Recent trends and challenges of algal biofuel conversion technologies

C H A P T E R

7 Recent trends and challenges of algal biofuel conversion technologies Pobitra Halder1,2, A.K. Azad3 1

2

Chemical and Environmental Engineering, School of Engineering, RMIT University, Melbourne, VIC, Australia; Department of Industrial and Production Engineering, Jessore University of Science and Technology, Jessore, Bangladesh; 3 School of Engineering and Technology, Central Queensland University, Melbourne, VIC, Australia

O U T L I N E 7.1 Introduction

167

7.2 Potential feedstock and their characteristics

169

7.3 Cultivation and harvesting of algal biomass

169

7.4 Algae biofuels and conversion process 7.4.1 Thermochemical conversion 7.4.2 Biochemical conversion 7.4.3 Chemical reaction

171 172 173 174

7.1 INTRODUCTION The need for clean energy as an alternative to fossil fuels throughout the world is necessary for the rapid population growth and technological development [1,2]. Currently, approximately 38% of the world’s total population (2.8 billion) have no clean cooking facilities and 14% of them (1.06 billion) have no electricity access [3]. It is also estimated that global energy needs are expected to increase by 44% from the year 2006e2030 [4]. The world’s primary energy consumption increased to 13511.2 million tons of oil equivalent (Mtoe) in the year 2017 with a growth rate of 2.2% over the previous year [5]. In the year 2017, the global carbon dioxide (CO2) emission increased by 1.6% over the previous year (33017.6 million tonnes of carbon dioxide) because of the high energy consumption rate. The transportation sector alone accounts for almost 15% of the world’s

Advanced Biofuels https://doi.org/10.1016/B978-0-08-102791-2.00007-6

7.5 Engine performance and emission characteristics using algal biofuel 174 7.6 Global algal biofuel activities

175

7.7 Challenges of biofuel production from algal biomass

175

7.8 Conclusion

176

References

176

total greenhouse gas emissions [6]. Therefore, the fast depletion rate of fossil fuels, as well as their high environmental emissions associated with climate change, is forcing the scientific community and policymakers to search for alternative and sustainable options. In this case, renewable energy can contribute to the mitigation of climate change as well as enhance future energy security, sustainability, and socioeconomic development. Over a period of time, a number of alternative energy resources have been explored to meet the increasing energy scarcity and environmental emissions. For instance, solar energy (i.e., photovoltaic solar cells and solar heat collectors), wind energy, hydropower, biomass, and geothermal resources have gained remarkable attention in recent years [7]. According to the report of the “Renewable Energy Policy Network for the 21st Century” (REN21), renewable energy contributes almost

167

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168

7. RECENT TRENDS AND CHALLENGES OF ALGAL BIOFUEL CONVERSION TECHNOLOGIES

18.2% of the global total energy consumption in the year 2016 [3]. This includes 10.4% new renewables which are mainly used for electricity generation while the remaining 7.8% is for traditional uses, for instance, cooking and heating in developing countries of the biomass. Biomass is considered as a carbon-neutral resource found abundantly on the earth, which can be converted to useful forms of energy (such as liquid, gaseous, and solid) with minimal SOx and NOx emissions compared to conventional fossil fuels. Bioenergy accounted for about 13% of the world’s total energy requirement in the year 2016 [3]. Biofuels contribute approximately 90% of the total share of renewable energy in the transportation sector. In the year 2015, biofuel production for transportation was about 2.7% of global transportation fuel and is expected to increase to 28% by the year 2050 [8]. Biofuels are found to be the most promising alternative because of the variability in forms of energy, abundance, and mitigation of environmental degradation. This has mainly triggered the urgency of searching different bioenergy feedstocks. The bioenergy feedstocks so far identified are classified into four groups, for example, first generation (1G), second generation (2G), third generation (3G), and fourth generation (4G) [9,10]. Fig. 7.1 shows the biofuel production outline of four generation feedstocks. Algal biomass (i.e., microalgae and macroalgae) is considered as the 3G biofuel feedstock which has gained substantial attention in recent times to overcome the challenges associated with 1G and 2G feedstocks [12]. Algal biomass has promising potential in the production of liquid fuels, various high-value

1G feedstocks

chemicals, animal feeds, fertilizers, as well as biogas from the residual biomass. Alternatively, algal biofuels provide a number of benefits, for example, (1) ability to grow in high CO2 concentrated environment throughout the year, (2) higher oil yield compared to other feedstocks, (3) less water consumption for growth, (4) ability to grow in wastewater and under harsh conditions, (5) no competition with cultivable land and food crops, and (6) lower recalcitrance compared to cellulosic feedstocks [13,14]. In the late 1950s, algal biomass was first used for converting its carbohydrate fraction into biogas through the anaerobic digestion process [15]. Later in the year 1978, Benemann et al. conducted the detailed engineering and technoeconomic analyses and suggested that production of biogas from algae can be competitive to fossil fuels from the economical point of view [16]. At the same time, the US Department of Energy explored the potential of biodiesel production from the lipid fraction of algae [13]. In the early 1980s, the emphasis was laid on the production of hydrogen fuel as well as biodiesel from algal biomass. During the last 10 years, significant advancements have been made in the cultivation of algal biomass; isolation and characterization of algal strain; and process development for future biofuel production. However, algal biofuel production is not still feasible in commercial scale, and it requires further improvement in cultivation and harvesting techniques, reactor design and process control, and genetic engineering [17]. Moreover, the algal biorefinery requires the integration of all the processes for producing fuels, chemicals, and other valuable products.

2G feedstocks Major features

Major features ● ● ● ●

Low production cost Developed technology Competition with food Geographical limitations Lignocellulosic biomass

Sugarcane, corn etc.

● ● ● ● ●

No competition with food Low geographical limitations High capital cost High recalcitrance Recently developed technology

4G feedstocks 3G feedstocks

● ● ● ● ● Microalgae and macroalgae

Major features

Major features

● ● ●

Can be produced anywhere High production cost Technology not fully developed CO2 High capital cost No competition with food

Low environmental impact High capital cost Emergent technology

Bioethanol Industrial waste CO2

FIGURE 7.1 Features of the biofuel production from various biomass resources [11].

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7.3 CULTIVATION AND HARVESTING OF ALGAL BIOMASS

This chapter summarizes the current status, future potential, and challenges of algal biomassebased biofuel production. Section 7.2 discusses the algal biomass potential, followed by biofuel conversion routes in Section 7.3, current global status and future trends in Section 7.4, and the challenges associated with the existing technologies and scale-up process development in Sections 7.5e7.7. Section 7.8 concludes the chapter with a summary.

7.2 POTENTIAL FEEDSTOCK AND THEIR CHARACTERISTICS Up to date, more than 50,000 microalgae species have been identified and out of them 30,000 species have been investigated for potential applications [18]. Microalgae contain about 20%e50% oil content and in some cases, it exceeds 80%. Unlike lignocellulosic biomass which consists of cellulose, hemicellulose, and lignin, microalgae mainly contain three major constituents such as carbohydrates in the cell wall or starch in the plastids, lipids, and proteins with a small quantity of ashes and acids. Carbohydrates and lipids can be converted to a wide variety of high energy content fuels, for example, syngas, methane, bioethanol, biodiesel, biobutanol, biogasoline, aviation biofuel, and solid biochar. In contrast, some algal biomass, for instance, red algae and certain green algae contain lignin in the cell wall [19,20]. Table 7.1 presents the percentage composition of various algal biomass species. Elementally, microalgae consist of elemental carbon (C), hydrogen (H), nitrogen (N), sulfur (S), and oxygen (O). • Carbohydrate: Carbohydrate mostly consists of cellulose and soluble polysaccharides in the cell wall of the algal biomass. The microalgae cell wall consists of two-wall layer, namely, the inner cell wall layer and an outer cell wall layer [21]. The inner cell wall layer usually contains cellulose and hemicellulose. Typically, carbohydrate is produced from photosynthesis and carbon fixation metabolism of the microalgae [22]. Fig. 7.2 illustrates the carbon fixation metabolism for carbohydrate accumulation and compositional structure of algal biomass. However, in the previous study, the researchers observed a competition between the accumulation of carbohydrate and lipids in microalgae biomass [38]. The percentage of the carbohydrate in microalgae varies from species to species; some microalgae contain carbohydrates to a large extent [38]. For instance, Chlorella vulgaris has 51% carbohydrate, Spirogyra sp. contains 33%e64% carbohydrate, Porphyridium cruentum comprises of 40%e57% carbohydrate, and Dunaliella salina can

169

accumulate 85.58% carbohydrate. Therefore, the large content of carbohydrate materials can be effectively converted into fermentable sugars during bioethanol production. • Lipids: Microalgae contains a substantial amount of intracellular lipids ranging from 20% to 60% on the dry matter basis [31,38]. Typically, two large groups of lipids, namely polar and nonpolar are found in microalgae biomass, which are different in their chemical properties. However, majority of the lipids are nonpolar in nature, which are similar to aliphatic compounds [39]. The polar lipids contain phosphatidyl inositol and phosphatidyl ethanolamine, glycolipids, and glycolipids ethers of fatty acids and glycerol, while, mono-, di-, triglycerides and isoprene lipids are the examples of the nonpolar lipids. Generally, the fats are insoluble in solvents; however, the polarity of the fats gradually increases with the increase of temperature. At subcritical conditions, the solvents increase the miscibility, which enhances the formation of glycerol during biodiesel production [40]. • Proteins: The protein content in microalgae biomass largely depends on the conditions of their growth and the culture medium of the microorganism cultivation [41]. Typically, the protein concentration in microalgae ranges from 20% to 50%; however, some species contain more than 50% protein. For example, Chlorella vulgaris residue contains 61.24% protein; Chlamydomonas reinhardtii has 64.70% protein on dry matter basis. The protein consists of various chains of peptide which are heterogeneous and complex in nature.

7.3 CULTIVATION AND HARVESTING OF ALGAL BIOMASS The efficient biofuel production from algal biomass consists of three major process steps namely, (1) algae cultivation, (2) biomass harvesting and dewatering, and (3) cell wall disruption for fractionation and conversion into biofuels and valuable products. So far, open ponds/raceway ponds, flat-plate photobioreactor, inclined tubular photobioreactor, and horizontal/continuous photobioreactor have been used for the cultivation of microalgae as shown in Fig. 7.3 [42]. The reactors produce dilute solution of microalgae ranging from 0.05% to 0.075% for raceway ponds and 0.3%e0.4% for closed photobioreactor [43]. The raceway ponds reactor is simple, easy to maintain, and has a low production cost compared to closed photobioreactor; however, the production rate is very low compared to closed photobioreactor [44]. For the development of efficient biofuel process, the cultivation

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170 TABLE 7.1

7. RECENT TRENDS AND CHALLENGES OF ALGAL BIOFUEL CONVERSION TECHNOLOGIES

Elemental and compositional analysis of algal biomass. Elemental analysis (%)

Algal biomass

C

H

N

O

Chlorella vulgaris

45.80

5.60

4.60

Chlorella vulgaris

53.80

7.72

Chlorella vulgaris

42.51

Chlorella vulgaris residue

Compositional analysis (%) S

Carbohydrate

Lipid

Protein

HCV (MJ/kg)

References

38.70

19.70

49.50

29.00

18.40

[23]

1.10

37.00

51.00

43.00

6.00

24.00

[23]

6.77

6.64

27.95

20.99

15.67

41.51

16.80

[24]

45.04

6.88

9.79

29.42

20.34

5.71

61.24

19.44

[24]

Chlorella vulgaris

43.90

6.20

6.70

43.30

15.50

54.90

18.00

[25]

Chlamydomonas reinhardtii (wild)

52.00

7.40

10.70

29.80

18.10

47.40

23.00

[25]

Chlamydomonas reinhardtii CW15þ

50.20

7.30

11.10

31.40

22.40

45.70

22.00

[25]

Dunaliella tertiolecta

39.00

5.37

1.99

53.20

21.69

2.87

61.32

14.24

[26]

Nannochloropsis oculata

39.90

5.50

6.20

17.00

20.00

39.00

16.80

[27]

Nannochloropsis oceanica

50.06

7.46

7.54

34.47

0.47

22.70

24.80

19.10

21.46

[28]

Spirulina platensis

46.16

7.14

10.56

35.44

0.74

30.21

13.30

48.36

20.52

[29]

Scenedesmus obliquus CNWeN

37.37

5.80

6.82

50.02

13.41

4.66

30.38

16.10

[30]

Chlorella sorokiniana CY1 residue

35.67

9.90

18.81

20.24

[31]

Chlamydomonas sp. JSC4 residue

35.70

6.85

12.18

17.41

[31]

Spirulina platensis

30.21

48.36

13.30

[32]

Dunaliella salina

32.00

9.00

57.00

[33]

Scenedesmus dimorphus

21e52

16e40

8e18

[33]

Chlamydomonas reinhardtii

22.60

12.60

64.70

[34]

Spirogyra sp.

33e64

11e21

6e20

[35]

Porphyridium cruentum

40e57

9e14

28e39

[35]

0.62

Dunaliella salina

85.58

11.47

8.46

[36]

Anabaena cylindrica

25e30

4e7

43e56

[37]

Synechococcus sp.

15

11

63

[37]

Spirulina maxima

13e16

6e7

60e71

[37]

Scenedesmus obliquus

10e17

12e14

50e56

[37]

Chlorella vulgaris

12e17

14e22

51e58

[37]

methods should have high productivity, low production and maintenance cost, simplicity in design and parameters control, and reliability [45]. Besides, the growth of microalgae is also dependent on the light, CO2, temperature, and pH [42]. The generalized conditions for algae cultivation are temperature 16
Ce27
C, salinity 12e40 g/L, light intensity 1000e10,000 lux, and pH 7e9. The main purpose of

the harvesting process is to concentrate the microalgae solution which can increase the solid matter in the solution up to 10%e25% [46]. Microalgae harvesting includes mechanical, chemical, biological, and electrical-based methods. The harvesting mainly comprises two process steps, namely thickening steps (flocculation, floatation, and sedimentation) and dewatering steps (centrifugation, pressure filtration,

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7.4 ALGAE BIOFUELS AND CONVERSION PROCESS

O O

Sunlight

CH OH

P, N , S

O

P

O

R

O

H2O

CO2

Lipids

O2 O

H

OH

O

H

H OH

H

OH

OH

H

O

OH H

OH

Carbohydrates

Protein

FIGURE 7.2 Structure of algal biomass.

(a)

(b)

(c)

(d)

FIGURE 7.3 Microalgae cultivation methods (a) open pond, (b) flat-plate photobioreactor, (c) inclined tubular photobioreactor, and (d) horizontal continuing photobioreactor [42].

vacuum filtration, and membrane filtration) [22,43]. Mechanical methods are the most widely implemented algal harvesting methods [44]. Biological methods are an emergent technology that can reduce the operating cost. Moreover, combining multiple operations can also lower the harvesting cost.

7.4 ALGAE BIOFUELS AND CONVERSION PROCESS Algal biomass has a wide range of applications including food nutrients, pharmaceuticals, chemicals, and different forms of renewable energy. Fig. 7.4 depicts

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7. RECENT TRENDS AND CHALLENGES OF ALGAL BIOFUEL CONVERSION TECHNOLOGIES

Processes

Thermochemical conversion

Conversion methods

Products/ applications

Pyrolysis

Liquid fuels Methanol Ethanol Butanol Bio-oil Biodiesel Syndiesel

Liquefaction Gassification Direct combustion

Algal biomass

Anaerobic digestion Biochemical conversion

Fermentation Photo-biological

Chemical reaction

Transesterification

Gaseous fuels Methanol Ethanol Butanol Bio-oil Biodiesel Syndiesel Heat and power electricity Solid fuel Bio-char

FIGURE 7.4 Algal biomass conversion routes for biofuels [47].

the possible routes of algal biomass conversion into fuels, heat, and power for establishing algal biorefinery concepts. The main components (carbohydrates, lipids, and proteins) of microalgae can be converted into bioethanol, biodiesel, biogas, biohydrogen, syngas through the biochemical, chemical, thermochemical, and direct combustion routes.

7.4.1 Thermochemical conversion Thermochemical conversion refers to the decomposition of organic matter of the algal biomass for the production of biofuels including liquid, gaseous, and solid fuels. Thermochemical conversion can be classified as pyrolysis, liquefaction, gasification, and direct combustion based on their temperature, pressure, and duration of heating [48]. Thermochemical conversion is considered as the simplest route for microalgae conversion into biofuel when compared to chemical and biochemical process. In pyrolysis of microalgae, the biomass sample is heated at 400
C to 600
C with a pressure of 0.1 MPa for 30e60 min in the absence of oxygen which mainly produces liquid oil, gas, and solid char. So far a wide number of studies investigated the slow and fast pyrolysis behavior of microalgae with or without catalyst and reported promising results [49e52]. The biooils produced from the pyrolysis of microalgae have a higher heating value of 31e42 MJ/kg with a viscosity of 0.060 Pa s and mainly contains hydrocarbons from lipids and nitrogenous compounds from protein [50,52]. More importantly, algal biooils are more stable compared to biooils produced from lignocellulosic biomass pyrolysis which is favorable for future applications [53]. The conversion rate of microalgae and the quality of biooils produced by the pyrolysis are

influenced by temperature, pressure, holding time, type of pyrolysis, and catalytic effects [54,55]. Miao et al. investigated the fast pyrolysis microalgae and lignocellulosic biomass and summarized that biooil from microalgae had low oxygen content with high calorific value compared to lignocellulosic biomass [52]. Pan et al. studied the catalytic pyrolysis of Nannochloropsis sp. and found that incorporation of catalyst lowered the oxygen content in biooils to 19 wt% from 30 wt% and increased the heating value to 32.5 MJ/kg from 24.6 MJ/kg [55]. Moreover, the microwave assisted pyrolysis of microalgae was found to be favorable in upgrading the biooils’ quality in terms of lower oxygen content and higher calorific value [56]. Liquefaction of microalgae is carried out at lower temperatures (300
Ce350
C) and high pressure (5e20 MPa) for 5e60 min in the presence of catalyst and solvent for converting the microalgae mostly into liquid fuels [57]. The microalgae biomass contains approximately 80%e90% moisture content [58] which is suitable feedstock for liquefaction conversion process as the feed materials used in reactor are mainly in slurry form. However, the reactor used in the liquefaction process is complex and costly [59]. Previous studies reported the successful liquefaction of microalgae for biofuels production. For instance, Dote et al. liquefied wet Botryococcus braunii at 300
C and achieved 64% biooil with a higher heating value of 45.8% MJ/kg [59]. Matsui et al. explored the catalytic effect of iron in the liquefaction of protein-rich Spirulina sp. at 350
C for 60 min under 5 MPa [59]. They found the biooil yield increased to 66.9 wt% from 52.3 wt% in the presence of Fe(CO)5eS catalyst. The biooil contained high carbon content and lower oxygen content with a heating value of 32e33 MJ/kg. Typically,

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7.4 ALGAE BIOFUELS AND CONVERSION PROCESS

gasification converts the carbonaceous materials in microalgae into clean fuel gas (H2, CO, CH4) at a high temperature ranging between 800
C and 1000
C in the presence of insufficient oxygen [59]. The conventional gasification of microalgae occurs in four stages such as drying, pyrolysis, combustion, and reduction. Hirano et al. investigated the effect of temperature on the gasification of Spirulina sp. and observed the increase in H2 content and decrease in CO2, CO, and CH4 content with the increase in temperature [60]. Besides, some researchers focused on the cogasification of microalgae with lignocellulosic biomass or lowrank coal in bubbling fluidized bed reactor and fluidized bed reactor, respectively [61,62]. However, because of the high moisture content in microalgae, the drying stage of conventional gasification consumes lot of heating energy. Therefore, supercritical water gasification has gained attention recently which completely avoids the drying stage [63]. Typically supercritical water gasification is carried out beyond the critical point of water (374
C and 22.1 MPa). Haiduc et al. studied the supercritical water gasification of microalgae and found that the process was able to avoid the formation of tar and char [64]. Guan et al. observed that the yields of H2 and CH4 from supercritical water gasification of Nannochloropsis sp. increased drastically in the presence of NaOH and KOH [65]. In the direct combustion of microalgae, the biomass is burnt around 1000
C in a furnace, boiler, or steam turbine in the presence of excess air to produce mainly heat energy [66]. The direct combustion accepts the microalgae only with less than 50% moisture content [59]. Therefore, drying and grinding of microalgae required for efficient combustion increase the energy demand and additional cost for the process [57]. However, the effective utilization of heat produced from the direct combustion of microalgae can reduce the additional cost required for drying and grinding [67]. Kadam suggested that the coal with algae cofiring can lower the emissions of greenhouse gases [68]. So far the studies related to the direct combustion of microalgae are very limited in the literature, which needs further investigation.

7.4.2 Biochemical conversion The biochemical conversion process of microalgae includes anaerobic digestion, fermentation, and photobiological technique. The conversion rate of these processes is low and requires long reaction time. In these conversion technologies, the microalgae are converted into biofuels through the microorganisms and enzymatic processes [69]. Recently, production of biogas from anaerobic digestion of microalgae has gained significant attention for a number of reasons. For instance, the process is appropriate for microalgae containing

173

around 80%e90% moisture content and microalgae contain high polysaccharides with no lignin [59]. However, the conversion of microalgae into biogas through anaerobic digestion is affected by a numbers of factors, for example, the recalcitrance of cell wall, carbonnitrogen ratio (C/N), and high protein content. It has been observed that the effective pretreatment before anaerobic digestion significantly enhances the biogas production [70]. Microalgae with high protein content result in low C/N ratio which is not favorable for anaerobic digestion of microalgae. However, codigestion of waste paper with algal biomass can substantially increase the biogas production [66,71]. With 50/ 50 blending of waste paper and microalgae, Yen and Brune were able to produce two times the biogas produced from only microalgae digestion [71]. In addition, high protein content increases the ammonium production which impedes anaerobic microorganisms; however, salt-adapted microorganisms can be a feasible solution for this issue [67]. The bioethanol production process includes pretreatment followed by enzymatic hydrolysis and then fermentation. Microalgae are considered as superior feedstock for enzymatic hydrolysis because of the absence of lignin in the cell wall which reduces the recalcitrance compared to lignocellulosic biomass. However, microalgae with low carbohydrate content are not suitable for bioethanol production. So far, many pretreatment methods including mechanical techniques, physical techniques, thermal pretreatment techniques, chemical techniques, and combined techniques have been investigated for the disruption of the cell wall and to remove lipids which favors the enzymatic hydrolysis route and bioethanol production [72]. Harun et al. studied the fermentation of Chlorococcum sp. and found 60% more ethanol from the lipids extracted from microalgae when compared with untreated intact microalgae [73]. This result indicates that lipids extraction for biodiesel with fermentation of leftover carbohydrate for bioethanol production can be a favorable route for algal biofuels. Hirano et al. identified C. vulgaris microalgae as a promising feedstock for bioethanol production with a conversion of 65% [74]. Hydrogen (H2) is the clean and efficient energy carrier that can be produced from microalgae through oxygenic photosynthesis which involves the utilization of hydrogenase or nitrogenase enzyme [74]. During the photosynthesis process, the water molecule of microalgae is converted into protons, electrons, and oxygen; a proton is then subsequently converted into H2 by hydrogenase enzymes [75]. However, the generation of oxygen inhibits the hydrogenase enzyme which mainly impedes the H2 production. This issue can be resolved through the proper separation of the oxygen produced during the photosynthesis processes.

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174

7. RECENT TRENDS AND CHALLENGES OF ALGAL BIOFUEL CONVERSION TECHNOLOGIES

7.4.3 Chemical reaction The chemical process of algal biofuel production includes the extraction of lipids from the microalgae and then conversion of lipids into biodiesel through transesterification. In this process, the transesterification reaction occurs between triglyceride (lipids) and alcohol in the presence of an appropriate catalyst which is acidic, alkaline, or enzyme-based to produce fatty acid methyl ester (FAME) and glycerol [66,67] as shown in Fig. 7.5. The byproduct glycerol can be used for pharmaceuticals and cosmetics [66]. Milano et al. obtained up to 90% FAME conversion from two different microalgae species such as Spirogyra and Oedigonium using alkaline catalyst [35]. In the case of ex situ transesterification, the pretreatment required for the lipids extraction is energy intensive and time-consuming, which consumes almost 80% of the total material preparation cost [76]. Jazzar et al. suggested that the in situ transesterification can overcome the pretreatment issue required for ex-situ process and make the overall process economically feasible [76]. However, Johnson and Wen investigated both in situ and ex situ transesterification for the production of biodiesel from Schizochytrium limacinum and found lower biodiesel yields in the case of in situ transesterification compared ex situ transesterification [77]. Salam et al. recommended that the biogas production from the residual microalgae of biodiesel production process can provide the energy required for the cultivation or separation of microalgae from dilute solution prior the in situ process [78]. Ehimen et al. reported that the effective stirring during in situ transesterification can boost the biodiesel from microalgae yields [79].

7.5 ENGINE PERFORMANCE AND EMISSION CHARACTERISTICS USING ALGAL BIOFUEL The biofuels produced from microalgae can be used as transportation fuel in replacement of petroleum fuels. However, number of factors such as engine performance (cylinder pressure, brake mean effective pressure, frictional mean effective pressure, power,

CH2

OCOR1

CH

OCOR2

+

3CH3OH Methanol

CH2

OCOR3

Triglyceride (lipids)

torque, brake specific fuel combustion, brake thermal efficiency) and emissions from microalgae-based fuels (particulate matter emissions, CO, CO2, NO, NOx) define the fuels’ characteristics and compatibility with petroleum fuels [80]. Islam et al. noticed low cylinder pressure at the early stage of the combustion and high pressure in the later stage with pure microalgae oil produced from Crypthecodinium cohnii [81]. Hariram and Kumar compared the cylinder peak pressure for both the pure petroleum diesel and the blends of algal oil methyl ester and petroleum diesel (15:85) [82]. They observed higher pressure at any load in the case of 15: 85 blends when compared to pure petroleum diesel. The indicated mean effective pressure was observed to be increased with 20% blending of algae biodiesel compared to petroleum diesel [80]. Kumar et al. reported that the microalgae biodiesel blend with butanol and pure diesel decreases the brake power and torque when compared to butanol and pure diesel as the microalgae biodiesel contains higher oxygenate [83]. Makarevic et al. stated the higher brake specific fuel combustion and lower brake thermal efficiency of algae biodiesel blend compared to diesel oil attributed to the lower calorific value of the blend [84]. In contrast, the increase in brake thermal efficiency of algae biodiesel blend was also reported when compared to pure diesel [82,85]. Jayaprabakar and Karthikeyan reported the higher NOx emissions of microalgae biodiesel blend due to the high oxygen content [85], while Islam et al. stated lower cetane number and ignition delay as the reasons for the increase in NO and NOx [81]. In contrast to these findings, Fisher et al. and Kumar et al. recently described the decrease in NO and NOx emissions of algae biodiesel when blended with butanol or diesel [83,86]. Moreover, a number of researchers investigated the CO and CO2 emission from microalgae biodiesel. They revealed that CO emission reduced significantly due to the conversion of CO into CO2 in the presence of additional oxygen in microalgae biodiesel when compared to petroleum diesel [35,81,83,85,87]. Moreover, Rahman et al. observed the increase in particle emissions from microalgae biodiesel due to the very low volatility, high boiling point, high density, and viscosity of microalgae biodiesel [88].

CH2

OH

CH

OH

CH2

OH

Catalyst

Glycerol

FIGURE 7.5 Transesterification for biodiesel production.

II. PRODUCTION

+

CH3

OCOR1

CH3

OCOR2

CH3

OCOR3

Methyl esters

7.7 CHALLENGES OF BIOFUEL PRODUCTION FROM ALGAL BIOMASS

175

7.6 GLOBAL ALGAL BIOFUEL ACTIVITIES

7.7 CHALLENGES OF BIOFUEL PRODUCTION FROM ALGAL BIOMASS

Biofuels production from algal biomass started from the mid-19th century; since then so many efforts have been made by the scientific community for the development of efficient and cost-effective algal biofuel technologies. In the year 1970s, utilization microalgae for biodiesel production gained special attention because of the scarcity of oil and gas supply [42]. The United States, the largest algal biofuel producing country, has been working on the research and development of algae-based biofuels. With the aim of producing biodiesel from microalgal lipids, the US Department of Energy (DOE) launched the “Aquatic Species Program” from the year 1978e1996 [17]. From the year 1968e1990, DOE supported another research project entitled “Marine Biomass Program” for the investigation of the technoeconomic feasibility of microalgae cultivation and biofuel conversion routes primarily, anaerobic digestion for biogas production [89]. In the year 2009, ExxonMobil Corporation invested US$600 million for algae-based transportation fuels [90]. During the year 1990e99, the Japanese government funded a project named “Biological CO2 Fixation and Utilisation” mainly for the research on the algal biofuel. However, the revolutionary advancement on algal biofuel happened in the year 2011 because of the crisis at the Fukushima nuclear power plant [91]. The United States financed number of projects during the last few years. For instance, US$48.6 million through National Alliance for Advanced Biofuels and Bioproducts consortium in the year 2010e13; US$11 million through Consortium for Algal Biofuels Commercialization in the year 2011e15 for the production of biofuels and other valuable products. Additionally, China, South Korea, the UK, Italy, the Philippines, and Nigeria have also developed their plans and started to finance in research and development for algal biofuels. Fig. 7.6 presents the current technological status of four generation biofuel production.

The algal biomass provides several advantages for the production of biofuels over the 1G and 2G biofuel feedstocks. So far, extensive research has been carried out and so many efforts have been made for the improvement of algal biofuel including the cultivation, harvesting, and biofuel production process integration. However, the algal biofuel is still not fully developed in large scale and yet to achieve feasibility from the technoeconomic point of view and need to revisit for addressing the issues associated with the technology. • Cultivation and harvesting: The higher cultivation cost of microalgae compared to other biofuel feedstocks is one of the major challenges for algal biofuel production. Additionally, the harvesting and dewatering of microalgae requires high energy inputs because of their small size and dilute concentration leading to the increase in capital expenditure. It has been estimated that only harvesting process contributes almost 20%e30% of the total production cost of algal biomass [47]. The harvesting methods so far developed and investigated have some specific weakness and none of them is feasible in terms of both efficiency and economic perspective. Therefore, a combination of multiple operation units can be an effective option for harvesting and dewatering. Preconcentration before dewatering of algal biomass can lower the harvesting cost significantly [43]. However, low cost and high efficient preconcentration methods are yet to be developed and the growth kinetics of cocultivation flocculants and microalgae are yet to be explored extensively. • Microalgae compositional characteristics: The production of biofuel from algal biomass is largely dependent on the amount of carbohydrates, lipids, and proteins accumulated in the biomass. For example, microalgae biodiesel production requires a high content of lipids. These compounds are

Fourth generation High cost

Third generation Second generation First generation

Low cost Research

Development

Demostration

FIGURE 7.6 Biofuel productiondtechnological status.

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Mature technology

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7. RECENT TRENDS AND CHALLENGES OF ALGAL BIOFUEL CONVERSION TECHNOLOGIES

accumulated through the photosynthesis and carbon fixation metabolism, and the content varies based on the conditions applied. For instance, the microalgae cultured at limited nitrogen conditions contain high carbohydrates and lipids compared to nitrogen-based compounds [11]. The carbohydrate metabolisms are in its infant stage and currently genetic engineering has been employed for the manipulation of the carbohydrate metabolisms [92]. Therefore, indepth investigation is required on the metabolism mechanism for the development the regulatory networks for estimating the carbohydrates and lipids accumulation. • Pretreatment for fractionation: The rigidity of algal biomass cell wall is another bottleneck for the industrial process efficiency of algal biofuel production. Pretreatment of microalgae is a key step for the production of algal bioethanol from carbohydrate compounds as the process increases the accessibility to microbial fermentation [41]. On the other hand, lipids need to be separated from the algal biomass structure for the production of biodiesel. The pretreatment methods used for the cell wall disruption and lipids extraction have some limitations including (i) high energy consumption, (ii) high cost, and (iii) high time requirement. Therefore, extensive research is required for the pretreatment process optimization to develop a costeffective pretreatment method which can increase the biofuel production as well as minimize the energy consumption during cell disruption. • Conversion technology and integrated biorefinery approach: As discussed earlier, each of the conversion technologies is different in process chemistry and produces different forms of biofuels from algal biomass. Therefore, the selection of appropriate conversion technology is necessary for the establishment of economically feasible and environment-friendly biofuel production routes from algal biomass. A previous study stated that thermochemical conversion process is more favorable compared to biochemical conversion process for many reasons, such as, no feedstock improvements requirement for thermochemical conversion and the low conversion efficiency of the biochemical process [93]. Wiley et al. reported that production of algal biogas through anaerobic digestion is more favorable than algal biodiesel production as anaerobic digestion requires no drying steps and consumes low energy [13]. In addition, lipid extraction for biodiesel production followed by biogas production from the residual biomass through anaerobic digestion is also found as feasible [94]. However, there is still no existent comprehensive comparison analysis from the technoeconomic point of view; therefore, further

detail investigations are required on the technoeconomic assessment, biorefinery-based production strategy, and photobioreactor design.

7.8 CONCLUSION The study concluded that the microalgae would be one of the potential feedstocks for energy extraction in the future. The study identified some of the key reasons, for example, this wet biomass consists of carbohydrates, lipids, and proteins; however no lignin in the cell wall which favours the conversion when compared with lignocellulosic biomass conversion. Earlier, microalgae were utilized for their nutritious and pharmaceutical value. Recently, biofuels production from microalgae has gained notable attention because of its various benefits from the economic and environmental point of view. However, largescale production still has some challenges mainly, the high energy consumption and high cost associated with microalgae cultivation, harvesting, and processing. Therefore, the study recommended for extensive investigation for the development of low-cost and high efficient cultivation and harvesting methods. The study also suggested that cultivation of algal biomass in wastewater as a culture medium, low-cost harvesting-dewatering method, cost-effective pretreatment technology for cell wall disruption, and biorefinery-based production approach need to be considered for the future development of algal biofuel production routes.

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