Bioresource Technology 151 (2014) 415–418
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Biodiesel production from lipids in wet microalgae with microwave irradiation and bio-crude production from algal residue through hydrothermal liquefaction Jun Cheng ⇑, Rui Huang, Tao Yu, Tao Li, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Cogeneration of biodiesel and bio-
crude was proposed to make full use of microalgae. Total energy recovery was increased to 67.73% by utilization of microalgae residue. Remaining nitrogen in liquid fuel decreased to 16.02% by the cogeneration process. Lipids and proteins were prevented from interacting to form low-grade liquid fuel.
a r t i c l e
i n f o
Article history: Received 23 August 2013 Received in revised form 7 October 2013 Accepted 10 October 2013 Available online 17 October 2013 Keywords: Microalgae Biodiesel Bio-crude Microwave Hydrothermal liquefaction
a b s t r a c t A cogeneration process of biodiesel and bio-crude was proposed to make full use of wet microalgae biomass. High-grade biodiesel was first produced from lipids in wet microalgae through extraction and transesterification with microwave irradiation. Then, low-grade bio-crude was produced from proteins and carbohydrates in the algal residue through hydrothermal liquefaction. The total yield (40.19%) and the total energy recovery (67.73%) of the cogenerated biodiesel and bio-crude were almost equal to those of the bio-oil obtained from raw microalgae through direct hydrothermal liquefaction. Upon microwave irradiation, proteins were partially hydrolyzed and the hydrolysates were apt for deaminization under the hydrothermal condition of the algal residue. Hence, the total remaining nitrogen (16.02%) in the cogenerated biodiesel and bio-crude was lower than that (27.06%) in the bio-oil. The cogeneration process prevented lipids and proteins from reacting to produce low-grade amides and other long-chain nitrogen compounds during the direct hydrothermal liquefaction of microalgae. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The traditional process of biodiesel production from algae often requires the consumption of a large amount of energy because of algae dewatering (Patil et al., 2012). As microwave irradiation has been reported to effectively extract lipids from wet algae and accelerate the transesterification reaction (Yuan et al., 2009; Lee et al., 2010), a process for the direct conversion of wet microalgae
⇑ Corresponding author. Tel.: +86 571 87952889; fax: +86 571 87951616. E-mail address: [email protected] (J. Cheng). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.10.033
biomass into biodiesel by microwave irradiation has been proposed in a previous study (Cheng et al., 2013). This simplified process has been shown to avoid the energy consumption by algae dewatering and increase both the biodiesel production rate and the yield via microwave irradiation. Although the lipids in wet algae are effectively converted into biodiesel in this process, the biomass residue (consisting mainly of carbohydrates and proteins) that remains after the extraction ends up as waste. The efficiency of utilizing the residue left after the lipid extraction has a significant influence on the overall energy balance of the microalgae-to-fuel conversion, according to an assessment by Xu et al. (2011).
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However, there have been few reported experiments regarding the algae residue that remains after lipid extraction. In recent years, the direct hydrothermal liquefaction of microalgae has attracted considerable interest for its low energy consumption and high oil yield. The bio-oil yield obtained through the hydrothermal process is 10–15% higher than the lipid content of the microalgae, by converting approximately 10% of the carbohydrates and 20% of the proteins into bio-oil (Biller et al., 2011). However, as oxygen-containing carbohydrates and nitrogen-containing proteins are converted into bio-oil, the bio-oil obtained from raw algae through direct hydrothermal liquefaction typically has high nitrogen (6 wt.%) and oxygen (12 wt.%) contents. The mixing of the carbohydrates and proteins with the lipids under a hydrothermal condition has also resulted in a complicated composition of the bio-oil (Brown et al., 2010). As this is undesirable in the final product, further deoxygenation and denitrification of this bio-oil has become a bottleneck in the development of a hydrothermal process. In order to utilize the algae residue that remains after the lipid extraction and to avoid the deterioration of lipids in the hydrothermal process, a cogeneration process of high-grade biodiesel from lipids in microalgae and low-grade bio-crude from proteins and carbohydrates in algal residue was proposed and comprehensively investigated in this paper. 2. Methods 2.1. Materials Nannochloropsis oceanica was purchased from Yantai Hearol Biotechnology Co., Ltd (China). The moisture content in the N. oceanica biomass was determined by oven drying at 105 °C until a constant mass was achieved. Distilled water was added to the dried N. oceanica cells to produce wet algae containing 80 wt.% water. Chloroform, methanol, and sulfuric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). 2.2. Biodiesel production from lipids in wet microalgae through extraction and transesterification with microwave irradiation Biodiesel was first produced from the lipids in wet microalgae through extraction and transesterification with microwave irradiation. As described in a previous study (Cheng et al., 2013), a WX4000 microwave digestion system (2.45 GHz, Shanghai Yiyao Microwave Chemistry Company) equipped with six 60-ml digestion reactors was used. Approximately 4 g of wet algae was placed in the digestion tank and mixed with 16 ml of chloroform, 16 ml of methanol, and 0.8 ml of sulfuric acid. The algal biomass was heated via microwave irradiation and maintained at 60 °C for 30 min. After the treatment, distilled water was added to the mixture, which was then centrifuged. Thereafter, the organic layer was transferred into a pre-weighed glass vial and evaporated in a baking oven at 70 °C. The aqueous phase and the algae residue remained in centrifuge tube were centrifuged to separate the algae residue. 2.3. Bio-crude production from algal residue through hydrothermal reaction Bio-crude was produced from the algal residue through a hydrothermal reaction. The algae residue obtained from the process described in Section 2.2 was washed with methanol (10 ml) three times in order to remove the residual chloroform. Then, it was washed three times with distilled water (10 ml) to remove the residual methanol.
Thereafter, the algae residue was mixed with 90 ml of water and added into a well-stirred batch reactor (500 ml) (Parr 4575A, USA). The residual air was removed from the sealed reactor by purging the reactor with nitrogen. Further, a liquefaction experiment was performed at 300 °C for 0.5 h. After the liquefaction, 20 ml of chloroform was added to the mixture in order to extract the bio-crude. Then, the reactor was washed five times with 10 ml of chloroform. All the used chloroform was collected and then centrifuged. Then, the organic layer was transferred into a pre-weighed glass vial; this layer was evaporated in oven at 70 °C. Finally, the weight of the biocrude was determined gravimetrically. 2.4. Bio-oil production from raw microalgae by hydrothermal liquefaction Approximately 50 g of wet algae was mixed with 40 ml of distilled water and was added into the reactor. The rest of the processes were the same as those described in Section 2.3. 2.5. Analytical chemistry The C, H, N, and S contents of the sample were measured using a CE Instruments Flash EA 1112 series elemental analyzer (Biller et al., 2011). The liquid fuel was analyzed using a GC–MS system (Trace DSQII, Thermo Scientific, USA) equipped with a 30 m 0.25 mm 0.25 lm Agilent DB-WAX capillary column (Wang et al., 2012). The oven temperature in the case of biodiesel was first maintained at 150 °C for 5 min. Then, the temperature was increased to 250 °C at the rate of 4 °C/min and was maintained at 250 °C for 5 min. The oven temperature in the cases of bio-crude and bio-oil was first maintained at 70 °C. Then, the temperature was increased to 250 °C at the rate of 3 °C/min and was maintained at 250 °C for 10 min. Data treatment was carried out using a computer with Xcalibur software and the NIST mass spectral library. All measurements were repeated in triplicate, and the mean values were reported. 3. Results and discussion 3.1. Comparison in oil yield N. oceanica biomass primarily contained 22.7% carbohydrates, 19.1% proteins, 24.8% lipids, and 16.2% moisture. This biomass was rehydrated to produce wet algae containing 80 wt.% water for experiments. As shown in Fig. 1, the yield of biodiesel obtained from the lipids in wet microalgae through extraction and transesterification was 29.63%, and the yield of bio-crude obtained from the microalgae residue through the hydrothermal reaction was 10.56%. The conversion efficiency of algal lipids into biodiesel was nearly 100%, which was higher than 90% obtained in supercritical alcohol conditions reported in recent literatures (Patil et al., 2011a, 2013). The yield of the bio-oil obtained from raw microalgae through direct hydrothermal liquefaction was 40.08%. These results revealed that the yield of bio-oil was higher than the yield of biodiesel. This could be explained by the conversion of carbohydrates and proteins into liquid fuels under a hydrothermal condition. The total yield (40.19%) of the cogenerated biodiesel and biocrude was almost equal to the yield (40.08%) of the bio-oil obtained from the raw microalgae through the direct hydrothermal liquefaction. This is most likely due to the following reasons: (1) The lipids in microalgae were effectively converted into liquid fuel in the cogeneration process and in the direct hydrothermal liquefaction of the raw microalgae, respectively. The lipids in the microalgae
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3.2. Elemental analysis and energy recovery Table 1 shows the elemental compositions and higher heating values (HHV) of feedstock and products. Biodiesel had the highest HHV (38.8 MJ/kg) along with the highest carbon content (76.92%), highest hydrogen content (10.36%), and lowest nitrogen content (1.59%). Bio-crude had the lowest HHV (28.6 MJ/kg) along with the lowest carbon content (63.62%), lowest hydrogen content (7.58%), and highest nitrogen content (6.98%). This indicated that the liquid fuels derived from the microalgae were successfully separated into two grades of fuel (biodiesel and bio-crude) in the cogeneration process. As shown in Fig. 1, the total energy recovery was increased from 53.6% to 67.73% by utilization of microalgae residue. The energy recovery of 67.73% in the cogeneration process of biodiesel and bio-crude was higher by 14.1% than that in only biodiesel
90
Oil yield, Element and Energy recovery(%)
were extracted and converted into high-grade biodiesel in the cogeneration process. The lipids in microalgae were hydrolyzed into fatty acids (some of the fatty acids decomposed into hydrocarbons) and became a part of bio-oil in the direct hydrothermal liquefaction (King et al., 1999; Watanabe et al., 2006). (2) The amount of liquid fuel obtained from the proteins decreased slightly in the cogeneration process of biodiesel and bio-crude. The proteins in the microalgae underwent a partial hydrolysis with microwave irradiation and acid during the lipids extraction in the cogeneration process (Xia et al., 2013). Since linear nitrogen-containing compounds were present in the form of the hydrolysates of proteins and were apt for the deaminization under a hydrothermal condition, a partial hydrolysis of the proteins prior to the hydrothermal liquefaction would reduce the nitrogen content in the liquid fuel (Dote et al., 1991). Further, the extraction of lipids prior to the hydrothermal liquefaction inhibited the polar nitrogen-containing organics from reacting with the fatty acids to form low-grade long-chain nitrogen compounds. This decreased the yield of the liquid fuel obtained from the proteins. (3) The amount of liquid fuel obtained from the carbohydrates was increased slightly in the cogeneration process of biodiesel and bio-crude. The ammonia derived from the deamination of the proteins promoted the conversion of the carbohydrates into the liquid fuel (Kruse et al., 2007). Further, the promoted deamination of the proteins resulted in an increase in ammonia in the aqueous phase. This increased ammonia in the aqueous phase in turn enhanced the yield of the liquid fuel obtained from the carbohydrates in the cogeneration process. Since the peptide bonds of the proteins were more stable than the glycosidic bonds in cellulose and starch, the carbohydrates present in the biomass had a tendency to be hydrolyzed faster than the proteins (Rogalinski et al., 2008; Toor et al., 2013). Thus, the carbohydrates tended to convert into a gas product at an early stage of the hydrothermal liquefaction without the effect of ammonia in the direct hydrothermal liquefaction of the raw microalgae.
Biodiesela Bio-crudeb Bio-oilc
80 70 60 50 40 30 20 10 0
Oil
Carbon
Hydrogen Nitrogen
Energy
Fig. 1. Comparison in oil yield, element and energy recovery between the cogeneration process and direct hydrothermal liquefaction of raw microalgae e,d. Notes: aBiodiesel was obtained from the lipids in wet microalgae through extraction and transesterification with microwave irradiation. bBio-crude was obtained from the algal residue though hydrothermal reaction. cBio-oil was obtained from the raw microalgae though direct hydrothermal liquefaction. dThe yield of the liquid fuel was calculated on the basis of the dry microalgae weight. eElement and energy recovery were defined as the ratios of carbon, hydrogen, nitrogen, and higher heating value (HHV) in liquid fuel to those in raw microalgae feedstock.
production reported in literature (Patil et al., 2011b). The total energy recovery (67.73%), carbon recovery (58.95%), and hydrogen recovery (51.88%) of the cogenerated biodiesel and bio-crude were almost equal to those of the bio-oil. However, compared with that in the bio-oil, the total remaining nitrogen in the cogenerated biodiesel and bio-crude was decreased from 27.06% to 16.02%. The total remaining oxygen (15.86%) in the cogenerated biodiesel and bio-crude was higher than that (14.02%) in the bio-oil. This could be explained by the following reasons: The hydrolysis of proteins prior to the hydrothermal liquefaction promoted the removal of the linear nitrogen contained in the proteins as ammonia and thus, decreased the remaining nitrogen in the cogenerated biodiesel and bio-crude. The promoted deamination of the proteins led to an increased yield of the liquid fuel derived from the oxygen-containing carbohydrates and thus, resulted in an increase in the remaining oxygen in the cogenerated biodiesel and bio-crude.
3.3. GC–MS analysis of the liquid fuel According to the GC–MS analyses, the biodiesel obtained from the lipids in the wet microalgae through extraction and transesterification using microwave irradiation mainly contained FAMEs such as C17H32O2 and C21H32O2. The bio-crude obtained
Table 1 Elemental compositions and higher heating values of feedstock and products.
Dried biomass of raw microalgae Biodiesel obtained from lipids in wet microalgae through extraction and transesterification with microwave irradiation Dried algal residue obtained after lipids extraction of raw microalgae with microwave irradiation Bio-crude obtained from algal residue though a hydrothermal reaction Bio-oil obtained from raw microalgae though direct hydrothermal liquefaction a
Sulfur (%)
Oxygena (%)
Higher heating valueb (MJ/kg)
7.54 1.59
0.47 0.21
34.47 10.92
21.46 38.86
6.55
11.07
0.56
36.58
18.17
7.58 9.75
6.98 5.09
0.69 0.52
21.13 12.06
28.62 36.35
Carbon (%)
Hydrogen (%)
50.06 76.92
7.46 10.36
45.24 63.62 72.58
Nitrogen (%)
Oxygen content was calculated by subtraction of carbon, hydrogen, nitrogen and sulfur contents from 100%. The higher heating value (HHV) was estimated with the Dulong formula (Biller et al., 2011): HHV (MJ/kg) = 0.338C + 1.428(H–O/8) + 0.095S, where C, H, O, and S were weight percentages of elemental compositions in materials. b
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from the microalgae residue through the hydrothermal reaction mainly contained heterocyclic nitrogen compounds (such as C11H18N2O2 and C8H7N). A large number of unidentified compounds were also detected in this bio-crude. The bio-oil obtained from the raw microalgae through the direct hydrothermal liquefaction contained large quantities of fatty acids (such as C16H30O2 and C16H32O2), some fatty acid amides (such as C16H33NO and C18H35NO), and heterocyclic nitrogen compounds. It revealed that the cogeneration process effectively separated the liquid fuel derived from the microalgae into two different grades of fuel (biodiesel and bio-crude) and thus, simplified the further utilization of these fuels. The biodiesel mainly contained FAMEs with the carbon-chain length ranging from 16 to 20. The bio-crude mainly contained pyrrole, indole, and other heterocyclic nitrogen compounds having a carbon-chain length ranging from 6 to 14. Almost no nitrogen compounds containing carbon chains having a length of 16–21 were detected in the bio-crude. It indicated that the cogeneration process prevented the lipids and the proteins from reacting to form low-grade amides and other long-chain nitrogen compounds and thus, increase the yield of the high-grade oil. The bio-oil contained 48.47% of the fatty acids having a carbon-chain length ranging from 16 to 18. However, 13.63% of the bio-oil was amides, pyrroles, and indoles, which contained carbon chains having a length of 16–21. This was attributed to the reaction between the lipids and the proteins under the hydrothermal condition during the direct hydrothermal liquefaction of the raw microalgae. This reaction formed amides and other nitrogen-containing compounds, and thus led to the deterioration of the lipids in the microalgae. A comparison revealed that the cogeneration process of biodiesel and bio-crude prevented the decomposition of some unsaturated fatty acids under the hydrothermal condition and produced abundant FAMEs. The biodiesel mainly contained FAMEs including C16:1, C20:5, C16:0, C20:4, C18:1, C18:2, and C20:3. However, only the corresponding fatty acids of C16:1, C16:0, C18:1, and C18:2 and a considerable amount of C14:0 were detected in the bio-oil. This could be explained by the decomposition of the highly unsaturated fatty acids (such as C20:5, C20:4, and C20:3) during the direct hydrothermal liquefaction of the microalgae. As lipids were extracted prior to the hydrothermal liquefaction in the cogeneration process, highly purified hydrocarbons could be obtained by further deoxidizing the lipids. The bio-oil contained approximately 4% hydrocarbons with a carbon-chain length of 15–20, whereas almost no hydrocarbons were detected in the bio-crude. This indicated that the hydrocarbons in the bio-oil were produced by deoxidation of the fatty acids under the hydrothermal condition. However, as hydrocarbons were mixed with the other hundreds of compounds in the bio-oil, it was difficult to utilize these hydrocarbons in the bio-oil. 4. Conclusion The cogeneration of high-grade biodiesel from lipids in microalgae and low-grade bio-crude from proteins and carbohydrates in algal residue is a promising process to make full use of microalgae biomass. The total yield (40.19%) and energy recovery (67.73%) of the cogenerated biodiesel and bio-crude were almost equal to those of the bio-oil obtained from microalgae through direct hydrothermal liquefaction. However, the total remaining nitrogen (16.02%) in the cogenerated biodiesel and bio-crude was lower than that (27.06%) in the bio-oil. Further research on the development of a lipid extraction method using non-chlorinated solvents and an upgrading process of bio-crude is required.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51176163), National High Technology R&D Program of China (2012AA050101), International Sci. & Tech. Cooperation Program of China (2012DFG61770 and 2010DFA72730), National Key Technology R&D Program of China (2011BAD14B02), Program for New Century Excellent Talents in University (NCET11-0446), Specialized Research Fund for the Doctoral Program of Higher Education (20110101110021), Science and Technology Project of Guangxi Province (1346011-1). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.biortech.2013.10.033. References Biller, P., Riley, R., Ross, A., 2011. Catalytic hydrothermal processing of microalgae: decomposition and upgrading of lipids. Bioresource Technology 102 (7), 4841– 4848. Brown, T.M., Duan, P., Savage, P.E., 2010. Hydrothermal Liquefaction and gasification of Nannochloropsis sp. Energy & Fuels 24 (6), 3639–3646. Cheng, J., Yu, T., Li, T., Zhou, J., Cen, K., 2013. Using wet microalgae for direct biodiesel production via microwave irradiation. Bioresource Technology 131, 531–535. Dote, Y., Yokoyama, S.-y., Ogi, T., Minowa, T., Murakami, M., 1991. Liquefaction of barley stillage and upgrading of primary oil. Biomass and Bioenergy 1 (1), 55– 60. King, J., Holliday, R., List, G., 1999. Hydrolysis of soybean oil. in a subcritical water flow reactor. Green Chemistry. 1 (6), 261–264. Kruse, A., Maniam, P., Spieler, F., 2007. Influence of proteins on the hydrothermal gasification and liquefaction of biomass. 2. Model compounds. Industrial & Engineering Chemistry Research 46 (1), 87–96. Lee, J.Y., Yoo, C., Jun, S.Y., Ahn, C.Y., Oh, H.M., 2010. Comparison of several methods for effective lipid extraction from microalgae. Bioresource Technology 101, S75–S77. Patil, P.D., Gude, V.G., Mannarswamy, A., Deng, S., Cooke, P., Munson-McGee, S., Rhodes, I., Lammers, P., Nirmalakhandan, N., 2011a. Optimization of direct conversion of wet algae to biodiesel under supercritical methanol conditions. Bioresource Technology 102 (1), 118–122. Patil, P.D., Gude, V.G., Mannarswamy, A., Cooke, P., Munson-McGee, S., Nirmalakhandan, N., Lammers, P., Deng, S., 2011b. Optimization of microwave-assisted transesterification of dry algal biomass using response surface methodology. Bioresource Technology 102 (2), 1399–1405. Patil, P.D., Gude, V.G., Mannarswamy, A., Cooke, P., Nirmalakhandan, N., Lammers, P., Deng, S.G., 2012. Comparison of direct transesterification of algal biomass under supercritical methanol and microwave irradiation conditions. Fuel 97, 822–831. Patil, P.D., Reddy, H., Muppaneni, T., Schaub, T., Omar Holguin, F., Cooke, P., Lammers, P., Nirmalakhandan, N., Li, Y., Lu, X., 2013. In-situ ethyl ester production from wet algal biomass under microwave-mediated supercritical ethanol conditions. Bioresource Technology 139, 308–315. Rogalinski, T., Liu, K., Albrecht, T., Brunner, G., 2008. Hydrolysis kinetics of biopolymers in subcritical water. The Journal of Supercritical Fluids 46 (3), 335–341. Toor, S., Reddy, H., Deng, S., Hoffmann, J., Spangsmark, D., Madsen, L., Holm-Nielsen, J.B., Rosendahl, L., 2013. Hydrothermal liquefaction of Spirulina and Nannochloropsis Salina under subcritical and supercritical water conditions. Bioresource Technology 131, 413–419. Wang, S., Guo, Z., Cai, Q., Guo, L., 2012. Catalytic conversion of carboxylic acids in bio-oil for liquid hydrocarbons production. Biomass and Bioenergy 45, 138– 143. Watanabe, M., Iida, T., Inomata, H., 2006. Decomposition of a long chain saturated fatty acid with some additives in hot compressed water. Energy Conversion and Management 47 (18), 3344–3350. Xia, A., Cheng, J., Lin, R., Lu, H., Zhou, J., Cen, K., 2013. Comparison in dark hydrogen fermentation followed by photo hydrogen fermentation and methanogenesis between protein and carbohydrate compositions in Nannochloropsis oceanica biomass. Bioresource Technology 138, 204–213. Xu, L., Wim Brilman, D.W., Withag, J.A., Brem, G., Kersten, S., 2011. Assessment of a dry and a wet route for the production of biofuels from microalgae: energy balance analysis. Bioresource Technology 102 (8), 5113–5122. Yuan, H., Yang, B.L., Zhu, G.L., 2009. Synthesis of biodiesel using microwave absorption catalysts. Energy & Fuels 23, 548–552.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Biodiesel production from lipids in wet microalgae with microwave irradiation and bio-crude production from algal residue through hydrothermal liquefaction Jun Cheng ⇑, Rui Huang, Tao Yu, Tao Li, Junhu Zhou, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Cogeneration of biodiesel and bio-
crude was proposed to make full use of microalgae. Total energy recovery was increased to 67.73% by utilization of microalgae residue. Remaining nitrogen in liquid fuel decreased to 16.02% by the cogeneration process. Lipids and proteins were prevented from interacting to form low-grade liquid fuel.
a r t i c l e
i n f o
Article history: Received 23 August 2013 Received in revised form 7 October 2013 Accepted 10 October 2013 Available online 17 October 2013 Keywords: Microalgae Biodiesel Bio-crude Microwave Hydrothermal liquefaction
a b s t r a c t A cogeneration process of biodiesel and bio-crude was proposed to make full use of wet microalgae biomass. High-grade biodiesel was first produced from lipids in wet microalgae through extraction and transesterification with microwave irradiation. Then, low-grade bio-crude was produced from proteins and carbohydrates in the algal residue through hydrothermal liquefaction. The total yield (40.19%) and the total energy recovery (67.73%) of the cogenerated biodiesel and bio-crude were almost equal to those of the bio-oil obtained from raw microalgae through direct hydrothermal liquefaction. Upon microwave irradiation, proteins were partially hydrolyzed and the hydrolysates were apt for deaminization under the hydrothermal condition of the algal residue. Hence, the total remaining nitrogen (16.02%) in the cogenerated biodiesel and bio-crude was lower than that (27.06%) in the bio-oil. The cogeneration process prevented lipids and proteins from reacting to produce low-grade amides and other long-chain nitrogen compounds during the direct hydrothermal liquefaction of microalgae. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The traditional process of biodiesel production from algae often requires the consumption of a large amount of energy because of algae dewatering (Patil et al., 2012). As microwave irradiation has been reported to effectively extract lipids from wet algae and accelerate the transesterification reaction (Yuan et al., 2009; Lee et al., 2010), a process for the direct conversion of wet microalgae
⇑ Corresponding author. Tel.: +86 571 87952889; fax: +86 571 87951616. E-mail address: [email protected] (J. Cheng). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.10.033
biomass into biodiesel by microwave irradiation has been proposed in a previous study (Cheng et al., 2013). This simplified process has been shown to avoid the energy consumption by algae dewatering and increase both the biodiesel production rate and the yield via microwave irradiation. Although the lipids in wet algae are effectively converted into biodiesel in this process, the biomass residue (consisting mainly of carbohydrates and proteins) that remains after the extraction ends up as waste. The efficiency of utilizing the residue left after the lipid extraction has a significant influence on the overall energy balance of the microalgae-to-fuel conversion, according to an assessment by Xu et al. (2011).
416
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However, there have been few reported experiments regarding the algae residue that remains after lipid extraction. In recent years, the direct hydrothermal liquefaction of microalgae has attracted considerable interest for its low energy consumption and high oil yield. The bio-oil yield obtained through the hydrothermal process is 10–15% higher than the lipid content of the microalgae, by converting approximately 10% of the carbohydrates and 20% of the proteins into bio-oil (Biller et al., 2011). However, as oxygen-containing carbohydrates and nitrogen-containing proteins are converted into bio-oil, the bio-oil obtained from raw algae through direct hydrothermal liquefaction typically has high nitrogen (6 wt.%) and oxygen (12 wt.%) contents. The mixing of the carbohydrates and proteins with the lipids under a hydrothermal condition has also resulted in a complicated composition of the bio-oil (Brown et al., 2010). As this is undesirable in the final product, further deoxygenation and denitrification of this bio-oil has become a bottleneck in the development of a hydrothermal process. In order to utilize the algae residue that remains after the lipid extraction and to avoid the deterioration of lipids in the hydrothermal process, a cogeneration process of high-grade biodiesel from lipids in microalgae and low-grade bio-crude from proteins and carbohydrates in algal residue was proposed and comprehensively investigated in this paper. 2. Methods 2.1. Materials Nannochloropsis oceanica was purchased from Yantai Hearol Biotechnology Co., Ltd (China). The moisture content in the N. oceanica biomass was determined by oven drying at 105 °C until a constant mass was achieved. Distilled water was added to the dried N. oceanica cells to produce wet algae containing 80 wt.% water. Chloroform, methanol, and sulfuric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). 2.2. Biodiesel production from lipids in wet microalgae through extraction and transesterification with microwave irradiation Biodiesel was first produced from the lipids in wet microalgae through extraction and transesterification with microwave irradiation. As described in a previous study (Cheng et al., 2013), a WX4000 microwave digestion system (2.45 GHz, Shanghai Yiyao Microwave Chemistry Company) equipped with six 60-ml digestion reactors was used. Approximately 4 g of wet algae was placed in the digestion tank and mixed with 16 ml of chloroform, 16 ml of methanol, and 0.8 ml of sulfuric acid. The algal biomass was heated via microwave irradiation and maintained at 60 °C for 30 min. After the treatment, distilled water was added to the mixture, which was then centrifuged. Thereafter, the organic layer was transferred into a pre-weighed glass vial and evaporated in a baking oven at 70 °C. The aqueous phase and the algae residue remained in centrifuge tube were centrifuged to separate the algae residue. 2.3. Bio-crude production from algal residue through hydrothermal reaction Bio-crude was produced from the algal residue through a hydrothermal reaction. The algae residue obtained from the process described in Section 2.2 was washed with methanol (10 ml) three times in order to remove the residual chloroform. Then, it was washed three times with distilled water (10 ml) to remove the residual methanol.
Thereafter, the algae residue was mixed with 90 ml of water and added into a well-stirred batch reactor (500 ml) (Parr 4575A, USA). The residual air was removed from the sealed reactor by purging the reactor with nitrogen. Further, a liquefaction experiment was performed at 300 °C for 0.5 h. After the liquefaction, 20 ml of chloroform was added to the mixture in order to extract the bio-crude. Then, the reactor was washed five times with 10 ml of chloroform. All the used chloroform was collected and then centrifuged. Then, the organic layer was transferred into a pre-weighed glass vial; this layer was evaporated in oven at 70 °C. Finally, the weight of the biocrude was determined gravimetrically. 2.4. Bio-oil production from raw microalgae by hydrothermal liquefaction Approximately 50 g of wet algae was mixed with 40 ml of distilled water and was added into the reactor. The rest of the processes were the same as those described in Section 2.3. 2.5. Analytical chemistry The C, H, N, and S contents of the sample were measured using a CE Instruments Flash EA 1112 series elemental analyzer (Biller et al., 2011). The liquid fuel was analyzed using a GC–MS system (Trace DSQII, Thermo Scientific, USA) equipped with a 30 m 0.25 mm 0.25 lm Agilent DB-WAX capillary column (Wang et al., 2012). The oven temperature in the case of biodiesel was first maintained at 150 °C for 5 min. Then, the temperature was increased to 250 °C at the rate of 4 °C/min and was maintained at 250 °C for 5 min. The oven temperature in the cases of bio-crude and bio-oil was first maintained at 70 °C. Then, the temperature was increased to 250 °C at the rate of 3 °C/min and was maintained at 250 °C for 10 min. Data treatment was carried out using a computer with Xcalibur software and the NIST mass spectral library. All measurements were repeated in triplicate, and the mean values were reported. 3. Results and discussion 3.1. Comparison in oil yield N. oceanica biomass primarily contained 22.7% carbohydrates, 19.1% proteins, 24.8% lipids, and 16.2% moisture. This biomass was rehydrated to produce wet algae containing 80 wt.% water for experiments. As shown in Fig. 1, the yield of biodiesel obtained from the lipids in wet microalgae through extraction and transesterification was 29.63%, and the yield of bio-crude obtained from the microalgae residue through the hydrothermal reaction was 10.56%. The conversion efficiency of algal lipids into biodiesel was nearly 100%, which was higher than 90% obtained in supercritical alcohol conditions reported in recent literatures (Patil et al., 2011a, 2013). The yield of the bio-oil obtained from raw microalgae through direct hydrothermal liquefaction was 40.08%. These results revealed that the yield of bio-oil was higher than the yield of biodiesel. This could be explained by the conversion of carbohydrates and proteins into liquid fuels under a hydrothermal condition. The total yield (40.19%) of the cogenerated biodiesel and biocrude was almost equal to the yield (40.08%) of the bio-oil obtained from the raw microalgae through the direct hydrothermal liquefaction. This is most likely due to the following reasons: (1) The lipids in microalgae were effectively converted into liquid fuel in the cogeneration process and in the direct hydrothermal liquefaction of the raw microalgae, respectively. The lipids in the microalgae
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3.2. Elemental analysis and energy recovery Table 1 shows the elemental compositions and higher heating values (HHV) of feedstock and products. Biodiesel had the highest HHV (38.8 MJ/kg) along with the highest carbon content (76.92%), highest hydrogen content (10.36%), and lowest nitrogen content (1.59%). Bio-crude had the lowest HHV (28.6 MJ/kg) along with the lowest carbon content (63.62%), lowest hydrogen content (7.58%), and highest nitrogen content (6.98%). This indicated that the liquid fuels derived from the microalgae were successfully separated into two grades of fuel (biodiesel and bio-crude) in the cogeneration process. As shown in Fig. 1, the total energy recovery was increased from 53.6% to 67.73% by utilization of microalgae residue. The energy recovery of 67.73% in the cogeneration process of biodiesel and bio-crude was higher by 14.1% than that in only biodiesel
90
Oil yield, Element and Energy recovery(%)
were extracted and converted into high-grade biodiesel in the cogeneration process. The lipids in microalgae were hydrolyzed into fatty acids (some of the fatty acids decomposed into hydrocarbons) and became a part of bio-oil in the direct hydrothermal liquefaction (King et al., 1999; Watanabe et al., 2006). (2) The amount of liquid fuel obtained from the proteins decreased slightly in the cogeneration process of biodiesel and bio-crude. The proteins in the microalgae underwent a partial hydrolysis with microwave irradiation and acid during the lipids extraction in the cogeneration process (Xia et al., 2013). Since linear nitrogen-containing compounds were present in the form of the hydrolysates of proteins and were apt for the deaminization under a hydrothermal condition, a partial hydrolysis of the proteins prior to the hydrothermal liquefaction would reduce the nitrogen content in the liquid fuel (Dote et al., 1991). Further, the extraction of lipids prior to the hydrothermal liquefaction inhibited the polar nitrogen-containing organics from reacting with the fatty acids to form low-grade long-chain nitrogen compounds. This decreased the yield of the liquid fuel obtained from the proteins. (3) The amount of liquid fuel obtained from the carbohydrates was increased slightly in the cogeneration process of biodiesel and bio-crude. The ammonia derived from the deamination of the proteins promoted the conversion of the carbohydrates into the liquid fuel (Kruse et al., 2007). Further, the promoted deamination of the proteins resulted in an increase in ammonia in the aqueous phase. This increased ammonia in the aqueous phase in turn enhanced the yield of the liquid fuel obtained from the carbohydrates in the cogeneration process. Since the peptide bonds of the proteins were more stable than the glycosidic bonds in cellulose and starch, the carbohydrates present in the biomass had a tendency to be hydrolyzed faster than the proteins (Rogalinski et al., 2008; Toor et al., 2013). Thus, the carbohydrates tended to convert into a gas product at an early stage of the hydrothermal liquefaction without the effect of ammonia in the direct hydrothermal liquefaction of the raw microalgae.
Biodiesela Bio-crudeb Bio-oilc
80 70 60 50 40 30 20 10 0
Oil
Carbon
Hydrogen Nitrogen
Energy
Fig. 1. Comparison in oil yield, element and energy recovery between the cogeneration process and direct hydrothermal liquefaction of raw microalgae e,d. Notes: aBiodiesel was obtained from the lipids in wet microalgae through extraction and transesterification with microwave irradiation. bBio-crude was obtained from the algal residue though hydrothermal reaction. cBio-oil was obtained from the raw microalgae though direct hydrothermal liquefaction. dThe yield of the liquid fuel was calculated on the basis of the dry microalgae weight. eElement and energy recovery were defined as the ratios of carbon, hydrogen, nitrogen, and higher heating value (HHV) in liquid fuel to those in raw microalgae feedstock.
production reported in literature (Patil et al., 2011b). The total energy recovery (67.73%), carbon recovery (58.95%), and hydrogen recovery (51.88%) of the cogenerated biodiesel and bio-crude were almost equal to those of the bio-oil. However, compared with that in the bio-oil, the total remaining nitrogen in the cogenerated biodiesel and bio-crude was decreased from 27.06% to 16.02%. The total remaining oxygen (15.86%) in the cogenerated biodiesel and bio-crude was higher than that (14.02%) in the bio-oil. This could be explained by the following reasons: The hydrolysis of proteins prior to the hydrothermal liquefaction promoted the removal of the linear nitrogen contained in the proteins as ammonia and thus, decreased the remaining nitrogen in the cogenerated biodiesel and bio-crude. The promoted deamination of the proteins led to an increased yield of the liquid fuel derived from the oxygen-containing carbohydrates and thus, resulted in an increase in the remaining oxygen in the cogenerated biodiesel and bio-crude.
3.3. GC–MS analysis of the liquid fuel According to the GC–MS analyses, the biodiesel obtained from the lipids in the wet microalgae through extraction and transesterification using microwave irradiation mainly contained FAMEs such as C17H32O2 and C21H32O2. The bio-crude obtained
Table 1 Elemental compositions and higher heating values of feedstock and products.
Dried biomass of raw microalgae Biodiesel obtained from lipids in wet microalgae through extraction and transesterification with microwave irradiation Dried algal residue obtained after lipids extraction of raw microalgae with microwave irradiation Bio-crude obtained from algal residue though a hydrothermal reaction Bio-oil obtained from raw microalgae though direct hydrothermal liquefaction a
Sulfur (%)
Oxygena (%)
Higher heating valueb (MJ/kg)
7.54 1.59
0.47 0.21
34.47 10.92
21.46 38.86
6.55
11.07
0.56
36.58
18.17
7.58 9.75
6.98 5.09
0.69 0.52
21.13 12.06
28.62 36.35
Carbon (%)
Hydrogen (%)
50.06 76.92
7.46 10.36
45.24 63.62 72.58
Nitrogen (%)
Oxygen content was calculated by subtraction of carbon, hydrogen, nitrogen and sulfur contents from 100%. The higher heating value (HHV) was estimated with the Dulong formula (Biller et al., 2011): HHV (MJ/kg) = 0.338C + 1.428(H–O/8) + 0.095S, where C, H, O, and S were weight percentages of elemental compositions in materials. b
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from the microalgae residue through the hydrothermal reaction mainly contained heterocyclic nitrogen compounds (such as C11H18N2O2 and C8H7N). A large number of unidentified compounds were also detected in this bio-crude. The bio-oil obtained from the raw microalgae through the direct hydrothermal liquefaction contained large quantities of fatty acids (such as C16H30O2 and C16H32O2), some fatty acid amides (such as C16H33NO and C18H35NO), and heterocyclic nitrogen compounds. It revealed that the cogeneration process effectively separated the liquid fuel derived from the microalgae into two different grades of fuel (biodiesel and bio-crude) and thus, simplified the further utilization of these fuels. The biodiesel mainly contained FAMEs with the carbon-chain length ranging from 16 to 20. The bio-crude mainly contained pyrrole, indole, and other heterocyclic nitrogen compounds having a carbon-chain length ranging from 6 to 14. Almost no nitrogen compounds containing carbon chains having a length of 16–21 were detected in the bio-crude. It indicated that the cogeneration process prevented the lipids and the proteins from reacting to form low-grade amides and other long-chain nitrogen compounds and thus, increase the yield of the high-grade oil. The bio-oil contained 48.47% of the fatty acids having a carbon-chain length ranging from 16 to 18. However, 13.63% of the bio-oil was amides, pyrroles, and indoles, which contained carbon chains having a length of 16–21. This was attributed to the reaction between the lipids and the proteins under the hydrothermal condition during the direct hydrothermal liquefaction of the raw microalgae. This reaction formed amides and other nitrogen-containing compounds, and thus led to the deterioration of the lipids in the microalgae. A comparison revealed that the cogeneration process of biodiesel and bio-crude prevented the decomposition of some unsaturated fatty acids under the hydrothermal condition and produced abundant FAMEs. The biodiesel mainly contained FAMEs including C16:1, C20:5, C16:0, C20:4, C18:1, C18:2, and C20:3. However, only the corresponding fatty acids of C16:1, C16:0, C18:1, and C18:2 and a considerable amount of C14:0 were detected in the bio-oil. This could be explained by the decomposition of the highly unsaturated fatty acids (such as C20:5, C20:4, and C20:3) during the direct hydrothermal liquefaction of the microalgae. As lipids were extracted prior to the hydrothermal liquefaction in the cogeneration process, highly purified hydrocarbons could be obtained by further deoxidizing the lipids. The bio-oil contained approximately 4% hydrocarbons with a carbon-chain length of 15–20, whereas almost no hydrocarbons were detected in the bio-crude. This indicated that the hydrocarbons in the bio-oil were produced by deoxidation of the fatty acids under the hydrothermal condition. However, as hydrocarbons were mixed with the other hundreds of compounds in the bio-oil, it was difficult to utilize these hydrocarbons in the bio-oil. 4. Conclusion The cogeneration of high-grade biodiesel from lipids in microalgae and low-grade bio-crude from proteins and carbohydrates in algal residue is a promising process to make full use of microalgae biomass. The total yield (40.19%) and energy recovery (67.73%) of the cogenerated biodiesel and bio-crude were almost equal to those of the bio-oil obtained from microalgae through direct hydrothermal liquefaction. However, the total remaining nitrogen (16.02%) in the cogenerated biodiesel and bio-crude was lower than that (27.06%) in the bio-oil. Further research on the development of a lipid extraction method using non-chlorinated solvents and an upgrading process of bio-crude is required.
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