(10a) a Comprehensive Review on Biodiesel as an Alternative Energy Resource and Its...

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2072 A.E. Atabani et al. / Renewable andSustainable EnergyReviews 16 (2012) 20702093

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Fig. 1. Total world, OECDand non-OECDtransportation sector energy consumption (GJ) between 2005 and 2035 [3,6].

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Fig. 2. Total world oil consumption,transportation oil consumptionand other sectors oil consumption (GJ) between 2007 and 2035 [3,6].

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A.E. Atabani et al. / Renewable and Sustainable Energy Reviews 16 (2012) 20702093 2073

(a) 2007 (b) 2035

Oil 35%

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23%

Coal 27%

Nuclear 5%

Renewables10%

Oil 30%

Natural Gas

22%

Coal 28%

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Renewables14%

Fig. 5. Breakdown of world marketed energy consumption by fuel in 2007 and 2035 [3].

Intergovernmental Panel on Climate Change, the transportationsector was responsible for about 23% ofenergy-related greenhousegas emissions in 2004. Passenger vehicles account for about 45% ofthis total [16]. In Europe, transportation sector accounts for morethan a fifth of greenhouse gas emissions. Between 1990 and 2001,emissions of greenhouse gases (GHGs) from transport (excluding

international transport) increased by 20% [17,18]. In Australia, Thetransport sector contributed to 13.7% ofAustralias net emissionsin 2006. Road transport was responsible for 87% ofthese emissionsor 12% of Australias total emissions [19]. In 2008, Almost 30% oftotal U.S. greenhouse gas (GHG) emissions come from the trans-portation sector, making transportation the second largest sourceofGHGemissionsintheUnited Statesafterthe electricpowersector(35%) [20].

The majority of transportation GHG emissions (95%) are com-posed of carbon dioxide (CO2). An additional one percent comesfrom methane (CH4) and nitrous oxides (N2O). The leakage of hydro fluorocarbons (HFCs) from vehicle air conditioning systemsis responsible for the remaining three percent of GHG emissions.Transportationsources alsoemit ozone, carbon monoxide (CO),and

aerosols. These substances are not countedas greenhouse gases butare believed to have an indirect effect on global warming, althoughtheir impact has not been quantified with certainty [20].

2. Biodiesel as an emerging energy resource

Globally, the awareness of energy issues and environmen-tal problems associated with burning fossil fuels has encouraged

many researchers to investigate the possibility of using alterna-tive sources ofenergy instead ofoil and its derivatives. Amongthem, biodiesel seems very interesting for several reasons: it ishighlybiodegradable and hasminimal toxicity,it can replace dieselfuel in many different applications such as in boilers and inter-nal combustion engines without major modifications and small

decrease in performances is reported, almost zero emissions ofsul-fates, aromatic compounds and other chemical substances that aredestructive to the environment, a small net contribution ofcarbondioxide (CO2) when the whole life-cycle is considered (includ-ing cultivation, production ofoil and conversion to biodiesel), itappears to cause significant improvement of rural economic poten-tial [9,21,22]. The invention of the vegetable oil fuelled engine bySir Rudolf Diesel dated back in the 1900s. However, full explo-ration ofbiodiesel only came into light in the 1980s as a result ofrenewed interest in renewable energy sources for reducing green-house gas (GHG) emissions, and alleviating the depletion offossilfuel reserves.Biodiesel is defined as mono-alkyl estersoflongchainfatty acids derived from vegetable oils or animal fats and alcoholwith or without a catalyst [2,9,10,2326]. Compared to diesel fuel,

biodiesel produces no sulfur, no net carbon dioxide, less carbonmonoxide, particulate matters, smoke and hydrocarbons emissionand more oxygen. More free oxygen leads to the complete combus-tion and reduced emission [27,28].

BiodieselhasbeeninuseinmanycountriessuchasUnitedStatesof America, Malaysia, Indonesia, Brazil, Germany, France, Italy andother Europeancountries. However, the potential for its productionand application is much more. Table 1 shows the list ofthe top 10

OECD (Organization for Economic Cooperation and Development)

Non-OECD (Countries outside theOrganization for Economic Cooperation and Development)

29.731.5

33.836.5

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metrictons)

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World total

Fig. 6. World, OECD and non-OECD carbondioxide emissionsfrom 2007 to 2035 [3].


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1520 27

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BiodieselProduction

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Fig. 7. Total world biodiesel productions (thousand barrels per day) between 2000 and 2008 [27,33].

Table 1

Top10 countriesin terms of biodiesel potential [8,2931].

Rank Country Biodieselpotential (ML)

Production($/L)

1 Malaysia 14,540 0.532 Indonesia 7595 0.493 Argentina 5255 0.624 USA 3212 0.705 Brazil 2567 0.62

6 Netherlands 2496 0.757 Germany 2024 0.798 Philippines 1234 0.539 Belgium 1213 0.78

10 Spain 1073 1.71

biodiesel producing countries. Form this table, it can be seen thatMalaysia is far ahead among the rest [8,2931].

Biodiesel has a massive potentiality to be a part of a sustainableenergymix in thefuture[32]. Globally, annual biodiesel productionincreased from 15 thousand barrel per day in 2000 to 289 thousandbarrel per day in 2008 as shown in Fig. 7 [27,33]. It is believed that,85% of biodiesel production comes from the European Union [9].The demand for biodiesel in European countries is expected to beup to 10.5 billion liters by 2010 [34].

3. Biodiesel feedstock

Globally, there are more than 350 oil-bearing crops identifiedas potential sources for biodiesel production. Table 2 shows mainfeedstocks of biodiesel. The wide range ofavailable feedstocks forbiodiesel production represents one of the most significant factorsof producing biodiesel [2,23,35]. As much as possible the feedstockshould fulfill two main requirements: low production costs andlarge production scale. The availability of feedstock for producingbiodiesel depends on the regional climate, geographical locations,local soil conditions and agriculturalpractices ofanycountry. Fromthe literature, it has been found that feedstock alone represents75% of the overall biodiesel production cost as shown in Fig. 8.Therefore, selecting the cheapest feedstock is vital to ensure low

Oil feedstock 75%

General overhead1%

Energy 2%

Direct labour 3%

Depreciation 7%

Chemicalfeedstocks12%

Fig. 8. General cost breakdown for production ofbiodiesel [9,27,40,46,47].

production cost ofbiodiesel. Fig.9 shows pictures of some biodieselfeedstocks. In general, biodiesel feedstock can be divided into fourmain categories as below [9,10,27,3644]:

1. Ediblevegetable oil: rapeseed, soybean, peanut, sunflower, palmand coconut oil.

2. Non-edible vegetable oil:jatropha, karanja, seamango, algae andhalophytes.

3. Waste or recycled oil.4. Animal fats: tallow, yellow grease, chicken fat and by-products

from fish oil.

Table 3 shows primary biodiesel feedstock for some selectedcountries around the world [9,45].

It is very important to consider some factors when comparingbetween different feedstocks. Each feedstock should be evaluatedbased on a full life-cycle analysis. This analysis includes: (1) avail-ability of land, (2) cultivation practices, (3) energy supply andbalance, (4) emission ofgreenhouse gases, (5) injection ofpesti-cides, (6) soil erosion and fertility, (7) contribution to biodiversityvalue losses, (8) logistic cost (transport and storage), (9) direct eco-nomic value of the feedstocks taking into account the co-products,

(10) creation or maintain of employment, (11) water require-ments and water availability, (12)effects of feedstock on airquality[9,53,54].

To consider any feedstock as a biodiesel source, the oil percent-age and the yield per hectare are important parameters. Table 4shows the estimated oil content and yields ofdifferent biodieselfeedstocks.

Edible oils resources such as soybeans, palm oil, sunflower,safflower, rapeseed, coconut and peanut are considered as thefirst generation ofbiodiesel feedstock because they were the firstcrops to be used for biodiesel production. Their plantations havebeen well established in many countries around the world suchas Malaysia, USA and Germany. Currently, more than 95% of theworldbiodiesel is produced fromedibleoils such as rapeseed (84%),

sunfloweroil (13%),palm oil(1%),soybeanoil andothers (2%). How-ever, their use raises many concerns such as food versus fuel crisisand major environmental problems such as serious destruction ofvital soil resources, deforestation andusageof much ofthe availablearable land. Moreover, in the last 10 years the pricesof vegetable oilplants have increased dramatically which will affect the economi-cal viability ofbiodiesel industry [29,53,61]. Furthermore, the useof such edible oils to produce biodiesel is not feasible in the long-term because of the growing gap between demand and supply ofsuch oils in many countries. Forinstance, dedicating all US soybeanto biodiesel production would meet only 6% of dieseldemands [62].

One ofthe possible solutions to reduce the utilization oftheedible oil for biodiesel production is by exploiting non-edible oils.Non-edible oils resources are gaining worldwide attention because

they are easily available in many parts of the world especially


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Table 2

Main feedstocks of biodiesel [2,10,43,45,46,48] .

Edible oils Non-edible oils Animal fats Other sources

Soybeans (Glycine max) Jatropha curcas Pork lard BacteriaRapeseed (Brassica napus L.) Mahua (Madhuca indica) Beef tallow Algae (Cyanobacteria)Safflower Pongamia (Pongamia pinnata) Poultry fat Microalgae (Chlorellavulgaris)Rice bran oil (Oryza sativum) Camelina (Camelina Sativa) Fish oil TarpenesBarley Cotton seed (Gossypiumhirsutum) Chicken fat PoplarSesame (Sesamum indicum L.) Karanja or honge (Pongamia pinnata) Switchgrass

Groundnut Cumaru MiscanthusSorghum Cynara cardunculus LatexesWheat Abutilonmuticum FungiCorn Neem (Azadirachta indica)Coconut Jojoba (Simmondsia chinensis)Canola Passion seed (Passiflora edulis)Peanut Moringa (Moringa oleifera)Palm and palm kernel (Elaeis guineensis) Tobacco seedSunflower (Helianthus annuus) Rubber seed tree (Hevcabrasiliensis)

Salmon oilTall (Carnegiea gigantean)Coffeeground (Coffea arabica)Nagchampa (Calophyllum inophyllum)Croton megalocarpus

Pachira glabra

Aleurites moluccana

Terminalia belerica

wastelands that are not suitable for food crops, eliminate com-petition for food, reduce deforstration rate, more efficient, moreenvironmentally friendly, produce useful by-products and they arevery economical comparable to edible oils. The main sources forbiodiesel production from non-edible oils are jatropha or ratan-

jyote or seemaikattamankku (Jatropha curcas), karanja or honge(Pongamiapinnata),Aleurites moluccana, Pachira glabra nagchampa(Calophyllum inophyllum), rubber seed tree (Hevca brasiliensis),Desert date (Balanites aegyptiaca), Croton megalocarpus, Rice bran,Seamango(Cerberaodollam),Terminaliabelerica, neem(Azadirachtaindica), Koroch seed oil (Pongamia glabra vent.), mahua (Madhucaindica and Madhuca longifolia), Tobacco seed (Nicotiana tabacum

L.), Chinese tallow, silk cotton tree (Ceiba pentandra),jojoba (Sim-mondsia chinensis), babassu tree andEuphorbia tirucalli. Non-edibleoils are regarded as the second generation of biodiesel feed-stocks. Animal fats such as beef tallow, poultry fat and pork lard,

Table 3

Current potential feedstocks for biodiesel worldwide [2,8,9,29,45,46,5052] .

Country Feedstock

USA Soybeans/waste oil/peanutCanada Rapeseed/animal fat/soybeans/yellow grease

and tallow/mustard/flaxMexico Animal fat/waste oilGermany RapeseedItaly Rapeseed/sunflower

France Rapeseed/sunflowerSpain Linseed oil/sunflowerGreece CottonseedUK Rapeseed/waste cooking oilSweden RapeseedIreland Frying oil/animal fatsIndia Jatropha/Pongamia pinnata

(karanja)/soybean/rapeseed/sunflower/peanutMalaysia Palm oilIndonesia Palm oil/jatropha/coconutSingapore Palm oilPhilippines Coconut/jatrophaThailand Palm/jatropha/coconutChina Jatropha/waste cooking oil/rapeseedBrazil Soybeans/palm oil/castor/cotton oilArgentina SoybeansJapan Waste cooking oilNew Zealand Waste c ooking o il/tallow

waste oils and grease are also considered second generation feed-stocks. The use of these types of feedstock eliminates the need todispose them. However, it has been reported that second genera-tion feedstocks may not be plentiful enough to satisfy the globalenergy demand. Moreover, biodiesel derived from vegetable oilsand animal fats has a relatively poor performance in cold weather.Furthermore, for many types ofanimal fats the transesterificationprocess is difficult because they contain high amount ofsaturatedfatty acids. In case ofwaste cooking oil, collection infrastructure

Table 4

Estimated oil content and yields of different biodiesel feedstocks[9,23,29,45,48,49,5260] .

Feedstocks Oil content (%) Oil yield(L/ha/year)

Castor 53 1413Jatropha Seed: 3540,

kernel: 50601892

Linseed 4044 Neem 2030 Pongamia pinnata (karanja) 2739 2252250a

Soybean 1520 446Sunflower 2535 952Calophyllum inophyllumL. 65 4680Moringa oleifera 40 Euphorbia lathyris L. 48 15002500a

Sapiumsebiferum L. Kernel 1229 Rapeseed 3846 1190Tung 1618 940Pachira glabra 4050 Palm oil 3060 5950Peanut oil 4555 1059Olive oil 4570 1212Corn (Germ) 48 172Coconut 6365 2689Cottonseed 1825 325Rice bran 1523 828Sesame 696Jojoba 4550 1818Rubber seed 4050 80120a

Sea mango 54 Microalgae (low oil content) 30 58,700Microalgae (medium oil content) 50 97,800Microalgae (high oil content) 70 136,900

a

(kg oil/ha).


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Fig. 9. Main biodiesel feedstocks: (a) edible and (b) non-edible [27,37,41,49].

and logistics could be hurdle as the sources are generally scattered[810,23,36,43,46,48,49,53,6073].

More recently, microalgae have emerged to be the third gen-eration of biodiesel feedstock. Microalgae are photosyntheticmicroorganisms that convert sunlight, water and CO2 to algalbiomass but they do it more efficiently than conventional cropplants. It represents a very promising feedstock because of itshigh photosynthetic efficiency to produce biomass, higher growth

rates and productivity and high oil content compared to edibleand non-edible feedstocks (Table 5). Microalgae have the poten-tial to produce an oil yield that is up to 25 times higher than theyield ofoil palm and 250 times the amount ofsoybeans as can beseen in Table 4. This is because microalgae can be grown in farmor bioreactor. Moreover, they are easier to cultivate than manyother plants. It is believed that microalgae can play an impor-tant role in solving the problem between the production of food


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Table 5

Comparison of biodiesel production from algae and oil plants [23,57].

Biodiesel producedfrom algae

Biodiesel producedfrom plants

Technology Cell bioengineering,automaticallyproduced in pilot plant

Agriculture in farm

Production period 57 days for a batchcultivation

Several months oryears

Oil content 30% (low oil content),50%(medium oilcontent),70% (high oilcontent)

Lessthan20% inseeds or fruits

Land occupied 0.0100.013 hectare forproducing 1103 L oila

2.24ha forproducing 1103 Loilb

Cost performance $2.4 per litermicroalgal oil

$0.60.8 per literplant oil

Developmentpotential Unlimited (work justbeginning)

Limited (manyworks have beendone)

a Based on soybean cultivation in farmland.b Based on projected area ofbioreactor in pilot plant.

and that of biodiesel in the near future. Moreover, among othergenerations of biodiesel feedstocks, microalgae appear to be theonly source of renewable biodiesel that is capable ofmeeting theglobal demand for transport fuels and can be sustainably devel-oped in the future. The main obstacle for the commercializationofmicroalgae is its high production cost from requiring high-oil-yielding algae strains and effective large-scale bioreactors. Recentstudies indicatethat algae forbiodiesel production cangrow on fluegas,giving opportunities in consuming greenhousegas as feedstock[8,9,23,36,39,43,46,49,53,6365,74,75]. Table 5 shows the compar-ison of biodiesel production from algae and oil plants.

Knothe [76] reported that the use of the terms first, second andthird generation are sometimes misleading and should not be usedto imply that biodiesel derived from second or third generationfeedstock may have superior fuel properties over first generation.For instance, biodiesel from Jatropha oil possesses poorer cold flowproperties than biodiesel derived from soybean, palm or rapeseedoil. Recently, there have been several publications which high-lighted the positive effects of blending different oils on the basicproperties of biodiesel [7779].

Janaun and Ellis [23] and Lin et al. [46] show that geneticallyengineered plants such as poplar, switchgrass, miscanthus and bigbluestem can be considered new feedstocks for biodiesel produc-tion. These feedstocks will create new bioenergy crops that are notassociated with food crops. Therefore, they are expected to rep-resent a sustainable biodiesel feedstock in the future. However,precaution on biosafety must be considered for these feedstocks.

4. Oil extraction methods

The second step in the production of biodiesel is oil extraction.In this process, the oil contained in the seeds has to be extracted.The main products arethe crude oil and the important by-productssuch as seeds or kernel cakes. There are three main methods thathave been identified forextraction ofthe oil: (i) Mechanical extrac-tion, (ii) solvent extraction and (iii) enzymatic extraction. Beforethe oil extraction takes place, seeds have to be dried. Seed can beeither dried in the oven (105 C) or sundried (3 weeks). Mechanicalexpellersor presses canbe fedwith eitherwholeseeds or kernels oramix of both, but commonpractice is to usewhole seeds.However,for chemical extraction only kernels are used as feed [80].

Table 6

Calculated oil yields (% of contained oil) of mechanical extraction methods[5,80,8284].

Press Oil yield (%) Necessary treatment

Engine drivenscrew press

68.080.079.0 Filterization and

degumming

Ram press 62.5

62.5

4.1. Mechanical extraction

Thetechniqueofoilextractionby mechanicalpresses is themostconventional one among other methods. In this type, either a man-ual ram press or an engine driven screw press can be used. It hasbeen found that, engine driven screw press can extract 6880%of the available oil while the ram presses only achieved 6065%(Table 6). This broader range is due to the fact that seeds can besubjected to a different numberofextractionsthrough the expeller.The oil extracted by mechanical presses needs further treatment offilterization and degumming. One more problems associated with

conventionalmechanical presses are,their designis suitedforsomeparticular seeds, and therefore the yield is affected if used for otherseeds. It has been found that, pretreatment of the seeds, such ascooking, can increase theoil yield ofscrew pressing up to 89%aftersingle pass and 91% after dual pass [80,81].

4.2. Solvent extraction (chemical extraction)

Solvent extraction is the technique ofremoving one constituentfrom a solid by means ofa liquid solvent. It is also called leaching.There are many factors influencing the rate of extraction such asparticle size, the type ofliquid chosen, temperature and agitationof the solvent. The small particle size is preferable because it allowsfora greater interfacial area between thesolid andliquid. Theliquid

chosen should be a good selective solvent and its viscosity shouldbe sufficiently low to circulate freely. Temperature also affects theextraction rate. The solubility ofthe material will increase withthe increasing temperature. Agitation ofthe solvent also affects, itincreases the eddy diffusion and therefore increases the transferof material from the surface ofthe particles. Solvent extraction isonly economical at a large-scale production ofmore than 50tonbiodiesel per day. There are three methods that are used in thistype as follow [80,81]:

(1) Hot water extraction.(2) Soxhlet extraction.(3) Ultrasonication technique.

4.3. Enzymatic oil extraction

Enzymatic oil extraction technique has emerged as a promisingtechnique for extraction of oil. In this process suitable enzymesare used to extract oil from crushed seeds. Its main advantagesare that it is environment friendly and does not produce volatileorganic compounds. However, the long process time is the maindisadvantage associated with this technique [81].

Table 7 shows the reaction temperature, reaction pH, timeconsumption and oil yield of different chemical and enzymaticextraction methods. It has been found that the chemical extrac-tion using n-hexane method results in the highest oil yield whichmakes it the most common type. However, this type consumesmuch more time compared to other types. Furthermore n-hexane

solvent extraction hasa negative environmentalimpacts as a result


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Table 7

Reported oil yields percentage for different chemical and enzymatic extractionmethods and different reaction parameters [80].

Extraction method Reactiontemperature(C)

ReactionpH

Timeconsumption(h)

Oil yield(%)

n-Hexane oil extraction(Soxhelt) apparatus

24 9599

1st acetone 48

2nd n-hexaneAqueous oil extraction(AOE)

2 3850 9 6 38

AOE with 10minofultrasonication aspretreatment

50 9 6 67

Aqueous enzymatic oilextraction (AEOE)(hemicellulase orcellulase)

60 4.5 2 73

AEOE (alkalineprotease)

60 7 2 8650 9 6 64

AEOE (alkaline protease)with5 min of ultrasonicas pretreatment

50 9 6 74

Three-phase partitioning 25 9 2 94

of the waste water generation, higher specific energy consumptionand higher emissions ofvolatile organic compounds and humanhealth impacts (working with hazardous and inflammable chemi-cals). Usingaqueousenzymaticoil extractionsgreatlyreduces theseproblems. In aqueous enzymatic oil extraction the use of alkalineprotease gave betterresults. Furthermore, ultrasonicationpretreat-ment is a more useful step in aqueous oil extraction [80,81].

5. Biodiesel production technologies

Globally, there are many efforts to develop and improve veg-etable oil properties in order to approximate the properties ofdiesel fuels. It has been remarked that high viscosity, low volatility

andpolyunsaturatedcharactersare the mostlyassociated problemswith crude vegetable oils. These problems can be overcome by fourmethods; pyrolysis, dilution with hydrocarbons blending, Micro-emulsion, and transesterification [29,36,46,51,8588]. Linetal. [46]conductacomparisonbetweendifferentbiodieselproductiontech-nologies. This comparison is given in Table 8.

5.1. Pyrolysis (thermal cracking)

Pyrolysis is the thermal decomposition of the organic mat-ters in the absence ofoxygen and in the presence ofa catalyst.The paralyzed material can be vegetable oils, animal fats, natu-ral fatty acids or methyl esters offatty acids. Many investigatorshave studied the pyrolysis oftriglycerides to obtain suitable fuels

for diesel engine. Thermal decomposition of triglycerides pro-duces alkanes, alkenes, alkadines, aromatics and carboxylic acids[29,51,58,65,68,81,8993]. It has been observed that pyrolysisprocess is effective, simple, wasteless and pollution free [43].According to Sharma et al. [75], pyrolysis of the vegetable oilcan produce a product that has high cetane number, low viscos-ity, acceptable amounts ofsulfur, water and sediments contents,acceptable copper corrosion values. However, ash contents, carbonresidues, and pour points were unacceptable.

5.2. Dilution

Mainly, vegetable oils are diluted with diesel to reduce the vis-cosity and improve the performance ofthe engine. This method

does notrequire anychemicalprocess [29,88]. Singh andSingh [43]

Table 8

Comparison ofmain biodiesel production technologies [46].

Technologies Advantage Disadvantage

Dilution (direstblending ormicro-emulsion

Simple process High viscosity Bad volatility Bad stability

Pyrolysis Simple process High temperature is

required No-polluting Equipment is expensive

Low purity

Transesterification

Fuel properties is closerto diesel

Low free fatty acid andwater content are required(for base catalyst)

High conversionefficiency

Pollutants will beproduced because productsmust be neutralized andwashed

Low cost Accompanied by sidereactions

Itis suitable forindustrialized production

Difficult reactionproducts separation

Supercriticalmethanol

No catalyst High temperature andpressure are required

Short reaction Time Equipment cost is high High conversion High energy consumption

Good adaptability

reported that substitutionof100%vegetable oilfor diesel fuel is notpractical. However a blend of20% vegetable oil and 80% diesel fuelwas successful. The use ofblends of diesel fuel with sunflower oil,coconut oil, African pear seed, rice bran oil, PP (Pistachia Palestine),waste cooking oil, palm oil, soybean oil, cottonseed oil, rubber seedoil, rapeseed oil,J. curcas oil, P. pinnataoil has been described in theliterature [29,58,65,88,89,91,94,95]. For instance, Ziejewski et al.[95] investigated the effects of the fuel blend of 25% sunflower oilwith 75% diesel fuel (25/75 fuel) in a direct injection diesel engine.The authors found that this blend is not suitable for long-term usein direct injection engines. This is because the viscosity at 313 Kwas 4.88cSt (maximum specified ASTM value is 4.0cSt at 313 K).Generally, direct use of vegetable oils and their blends have beenconsidered to be difficult to use in both direct and indirect dieselengines [58,65].

5.3. Micro-emulsion

A micro-emulsion is defined as a colloidal equilibrium disper-sion of optically isotropic fluid microstructure with dimensionsgenerally into 1150 nm range formed spontaneously from twonormally immiscible liquids and one and more ionic or more ionic

amphiphiles. Micro-emulsions using solvents such as methanol,ethanol, hexanol, butanol and 1-butanol have been investigated bymany researchers.Micro-emulsionwith these solvents has met themaximum viscosity requirement fordieselfuel. It has been demon-strated that short-term performances of both ionic and non-ionicmicro-emulsions ofaqueous ethanol in soybean oil are nearly aswell as that ofNo. 2 diesel fuel [29,43,65,68,81,89,91].

The fuel properties ofthe liquid product fractions ofthe ther-mally decomposed vegetable oil are likely to approach diesel fuels.Soybean oilpyrolyzed distillate had a cetane number of 43, exceed-ing that ofsoybean oil (37.9) and the ASTM minimum value of40. However, the viscosity of the distillate was 10.2mm2/s at311K, which is higher than the ASTM specification for No. 2 dieselfuel (1.94.1mm2/s) but considerably below that of soybean oil(32.6mm2/s) [29].


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CATALYST

Diglycerides + FAMETriglycerides + ROH

CATALYST

Monoglycerides + FAMEDiglycerides + ROH

CATALYST

Glycerol + FAMEMonoglycerid + ROH

Fig. 10. Equation of transestetification reaction [2,29,43,51,75,87,96] .

5.4. Transesterification (alcoholysis)

Transesterification is regarded as the best method among otherapproaches due to its low cost and simplicity [2,8,29,65,93].Biodiesel is the main product of this process. Transesterificationconsists of a number of consecutive, reversible reactions. In thesereactions, the triglycerides are converted step wise to diglycerides,monoglyceride and finally glycerol which sinks to the bottom andbiodiesel which floats on top and can be siphoned offas shown inFig. 10. Glycerol is an important by-product and can be burned forheat or be used as feedstock in the cosmetic industry. In this reac-tion, methanol and ethanol are the two main light alcohols used fortransesterification process due to their relatively low cost. How-ever, propanol, isopropanol, tert-butanol, branched alcohols andoctanol and butanol can also be employed but the cost is muchhigher [2,29,43,45,58,75,87,96].

In this reaction, 100lb offat or oil are reacted with 10l b of ashort chain alcohol in the presence of a catalyst to produce 10lbof glycerin and 100 lb of biodiesel. As per the transesterificationreaction, 3 moles ofmethanol were required to react with the veg-etable oil [54]. Generally, transesterification process includes twomain processes; catalytic and non-catalytic method. A catalyst isused to commence the reaction. The catalyst is vital as alcohol isbarely soluble in oil or fat. The catalyst enhances the solubility ofalcohol and thus increases the reaction rate. The most frequently

used process is the catalytic transesterification process. Fig. 11shows the detailed classification oftransesterification processes[41,45].

Alkaline catalysts include catalysts such as NaOH, NaOCH3,KOCH3, KOH, NaMeO and K2CO3. Most of the biodiesel producersuse sodium hydroxide or potassium hydroxide. Even though someauthors reported that sodium hydroxide is better than potassiumhydroxide and some are ofthe view that potassium hydroxide isbetter than sodium hydroxide. However, most ofthe researchersreported that both sodium and potassium hydroxide performequally well. Sodium and potassium methoxides return betteryield than all catalysts but they are costly, so they are not veryfrequently used. Many researchers have found that alkaline cat-alytic method is the fastest and most economical catalyst thanother catalysts. An alkaline catalyst proceeds at around 4000 timesfaster than with the same amount of acid catalyst. Moreover, thismethod can achieve high purity and yield of biodiesel productin a short time (3060min). Therefore, it dominates the currentbiodiesel production methods. However, to use alkaline catalysts,free fatty acid (FFA) level should be below a desired limit (rangingfrom less than 0.5% to less than 3%). Beyond this limit the reac-tion will not take place and the product formed will be soap andwater instead ofesters and the yield of esters will be too less.In addition to, this reaction has several drawbacks; it is energyintensive; recovery of glycerol is difficult; the catalyst has to beremoved from the product; alkaline wastewater requires treat-ment and the level offree fatty acids and water greatly interferewith the reaction [2,8,23,29,31,36,43,45,54,61,75,81,85,89,91,93,98100].

Acid catalysts include sulfuric, hydrochloric, ferric sulfate, phos-phoric and organic sulfonic acid. Some researchers have claimedthat acid catalysts are more tolerant than alkaline catalysts for veg-etable oils having high free fatty acids and water. Therefore, acidcatalyst is used to reduce the free fatty acids contents to a levelsafe enough for alkaline transesterification which is preferred overtheacid catalyst afterthe acid value is reduced to thedesired limit.It has been reported that acid-catalyzed reaction gives very highyield in esters. However, the reaction is slow (348 h). It has beenreported that the homogeneous transesterification consumes largeamountofwater forwet washing to removethe salt produced from

the neutralization process, and the residual acid or base catalyst.Nevertheless, there are many companies around the world com-mercializing this technology because of its relatively lower energy

Table 9

Enzymatic transesteritication reactions using various types of alcohols and lipases [43].

No. Oil Alcohol Lipase Conversion (%) Solvent

1 Rapeseed 2-Ethyl-1-hexanol C. rugosa 97 None2 Palm oil Methanol Rhizopus oryzae 55 (w/w) Water3 Sunflower Ethanol M.meihei (Lypozyme) 83 None4 Soybean oil Methanol C. antarctica lipase 93.80 >1/2 molar

equivalent MeOH5 Fish Ethanol C. anturctica 100 None6 Palm kernel Methanol I. cepuciu 15 None

7 Recycled restaurant grease Ethanol J. cepacia (Lipase PS-30) +C.anturclica (Lipase SP435) 85.4 None

Table 10

Summary of transesterification method [43,75].

No. Sample Catalyst Alcohol Temperature (C) Ration of alcohol to oil Yields (%)

1 Microalgae Sulfuric acid Methanol 30 56:1 602 Pongamia pinnata KOH (1%, w/w) Methanol 105 10:1 923 Soybean oil Absence of catalyst Supercritical methanol 280 24 and CO2/methanol = 0.1 984 Sunflower oil No catalyst Supercritical methanol

and ethanol200400 (pressure200bar)

40:1 7896

5 Rice bran Sulfuric acid Methanol 60 10:1


11/24


Transesterification

Catalytic method

Noncatalytic method

Homogeneous catalysts

Heterogeneous catalysts

Supercritical

methanol (SCM)

Ethanol

Propanol

Butanol

Alkaline catalyst

Acid catalyst

Enzymes

Titanium silicates, alkaline earth metal

(MgO, CaO, SrO), sulfated,

amorphous zirconia, titanium and

potassium zirconias.

KOH, NaOH

H2SO4, HCL

BIOX co-solvent process

Fig. 11. Classification oftransesterification processes [29,41,65,97].

use, high conversion efficiency and cost effective reactants and cat-alyst [2,8,23,29,36,43,45,81,85,91,98,99].

Lipase catalysts such as Diazomethane CH2N2have shown goodtolerance for the free fatty acid level ofthe feedstock. Moreover,they areknown to have a propensity to acton long-chain fatty alco-hols better than on short-chain ones. Therefore, the efficiency ofthe transesterification of triglycerides with ethanol is higher com-pared to that with methanol in systems with or without a solvent(Table 9). In this reaction, there is no need for complex operationsforthe recovery of glycerol andthe elimination of catalyst andsoap.However, the reaction yields and the reaction times are still unfa-vorable compared to the alkaline catalysts. Moreover, lipases arevery expensive for large scale industrial production and they are

unable to provide the degree ofreaction completion required tomeet ASTM fuel specifications. Recently, it has been found thatthe use of solvent-tolerant lipases, multiple enzymes and immo-bilized lipases-making catalysts can be developed as cost-effectiveenzymes [2,29,43,45,65,96]. In general, chemically catalyzed pro-cesses, includingalkali catalyzedand acid catalyzedones have beenproved to be more practical than the enzyme catalyzed process[100].

The catalytic transesterification has some problems such as:high time consumption, lag of reaction time caused by theextremely low solubility ofthealcohol in thetriglyceridephaseandthe need for separation of the catalyst and saponified impuritiesfrom biodiesel. These problems are not faced in the non-catalytictransesterification methods. For instance, supercritical methanol

method uses lower energy and completes in a very short time

(24 min) compared to catalytic transesterification. Further, sinceno catalyst is used, the purification of biodiesel and the recoveryofglycerol are much easier, trouble free and environment friendly[2,45,81,87]. However, the method has a high cost in reactor andoperation (due to high pressures and high temperatures), and highmethanol consumption (e.g., high methanol/crude-oil molar ratioof 40/1) [2,61].

There are a variety ofstudies have been conducted for trans-esterification reaction for different catalysts, alcohols and molarratios at different temperatures. The summary of these studiesis given in Table 10. Table 11 presents a comparison between

Table 11

Comparisons between chemically catalytic processes and supercritical methanolmethod forbiodiesel production from vegetable oils by transesterification [2,101].

Alkali catalyticmethod

Acid catalyticmethod

Supercriticalmethod

Reaction temperature (K) 303338 338 523573Reaction pressure (MPa) 0.1 0.1 1025Reaction time (min) 60360 4140 715Methyl ester yield (wt%) 96 90 98Removal of Methanol,

catalystMethanol,catalyst

Methanol

Purification Glycerol, soaps GlycerolFree fatty acids Saponified

productsMethyl esters,water

Methyl esters,water

Process Complicated Complicated SimpleYield Normal Normal High


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chemically catalyzed processes and a non-catalytic supercriticalmethanol method for biodiesel production.

5.4.1. Variables affecting the transesterification reaction

Transesterification reaction is affected by various param-eters depending upon the reaction conditions. The reactionis either incomplete or the yield is reduced to a significantextent if the parameters are not optimized. Each parameter is

equally important to achieve a high quality biodiesel whichmeets the regulatory standards. The most important parametersthat affect the transesterification process are mentioned below[2,8,29,51,68,75,89,91,93,96,99,102109]:

1. Free fatty acids, moisture and water content.2. Type of alcohol and molar ratio employed.3. Type and concentration of catalysts.4. Reaction temperature and time.5. Rate and mode of stirring.6. Purification of the final product.7. Mixing intensity.8. Effect of using organic co-solvents.9. Specific gravity.

6. Properties and qualities of biodiesel

The advancements of biodiesel quality are being developedglobally to maintain the quality of the end product and to ensurebetter criteria of biodiesel storage and feedstock for consumersconfidence and successful commercialization of biodiesel. Sincebiodiesel is produced from quite differently scaled plants of vary-ing origins and qualities, it is necessary to install a standardizationof fuel quality to guarantee an engine performance without anydifficulties [29,46]. Austria was the first country in the world todefine andapprove the standardsfor rapeseed oilmethyl estersas adiesel fuel. The guidelines forstandards and the quality of biodiesel

have also been defined in other countries such as in Germany,Italy, France, the Czech Republic and the United States [99]. Cur-rently, the properties and qualities ofbiodiesel must adhere withthe international biodiesel standard specifications. These specifica-tions include the American Standards for Testing Materials (ASTM6751-3) or the European Union (EN 14214) Standards for biodieselfuel [35]. However, there are some other standards available glob-ally such as in Germany (DIN 51606), Austria (ON) and Czechrepublic (CSN) [46]. The properties ofbiodiesel are characterizedby physicochemical properties. Some of these properties include;caloric value (MJ/kg), cetane number, density (kg/m3), viscosity(mm2/s), cloud and pour points (C), flash point (C), acid value(mg KOH/g-oil), ash content (%), copper corrosion, carbon residue,water content and sediment, distillation range, sulfur content,

glycerine (% m/m), phosphorus (mg/kg) and oxidation stability.The physical and chemical fuel properties of biodiesel basicallydepend on the type of feedstock and their fatty acids composition[35,41,46,50,79,108,110114]. Table12 shows thegeneral parame-ters forthe quality of biodiesel in different countries. A summary ofphysicochemical properties ofdiesel and biodiesel produced fromdifferent feedstocks, ASTM 6751-3 and EN 14214 specifications areshown in Table 13.

The following section gives explanations ofsome general prop-erties of biodiesel.

6.1. Kinematic viscosity

Viscosity is the most important property ofany fuel as it indi-

cates the ability of a material to flow. It therefore affects the

operation ofthe fuel injection equipment and sprayaoutomization,particularly at low temperatures when the increase in viscosityaffects the fluidityofthe fuel. The kinematic viscosityofbiodiesel is1015 times greater than that of diesel fossil fuels. This is becauseofits large molecular mass and large chemical structure. In somecases at low temperatures biodiesel can becomes very viscousor even solidified. Some literatures thought that higher viscos-ity of biodiesel can affect the volume flow and injection spraycharacteristics in the engine. At low temperature it may evencompromise the mechanical integrity of the injection pump drivesystems. The maximum allowable limit according to ASTM D445ranges are (1.96.0 mm2/s) and (3.55.0mm2/s) in EN ISO 3104[5,29,41,53,87,108,112,116120].

6.2. Density and relative density

Density is the weight per unit volume. Oils that are densercontain more energy [111]. Density is measured according to ENISO 3675/12185 and ASTM D1298. Following this standard, den-sity should be tested at the temperature reference of 15 or 20C[113,120].

Relative density is the density of the component compared to

the density ofwater. The relative density of biodiesel is neededto make mass to volume conversions, calculate flow and viscosityproperties, and is used tojudge the homogeneity ofbiodiesel tanks[121].

6.3. Flash point (FP)

Flash point ofa fuel is the temperature at which it will ignitewhen exposed to a flame or a spark. Flash point varies inverselywith the fuels volatility. The flash point ofbiodiesel is higher thanthe prescribed limit ofdiesel fossil fuel, which is safe for transport,handling and storage purpose [35,99,108,111]. Usually biodieselhas a flash point more than 150 C, while conventional diesel fuelhas a flash point of 5566 C [121]. Demirbas [122] stated that theflash point values of fattyacid methyl esters are significantly lowerthan those ofvegetable oils. The limit offlash point ranges in ASTMD93 is 93 C andin EN ISO 3679 is 120 C [41,116,120,123].

6.4. Cloud point (CP) pour point (PP) and cold filter plugging

point (CFPP)

The behavior ofbiodiesel at low temperature is an importantquality criterion. This is because partial or full solidification ofthefuel may cause blockage of the fuel lines and filters, leading to fuelstarvation, problems ofstarting, driving and engine damage dueto inadequate lubrication. The cloud point is the temperature atwhich wax crystals first becomes visible when the fuel is cooled.

Pour point is the temperature at which the amount ofwax out ofsolution is sufficientto gelthe fuel,thus it is thelowest temperatureat which the fuel can flow. Cloud and pour points are measuredusing ASTM D2500 EN ISO 23015 and D97 procedures. Generally,biodiesel has higher CP and PP compared to conventional diesel[37,41,116,120,121,124].

Cold filter plugging point (CFPP) refers to the temperature atwhich thetestfilter starts to plug due to fuel componentsthat havestarted to gel or crystallize. It is a commonly used as indicator oflowtemperature operability of fuels and reflectstheir cold weatherperformance. At low operating temperature fuel may thicken andmight not flow properly affecting the performanceoffuel lines, fuelpumps and injectors. CFPP defines the fuels limit of filterability,having a better correlation than cloud point for biodiesel as well asdiesel. CFPP is measured using ASTM D6371 [41,99,108,121,125].


13/24


Table 12

General parameters of thequality ofbiodiesel [68,93,115].

Parameters Austria (ON) Czech republic(CSN)

India ( BIS) France ( generalofficial)

Germany (DIN) Italy (UNI) USA (ASTM)

Density at 15Cg/cm3 0.850.89 0.870.89 0.870.89 0.870.89 0.8750.89 0.860.90 Viscosity at 40mm2/s 3.55.0 3.55.0 3.55.0 3.55.0 3.55.0 1.96.0Flash point (C) 100 110 100 110 100 130Pour point (C) 10 0/5 Cetane number 49 48 49 49 47

Conradson carbon residue (%) 0.05 0.05 0.05 0.05Iodine number 120 115 115 Methanol/ethanol (mass%) 0.2 0.1 0.3 0.2 Ester content (mass%) 96.5 98 Monoglyceride (mass%) 0.8 0.8 0.8 Diglyceride (mass%) 0.2 0.4 0.2 Triglyceride (mass%) 0.2 0.4 0.1 Free glyceride (mass%) 0.02 0.02 0.02 0.02 0.05 0.02Total glycerol (mass%) 0.24 0.24 0.25 0.25 0.24

6.5. Titer

Titer is thetemperature at which oilchangesfromsolidtoliquid.Titer is important because the transesterification process is fun-damentally a liquid process, and oils with high titer may require

heating, which increase the energy requirements and productioncosts for a biodiesel plant [45].

6.6. Cetane number (CN)

The cetane number (CN) is the indication of ignition character-istics or ability of fuel to auto-ignite quickly after being injected.Better ignition quality ofthe fuel is always associated with higherCN value. It is one of themost important parameters, which is con-sidered during the selection procedure of methyl esters for usingas biodiesel [29,53,99,108,116,126]. Cetane number increases withincreasing chain length of fatty acids and increasing saturation.A higher CN indicates shorter time between the ignition and theinitiation of fuel injection into the combustion chamber [45,127].

Biodiesel has higher cetane number than conventional diesel fuel,which results in higher combustion efficiency [124,127]. The CNof diesel, specified by ASTM D613 is 47min and EN ISO 5165 is51.0min [15,120,127].

6.7. Oxidation stability (OS)

The oxidation ofbiodiesel fuel is one ofthe major factors thathelps assess the quality of biodiesel. Oxidation stability is an indi-cation of the degree ofoxidation, potential reactivity with air, andcan determine the need for antioxidants. Oxidation occurs due tothe presence of unsaturated fatty acid chains and the double bondin the parent molecule, which immediately react with the oxygenas soon as it is being exposed to air [35,121]. The chemical com-

position of biodiesel fuels makes it more susceptible to oxidativedegradation than fossil diesel fuel [41]. The Rancimat method (ENISO 14112) is listed as the oxidative stability specification in ASTMD6751 and EN 14214. A minimum IP (110C) of 3 h is required forASTM D6751, whereas a more stringent limit of6h or greater isspecified in EN 14214 [125].

6.8. Lubrication properties

Atadashi et al. [35] stated that the lubrication properties ofthe biodiesel are better than diesel, which can help to increasethe engine life. Lapuerta et al. [127] reported that fatty acid alkylesters (biodiesel) have improved lubrication characteristics, butthey can contribute to the formation of deposits, plugging of

filters, depending mainly on degradability, glycerol (and other

impurities) content, cold flow properties, etc. Demirbas [110]stated that biodiesel provides significant lubricity improvementover diesel fuel. Xue et al. [119] shows that high lubricity ofbiodiesel might result in the reduced friction loss and thus improvethe brake effective power.

6.9. Acid value

Acid number or neutralization number is a measure of freefatty acids contained in a fresh fuel sample. Free fatty acids (FFAs)are the saturated or unsaturated monocarboxylic acids that occurnaturally in fats, oils or greases but are not attached to glycerolbackbones. Fatty acids vary in carbon chain length and in thenumber ofunsaturated bonds (double bonds). The structures ofcommon fatty acids are given in Table 14. Higher amount offreefatty acids leads to higher acid value. Acid value is expressed asmg KOH required for neutrlizing 1 g ofFAME. Higher acid contentcan cause severe corrosion in fuel supply system ofan engine. Theacid value is determined using the ASTM D664 and EN 14104.Both

standards approved a maximum acid value for biodiesel of0.50mgKOH/g [8,41,79,89,116,128]. Fig. 12 shows fatty acid profile of someselected biodiesel feedstocks.

6.10. Heating value, heat of combustion

Heating value, heat ofcombustion is the amount of heatingenergy released by the combustion of a unit value offuels. Oneof the most important determinants ofheating value is the mois-ture content of the feedstock oil [45,111]. The heating value is notspecified in the biodiesel standards ASTM D6751 and EN 14214 butis prescribed in EN 14213 (biodiesel for heating purpose) with aminimum of 35MJ/kg [128].

6.11. Free glycerin

Free glycerol refers to the amount of glycerol that is left inthe finished biodiesel. The content of free glycerol in biodiesel isdependent on the production process. The high yield ofglycerolin biodiesel may be resulted from insufficient separation duringwashing of the ester product. Glycerol is essentially insoluble inbiodiesel so almost all of the glycerol is easily removed by settlingor centrifugation. Free glycerol may remain either as suspendeddroplets or as the very small amount that is dissolved in thebiodiesel. High free glycerol can cause injector coking and dam-age to the fuel injection. The ASTM specification requires that thetotal glycerol be less than 0.24% ofthe final biodiesel product asmeasured using a gas chromatographic method described in ASTMD6584 and EN 14105/14106 has limit max. 0.02% [41,116,131,132].


14/24


Table 13

Properties and qualities of biodiesel produced from different feedstocksin comparison with diesel [5,9,10,26,29,35,40,44,45,47,51,53,56,59,65,78,89,94,99,101,108,110,112,114,119,121,125129,132134,136178] .

Fuel properties Diesel fuel Biodiesel Test method

ASTM D975 ASTM D6751 DIN 14214 ASTM DIN

Density 15 C (kg/m3) 850 880 860900 D1298 EN ISO 3675/12185Viscosity at 40C (cSt) 2.6 1.96.0 3.55.0 D-445 EN ISO 3104Cetane number 4055 Min. 47 Min. 51 D-613 EN ISO 5165Iodine number 38.3 Max. 120 EN 14111Calorific value (MJ/kg) 4246 35 EN 14214Acid (neutralization) value (mg KOH/g) 0.062 Max.0.50 Max.0.5 D-664 EN 14104Pour point (C) 35 15 to 16 D-97 Flash point (C) 6080 Min. 100170 >120 D-93 ISO DIS 3679Cloud point (C) 20 3 to 12 D-2500 Cold filter plugging point (C) 25 19 Max. +5 D-6371 EN 14214Copper strip corrosion (3h at 50C) 1 Max. 3 Min.1 D-130 EN ISO 2160Carbon (wt%) 8487 77 Hydrogen (wt%) 1216 12 Oxygen (wt%) 00.31 11 Methanol content % (m/m) Max. 0.20 EN 14110Water and sediment content (vol%) 0.05 Max. 0.05 Max. 500b D2709 EN ISO 12937Ash content % (w/w) 0.01 0.02 0.02 EN 14214Sulfur % (m/m) 0.05 Max. 0.05 10b D 5453 EN ISO 20846Sulfated ash% (m/m) Max. 0.02 Max. 0.02 D-874 EN ISO 3987Phosphorus content Max. 0.001 10b D-4951 EN 14107Free glycerin % (m/m) Max. 0.02 Max.0.02 D-6584 EN 14105/14106

Total glycerin % (m/m) Max. 0.24 0.25 D-6584 EN 14105Monoglyceride % (m/m) 0.52 0.8 EN 14105Diglyceride % (m/m) 0.2 EN 14105Triglyceride % (m/m) 0.2 EN 14105CCR100% (mass%) 0.17 (0.1)d Max. 0.05 Max. 0.03 D-4530 EN ISO10370Distillation temperature (%) Max. 360 C D-1160 Oxidation stability (h, 110C) 3 min 6 min D-675 EN 14112Lubricity (HFRR;m) 685 314

Fuel properties JatrophaFAME

Callophylum

inaphyllum

FAME

MadhucaFAME

MesuaFAME

RubberFAME

Camelina sativa

FAMECanola sativa

FAMEIdesia polycarpa

var. vestitafruit FAME

Density 15 C (kg/m3) 879.5 888.6 874 898 886.2Viscosity at 40C (cSt) 4.8g 7.724 3.98 6.2 5.81 4.15 4.42 4.12Cetane number 51.6 51.9 65 54 52.8 47Iodine number 104 85 Calorific value (MJ/kg) 39.23 36.8 42.23 36.5 Acid (Neutralization)

value (mg KOH/g)0.4 0.76 0.41 0.01 0.31 0.01 0.27

Pour point (C) 2 6 3 8 4 9 Flash point (C) 135 151 208 112 130 >160 >160 >174Cloud point (C) 2.7 38 4 3 3.3 4Cold filter plugging

point (C)0 3 7 2

Copper strip corrosion(3hat50 C)

1a 1b 1a 1a 1a

Carbon (wt%) Hydrogen (wt%) Oxygen (wt%) Methanol content %

(m/m)

Water and sedimentcontent (vol%)


15/24


Table 13 (Continued )

Fuel properties Peanut (Arachishypogea L.)FAME

Coriander(Coriandrumsativum L.)FAME

MacluraFAME

Okra (Hibiscusesculentus)FAME

Terminalia

catappa L.FAME

Terminalia

belerica robxFAME

Pumpkin(Cucurbita pepoL.) FAME

Tung (Verniciafordii) FAME

Density 15 C (kg/m3) 0.8485h 0.889h 87614.9 873f 882.8 883.7 901f

Viscosity at 40C (cSt) 4.42 4.21 4.66 4.010.10 4.3 5.17 4.41 7.07Cetane number 53.59 53.3 48 55.22.00 53 Iodine number 67.45 89 125 83.2 115

Calorific value (MJ/kg) 40.1 36.97 39.22 38.08 Acid (neutralization)value (mg KOH/g)

0.28 0.10 0.4 0.390.05 0.23 0.48 0.11

Pour point (C) 8 19 9 2.000.12 6 Flash point (C) 166 180 1563.80 90 >120 167Cloud point (C) 0 5 1.000.10 Cold filter plugging

point (C) 15 1.000.10 9 19

Copper strip corrosion(3hat50 C)

1a 1a

Carbon (wt%) 62.1 Hydrogen (wt%) Oxygen (wt%) Methanol content %

(m/m) 0.1810.004

Water and sedimentcontent (vol%)

0.126b 490k

Ashcontent % (w/w) 0.0100.001 Sulfur% (m/m) 0 4a 0.0120.001 96a 2k 0.002Sulfated ash % (m/m) Phosphorus content 0 3k Free glycerin % (m/m) 0.005 0.0120.001 0 Total glycerin % (m/m) 0.119 0.2200.020 0.16 Monoglyceride % (m/m) 0.380.05 0.47 Diglyceride % (m/m) 0.130.02 0.15 Triglyceride % (m/m) 0.070.01 0.14 0CCR100% (mass%) 0.0085 Oxidation stability (h,

110 C) 14.6 1.710.15

Lubricity (HFRR;m) 1383.50

Fuel properties Perilla seedFAME

StillingiaFAME

Poultry fatFAME

Choice whitegrease FAME

HempFAME

Hepar, highIV FAME

Hepar, lowIV FAME

Corn,distillersFAME

Density 15

C (kg/m3

) 0.900h

Viscosity at 40C (cSt) 3.937g 4.81g 4.496g 4.536g 3.874g 4.422g 4.643g 4.382g

Cetane number 50 Iodine number Calorific value (MJ/kg) Acid (neutralization) value (mg KOH/g) 0.293 0.007 0.044 0.021 0.097 0.062 0.165 0.283Pour point (C) Flash point (C) >160 137 >160 >160 >160 >160 >160 >160Cloud point (C) 8.5 13 6.1 7 1.3 16 6.7 2.8Cold filter plugging point (C) 11 10 2 6 6 13 6 3Copper strip corrosion (3h at 50 C) 1a 1a 1a 1a 1a 1a 1aCarbon (wt%) Hydrogen (wt%) Oxygen (wt%) Methanol content % (m/m) Water and sediment content (vol%)


16/24



Fuel p roperties RapeseedFAME

SunflowerFAME

TobaccoFAME

SafflowerFAME

Cynara

cardunculus L.FAME

Rice branFAME

Microalgal 1FAME

Vernicia foriiFAME

Density 15 C (kg/m3) 882 880.0 888.5 888.5 0.889h 0.872c,h 864Viscosity at 40C (cSt) 4.439g 4.439 4.23g 5.8g 5.101g 4.81lg,i 4.519 2.5Cetane number 54.4 49 51.6 56 59 51.6 53Iodine number 136 117 Calorific value (MJ/kg) 37 38.122 41.38

Acid (neutralization) value(mg KOH/g) 0.027 0.3 0.48 0.022 0.19

Pour point (C) 12 269j Flash point (C) 170 >160 165.4 148 182 430j >160 185Cloud point (C) 3.3 3.4 5 4 5.2 Cold filter plugging point

(C)13 3 5 10 7

Copper strip corrosion (3hat50 C)

1a 1a 1a

Carbon (wt%) 81 59.5 Hydrogen (wt%) 12 Oxygen (wt%) 7 Methanol content % (m/m)


17/24



Fuel properties Pongamia(karanja)FAME

PalmFAME

SoybeanFAME

NeemFAME

Hibiscus

sabdariffa

LFAME(Rossele)

Microalgal 2FAME

Beef tallowFAME

BorageFAME

EveningprimroseFAME

Calorific value (MJ/kg) 43.42 39.76 39.81 Acid (neutralization) value (mg KOH/g) 0.42 0.24 0.266 0.649 0.43 0.003 0.147 0.138 0.37Pour point (C) 3 15 2 1 Flash point (C) 95 135 254 76 >130 >160 >160 >160 >160

Cloud point (

C) 7 16 0.9 9 3.9 16

1.3

7.5Cold filter plugging point (C) 12 4 11 2 14 4 10Copper strip corrosion (3h at 50 C) 1a 1a 1b 1a 1a 1a 1a 1aCarbon (wt%) Hydrogen (wt%) Oxygen (wt%) Methanol content % (m/m) Water and sediment content (vol%)


18/24


0

10

20

30

40

50

60

70

80

90

100

Moringa

oleifera

Used fryingCoconutPeanutSunflowerSoybeanRapeseedPalm oil

oil

Tallow

%

Other

C22:1

C22:0

C20:1

C20:0

C18:3

C18:2

C18:1

C18:0

C16:0

C14:0

C12:0

Fig. 12. Fatty acidprofile ofsome biodiesel feedstocks [44,46,130].

Table 14

The chemical structures ofcommon fatty acids [38,46,51,75,89,129] .

Fatty acid Structure Systematic

name

Chemical structure

Lauric (12:0) Dodecanoic CH3(CH2)10COOHMyristic (14:0) Tetradecanoic CH3(CH2)12COOHPalmitic (16:0) Hexadecanoic CH3(CH2)14COOHStearic (18:0) Octadecanoic CH3(CH2)16COOHOleic (18:1) cis-9-

OctadecenoicCH3(CH2)7CH CH(CH2)7COOH

Linoleic (18:2) cis-9-cis-12-Octadecadienoic

CH3(CH2)4CH CHCH2CH CH(CH2)7COOH

Linolenic (18:3) cis-9-cis-12 CH3CH2CH CHCH2CH CHCH2CH CH(CH2)7COOH

Arachidic (20:0) Eicosanoic CH3(CH2)18COOHBehenic (22:0) Docosanoic CH3(CH2)20COOHErucic (22:1) cis-13-

DocosenoicCH3(CH2)7CH CH(CH2)11COOH

Lignoceric (24:0) Tetracosanoic CH3(CH2)22COOH

6.12. Total glycerol

Total glycerin is a measurement of how much triglycerideremains unconvertedinto methylesters. Totalglycerinis calculatedfromthe amount of free glycerin,monoglycerides,diglycerides, andtriglycerides [133]. The reactions are shown below:

Each reaction step produces a molecule of a methyl ester ofa fatty acid. If the reaction is incomplete, then there will betriglycerides, diglycerides, and monoglycerides left in the reactionmixture. Eachofthese compoundsstillcontainsa glycerol moleculethathasnotbeenreleased.Theglycerolportionofthesecompoundsis referred to as bound glycerol. When the bound glycerol is addedtothefreeglycerol,thesumisknownasthetotalglycerol.TheASTMspecificationrequires that thetotalglycerol beless than 0.24% ofthefinal biodiesel product as measured using a gas chromatographicmethod described in ASTM D 6584 and 0.25% in EN 14105. Fuelsthat do notmeet these specifications areproneto coking; thus, maycause the formation of deposits on the injector nozzles, pistons andvalves [41,116,131].

6.13. Water content and sediment

Water and sediment contamination are basically housekeeping

issues for biodiesel. Water can present in two forms, either asdissolved water or as suspended water droplets. While biodieselis generally considered to be insoluble in water, it actually takesup considerably more water than diesel fuel. Biodiesel can containas much as 1500ppm ofdissolved water while diesel fuel usuallyonly takes up about 50ppm. Sediment may consist ofsuspendedrust and dirt particles or it may originate from the fuel as insolublecompounds formed during fuel oxidation [116,131,132]. Waterand sediment testing is done using 100 mLof biodiesel and cen-trifuging it at 1870 rpm for 11min. Ifthe water and sediment levelis below 0.005 vol% (vol),the resultis reported as


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Table 15

Known problems,probable cause and potential solutionsfor using straight vegetable oil in diesel engines [58,81,89,91,179].

Problem Probable cause Potential solution

Short-term

1. Cold weather starting High viscosity, lowcetane number and lowflash point of vegetable oils

Preheat fuel prior to injection

2. Plugging and gumming offilters, lines and injectors Natural gums (phosphatides) in vegetable oil.Other ash

Refinethe oil partially to removegums

3. Engine knocking Low cetane numbers. Improper injection

timing

Adjust injection timing

Use higher compression engines. Preheat fuelprior to injection. Chemically alter fuel to anester

Longterm

4. Cooking of injector on piston and enginehead

High viscosity ofvegetable oil Heat fuel prior to injection Incomplete combustion offuel Switch engine to diesel fuel when operation

at part loads Poor combustion at part load with vegetableoils

Chemically alter thevegetable oil to an ester

5. Carbondeposits on pistonand head ofengine

High viscosity ofvegetable oil Heat fuel prior to injection. Switch enginetodiesel fuel when operation at part loads.Chemically alter thevegetable oil to an ester

Incomplete combustion offuel Poor combustion at part load with vegetableoils

6. Excessive engine wear

High viscosity ofvegetable oil, incompletecombustion offuel. Poor combustion at partload with vegetableoils. Possibly free fattyacids in vegetableoil. Diulution of enginelubricating oil dueto blow-by of vegetableoil

Heat fuel prior to injection Switch engine to diesel fuel when operationat part loads Chemically alter thevegetable oil to an ester.Increase motor oil changes Motor oil additivesto inhibit oxidation

7. Failure ofenginelubricatingoil due topolymerization

Collection ofpolyunsaturated vegetable oilblow-by in crankcaseto thepointwherepolymerization occurs

Heat fuel prior to injection Switch engine to diesel fuel when operationat part loads Chemically alter thevegetable oil to an ester Increase motor oil changes Motor oil additivesto inhibit oxidation

the percentage of sulfated ash present in the biodiesel. The ASTMD874 standard mentions that the samples can have a maximum0.02% of sulfated ash [116,121].

6.15. Carbon residue

Carbon residue of the fuel is indicative of carbon deposit-ing tendencies ofthe fuel after combustion. Canradsons carbonresidue for biodiesel is more important than that in diesel fuelbecause it shows a high correlation with presence of free fattyacids, glycerides, soaps, polymers, higher unsaturated fatty acidsand inorganic impurities. Although this residue is not solely com-posed of carbon, the term carbon residue is found in all standardsbecause it has long been commonly used. The range oflimit stan-dard ASTM D4530 is max. 0.050% (m/m) and EN ISO10370 is max.0.30% (m/m) [41,99,108,116,121].

6.16. Copper strip corrosion

The copper corrosion test measures the corrosion tendency of

fuel when used with copper, brass, or bronze parts. A copper stripis heated to 50 C in a fuel bath for 3h followed by comparisonwith a standard strips to determine the degree ofcorrosion. Cor-rosion resulting from biodiesel might be induced by some sulfurcompounds by acids; hence this parameter is correlated with acidnumber. The ASTM D130 standard mentions that the samples canhave class 3 and EN ISO 2160 has class 1 [41,116,121].

6.17. Cold soak filtration

Cold soak filtration is the newest biodiesel requirement set inASTM D6751. The cold soak filtration test is done to determine ifcrystals form at low temperatures and do not redissolve when thebiodiesel returns to a higher temperature. The ASTM D6751 proce-

dureinvolveschilling300mLofbiodieselfor16hat40

F, removing

the sample and letting thesample warm back up to room tempera-ture.Whenthesamplehaswarmedbackup to2022 C,itis filteredthrough a 0.7m filter paper. The sample is timed as it passesthrough the filter paper and when all 300 mLpasses through thepaper, the result is reported (in seconds). The maximum allowabletest result for cold soak filtration is 360s [121].

6.18. Visual inspection

The visual inspection test is used to determine the presence ofwater and particulates in biodiesel. It is measured as a haze valuebyplacinga line chart behind a clearjarofbiodiesel and referencinghow the lines compare to six different pictures with haze ratingsfrom1to6,with1beingtheleastamountofparticulatesand6beingthe highest. A haze rating of 1 is the clearest; while a haze ratingof 6 means that the biodiesel is very cloudy. Visual inspection ofbiodiesel is determined by ASTM D4176, Standard Test. Method forFree Waterand ParticulateContamination in Distillate Fuels(VisualInspection Procedures), Procedure 2 [121].

6.19. Phosphorous, calcium, andmagnesium

The specifications from ASTM D6751 state that phosphorouscontent in biodiesel must be less than 10ppm, and calcium andmagnesium combined must be less than 5 ppm. Phosphorous wasdetermined using ASTM D4951, calcium and magnesium weredetermined using EN Standard 14538 [121].

6.20. Moisture contents

Moisture is the amount of water which cannot be convertedto biodiesel. Moisture can react with the catalyst during transes-terification which can lead to soap formation and emulsions. Themoisture in the biodiesels is measured in accordance with ASTM

E203 standard test method for water (up to 1500 ppm). In Europe,


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Table 16

Advantages and disadvantages ofbiodiesel.

Reference

Advantagesof biodiesel

Biodiesel has1011% ofoxygen; this makes biodiesel a fuel with high combustion characteristics [29,35,53,64,75,76,79,114,122,126,149,154,157,176]Biodiesel reduces netcarbon-dioxide emissionsby 78% on a lifecycle basis when compared to

conventional diesel fuel and reduces smoke dueto free soot[35,131,154]

Biodiesel is renewable, non-toxic, non-flammable, portable, readily available, biodegradable,sustainable,eco-friendly and free from sulfur and aromatic content, this makes it an ideal fuel

for heavily polluted cities. Biodiesel also reduces particular matter content in theambient airand hence reduces air toxicity. It provides a 90%reduction in cancer risks and neonatal defectsdueto its less polluting combustion

[2,10,27,29,35,53,58,64,76,115,123,154,179182]

Biodiesel helps rural development to restore degraded lands over a period. Moreover, it hasgoodpotential for rural employment generation

[76]

Biodiesel serves as climatic neutral in view oftheclimatic changethat is presently an importantelement ofenergy use and development

[29,35,76,108,125,149,154,157,176]

Biodiesel hashigher cetanenumber (about6065 dependingon the vegetable oil) than petroleumdiesel (53) which reduces theignitiondelay

[2,27,29,35,41,53,58,65,122,126,149,176]

Production can be raised easily and is less time consumingNo need for drilling, transportation, or refining like petroleum diesel. Therefore, each country has

the ability to produce biodiesel as a locally produced fuel. Moreover, there is no need to paytariffs or similar taxes to thecountries from which oil and petroleum dieselis imported

[27,79,93,176]

Biodiesel has superior better lubricity properties. This improves lubrication in fuel pumps andinjector units, which decreases engine wear, tear and increases engine efficiency

[29,58,79,122,126,149,162,179]

Biodiesel is safe for transportation, handling, distribution, utilization and storage due to its higherflash point (above 100170C) than petroleum diesel (6080 C)

[2,27,29,53,122,123,149,154,179,181]

Biodiesel reduces the environmental effect ofa waste product and can bemadeout ofusedcooking oils and lards

[79,93,128,154,176]

Biodiesel may not require engine modification up to B20. However, higher blendsmay need someminor modification

[93]

Disadvantagesof biodiesel

Biodiesel has12% lower energy content than diesel, this leads to an increase in fuel consumptionof about 210%. Moreover, biodiesel hashigher cloud point and pour point,higher nitrogenoxide emissions than diesel. It haslower volatilities that cause theformation ofdeposits inengines dueto incomplete combustion characteristics

[2,29,47,53,58,79,93,118,131,134,180]

Biodiesel causes excessive carbon deposition and gum formation (polymerization) in engines andthe oil gets contaminatedand suffers from flowproblem. It hasrelatively higherviscosity(1118 times diesel) and lower volatility than diesel andthus needs higherinjectorpressure

[31,176]

Oxidation stability ofbiodiesel is lower than that ofdiesel. Itcan beoxidized into fatty acidsin thepresence of air and causescorrosion offuel tank, pipe and injector

[108,176]

Due to thehigh oxygen content in biodiesel, advance in fuel injection and timingand earlier startofcombustion, biodiesel produces relatively higher NOx levels than diesel in the range of1014% during combustion

[27,53,93,99,108,111,113,119,135,149,182]

Biodiesel can cause corrosion in vehicle material (cooper and brass)such as fuel systemblockage,

seal failures, filter clogging and deposits at injection pumps

[79,93,108]

Use of biodiesel in internal combustion engine may lead to engine durability problems includinginjector cocking, filter plugging and pistonring sticking,etc.

[2,28]

As more than 95% of biodieselis made from edibleoil, there have been many claimsthat this maygive rise to further economic problems.By converting edibleoils into biodiesel, food resourcesare being used as automotive fuels. It is believed that large-scale production ofbiodiesel fromedibleoils may bring about a global imbalancein the food supply-and-demand market

[58]

Lower engine speed andpower, high price, high engine wear, engine compatibility [58]Transesterification process is expensive (cost offuel increases), these oils require expensive fatty

acid separation or useofless effective (or expensive acid catalysts)[108,112,118,124]

The transesterification has some environmental effects such as waste disposal and waterrequirement for washing, soap formation, etc.

[108,117]

standard EN 14214 has a Karl Fischer moisture specification of0.050 wt% maximum tried [45,108,121].

7. Problems and potential solutions of using vegetable oils

The direct use of vegetable oils or blends has generally beenconsidered to be impractical for both direct and indirect dieselengines. The high viscosity, low volatility, acid composition, freefatty acid and moisture content, gum formation due to oxida-tion and polymerization during storage and combustion, poor coldengine start-up, misfire, ignition delay, incomplete combustion,carbon deposition around the nozzle orifice, ring sticking, injectorchoking in engineand lubricating oilthickening arethe majorprob-lems of using vegetable oils. In general, the problems associatedwith using straight vegetable oil in diesel engines areclassified intoshort term and long term. Table 15 highlights the problems, proba-ble causes and the potential solutions [8,43,53,58,80,81,88,89,99].

8. Advantages anddisadvantages of biodiesel

Table 16 gives a summary ofthe advantages and disadvantagesof biodiesel.

9. Economical viability of biodiesel

Biodiesel is an attractive renewable energy resource. However,there are some challenges that face this vital resource. These chal-lenges include the high cost and limited availability of biodieselfeedstock beside the cheaper prices ofcrude petroleum. There arevarious factors contributing to the cost ofbiodiesel. These factorsinclude feedstock prices, plants capacity, feedstock quality, pro-cessing technology, net energy balance nature ofpurification andits storage, etc. However, the two main factors are the costs of feedstocks and the cost of processing into biodiesel. It has been

found that the cost of feedstocks accounts for 75% of the total cost


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of biodiesel fuel. Therefore, selecting the best feedstock is vitalto ensure low biodiesel cost. It has been found that, consideringnon-edible oils as a feedstock for biodiesel can reduce this cost.In terms of production cost, there also are two aspects, the trans-esterification process cost and by-the product (glycerol) recovery.The continuous transesterification process is one choice to lowerthe production cost. The foundations ofthis process are a shorterreaction timeand greater production capacity.The recovery ofhighquality glycerol is another way to lower biodiesel production cost.Therefore,biodiesel plantmust haveits own glycerol recovery facil-ity [9,21,29,53,58,75,76,108].

AccordingtoIEA [183], biodiesel produced withcurrent technol-ogy in OECD countries are still two to three times more expensivethan gasoline and diesel. Moreover, a review of many economicfeasibility studies around the world shows that biodiesel usuallycosts over US$0.5 per liter compared to US $0.35 per liter for nor-mal diesel. Currently biodiesel cost is 1.53 times higher than thefossil diesel cost in developed countries. Biodiesel is thus currentlynot economically feasible, and more research and technologicaldevelopment are needed. Thus supporting policies such as taxcredits are important to promote bio-diesel research and maketheir prices competitive with other conventional sources ofenergy[53,58,75,89,122,130,179,184].

10. Future of biodiesel

Acceptance of Kyoto protocol and clean development mech-anism (CDM) will lead to more biodiesel production around theworld. For instance, it is anticipated that this policy will lead to atotal bio-fuel demand in EU of around 19.5 and 30.3 million tonsin 2012 and 2020 respectively. Biodiesel production is expand-ing rapidly around the world, driven by energy security and otherenvironmental concerns. Given geographic disparities betweendemand and supply potential, and supply cost, expanded trade inbiodiesel appears to make sense. Global potential in biodiesel pro-duction is very unclear, but in the long run it could be a substantial

percentage of transport fuel demand. Currently, biodiesel can bemore effective ifused as a complement to other energy sources.With theincrease in globalhumanpopulation,more land will be

needed to produce food for human consumption. Thus, the insuffi-cient lands could increase the production cost ofbiodiesel plants.This problem already exists in Asia where vegetable oil prices arerelatively high. The same trend will eventually happen in the restof the world. This is the potential challenge to biodiesel produc-tion. Therefore, non-edible oil, genetically engineered plants andmicroalgae feedstocks canbe propersolutionsfor this problem andcan ensure the sustainability of biodiesel production in the future[43,75,183,185].

11. Conclusion

Energy is an indispensable factor for human to preserve eco-nomic growth and maintain standard of living. Globally, thetransportation sector is the second largest energy consuming sec-tor after the industrial sector and accounts for 30% ofthe worldstotal delivered energy.This sector hasexperienced a steady growthin the past 30 years. It has been estimated that the global trans-portation energy use is expected to increase by an average of1.8%per year from 2005 to 2035. Nearly all fossil fuel energy consump-tion in the transportation sector is from oil (97.6%). However, theexpected depletion of fossil fuels and the environmental problemsassociated with burning them has encouraged many researchers toinvestigate the possibility ofusing alternative fuels. Among them,biodieselseems a very promisingresource. Thewide range of avail-

able feedstock for biodiesel production represents one of the most

important advantages of producing biodiesel. From the literature,it has been found that feedstock alone represents more than 75% ofthe overall biodiesel production cost. Therefore, selecting the bestfeedstock is vital to ensure low production cost of biodiesel.

From this overview, it can be concluded that searching forbiodiesel feedstocks should focus on those feedstocks that do notcompete with food crops, do not lead to land-clearing and providegreenhouse-gas reductions. Non-edible oils such as J. curcas andC. inophyllum, and more recently microalgae and genetically engi-neered plants such as poplar and switchgrass have emerged to bevery promising feedstocks for biodiesel production.

Oil extraction is the second step ofbiodiesel production. Thereare three main methods that have been identified for extraction ofthe oil,mechanical, solvent and enzymatic oil extraction.It hasbeenfound that the solvent extraction using n-hexane method results inthe highest oil yield which makes it the most common type.

Biodiesel properties must adhere with the internationalbiodiesel standard specifications such as the American Standardsfor Testing Materials (ASTM 6751-3) or the European Union (EN14214) for biodiesel fuel. These propertiescan be improved by fourmethods: pyrolysis, dilution with hydrocarbons blending, Micro-emulsion, and transesterification. Transesterification is regarded asthe best method among other approaches due to its low cost andsimplicity. The properties ofbiodiesel are characterized by physic-ochemical properties. In this paper, some ofthese properties suchas cetane number, density, viscosity, cloud and pour points, flashpoint,acid value, copper corrosion,glycerineand oxidationstabilityhave been presented.

Biodiesel is currently not economically feasible, and moreresearch and technological development are needed. As a recom-mendation, supporting policies are important to promote biodieselresearch and make their prices competitive with other conven-tional sources ofenergy.

Acknowledgment

The authors would like to acknowledge for the Min-istry of Higher Education of Malaysia and The University ofMalaya, Kuala Lumpur, Malaysia for the financial support underUM.C/HIR/MOHE/ENG/06 (D000006-16001).

References

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[3] U.S. Energy Information Administration. International energy outlook 2010.Available from: http://www.eia.doe.gov/oiaf/ieo/pdf/0484%282010%29.pdf;2010 [cited 22.02.11].

[4] An F. Global overview on vehicle fuel economy and emission standards.Availablefrom: http://www.un.org/esa/dsd/susdevtopics/sdt pdfs/meetings/egm0809/feng Global%20Overview%20UN%20NYC%20Aug-09.pdf; 2010[cited 15.01.11].

[5] Rabe ELM. Jatropha oilin compressionignitionengines, effects on theengine,environment and Tanzania as supplyingcountry. Eindhoven: EindhovenUni-versity of Technology; 2010.

[6] Atabani AE, Badruddin IA, Mekhilef S, Silitonga AS. A review on global fueleconomy standards, labels and technologies in the transportation sector.Renew Sustain Energy Rev 2011;15(9):4586610.

[7] British Petroleum. BP statistical reviewofworldenergy, June2010. Availablefrom: http://www.bp.com/productlanding.do?categoryId=6929&contentId=7044622; 2010 [cited 20.01.11].

[8] SharmaYC, Singh B. Developmentof biodiesel: current scenario.Renew Sus-tain EnergyRev 2009;13(67):164651.

[9] Ahmad AL, Mat Yasin NH, Derek CJC, Lim JK. Microalgae as a sustainableenergy source for biodiesel production: a review. Renew Sustain Energy Rev2011;15(1):58493.

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