1. Introduction
The most desired properties of materials, products and industrial processes are sustainability, eco-efficiency and industrial ecology. Achieving a sustainable, resource-efficient and low-carbon economy is one of the key fundamental challenges in the 21st century [1] . For many years, fossil fuel has been the major source of energy worldwide [2] . Large utilisation of fossil fuels due to increasing human activities has contributed to national energy security concern, environmental pollution and depleting oil reserves. This has, in turn, led to an increase in demand for viable and sustainable alternatives. Recently, many countries have explored renewable sources such as solar, wind, geothermal and agricultural feedstocks as alternative fuel sources. However, the agricultural feedstock can also help in generating an ecofriendly and efficient fuel that can compete effectively with fossil fuel and even capture market currently dominated by the latter. Fuel from plant resources such as vegetable oil is known as a cheaper renewable source of energy [3] .
Vegetable oils have long been in use as a substitute for diesel fuel as far back as 1930 [4] . Many vegetable oils from palm, groundnut, rapeseed, and soybean with good yields have been investigated. Non-edible oil such as jojoba and karanja oils have also been studied. Due to the problems associated with their use directly in conventional engines, vegetable oils are modified into biodiesel. Biodiesel is a promising alternative fuel. In the recent time, it is attracting the attention of many researchers because of its potential to address the growing concern of depletion of fossil fuel reserves and environmental problems [5] . Several conversion processes such as hydrotreatment and isomerisation, Fischer-Tropsch [6] and transesterification [3] have been reported. The most effective of these methods is transesterification, a reaction of oil with an alcohol to form ester and glycerol in the presence of an acid or base catalyst, due to its simplicity. It has been in commercial use not only in developed nations (e.g. Germany, France, Canada, Italy, New Zealand, etc.) but also in developing nations (India, Brazil, etc.). In some major oil exporting countries, biodiesel generates high foreign exchange savings. It has however been more attractive recently because of its environmental benefits, renewability, biodegradability, non-toxic nature and cost-effectiveness. Interestingly also, it has been promoted as a technical option to respond to climate change.
Jatropha curcas is a hardy plant that has the potential to grow under both arid and semi-arid conditions [7] . It belongs to the family of Euphorbiaceae [8] [9] . It is cultivated both in tropical and subtropical regions of the world and is high yielding. Jatropha is grown mainly to demarcate plots of land in Nigeria. Its oil has not been exploited for fuel purpose in the country [3] . Ideally, prime conditions for the choice of plant resources for the production of renewable fuel are abundance and low-cost. In sub-Saharan Africa, Jatropha curcas is readily available due to its ease of cultivation. It has high oil yield and maintains no competition with food. However, for a monocultured economy, such as Nigeria, in need of diversification, the use of Jatropha can be a viable alternative by harnessing its potential for biodiesel production on a large scale. Biodiesel ethyl esters of some vegetable oils using absolute ethanol have been reported [10] and often the choice of vegetable oils for biodiesel production is a function of its abundance in the region of testing [11] . Based on the aforementioned excellent features, Jatropha curcas is thought to be an appropriate and sustainable alternative feedstock for the production of biodiesel. Consequently, the potential of using Jatropha curcas oil as feedstock for biodiesel production is the focus of this study.
This work provides technical data that can assist Nigeria in reaching informed decisions on industrial exploitation of local Jatropha seed for biodiesel production. It aimed at studying the physicochemical properties of Jatropha curcas seed oil sourced locally in Nigeria, to synthesise its ethyl ester biodiesel via base-catalysed transesterification process and to investigate the regulated and unregulated properties of the biodiesel thereof using standard techniques. This research provides a sustainable strategy, a viable alternative and renewable source for energy generation against over-dependence on fossil fuel that contributes to national energy security concern and environmental degradation.
2. Materials and Methods
2.1. Materials
All reagents (n-hexane, absolute ethanol and sodium sulphate) used were of analytical grade. All analyses were conducted according to standard test methods. The flash point was determined using a Pensky-Martens Closed Flash Tester (K-16270) by Kehler Instrument Company. A 6200 Automatic Isoperibol Oxygen Bomb Calorimeter was used to determine the higher heating value of the biodiesel. ASTM D613 was used to determine cetane number [12] , for iodine value, ASTM D445 for kinetic viscosity and the pour point determination was carried out according to ASTM D97.
2.2. Oil Extraction and Pretreatment
Jatropha curcas seeds were collected from Ado-Ekiti and Igbara Odo, both towns in Ekiti State, Nigeria. The seeds were identified according to the flora of West Africa in the Department of Plant Science and Biotechnology, Ekiti State University, Ado-Ekiti, Ekiti State. The seed shells were cracked to remove the seed (endosperms), air dried and later oven dried at 60˚C to constant weight. The dried seeds were crushed into cake with mortar and pestle before extracting the oil by Soxhlet extraction using n-hexane at 60˚C. The extracted oil was refined by alkaline refining process as follows: 0.16 g of 18 M NaOH was added to 30 g of oil and agitated for 30 min. The mixture was heated to 75˚C and centrifuged. The physicochemical analyses conducted on the oil include acid value, % FFA as oleic acid, peroxide value, saponification value and specific gravity at 15˚C. These analyses were conducted according to the methods described by the Association of Official Analytical Chemists [13] .
2.3. Synthesis of Biodiesel Ethyl Ester of Refined Jatropha curcas Seed Oil
Base-catalysed transesterification was adopted for this process. Ethanol was dried over anhydrous sodium sulphate prior to use. Potassium ethoxide was prepared by dissolving 0.37 g KOH (catalyst) in 16.6 ml absolute ethanol in a 250 ml beaker. The potassium ethoxide was gently introduced into 200 ml refined oil in a 1 L round bottom flask. The mixture was stirred and refluxed at 70˚C for 30 min. This was later transferred into a separating funnel and allowed to stand for 1 h in order to separate the mixture into layers. The lower layer was run off while the upper layer ran into a round bottom flask and refluxed again at 70˚C at 30 min with another potassium ethoxide catalyst. The ethyl ester was washed with 20 ml distilled water five times, drained into a beaker and dried over anhydrous sodium sulphate. The fatty acid ethyl ester was analysed for its fuel properties (specific gravity, flash point, pour point, kinetic viscosity, cetane number, iodine value and higher heating value) and tested according to India, ASTM 6751 and EN 41214 Biodiesel Standards.
2.4. Gas Chromatograph Analysis
Agilent 6890 Gas chromatograph System equipped with HP INNOWAX (30 m × 0.25 mm × 0.25 µm) and Agilent ChemStation software was used for the analysis using the following conditions: split injection (split ratio: 20:1), carrier gas: nitrogen, inlet temperature: 250˚C; oven programme: initial temperature at 60˚C, ramped at 12˚C/min for 20 min and then maintained for 2 min, ramped again at 15˚C/min for 3 min and then maintained for 8 min, detector: FID, detector temperature: 320˚C, hydrogen pressure: 22 psi, compressed air: 35 psi. The fatty acid was determined as ethyl ester. The ethyl ester was prepared following a standard procedure for GC analysis as follows: 20 mg of the oil was mixed with 2 cm3 of toluene Then 2 cm3 of 1.5% sulphuric acid in ethanol was added. The mixture was stirred and incubated at 55˚C overnight. 4 cm3 saturated solution of sodium chloride was added and vortexed. 2 cm3 HPLC grade hexane was added followed by addition of 3 cm3 of 2% NaHCO3. The mixture was also vortexed, and 180 µl of the upper phase was taken for GC analysis.
3. Results and Discussion
3.1. Physicochemical Analysis
Results of physicochemical analyses carried out on the oil as shown in Table 1 include acid value, peroxide value, saponification value and specific gravity. The
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Table 1. Physicochemical properties of refined Jatropha curcas seed oil.
aResult of this study, Crude Oil Yield = 58.16%; b [16] ; c [17] .
yield of the oil is an important parameter that can determine its cost for biodiesel production. The oil content of Jatropha curcas seed (58.16%) was found to be higher than 47.25% and 49.1% reported by Akintayo and Martin et al. respectively [14] [15] but lower than that of Malaysian Jatropha curcas crude oil (63.16%) as reported by Akbar et al. [16] . After alkaline pretreatment, the yield was found to be 52.5%. However, the high oil content of Jatropha curcas indicates its suitability as a non-edible vegetable oil feedstock for biodiesel production.
Acid value is a measure of the acidic component in an oil sample. It is determined as the mass in mg of KOH required to neutralise the free fatty acid components in one gram of sample and is used to monitor oil degradation during storage. This was carried out by titration according to ASTM D664. The experimental result revealed a very high acid value of the extracted oil of 4.56 mg KOH/g. This high value explains the reason why the oil was subjected to alkaline refining prior to the production of biodiesel. Due to the chosen strength of potassium hydroxide used for refining (i.e. to neutralise some of the free fatty acids), the acid value of Jatropha curcas oil was reduced to 1.16 mg KOH/g and the % FFA (as oleic acid) reported as 0.58, values appropriate to complete the reaction with base catalysts. About 74.6% reduction was achieved for 1 h after refining. Jatropha curcas oil with FFA level of <1% has been reported to give a high yield of biodiesel [9] . This value was far lower than 35.8 mg KOH/g and 9.78 mg KOH/g obtained by Aransiola and Lau et al. [17] [18] respectively, for crude jatropha oil.
Saponification value is a measure of the mean molecular weight of the fatty acid present in an oil sample. Result obtained for oil under investigation revealed a saponification value of 159.9, which was lower than 193 obtained by [16] for Malaysian Jatropha curcas oil. A lower saponification value indicates a lower tendency of soap formation.
Peroxide value is an indicator of the deterioration of oil due to oxidation at double bond of an unsaturated fatty acid; rancidity. The peroxide value for refined Jatropha curcas oil was 1.92 mE/kg which is 0.05% lesser than 1.93 mE/kg obtained by [16] for Malaysian Jatropha curcas oil. The specific gravity of the refined Jatropha curcas oil was found to be 0.882.
3.2. Fuel Properties of Jatropha curcas (Ethyl Ester) Biodiesel
The result of measured fuel analyses for biodiesel produced is shown in Table 2.
A yield of 57.6% was obtained, and its specific gravity (15˚C) was determined to be 0.882.
Flash point is the lowest temperature at which fuel begins to evolve vapour in sufficient quantity to form an explosive or inflammable mixture with air. It measures the response of fuels to heat and assesses their flammability hazard levels and safety conditions. It has been reported that residual alcohol can reduce flash point of biodiesel [19] . This implies that purity of biodiesel can be measured
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Table 2. Results of fuel properties of ethyl ester of Jatropha curcas seed oil.
dResult of this study; eIBS: India Biodiesel Standard.
using flash point. High flash point makes biodiesel safer for storage, handling and transportation [20] . The flash point for Jatropha curcas (ethyl ester) biodiesel was measured to be at 215˚C. This value is within ASTM D6751, EN 14214 and India Biodiesel specifications.
The pour point is the lowest temperature at which fuel start to flow. It is used to measure the cold flow operability of fuel. The cold property is one of the most critical obstacles against the widespread of fuel usage. Pour point is mostly related to saturation of biodiesel [19] . Jatropha curcas (ethyl ester) biodiesel has a high amount of unsaturated fatty acids and low amount of saturated fatty acids. Pour point was therefore observed at −6˚C which is lower than the temperature attainable in the region of testing. This value met the recommended specification.
Viscosity is a significant internal property of a fluid which indicates the degree of resistance to flow [2] . The result of the analysis reveals kinematic viscosity at 40˚C to be 5.01 mm2/s. This falls within the threshold values of ASTM 6751 (1.9 - 6.0 mm2/s), EN 14214 (3.5 - 5.0 mm2/s) and Indian Biodiesel specifications (IBS) (2.5 - 6.0 mm2/s). IBS is used mainly in this discussion because of similarities in the climate and the level of development of Nigeria and India.
Cetane number is an indicator for measuring the ignition performance of fuel in a combustion engine. The cetane number of Jatropha curcas (ethyl ester) biodiesel was 49.77. This met ASTM D6751 standard of 47 but lower than the minimum of 51 recommended by EN 14214 and India Biodiesel specification (IBS).
Iodine value is a measure of the unsaturation of oils. High iodine value indicates high unsaturation of oils [21] . The experimental result obtained (99.20 g/100 g) using Wij’s reagent showed a value higher than that of conventional diesel fuel. High unsaturation oil polymerises on heating. This problem could be worse with an increase in the number of double bonds in the fatty acid chains. However, a report proposed a limit for unsaturated fatty acid particularly oleic and linoleic acids in biodiesel to ensure the quality of biodiesel. The experimental result of iodine value met EN 14214 specification, indicating the suitability of this biodiesel for fuel purpose.
Heating value is an important parameter that measures the capacity of fuel to generate heat. It can be used to adjudge the suitability of biodiesel as an alternative to diesel fuel. The heating values of biodiesel are usually lower than that of diesel fuel―5 MJ/kg [2] [11] and this has been attributed to their higher oxygen content [22] [23] . The result obtained in this work revealed the heating value of Jatropha curcas (ethyl ester) biodiesel to be 42.77 MJ/kg. This result met the recommended specifications according to ASTM D6751, EN 14214 and India Biodiesel standards.
3.3. Fatty Acid Compositions
Results of the fatty acid composition of the ethyl ester of Jatropha curcas oil is shown in Table 3, containing mainly fatty acids with more than 16 carbon
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Table 3. Fatty acid composition as ethyl ester of Jatropha curcas seed oil.
fxx: y indicates xx carbons in the fatty acids with y double bonds. Total saturated fatty acids = 19.53%. Total unsaturated fatty acids = 80.45%.
atoms. The total unsaturated fatty acids consisting of oleic, palmitoleic, linoleic, linolenic, arachidonic and erucic acids made up 80.45%, which is relatively higher than 78.5% obtained by Lau [18] . The fatty acid composition of Jatropha curcas (ethyl ester) biodiesel has oleic acid (44.85%) as the dominant fatty acid and this matched with the results reported by Aransiola et al. and Ong et al. [17] [24] . The presence of a large fraction of these high molecular fatty acids renders biodiesel less volatile. Fatty acid has been reported to influence regulated properties of biodiesel [3] [11] such as viscosity, flash point, cetane number and density.
4. Conclusion
The utilisation of non-edible Jatropha curcas seed oil for biodiesel production is sustainable. Biodiesel from refined jatropha oil can serve as a smart alternative to diesel fuel, which is non-renewable. The measured fuel properties of Jatropha curcas biodiesel compared favourably with those worked upon by other researchers and met both ASTM D6751 and EN 14214 standards as well as the India biodiesel specification (IBS). The pre-treatment of the extracted oil via alkaline refining process helped to reduce the free fatty acid content, which in turn enhanced the quality of the biodiesel produced thereof. The use of simple transesterification process using potassium hydroxide (base-catalyst) has proven to be an effective conversion route to obtain biodiesel from Jatropha curcas oil. The presence of high molecular fatty acids in the biodiesel can significantly affect fuel properties of the fuel. Although the biodiesel has comparatively compatible fuel properties with conventional diesel fuel, it was found to be less volatile than diesel fuel. But better still, the biodiesel produced from refined Jatropha curcas seed oil can be used as fuel blend in diesel engine. However, there is a need to evaluate the suitability of biodiesel as a blend in different diesel fuels and the blend proportion with optimum efficiency.