Variables Affecting Biodiesel Production from Zanthoxylum

Article pubs.acs.org/IECR

Variables Affecting Biodiesel Production from Zanthoxylum bungeanum Seed Oil with High Free Fatty Acids Junhua Zhang,* Lei Zhang, and Lili Jia College of Forestry, Northwest A&F University, Yangling 712100, China ABSTRACT: The Zanthoxylum bungeanum seed oil (ZSO) having high free fatty acids was successfully transformed into biodiesel by acid-catalyzed esterification and alkali-catalyzed transesterification and high conversion to biodiesel was obtained in a short time compared to acid-catalyzed transesterification. Methanol-to-oil molar ratio, catalyst amount, and reaction time were found to clearly affect the ferric sulfate-catalyzed esterification of ZSO. The acid value of ZSO was reduced to less than 2 mg of KOH/g with a methanol-to-oil molar ratio of 16:1, catalyst amount of 2%, and reaction time of 3 h, following transesterification using an alkaline catalyst. The conversion to biodiesel by potassium hydroxide-catalyzed transesterification was achieved above 98% under optimum conditions: a methanol-to-oil molar ratio of 6:1, catalyst amount of 1.5%, reaction time of 60 min, and reaction temperature of 60 °C. The biodiesel production process was also confirmed by 1H NMR and FTIR.

1. INTRODUCTION The possibility of developing alternative energy sources to replace traditional fossil fuels has been receiving great interest in recent decades. Biodiesel (fatty acid methyl esters, FAME) shows large potential applications as diesel substitutes and is receiving increasing attention as an alternative, nontoxic, biodegradable, and renewable diesel fuel. Biodiesel has a higher cetane number than does diesel fuel, no aromatics, no sulfur, and contains 10% to 11% oxygen by weight. These characteristics of biodiesel reduce the emissions of carbon monoxide, unburned hydrocarbons, and particulate matter in the exhaust gas compared to diesel fuel. In addition, biodiesel can be used directly in most diesel engines without requiring extensive engine modifications. Production costs of biodiesel are still rather high, compared to costs of petroleum-based diesel fuel. One way of reducing the biodiesel production costs is to use inexpensive, nonedible raw feedstocks. There are several nonedible vegetable oils that could be utilized as feedstocks for bioidiesel production, such as Jatropha curcas seed oil, Pistacia chinensis seed oil, Cornus wilsoniana fruit oil, Xanthoceras sorbifolia seed oil, and Camellia oleifera seed oil.1 Besides those feedstocks, Zanthoxylum bungeanum seed oil (ZSO) is also a potential valuable vegetable oil for biodiesel production. ZSO is a byproduct of Z. bungeanum, the pericarps of which is widely used as a spice in indigenous kitchens in China. Z. bungeanum seed has an estimated annual production potential of one million metric tons in China and most of seeds are used as muck and solid fuel. For biodiesel production, alkali-catalyzed transesterification is much faster than acid-catalyzed transesterification and is most often used commercially.2 The high content of free fatty acids (FFA) in ZSO will complicate the production of biodiesel by alkali-catalyzed transesterification. During alkali-catalyzed transesterification, the high content of FFA will react with alkali catalysts to produce soap, and soap will inhibit the transesterification for biodiesel production.3 In addition, excessive soap in the products can inhibit later processing of the © 2012 American Chemical Society

biodiesel, including glycerol separation and water washing. The soap of saturated fatty acids tends to solidify at ambient temperatures so a reaction mixture with excessive soap may gel and form a semisolid mass that is very difficult to recover. Therefore, to avoid the formation of soap, a process combining acid-catalyzed esterification with alkali-catalyzed transesterification for biodiesel production from raw feedstocks having high FFA content was recommended by many authors.4−11 Ferric sulfate was used as a solid acid catalyst to catalyze the esterification of FFA in different raw feedstocks and showed a high catalytic activity.9,12−14 In addition, ferric sulfate has a lower price and could be easily recovered due to its very low solubility in oil. As a promising solid acid catalyst, there was little knowledge about the effect of ferric sulfate on the esterification of FFA in ZSO. There are three kinds of catalysts that can be used in a transesterification reaction, namely, strong alkaline catalyst, strong acid, and enzyme.2,15 The main advantages of using a strong alkali as a catalyst are shorter reaction time and less amount of catalyst required in the transesterification reaction. Potassium hydroxide and sodium hydroxide are the commonly used alkaline catalysts, and methanol is normally the alcohol of choice for biodiesel production because it is inexpensive and affords a high level of conversion.2,16 In this work, the variables affecting ferric sulfate-catalyzed esterification of FFA in ZSO including methanol-to-oil molar ratio, catalyst amount, and reaction time were investigated. The effects of methanol-to-oil molar ratio, catalyst amount, reaction temperature, and reaction time on conversion to biodiesel by potassium hydroxide-catalyzed transesterification were also investigated and optimized. The process contributes to the production of biodiesel from nonedible ZSO, a viable and valuable raw feedstock for biodiesel production. Received: Revised: Accepted: Published: 3525

February February February February

12, 2011 3, 2012 4, 2012 4, 2012

dx.doi.org/10.1021/ie200306w | Ind. Eng. Chem. Res. 2012, 51, 3525−3530

Industrial & Engineering Chemistry Research

Article

2. MATERIALS AND METHODS 2.1. Materials. Methanol, Fe2(SO4)3·XH2O, potassium hydroxide, anhydrous sodium sulfate, and other chemicals were of analytical reagent grade. Zanthoxylum bungeanum seed oil (ZSO) was provided by a local company located in Hancheng, Shaanxi Province, China. This is a high yield area for Z. bungeanum Maxin. ZSO is brown in color with an initial acid value of 16 mg KOH/g, corresponding to a FFA level of 8.0%. 2.2. Esterification of ZSO. The 70 g of ZSO mixed with methanol and ferric sulfate was poured into a 250 mL flat bottom, one-neck glass container equipped with a reflux condenser. The glass container was kept in a water bath. To obtain a high reaction rate and low production cost, a 95 °C reaction temperature was selected, at which temperature the reaction can be carried out at local atmospheric pressure without additional supercharging equipment. The reaction was carried out without any stirring due to the boiling of methanol. During the reaction, any evaporated methanol was returned to the glass container by the condenser. After the reaction, the mixture was allowed to settle in a separating funnel overnight and the top methanol−water layer was removed. The insoluble catalyst in the mixture was removed by filtration and the methanol in the lower layer was recovered under vacuum at 50 °C with a rotational evaporator. Then, the lower layer was washed four times using hot water (80 °C) and dried with enough anhydrous sodium sulfate. The variables affecting the ferric sulfate-catalyzed esterification such as methanol-to-oil molar ratio (4:1−24:1), catalyst amount (0.5%−3.0% wt. of oil), and reaction time (0.5−4.0 h) were investigated. 2.3. Transesterification of ZSO. The 80 g of pretreated ZSO was poured into a round-bottomed flask equipped with a reflux condenser and heated to the reaction temperature. The potassium hydroxide in methanol solution was preheated and added to the flask. An additional amount of potassium hydroxide was added to neutralize the residual FFA in the oil mixture (the molecular weight of FFA was assumed to be 282). The mixture was stirred at the same speed of 600 rpm for all test runs. After the completion of the reaction, the product was allowed to settle overnight in a separating funnel for the separation of biodiesel. The lower glycerol layer was drawn off. The excess of methanol in the upper biodiesel layer was recovered under vacuum at 50 °C with a rotational evaporator. After that, the biodiesel layer was washed four times using hot water (80 °C) and dried with enough anhydrous sodium sulfate. The variables affecting the alkali-catalyzed transesterification such as methanol-to-oil molar ratio (3:1−8:1), catalyst amount (0.3−1.8 wt % of oil), reaction temperature (45−70 °C) and reaction time (30−150 min) were investigated by a one-factorat-a-time method. After that, transesterification of ZSO for biodiesel production was optimized by orthogonal experiment design. 2.4. Analysis. The acid value was determined according to the national standard of China.17 The conversion of the ZSO to ZSO biodesel was calculated by C(%) = (Goil − G biodiesel)/Goil × 100

glycerol and glycerol bound as mono-, di-, and triacylglycerides in the samples. The total glycerol contents of ZSO and ZSO biodiesel were determined according to the iodometric−periodic acid method,18 which was based on the official method Ca 14-56 for total, free, and combined glycerol from the American Oil Chemists’ Society (AOCS).19 1 H NMR analyses were performed on a Varian INOVA-400 MHz spectrometer (Varian, USA) using CDCl3 as the solvent and tetramethylsilane as the internal standard. The FTIR spectra were obtained in a Nicolet Avatar-330 spectrometer (Thermo Electron Corporation, USA) using KBr tablets in the 4000−500 cm−1 range.

3. RESULTS AND DISCUSSION 3.1. Esterification of ZSO. 3.1.1. Effect of Methanol-toOil Molar Ratio. An esterification process of FFA with methanol is represented by k1

CH3OH + CH3OH ⇄ H2O + RCOOCH3 k2

FFA

methanol

water

FAME

(3)

During the esterification, FFA is converted to biodiesel (fatty acid methyl ester, FAME) and a byproduct, water, is also produced. Water formation is the major handicap of the completion of acid-catalyzed esterification of FFA.20 In addition, the reaction is reversible and methanol should be in excess so as to drive the equilibrium toward products. The molar ratio of methanol-to-oil has been reported to be one of the most important variables affecting the esterification of FFA.6,7,14 The acid values of ZSO after ferric sulfate-catalyzed esterification at six different molar ratios were investigated (Figure 1). With a methanol-to-FFA molar ratio of 16:1 and

Figure 1. Effect of methanol-to-oil molar ratio on the reduction of the acid value of ZSO (catalyst amount, 2%; reaction temperature, 95 °C).

catalyst amount of 2% (based on the weight of oil), the acid value of ZSO was reduced from 16 to 1.92 mg KOH/g after 3 h of reaction, which satisfies transesterification using an alkaline catalyst. When the methanol-to-oil molar ratio was 32:1, the acid value of ZSO achieved a reached 1.66 mg KOH/g after 3 h of reaction. As expected, a high methanol-to-oil molar ratio was important to improve the esterification of FFA with methanol. The acid value of ZSO cannot be reduced to less than 2 mg

(2)

in which C is the conversion to biodiesel, Goil is the total glycerol content of ZSO, and Gbiodiesel is the total glycerol content of ZSO biodiesel. Total glycerol is the sum of free 3526



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KOH/g even though the methanol-to-oil molar ratio increased to 32:1 after 1.5 h reaction, since the reaction was inhibited by the water formation during esterification.20 The results presented here indicated that a relatively long reaction time was important to esterification. In the process of esterification of waste cooking oil with ferric sulfate at a methanol-to-FFA molar ratio of 10:1, and catalyst amount of 2% and reaction temperature of 95 °C, the conversion of FFA achieved 97% (corresponding to an acid value of 2.11 mg KOH/g) after 4 h of reaction.9 The reported results also indicated ferric sulfate showed a higher catalytic activity in the esterification of FFA in waste cooking oil. Our results indicated that ferric sulfate had good catalytic capacity of esterification of FFA with methanol, which was in agreement with our previous results14 and the reported results.9 3.1.2. Effect of Catalyst Amount. When methanol-to-oil molar ratio was 24:1 and the catalyst amount was 1.5%, the acid value of ZSO reached to 1.95 mg KOH/g after 3 h of reaction (Figure 2). With a further increase in the catalyst amount there

Figure 3. Effect of reaction time on the reduction of the acid value of ZSO (methanol-to-oil molar ratio, 24:1; catalyst amount, 2%; reaction temperature, 95 °C).

produced during the esterification of FFA in ZSO, which prevented further esterification.20 As can be observed, ferric sulfate-catalyzed esterification was a practical method to reduce the acid value of ZSO with high FFA to a low level, satisfying transesterification using an alkaline catalyst. To investigate the variables affecting the transesterification of ZSO for biodiesel production, ZSO with high FFA was pretreated by esterification and used for following transesterification. 3.2. Confirmation of Esterification by 1H NMR. It has been demonstrated that the formation of methyl ester in esterification can be monitored by the signals of 1H NMR at approximately 3.7 ppm and 1H NMR is suitable for monitoring esterification process.21,22 The application of 1H NMR for monitoring esterification is based on the analysis of the glycerin protons. In the 1H NMR spectra of fatty acid and their derivatives, signals at approximately 3.7 ppm and 2.3 ppm are related to protons of methyl ester and α-carbonyl methylene, and signals at 4.1−4.4 ppm are assigned to glyceridic protons.23 After ferric sulfate-catalyzed esterification, signals at 3.66 ppm in the 1H NMR spectrum of ZSO sample indicated that FFA in ZSO was transformed into methyl ester during the esterification, which was not observed in original ZSO (data not shown). As expected, some glyceryl-related signals in the 1 H NMR spectrum of ZSO after esterification were observed at 4.1−4.4 ppm, indicating the presence of glycerin derivatives and following transesterification was necessary for biodiesel production. 3.3. Transesterification of ZSO. 3.3.1. Effect of Methanol-to-Oil Molar Ratio. Methanol-to-oil molar ratio is one of the important factors that affect the transesterification process. The stoichiometric ratio for transesterification requires three moles of alcohol and one mole of triglyceride to yield three moles of FAME and one mole of glycerol. However, transesterification is an equilibrium reaction in which a large excess of alcohol is required to drive the reaction to the right side of reaction products.24 In the present work, the effect of a methanol-to-oil molar ratio varied from 3:1 to 8:1 on conversion to biodiesel was studied. When the methanol-tooil molar ratio was 3:1, the conversion to biodiesel was 86.1% (Figure 4). When the molar ratio was over 4:1, the increment of conversion to biodiesel was slight. The maximum conversion

Figure 2. Effect of catalyst amount on the reduction of the acid value of ZSO (methanol-to-oil molar ratio, 24:1; reaction temperature, 95 °C).

was little improvement in the reduction of the acid value of ZSO. At catalyst amount 0.5%−3.0%, the acid value of ZSO could not go below 2 mg KOH/g during 1.5 h of reaction. As expected, when the catalyst amount exceeded 1.5%, there was little improvement in the reduction of the acid value of ZSO. This could be due to several reasons including insufficient dissolve of ferric sulfate in the reaction mixture. Our results are in accordance with the reported results that increasing catalyst amount beyond 2% did not have much effect on reducing the acid value.9 3.1.3. Effect of Reaction Time. FFA in ZSO easily reacted with methanol in the first 1 h and the acid value of ZSO decreased to 3.80 mg KOH/g (Figure 3). This could be because the ferric sulfate had water of crystallization and can be rapidly converted to the [Fe(H2O)6]3+ complex and consequently release protons, thus increasing the esterification of FFA.13 The rate of esterification slowed from 1 to 2 h. The acid values of ZSO achieved 2.04 and 1.81 mg KOH/g after 2 and 3 h of reaction, respectively. It was also observed that increasing reaction time beyond 2 h did not have much effect on reducing the acid value. This might be due to the effect of water 3527



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Figure 6. Effect of reaction temperature on conversion to biodiesel during alkali-catalyzed transesterification (methanol-to-oil molar ratio, 6:1; KOH, 0.9%; reaction time, 2 h; and rate of stirring, 600 rpm).

Figure 4. Effect of methanol-to-oil molar ratio on conversion to biodiesel during alkali-catalyzed transesterification (KOH, 0.9%; reaction temperature, 60 °C; reaction time, 2 h; and rate of stirring, 600 rpm).

increase in reaction temperature, there was little increase in conversion to biodiesel. The reaction temperature above the boiling point of methanol (65 °C) should be avoided since at higher temperature, methanol tends to be lost and high temperature accelerates saponification of the glycerides by alkaline catalyst.25 Our results confirmed that higher a temperature of 70 °C was not beneficial for transesterification and little decrease in conversion to biodiesel was observed. 3.3.4. Effect of Reaction Time. The reaction was very fast during the first 30 min, and the conversion to biodiesel reached a high level of 97.7% (Figure 7). The conversion to biodiesel

to biodiesel was obtained at 6:1 and a further increment did not result in higher conversion to biodiesel. 3.3.2. Effect of Catalyst Amount. The effect of catalyst amount varied in the range of 0.3%−1.8% for six different values was investigated (Figure 5). When catalyst amount was

Figure 5. Effect of catalyst amount on conversion to biodiesel during alkali-catalyzed transesterification (methanol-to-oil molar ratio, 6:1; reaction temperature, 60 °C; reaction time, 2 h; and rate of stirring, 600 rpm).

0.3%, the conversion to biodiesel was 90.5%. With a catalyst amount 0.6%, the conversion to biodiesel increased to 96.2%. With further increase in catalyst amount there was little improvement in the conversion to biodiesel. While catalyst amount exceeded 1.5%, the conversion to biodiesel decreased reversely. The results supported that when excess alkali was used, the resulting soap formed by saponification produced emulsification of the ester and thereby inhibiting the transesterification. 3.3.3. Effect of Reaction Temperature. The reaction temperature was varied at six different levels to determine the effect of reaction temperature on ZSO biodiesel production. As can be seen from Figure 6, when the reaction temperature was 50 °C, the conversion to biodiesel reached 96.0%. With further

Figure 7. Effect of reaction time on conversion to biodiesel during alkali-catalyzed transesterification (methanol-to-oil molar ratio, 6:1; KOH, 0.9%; reaction temperature, 60 °C; and rate of stirring, 600 rpm).

reached the maximum value of 98.4% at 60 min. With further increase in reaction time, there was little decrease in conversion to biodiesel. 3.4. Optimal Parameters of Transesterification. Orthogonal experiment design and results are showed in Table 1. According to the range of the factors investigated, catalyst amount was the primary factor and reaction time was secondary. The order of the effect on conversion to biodiesel was catalyst amount > reaction time > reaction temperature > 3528



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Table 1. Orthogonal Experiment Design L9(34) and Results of Transesterification trail no.

methanol-tooil molar ratio

amount of catalyst (%)

reaction time (min)

reaction temperature (°C)

1 2 3 4 5 6 7 8 9

5:1 5:1 5:1 6:1 6:1 6:1 7:1 7:1 7:1

1.5 1.2 0.9 1.5 1.2 0.9 1.5 1.2 0.9

60 90 120 90 120 60 120 60 90

65 60 55 55 65 60 60 55 65

k1 k2 k3 R

97.72 97.77 98.02 0.30

98.58 97.47 97.46 1.12

98.10 98.03 97.38 0.72

97.73 97.90 97.88 0.17

ZSO was transformed into ZSO biodiesel. The bands at 1196 and 1435 cm−1 were observed in the FTIR of the ZSO biodiesel indicating the presence of O−CH3 and CH3 in biodiesel, which were not observed in ZSO. The results presented here confirmed that ZSO was transformed into FAME. A few studies have already reported the use of 1H NMR to monitor transesterification.23,27−30 The formation of methyl ester in transesterification can be monitored by the signal at approximately 3.7 ppm. After the alkali-catalyzed transesterification under optimum conditions, the 1H NMR spectrum of ZSO biodiesel is shown in Figure 9. No visible

conversion to biodiesel (%) 98.62 97.60 96.93 98.76 96.84 97.72 98.37 97.96 97.74

methanol-to-oil molar ratio. The optimal parameters of conversion to biodiesel obtained in the range analysis were the following: methanol-to-oil molar ratio, 7:1; catalyst amount, 1.5%; reaction time, 60 min; and reaction temperature, 60 °C. In this experiment, the methanol-to-oil molar ratio was not a main factor affecting the conversion to biodiesel in the tested range. For economical reasons, the methanol-to-oil molar ratio 6:1 was chosen, and the combination of methanol-to-oil molar ratio of 6:1, catalyst amount of 1.5%, reaction time of 60 min, and reaction temperature of 60 °C was selected as the optimum for potassium hydroxide-catalyzed transesterification. Under the optimum conditions, the conversion to biodiesel was achieved above 98%. 3.5. Confirmation of Transesterification by FTIR and 1 H NMR. Fourier-transformed infrared spectroscopy (FTIR) has been reported as a fast and accurate method to monitor the transesterification of vegetable oils.23,26 The FTIR spectra of ZSO and ZSO biodiesel in the region from 2000 to 660 cm−1 are shown in Figure 8. A strong band at 1740 cm−1 was

Figure 9. 1H NMR spectrum of ZSO biodiesel.

glyceryl-related signals were found at 4.1−4.4 ppm indicating its high conversion into methyl esters, as can be noticed in the spectrum at 3.66 ppm. The conversion of transesterification was above 98% as calculated from the integration values of the glyceridic and methyl ester protons in 1H NMR by Knothe,27 which was in accordance with the quantification by calculating the total glycerol content.

4. CONCLUSIONS The vegetable oil having high free fatty acids can be successfully transformed into biodiesel with acid-catalyzed esterification followed by alkali-catalyzed transesterification. High conversion to biodiesel achieved in a short time compared to acid-catalyzed transesterification. The esterification catalyzed by ferric sulfate was effective to reduce the high amount of FFA in feedstock to a low level, satisfying alkali-catalyzed transesterification. Variables, that is, methanol-to-oil molar ratio, catalyst amount, and reaction time, were investigated as they were found to clearly affect final acid value after esterification. The effects of methanol-to-oil molar ratio, catalyst amount, reaction temperature, and reaction time on conversion to biodiesel by potassium hydroxide-catalyzed transesterification were investigated. High conversion to biodiesel above 98% was obtained under optimum conditions (methanol-to-oil molar ratio, 6:1; catalyst amount, 1.5%; reaction time, 60 min; and reaction temperature, 60 °C), which was also confirmed by FTIR and 1H NMR spectra of ZSO biodiesel.

Figure 8. FTIR spectra of ZSO and ZSO biodiesel.

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observed, which was related to the carbonyl group in ZSO and ZSO biodiesel. The presence of the band at 725 cm−1 was related to the group (CH2)n in these samples. The band at 1169 cm−1 attributed to the C−CH2−O vibration was reduced after

AUTHOR INFORMATION

Corresponding Author

*Address: Junhua Zhang, College of Foresty, Northwest A&F University, Taicheng Road 3, Yangling, 712100, Shaanxi 3529



Article

method). In Of f icial Methods and Recommended Practices of the AOCS; American Oil Chemists’ Society: Champaign, IL, 1998. (20) Canakci, M.; Van Gerpen, J. Biodiesel productio via acid catalysis. Trans. ASAE 1999, 42, 1203. (21) Zhang, J.; Jiang, L. Acid-catalyzed esterification of Zanthoxylum bungeanum seed oil with high free fatty acids for biodiesel production. Bioresour. Technol. 2008, 99, 8995. (22) Mello, V. M.; Oliveira, F. C. C.; Fraga, W. G.; do Nascimento, C. J.; Suarez, P. A. Z. Determination of the content of fatty acid methyl esters (FAME) in biodiesel samples obtained by esterification using 1 H-NMR spectroscopy. Magn. Reson. Chem. 2008, 46, 1051. (23) Gelbard, G.; Bres, O.; Vargas, R. M.; Vielfaure, F.; Schuchardt, U. F. 1H-Nuclear magnetic resonance determination of the yield of the transesterification of rapeseed oil with methanol. J. Am. Oil Chem. Soc. 1995, 72, 1239. (24) Karavalakis, G.; Anastopoulos, G.; Stournas, S. Methyl ester production from sunflower and waste cooking oils using alkali-doped metal oxide catalysts. Ind. Eng. Chem. Res. 2010, 49, 12168. (25) Dorado, M. P.; Ballesteros, E.; Lopez, F. J.; Mittelbach, M. Optimization of alkali-catalyzed transesterification of Brassica carinata oil for biodiesel production. Energy Fuels 2004, 18, 77. (26) Zagonel, G. F.; Peralta-Zamora, P.; Ramos, L. P. Multivariate monitoring of soybean oil ethanolysis by FTIR. Talanta 2004, 63, 1021. (27) Knothe, G. Monitoring a progressing transesterification reaction by fiber-optic near infrared spectroscopy with correlation to 1H nuclear magnetic resonance spectroscopy. J. Am. Oil Chem. Soc. 2000, 77, 489. (28) Costa Neto, P. R.; Balparda Caro, M. S.; Mazzuco, L. M.; Graca Nascimento, M. Quantification of soybean oil ethanolysis with 1H NMR. J. Am. Oil Chem. Soc. 2004, 81, 1111. (29) Morgenstern, M.; Cline, J.; Meyer, S.; Cataldo, S. Determination of the kinetics of biodiesel production using proton nuclear magnetic resonance spectroscopy (1H NMR). Energy Fuels 2006, 20, 1350. (30) Jin, F.; Kawasaki, K.; Kishida, H.; Tohji, K.; Moriya, T.; Enomoto, H. NMR spectroscopic study on methanolysis reaction of vegetable oil. Fuel 2007, 86, 1201.

Province, China. Tel: +86-29-8708 1511. Fax: +86-29-8708 2892. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Northwest A&F University Key Research Project for Young Staff. NOMENCLATURE FAME = fatty acid methyl esters FFA = free fatty acids FTIR = Fourier-transformed infrared spectroscopy ZSO = Zanthoxylum bungeanum seed oil

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Variables Affecting Biodiesel Production from Zanthoxylum

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