Garden cress (Lepidium sativum Linn.) seed oil as a potential feedstock for biodiesel production

Garden cress (Lepidium sativum Linn.) seed oil as a potential feedstock for biodiesel production

Bioresource Technology 126 (2012) 193–197 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.c...

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Bioresource Technology 126 (2012) 193–197

Contents lists available at SciVerse ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Garden cress (Lepidium sativum Linn.) seed oil as a potential feedstock for biodiesel production Imededdine Arbi Nehdi a,⇑, Hassen Sbihi a, Chin Ping Tan b, Saud Ibrahim Al-Resayes a a b

King Saud University, College of Science, Chemistry Department, P.O. Box 2455, 1145 Riyadh, Saudi Arabia Department of Food Technology, Faculty of Food Science and Technology, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia

h i g h l i g h t s " Lepidium sativum seeds contain 26.77% of oil. " Lepidium sativum seed oil (LSO) was transesterified into its methyl esters (LSOME). " The properties of LSOME were evaluated against ASTM standards for biodiesel. " LSOME fuel properties indicate that it is suitable for use as a biodiesel fuel.

a r t i c l e

i n f o

Article history: Received 5 March 2012 Received in revised form 19 July 2012 Accepted 24 August 2012 Available online 11 September 2012 Keywords: Lepidium sativum L. Biofuel Vegetable oil Fatty acid methyl ester Transesterification

a b s t r a c t Lepidium sativum L. (garden cress) is a fast growing annual herb, native to Egypt and west Asia but widely cultivated in temperate climates throughout the world. L. sativum seed oil (LSO) extracted from plants grown in Tunisia was analyzed to determine whether it has potential as a raw material for biodiesel production. The oil content of the seeds was 26.77%, mainly composed of polyunsaturated (42.23%) and monounsaturated (39.62%) fatty acids. Methyl esters (LSOMEs) were prepared by base-catalyzed transesterification with a conversion rate of 96.8%. The kinematic viscosity (1.92 mm2/s), cetane number (49.23), gross heat value (40.45), and other fuel properties were within the limits for biodiesel specified by the ASTM (American Standard for Testing and Materials). This study showed that LSOMEs have the potential to supplement petroleum-based diesel. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Vegetable oils such rapeseed, soybean and sunflower oil are the most used raw materials for biodiesel production, but these oils are expensive and also used for food production. Therefore, non-food oils have been evaluated as renewable resources for biodiesel production (Ruan et al., 2008, 2012). One of the possible alternative oils for biodiesel production is oil from the seeds of garden cress (Lepidium sativum L.), a fast growing annual herb belonging to the Brassicaceae family that is native to Egypt and west Asia. Garden cress is also widely cultivated in temperate countries for various culinary and medicinal uses (Gokavi et al., 2004). The brownish-red oval seeds of L. sativum contain approximately 22.7% oil (LSO), mainly consisting of unsaturated fatty acids such us linolenic, linoleic, gadoleic, oleic, and erucic acid (Moser et al., 2009; Diwakar et al., 2010). Shehzad et al. (2011) found that a maximum seed yield of 305.9 kg per hectare can be obtained when ⇑ Corresponding author. Tel.: +966 14697118; fax: +966 4675992. E-mail address: [email protected] (I.A. Nehdi). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.08.113

grown without competition from weeds. Garden cress can be sown and harvested several times throughout the year, although January, February and November are the most suitable months of the year to sow garden cress in a Mediterranean climate (Tuncay et al., 2011). These findings show that L. sativum seed is a potential source of oil and hence justifies the study on possible industrial uses. This paper presents the process development in the production of biodiesel from L. sativum seed oil. The first objective of this study was to optimize the reaction condition for weight ratio of catalyst to methanol at 65 °C. The second objective was to analyze the fuel properties such us gross heat value, cetane number, viscosity, density, flash point, cloud point and pour point. 2. Methods 2.1. Materials Mature pods of L. sativum were collected in August from different plants in the same field located at latitude 48°240 N; longitude 13°740 E; altitude: 17 m in Sidi Thabet (Tunisia). The mature pods

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were harvested one month after sowing. Damaged seeds were removed and the remaining seeds were oven-dried at 60 °C for 24 h. The dried seeds were milled in a K/IKA WERKE M20 grinder and oil was extracted with hexane for 8 h in a Soxhlet apparatus. Hexane was removed by rotary evaporation (R-210 BUCHI, Flawil, Switzerland) at 80 °C to obtain the LSO. Fatty acids methyl ester standards were purchased from Supelco (Bellefonte, USA). All chemicals and reagents were of analytical grade and were obtained from Merck (Darmstadt, Germany). 2.2. Oil characterization The fatty acid methyl ester (FAME) composition was determined by converting the oil to fatty acid methyl esters, as described previously (Nehdi, 2011). The esters were separated using a Shimadzu GC-2014 chromatograph equipped with an FID detector and Rtx-1 column (30 m  0.25 mm i.d., 0.25 lm film thickness). The carrier gas was He at a flow rate of 1.41 ml/min. The oven temperature was ramped from 150 to 180 °C at 15 °C per min, followed by an increase to 210 °C at 1 °C per min. The temperatures of the injector and detector were 220 °C and 275 °C, respectively. Peaks were identified by comparison to the retention times of reference standards and FAMEs from conventional oils. This identification was confirmed by GC–MS (GCMSQP2010 Ultra, Shimadzu, Kyoto, Japan) operating under similar conditions as used for the GC–FID. Samples were analyzed in triplicate and mean values are reported. The FTIR spectra were recorded on a Bruker Tensor 27 FTIR spectrometer equipped with an ATR sampling accessory with a removable ZnSe crystal. The samples were dissolved in CCl4. The spectrum was collected with 64 scans at a spectral resolution of 4 cm1. The average spectrum from triplicate analyses in the range 4000–400 cm1 was analyzed using Opus 6.5 Software (Bruker, Rheinstetten, Germany). International Organization for Standardization (ISO) standards were used to determine peroxide value (ISO 3960), acidity (percentage of free fatty acids was calculated as oleic acid) (ISO 660) and unsaponifiable matter (ISO 3961). The refractive index of the seed oil and its methyl esters was determined using an Abbe refractometer (Bellingham and Stanley Ltd, Kent, England). The viscosity and the density of the oil were measured following ASTM D 445 and ASTM D 5002. Thermal properties were analyzed with a thermogravimetric analyzer TGA-50 (Shimadzu, Kyoto, Japan) in the 25–600 °C temperature range, using a synthetic zero air atmosphere (100 ml/ min), alumina crucibles, a heating rate 10 °C/min, and a sample mass of about 5 mg. TGA curves, as well as derivate thermogravimetric (DTG) curves were used in this study. Curves were prepared using normalized data of the average of three independent measurements.

moval of the catalyst. The washed methyl esters were dried over anhydrous sodium sulfate. The weights of the separated fractions were measured and the percentage yield of esters was determined as follows:

Yield ð%Þ ¼ ðTotal weight of methyl esters=Total weight of oil in the sampleÞ  100%: 2.4. Biodiesel characterization The fuel properties of the methyl esters were tested according to ASTM standards. The properties included density (ASTM D 5002), kinematic viscosity (ASTM D 445), flash point (D 93), cloud point (ASTM D 2500), pour point (ASTM D 97), sulfur content (ASTM D 4294), carbon residue (ASTM D4530), gross heat value (ASTM D4809), water and sediment (D 2709), ash content (ASTM D874), distillation curves (ASTM D1160), gross heat value (ASTM D240) and water and sediment (ASTM D2709). The iodine and saponification values were determined according to ISO 3596 and 3657, respectively. The cetane number (CN) was calculated (Mohibbe Azam et al., 2005) from the following equation by using the saponification value (SV) and iodine value (IV):

CN ¼ 46:3 þ 5458=SV  0:225 IV

ð1Þ

The average calculated molecular weight (MWcal, g/mol) was determined by a weighted average method utilizing the FA profile shown in Table 2. 3. Results and discussion 3.1. L. sativum seed oil Table 1 summarizes the composition and some physicochemical properties of the oil extracted from L. sativum seeds. The seeds contained 26.77% (w/w) crude oil, which is more than that reported in two previous studies (Moser et al., 2009; Diwakar et al., 2010). This difference may be due to different conditions for plant growth. The tested LSO had low acidity (0.25%), which enabled direct base-catalyzed transesterification for biodiesel production without acid pretreatment (Tiwari et al., 2007; Ghadge and Raheman, 2005). The LSO also showed very low kinematic viscosity (15.41 mm2/s) compared to that reported for soybean, sunflower, and Jatropha oils which ranged from 30 to 40 mm2/s at 40 °C (Karmakar et al., 2010). The LSO contained high percentages of polyunsaturated (42.23%) and monounsaturated (39.62%) fatty acids. Kim et al. (2010) suggests that the oil with more double bonds appeared to have lower viscosity due to their loosely-packed structure.

2.3. Transesterification experiments

3.2. Effect of catalyst concentration

The methyl esters were synthesized in a conical flask equipped with magnetic stirrer (600 rpm)and using a reflux condenser under standard conditions employing a 6:1 M ratio of methanol to oil with (0.5, 0.75, 1 and 1.25 wt.%) sodium hydroxide as catalyst for 2 h at 65 °C. At the end of experiment, the reaction mixture was transferred to a separator funnel and allowed to settle at room temperature for a minimum of 12 h until the layers were clearly separated. The upper layers were the transparent methyl esters, while dark-brown glycerin formed the bottom layer. After draining the glycerin, the fatty acid methyl esters were washed gently with three volumes of distilled water to remove residual NaOH, glycerol, methanol, and soap. A few drops of sulfuric acid were added to the second wash to neutralize any remaining soap and other catalytic impurities. A pH indicator was used to check for the complete re-

The effect of the NaOH concentration on the conversion of methyl esters is depicted in Fig. 1. Keeping other parameters constant, it was observed that using a lower concentration (0.5 wt.%) led to an incomplete reaction and resulted in a lower yield of methyl esters. The ester content reached a maximum value (96.8%) as the sodium hydroxide concentration reached 1 wt.%, and further increases in catalyst concentration caused decreases in ester production (Fig. 1). A large amount of soap was observed when an excess amount of sodium hydroxide (1.25 wt.%) was added. 3.3. L. sativum methyl ester profile The FA profile of LSO (Table 1) did not change with transesterification. The major peaks in the FTIR spectra of the LSO and LSOME

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Table 1 Composition and some physicochemical properties of Lepidium sativum oil (LSO). Physicochemical properties

Value

Viscosity at 40 °C (mm2/s) Density at 25 °C (kg/m3) Free fatty acids (% as oleic acid) Peroxide value (meq.O2/kg) Unsaponifiable matter (wt.%) Molecular weight (g/mol)

15.41 0.879 0.25 0.561 3.53 890.50

Fatty acid composition C 10:0 C11:0 C12:0 C14:0 C14:1 D11 C15:0 C15:1 D12 C16:0 C16:1 D7 C16:1 D9 C16:1 D11 C16:2 D7 10 C16:3 D7 10 13 C17:0 C17:1 D11 C18:0 C18:1 D9 C18:1 D7 C18:2 D9,12 C18:3 D9,12,15 C20 C20:1 D5 C20:1 D11 C20:2 D11,14 C20:3 D11,14,17 C22 C22:1 D13 Unknown (sum) Total saturated Total monounsaturated Total polyunsaturated U/S

Value (%) 0.04 ± 0.00 0.07 ± 0.00 0.04 ± 0.00 0.11 ± 0.01 0.04 ± 0.00 0.05 ± 0.00 0.04 ± 0.00 9.03 ± 0.41 0.15 ± 0.02 0.07 ± 0.00 0.04 ± 0.00 0.04 ± 0.00 0.05 ± 0.00 0.05 ± 0.00 0.04 ± 0.00 3.28 ± 0.12 21.21 ± 0.69 1.30 ± 0.11 11.17 ± 0.31 30.11 ± 0.51 3.92 ± 0.31 0.21 ± 0.05 12.01 ± 0.31 0.41 ± 0.03 0.45 ± 0.05 0.99 ± 0.03 4.51 ± 0.39 0.65 17.58 39.62 42.23 4.65

U/S: unsaturated:saturated ratio of fatty acids. Molecular weight is calculated from the FA composition.

1

were at 3010 cm ([email protected]–H stretching (asymmetry) (cis)), 2958 cm1 (C–H stretching (asymmetry) of aliphatic of CH3 group), 2926 cm1 (C–H stretching (asymmetry) of aliphatic CH2 group), 2854 cm1 (C–H stretching symmetry) of aliphatic CH2 group), 1746–1743 cm1 ([email protected] stretching), 1655 cm1 ([email protected] stretching (cis)), 1464 cm1 (C–H bending (scissoring) of CH2 and CH3 groups), 1377–1363 cm1 (C–H bending of CH2 group), 1246–1238 cm1 (C–O stretching), 1171–1163 cm1 (C–O stretching and C–H bending), 1099 cm1 (C–O stretching), and 794– 722 cm1 (C–H rocking of CH2 group) (Vlachos et al., 2006; Yang et al., 2005). The position of the carbonyl group in FTIR is sensitive to substituent effects (Bianchi et al., 1995). Therefore, there were some differences between the FTIR spectra of the oil and its methyl esters. The absorption peaks of the oil sample at 1746, 1377, 1238 and 1163 cm1 shifted to 1743, 1363, 1246 and 1171 cm1, respectively, in the biodiesel sample. In the spectrum of the biodiesel sample, the absence of peaks at 1099 cm1 and 1163 cm1 and the appearance of new bands at 1437, 1196, 1171, and 1016 cm1, corresponding to the C–O bond of the group CH3–O near the carbonyl ester group, was an indication of conversion of the oil into its methyl ester form. The thermal stability and volatility characteristics influence the ignition quality of fuels (Rodriguez et al., 2009). The thermogravimetric data (TGA) and first derivative data (DTG) of LSO and LSOME samples at dry heating air atmosphere indicated that LSO was ther-

% Yield of biodiesel

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% NaOH (w/w NaOH / methanol) Fig. 1. The effect of catalyst concentration on the esters yield: the temperature was 65 °C temperature, 600 rpm agitation speed, 120 min reaction time was 120 min and 6:1 methanol/oil.

mally stable up to 250.8 °C, and LSOME up to 159.3 °C, with a mass loss of 5%. A 90% mass loss occurred at 496.4 °C for the oil and 292.3 °C for its methyl esters. The TGA/DTG curves of the LSO showed three mass-loss steps, with maxima at 360.6, 413.9, and 539.8 °C, which represent the volatilization and/or combustion of triglycerides. The same behavior was observed for rambutan seed fat (Solís-Fuentes et al., 2010). For the LSOME, the changes in weight occurred because of evaporation and/or combustion of the methyl esters, mainly of methyl linolenate and oleate, the most abundant components. The LSOME thermogram consisted of three steps. About 5% mass loss occurred during its first evaporation stage below 159.3 °C, a rapid mass loss (85%) between 159.8 and 292.3 °C, with a maximum rate at 225.4 °C, and a slower weight loss reduction at higher temperature. The TGA/DTG curves of methyl esters derived from corn (Jain and Sharma, 2011) and Jatropha cursas (Jain and Sharma, 2012) oils showed the same behavior. The differences between the thermograms of the LSO and LSOME confirmed that the oil was converted into methyl esters. 3.4. Fuel properties of LSOME The fuel properties of LSOME are summarized in Table 2, along with comparisons to the biodiesel standards in ASTM D6751. 3.4.1. Density Density is an important parameter for airless combustion systems because it influences the efficiency of fuel atomization. The density of LSOME was 845 kg/m3, which was a lower value than those reported for other common biodiesels such as rapeseed (882 kg/m3), palm (876 kg/m3) and soybean (884 kg/m3) methyl esters (Karmakar et al., 2010), but similar to that of peanut methyl ester (848 kg/m3) (Kaya et al., 2009). This value was within the range for No. 2 petroleum diesel (0.82–0.86 kg/m3). The light LSOME can easy flow through pipelines and injector nozzle openings for atomization in the cylinder of diesel engines. 3.4.2. Kinematic viscosity Kinematic viscosity is the most important property of biodiesels because it affects the operation of fuel injection equipment, particularly at low temperatures, when an increase in viscosity can affect the fluidity of the fuel. High viscosity leads to poor atomization of the fuel spray, incomplete combustion, less accurate operation of fuel injectors, ring carbonization, and ultimately, can result in formation of soot and engine deposits (Demirbas, 2009; Karaosmanoglu, 1999). The alkali-catalyzed transesterification reduced the viscosity of the crude oil from 15.42 to 1.92 mm2/s (approximately one-eighteenth of its initial value). This value was satisfactory

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Table 2 Comparison of the fuel properties of Lepidium sativum oil methyl esters (LSOME) with standard biodiesel. Property

LSOME

ASTM D6751

Kinematic viscosity at 40 °C (mm2/s) Density at 15 °C (kg/m3) Flash point (°C) Cloud point (°C) Pour point (°C) Distillation temperature (90% recovered) (°C) Sulfur content (wt.%) Carbon residue (wt.%) (on 100% distillation residue) Gross heat value (MJ/kg) Ash content (wt.%) Refractive index at (25 °C) Water and sediment, (%volume) Saponification value (mg KOH/g) Iodine value (g iodine/100 g) Cetane number

1.921 0.845 176 1 6 359 0.018 0.04 40.45 0.001 1.458 Trace 172 128 49.23

1.9–6.0 NS 130 min NS NS 360 max 0.1 max 0.05 max NS 0.02 max NS 0.05 max

Min 47

according to the specified range in ASTM D6751 (1.9–6 mm2/s), and was lower than those of soybean (4.03 mm2/s), sunflower (4.43 mm2/s), palm (5.70 mm2/s) and Jatropha (4.80 mm2/s) methyl esters (Karmakar et al., 2010), probably because of the high content of linolenic acid (30.11%). This low kinematic viscosity of LSOME is an important property of this newly produced biodiesel, making it easier to pump and allowing it to form finer droplets. 3.4.3. Cold flow properties The tendency of a fuel to solidify or gel at low temperatures can be quantified by two fuel parameters, cloud and pour points (CP and PP). CP is the temperature at which crystals become visible (diameter (d) P 0.5 lm) (Knothe et al., 2005), and PP is the temperature at which crystal agglomeration is extensive enough to prevent free pouring of fluid. The LSOME had CP and PP values of 1 and 6 °C, respectively. These were indicative of a comparatively low concentration of saturated and a high concentration of unsaturated fatty acid esters. These values are similar to those of canola methyl esters, but lower than those of soybean methyl esters (CP = 0 °C, PP = 2 °C) (Knothe et al., 2005). These results indicated that the LSOME would be suitable as a fuel for use in cold-weather conditions.

which was higher than those of non-conventional biodiesel such us Rhus thyfina (45.6), Kosteletzkya pentacarpos (46.9) and Xanthium sibiricum (46.5) (Ruan et al., 2012) but lower than that of Kosteletzkya virginica (56) (Ruan et al., 2008). Furthermore, this cetane value was comparable to this of soybean (49) but higher than this of sunflower (45) methyl esters (Karmakar et al., 2010). 3.4.6. Gross heat of combustion The gross heat of combustion (higher heating value; HHV) is a property indicating the suitability of a fatty acid ester for use as a diesel fuel. The heat content of alkyl esters is nearly 90% of that of petroleum-based diesel (45.34 MJ/kg) (Knothe et al., 2005). In the case of LSOME, the heating value was slightly lower, 40.45 MJ/kg, representing 89.21% of that of petro-diesel. This value was attributed to the oxygen content of the LSOME. The presence of oxygen in biodiesel enables the complete combustion of fuel in the engine (Ramadhas et al., 2005). The LSOME heating value was higher than those of sunflower (38.47 MJ/kg), soybean (39.72 MJ/kg), and canola (39.87 MJ/kg) methyl esters (Knothe et al., 2005). 3.4.7. Distillation curve of LSOME Distillation includes the determination of the range of boiling points for fuels. The distillation range of diesel fuels affects their properties such as viscosity and density (Gerpen et al., 2004; Ali et al., 1995). The distillation curve of LSOME showed the same behavior as that of linseed methyl esters (Demirbas, 2009). The maximum temperature must be 360 °C, according to the ASTM D6751 standard specification for the 90% distillation fraction. In this study, the 90% distillation fraction temperature (359 °C) was below this value; however, this value was similar to those of sunflower, canola, soybean, corn, and cottonseed methyl esters (357– 359 °C) (Alptekin and Canakci, 2009).

3.4.4. Flash point The flash point which is determined by ASTM D93 is the temperature at which the fuel is mixture-like and is ready to ignite when exposed to a flame. Table 2, indicates that the biodiesel produced from LSO had a flash point of 176 °C which is satisfactory according to the specified range in ASTM D6751 (130 °C minimum) and is comparable to those of olive (178 °C), soybean (178 °C) and peanut (176 °C) methyl esters (Karmakar et al., 2010). It is generally accepted that a higher value of flash point decreases the risk of fire. This is an advantage of biodiesel over petro-diesel.

3.4.8. Sulfur and ash contents Most of the sulfur in biodiesel is derived from the crops used as feedstock (Schinas et al., 2009). The concentration of sulfur in the LSOME sample was measured by ASTM D 4294. As shown in Table 2, unlike the conventional petro-diesel fuels with high sulfur content, the biodiesel produced from LSO had a negligible sulfur content (0.018%). The sulfur content in the LSOME was less than 1 ppm, which is one of the main advantages of biodiesel. The ash content reflects the amount of inorganic contaminants, such as abrasive solids and catalyst residues, and the amount of soluble metals contained in a fuel sample (Anwar et al., 2010). High concentrations of these materials can cause injector tip plugging, combustion deposits, and injection system wear. The ash content is important for the heating value, as it decreases with increasing ash content. As shown in Table 2, the LSOME had a negligible ash content (0.001% by weight), similar to that of sunflower methyl ester (0.001%) but lower than those of Jatropha (0.002%) and rapeseed (<0.01%) methyl esters (Sarin et al., 2010).

3.4.5. Cetane number The cetane number (CN) is the ability of fuel to ignite quickly after being injected. Better ignition quality of the fuel is always associated with higher CN value. The cetane number of biodiesel varies widely in the range of 48–67 depending upon various parameters including oil processing technology and climatic conditions where feedstock is collected (Eevera et al., 2009). CN affects a number of engine performance parameters like combustion, stability, drivability, white smoke, noise and emissions of CO and hydrocarbons (Antolin et al., 2002; Ma and Hanna, 1999). The cetane number (CN) of methyl ester was calculated on the basis of SV and IV which were 172 and 128, respectively (Table 2). The cetane number of the produced biodiesel sample was 49.23 (Table 2)

3.4.9. Carbon residue, water and sediment content and refractive index The tendency of a biodiesel fuel to form carbon deposits in an engine can be estimated by the carbon residue after the complete combustion of the fuel. The synthesized LSOME had 0.04% carbon residue, which is in the specified ranges in ASTM D6751 (0.05% max). This low value was comparable to those of methyl esters of soybean (0.038%), sunflower (0.035%), and corn (0.041%) oils (Bazooyar et al., 2011). The LSOME sample showed trace water and sediment contents, similar to those of corn, olive, and grapeseed oil methyl esters (Bazooyar et al., 2011). The refractive index of the LSOME was 1.458, higher than that of oleander methyl ester (1.446) (Deka and Basumatary, 2011) but lower than that of Maclura pomifera methyl ester (1.470) (Fatnassi

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et al., 2010). The value of the refractive index is proportional to the level of unsaturation of the methyl ester oils (Karleskind, 1992). 4. Conclusion The characteristics of L. sativum seed oil (LSO) were investigated to determine whether it has the potential to be used as a raw material for biodiesel production. LSO was extracted and chemically converted via an alkaline transesterification reaction to its fatty acid methyl esters (LSOME). All the determined parameters of the LSOME were within the ASTM D6571 limits for biodiesel. A significantly lower kinematic viscosity of LSOME as compared to the ASTM standards indicates that it will be easily pumped; subsequently it will atomize readily, and will form finer droplets for complete combustion in diesel engines. From the physicochemical properties studied, the LSOME has been shown to be a good substitute for imported petroleum diesel. Further investigation of biodiesel originated from LSO can be widened by the examination of different catalyst types and the impact of LSOME on the exhaust emissions in comparison with other biodiesels. Acknowledgements The work was supported by the National Plan for Science and Technology (NPST) funded by King Abdulaziz City for Science and Technology (KACST) through Project number 11-ENE2115-02. References Ali, Y., Hanna, M.A., Cuppett, S.L., 1995. Fuel properties of tallow and soybean oil esters. J. Am. Oil Chem. Soc. 72, 1557–1564. Alptekin, A., Canakci, M., 2009. Characterization of the key fuel properties of methyl ester–diesel fuel blends. Fuel 88, 75–80. Antolin, G., Tinaut, F.V., Briceno, Y., Castano, V., Perez, C., Ramirez, A.I., 2002. Optimisation of biodiesel production by sunflower oil transesterification. Bioresour. Technol. 83, 111–114. Anwar, F., Rashid, U., Ashraf, M., Nadeem, M., 2010. Okra (Hibiscus esculentus) seed oil for biodiesel production. Appl. Energy 87, 779–785. Bazooyar, B., Ghorbani, A., Shariati, A., 2011. Combustion performance and emissions of petro-diesel and biodiesels based on various vegetable oils in a semi industrial boiler. Fuel 90, 3078–3092. Bianchi, G., Howarth, O.W., Sameul, C.J., Vlahov, G., 1995. Long range c-inductive interactions through saturated C–C bonds in polymethylene chains. J. Chem. Soc., Perkin Trans. 2, 1427–1432. Deka, D.C., Basumatary, S., 2011. High quality biodiesel from yellow oleander (Thevetia peruviana) seed oil. Biomass Bioenergy 35, 1797–1803. Demirbas, A., 2009. Progress and recent trends in biodiesel fuels. Energy Convers. Manage. 50, 14–34. Diwakar, B.T., Dutta, P.K., Lokesh, B.R., Naidu, K.A., 2010. Physicochemical properties of garden cress (Lepidium sativum L.) seed oil. J. Am. Oil Chem. Soc. 87, 539–548. Eevera, T., Rajendran, K., Saradha, S., 2009. Biodiesel production process optimization and characterization to assess the suitability of the product for varied environmental conditions. Renewable Energy 34, 762–765. Fatnassi, S., Chatti, S., Zarrouk, H., 2010. Methyl ester of [Maclura pomifera (Rafin.) Schneider] seed oil: biodiesel production and characterization. Bioresour. Technol. 101, 3091–3096. Gerpen, J.V., Shank, B., Pruszko, R., Clements, D., Knothe, G., 2004. Biodiesel production technology. Colorado: National Renewable Energy Laboratory Report.

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