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Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char
Energy Conversion and Management xxx (2015) xxx–xxx
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Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char Tao Dong a, Difeng Gao a, Chao Miao a, Xiaochen Yu a, Charles Degan a, Manuel Garcia-Pérez a, Barbara Rasco b, Shyam S. Sablani a, Shulin Chen a,⇑ a b
Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164, USA School of Food Science, Washington State University, Pullman, WA 99164, USA
a r t i c l e
i n f o
Article history: Received 30 March 2015 Accepted 24 June 2015 Available online xxxx Keywords: Microalgae Biodiesel Bio-char catalyst Catalyst-fouling Refinery Chlorophyll
a b s t r a c t An efficient process for biodiesel production from fast-refined microalgal oil was demonstrated. A low cost catalyst prepared from pyrolysis-derived bio-char, was applied in pre-esterification to reduce free fatty acid (FFA) content. Results showed that the bio-char catalyst was highly active in esterification; however, the performance of the catalyst significantly reduced when crude microalgal oil was used as feedstock. To solve the problem caused by catalyst-fouling, a fast and scalable crude oil refinery procedure was carried out to remove chlorophyll and phospholipids that might degrade the catalyst and the quality of biodiesel. The activity and reusability of bio-char catalyst were remarkably improved in the fast-refined oil. FFA content in the refined microalgal oil was reduced to less than 0.5% after pre-esterification. The bio-char catalyst could be reused for 10 cycles without dramatic loss in activity. The pre-esterification fits the first-order kinetic reaction with activation energy of 42.16 kJ/mol. The activity of bio-char catalyst was superior to commercial Amberlyst-15 under the same reaction condition. A total fatty acid methyl ester (FAME, namely biodiesel) yield of 99% was obtained following the second-step CaO-catalyzed transesterification. The cost-effective bio-char catalyst has great potential for biodiesel production using feedstocks having high FFA content. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Fatty acid methyl esters (FAME) form the primary component of biodiesel, and have been produced from vegetable oil, animal fat, and waste cooking oil [1–3]. However, the limited supply of these feedstocks presents a major bottleneck for increasing biodiesel production [4]. Oleaginous microalgae have gained increasing attention as a next-generation renewable biofuel resource due to their fast growth rate and high lipid content, as well as their environmentally benign nature [5–7] compared to other energy crops. Many microalgae oils have been characterized as feedstock with high free fatty acid (FFA) content. For example, FFA content as high as 70% of total lipids was reported [4]. Unfortunately, highly efficient base catalysts are not practical under such conditions due to saponification. However, a two-step approach, in which acid-catalyzed pre-esterification occurs prior to the
⇑ Corresponding author at: Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164-6120, USA. Tel.: +1 (509) 335 3743; fax: +1 (509) 335 2722. E-mail address: [email protected] (S. Chen).
base-catalyzed transesterification, could lead to a higher FAME yield with a shorter reaction time [8]. The application of homogeneous acid catalysts such as sulfuric acid complicates downstream processing by generating an acidic waste stream requiring further treatment. Water washing tanks, one expense associated with handling waste streams, alone cost about 20% of the total capital in biodiesel production [9]. In contrast, heterogeneous catalysts can be easily separated after the reaction, bypassing the water washing and neutralization steps, simplifying the downstream process and reducing waste disposal costs and environmental impacts. Thus, a heterogeneous catalyst based process may be more economical compared to traditional homogeneous and supercritical conversion processes, especially for feedstocks with high FFA content [10]. Many studies have explored highly efficient heterogeneous catalysts, including super acids, zeolites, metal oxides [11], ion-exchange resins [2], and mesoporous silica [12]. However, the acidic site density of metal oxide and zeolite catalysts is usually low. Further, these catalysts are sensitive to water due to the hydrophilic surface. Although ion exchange resins possess strong acidic sites, they have low heat stability. Furthermore, the
http://dx.doi.org/10.1016/j.enconman.2015.06.072 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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supporting materials for solid acids, such as silicon oxides for perfluorinated alkanes [13] and zirconia for heteropoly acids are very expensive [14]. Carbonaceous materials are relatively inexpensive, widely available, and can be easily functionalized with –SO3H groups [15]. Recently, a new class of sulfonated carbons has been successfully developed as heterogeneous acid catalysts [15,16]. Partial carbonization of sugar forms a rigid carbon matrix consisting of polycyclic aromatic carbon sheets arranged in a three dimensional sp3–bonded structure. Sulfonation of these carbon materials results in the formation of a stable solid acid catalyst with a high density of –SO3H that display remarkable catalytic performance for esterification [15]. Unfortunately, the carbonization process is usually time-consuming and produces large amounts of waste [17,18]. Fast pyrolysis is a thermo-chemical process in which biomass is converted into bio-oil, bio-char and non-condensable gases by heating the biomass to approximately 500 °C in the absence of oxygen [19]. Bio-char is co-produced from the pyrolysis process and is currently used as a household fuel, for industrial applications or as a soil amendment. Bio-char is extensively used as a reducing agent in the iron and steel industries, and in the manufacture of carbon disulphide, sodium cyanides, carbides and activated carbon [20]. Bio-char can also provide a potential cost-effective carbonaceous catalyst support for preparing catalysts [21]. The carbonaceous nature and potential for sulfonation of bio-char makes it a promising catalyst for converting microalgal oil into biodiesel. Since microalgae oil differs from common biodiesel feedstocks such as vegetable oil in terms of both FFA and chlorophyll contents, the performance of the bio-char catalyst in this feedstock should be evaluated. This study investigated the microalgae biodiesel production process using acidic bio-char as a catalyst. The study also explored the effects of contaminants in the microalgal oil on the bio-char catalyst and on the pre-esterification process.
quantified by in situ methylation method [23]. Freeze-dried biomass was grinded in a motor with a pestle, and extracted with chloroform/methanol (2:1, v/v) using a Soxhlet extraction apparatus [24]. Crude microalgal oil was mixed with 1% (v/v) of pure phosphoric acid in a round bottom flask and heated on a vacuum evaporator at 100 °C for 30 min to remove chlorophyll. Dark color floccule was formed and adhered to the surface of the flask. An oil phase with a light orange color was obtained and easily separated from the dark precipitation. Next, the acidified oil was transferred to a second round bottom flask and mixed with 20% (v/v) water. The mixture was stirred at 1000 rpm at 80 °C for 1 h to hydrate phospholipids forming gum. Then the refined microalgal oil was separated from the water phase and gum by centrifugation at 4000 rpm for 2 min. The refined microalgal oil was obtained by washing three times with an equal volume of water to remove residue phosphoric acid. Fractionation and analysis of the FFAs and acylglycerols was performed on the solid phase extraction (SPE) cartridge, followed by methylation and GC analysis on an Agilent 7890A GC equipped with a FAMEWAX column (30 m 320 lm 0.25 lm) (Restek, Bellefonte, PA). Tridecanoic acid methyl ester (C13:0) was used as an internal standard [8]. The chlorophyll and total carotenoid content in the crude and refined microalgal oils were determined by UV–vis spectroscopy with calculations shown below (Wellburn, 1994). The oil was diluted with heptane and scanned from 300 to 800 nm with a Shimadzu UV–2550 spectrophotometer (Shimadzu, Kyoto, Japan).
Chlorophyll aðCa Þðlg mL1 Þ ¼ 10:05ðOD662 Þ 0:77ðOD644 Þ Chlorophyll bðCb Þðlg mL1 Þ ¼ 16:37ðOD644 Þ 3:14ðOD662 Þ Total carotenoidsðCxþc Þðlg mL1 Þ ¼ ð1000OD470 1:28Ca 56:7Cb Þ=205
ð1Þ
2. Materials and methods 2.1. Materials Douglas fir was used as feedstock for the production of the bio-char based catalyst. Douglas fir was donated by Herman Brothers Logging & Construction, Incorporated (Port Angeles, WA). A pioneer mill (Model number 400 HD, serial number 2404, Bliss Industries, Inc., Ponca, OK) was used to grind the feedstock to 2 mm or less. Green microalgae Chlorella sorokiniana (UTEX 1602) was obtained from the Culture Collection of Alga at the University of Texas (Austin, TX). Agilent Bond Elut NH2 cartridge (100 mg) was purchased from Agilent (Santa Clara, CA). Fatty acid standards for GC analysis, consisting of tridecanoic acid, myristic acid, palmitic acid, hexadecenoic acid, heptadecanoic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, eicosanoic acid and their methyl esters, were purchased from Sigma Chemical Co. (St. Louis, MO). The Amberlyst-15 catalyst used in the pre-esterification step as a control catalyst was obtained from Acros Organics (Fair Lawn, NJ). All solvents and reagents were either of HPLC grade or analytical reagent grade. 2.2. Preparation and characterization of crude and refined microalgal oil The microalgae were inoculated in a 5 L fermenter (New Brunswick Scientific, CT) and grown heterotrophically on glucose for 4 days at 37 °C [8,22]. The microalgae biomass was harvested by centrifugation. The total FAME content in the biomass was
2.3. Co-production of bio-char via fast pyrolysis Douglas fir wood chips were soaked in deionized water and autoclaved at 121 °C, 15 psi for 30 min to reduce the content of alkaline metals [25]. The biomass was dried and pyrolyzed using an Auger pyrolysis reactor at 600 °C with a residence time of 1 min inside the reactor, as described elsewhere [26]. The pyrolysis vapors were condensed in a three stage condensation unit. The charred particles were collected, allowed to cool to room temperature for two hours, and weighed. 2.4. Preparation of the acidic bio-char catalyst The bio-char (10 g) was grounded in a motor and then mixed with 150 mL of concentrated sulfuric acid (Sigma, St. Louis, MO, US) in a 500 mL flat bottom flask. The flask was heated on a magnetic (100 rpm) heating plate for 24 h at 150 °C. After the sulfonation, the slurry was carefully poured into 500 mL of water and filtered. The bio-char catalyst was washed with hot deionized water until the pH was neutral [27]. The bio-char catalyst was then dried in an oven at 105 °C for 4 h and stored in desiccators before use. The surface area was measured using a Coulter SA 3100 surface area analyzer (Beckman Coulter, US) and calculated with the single point Brunauer–Emmett–Teller (BET) method. Scanning electron microscopy (SEM) was used to assess the physical structure and pore size of the catalysts. The elemental composition was determined with an ECS 4010 elemental analyzer (Costech Analytical, Valencia, CA). An FTIR Thermo Nicolet Avatar 370
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
T. Dong et al. / Energy Conversion and Management xxx (2015) xxx–xxx
spectrometer was applied to analyze the bonding groups (Thermo Scientific, US). The total acid site density, consisting of carboxylic, phenolic, and sulfonic groups was measured with titration [18].
2.5. Bio-char catalyzed pre-esterification A mixture of refined microalgal oil and FFAs produced by hydrolysis of C. sorokiniana reined oil was prepared; oil samples had FFA concentrations as high as 44.6% (w/w) [4]. A 300 mg of high-FFA microalgae oil or pure oleic acid (used as a model FFA control feedstock for comparison), methanol, bio-char catalyst and a PTFE coated stir bar were fed into a screw-cap Pyrex tube reactor. Pre-esterification experiments were performed by heating the reactant at 300 rpm in a silicone oil bath. Different reaction operating parameters were investigated, including the catalyst loading amount in the range of 3–7 wt.%, the reaction temperature in the range of 80–120 °C, the methanol to oil molar ratio in the range of 5:1–30:1, and the reaction time in the range of 10– 90 min. After the reaction, the reactor was frozen to 20 °C to stop the reaction and 1 mL of tridecanoic acid methyl ester methanol solution (4 mg/mL) and 2 mL of sodium chloride water solution (5%, w/v) were added. The FFA and FAME were extracted by 3 mL of hexane for GC analysis. [8]. The Amberlyst-15 catalyst was used in the pre-esterification as a control catalyst for comparison. The FAME yield was calculated using Eq. (1)
FAME yield % ¼
3
3. Results and discussion 3.1. Preparation and characterization of the bio-char catalyst The yield of bio-char was 16% and the yield of bio-oil was 47% with the Auger pyrolysis reactor. The lower yield of oil compared to that reported by Liaw et al. (2012) can be explained by the higher temperature (600 °C), which is not optimal for bio-oil production but produces a bio-char with a higher surface area. This process can be adjusted to favor bio-char, bio-oil, or gas production [19,27]. Bio-char is considered to be a low-cost and environmentally friendly carbonaceous co-product from pyrolysis that has been used for soil amendment and carbon sequestration [19,29], and in this study as a catalyst.
Produced FAME ðgÞ 100% Total FAME potential in starting oilðgÞ ð1Þ
The total FAME potential is the total FAME can be converted into.
2.6. Alkali-catalyzed transesterification After pre-esterification, the mixture was centrifuged at 1500 rpm for 10 min to separate the bio-char catalyst from the liquid phase. The separated bio-char catalyst was washed with hexane, and the washing solvent was transferred to a second Pyrex reactor and evaporated by nitrogen. The reactant mixture was allowed to stand for phase separation. The upper phase consisting of methanol and H2O and the bottom phase of mainly oil and esterified fatty acids were separated [28]. The bottom phase was also transferred to the second Pyrex reactor. Methanol and CaO catalyst (3 wt.%) was then added to promote transesterification at 65 °C under continuous stirring at 300 rpm. Samples of the reaction mixture were collected at 20 min intervals, and FAME content was determined. At the end of the reaction, methanol was evaporated and FAMEs accumulated in the upper phase were separated from the bottom glycerol phase. The FAME production was then washed with water before GC–MS analysis.
2.7. GC–MS analysis The microalgae biodiesel samples were analyzed by a GC–MS system with an Agilent 5975 C MSD (Triple-Axis Detector) and an Agilent 7890 A GC equipped with a capillary column (J&W 122-5536G, 30 m 250 lm 0.5 lm). Helium was used as the carrier gas. The injection volume was 1 lL with split ratio at 20. The parameters of the oven temperature program started at 40 °C with 3 °C min1 intervals up to 250 °C and held for 20 min. The temperatures of the injector and detector were set at 300 °C and 250 °C, respectively [8].
Fig. 1. SEM images of: (a) bio-char catalyst indicating pore sizes; and (b) bio-char catalyst emphasizing fibrous channels and pore networks.
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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Table 1 Surface area, elemental analysis, and total acidity of bio-char catalyst.
a b
Sample
Surface area
Bio-char catalyst
3.51
a
(m2 g1)
Total acidityb (mmol/g)
Elemental composition (wt.%) C
H
N
O
S
59.14
2.16
0.30
33.44
2.02
1.37
Single point BET analysis. Back titration method.
Table 2 Esterification activity of bio-char catalyst in the crude and refined microalgal oil. Cycle no.
1 2 3
FFA conversion % Oleic acid
Crude microalgae oil
Refined microalgae oil
99.22 ± 0.32 97.74 ± 0.73 96.04 ± 0.95
98.61 ± 1.02 65.99 ± 5.06 8.00 ± 1.62
98.40 ± 0.68 97.60 ± 0.53 97.13 ± 0.68
Pre-esterification conditions: temperature 100 °C, methanol to oil ratio of 20:1, 10 wt.% of catalyst and 60 min. The bio-char catalyst was washed with hexane and dried at 105 °C for 1 h after each cycle. Experiments were triplicated.
showed that the sulfur content was 2.02 wt.% (Table 1). Since all sulfur atoms in sulfonated carbon catalysts are likely to occur in –SO3H groups [30], the mass content of –SO3H groups and the acid site concentration was calculated to be about 5.11% and 0.63 mmol g1, based upon the sulfur content, respectively. However, the results of titration indicate a total acidity of 1.37 mmol g1 (Table 1), which is higher than that calculated from the sulfur content. This is because a large fraction of the acid groups are likely associated with weak acid groups such as phenolic and carboxylic groups, which make the carbonaceous microsphere polar. Although only the –SO3H acid groups are active for esterification [18], these weak acid groups can improve the catalytic activity by increasing the acidity and improving the adsorption of reactant molecules on the catalyst surface [27]. FTIR spectra of the bio-char catalyst before and after sulfonation are depicted in Fig. A1. The strong peak at around 1712 cm1 and the weak peak at around 1207 cm1 can be assigned to the typical stretching modes of –SO3H groups [17]. FTIR shows that the –SO3H groups were immobilized on the bio-char catalyst following sulfonation.
3.2. Refining the microalgal oil
Fig. 2. Absorbance of crude microalgal oil before and after phosphoric acid discoloration treatment.
Table 3 Chemical composition of crude and refined microalgae oil. Samples
Crude oil Refined oil High-FFA oila
Fatty acid content (% of oil) Neutral lipids
FFA
Phospholipids
79.96 ± 0.62 87.26 ± 1.01 55.15 ± 0.82
10.81 ± 0.57 12.63 ± 1.04 44.6 ± 0.36
3.39 ± 0.35 ND ND
ND: not detected a The high-FFA oil was prepared by mixing refined oil with FFA hydrolyzed from refined oil to obtain a high FFA content. This oil was used to carry out the two-step biodiesel production.
Fig. 1 shows the morphology and pore size of the bio-char catalyst obtained by SEM images. The biochar has a fibrous surface structure with an irregular shape characterized by a highly complex network of pores, channels, and fibrous ridged surfaces similar to the raw wood structure [19]. This structure differs from the planar structure of the carbonaceous material derived from sugar [16]. The bio-char catalyst had a specific area of 3.51 m2 g1 based on calculations from adsorption isotherms using the standard BET equation (Table 1). Elemental analysis (EA) of the bio-char catalyst
The total FAME in the biomass was 33.6% (based on dry weight) and 94.1% of FAME was extracted into the crude oil. The crude microalgae oil was a sticky liquid with a dark green color. In preliminary experiments, the crude microalgae oil was directly used as a feedstock to test the esterification activity of the bio-char catalyst. The activity of recycled bio-char catalyst decreased dramatically in the crude microalgae oil (Table 2). In contrast, the activity of the bio-char catalyst remained constant in pure oleic acid after being recycled three times. The results showed that the bio-char catalyst could absorb catalyst poisons from the crude microalgal oil. This result is in accordance with previous report that compounds in microalgae biomass could reduce the activity of Amberlyst-15 catalyst [8]. Washing the recycled bio-char catalyst from crude microalgal oil with hot acetone could only partially recover the esterification activity (data not shown). This result indicates that impurities in the crude microalgae oil could irreversibly block the active sites on the catalyst. Thus, refining crude microalgae via a practical and scalable approach is indispensable for commercialization of microalgal biodiesel. In this study, an integrated and simple refinery was conducted to remove impurities before the oil conversion step with the bio-char catalyst. As illustrated in Fig. 2, the crude oil had strong absorption peaks at 667 nm and 470 nm due to the chlorophyll and carotenoid pigments. Through the simple phosphoric acid discoloration pre-treatment, the total chlorophyll and carotenoid contents in the crude oil could be decreased from 2290 and 2254 ppm to 7 ppm and 754 ppm, respectively, indicating a 99.7% and 66.6% removal efficiency for chlorophyll and carotenoid pigments. In the first phosphoric acid pretreatment (discoloration) step, a dark green floccule formed very fast and became separated from the oil. Since the dark green precipitate was not soluble in oil, removal
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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Fig. 3. The effects of pre-esterification conditions on the FFA reduction and FAME yield. (A) The effect of catalyst loading amount on FFA reduction; (B) The effect of catalyst loading amount on FAME yield; (C) The effect of methanol to oil molar ratio on FFA reduction; (D) The effect of methanol to oil molar ratio on FAME yield; (E) The effect of temperature on FFA reduction; (F) The effect of temperature on FAME yield. Reaction condition for (A) and (B): methanol to oil molar ratio 20:1, temperature 100 °C; reaction condition for (C) and (D): catalyst loading amount 5 wt.%, temperature 100 °C; reaction condition for (E) and (F): catalyst loading amount 5 wt.%, methanol to oil molar ratio 20:1.
of the chlorophyll floccule from the clear oil was simple. The refined oil had an orange color indicative of carotenoids (Fig. A.2). As shown in Table 3, the phospholipid was completed removed in the second phosphoric acid pretreatment (degumming) step, indicating that the traditional degumming process was compatible with chlorophyll removal process described above. The 94.9% of FAME in crude oil was retained in the refined oil after the two-step refining. By using a sequential phosphoric acid discoloration-degumming pretreatment process, the chlorophyll and phospholipids were removed to obtain a refined a microalgal biodiesel feedstock free of chlorophyll and phospholipids that might pollute the catalyst. The bio-char catalyst could be used three times without losing activity if the microalgal oil was first refined (Table 2). Compared to the performance of catalyst in crude oil, this clearly demonstrates that the life of catalyst can be prolonged by refining the crude microalgal oil. This indicates that the chlorophyll and phosphoric lipids might be the major compounds that fouled the catalyst. However, the other compounds with fouling effects cannot necessarily be ruled out. The catalyst fouling mechanism is worthy of investigation in a future study. Microalgae oil differs from commercial biodiesel feedstocks, such as those from vegetable oils, mainly in terms of chlorophyll and FFA content. Most microalgae can produce large amounts of chlorophyll that are co-extracted with lipids. Since the chlorophyll contamination can degrade the quality of biodiesel [31] the removal of chlorophyll is important [4,32]. Chen et al. employed bleaching earth to absorb the chlorophyll from microalgal biodiesel [4]. Sathish developed a multi-step process to remove chlorophyll from microalgal crude oil [32]. However, a process that can be scaled up and compatible with current oil refinery infrastructure is still under development. In this study, phosphoric acid was used as the protonation reagent for chlorophyll de-alkylation [33]. The non-polar aliphatic
side chain can be cleaved from the porphyrin structure with a carboxylic acid side-chain to form pheophorbide, which is not soluble in oil. Thus, this precipitable form of chlorophyll can be easily separated from the oil. Moreover, the phosphoric acid discoloration treatment is compatible with conventional oil refining practice, since phosphoric acid is routinely added for oil degumming. The recovered pheophorbide and phospholipids may have pharmaceutical [34] or food applications, and may add value to the microalgae biodiesel production. The successful separation of chlorophyll and phospholipids may not only prolong the life of the solid catalyst and increase the quality of biodiesel, but also produce value-added co-products. 3.3. Effect of reaction conditions on pre-esterification 3.3.1. Effect of catalyst loading amount As shown in Fig. 3a and b, all reactions reached equilibrium in 90 min under the given conditions, but more catalyst loading could increase the FFA conversion rate, especially at the start of the pre-esterification stage. The catalyst loading amount should not be less than 5 wt.% to reduce the FFA content below 0.5% in a relative shorter time (60 min). It was notable that although the catalyst loading of 7 wt.% could lead to the fastest FFA conversion in the early stage, the same equilibrium time could be obtained by using 5 wt.% of catalyst loading amount. Thus, the 5 wt.% loading amount was selected in this work. 3.3.2. Effect of methanol to oil ratio Higher methanol ratios favored the reduction of FFA to produce FAME (Fig. 3c and d). Although the equal molar ratio of methanol to oil was enough to convert the FFA into FAME, an excess of methanol was preferable due to the reversible nature of the esterification step. At the initial stage of the reaction (10 min), a faster
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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100 °C when the methanol ratio was higher than 20:1. Thus, the methanol ratio of 20:1 was selected for the following experiments. The miscibility of triglyceride and methanol is a crucial limiting factor for transesterification [8]. As shown in Fig. 3d, the yield of FAME was much higher when the methanol ratio was above 5:1, because more triglyceride was dissolved at a higher methanol to oil ratio, therefore increasing the rate of transesterification.
3.3.3. Effect of reaction temperature The pre-esterification rate was strongly dependent upon reaction temperature (Fig. 3e and f). The FFA content was reduced to less than 0.5% in 60 min at temperatures higher than 100 °C. The reaction rate was accelerated by increased temperature. The effect of temperature on the kinetic constants was determined by fitting the results to the Arrhenius equation. The esterification fits the first-order kinetic reaction, and the calculated reaction activation energy is 42.16 kJ/mol with R2 = 0.9878. Although a faster conversion rate was observed at 120 °C, there was only a marginal difference in FFA content between 100 and 120 °C at 60 min. As shown in Fig. 3e and f, for the reactions at temperature 100 and 120 °C, FFA content was reduced to below 0.5% after 60 min. However, the FAME yield at 120 °C was about 5% higher than that observed at 100 °C after 60 min. These results indicate a slight transesterification activity of the bio-char catalyst, probably due to the increased miscibility of triglyceride in methanol at elevated temperatures. This accords with previous reports that esterification and transesterification could occur simultaneously when a solid acidic catalyst is applied [8,27]. Since the acid catalyzed transesterification was much slower than esterification, the two-step process was applied in this study to obtain a high FAME yield in a short time. Fig. 4. The activity of bio-char catalyst and Amberlyst-15 for the pre-esterification step. (A) FFA reduction in pre-esterification; (B) FAME yield in pre-esterification. Reaction condition: catalysts loading 5 wt.%, methanol to oil ratio 20: 1, the reaction temperature 100 °C.
3.3.4. Comparison of bio-char catalyst with Amberlyst-15 in the preesterification step As shown in Fig. 4a and b, the performance of bio-char catalyst was superior to Amberlyst-15 in the pre-esterification process. The FFA content could be reduced to less than 0.5% in 60 min when bio-char was used as catalyst. In contrast, under the same reaction conditions, the Amberlyst-15 could only reduce the FFA content to 2.8%. Similarly, a higher FAME yield could be obtained when bio-char catalyst was applied. The higher catalytic activity of the bio-char catalyst may be associated with better dispersion in the liquid-phase reaction mixture compared to Amberlyst-15. The results demonstrate that the bio-char catalyst had a higher catalytic activity than Amberlyst-15. However, the price of bio-char catalyst is expected to be much lower than Amberlyst-15, and
Fig. 5. Reusability of bio-char catalyst in pre-esterification. Pre-esterification condition: temperature 100 °C, methanol to oil ratio of 10:1, 5 wt.% of catalyst and 60 min. The bio-char catalyst was dried at 105 °C for 1 h after each cycle.
FFA reduction rate was observed with lower methanol volume, because less methanol could increase the concentration of FFA surrounding the solid catalyst and therefore accelerate esterification [8,35]. The FAME conversion was higher with more methanol after 30 min, because higher concentration of reactant is preferred to proceed the reversible esterification and result in a low final FFA content when the reaction reached equilibrium. FFA content could be reduced to lower than 0.5% in a relatively short time (60 min) at
Fig. 6. The effect of the methanol to oil ratio on the base-catalyzed transesterification. Reaction condition: reaction temperature 65 °C, CaO loading 3 wt.%.
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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therefore shows great potential for applications in the biodiesel industry. 3.3.5. Reusability of bio-char catalyst As displayed in Fig. 5, the FFA conversion rate dropped from 98.2% to 83.9% after three cycles; however, the catalyst activity remained relatively stable in the subsequent experiments up to 10 cycles. It was likely that the –SO3H not tightly combined with the bio-char were washed off in the first few cycles, but the rest of the active sites attached tightly to the carbon support and could not be leached. These results indicate that the bio-char catalyst is durable through several pre-esterification steps and can be recycled a number of times. Moreover, compared with crude microalgae oil (Table 1), durability of the bio-char catalyst increased significantly when the refined microalgae oil was used as feedstock. This was due to the reduced fouling from the catalyst inhibitors. Therefore, the customized refining process was crucial for efficient microalgal biodiesel production. 3.4. Transesterification of pretreated microalgal oil CaO was applied as catalyst in transesterification because it is a great heterogeneous alkaline catalyst for biodiesel production [36,37]. As shown in Fig. 6, a FAME yield of 99% could be obtained with 3 wt.% of CaO at 65 °C with a methanol to oil ratio of 10:1 and 20:1. The reaction could reach equilibrium in a shorter time with a higher methanol ratio, since additional methanol tended to increase the miscibility of triglyceride and methanol. However, a lower methanol to biomass ratio is preferable in order to reduce cost and energy requirements for solvent recovery. Thus, the methanol to oil ratio of 10:1 was effective enough for the base-catalyzed transesterification. Compared to the homogeneous alkaline catalysts, the CaO could be easily recycled after the reaction. Applying heterogeneous alkaline catalyst can also preclude the need for the water washing step after the reaction, as well as reduce capital and operational costs.
7
The application of bio-char catalyst instead of sulfuric acid can also reduce equipment corrosion and reduce environmental impacts by reducing the corrosive waste stream.
4. Conclusions In this study, bio-char was produced from the Auger pyrolysis of Douglas fir at 600 °C, followed by sulfonation. The resultant bio-char exhibited catalytic activity higher than that of Amberlyst-15 in the pre-esterification process. Impurities such as chlorophyll and phospholipids might be the major poisons for catalyst fouling, indicating that a refining process for the crude oil was crucial. A simple and practical refinery process was developed to improve the durability of the catalyst and quality of biodiesel. The resulting bio-char catalyst reduced the FFA content to below 0.5% in pre-esterification step. A total FAME yield of 99% was obtained in the second step CaO-catalyzed transesterification. The application of bio-char catalyst may also reduce environmental impact by decreasing the amount of corrosive acid in waste streams.
Appendix A See Figs. A.1 and A.2 and Table A.1.
3.5. Characterization of microalgal biodiesel The GC–MS analysis of the biodiesel produced from C. sorokiniana with the reported method is summarized in Table A.1. FAME was the major component, with olefins and fatty alcohols in minor quantities. The majority of the FAME was derived from fatty acids with 16 and 18 carbons (95% of total FAME), indicating a similar composition to that produced from corn and soybean oil [38]. The physical and chemical properties of biodiesel, such as iodine values, cetane numbers, kinematic viscosity, density and higher heating values were estimated with empirical Equations [24,39,40]. As shown in Table 4, compared with the European standard (EN 14214:2008), the corresponding specifications of the microalgal biodiesel were all qualified. This suggests that producing microalgal biodiesel from C. sorokiniana via the two-step process described here is appropriate. This can be done by employing heterogeneous acidic and basic catalysts. Compared to the traditional two-step process with homogeneous catalysts, this approach precludes the neutralization and water washing steps.
Fig. A1. FTIR scan of original bio-char (dotted line) and bio-char catalyst (solid line).
Table 4 Biodiesel characterization. Parameter
Biodiesel
EN 14214:2008
Iodine value Cetane number Kinematic viscosity (mm2/s) Density (g/cm3) Higher heating value (MJ/kg)
89.2 60.7 4.39 0.87 39.8
6120 P51 3.5–5.0 0.86–0.90
Fig. A2. Crude microalgal oil and refined microalgal oil Crude oil was refined by decoloration and degumming. Residue phosphoric acid was removed by water washing.
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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Table A1 GC–MS analysis of microalgal biodiesel. Peak
Retention time
Area (%)
Name
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
39.738 43.461 44.292 45.297 45.543 46.579 48.384 51.676 52.025 52.61 53.564 57.42 58.169 63.379 68.23
0.10 0.02 0.09 1.05 3.97 25.54 1.64 20.12 39.72 5.65 0.47 0.47 0.85 0.20 0.10
Pentadecanoic acid methyl ester Pentadecanal 3,7,11,15-Tetramethyl-2-hexadecen-1-ol 7, 10-Hexadecadienoic acid methyl ester cis-9-Hexadecenoic acid methyl ester Hexadecanoic acid methyl ester Heptadecanoic acid methyl ester cis,cis-9,12-Octadecadienoic acid, methyl ester cis-9-Octyldecenoic acid, methyl ester Octadecanoic acid methyl ester Nonadecanoic acid cis-11-Eicosenoic acid methyl ester Eicosanoic acid methyl ester Docosanoic acid methyl ester Tetracosanoic acid methyl ester
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Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char Tao Dong a, Difeng Gao a, Chao Miao a, Xiaochen Yu a, Charles Degan a, Manuel Garcia-Pérez a, Barbara Rasco b, Shyam S. Sablani a, Shulin Chen a,⇑ a b
Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164, USA School of Food Science, Washington State University, Pullman, WA 99164, USA
a r t i c l e
i n f o
Article history: Received 30 March 2015 Accepted 24 June 2015 Available online xxxx Keywords: Microalgae Biodiesel Bio-char catalyst Catalyst-fouling Refinery Chlorophyll
a b s t r a c t An efficient process for biodiesel production from fast-refined microalgal oil was demonstrated. A low cost catalyst prepared from pyrolysis-derived bio-char, was applied in pre-esterification to reduce free fatty acid (FFA) content. Results showed that the bio-char catalyst was highly active in esterification; however, the performance of the catalyst significantly reduced when crude microalgal oil was used as feedstock. To solve the problem caused by catalyst-fouling, a fast and scalable crude oil refinery procedure was carried out to remove chlorophyll and phospholipids that might degrade the catalyst and the quality of biodiesel. The activity and reusability of bio-char catalyst were remarkably improved in the fast-refined oil. FFA content in the refined microalgal oil was reduced to less than 0.5% after pre-esterification. The bio-char catalyst could be reused for 10 cycles without dramatic loss in activity. The pre-esterification fits the first-order kinetic reaction with activation energy of 42.16 kJ/mol. The activity of bio-char catalyst was superior to commercial Amberlyst-15 under the same reaction condition. A total fatty acid methyl ester (FAME, namely biodiesel) yield of 99% was obtained following the second-step CaO-catalyzed transesterification. The cost-effective bio-char catalyst has great potential for biodiesel production using feedstocks having high FFA content. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Fatty acid methyl esters (FAME) form the primary component of biodiesel, and have been produced from vegetable oil, animal fat, and waste cooking oil [1–3]. However, the limited supply of these feedstocks presents a major bottleneck for increasing biodiesel production [4]. Oleaginous microalgae have gained increasing attention as a next-generation renewable biofuel resource due to their fast growth rate and high lipid content, as well as their environmentally benign nature [5–7] compared to other energy crops. Many microalgae oils have been characterized as feedstock with high free fatty acid (FFA) content. For example, FFA content as high as 70% of total lipids was reported [4]. Unfortunately, highly efficient base catalysts are not practical under such conditions due to saponification. However, a two-step approach, in which acid-catalyzed pre-esterification occurs prior to the
⇑ Corresponding author at: Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164-6120, USA. Tel.: +1 (509) 335 3743; fax: +1 (509) 335 2722. E-mail address: [email protected] (S. Chen).
base-catalyzed transesterification, could lead to a higher FAME yield with a shorter reaction time [8]. The application of homogeneous acid catalysts such as sulfuric acid complicates downstream processing by generating an acidic waste stream requiring further treatment. Water washing tanks, one expense associated with handling waste streams, alone cost about 20% of the total capital in biodiesel production [9]. In contrast, heterogeneous catalysts can be easily separated after the reaction, bypassing the water washing and neutralization steps, simplifying the downstream process and reducing waste disposal costs and environmental impacts. Thus, a heterogeneous catalyst based process may be more economical compared to traditional homogeneous and supercritical conversion processes, especially for feedstocks with high FFA content [10]. Many studies have explored highly efficient heterogeneous catalysts, including super acids, zeolites, metal oxides [11], ion-exchange resins [2], and mesoporous silica [12]. However, the acidic site density of metal oxide and zeolite catalysts is usually low. Further, these catalysts are sensitive to water due to the hydrophilic surface. Although ion exchange resins possess strong acidic sites, they have low heat stability. Furthermore, the
http://dx.doi.org/10.1016/j.enconman.2015.06.072 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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supporting materials for solid acids, such as silicon oxides for perfluorinated alkanes [13] and zirconia for heteropoly acids are very expensive [14]. Carbonaceous materials are relatively inexpensive, widely available, and can be easily functionalized with –SO3H groups [15]. Recently, a new class of sulfonated carbons has been successfully developed as heterogeneous acid catalysts [15,16]. Partial carbonization of sugar forms a rigid carbon matrix consisting of polycyclic aromatic carbon sheets arranged in a three dimensional sp3–bonded structure. Sulfonation of these carbon materials results in the formation of a stable solid acid catalyst with a high density of –SO3H that display remarkable catalytic performance for esterification [15]. Unfortunately, the carbonization process is usually time-consuming and produces large amounts of waste [17,18]. Fast pyrolysis is a thermo-chemical process in which biomass is converted into bio-oil, bio-char and non-condensable gases by heating the biomass to approximately 500 °C in the absence of oxygen [19]. Bio-char is co-produced from the pyrolysis process and is currently used as a household fuel, for industrial applications or as a soil amendment. Bio-char is extensively used as a reducing agent in the iron and steel industries, and in the manufacture of carbon disulphide, sodium cyanides, carbides and activated carbon [20]. Bio-char can also provide a potential cost-effective carbonaceous catalyst support for preparing catalysts [21]. The carbonaceous nature and potential for sulfonation of bio-char makes it a promising catalyst for converting microalgal oil into biodiesel. Since microalgae oil differs from common biodiesel feedstocks such as vegetable oil in terms of both FFA and chlorophyll contents, the performance of the bio-char catalyst in this feedstock should be evaluated. This study investigated the microalgae biodiesel production process using acidic bio-char as a catalyst. The study also explored the effects of contaminants in the microalgal oil on the bio-char catalyst and on the pre-esterification process.
quantified by in situ methylation method [23]. Freeze-dried biomass was grinded in a motor with a pestle, and extracted with chloroform/methanol (2:1, v/v) using a Soxhlet extraction apparatus [24]. Crude microalgal oil was mixed with 1% (v/v) of pure phosphoric acid in a round bottom flask and heated on a vacuum evaporator at 100 °C for 30 min to remove chlorophyll. Dark color floccule was formed and adhered to the surface of the flask. An oil phase with a light orange color was obtained and easily separated from the dark precipitation. Next, the acidified oil was transferred to a second round bottom flask and mixed with 20% (v/v) water. The mixture was stirred at 1000 rpm at 80 °C for 1 h to hydrate phospholipids forming gum. Then the refined microalgal oil was separated from the water phase and gum by centrifugation at 4000 rpm for 2 min. The refined microalgal oil was obtained by washing three times with an equal volume of water to remove residue phosphoric acid. Fractionation and analysis of the FFAs and acylglycerols was performed on the solid phase extraction (SPE) cartridge, followed by methylation and GC analysis on an Agilent 7890A GC equipped with a FAMEWAX column (30 m 320 lm 0.25 lm) (Restek, Bellefonte, PA). Tridecanoic acid methyl ester (C13:0) was used as an internal standard [8]. The chlorophyll and total carotenoid content in the crude and refined microalgal oils were determined by UV–vis spectroscopy with calculations shown below (Wellburn, 1994). The oil was diluted with heptane and scanned from 300 to 800 nm with a Shimadzu UV–2550 spectrophotometer (Shimadzu, Kyoto, Japan).
Chlorophyll aðCa Þðlg mL1 Þ ¼ 10:05ðOD662 Þ 0:77ðOD644 Þ Chlorophyll bðCb Þðlg mL1 Þ ¼ 16:37ðOD644 Þ 3:14ðOD662 Þ Total carotenoidsðCxþc Þðlg mL1 Þ ¼ ð1000OD470 1:28Ca 56:7Cb Þ=205
ð1Þ
2. Materials and methods 2.1. Materials Douglas fir was used as feedstock for the production of the bio-char based catalyst. Douglas fir was donated by Herman Brothers Logging & Construction, Incorporated (Port Angeles, WA). A pioneer mill (Model number 400 HD, serial number 2404, Bliss Industries, Inc., Ponca, OK) was used to grind the feedstock to 2 mm or less. Green microalgae Chlorella sorokiniana (UTEX 1602) was obtained from the Culture Collection of Alga at the University of Texas (Austin, TX). Agilent Bond Elut NH2 cartridge (100 mg) was purchased from Agilent (Santa Clara, CA). Fatty acid standards for GC analysis, consisting of tridecanoic acid, myristic acid, palmitic acid, hexadecenoic acid, heptadecanoic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, eicosanoic acid and their methyl esters, were purchased from Sigma Chemical Co. (St. Louis, MO). The Amberlyst-15 catalyst used in the pre-esterification step as a control catalyst was obtained from Acros Organics (Fair Lawn, NJ). All solvents and reagents were either of HPLC grade or analytical reagent grade. 2.2. Preparation and characterization of crude and refined microalgal oil The microalgae were inoculated in a 5 L fermenter (New Brunswick Scientific, CT) and grown heterotrophically on glucose for 4 days at 37 °C [8,22]. The microalgae biomass was harvested by centrifugation. The total FAME content in the biomass was
2.3. Co-production of bio-char via fast pyrolysis Douglas fir wood chips were soaked in deionized water and autoclaved at 121 °C, 15 psi for 30 min to reduce the content of alkaline metals [25]. The biomass was dried and pyrolyzed using an Auger pyrolysis reactor at 600 °C with a residence time of 1 min inside the reactor, as described elsewhere [26]. The pyrolysis vapors were condensed in a three stage condensation unit. The charred particles were collected, allowed to cool to room temperature for two hours, and weighed. 2.4. Preparation of the acidic bio-char catalyst The bio-char (10 g) was grounded in a motor and then mixed with 150 mL of concentrated sulfuric acid (Sigma, St. Louis, MO, US) in a 500 mL flat bottom flask. The flask was heated on a magnetic (100 rpm) heating plate for 24 h at 150 °C. After the sulfonation, the slurry was carefully poured into 500 mL of water and filtered. The bio-char catalyst was washed with hot deionized water until the pH was neutral [27]. The bio-char catalyst was then dried in an oven at 105 °C for 4 h and stored in desiccators before use. The surface area was measured using a Coulter SA 3100 surface area analyzer (Beckman Coulter, US) and calculated with the single point Brunauer–Emmett–Teller (BET) method. Scanning electron microscopy (SEM) was used to assess the physical structure and pore size of the catalysts. The elemental composition was determined with an ECS 4010 elemental analyzer (Costech Analytical, Valencia, CA). An FTIR Thermo Nicolet Avatar 370
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
T. Dong et al. / Energy Conversion and Management xxx (2015) xxx–xxx
spectrometer was applied to analyze the bonding groups (Thermo Scientific, US). The total acid site density, consisting of carboxylic, phenolic, and sulfonic groups was measured with titration [18].
2.5. Bio-char catalyzed pre-esterification A mixture of refined microalgal oil and FFAs produced by hydrolysis of C. sorokiniana reined oil was prepared; oil samples had FFA concentrations as high as 44.6% (w/w) [4]. A 300 mg of high-FFA microalgae oil or pure oleic acid (used as a model FFA control feedstock for comparison), methanol, bio-char catalyst and a PTFE coated stir bar were fed into a screw-cap Pyrex tube reactor. Pre-esterification experiments were performed by heating the reactant at 300 rpm in a silicone oil bath. Different reaction operating parameters were investigated, including the catalyst loading amount in the range of 3–7 wt.%, the reaction temperature in the range of 80–120 °C, the methanol to oil molar ratio in the range of 5:1–30:1, and the reaction time in the range of 10– 90 min. After the reaction, the reactor was frozen to 20 °C to stop the reaction and 1 mL of tridecanoic acid methyl ester methanol solution (4 mg/mL) and 2 mL of sodium chloride water solution (5%, w/v) were added. The FFA and FAME were extracted by 3 mL of hexane for GC analysis. [8]. The Amberlyst-15 catalyst was used in the pre-esterification as a control catalyst for comparison. The FAME yield was calculated using Eq. (1)
FAME yield % ¼
3
3. Results and discussion 3.1. Preparation and characterization of the bio-char catalyst The yield of bio-char was 16% and the yield of bio-oil was 47% with the Auger pyrolysis reactor. The lower yield of oil compared to that reported by Liaw et al. (2012) can be explained by the higher temperature (600 °C), which is not optimal for bio-oil production but produces a bio-char with a higher surface area. This process can be adjusted to favor bio-char, bio-oil, or gas production [19,27]. Bio-char is considered to be a low-cost and environmentally friendly carbonaceous co-product from pyrolysis that has been used for soil amendment and carbon sequestration [19,29], and in this study as a catalyst.
Produced FAME ðgÞ 100% Total FAME potential in starting oilðgÞ ð1Þ
The total FAME potential is the total FAME can be converted into.
2.6. Alkali-catalyzed transesterification After pre-esterification, the mixture was centrifuged at 1500 rpm for 10 min to separate the bio-char catalyst from the liquid phase. The separated bio-char catalyst was washed with hexane, and the washing solvent was transferred to a second Pyrex reactor and evaporated by nitrogen. The reactant mixture was allowed to stand for phase separation. The upper phase consisting of methanol and H2O and the bottom phase of mainly oil and esterified fatty acids were separated [28]. The bottom phase was also transferred to the second Pyrex reactor. Methanol and CaO catalyst (3 wt.%) was then added to promote transesterification at 65 °C under continuous stirring at 300 rpm. Samples of the reaction mixture were collected at 20 min intervals, and FAME content was determined. At the end of the reaction, methanol was evaporated and FAMEs accumulated in the upper phase were separated from the bottom glycerol phase. The FAME production was then washed with water before GC–MS analysis.
2.7. GC–MS analysis The microalgae biodiesel samples were analyzed by a GC–MS system with an Agilent 5975 C MSD (Triple-Axis Detector) and an Agilent 7890 A GC equipped with a capillary column (J&W 122-5536G, 30 m 250 lm 0.5 lm). Helium was used as the carrier gas. The injection volume was 1 lL with split ratio at 20. The parameters of the oven temperature program started at 40 °C with 3 °C min1 intervals up to 250 °C and held for 20 min. The temperatures of the injector and detector were set at 300 °C and 250 °C, respectively [8].
Fig. 1. SEM images of: (a) bio-char catalyst indicating pore sizes; and (b) bio-char catalyst emphasizing fibrous channels and pore networks.
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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Table 1 Surface area, elemental analysis, and total acidity of bio-char catalyst.
a b
Sample
Surface area
Bio-char catalyst
3.51
a
(m2 g1)
Total acidityb (mmol/g)
Elemental composition (wt.%) C
H
N
O
S
59.14
2.16
0.30
33.44
2.02
1.37
Single point BET analysis. Back titration method.
Table 2 Esterification activity of bio-char catalyst in the crude and refined microalgal oil. Cycle no.
1 2 3
FFA conversion % Oleic acid
Crude microalgae oil
Refined microalgae oil
99.22 ± 0.32 97.74 ± 0.73 96.04 ± 0.95
98.61 ± 1.02 65.99 ± 5.06 8.00 ± 1.62
98.40 ± 0.68 97.60 ± 0.53 97.13 ± 0.68
Pre-esterification conditions: temperature 100 °C, methanol to oil ratio of 20:1, 10 wt.% of catalyst and 60 min. The bio-char catalyst was washed with hexane and dried at 105 °C for 1 h after each cycle. Experiments were triplicated.
showed that the sulfur content was 2.02 wt.% (Table 1). Since all sulfur atoms in sulfonated carbon catalysts are likely to occur in –SO3H groups [30], the mass content of –SO3H groups and the acid site concentration was calculated to be about 5.11% and 0.63 mmol g1, based upon the sulfur content, respectively. However, the results of titration indicate a total acidity of 1.37 mmol g1 (Table 1), which is higher than that calculated from the sulfur content. This is because a large fraction of the acid groups are likely associated with weak acid groups such as phenolic and carboxylic groups, which make the carbonaceous microsphere polar. Although only the –SO3H acid groups are active for esterification [18], these weak acid groups can improve the catalytic activity by increasing the acidity and improving the adsorption of reactant molecules on the catalyst surface [27]. FTIR spectra of the bio-char catalyst before and after sulfonation are depicted in Fig. A1. The strong peak at around 1712 cm1 and the weak peak at around 1207 cm1 can be assigned to the typical stretching modes of –SO3H groups [17]. FTIR shows that the –SO3H groups were immobilized on the bio-char catalyst following sulfonation.
3.2. Refining the microalgal oil
Fig. 2. Absorbance of crude microalgal oil before and after phosphoric acid discoloration treatment.
Table 3 Chemical composition of crude and refined microalgae oil. Samples
Crude oil Refined oil High-FFA oila
Fatty acid content (% of oil) Neutral lipids
FFA
Phospholipids
79.96 ± 0.62 87.26 ± 1.01 55.15 ± 0.82
10.81 ± 0.57 12.63 ± 1.04 44.6 ± 0.36
3.39 ± 0.35 ND ND
ND: not detected a The high-FFA oil was prepared by mixing refined oil with FFA hydrolyzed from refined oil to obtain a high FFA content. This oil was used to carry out the two-step biodiesel production.
Fig. 1 shows the morphology and pore size of the bio-char catalyst obtained by SEM images. The biochar has a fibrous surface structure with an irregular shape characterized by a highly complex network of pores, channels, and fibrous ridged surfaces similar to the raw wood structure [19]. This structure differs from the planar structure of the carbonaceous material derived from sugar [16]. The bio-char catalyst had a specific area of 3.51 m2 g1 based on calculations from adsorption isotherms using the standard BET equation (Table 1). Elemental analysis (EA) of the bio-char catalyst
The total FAME in the biomass was 33.6% (based on dry weight) and 94.1% of FAME was extracted into the crude oil. The crude microalgae oil was a sticky liquid with a dark green color. In preliminary experiments, the crude microalgae oil was directly used as a feedstock to test the esterification activity of the bio-char catalyst. The activity of recycled bio-char catalyst decreased dramatically in the crude microalgae oil (Table 2). In contrast, the activity of the bio-char catalyst remained constant in pure oleic acid after being recycled three times. The results showed that the bio-char catalyst could absorb catalyst poisons from the crude microalgal oil. This result is in accordance with previous report that compounds in microalgae biomass could reduce the activity of Amberlyst-15 catalyst [8]. Washing the recycled bio-char catalyst from crude microalgal oil with hot acetone could only partially recover the esterification activity (data not shown). This result indicates that impurities in the crude microalgae oil could irreversibly block the active sites on the catalyst. Thus, refining crude microalgae via a practical and scalable approach is indispensable for commercialization of microalgal biodiesel. In this study, an integrated and simple refinery was conducted to remove impurities before the oil conversion step with the bio-char catalyst. As illustrated in Fig. 2, the crude oil had strong absorption peaks at 667 nm and 470 nm due to the chlorophyll and carotenoid pigments. Through the simple phosphoric acid discoloration pre-treatment, the total chlorophyll and carotenoid contents in the crude oil could be decreased from 2290 and 2254 ppm to 7 ppm and 754 ppm, respectively, indicating a 99.7% and 66.6% removal efficiency for chlorophyll and carotenoid pigments. In the first phosphoric acid pretreatment (discoloration) step, a dark green floccule formed very fast and became separated from the oil. Since the dark green precipitate was not soluble in oil, removal
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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Fig. 3. The effects of pre-esterification conditions on the FFA reduction and FAME yield. (A) The effect of catalyst loading amount on FFA reduction; (B) The effect of catalyst loading amount on FAME yield; (C) The effect of methanol to oil molar ratio on FFA reduction; (D) The effect of methanol to oil molar ratio on FAME yield; (E) The effect of temperature on FFA reduction; (F) The effect of temperature on FAME yield. Reaction condition for (A) and (B): methanol to oil molar ratio 20:1, temperature 100 °C; reaction condition for (C) and (D): catalyst loading amount 5 wt.%, temperature 100 °C; reaction condition for (E) and (F): catalyst loading amount 5 wt.%, methanol to oil molar ratio 20:1.
of the chlorophyll floccule from the clear oil was simple. The refined oil had an orange color indicative of carotenoids (Fig. A.2). As shown in Table 3, the phospholipid was completed removed in the second phosphoric acid pretreatment (degumming) step, indicating that the traditional degumming process was compatible with chlorophyll removal process described above. The 94.9% of FAME in crude oil was retained in the refined oil after the two-step refining. By using a sequential phosphoric acid discoloration-degumming pretreatment process, the chlorophyll and phospholipids were removed to obtain a refined a microalgal biodiesel feedstock free of chlorophyll and phospholipids that might pollute the catalyst. The bio-char catalyst could be used three times without losing activity if the microalgal oil was first refined (Table 2). Compared to the performance of catalyst in crude oil, this clearly demonstrates that the life of catalyst can be prolonged by refining the crude microalgal oil. This indicates that the chlorophyll and phosphoric lipids might be the major compounds that fouled the catalyst. However, the other compounds with fouling effects cannot necessarily be ruled out. The catalyst fouling mechanism is worthy of investigation in a future study. Microalgae oil differs from commercial biodiesel feedstocks, such as those from vegetable oils, mainly in terms of chlorophyll and FFA content. Most microalgae can produce large amounts of chlorophyll that are co-extracted with lipids. Since the chlorophyll contamination can degrade the quality of biodiesel [31] the removal of chlorophyll is important [4,32]. Chen et al. employed bleaching earth to absorb the chlorophyll from microalgal biodiesel [4]. Sathish developed a multi-step process to remove chlorophyll from microalgal crude oil [32]. However, a process that can be scaled up and compatible with current oil refinery infrastructure is still under development. In this study, phosphoric acid was used as the protonation reagent for chlorophyll de-alkylation [33]. The non-polar aliphatic
side chain can be cleaved from the porphyrin structure with a carboxylic acid side-chain to form pheophorbide, which is not soluble in oil. Thus, this precipitable form of chlorophyll can be easily separated from the oil. Moreover, the phosphoric acid discoloration treatment is compatible with conventional oil refining practice, since phosphoric acid is routinely added for oil degumming. The recovered pheophorbide and phospholipids may have pharmaceutical [34] or food applications, and may add value to the microalgae biodiesel production. The successful separation of chlorophyll and phospholipids may not only prolong the life of the solid catalyst and increase the quality of biodiesel, but also produce value-added co-products. 3.3. Effect of reaction conditions on pre-esterification 3.3.1. Effect of catalyst loading amount As shown in Fig. 3a and b, all reactions reached equilibrium in 90 min under the given conditions, but more catalyst loading could increase the FFA conversion rate, especially at the start of the pre-esterification stage. The catalyst loading amount should not be less than 5 wt.% to reduce the FFA content below 0.5% in a relative shorter time (60 min). It was notable that although the catalyst loading of 7 wt.% could lead to the fastest FFA conversion in the early stage, the same equilibrium time could be obtained by using 5 wt.% of catalyst loading amount. Thus, the 5 wt.% loading amount was selected in this work. 3.3.2. Effect of methanol to oil ratio Higher methanol ratios favored the reduction of FFA to produce FAME (Fig. 3c and d). Although the equal molar ratio of methanol to oil was enough to convert the FFA into FAME, an excess of methanol was preferable due to the reversible nature of the esterification step. At the initial stage of the reaction (10 min), a faster
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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100 °C when the methanol ratio was higher than 20:1. Thus, the methanol ratio of 20:1 was selected for the following experiments. The miscibility of triglyceride and methanol is a crucial limiting factor for transesterification [8]. As shown in Fig. 3d, the yield of FAME was much higher when the methanol ratio was above 5:1, because more triglyceride was dissolved at a higher methanol to oil ratio, therefore increasing the rate of transesterification.
3.3.3. Effect of reaction temperature The pre-esterification rate was strongly dependent upon reaction temperature (Fig. 3e and f). The FFA content was reduced to less than 0.5% in 60 min at temperatures higher than 100 °C. The reaction rate was accelerated by increased temperature. The effect of temperature on the kinetic constants was determined by fitting the results to the Arrhenius equation. The esterification fits the first-order kinetic reaction, and the calculated reaction activation energy is 42.16 kJ/mol with R2 = 0.9878. Although a faster conversion rate was observed at 120 °C, there was only a marginal difference in FFA content between 100 and 120 °C at 60 min. As shown in Fig. 3e and f, for the reactions at temperature 100 and 120 °C, FFA content was reduced to below 0.5% after 60 min. However, the FAME yield at 120 °C was about 5% higher than that observed at 100 °C after 60 min. These results indicate a slight transesterification activity of the bio-char catalyst, probably due to the increased miscibility of triglyceride in methanol at elevated temperatures. This accords with previous reports that esterification and transesterification could occur simultaneously when a solid acidic catalyst is applied [8,27]. Since the acid catalyzed transesterification was much slower than esterification, the two-step process was applied in this study to obtain a high FAME yield in a short time. Fig. 4. The activity of bio-char catalyst and Amberlyst-15 for the pre-esterification step. (A) FFA reduction in pre-esterification; (B) FAME yield in pre-esterification. Reaction condition: catalysts loading 5 wt.%, methanol to oil ratio 20: 1, the reaction temperature 100 °C.
3.3.4. Comparison of bio-char catalyst with Amberlyst-15 in the preesterification step As shown in Fig. 4a and b, the performance of bio-char catalyst was superior to Amberlyst-15 in the pre-esterification process. The FFA content could be reduced to less than 0.5% in 60 min when bio-char was used as catalyst. In contrast, under the same reaction conditions, the Amberlyst-15 could only reduce the FFA content to 2.8%. Similarly, a higher FAME yield could be obtained when bio-char catalyst was applied. The higher catalytic activity of the bio-char catalyst may be associated with better dispersion in the liquid-phase reaction mixture compared to Amberlyst-15. The results demonstrate that the bio-char catalyst had a higher catalytic activity than Amberlyst-15. However, the price of bio-char catalyst is expected to be much lower than Amberlyst-15, and
Fig. 5. Reusability of bio-char catalyst in pre-esterification. Pre-esterification condition: temperature 100 °C, methanol to oil ratio of 10:1, 5 wt.% of catalyst and 60 min. The bio-char catalyst was dried at 105 °C for 1 h after each cycle.
FFA reduction rate was observed with lower methanol volume, because less methanol could increase the concentration of FFA surrounding the solid catalyst and therefore accelerate esterification [8,35]. The FAME conversion was higher with more methanol after 30 min, because higher concentration of reactant is preferred to proceed the reversible esterification and result in a low final FFA content when the reaction reached equilibrium. FFA content could be reduced to lower than 0.5% in a relatively short time (60 min) at
Fig. 6. The effect of the methanol to oil ratio on the base-catalyzed transesterification. Reaction condition: reaction temperature 65 °C, CaO loading 3 wt.%.
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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therefore shows great potential for applications in the biodiesel industry. 3.3.5. Reusability of bio-char catalyst As displayed in Fig. 5, the FFA conversion rate dropped from 98.2% to 83.9% after three cycles; however, the catalyst activity remained relatively stable in the subsequent experiments up to 10 cycles. It was likely that the –SO3H not tightly combined with the bio-char were washed off in the first few cycles, but the rest of the active sites attached tightly to the carbon support and could not be leached. These results indicate that the bio-char catalyst is durable through several pre-esterification steps and can be recycled a number of times. Moreover, compared with crude microalgae oil (Table 1), durability of the bio-char catalyst increased significantly when the refined microalgae oil was used as feedstock. This was due to the reduced fouling from the catalyst inhibitors. Therefore, the customized refining process was crucial for efficient microalgal biodiesel production. 3.4. Transesterification of pretreated microalgal oil CaO was applied as catalyst in transesterification because it is a great heterogeneous alkaline catalyst for biodiesel production [36,37]. As shown in Fig. 6, a FAME yield of 99% could be obtained with 3 wt.% of CaO at 65 °C with a methanol to oil ratio of 10:1 and 20:1. The reaction could reach equilibrium in a shorter time with a higher methanol ratio, since additional methanol tended to increase the miscibility of triglyceride and methanol. However, a lower methanol to biomass ratio is preferable in order to reduce cost and energy requirements for solvent recovery. Thus, the methanol to oil ratio of 10:1 was effective enough for the base-catalyzed transesterification. Compared to the homogeneous alkaline catalysts, the CaO could be easily recycled after the reaction. Applying heterogeneous alkaline catalyst can also preclude the need for the water washing step after the reaction, as well as reduce capital and operational costs.
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The application of bio-char catalyst instead of sulfuric acid can also reduce equipment corrosion and reduce environmental impacts by reducing the corrosive waste stream.
4. Conclusions In this study, bio-char was produced from the Auger pyrolysis of Douglas fir at 600 °C, followed by sulfonation. The resultant bio-char exhibited catalytic activity higher than that of Amberlyst-15 in the pre-esterification process. Impurities such as chlorophyll and phospholipids might be the major poisons for catalyst fouling, indicating that a refining process for the crude oil was crucial. A simple and practical refinery process was developed to improve the durability of the catalyst and quality of biodiesel. The resulting bio-char catalyst reduced the FFA content to below 0.5% in pre-esterification step. A total FAME yield of 99% was obtained in the second step CaO-catalyzed transesterification. The application of bio-char catalyst may also reduce environmental impact by decreasing the amount of corrosive acid in waste streams.
Appendix A See Figs. A.1 and A.2 and Table A.1.
3.5. Characterization of microalgal biodiesel The GC–MS analysis of the biodiesel produced from C. sorokiniana with the reported method is summarized in Table A.1. FAME was the major component, with olefins and fatty alcohols in minor quantities. The majority of the FAME was derived from fatty acids with 16 and 18 carbons (95% of total FAME), indicating a similar composition to that produced from corn and soybean oil [38]. The physical and chemical properties of biodiesel, such as iodine values, cetane numbers, kinematic viscosity, density and higher heating values were estimated with empirical Equations [24,39,40]. As shown in Table 4, compared with the European standard (EN 14214:2008), the corresponding specifications of the microalgal biodiesel were all qualified. This suggests that producing microalgal biodiesel from C. sorokiniana via the two-step process described here is appropriate. This can be done by employing heterogeneous acidic and basic catalysts. Compared to the traditional two-step process with homogeneous catalysts, this approach precludes the neutralization and water washing steps.
Fig. A1. FTIR scan of original bio-char (dotted line) and bio-char catalyst (solid line).
Table 4 Biodiesel characterization. Parameter
Biodiesel
EN 14214:2008
Iodine value Cetane number Kinematic viscosity (mm2/s) Density (g/cm3) Higher heating value (MJ/kg)
89.2 60.7 4.39 0.87 39.8
6120 P51 3.5–5.0 0.86–0.90
Fig. A2. Crude microalgal oil and refined microalgal oil Crude oil was refined by decoloration and degumming. Residue phosphoric acid was removed by water washing.
Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072
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Table A1 GC–MS analysis of microalgal biodiesel. Peak
Retention time
Area (%)
Name
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
39.738 43.461 44.292 45.297 45.543 46.579 48.384 51.676 52.025 52.61 53.564 57.42 58.169 63.379 68.23
0.10 0.02 0.09 1.05 3.97 25.54 1.64 20.12 39.72 5.65 0.47 0.47 0.85 0.20 0.10
Pentadecanoic acid methyl ester Pentadecanal 3,7,11,15-Tetramethyl-2-hexadecen-1-ol 7, 10-Hexadecadienoic acid methyl ester cis-9-Hexadecenoic acid methyl ester Hexadecanoic acid methyl ester Heptadecanoic acid methyl ester cis,cis-9,12-Octadecadienoic acid, methyl ester cis-9-Octyldecenoic acid, methyl ester Octadecanoic acid methyl ester Nonadecanoic acid cis-11-Eicosenoic acid methyl ester Eicosanoic acid methyl ester Docosanoic acid methyl ester Tetracosanoic acid methyl ester
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Please cite this article in press as: Dong T et al. Two-step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived bio-char. Energy Convers Manage (2015), http://dx.doi.org/10.1016/j.enconman.2015.06.072