Biodiesel: Fuel for the Future (A Brief Review)

Several attempts have been made by researchers across the globe to counter the effects of the growing informat ion related to the finite nature of the present fossil fuel reserve and the associated hazards. Biofuel has gained global popularity in this respect as a biomass based fuel that have been tipped as a timely candidate. This review strictly focuses on biodiesel of all b iomass derived biofuel.The major feedstock and their supply lines in a b id to create a balance between food and energy was considered. Various catalyst used including biocatalyst for the simple trans-esterification react ion of the feedstock (Vegetable/Used Oil; Edible/Inedib le Oil and Alcohol) were analyzed. In fluence of the feedstock molar rat io, temperature and t ime of the reaction on the conversion/yield and product purity and quality were equally reviewed. The thermo-kinetics of the biodiesel production, fuel properties of biodiesel and the trials subjected to biodiesel in terms of workab ility were considered for review in th is paper.


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 hard er and mo re expens ive to reco ver as convent ion al reserves are depleted and as foreign suppliers increase the price for their declining reserves [1].The drastic ju mp in o il prices between 1973 and 1979 started the agitation that made many govern ments to adopt policies to develop alternative energy sources. W ith th e wide majo rity o f at mospheric scientist now agreeing that global warming is already well underway ,there are now mo re strident calls to replace crude oil as our liquid fuel source in order to reduce the build-up of greenhouse gases in the environ ment [1].Thus an additional emphasis is being placed on the develop ment, production and the use of alternative fuel considered being friendlier to the env iron ment th an fossil fuel.. In the rev iew o f [2], necessity to strict ly limit the g lobal warming was high ly emphasized of wh ich accelerated release of fossil entombed CO 2 h a s b e en g en e r a lly a c c ep t ed as a ma jo r cont ribut or [3].Generally , b io -sou rced fuels are termed biofu el examp les of wh ich are b io methanol, b ioethano l, biobutanol, bio methane, b iohydrogen, b iod iesel etc. This paper centers on b iodiesel not on ly because its exh ibits d efined ch emical and p hys ical p rop erties 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 fo r scientists to keep researching into the world of b iodiesel. Though, numerous popular articles and scientific papers have cautioned against the global drive towards a biofuel econo my generally, highlighting the potential impacts on food security. Since vegetable oils which are the major feed stock for b iodiesel 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 o il as major feedstock for biodiesel. Ku mar et al. [4] discussed the importance of non-edible oil Jatropha curcus in biodiesel production. Shah et al. [5] produced biodiesel fro m jatropha by enzy me in a solvent free system. Attempts have also been made to produce biodiesel fro m non-edible sources like used frying oil, greases, tallow and lard [6][7][8][9]. Soap nut which is reported to be wildly gro wn in forest areas in Nepal in the elevation of 300-1900m [10] have also been presented as a suitable feedstock for biodiesel production [11]. Co mpared to petroleu m-based diesel, the high cost of biodiesel is a major barrier to its co mmercializat ion. It costs approximately one and a half times that of petroleu m-based diesel depending on feedstock oils [12][13], nevertheless, numerous literatures [14][15][16][17][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 , v irgin 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.

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 o il, and, vegetable oil. Dr. Diesel's first engine experiments were catastrophic failures. But by the time he showed his engine at the World Exh ibition 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 agricu lture 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 h is invention provided the foundation for a society fuelled with clean, renewab le, 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 o il fro m animal and plant respectively. Tables below show various oil used for biodiesel production. Source: [20 ,74] Fatty acid composition usually obtained by gas chromatography is the majo r indicator of the properties of biodiesel [27]. Such composition of o il has an important ro le in the performance of biodiesel in d iesel engines. Saturation fatty acid methyl esters increase the cloud point, cetane number, and imp rove stability whereas mo re polyunsaturation reduce the cloud point, cetane number, and stability [28][29][30], the details of wh ich 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. Ho wever, 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 fro m this commodity crop [32]. There is still so me 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 roo m 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.  In all, contained in the article o f 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 simp le which involves mixing alone and hence the equipment cost is low. But d irect 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 gu m format ion by o xidation and poly merization and carbon deposition. Hence vegetables oils are processed so as to acquire properties (v iscosity and volatility) similar to that of fossil fuels and the processed fuel can be directly used in the d iesel engines. Three processing techniques are main ly used to convert vegetable oils to fuel form [62] namely, pyrolysis, micro-emulsificat ion and trans-esterification. Pyrolysis refers to chemical change caused by application of heat to get simp ler co mpounds from a co mplex co mpound. 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 carboxy lic 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-treat ment before use.All oils contains certain percent of gum which are expected to be removed irrespective of how little they may be.Gu m free o il will synthesize biodiesel o f improved cloud point an pour point.

Synthesis of Biodiesel
Biodiesel are produced through a simp le technology called trans-esterificat ion reaction. Degummed oil free of all forms of impu rit ies is reacted with a reasonable alcohol (ethanol, methanol, butanol etc.) [29].Chemically, the react ion 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, name ly [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 0 C -65 o C. The temperature which is higher than the normal boiling point of methanol (68 o C) causes more vaporization o f methanol (loss). On the other hand, the temperature wh ich is lower than 50 o C causes higher viscosity of biodiesel [34].The rat io of methanol to oil also affects the reaction, the higher mo lar rat io, the h igher conversion of alcohol. The rat ios, 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 tallo w, 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 appro ximate yield of 80 % was observed after 1 minute for soybean and sunflower oils at methanol to oil rat io of 6:1. After 1 hour, the conversions were almost the same (93%-98%).The effect of reaction time fo r palm oil at 40:1 methanol: o il with 5% H 2 SO 4 (v/v) at 95 ℃ for 9 hours and obtained a maximu m yield of 97 % [42]. However,further efforts is required to demonstrate the reactor ideal enough for biodiesel synthesis and its design and fabrication.

Thermo-Kinetics of Trans-Esterification
The feasibility of a reaction is determined fro m the thermodynamic parameters. Since both reactants and products are liquids, entropy change will tend to zero, hence equilibriu m constant will be low [20]. Kinetics of trans-esterificat ion 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 react ion, which again should confirm the low possibility o f reverse reactions. So me 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-esterificat ion kinetics of soybean oil with five d ifferent catalysts have also been studied [40].Similarly, Divya and Tyagi [20] reported the study carried out by Ko mers et al; [41] where kinetics and mechanism o f KOH catalyzed methanolysis of rapeseed oil for b iodiesel production was investigated. The first sequence of the reaction expressed the methanolysis of rapeseed oil to biodiesel wh ile 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 kinet ic equations which were solved numerically by two independent computing methods.Nonetheless,this aspect is yet to be fully exp loited by researchers.Highly limited art icles on elementary steps of the trans-esterification react ion and kinetic modeling are available in open literatures.

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 emu lsion than KOH and makes comp licated to separate biodiesel fro m g lycerin. For th is 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 mo isture 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-esterificat ion process was stated. This will give a rough guide on the amount of catalyst that will give an optimu m 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 d issolves 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 hydro xide catalysts was approximately 86%. Zheng et al. [45] showed that methyl ester conversion of waste cooking oil in acid cataly zed 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 format ion associated with biodiesel production, some non-enzymatic heterogeneous catalysts have been investigated. ZrO 2 , ZnO, SO 4 2-/SnO 2 ,SO4 2-/ZrO 2 , zeolite, and KNO 3 /ZrO 2 are some solid catalysts that were studied in the trans-esterificat ion of palm and coconut oil [46]. The reaction was carried out at 200 ℃, 50 bar, 3 wt% catalysts, and a 6:1 mo lar ratio of methanol to oil. A ll 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. ZrO 2 gave 86.3 % yield for coconut oil and 90.3 % yield for palm oil.
What seems to be left out is the technology for the eventual separation of the lye catalyst fro m the glycerol.Most researchers are silent about it probably because of their wide availability and low cost.

Biodiesel Fuel Properties
The fuel properties of b iodiesel are d iscussed below; • Specific Gravity, Density and API (America Petroleu m 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 grav ity, wh ich is the ratio of the weight of a given volume of a material to the weight of the same volu me of water measured at the same temperature. Specific grav ity is used to calculate the mass of oils. The API (A merican Petro leu m 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 fro m 0.82 fo r lighter Crudes to over 1.0 for heavier o il.
• Flash Point: This is the minimu m temperature at wh ich the vapour fro m oil sample will give a mo mentary 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 wh ich 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 wh ich wax crystals begin to form on the surface of the biodiesel is the cloud point.
• Aniline Point, Diesel Index: Aniline Point is the minimu m temperature at which equal volu mes 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
= Refract ive 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 fro m various sources.

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 o il fro m 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].

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 fro m jatropha seed oil was used to test run a fossil diesel generator for hours [72].

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 b iofuel, has been described in this review as a fuel with necessary potentials to replace fossil diesel in future. The trials b iodiesel and its blend have undergone is a confirmatory test to all advantages including environ mental benefits accrued to it thereby plays a vital ro le in meet ing future fuel requirements. The availability of major feedstock namely oil fro m bio-sources and simp licity 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.