1. Introduction

Liquid fuel is a very precious resource used abundantly and somehow indiscriminately by modern man. The primary source of liquid fuel is currently crude oil which is becoming harder and more expensive to recover as conventional reserves are depleted and as foreign suppliers increase the price for their declining reserves[1].The drastic jump in oil prices between 1973 and 1979 started the agitation that made many governments to adopt policies to develop alternative energy sources. With the wide majority of atmospheric scientist now agreeing that global warming is already well underway ,there are now more strident calls to replace crude oil as our liquid fuel source in order to reduce the build-up of greenhouse gases in the environment[1].Thus an additional emphasis is being placed on the development, production and the use of alternative fuel considered being friendlier to the environment than fossil fuel.. In the review of[2], necessity to strictly limit the global warming was highly emphasized of which accelerated release of fossil entombed CO₂ has been generally accepted as a major contributor[3].Generally, bio-sourced fuels are termed biofuel examples of which are biomethanol, bioethanol, biobutanol, biomethane, biohydrogen, biodiesel etc. This paper centers on biodiesel not only because its exhibits defined chemical and physical properties to meet the demands of engine application but because it is presently produced as a fuel on industrial scale. The functionality of biodiesel as a possible and likely candidate to replace fossil fuels as primary energy source for machineries and vehicular flow remains a driving force for scientists to keep researching into the world of biodiesel. Though, numerous popular articles and scientific papers have cautioned against the global drive towards a biofuel economy generally, highlighting the potential impacts on food security. Since vegetable oils which are the major feed stock for biodiesel are widely grown and used for food and animal feed, hence there is the current debate “Food or Fuel?” Nonetheless, lengthy list of academia have worked on surplus and yet inedible oil as major feedstock for biodiesel. Kumar et al.[4] discussed the importance of non-edible oil Jatropha curcus in biodiesel production. Shah et al.[5] produced biodiesel from jatropha by enzyme in a solvent free system. Attempts have also been made to produce biodiesel from non-edible sources like used frying oil, greases, tallow and lard[6-9]. Soap nut which is reported to be wildly grown in forest areas in Nepal in the elevation of 300-1900m[10] have also been presented as a suitable feedstock for biodiesel production[11]. Compared to petroleum-based diesel, the high cost of biodiesel is a major barrier to its commercialization. It costs approximately one and a half times that of petroleum- based diesel depending on feedstock oils[12-13], nevertheless, numerous literatures[14-18] revealed the importance of biodiesel especially with respect to the future energy needs.

In this review article, we examine various sources of biodiesel; edible and inedible oil , virgin oil and used oil, that is gaining increasing importance, thermo-kinetics studies and catalysis of biodiesel processing, fuel properties other process parameters, and its chances as future fuel.

2. History of Biodiesel

Dr. Rudolf Diesel actually invented the diesel engine to run on a myriad of fuels including coal dust suspended in water, heavy mineral oil, and, vegetable oil. Dr. Diesel’s first engine experiments were catastrophic failures. But by the time he showed his engine at the World Exhibition in Paris in 1900, his engine was running on 100% peanut oil. Dr. Diesel was visionary. In 1911 he stated “The diesel engine can be fed with vegetable oils and would help considerably in the development of agriculture of the countries which use it.” In 1912, Diesel said, “The use of vegetable oils for engine fuels may seem insignificant today. But such oils may become in course of time as important as petroleum and the coal tar products of the present time.” No doubt,this statement has come to stay.Since Dr. Diesel’s untimely death in 1913, his engine has been modified to run on the polluting petroleum fuel we now know as “diesel.” Nevertheless, his ideas on agriculture and his invention provided the foundation for a society fuelled with clean, renewable, locally grown fuel. Today throughout the world, countries are returning to using this form of fuel due to its renewable source and reduction in pollution[19].

3. Feedstocks for Biodiesel

The major feedstock for biodiesel is fat or oil from animal and plant respectively. Tables below show various oil used for biodiesel production.

Table 1. Production of biodiesel in Different countries

Fatty acid composition usually obtained by gas chromatography is the major indicator of the properties of biodiesel[27]. Such composition of oil has an important role in the performance of biodiesel in diesel engines. Saturation fatty acid methyl esters increase the cloud point, cetane number, and improve stability whereas more polyunsaturation reduce the cloud point, cetane number, and stability[28-30], the details of which will be treated in the latter part of this review .The presence of free fatty acids and water in the feedstock result in the production of soap in the presence of alkali catalyst. Thus, additional steps to remove any water and either the free fatty acids or soap from the reaction mixture are required. However, the tools of biotechnology could be utilized to modify the fatty acid profile of soybean for performance enhancement, which may increase the attractiveness of biodiesel derived from this commodity crop[32]. There is still some level of disagreement in the literature over the suitability of animal fats as feedstock for biodiesel. Animal fats are solid but oil is liquid at room temperature. Thus animal fats cannot be used as fuel in its original form.In the report of Mohd Gadaffi[24],Fangrui Ma and Milford A. Hanna[35] submitted that animal fats contains more saturated fatty acid than vegetable oil and thus several problem will occur such as carbon deposits in the engine, engine durability and lubricating oil contamination because of incompatibility with the engine.

Table 2. Selected Restaurants in the City of Gainesville (U.S.A) to Determine Supply Lines for Waste Cooking Oil

Table 3. Fatty Acid Composition of Selected Oil

In all, contained in the article of Srivathsan[61] is that the direct usage of vegetable oils as biodiesel is possible by blending it with conventional diesel fuels in a suitable ratio and these blends are stable for short term usage. The blending process is simple which involves mixing alone and hence the equipment cost is low. But direct usage of these triglyceric esters (oils) is unsatisfactory and impractical for long term usages in the available diesel engines due to high viscosity, acid contamination, and free fatty acid formation resulting in gum formation by oxidation and polymerization and carbon deposition. Hence vegetables oils are processed so as to acquire properties (viscosity and volatility) similar to that of fossil fuels and the processed fuel can be directly used in the diesel engines. Three processing techniques are mainly used to convert vegetable oils to fuel form[62] namely, pyrolysis, micro-emulsification and trans-esterification. Pyrolysis refers to chemical change caused by application of heat to get simpler compounds from a complex compound. The process is also known as cracking. Vegetable oils can be cracked to reduce viscosity and improve cetane number. The products of cracking include alkanes, alkenes, and carboxylic acids. Soyabean oil, cottonseed oil, rapeseed oil and other oils are successfully cracked with appropriate catalysts to get biodiesel[62]. By using this technique good flow characteristics were achieved due to reduction in viscosity. We have equally considered in this review that a lot of plant derived oil especially the inedible ones are yet to be considered as equal candiadate for biodiesel synthesis.The few ones already considered were not subjected to full pre-treatment before use.All oils contains certain percent of gum which are expected to be removed irrespective of how little they may be.Gum free oil will synthesize biodiesel of improved cloud point an pour point.

4. Synthesis of Biodiesel

Biodiesel are produced through a simple technology called trans-esterification reaction. Degummed oil free of all forms of impurities is reacted with a reasonable alcohol (ethanol, methanol, butanol etc.)[29].Chemically, the reaction can be represented as;

Several other side reaction occurs which if uncontrolled hampers conversion, product yield and quality[20]. Usually 3 parameters have effect on the trans-esterification, namely[33];

• temperature (T), reaction

• time (t) and

• ratio of oil to alcohol.

The reaction temperature plays an important role on the quality of the products. Kapilakarn K and .Peugtong[33] reported that normally, the range of the temperature used in the process is between. 50 ⁰C – 65 ^oC. The temperature which is higher than the normal boiling point of methanol (68 ^oC) causes more vaporization of methanol (loss). On the other hand, the temperature which is lower than 50 ^oC causes higher viscosity of biodiesel[34].The ratio of methanol to oil also affects the reaction, the higher molar ratio, the higher conversion of alcohol. The ratios, normally used, are between 5:1 to 10:1[35]. However using too high excess methanol can obstruct glycerin separation[36]. Divya,B and Tyagi,V.K[20] investigated the conversion rate into biodiesel using beef tallow, sunflower , and soybean feed stocks. For beef tallow, reaction rate was very slow during the first minute due to the mixing and dispersion of methanol into beef tallow, the reaction proceeded very fast for the next five minutes. An approximate yield of 80 % was observed after 1 minute for soybean and sunflower oils at methanol to oil ratio of 6:1. After 1 hour, the conversions were almost the same (93%-98%).The effect of reaction time for palm oil at 40:1 methanol: oil with 5% H₂SO₄ (v/v) at 95 ℃ for 9 hours and obtained a maximum yield of 97 %[42]. However,further efforts is required to demonstrate the reactor ideal enough for biodiesel synthesis and its design and fabrication.

5. Thermo-Kinetics of Trans-Esterification

The feasibility of a reaction is determined from the thermodynamic parameters. Since both reactants and products are liquids, entropy change will tend to zero, hence equilibrium constant will be low[20]. Kinetics of trans-esterification reaction has at least 3 main reactions as shown in the equations stated below[37];

(1)

(2)

(3)

Frequency factor, which is a measure of collisions between reactants, is always high for the forward reactions; this indicates that the reverse reaction is less favoured. Activation energy for the reverse reaction is higher than that for the forward reaction, which again should confirm the low possibility of reverse reactions. Some of the few kinetics studies that have been performed in recent times include esterification of free fatty acids in sunflower oil and oleic acid[38-39]. Trans-esterification kinetics of soybean oil with five different catalysts have also been studied[40].Similarly, Divya and Tyagi[20] reported the study carried out by Komers et al;[41] where kinetics and mechanism of KOH catalyzed methanolysis of rapeseed oil for biodiesel production was investigated. The first sequence of the reaction expressed the methanolysis of rapeseed oil to biodiesel while the second sequence described the always present side reaction separation of glycerides and biodiesel by KOH. The proposed chemical model was described by a system of differential kinetic equations which were solved numerically by two independent computing methods.Nonetheless,this aspect is yet to be fully exploited by researchers.Highly limited articles on elementary steps of the trans-esterification reaction and kinetic modeling are available in open literatures.

6. Catalysis of Trans-esterification

In the study carried out by Hossain and Boyce[47] inspite of higher yield, using NaOH as catalyst during biodiesel synthesis from waste sunflower cooking oil causes more emulsion than KOH and makes complicated to separate biodiesel from glycerin. For this reason, KOH has been screened as a catalyst whose effect in terms of concentration can be studied with respect to biodiesel production. The solution of alkaline catalyst in methanol was recommended to be prepared freshly in order to avoid the moisture absorbance and to maintain the catalytic activity[43-44]. In the article of Twar et al;[63], the essence of the titration process to get the number of gram of NaOH that will be used per liter of oil in the trans-esterification process was stated. This will give a rough guide on the amount of catalyst that will give an optimum yield. The experimental details goes thus; Dissolve 1 gram of NaOH in 1 liter of distilled water solution (0.1% NaOH). Phenolphthalein solution was used to get the end point. In a smaller beaker, 1ml of oil was dissolved in 10ml of pure isopropyl alcohol. The beaker was warmed gently by standing it in some hot water, stir until all the oil dissolves in the alcohol and the mixture turns clear. 2 drops of phenolphthalein solution was added. Using a burette, 0.1% NaOH solution was added drop by drop to the oil alcohol phenolphthalein solution, stirring all the time, until the solution stays pink for 10 seconds. The number of mls of 0.1% NaOH solution used added to 5.0 will give the number of NaOH to be used per liter of oil[63].

Leung and Gau[26] reported that the conversion of waste cooking oil using sodium hydroxide catalysts was approximately 86%. Zheng et al.[45] showed that methyl ester conversion of waste cooking oil in acid catalyzed transesterifications can reach up to 99%. This process was carried out using a very high methanol to oil ratio of 250:1. In an attempt to reduce the problems with separation and soap formation associated with biodiesel production, some non-enzymatic heterogeneous catalysts have been investigated. ZrO₂, ZnO, SO₄^2-/SnO₂,SO4^2-/ZrO₂, zeolite, and KNO₃/ZrO₂ are some solid catalysts that were studied in the trans-esterification of palm and coconut oil[46]. The reaction was carried out at 200 ℃, 50 bar, 3 wt% catalysts, and a 6:1 molar ratio of methanol to oil. All the solid catalysts exhibited some activity for both palm and coconut oil. The sulfonated metal catalysts gave the highest fatty acid methyl ester yields overall. ZrO₂ gave 86.3 % yield for coconut oil and 90.3 % yield for palm oil.

Bio-catalyzed trans-esterification as new biochemical routes to biodiesel production, are similarly gradually gaining attention as reported by Palligarnai and Michael Briggs[48] and[49-53]. Most of the articles published have used a variety of substrates such as rice bran oil, canola, sunflower oil, soybean oil, olive oil, and castor oil. Several lipases from microbial strains, including Pseudomonas fluorescens[54-55], Pseudomonas cepacia[56], Rhizomucor miehei[55], Rhizopus oryzae[57], Candida rugosa[58], Thermomyces lanuginosus[59], and Candida antarctica[60], have been reported to have trans-esterification activity. As shown in table 4, Ojolo et al. [75],explained that the catalyst concentration increase influences the ester yield in a positive manner up to 0.80% NaOH for jatropha oil after that it decreases.

What seems to be left out is the technology for the eventual separation of the lye catalyst from the glycerol.Most researchers are silent about it probably because of their wide availability and low cost.

Table 4. Yield % of Biodiesel with Varying Amount of Catalyst

7. Biodiesel Fuel Properties

The fuel properties of biodiesel are discussed below;

• Specific Gravity, Density and API (America Petroleum Institute) gravity: Density is the mass of unit volume of a material at a specific temperature. A more useful unit used by the petroleum industry is specific gravity, which is the ratio of the weight of a given volume of a material to the weight of the same volume of water measured at the same temperature. Specific gravity is used to calculate the mass of oils. The API (American Petroleum Institute) gravity is another way to express the relative masses of oils. The API gravity could be calculated mathematically using the equation:

A low API gravity indicates heavier oil, while a higher API gravity means a lighter crude or product. Specific gravities of crude oils roughly range from 0.82 for lighter Crudes to over 1.0 for heavier oil.

• Flash Point: This is the minimum temperature at which the vapour from oil sample will give a momentary flash on application of a standard flame under specific test conditions. This is used to predict the possible fire hazard during transportation, storage and handling.

• Pour Point, Cloud Point: The pour point of a crude oil or product is the lowest temperature at which oil is observed to flow under the conditions of the test. Handling and transporting oils and heavy fuels is difficult at temperatures below their pour points .Often, chemical additives known as pour point depressants are used to improve the flow properties of the fuel. The temperature at which wax crystals begin to form on the surface of the biodiesel is the cloud point.

• Aniline Point, Diesel Index: Aniline Point is the minimum temperature at which equal volumes of anhydrous aniline and oil mix together. A low aniline point indicates low diesel index. Diesel index is a measure of ignition quality of a fuel. Aniline point can also predict the amount of carbon present in the oil as given by the equation

Where = Refractive index at 20 ℃ and Density at 20℃.

• Viscosity: This is the resistance to flow of oil. Ease of starting depends on viscosity. Glycerin contamination may cause biodiesel viscosity to increase[20].Production of biodiesel with much reduced viscosity and cloud point can be found in the patent of Nourreddini[68]. Tables 4-5 shows the fuel properties of biodiesel from various sources.

Table 5. Fuel Properties of Biodiesel from various sources

Table 6. Heat of a Combustion and Specific gravity of other vegetable biodiesel fuel

8. The Energy Balance of Biodiesel

The net energy balance of various ethanol and biodiesel feedstock has been a center of debate within scientific and policy circles. The energy balance denotes to “a comparison of the energy stored in a fuel to the energy required to grow, process and distribute that fuel”[69]. According to most of the sources, biodiesel provides a positive energy balance: for every unit of energy needed to produce biodiesel, 2.5 to 3.2 units of energy are gained. Evidence suggests that virgin oil from sources other than Soya may have even higher energy content. Overall, biodiesel is said to have the highest energy yield than any liquid fuel[70].

9. Field Trials of Biodiesel

In recent years, trials on automobiles using biodiesel have been conducted by several institutions in India which have confirmed that biodiesel can reduce wear and tear of engines and reduce oil pollution significantly[71]. At the University of Ilorin , Kwara State, Nigeria, biodesel obtained from jatropha seed oil was used to test run a fossil diesel generator for hours[72].

10. Conclusions and Future Prospects

Energy is an essential factor in industrial growth and in provision of required services that improve the quality of life of mankind. Biodiesel, of the family of biofuel, has been described in this review as a fuel with necessary potentials to replace fossil diesel in future. The trials biodiesel and its blend have undergone is a confirmatory test to all advantages including environmental benefits accrued to it thereby plays a vital role in meeting future fuel requirements. The availability of major feedstock namely oil from bio-sources and simplicity of the trans-esterifcation technology that ensures its conversion to biodiesel are added advantage in terms of the future needs of biodiesel. The use of inedible oil and waste frying/cooking oil has equally assisted in establishing a balance between energy and food security.However, serious efforts have to be intensified on design of large scale bio-refineries for future biodiesel production.