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Thermochemical Catalytic Liquefaction of the Marine Microalgae Dunaliella tertiolecta and Characterization of Bio-oils Shuping Zou,†,‡ Yulong Wu,*,† Mingde Yang,*,† Chun Li,§ and Junmao Tong| Institute of Nuclear and New Energy Technology, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China, School of Life Science and Technology, Beijing Institute of Technology, Beijing 100081, People’s Republic of China, and Food College, Shihezi UniVersity, Shihezi 832000, Xijiang, People’s Republic of China ReceiVed January 4, 2009. ReVised Manuscript ReceiVed May 13, 2009
Thermochemical catalytic liquefaction (TCL) of the marine microalgae Dunaliella tertiolecta was performed in ethylene glycol acidified with H2SO4 as a catalyst. The mathematical model for predicting the liquefaction yield was set up by a central composite rotatable design (CCRD) and response surface analysis (RSA). A total of 23 individual experiments were conducted to study the effect of H2SO4 concentration, reaction temperature, and reaction time on the liquidation yield. From a regression analysis, the conversion yield of microalgae cells into liquid is simply expressed as a function of the operating variables by polynomial containing quadratic terms. The highest liquefaction yield of microalgae would be 97.05%, at the following optimized reaction conditions: an amount of added H2SO4 of 2.4%, and a reaction temperature of 170 °C, with a 33 min reaction time. To put bio-oils into wide application, the various physical and chemical characteristics of bio-oils at the conditions of the maximum product yields were determined, and the detailed chemical compositional analysis of bio-oils was performed by various spectroscopic techniques such as Fourier transform infrared spectroscopic analysis (FT-IR), carbon-13 nuclear magnetic resonance (13C NMR), and gas chromatography - mass spectrometry (GC-MS). The bio-oils were composed of benzofuranone, fatty acid methyl ester, and fatty acid hydroxyethyl ester, with a long chain from C14 to C18. These bio-oils were presented as an environmentally friendly feedstock candidate for biofuels and chemicals.
1. Introduction Biomass has recently been considered to be an alternative energy source, since it is a renewable resource and it fixes CO2 in the atmosphere through photosynthesis. Moreover, because fuels from biomass have low sulfur and nitrogen content, its energy utilization also creates less environmental pollution than fossil fuels. Biomass resources include wood and wood wastes, energy crops, aquatic plants, agricultural crops and their waste byproduct, municipal wastes, and animal wastes. Among these biomass resources, microalgae have been suggested as very good candidates for fuel production because of their advantageous higher photosynthetic efficiency, higher biomass production, and faster growth, compared to lignocellulosic material, such as trees.1 As a good source of single-cell protein and a microalga of high commercial value, Dunaliella has been massively cultured in many countries and areas. The cultural technique for this alga has been greatly developed in the past decades. If bio-oils can be recovered from Dunaliella, an energy production system from mass cultivated Dunaliella can be created. Recently, many efforts have been put into the production of fuel from microalgae. Work to obtain bio-oils from microalgae * To whom correspondence should be addressed. Phone: 86-01089796088; fax: 86-10-69771464; e-mail:
[email protected] (Y.W.),
[email protected] (M.Y.). † Tsinghua University. ‡ Tianjin University. § Beijing Institute of Technology. | Shihezi University. (1) Calvin, M.; Taylor, S. E. Fuel from algae. In Algal and cyanobacterial biotechnology; Creswell, R. C., Rees, T. A. V., Shah, N. Eds.; Longman Scientific & Technical Wiley: New York, 1989; pp 136-160.
through pyrolysis has been reported.2-4 However, microalgae usually have a high moisture content. As a result, the pyrolysis requires a drying process that needs a great deal of energy to vaporize the water. Some researchers studied the production of fuel oil from microalgae through direct liquefaction in pure water, in conditions close to its critical state.5-7 However, technical problems may arise due to the high pressure and corrosion effects of water. Therefore, more appropriate processes must be developed to obtain bio-oils from microalgae. The thermochemical catalytic liquefaction (TCL) of biomass under atmospheric pressure is of considerable current interest.8-11 Rezzoug studied a biofuel partly soluble in gasoline, obtained by liquefying wood in anhydrous glycerin.11 Due to the fact that there is no need for high pressure and a drying process, TCL may prove to be an economical method for producing bio(2) Ginzburg, B. Z. Renew. Energy 1993, 3 (2), 249–252. (3) Miao, X. L.; Wu, Q. Y.; Yang, C. Y. J Anal Appl Pyrolysis 2004, 71 (2), 855–863. (4) Miao, X. L.; Wu, Q. Y. J. Biotechnol. 2004, 110 (1), 85–93. (5) Minowa, T.; Yokoyama, S.; Kishimoto, M. Fuel 1995, 74 (12), 1735– 1738. (6) Sawayama, S.; Minowa, T.; Yokoyama, S. Y. Biomass Bioenergy 1999, 17 (1), 33–39. (7) Yang, Y. F.; Feng, C. P.; Inamori, Y.; Maekawa, T. Resour. ConserV. Recycl. 2004, 24, 21–33. (8) Demirbas, A. Energy ConVers. Manage. 2000, 41, 1741–1748. (9) Kurimoto, Y.; Takeda, M.; Koizumi, A. Bioresour. Technol. 2000, 74, 151–157. (10) Ge, J. J.; Wu, R.; Deng, B. L. Polym. Mater. Sci. Eng. 2003, l9 (2), 194–198. (11) Rezzoug, S. A.; Capart, R. Energy ConVers. Manage. 2003, 44, 781–792.
10.1021/ef9000105 CCC: $40.75 2009 American Chemical Society Published on Web 06/10/2009
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Zou et al. Table 1. Analyses Results of Dunaliella tertiolecta
element analysis (wt % dry basis) moisture ash volatile matter fixed carbona a
elemental analyze (wt % dry ash free)
4.98 13.54 54.48 27.00
carbon hydrogen nitrogen sulfur aoxygen
39.00 5.37 1.99 0.62 53.02
chemical components (wt % organic basis) crude protein crude lipid carbohydrate
61.32 2.87 21.69
Calculated by difference.
oils and chemical feedstocks from biomass. However, there have been very few studies on the TCL conditions of microalgae and the characterization of the bio-oils obtained. The physical and chemical properties of bio-oils are strongly dependent on feedstock and production circumstances; these properties give important indications about the TCL process parameters and information about the quality of the product, therefore, analysis and characterization of bio-oils is very important.12-16 The characterization of bio-oils is currently underway, due to its significance in various applications in thermochemical conversion processes. Various analytical techniques have been attempted to overcome the problems encountered in the characterization of biomass-derived liquid products. In this paper, the marine microalga, D. tertiolecta, was chosen as the renewable energy source. The TCL of microalgae was investigated, and some features of obtained bio-oils were characterized by using some spectroscopic techniques such as Fourier transform infrared spectroscopic analysis (FT-IR), Carbon-13 nuclear magnetic resonance (13C NMR), gas chromatography - mass spectrometry (GC-MS), and elemental analysis. 2. Experimental Section 2.1. Material. Microalgae cells of D. tertiolecta were provided from Tanjing Microalgae Biotechnologies Co., Ltd. (Tianjing, China). Samples were prepared by pulverization in a mortar to
