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Rapid pyrolysis behavior of oleaginous microalga, Chlorella sp. KR-1 with different triglyceride contents
Renewable Energy 81 (2015) 779e784
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Rapid pyrolysis behavior of oleaginous microalga, Chlorella sp. KR-1 with different triglyceride contents Jeong-Geol Na a, Young-Kwon Park b, Doo Il Kim a, You-Kwan Oh a, Sang Goo Jeon a, Jin Woo Kook c, Ji Hoon Shin c, See Hoon Lee c, * a b c
Clean Fuel Department, Korea Institute of Energy Research, Republic of Korea School of Environmental Engineering, University of Seoul, Republic of Korea Dept. of Mineral Resources and Energy Eng., Chonbuk National University, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 July 2014 Accepted 31 March 2015 Available online
Rapid pyrolysis of two Chlorella sp. KR-1 samples, with triglyceride content of 23.8 and 44.9wt%, was conducted using pyrolysis gas chromatograph-mass spectrometry over a temperature range (400 e600 C). The effects of triglyceride content of the microalgal cells and pyrolysis temperature on the structure and distribution of products were analyzed and the pyrolysis products were classified according to number of carbons and chemical species. The triglyceride content of microalgal cells was found to affect the concentration of free fatty acids, hydrocarbons, and derivatives of carbohydrates. In the case of the lipid-rich sample, free fatty acids originating from triglyceride decomposition were found to be the predominant compounds whereas sugar-dehydrated products such as anhydroglucose were the predominant compounds in the case of the lipid-poor sample. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Rapid pyrolysis Gas chromatograph-mass spectrometry Microalgae Free fatty acid Hydrocarbon
1. Introduction Research and development has recently seen a considerable emphasis placed on the use of biomass as a fuel source to ease the depletion of global fossil fuel reserves. From a feedstock supply and ethical standpoint, much attention has been placed on algae because it has high photosynthetic yields, and can be cultivated on non-arable land. Additionally, some algae species can accumulate large amounts of lipids intracellularly, which could potentially be converted into various biofuels [1e4]. By using the Delphi method, most of the experts believe that the production of microalgae for biofuels will only achieve full commercial scale until 2020, whereafter it could represent an important share of the total worldwide fuel production despite some remaining challenges [5]. The conventional process for producing biofuels from microalgae biomass involves extracting lipids using an organic solvent followed by transesterification reactions. This is a traditional technology, having been developed commercially for the conversion of vegetable oils into biodiesel. Solvent extraction, however, is not very efficient for microalgae. This is because of the thicker cell
* Corresponding author. Tel.: þ82 63 270 3632; fax: þ82 63 270 2366. E-mail address: [email protected] (S.H. Lee). http://dx.doi.org/10.1016/j.renene.2015.03.088 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
wall and structural differences of the microalgae compared with their terrestrial biomass counterparts. The excessive amount of water in collected microalgae is one of main bottlenecks, because the energetic output only slightly exceeds the required fuel input for the production and processing of microalgae [6e8]. Although less energy-intensive dewatering methods are being developed, several thermochemical processes have been studied and proposed because of their yield in chemicals and fuels [8e11]. In the case of woody biomass, thermochemical conversions such as pyrolysis, gasification and combustion have already been developed at a commercial level. Of these processes, pyrolysis has received attention in recent years because of its simplicity and speed. In addition, unlike other thermochemical conversion methods, pyrolysis of biomass could produce versatile liquid products that are easy to handle and that are feedstocks for chemicals [12e14]. Generally, microalgae biomass contains a large amount of triglycerides, whereas their lignin content is typically low, although this is species dependent. In addition to triglycerides, microalgae possess carbohydrates and proteins. These imply that the thermal behavior of microalgae biomass is different from that of lignocellulosic biomass. According to Ross et al. [10] the pyrolysis of microalgae underwent a step-wise process where carbohydrates, proteins and lipids decomposed. Also the bio-oil from microalgae
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contained more oxygen and nitrogen and less sulfur than palm oils and there were high yield of fatty oxygenates and nitrogenous compounds in the bio-oil due to high contents of lipids and proteins [15]. The pyrolysis characteristics of algal biomass have been investigated in a number of studies using various experimental conditions [9,10,16e21]. Ross et al. [10] reported the thermochemical behavior of microalgae, Fucus vesiculosus, Chorda flum, Laminaria digitata, F. serratus, L. hyperborea, and Macrocystis pyrifera, and concluded that flash pyrolysis produced a number of interesting compounds and a range of low molecular weight oxygenated and nitrogen containing compounds. Li et al. [9,16] studied the pyrolysis characteristics and kinetics of marine algae and found that there were three stages during the pyrolysis of macroalgae. These were moisture evaporation, primary devolatilization and residual decomposition. Additionally, it was found that the average active energy range for a pyrolysis reaction was 154e250 kJ/mol. In experiments of a fluidized bed pyrolyzer of seaweed [19], the composition on liquids obtained varied with the seaweed species. Maddi et al. [17] compared the pyrolysis characteristics of algal and lignocellulose feedstocks and suggested that algae pyrolysis could produce liquid fuel in appreciable yield, and that the technologies being developed for thermochemical conversion of lignocellulose materials could be applied to algae as well. Most studies of algal pyrolysis have compared only the characteristics of different algae species, especially for macroalgae. There are very few studies in which the effects of lipid content on the product distribution following pyrolysis with same microalgae species have been investigated. In the present study, pyrolysis of two Chlorella sp. KR-1 samples, differing in triglyceride content, was conducted at different temperatures using pyrolysis gas chromatograph-mass spectrometry (PyGC/MS). The pyrolysis products were classified according to number of carbons and chemical species. The effect of triglyceride content and pyrolysis temperature on the reaction pathway is also discussed. Based on these results, the pyrolysis process for the conversion of microalgae to potentially useful products was evaluated. 2. Experimental 2.1. Preparation of microalgae biomass Chlorella sp. KR-1, a freshwater microalgae isolated from water samples collected in a region nearby Youngwol, Korea was used in this work. The microalgae was cultivated in a bubble-column reactor (working volume, 500 mL), which was made of Pyrex glass. The reactor was placed in a growth chamber equipped with six fluorescent lamps (light intensity: ca. 40 mmol photons/m2s) positioned at the front and side of the chamber. A mixture of CO2 (10 vol%) in air was supplied to the reactor at a flow rate of 0.4 L/ min. The cultivation temperature was 25 C. Two microalgae samples containing respectively 23.8% and 44.9wt% of triglyceride were obtained by adjusting the nitrate concentration to 4 mM and 1 mM, respectively. The microalgae cells were harvested by
centrifugation (3800 g, 10 min) and washed with distilled water (repeated twice). The cells were then dried in a freeze-dryer (FD5512, IlShinBioBase Co., Korea) for at least four days. The freeze-dried cells were then analyzed by pyGC/MS. The triglyceride content in the KR-1 samples was analyzed using the modified direct transesterification method. Detailed information on Chlorella sp. KR-1 and growth conditions can be found in Lee et al. [22] and Na et al. [23]. 2.2. Py-GC/MS analyses A double-shot pyrolyzer (Frontier-Lab Co., Py-2020iD), coupled directly to GC/MS, was used for identification of the pyrolysis products. The Chlorella sp. KR-1 (2 mg) was placed in a sample cup and then into a furnace at different temperatures (400, 500 and 600 C) under a He atmosphere. The gaseous species generated during the pyrolysis were introduced via a GC inlet port (split ratio of 1/100) into a metal capillary column (Ultra ALLOY-5MS/HT; 5% diphenyl and 95% dimethylpolysiloxane, length 30 m, i.d. 0.25 mm, film thickness 0.5 um, Frontier Laboratories Ltd. Japan). To prevent condensation of products, the interface and inlet temperatures were both maintained at 300 C. The column temperature was programmed to change from 40 (5 min) to 320 C (10 min), at a heating rate of 5 C/min. The temperature of the GC/MS interface was 280 C, with the MS operated in the EI mode at 70 eV. The program was run in the scanning range from 29 to 400 a.m.u. at a rate of 2 scans/sec. The identification of peaks was performed using the NISTMS library. The experiments were conducted at least three times for each run to confirm the reproducibility of the reported procedures. The average values of the peak area and peak area % as received were calculated for each identified product. 3. Results and discussion The fatty acid composition in Chlorella sp. KR-1 samples is shown in Table 1. Palmitic acid (C16:0) content is higher than other fatty acid content, but C18 fatty acids such as stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2) and linolenic acid (C18:3) are more abundant in the KR-1 samples than C16 fatty acids. In the microalgae cells containing 23.8% lipids (Chlorella 1), the content of C16 and C18 fatty acids were found to be 29.6% and 59.7%, respectively. Conversely, in the microalgae cells with a lipid content of 44.9% (Chlorella 2), C16 and C18 fatty acids were found to be 35.3% and 56.0% respectively. The ratio of C16 and C18 fatty acids in the two samples is therefore comparable. In general pyrolysis of lignocellulose material is performed at reaction temperatures between 400 C and 600 C [12e14]. For this reason, pyrolysis with Chlorella sp. KR-1 was carried out at the same temperatures. Fig. 1 shows the distribution of structural classes in the products obtained from rapid pyrolysis experiments using PyGC/MS. The products were classified as free fatty acids, carbohydrates, hydrocarbons, alcohols, aldehydes, esters, ketones, and others depending on their functional groups. The lipid content of the algae was found to affect the concentration of free fatty acids, hydrocarbons, and derivatives of carbohydrates (Fig. 1). Similar to
Table 1 Fatty acid composition in Chlorella sp. KR-1 cells. Sample Chlorella 1 Chlorella 2
Cellular content (mg/g cell) Relative content (%) Cellular content (mg/g cell) Relative content (%)
Myristic acid (C14:0)
Palmitic acid (C16:0)
Myristoleic acid (C16:1)
Stearic acid (C18:0)
Oleic acid (C18:1)
Linoleic acid (C18:2)
Linolenic acid (C18:3)
Others
0.93 0.4 e e
64.42 29.6 130.74 32.1
e e 13.31 3.2
16.60 7.1 29.40 6.8
53.69 24.0 96.31 23.0
66.49 25.2 96.16 19.5
11.72 3.4 43.88 6.7
24.33 10.2 38.87 8.8
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Fig. 1. Class distributions in the product from rapid pyrolysis by PyGC/MS. (a) 23.8% and (b) 44.9%.
this study, a range of fragments derived from lipids, including derivatives of alkyl ester of benzoic acid, octadecanoic acid octyl ester and aliphatic hydrocarbons and squalence, were generated from macroalgae pyrolysis [20]. In the case of the lipid-rich sample, Chlorella 2, free fatty acids originating from lipid decomposition were found to be the predominant compounds. In the case of the lipid poor algae, Chlorella 1, sugars derivatives were the predominant compounds. Of the carbohydrates, levoglucosan, a dehydration product of glucose, was found as a major compound. In addition, esters, alcohols and ketones such as furanones, pyranones
and furan aldehydes were found, giving further evidence of sugar decomposition. In the pyrolysis products of Chlorella 1, an abundance of alcohols such as propanol and benzenediol were found. As can be seen from the Py-GC/MS peak areas of free fatty acids and hydrocarbons shown in Fig. 2, total areas are almost identical regardless of pyrolysis temperature. The peak areas of free fatty acids in the product from Chlorella 1 and Chlorella 2 were 42% and 70%, respectively. The increase in hydrocarbons compensated for the decrease in free fatty acids at high temperatures, suggesting that the origin of hydrocarbons from the pyrolysis of the KR-1 samples stems for the most part from free fatty acids.
Fig. 2. GC/MS peak areas of free fatty acids and hydrocarbons.
Fig. 3. Classification of ester compounds.
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Fig. 4. Peak areas of C16 and C18 free fatty acids.
From both samples, substantial amounts of pure hydrocarbons were generated. In fact in the Chlorella bearing the higher lipid content, the hydrocarbon carbon content was even greater, implying that pure hydrocarbons are generated from deoxygenation reactions of free fatty acids. Abundant C15 and C17 hydrocarbons were found as products of pyrolysis due to the free fatty acid deoxygenation of C16 and C18 fatty acids. It was observed that the deoxygenation reaction of the free fatty acids became more vigorous with increasing pyrolysis temperature, and pure hydrocarbons also increased while free fatty acids decreased. Although other compounds showed a decrease with temperature, this was not as extensive as that seen in the case of the free fatty acids and hydrocarbons. It is proposed that the origin of the compounds was sugar and their decomposition completed below 400 C [23].
Classification of ester compounds from microalga rapid pyrolysis was shown in Fig. 3. As can be seen, cyclic esters, such as furanones and pyranones, which are derived from the decomposition of sugars, were found as major ester components in the pyrolysis products of Chlorella 1. Fatty acid alkyl esters, condensation products of free fatty acids and alcohols, however, were generated in ample quantities during the pyrolysis of Chlorella 2. The peak areas of C16 and C18 free fatty acids from rapid pyrolysis of Chlorella sp. KR-1 were shown in Fig. 4. The composition analysis of the KR-1 samples showed the C18 free fatty acids to be more abundant than C16 free fatty acids. C18 free fatty acids decreased with increasing pyrolysis temperature. However, C16 free fatty acids decreased between 400 C and 500 C and then were unvaried at 600 C. Distribution of the products by carbon number (Fig. 5) indicates C6, C16 and C18 are most prominent. C6 compounds are derived from decomposition of sugars and the most abundant of these was levoglucosan. The main C16 and C18 compounds were free fatty acids. These free fatty acids were found (as expected) to be more abundant in the products from lipid-rich Chlorella 2. In addition, the concentration of C6, C16 and C18 compounds decreased whereas the concentration of other compounds increased with increasing pyrolysis temperature due to decomposition reactions. The pure hydrocarbons were subdivided into linear, naphthenic and aromatic hydrocarbons. As shown in Fig. 6, linear alkanes and olefins were the most common hydrocarbons seen as products of pyrolysis, especially in the case of Chlorella 2. There were no significant differences between the naphthenic levels of the lipid-rich and lipid-poor samples, meaning naphtens might be produced by the decarbonization of other components. Cyclic compounds were produced in the pyrolysis of various biomass materials [24,25]. tova et al. [26] proposed that aromatics could be generated Kuba through the intramolecular cyclization of alkenyl and alkadienyl radicals formed during pyrolysis. The content of hydrocarbons increased with increasing pyrolysis temperature due to the deoxygenation reactions becoming more vigorous in nature. There were only a few cyclic hydrocarbons possessing multiple ring structures in the temperature range studied and the major aromatic hydrocarbons instead were toluene. The distribution of the types of pure hydrocarbons generated from the pyrolysis was shown in Fig. 7. The C15 and C17
Fig. 5. Carbon number distributions of the products. (a) 23.8% and (b) 44.9%.
J.-G. Na et al. / Renewable Energy 81 (2015) 779e784
783
Fig. 6. Class distributions in the pure hydrocarbons. (a) 23.8% and (b) 44.9%.
hydrocarbons, the decarboxylated products of C16 and C18 fatty acids, were the main type of pure hydrocarbons found. As pyrolysis temperature increased, the concentration of lower molecular weight hydrocarbons increased. Since summation of the peak areas of the free fatty acids and hydrocarbons were found to be similar (Fig. 2), this is thought to indicate the further fragmentation of deoxygenated products rather than deoxygenation of sugars and proteins. By using the analysis results of microalgae, a tentative reaction for the reaction products was developed, as shown in Fig 8. Sugars in microalgae might be dehydrated and deoxygenated at relatively low temperature and results in levoglucosan or furanic compounds. As the reaction temperature increases, cyclic hydrocarbons might be converted from the sugars by secondary reaction. Lipids in microalgae are pyrolyzed at higher reaction temperature and are
converted into free fatty acids. Subsequently free fatty acids are converted into pure hydrocarbons by decarbonization or cracking reaction. The content of aromatic compounds moreover increases at higher temperature because of the aromatization of olefinic hydrocarbons. 4. Conclusions Rapid pyrolysis of Chlorella sp. KR-1 by using Py-GC/MS can result in a wide range of products, depending on the reaction temperatures and their triglyceride contents. During rapid pyrolysis, lipids were converted to free fatty acids and hydrocarbons by cracking and decarboxylation. Total amounts of free fatty acids and hydrocarbons estimated were almost identical regardless of pyrolysis temperature, suggesting that the origin of hydrocarbons
Fig. 7. Carbon number distributions of the pure hydrocarbons. (a) 23.8% and (b) 44.9%.
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Fig. 8. Proposed reaction pathway for the pyrolysis of the cell constituents of Chlorella sp. KR-1.
from the pyrolysis of the microalgae stems for the most part from the free fatty acids. It was observed that the deoxygenation reaction of the free fatty acids became more vigorous with increasing pyrolysis temperature, and pure hydrocarbons also increased while free fatty acids decreased. In conclusion, the triglycerides in the microalgae could be extracted in the form of suitable raw materials for biofuels without solvents by pyrolysis. Acknowledgments This paper was supported by research funds of Chonbuk National University in 2011, Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A1009749), and the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20132020000170). References [1] Haag AL. Algae bloom again. Nature 2007;447:520e1. [2] Bartle JR, Abadi A. Toward sustainable production of second generation bioenergy feedstocks. Energy Fuels 2010;24:2e9. [3] Tran NH, Bartlett JR, Kannangara GSK, Milev AS, Volk H, Wilson MA. Catalytic upgrading of biorefinery oil from micro-algae. Fuel 2010;89:265e74. [4] Mata TM, Martins AA, Caetano NS. Microalgae for biodiesel production and other applications. Renew Sustain Energy Rev 2010;14:217e32. [5] Ribeiro LA, Silva PP, Mata TM, Martins AA. Prospects of using microalgae for biofuels production: results of a Delphi study. Renew Energy 2015;75: 799e804. [6] Huntley ME, Redalje DG. CO2 mitigation and renewable oil from photosynthetic microbs: a new appraisal. Mitig Adapt Strat Glob Change 2007;12: 573e608. [7] Lee JN, Lee JS, Shin CS, Park SC, Kim SW. Effects of SO2 and NO on growth of highly CO2 tolerant microalgae. J Microbiol Biotechnol 2000;10:338e43. [8] Xu L, Brilman DWF, Withag JAM, Brem G, Kersten S. Assessment of a dry and a wet route for the production of biofuels from microalgae: energy balance analysis. Bioresour Technol 2011;102:5113e22.
[9] Li D, Chen L, Yi X, Zhang X, Ye N. Pyrolytic characteristics and kinetics of two brown algae and sodium alginate. Bioresour Technol 2010;101:7131e6. [10] Ross AB, Jones JM, Kubacki ML, Bridgeman T. Classification of macroalgae as fuel and its thermochemical behavior. Bioresour Technol 2008;99:6494e504. [11] Wang X, Zhao B, Tang X, Yang X. Comparison of direct and indirect pyrolysis of micro-algae Isochrysis. Bioresour Technol 2015;179:58e62. [12] Lee SH, Eom MS, Yoo KS, Kim NC, Jeon JK, Park YK, et al. The yields and composition of bio-oil produced from Quercus Acutissima in a bubbling fluidized bed pyrolyzer. J Anal Appl Pyrolysis 2008;83:110e4. [13] Park DK, Kim SD, Lee SH, Lee JG. Co-pyrolysis characteristics of sawdust and coal blend in TGA and a fixed bed reactor. Bioresour Technol 2010;101: 6151e6. [14] Eom MS, Lee SH, Yoo KS, Park YK, Lee JG, Kim SS, et al. Production of bio oil by using Larch sawdust in a bubbling fluidized bed reactor. Energy Sources Part A 2013;35:1225e32. [15] Kim SW, Koo BS, Lee DH. A comparative study of bio-oils from pyrolysis of microalgae and oil seed waste in a fluidized bed. Bioresour Technol 2014;162: 96e102. [16] Li D, Chen L, Zhang X, Ye N, Xing F. Pyrolytic characteristics and kinetic studies of three kinds of red algae. Biomass Bioenergy 2011;35:1765e72. [17] Maddi B, Viamajala S, Varanasi S. Comparative study of pyrolysis of algal biomass from natural lake blooms with lignocellulosic biomass. Bioresour Technol 2011;102:11018e26. [18] Anastasakis K, Ross AB, Jones JM. Pyrolysis behavior of the main carbohydrates of brown macro-algae. Fuel 2011;90:598e607. [19] Yanik J, Stahl R, Troeger N, Sinag A. Pyrolysis of algal biomass. J Anal Appl Pyrolysis 2013;103:134e41. [20] Kebelmann K, Hornung A, Karsten U, Griffiths G. Thermo-chemical behavior and chemical product formation from polar seaweeds during intermediate pyrolysis. J Anal Appl Pyrolysis 2013;104:131e8. [21] Chaiwong K, Kiatsiriroat T, Vorayos N, Thararax C. Study of bio-oil and biochar production from algae by slow pyrolysis. Biomass Bioenergy 2013;56: 600e6. [22] Lee YC, Lee KB, Hwang YH, Andersen HR, Kim BH, Lee SY, et al. Aminoclaytemplated nanoscale zero-valent iron (nZVI) synthesis for efficient harvesting of oleaginous microalga, Chlorella sp. KR-1. RSC Adv 2014;4:4122e7. [23] Na JG, Lee HS, Oh YK, Park JY, Ko CH, Lee SH, et al. Rapid estimation of triacylglycerol content of Chlorella sp. By thermogravimetric analysis. Biotechnol Let 2011;33:957e60. [24] Pütün E, Ates¸ F, Pütün AE. Catalytic pyrolysis of biomass in inert and steam atmospheres. Fuel 2008;87:815e24. [25] Vardon DR, Sharma BK, Blazina GV, Rajagopalan K, Strathmann TJ. Thermochemical conversion of raw and defatted algal biomass via hydrothermal liquefaction and slow pyrolysis. Bioresou Technol 2012;109:178e87. tov vova J, Seames WS, Luo Y, Sadrameli SM, Linnen MJ, et al. [26] Kuba a A, St'a Triacylgrlyceride thermal cracking: pathways to cyclic hydrocarbons. Energy Fuels 2012;26:672e85.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Rapid pyrolysis behavior of oleaginous microalga, Chlorella sp. KR-1 with different triglyceride contents Jeong-Geol Na a, Young-Kwon Park b, Doo Il Kim a, You-Kwan Oh a, Sang Goo Jeon a, Jin Woo Kook c, Ji Hoon Shin c, See Hoon Lee c, * a b c
Clean Fuel Department, Korea Institute of Energy Research, Republic of Korea School of Environmental Engineering, University of Seoul, Republic of Korea Dept. of Mineral Resources and Energy Eng., Chonbuk National University, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 July 2014 Accepted 31 March 2015 Available online
Rapid pyrolysis of two Chlorella sp. KR-1 samples, with triglyceride content of 23.8 and 44.9wt%, was conducted using pyrolysis gas chromatograph-mass spectrometry over a temperature range (400 e600 C). The effects of triglyceride content of the microalgal cells and pyrolysis temperature on the structure and distribution of products were analyzed and the pyrolysis products were classified according to number of carbons and chemical species. The triglyceride content of microalgal cells was found to affect the concentration of free fatty acids, hydrocarbons, and derivatives of carbohydrates. In the case of the lipid-rich sample, free fatty acids originating from triglyceride decomposition were found to be the predominant compounds whereas sugar-dehydrated products such as anhydroglucose were the predominant compounds in the case of the lipid-poor sample. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Rapid pyrolysis Gas chromatograph-mass spectrometry Microalgae Free fatty acid Hydrocarbon
1. Introduction Research and development has recently seen a considerable emphasis placed on the use of biomass as a fuel source to ease the depletion of global fossil fuel reserves. From a feedstock supply and ethical standpoint, much attention has been placed on algae because it has high photosynthetic yields, and can be cultivated on non-arable land. Additionally, some algae species can accumulate large amounts of lipids intracellularly, which could potentially be converted into various biofuels [1e4]. By using the Delphi method, most of the experts believe that the production of microalgae for biofuels will only achieve full commercial scale until 2020, whereafter it could represent an important share of the total worldwide fuel production despite some remaining challenges [5]. The conventional process for producing biofuels from microalgae biomass involves extracting lipids using an organic solvent followed by transesterification reactions. This is a traditional technology, having been developed commercially for the conversion of vegetable oils into biodiesel. Solvent extraction, however, is not very efficient for microalgae. This is because of the thicker cell
* Corresponding author. Tel.: þ82 63 270 3632; fax: þ82 63 270 2366. E-mail address: [email protected] (S.H. Lee). http://dx.doi.org/10.1016/j.renene.2015.03.088 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
wall and structural differences of the microalgae compared with their terrestrial biomass counterparts. The excessive amount of water in collected microalgae is one of main bottlenecks, because the energetic output only slightly exceeds the required fuel input for the production and processing of microalgae [6e8]. Although less energy-intensive dewatering methods are being developed, several thermochemical processes have been studied and proposed because of their yield in chemicals and fuels [8e11]. In the case of woody biomass, thermochemical conversions such as pyrolysis, gasification and combustion have already been developed at a commercial level. Of these processes, pyrolysis has received attention in recent years because of its simplicity and speed. In addition, unlike other thermochemical conversion methods, pyrolysis of biomass could produce versatile liquid products that are easy to handle and that are feedstocks for chemicals [12e14]. Generally, microalgae biomass contains a large amount of triglycerides, whereas their lignin content is typically low, although this is species dependent. In addition to triglycerides, microalgae possess carbohydrates and proteins. These imply that the thermal behavior of microalgae biomass is different from that of lignocellulosic biomass. According to Ross et al. [10] the pyrolysis of microalgae underwent a step-wise process where carbohydrates, proteins and lipids decomposed. Also the bio-oil from microalgae
780
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contained more oxygen and nitrogen and less sulfur than palm oils and there were high yield of fatty oxygenates and nitrogenous compounds in the bio-oil due to high contents of lipids and proteins [15]. The pyrolysis characteristics of algal biomass have been investigated in a number of studies using various experimental conditions [9,10,16e21]. Ross et al. [10] reported the thermochemical behavior of microalgae, Fucus vesiculosus, Chorda flum, Laminaria digitata, F. serratus, L. hyperborea, and Macrocystis pyrifera, and concluded that flash pyrolysis produced a number of interesting compounds and a range of low molecular weight oxygenated and nitrogen containing compounds. Li et al. [9,16] studied the pyrolysis characteristics and kinetics of marine algae and found that there were three stages during the pyrolysis of macroalgae. These were moisture evaporation, primary devolatilization and residual decomposition. Additionally, it was found that the average active energy range for a pyrolysis reaction was 154e250 kJ/mol. In experiments of a fluidized bed pyrolyzer of seaweed [19], the composition on liquids obtained varied with the seaweed species. Maddi et al. [17] compared the pyrolysis characteristics of algal and lignocellulose feedstocks and suggested that algae pyrolysis could produce liquid fuel in appreciable yield, and that the technologies being developed for thermochemical conversion of lignocellulose materials could be applied to algae as well. Most studies of algal pyrolysis have compared only the characteristics of different algae species, especially for macroalgae. There are very few studies in which the effects of lipid content on the product distribution following pyrolysis with same microalgae species have been investigated. In the present study, pyrolysis of two Chlorella sp. KR-1 samples, differing in triglyceride content, was conducted at different temperatures using pyrolysis gas chromatograph-mass spectrometry (PyGC/MS). The pyrolysis products were classified according to number of carbons and chemical species. The effect of triglyceride content and pyrolysis temperature on the reaction pathway is also discussed. Based on these results, the pyrolysis process for the conversion of microalgae to potentially useful products was evaluated. 2. Experimental 2.1. Preparation of microalgae biomass Chlorella sp. KR-1, a freshwater microalgae isolated from water samples collected in a region nearby Youngwol, Korea was used in this work. The microalgae was cultivated in a bubble-column reactor (working volume, 500 mL), which was made of Pyrex glass. The reactor was placed in a growth chamber equipped with six fluorescent lamps (light intensity: ca. 40 mmol photons/m2s) positioned at the front and side of the chamber. A mixture of CO2 (10 vol%) in air was supplied to the reactor at a flow rate of 0.4 L/ min. The cultivation temperature was 25 C. Two microalgae samples containing respectively 23.8% and 44.9wt% of triglyceride were obtained by adjusting the nitrate concentration to 4 mM and 1 mM, respectively. The microalgae cells were harvested by
centrifugation (3800 g, 10 min) and washed with distilled water (repeated twice). The cells were then dried in a freeze-dryer (FD5512, IlShinBioBase Co., Korea) for at least four days. The freeze-dried cells were then analyzed by pyGC/MS. The triglyceride content in the KR-1 samples was analyzed using the modified direct transesterification method. Detailed information on Chlorella sp. KR-1 and growth conditions can be found in Lee et al. [22] and Na et al. [23]. 2.2. Py-GC/MS analyses A double-shot pyrolyzer (Frontier-Lab Co., Py-2020iD), coupled directly to GC/MS, was used for identification of the pyrolysis products. The Chlorella sp. KR-1 (2 mg) was placed in a sample cup and then into a furnace at different temperatures (400, 500 and 600 C) under a He atmosphere. The gaseous species generated during the pyrolysis were introduced via a GC inlet port (split ratio of 1/100) into a metal capillary column (Ultra ALLOY-5MS/HT; 5% diphenyl and 95% dimethylpolysiloxane, length 30 m, i.d. 0.25 mm, film thickness 0.5 um, Frontier Laboratories Ltd. Japan). To prevent condensation of products, the interface and inlet temperatures were both maintained at 300 C. The column temperature was programmed to change from 40 (5 min) to 320 C (10 min), at a heating rate of 5 C/min. The temperature of the GC/MS interface was 280 C, with the MS operated in the EI mode at 70 eV. The program was run in the scanning range from 29 to 400 a.m.u. at a rate of 2 scans/sec. The identification of peaks was performed using the NISTMS library. The experiments were conducted at least three times for each run to confirm the reproducibility of the reported procedures. The average values of the peak area and peak area % as received were calculated for each identified product. 3. Results and discussion The fatty acid composition in Chlorella sp. KR-1 samples is shown in Table 1. Palmitic acid (C16:0) content is higher than other fatty acid content, but C18 fatty acids such as stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2) and linolenic acid (C18:3) are more abundant in the KR-1 samples than C16 fatty acids. In the microalgae cells containing 23.8% lipids (Chlorella 1), the content of C16 and C18 fatty acids were found to be 29.6% and 59.7%, respectively. Conversely, in the microalgae cells with a lipid content of 44.9% (Chlorella 2), C16 and C18 fatty acids were found to be 35.3% and 56.0% respectively. The ratio of C16 and C18 fatty acids in the two samples is therefore comparable. In general pyrolysis of lignocellulose material is performed at reaction temperatures between 400 C and 600 C [12e14]. For this reason, pyrolysis with Chlorella sp. KR-1 was carried out at the same temperatures. Fig. 1 shows the distribution of structural classes in the products obtained from rapid pyrolysis experiments using PyGC/MS. The products were classified as free fatty acids, carbohydrates, hydrocarbons, alcohols, aldehydes, esters, ketones, and others depending on their functional groups. The lipid content of the algae was found to affect the concentration of free fatty acids, hydrocarbons, and derivatives of carbohydrates (Fig. 1). Similar to
Table 1 Fatty acid composition in Chlorella sp. KR-1 cells. Sample Chlorella 1 Chlorella 2
Cellular content (mg/g cell) Relative content (%) Cellular content (mg/g cell) Relative content (%)
Myristic acid (C14:0)
Palmitic acid (C16:0)
Myristoleic acid (C16:1)
Stearic acid (C18:0)
Oleic acid (C18:1)
Linoleic acid (C18:2)
Linolenic acid (C18:3)
Others
0.93 0.4 e e
64.42 29.6 130.74 32.1
e e 13.31 3.2
16.60 7.1 29.40 6.8
53.69 24.0 96.31 23.0
66.49 25.2 96.16 19.5
11.72 3.4 43.88 6.7
24.33 10.2 38.87 8.8
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Fig. 1. Class distributions in the product from rapid pyrolysis by PyGC/MS. (a) 23.8% and (b) 44.9%.
this study, a range of fragments derived from lipids, including derivatives of alkyl ester of benzoic acid, octadecanoic acid octyl ester and aliphatic hydrocarbons and squalence, were generated from macroalgae pyrolysis [20]. In the case of the lipid-rich sample, Chlorella 2, free fatty acids originating from lipid decomposition were found to be the predominant compounds. In the case of the lipid poor algae, Chlorella 1, sugars derivatives were the predominant compounds. Of the carbohydrates, levoglucosan, a dehydration product of glucose, was found as a major compound. In addition, esters, alcohols and ketones such as furanones, pyranones
and furan aldehydes were found, giving further evidence of sugar decomposition. In the pyrolysis products of Chlorella 1, an abundance of alcohols such as propanol and benzenediol were found. As can be seen from the Py-GC/MS peak areas of free fatty acids and hydrocarbons shown in Fig. 2, total areas are almost identical regardless of pyrolysis temperature. The peak areas of free fatty acids in the product from Chlorella 1 and Chlorella 2 were 42% and 70%, respectively. The increase in hydrocarbons compensated for the decrease in free fatty acids at high temperatures, suggesting that the origin of hydrocarbons from the pyrolysis of the KR-1 samples stems for the most part from free fatty acids.
Fig. 2. GC/MS peak areas of free fatty acids and hydrocarbons.
Fig. 3. Classification of ester compounds.
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Fig. 4. Peak areas of C16 and C18 free fatty acids.
From both samples, substantial amounts of pure hydrocarbons were generated. In fact in the Chlorella bearing the higher lipid content, the hydrocarbon carbon content was even greater, implying that pure hydrocarbons are generated from deoxygenation reactions of free fatty acids. Abundant C15 and C17 hydrocarbons were found as products of pyrolysis due to the free fatty acid deoxygenation of C16 and C18 fatty acids. It was observed that the deoxygenation reaction of the free fatty acids became more vigorous with increasing pyrolysis temperature, and pure hydrocarbons also increased while free fatty acids decreased. Although other compounds showed a decrease with temperature, this was not as extensive as that seen in the case of the free fatty acids and hydrocarbons. It is proposed that the origin of the compounds was sugar and their decomposition completed below 400 C [23].
Classification of ester compounds from microalga rapid pyrolysis was shown in Fig. 3. As can be seen, cyclic esters, such as furanones and pyranones, which are derived from the decomposition of sugars, were found as major ester components in the pyrolysis products of Chlorella 1. Fatty acid alkyl esters, condensation products of free fatty acids and alcohols, however, were generated in ample quantities during the pyrolysis of Chlorella 2. The peak areas of C16 and C18 free fatty acids from rapid pyrolysis of Chlorella sp. KR-1 were shown in Fig. 4. The composition analysis of the KR-1 samples showed the C18 free fatty acids to be more abundant than C16 free fatty acids. C18 free fatty acids decreased with increasing pyrolysis temperature. However, C16 free fatty acids decreased between 400 C and 500 C and then were unvaried at 600 C. Distribution of the products by carbon number (Fig. 5) indicates C6, C16 and C18 are most prominent. C6 compounds are derived from decomposition of sugars and the most abundant of these was levoglucosan. The main C16 and C18 compounds were free fatty acids. These free fatty acids were found (as expected) to be more abundant in the products from lipid-rich Chlorella 2. In addition, the concentration of C6, C16 and C18 compounds decreased whereas the concentration of other compounds increased with increasing pyrolysis temperature due to decomposition reactions. The pure hydrocarbons were subdivided into linear, naphthenic and aromatic hydrocarbons. As shown in Fig. 6, linear alkanes and olefins were the most common hydrocarbons seen as products of pyrolysis, especially in the case of Chlorella 2. There were no significant differences between the naphthenic levels of the lipid-rich and lipid-poor samples, meaning naphtens might be produced by the decarbonization of other components. Cyclic compounds were produced in the pyrolysis of various biomass materials [24,25]. tova et al. [26] proposed that aromatics could be generated Kuba through the intramolecular cyclization of alkenyl and alkadienyl radicals formed during pyrolysis. The content of hydrocarbons increased with increasing pyrolysis temperature due to the deoxygenation reactions becoming more vigorous in nature. There were only a few cyclic hydrocarbons possessing multiple ring structures in the temperature range studied and the major aromatic hydrocarbons instead were toluene. The distribution of the types of pure hydrocarbons generated from the pyrolysis was shown in Fig. 7. The C15 and C17
Fig. 5. Carbon number distributions of the products. (a) 23.8% and (b) 44.9%.
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Fig. 6. Class distributions in the pure hydrocarbons. (a) 23.8% and (b) 44.9%.
hydrocarbons, the decarboxylated products of C16 and C18 fatty acids, were the main type of pure hydrocarbons found. As pyrolysis temperature increased, the concentration of lower molecular weight hydrocarbons increased. Since summation of the peak areas of the free fatty acids and hydrocarbons were found to be similar (Fig. 2), this is thought to indicate the further fragmentation of deoxygenated products rather than deoxygenation of sugars and proteins. By using the analysis results of microalgae, a tentative reaction for the reaction products was developed, as shown in Fig 8. Sugars in microalgae might be dehydrated and deoxygenated at relatively low temperature and results in levoglucosan or furanic compounds. As the reaction temperature increases, cyclic hydrocarbons might be converted from the sugars by secondary reaction. Lipids in microalgae are pyrolyzed at higher reaction temperature and are
converted into free fatty acids. Subsequently free fatty acids are converted into pure hydrocarbons by decarbonization or cracking reaction. The content of aromatic compounds moreover increases at higher temperature because of the aromatization of olefinic hydrocarbons. 4. Conclusions Rapid pyrolysis of Chlorella sp. KR-1 by using Py-GC/MS can result in a wide range of products, depending on the reaction temperatures and their triglyceride contents. During rapid pyrolysis, lipids were converted to free fatty acids and hydrocarbons by cracking and decarboxylation. Total amounts of free fatty acids and hydrocarbons estimated were almost identical regardless of pyrolysis temperature, suggesting that the origin of hydrocarbons
Fig. 7. Carbon number distributions of the pure hydrocarbons. (a) 23.8% and (b) 44.9%.
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Fig. 8. Proposed reaction pathway for the pyrolysis of the cell constituents of Chlorella sp. KR-1.
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