Third-Generation Biodiesel: Different Production Processes

Abstract

The production of biodiesel from microalgae faces several problems to be solved, among them is the necessity of increasing their lipid content, optimizing the harvesting, and improving the conversion of lipids to bioenergy, therefore reducing the energy cost of the production process prior to its commercial launch. Research focused on optimizing the biodiesel production process known as transesterification has various objectives such as eliminating the biomass drying stage, unifying the extraction and transesterification stages, improving the reaction yield using supercritical conditions, providing heating with microwave and ultrasonic radiation, reusing enzymatic and heterogeneous catalysts, among others. This chapter aims to summarize the advances that have been achieved with the various operating conditions for the in situ, direct, and supercritical oil transesterification process of microalgae from the genera Chlorella, Scenedesmus, Spirulina, and Nannochloropsis.

1. Introduction

Research and development on microalgae as a feedstock for third-generation biofuel have focused on increasing their production, this is due to their ability to synthesize and accumulate lipids [1]. The main interest of these studies is their doubling rate, which varies from 3.5 to 24 h, resulting in a high growth rate and high biomass yield in short periods [1, 2, 3].

The overall success of biodiesel production from microalgae begins with the selection of the right strain, which must exhibit optimal biomass production performance, as well as a high accumulation of triglycerides, and must adapt to extreme environments [4].

Obtaining lipids from microalgae begins with their growth, followed by harvesting, filtration, drying, and, finally, oil extraction [5]. Microalgal biomass production cost is estimated at $5.8 USD/kg, with an energy requirement of 33 MJ/kg dry biomass for lipid recovery [6].

Biodiesel has similar physicochemical properties to diesel, making it suitable for use in compression-ignition internal combustion engines; it is also considered to have a net contribution of carbon to the environment, high flash point, complete combustion with low sulfur emissions, and high cetane number [7]. Emissions from its combustion depend on the percentage used and the oil used for its combustion [8]. In addition, its calorific value is lower than that of diesel, being 39.45 MJ/kg versus 44.24 G/kg for diesel [9]. The biodiesel obtained from castor oil has an energy of 26.03 MJ/kg [10].

To optimize the production of biodiesel by means of transesterification, researchers have used different reaction conditions, as explained below.

2. Microalgae

Microalgae are unicellular photosynthetic eukaryotic microorganisms that inhabit freshwater and brackish water aquatic environments [11]. They produce fatty acids as precursors for the synthesis of several types of lipids, which vary between species: polar and neutral lipids, waxes, sterols, phospholipids, glycolipids, carotenoids, terpenes, carotenoids, tocopherols, and quinones, for the synthesis of proteins and cell growth [12], of which only neutral lipids are suitable for biodiesel production.

Chemical stimuli such as nutrient deficiency (nitrogen, phosphorus, sulfur, and silicon), salinity, and pH, as well as physical stimuli such as temperature and light intensity, influence the composition of lipids and fatty acids contained in microalgae [13, 14]. The C:N ratio is the main factor responsible for the synthesis and accumulation of intracellular lipids [15]; both elements are important in the photosynthetic metabolic pathway of microalgae [16]. Several strains of microalgae have been studied in different culture media, with the intention of increasing biomass productivity or lipid productivity; however, these studies have not focused on the utilization of sunlight and contaminated natural waters.

The fatty acids present in microalgae include linear molecules with a length of 12 to 22 carbon atoms in even numbers, both saturated and unsaturated. Unsaturated fatty acids vary in the number and position of double bonds in the main carbon chain, monounsaturated fatty acid contains only one double bond, while polyunsaturated fatty acids contain two or more double bonds [17]. The lipid contents of various species of microalgae have been reported to range from 11 to 63% on a dry weight basis and are listed in Table 1.

Microalgae	% Lipids	Reference
Chlamydomonas reinhardtii	21	[18]
Chlorella vulgaris	36–40	[11]
Scenedesmus sp.	40	[12]
Chlorella sp.	28–32	[13]
Scenedesmus dimorphus	31	[14]
	10.23	[15]
Chlorella minutissima	31	[16]
Scenedesmus obliquus	12–14	[18]
Nannochloropsis salina	50–55	[17]

Table 1.

Percentage weight basis.

Among the fatty acids present in vegetable oils are the saturated palmitic (C16:0) and stearic (C18:0) fatty acids, the saturated fatty acids oleic (C18:1) and linoleic (C18:2), as well as the polyunsaturated fatty acids linolenic (C18:3), eicosapentaenoic (C20:5) and docohexanoic (C22:6) [19, 20, 21]. Microalgae with a high percentage of fatty acids, mainly those containing acids from C16 to C18, are suitable for the production of biodiesel [22].

Scenedesmus dimorphus contains fatty acids mainly from C16 to C18 and can also be obtained from C20 to C22 depending on the culture conditions. In BG-11 medium, it can accumulate from C14 to C20, the ones found in higher percentage were palmitic acid at 24.23%, linolenic acid at 44.01%, hexacosanoic acid (C26:0) at 19.12%, while when cultured in lactic acid-rich wastewater the lipid profile deviates to the accumulation of palmitic acid with 25.01% and oleic acid with 53.24% [23]. In another research where it is cultivated in municipal wastewater, the lipids obtained range from C15 to C18, predominantly palmitic acid with 21.6%, oleic acid with 23.3%, and linolenic acid with 24.01%, when the biomass is subjected to salt stress with NaCl at 5% P/V, the lipid profile changes, increasing the percentage of palmitic acid to 40.5% and stearic acid from 2.5 to 11.2%, while linolenic acid decreases to 4.16% [24]. In other words, the lipids of Scenedesmus dimorphus include palmitic, oleic, and linolenic acid in different percentages depending on the growth metabolism.

A comparative study of biomass and lipid content of Chlorella vulgaris reported higher biomass productivity with the use of mixotrophic growth (53%, 3 g/L) in comparison to autotrophic (20%, 0.8 g/L) and heterotrophic (16%, 0.2 g/L) growth, highlighting that microalgae can use both organic and inorganic carbon, nevertheless, in both cases, light is required for growth [25]. In another experiment, it was reported that Chlorella strains are capable of bio-sequestration of heavy metals such as Hg, Cd, Pb, Au, Ag, Cu, Cr (VI), and Ni [26]. The lipid profile of Chlorella vulgaris contains saturated and unsaturated fatty acids, specifically palmitic acid, linoleic acid, oleic acid, and stearic and linolenic acids. Generally, the acids present range from C16 to C18, with a higher percentage of palmitic acid [27, 28, 29, 30].

Chlorella minutissima presented 57.1% palmitic acid (C16:0), 5.92% stearic acid (C18:0), 26.3% oleic acid (C18:1) when grown in Guillard f/2 medium under 150 Lux illumination [31]. Meanwhile, Scenedesmus sp stores 25.35% of palmitic acid and 74.67% of oleic acid [32]. Finally, Spirulina platensis has been shown to store palmitic acid and oleic acid [33].

Different species of green microalgae and their growth metabolisms vary in lipid contents; however, it is observed that they tend to accumulate palmitic and oleic acid as main sources.

3. Conversion of biomass to bioenergy

Biodiesel is a chemically long-chain fatty acid monoalkyl ester, one of the processes to obtain it is transesterification, which consists of combining vegetable oil with alcohol, obtaining biodiesel and the value-added residue glycerol [27, 32, 34, 35].

Among the different alternatives for the production of biodiesel from microalgae as raw material, there is the conventional transesterification of oils, transesterification under supercritical operating conditions, direct transesterification of wet biomass, and in situ transesterification of dried microalgae, in addition to the effect of different types of catalysis.

Cao et al. investigated the possibility of avoiding the steps of dehydration of the biomass and extraction of its lipids by proposing a method of direct transesterification of wet biomass, observing a yield inversely proportional to the humidity, the yield of the reaction remains stable for humidities lower than 30%, the advantage of this alternative is the reduction of the energy requirements for the production of biodiesel [21]. Direct transesterification is a method that does not require dehydration of the biomass and extraction of lipids, in Table 2 it is observed that direct transesterification has been studied in supercritical conditions, in these conditions a catalyst is not required for the reaction, the reason for which the product does not require purification.

Microalgae	Operating conditions	Performance	Reference
Transesterification under supercritical conditions
Nannochloropsis	Algae moisture/methanol (W/V) 1:8, 250°C, 25 min.	84.16%	[36]
Nannochloropsis sp	Wet algae/ethanol (W/V) 1:9, 25 min, 265°C, 80 Bar	30.9%	[17]
Chlorella sp	20 gr algae mesh 40, 2 L/min CO2, 60°C, 20 MPa	63.7%	[37]
Schizochytrium limacinum	Biomass 1.6% moisture: methanol 1:6, 210°C, 7.6 MPa, 60 min.	84%	[38]
Chlorella prothothecoides	Microalgae oil: methanol 1:19, 320°C, 152 bar, 31 min	90.8%	[39]
	Microalgae oil: ethanol 1:33, 340°C, 170 bar, 35 min	87.8%
Chlorella sp	Biomass 75% moisture: methanol 1:10 P/V, 265°C, 50 min	42.62%	[40]
Nannochloropsis oculata	Biomass 75% moisture: methanol 1:10 P/V, 265°C, 50 min	21.67%
Spirulina platensis	Biomass: methanol 1gr:8 ml, cosolvent hexane 4 ml: gr biomass 40% moisture, 244.8 °C, 6.61 MPa	99.32%	[41]
Chlorella	Microalgae oil: methanol 1:30, 270°C, 8.1 MPa, 30 min	97.1%	[42]
Spirulina platensis	Microalgae oil: methanol 1:40, 0.003 g CO2/gr methanol, 300°C, 20 MPa, 30 min	72%	[43]
	Microalgae oil: ethanol 1:40, 0.001 g CO2/gr methanol, 300°C, 20 MPa, 30 min	68%
Schizochytrium limacinum	Biomass: methanol 1:10 P/V, 260°C, 20 MPa, 20 min	61.4%	[44]
	Biomass: methanol 1:10 P/V, 270°C, 20 MPa, 40 min	>90%
	Biomass: dimethyl carbonate 1:10 P/V, 270°C, 20 MPa, 30 min	>50%
	Biomass: methyl acetate 1:10 P/V, 270°C, 20 MPa, 30 min	>40%
Schizochytrium limacinum	Biomass: etanol 1:10 P/V, 320°C, 20 MPa, 50 min	>95%	[45]
	Biomass: etanol 1:10 P/V, 380°C, 20 MPa, 60 min	>60%
Direct transesterification
Chlorella vulgaris	200 mg biomass with 5.3% moisture:3 ml methanol, H2SO4 (mol:80.8 mol), 60°C, 120 min, 200 rpm	94.6%	[23]
	200 mg biomass with 5.3% moisture:3 ml methanol, KOH (mol:19.9 mol), 60°C, 120 min, 200 rpm	70.5%
Chlorella vulgaris	Biomass at 86–91% moisture: methanol 1:66.9, lipase Burkholderia 2028 U/g, 45°C, hexane: methanol 1:65, 500 rpm, 48 h	58.21%	[46]
Chlorella vulgaris	Biomas moisture 40.6%, Biomass: methanol 1:9, biomass: IL (Phosphonium carboxylate) 1:8 W, 102.4°C, 4.6 h	98.6 ± 1.82%	[47]
In situ transesterification
Chlorella sp.	Biomass: methanol 100 mg:12 ml, H2 SO4 3%, 65°C, 2 h	45.26%	[19]
	Biomass: methanol 100 mg:12 ml, H2 SO4 4%, 65°C, 2 h	53.19%
	Biomass: methanol 100 mg:12 ml, H2 SO4 3.5%, 65°C, 2 h	60.71%
	Biomass: methanol 100 mg:12 ml, H3 PO3 3%, 65°C, 2 h	39.12%
	Biomass: methanol 100 mg:12 ml, H2 SO4 3% (E1), NaOH 1%W (E2), 65°C, 2 h	43.92%
Spirulina platensis	Biomass: methanol 1gr:5 ml, NaOH 0.5 gr/gr, 60°C, 3 h	30%	[20]
	Biomass: methanol 1 g:5 ml, H2 SO4, 1 gr/gr, 60°C, 3 h	79.5%
	Biomass: methanol 1gr:5 ml, HCl, 1 gr/gr, 60°C, 3 h	58.6%
	Biomass: methanol 1 g:5 ml, HNO3 1 g/gr, 60°C, 3 h	40.3%
	Biomass: methanol 1 g:5 ml, H3 PO4, 1 gr/gr, 60°C, 3 h	3.6%
Scenedesmus sp.	0.5 gr algae, 5 ml DMC1/gr microalgae, 1% W/W water/microalgae, 50°C, 36 h, 180 rpm	92%	[12]
Chlorella pyrenoidosa	4 ml methanol, 8 ml n-hexane, H2 SO4 0.5 M, 120°C, 180 min	92.5%	[21]
Nannochloropsis gaditana (11.1% saponifiable lipids LS and 75% moisture)	Methanol/LS 171.1 mg/gr, 5% V/V acetyl chloride, 100°C, 105 min, 2.5 atm, 104 ml/gr hexane	100%	[22]
Nannocloropsis sp	1gr biomass, lipid: methanol 1:200, KOH 2%W, methanol: hexane 1:0, 60°C, 2 h	49.9%	[48]
	1gr biomass, lipid: methanol 1:200, KOH 2%W, methanol: hexane 1:1, 60°C, 2 h	56.1%
	1gr biomass, lipid: methanol 1:200, KOH 2%W, methanol: hexan 1:1.5, 60°C, 2 h	48.43%
	1gr biomass, lipid: methanol 1:300, KOH 2%W, methanol: hexan 1:1.5, 60°C, 2 h	76.77%
	1gr biomass, lipid: methanol 1:400, KOH 2%W, methanol: hexan 1:1.5, 60°C, 2 h	78.87%
	1gr biomass, lipid: methanol 1:400, KOH 2%W, methanol: hexane 1:1.5, 60°C, 4 h	90.95%
Spirulina sp	Biomass: methanol 50gr:100 ml, KOH 2%W, biomass: cosolvent 1:200 chloroform: methanol, 180 W ultrasound, 15 min	1.6% W Biomass 95.2% FAME content	[49]
Chlorella sp	Biomass: methanol 1gr:120 ml, 10%W catalyst2, 80°C, 3 h	33%	[50]
	Biomass: methanol 1gr:120 ml, 20%W catalyst2, 80°C, 3 h	47%
Chlorella vulgaris, Scenedesmus bijurjos, Euglena sp, Oscillatoria quadripuntctulata, Microcystisaeruginosa and Chlamydomonas debaryana	Biomass: methanol 1gr:12 ml, 4%W Fe2O3-Al2O3, 6 h	95.6%	[51]
Chlorella pyrenoidosa	1 gr biomass, methanol: CPME3: HCl 10:1.5:1, 80°C, 380 rpm, 150 min	92%	[52]
	1 gr biomass, methanol: 2-MeTHF4: HCl 10:1.5:1, 80°C, 380 rpm, 150 min	91%
	1 gr biomass, methanol: chloroform: HCl 10:1.5:1, 80°C, 380 rpm, 150 min	76%
Microwave-assisted transesterification
Nannochloropsis	Dry biomass/methanol (W/V) 1:12, KOH 2%W, 6 min, 60–64°C	80.1%	[36]
Chlorella pyrenoidosa	Wet biomass (77%): methanol+ H2SO4 3% V, (2gr:6 ml), 70°C, 30 min.	50%	[24]
	Wet biomass (77%): ethanol+ H2SO4 3% V, (2gr:6 ml), 70°C, 30 min.	80.6%
	Wet biomass (77%): isopropanol+ H2SO4 3% V, (2gr:6 ml), 70°C, 30 min.	85.7%
Transesterification of oils with homogeneous catalyst
Chlorella vulgaris	Methanol: oil 6:1, NaOH 1%, 55°C, 15 min.	>98%	[25]
Tetraselmis sp	Methanol: oil 6:1, 0.5 g NaOH, 65°C, 300 rpm, 3 h	Not specified	[26]
Chlorella vulgaris	Methanol: oil 10:1, 1.5% H2SO4 V/V, 60°C, 40 min (E1); Methanol: oil 10:1, KOH 1.5%, 60°C, 15 min microwave 700 W (E2)	84.01%	[28]
	Methanol: oil 10:1, 1.5% H2 SO4 V/V, 60°C, 40 min (E1); Methanol: oil 10:1, KOH 1.5%, 60°C, 3 h (E2)	83.23%
Chlorella vulgaris	38 mg oil:3 ml methanol, H2SO4 (mol:50.7 mol), 60°C, 120 min, 200 rpm	96.6%	[23]
	38 mg oil:3 ml methanol, KOH (mol:2.5 mol), 60°C, 120 min, 200 rpm	90.4%
Chlorella sp	Methanol: oil 30:1, H2SO4 10%P, 4 h	87.5%	[53]
	Ethanol: oil 30:1, H2SO4 10%P, 4 h	75.4%
Transesterification of oils with heterogeneous catalyst
Nannochloropsis oculata	Methanol: oil 6:1, 80% CaO/Al2O3 2% W/W, 50°C, 4 h, 1100 rpm	23	[29]
	Methanol: oil 30:1, 80% CaO/Al2O3 2% W/W, 50°C, 4 h, 1100 rpm	97.5%
Tetraselmis sp	Isooctane: oil 5 ml:10 ml, 1gr immobilized Candida rugosa, 60 ml methanol, 40°C, 180 rpm, 10 h	Not specified	[26]
Nannochloropsis oculata	Methanol: oil 15:1, 3.5% W/W ZnO: Mn^+2 encapsulated in PEG5, 60°C, 4 h	87.5%	[30]
Scenedesmus sp.	Methanol: oil 11:1, goat bone nanocatalyst 2% P, 60°C, 1500 rpm, 3 h	92%	[31]
Chlorella sp., Scenedesmus sp. Synechocystiss sp., Spirulina sp.	Methanol: oil 20:1, 5% P sulfonated peanut shell biochar, 65°C, 4 h	92%	[33]
Chlorella vulgaris	Methanol: oil in hexane (5 mg: ml) 0.15 ml:0.5 ml, 8.6 mg ROL/MNP-AP-GA6, 45°C, 24 h	69.8%	[54]

Table 2.

Third-generation biodiesel production processes.

¹

Dimethylcarbonate.

²

LiOH-pumice.

³

Cyclopentyl methyl ether.

⁴

2-Methyltetrahydrofuran.

⁵

Polyethylene glycol.

⁶

Rhizopus oryzae lipase (ROL)-Superparamagnetic nanoparticles with 3-aminopropyl triethylenesilane-gutaraldehyde (MNP-AP-GA).

Sivaramakrishnan et al. investigated the direct transesterification of the microalgae Scenedesmus sp. using dimethyl carbonate (DMC) and lipase (Lipozyme-Novozyme CAL-B from Candida antarctica, Novozymes Denmark) immobilized on Celite (commercial brand of diatoms), with 1% water for 36 h at 50°C, stirring at 180 rpm obtaining a yield of 92%, before the synthesis, a process of cell disruption is carried out with ultrasound, the use of lipase in this process showed a yield of 90% up to the fifth cycle, addition, this process generates glycerol carbonate as a by-product [12]. he most common methylating agent is methanol, as a by-product of the transesterification reaction, glycerin is obtained, which is oversupplied in the market, consequently, its price has decreased, the use of different methylating agents such as dimethyl carbonate, and methyl acetate produced as a by-product glycerol dicarbonate and triacetin [44].

Another alternative method considers the use of supercritical methanol coupled to oil extraction with CO₂ under supercritical conditions, for the case of biodiesel production from Chlorella sp. the advantage of this process is the utilization of the extraction residues in the production of drugs because carbohydrates, trace elements, and minerals are not damaged in the process and can be easily separated from CO₂ by decompression [37]. Supercritical transesterification of microalgae oils offers better yields than when using dry or wet biomass, on the other hand, the use of cosolvents improves the process.

Patil et al. compared the yield of a simultaneous microwave-assisted extraction and transesterification reaction under supercritical conditions of the microalgae Nannocloropsis obtaining yields of 80.13 and 84.15% respectively, they conclude that microwave radiation is more effective in cell lysis, due to that the reaction time is shorter, the process in supercritical conditions consumes 600 KJ while the assisted transesterification consumes only 254.5 KJ, that is, this alternative is an easy, fast and efficient process with a lower energy requirement [36].

Biodiesel is influenced by the physicochemical characteristics of the feedstock from which it is produced. Because of this relationship, microalgae oil should contain long-chain fatty acids with a low degree of unsaturation, preferably palmitoleic (16:1), oleic (18:1), and myristic (14:1) acids, thus allowing the reduction of toxic emissions, improving the cetane number and oxidative stability, without affecting lubricity, viscosity, and cloud point. Common methyl esters in biodiesel are methyl palmitate, methyl stearate, methyl oleate, methyl linoleate, methyl linolenate, and methyl laureate [55].

4. Conclusions

The production of third-generation biodiesel has focused on the variation of operating conditions to increase yield. Direct transesterification has the advantage of converting wet biomass directly avoiding the drying and extraction stages, as well as transesterification under supercritical conditions, another advantage of the latter is that it does not require the use of a catalyst, consequently, the biodiesel does not need purification. Both processes have yields inversely proportional to the amount of moisture in the biomass, the use of a cosolvent increases the reaction yield.

Microwave and ultrasound-assisted transesterification have yields greater than 75% in shorter reaction times, which is attributed to the higher mass transfer. The transesterification of oils with homogeneous and heterogeneous catalysis presents yields higher than 87%, the advantage of heterogeneous catalysts is their reusability, easy recovery, and elimination of the biodiesel purification stage.

Among the variables that affect the reaction yield is that a high temperature favors mass transfer, and an excess of alcohol favors the formation of products, however, it must be optimized to avoid yield losses due to the dissolution of biodiesel in alcohol, likewise an excess of catalyst can reduce the reaction yield.

Acknowledgments

We are grateful for the support of the National Council of Science and Technology (CONACYT), for the scholarship granted to Sheila Genoveva Pérez Bravo to study the Doctorate in Engineering Sciences within the doctoral program of the Instituto Tecnológico de Ciudad Madero, included in the PNPC, under agreement 21542.

Conflict of interest

The authors declare no conflict of interest.