FAMEs are collected from the top of the column through valve 40 and the high value by-product pigments are removed from the bottom through valve 43. Three parts of the microalgae production process with corresponding SCFs are efficient and environment friendly, which need no more separation steps and maintains the maximum amount of value-added components.

Fig. 2 Global scheme of the microalgal biodiesel production by supercritical fluid technology 1-microalgae; 2-high-pressure slush pump; 3-fresh water; 4,15, 17, 23, 31-delivery pump; 5-subcritical water reactor; 6, 8, 9, 18, 21, 24, 26, 32, 40, 43, 45, 50-line valve; 7-gravity clarifier; 10, 33-mixer; 11, 34-pre-heater; 12-supercritical methanol reactor; 13, 36, 38, 41-back-pressure regulator; 14-methanol; 16-methanol recovery tank; 19-water washing; 20-crude biodiesel and other oil soluble fractions; 22,44-flowmeter; 25-CO₂ gas cylinder; 27,46-check valve; 28-filter; 29,48-cooler; 30-CO₂ storage tank; 35-extraction column; 37-fractionation column; 39,42-separator; 47-rectification column; 49-reboiler.

5. Economics of microalgal biodiesel production

Although many challenges remain in microalgal biodiesel production, a growing number of researchers committed to believe that the rewards would eventually outweigh the risks. Research on the microalgal biodiesel production dates back to the energy crisis in 1970s.⁸⁰ Every year, more than 7.5 × 10⁶ tons of algae are harvested, representing a world market of US$ 6 × 10⁹ per year. The main bottleneck for the production of biodiesel from microalgae is the economic viability for developing and establishing this technology at an industrial level.⁵² The main two factors contributing to the cost of biodiesel are algal productivity and processing technology, indicating that research should focus on various aspects of algal biology, which have the greatest impact upon growth rate and lipid biosynthesis.⁸¹

Recently, there have been many studies on the economics of algal production in open ponds (OP), photobioreactor (PBR) systems and some evaluation of hybrid systems combining the use of both OP and PBR systems. Detailed economic analysis on the costs for the production of microalgal biodiesel has also been reported. Table 3 summarizes the comparison between the costs for microalgal biodiesel production by OP, PBR and Hybrid systems on the basis of the published data in recent years. It is assumed that the density of microalgal biodiesel is 0.85 g cm⁻³. The calculated cost data from Table 3 indicate that the total cost of lipid production in the OP systems is lower than that of in the PBR systems. If the price of crude oil rises to $80/barrel as predicted, then microalgal oil costing $87/barrel is possible to economically substitute for crude petroleum.⁸² Therefore, since the oil price has been recently around US $100 per barrel, microalgal oil can in theory already be economically produced.

Table 3 Comparison between the costs for microalgal biodiesel production according to the published data in recent years

	Lipids content/dwt%	Productivity^a/( g m^−2 per day)	Lipids production cost/$ per gal	Cost of biodiesel ($ per ton)			Ref.
				OP	PBR	Hybrid
a Productivity is on an areal basis (g m^−2 per day) for open ponds and a volumetric basis (kg m^−3 per day) for PBRs.
Benemann and Oswald (1996)	50	30	—	541–671	—	—	92
	50	60	—	376–400	—
Chisti (2007)	30	N/A	—	3294	3776	—	82
	30–70	35	—	8024–16 471	—	—
	30–70	1.535 (kg m^−3 per day)	—	—	6600–13 176	—
Huntley and Redalje (2007)	40	18.5	—	—	—	280	93
	40	60	—	—	—	910
Pienkos (2008)	25	20	—	718	—	—	94
	50	40	—	1082	—	—
	60	60	—	3259		—
Pienkos & Darzins (2009)	15	10	25	7500	—	—	95
	25	25	7.5	2250	—	—
	50	50	2.5	750	—	—
Davis et al. (2011)	25	25	8.52	3058	—	—	84
	—	1.25 (kg m^−3 per day)	18.10	—	6380	—
Sun et al. (2011)	25–60 (OP)	20–60	2.4–10.6	—	—	—	96
	35–60 (OP)	35–58	9.7–38.7	—	—	—
	16–47 (Hybrid)	25–40	0.9–31.8	—	—	—
Amer et al. (2011)	50	24	24.1	4176			97
Delrue et al. (2012)	20–50	20–30	10.2	3412			98
	20–50	N/A	17.7		5706
	20–50	—	16.9			5482
Sawaengsak et al. (2014)	25	25	—	2500			86
	25	1.25 (kg m^−3 per day)	—		8235

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However, studies on industrial feasibility vary enormously with the precise parameters. Richardson et al.⁸³ used a multi-year, Monte Carlo financial feasibility model to estimate the costs of production and chance of economic success for large-scale algal biofuel facilities in the US Southwest. The average total costs of production for lipids were $12.73/gal and $31.61/gal for OP and PBRs, respectively. The values are higher than those ones reported by Davis et al.⁸⁴ This is due to fully accounting for financial costs and risks, including high amounts of capital expenses, which are generally excluded from techno-economic studies. It is noted that algae production is technically feasible, but it is not yet economically feasible.

Additionally, Rios et al.⁸⁵ used computational tools to model different scenarios of the harvesting, oil extraction and transesterification of microalgal biodiesel. The analysis indicated that the OP cultivation technology has encountered a bottleneck in the development of biological and engineering production; conversely, the PBR systems are more appropriate to improve the production rates and lipid content from the single selected strain without contamination.

Sawaengsak et al.⁸⁶ evaluated the economic feasibility of microalgal biodiesel production with high-value by-products in both OP and PBRs in Thailand. The life cycle cost showed that although the ω-3 fatty acid production gained higher revenue, the capital and operating cost need to be reduced by more than 50% to make the systems beneficial. Further research is necessary to find the improvement in the system to make it profitable.

Torres et al.⁸⁷ found that it is significant to formulate realistic scenarios regarding the biomass composition (i.e. ash-free, dry weight) due to the final products being very sensitive to the lipid content. Economic profitability of microalgal biodiesel still needs to be improved. Walker's study found the reliable evidence that if all of the inputs are considered, the net energy gain of liquid biofuels is either very modest or non-existent. Therefore, it leads to little or no sparing of carbon dioxide emissions.⁸⁸ Energy ratios, which range from 3.3 to 7.5 depend on various parameters, such as a real biomass productivity value, resulting biogas yield, the harvesting and extraction processes, algal cell oil yields, waste-water treatment, and fertilizer/nutrient recycling.⁸⁹

It is still too early to conclude that microalgae biodiesel production has achieved the ultimate goal and is already economically feasible. Further investigations need to be performed to optimize the current technologies in order to overcome the limitation of microalgal biodiesel industrialization. Some strategies are addressed here for some inspiration for microalgal biodiesel.

6. Feasibility strategy of microalgal biodiesel

From the comparative analysis of microalgal production and to reduce the total cost of this project, research should focus on increasing the microalgae lipid content, production and downstream processes, even with combining the exploitation of high value by-products. Some strategies are provided as follows.

(1) Sewage drained from municipal, agricultural and industrial activities potentially provide cost-effective and sustainable means of algal growth for biofuels. In addition, it is also potential for combining sewage treatment by algae, such as nutrient removal with biofuel production. Studies also have shown that microalgae could grow and efficiently remove nutrients from primary settled sewage.⁹⁰ Cai⁹¹ developed an economical method for microalgae cultivation and biodiesel refinement that efficiently treats polluted water. It reduces the emission of CO₂ and enables a clean production of biodiesel by efficiently using sewage, waste gas, dregs, waste heat, and the crude glycerin by-product.

(2) The transesterification reaction of microalgae with SCFs is more viable because it can lower greenhouse gases emissions and simplify the separation step since there is no catalyst present. Contrarily, the alkali-catalysed process has a slightly higher production cost because of the numerous unit operations and processing steps.¹⁷ Especially, the two-step process that combines hydrolysis and esterification may become a promising alternative for producing biodiesel due to its milder reaction conditions and the same tolerance as the direct transesterification for microalgal oil with high FFAs and water.

(3) An integration of the conversion of microalgal oils to biodiesel along with the extraction of high value functional ingredients can reduce the total cost of the microalgae biodiesel production industry. Currently, many industries are devoted to the cultivation of microalgae for different purposes, e.g. some industries produce microalgae biomass as a final product, others take the biomass to obtain high value by-products, such as proteins, vitamins or carotenoids.⁵² The co-production of some high value fractions and their marketing are also significant. Although there is 50% oil content in algae species, the additional 50% of the biomass remains. This biomass fraction contains valuable proteins for livestock, poultry and fish feed additives valued from $ 800 up to $ 2500 per tonne.⁹⁹ When microalgae are used for the production of high value chemicals with the by-product biomass, the overall process is expected to be economically feasible.^100–102 Some high-value by-products extracted from microalgae are presented in Table 4.

Table 4 Some high value by-products extracted from microalgae

Product group	Product	Examples (producer)	Applications	Ref.
Carotenoids	Astaxanthin	Haematococcus pluvialis	Pigments, cosmetics, pro-vitamins, pigmentation	103
	Canthaxanthin	Chlorella vulgaris		104
	Violaxanthin	Chlorella ellipsoidea		105
	β-Carotene	Dunaliella salina		104
	Lutein	Chlorella pyrenoidosa		106
Polyunsaturated fatty acids (PUFAs)	γ-Linolenic acid (GLA)	Arthrospira maxima	Food additive, nutraceuticals pharmaceuticals	104
	Eicosapentaenoic acid (EPA)	Nannochloropsis oculata		107
	Arachidonic acid (AA)	Nannochloropsis oculata		107
	Docosahexaenoic acid (DHA)	Nannochloropsis oculata		108
Vitamins	Pro-vitamin A	Dunaliella salina	Food, health, nutrition	104
	α-Tocopherol (Vitamin E)	Spirulina platensis		109
Proteins	Phycobiliproteins	Nostoc sp	Food, cosmetics, medicine	110

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7. Conclusion

As the world energy consumption is steadily increasing with the rapid promotion of global economics, the contradiction between diminishing fossil fuel supplies and increasing demand will further deteriorate. The rise of crude oil prices brings new opportunities for the development of renewable energy sources, especially biodiesel. The transesterification for microalgae conversion into biodiesel with SCFs is regarded as the most promising process for an industrial application. Although economic analysis of microalgae production indicates that the present technologies still have drawbacks, feasible strategies are illuminated here using SCF technology. Following these developments, commercial production of microalgal biodiesel, simultaneously associated with CO₂ emission reduction, waste water treatment and high-valuable products' extraction, will become competitive in comparison with other conventional sources of energy in the foreseeable future.

Acknowledgements

The authors would like to acknowledge the following financial supports: National Natural Science Foundation of China (no. 21376186), the Ministry of Education (Doctoral Special Research Foundation no. 20110201110032), China and the Fundamental Research Funds for the Central Universities (New Teacher Research Support Plan no. 08141002, International Cooperation Project no. 2011jdhz37 and Integrated Cross Project xjj2014136 in Xi'an Jiaotong University), Natural Science Basic Research Plan in Shaanxi Province of China (no. 2012JM2010), and the Ministry of Human Resources and Social Security of China (Sci. & Tech. Project for Overseas Scholars, no. 19900001).

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