La

b

c

d Schooe Interd

a r t

ARReceived in revised form 28 September 2010 tages. This review highlights some of the perspectives for the biodiesel industry to thrive as an alternative

and the conversion technologies of biodiesel industry. More specically, an overview is given on possibleenvironmental and social impacts associated with biodiesel production, such as food security, land

. . . . . .

. . . . . .develo

1. Introduction

Since the commencement of industrial revolution in the late18th and early 19th century, energy has become an indispensablefactor for mankind to preserve economic growth and maintainstandard of living. The most of global primary energy production

derives from fossil energy. As shown in Fig. 1, fossil fuels accountedfor 88% of the primary energy consumption, with oil (35% share),coal (29%) and natural gas (24%) as the major fuels, while nuclearenergy and hydroelectricity account for 5% and 6% of the total pri-mary energy consumption, respectively [1]. However, due to thelimited traditional fossil energy resources and increased environ-mental concerns, a requirement for alternative energy sourceshas been paid a great attention in recent years. Developing alterna-tive energy is an inevitable choice for sustainable economic growth

Corresponding authors. Tel.: +45 8942 3702 (L. Lin).

Applied Energy 88 (2011) 10201031

Contents lists availab

lseE-mail addresses: Lin780530@126.com (L. Lin), dong@inano.au.dk (D.Mingdong).2.2. Feedstocks of biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10222.3. Biodiesel conversion technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025

3. Key drivers and challenges of biodiesel industry development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10263.1. Security of energy supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10263.2. Environmental effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10273.3. Food security, land use changes and water source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1027

4. Policy and government incentives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10285. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1028

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1028References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1028Keywords:BiodieselEnergy securityEnvironmentFoodLand changePolicy

Contents

1. Introduction . . . . . . . . . . . . . . . . .2. Profile of biodiesel . . . . . . . . . . . .

2.1. Historical background and0306-2619/$ - see front matter 2010 Elsevier Ltd. Adoi:10.1016/j.apenergy.2010.09.029change and water source. Further emphasis is given on the need for governments incentives and publicawareness for the use and benets of biodiesel, while promoting policies that will not only endorse theindustry, but also promote effective land management.

2010 Elsevier Ltd. All rights reserved.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1020

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1021pment of biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1021Available online 30 October 2010fuel, while discussing opportunities and challenges of biodiesel. This review is divided in three parts. Firstoverview is given on developments of biodiesel in past and present, especially for the different feedstocksAccepted 28 September 2010rticle history:eceived 7 July 2010

Fossil fuel resources are decreasing daily. As a renewable energy, biodiesel has been receiving increasingattention because of the relevance it gains from the rising petroleum price and its environmental advan-i c l e i n f o a b s t r a c ty of Industrial Technology, Chiangrai Rajabhat University, Chiangrai 57100, Thailandl of Material Science & Engineering, Jiangsu University, Zhenjiang 212013, PR Chinaisciplinary Nanoscience Center, Aarhus University, Aarhus 8000, Denmarkin Lin a,, Zhou Cunshan b, Saritporn Vittayapadung c, Shen Xiangqian d, Dong Mingdong e,School of Food & Biological Engineering, Jiangsu University, Zhenjiang 212013, PR ChinaSchool of Agriculture and Food Science, Zhejiang A & F University, Linan 311300, PR ChinaFacultOpportunities and challenges for biodiesel fuelApplied

journal homepage: www.ell rights reserved.le at ScienceDirect

Energy

vier .com/ locate/apenergy

prod

L. Lin et al. / Applied Energy 88 (2011) 10201031 1021in human society. In addition, it is also important for the harmoni-ous coexistence of human and environment as well as for the sus-tainable development [2]. Considerable attention was focused onthe development of biofuel, with particular referring to the biodie-sel. Biodiesel offers a number of technical and environmental ben-ets over conventional fossil-based fuels. Especially, similaritiesbetween the combustion properties of biodiesel and fossil-baseddiesel have made the former one of the most promising alterna-tives to a renewable and sustainable fuel for the automobile. In re-cent years, the biodiesel industry developed rapidly. According tothe International Energy Agencys (IEA) report on biodiesel produc-tion that looked at the 21 leading biofuel producing countries, glo-bal biodiesel production has increased tenfold from 2000 to 2008and could be doubled to 21.8 bn liters by 2012 [3]. The motivationsfor governments to aggressively pursue biodiesel development arecomplex and multidimensional. The key driving force behind thegovernment policies to develop biodiesel can be divided into threepoints as follow: Firstly, with the global energy crisis approaching,biodiesel fuel will play a more important role in strengthening anations energy security. Secondly, as a renewable energy, biodieselis derived from plant materials which can contribute to the reduc-tion of greenhouse gas (GHG) emissions when replacing fossil oil ifthey are sustainably managed. Thirdly, the increased demand foroil crops for biodiesel production clearly has a positive effect onnet farm income and also reduces government outlays to farmersby raising the market price of oil crops.

However, biodiesel could be a double-edged sword. While wefocused on the advantages of biodiesel, the debates on biodiesel

Fig. 1. World primary energyhave been intensively discussed in both the scientic world andthe media. With the global increase in the scale of biodiesel pro-duction, biodiesel has become a systemic risk with respect to itseconomic, ecological, and socio-political impacts. Opportunities,challenges, and even threats have been raised. Especially those

Fig. 2. Transesterication of triacylglycerols tlinked to rst-generation biodiesel, have received considerablecriticisms recentlymost notably the biodiesel potential toincrease food prices and damage biodiversity; their relatively lowGHG abatement capacity yet high marginal carbon abatementcosts; their continuing need for signicant government supportand subsides; their direct and indirect impacts on land use changeand related greenhouse gas emissions [4,5]

Here we highlight the important aspects of the biodiesel whichwill reveal the perspective as the next generation green fuel. Threemajor areas are discussed in the following:

(1) Prole of biodiesel.(2) Key drivers and challenges of biodiesel industry devel-

opment.(3) Policy and government incentives.

2. Prole of biodiesel

2.1. Historical background and development of biodiesel

Biodiesel, which can also be known as fatty acid methyl ester(FAME), is produced from transesterication of vegetable oils oranimal fats with the addition of methanol as shown in Fig. 2 [6].Biodiesel is quite similar to petroleum-derived diesel in its maincharacteristics such as cetane number, energy content, viscosityand phase changes [7]. Biodiesel contains no petroleum products,but it is compatible with conventional diesel and can be blended

uction in 2009 by source [1].in any proportion with fossil-based diesel to create a stable bio-diesel blend. Therefore, biodiesel has become one of the most com-mon biofuels in the world. The key milestones in the developmentof biodiesel industry were shown in Table 1.

In fact, the usage of vegetable oils in diesel engine could be da-ted back to August 10, 1893. In this day, Rudolf Diesel, the inventor

o yield fatty acid alkyl esters (biodiesel).

ergyof the engine that beared his name, ran on his prime engine model

Table 1Key milestones in the development of biodiesel industry.

Date Event

August 10,1893

Rudolf Diesels prime diesel engine model, which was fueledby peanut oil, ran on its own power for the rst time inAugsburg, Germany

1900 Rudolf Diesel showed his engine at the world exhibition inParis, his engine was running on 100% peanut oil

August 31,1937

A Belgian scientist, G. Chavanne, was granted a patent for aProcedure for the transformation of vegetable oils for theiruses as fuels. The concept of what is known as biodieseltoday was proposed for the rst time

1977 A Brazilian scientist, Expedito Parente, applied for the rstpatent of the industrial process for biodiesel

1979 Research into the use of transesteried sunower oil, andrening it to diesel fuel standards, was initiated in SouthAfrica

1983 The process for producing fuel-quality, engine-tested biodieselwas completed and published internationally

November,1987

An Austrian company, Gaskoks established the rst biodieselpilot plant

April, 1989 Gaskoks established the rst industrial-scale plant1991 Austrias rst biodiesel standard was issued1997 A German standard, DIN 51606, was formalized2002 ASTM D6751 was rst publishedOctober,

2003A new Europe-wide biodiesel standard, DIN EN14214 waspublished

September,2005

Minnesota became the rst US state to mandate that all dieselfuel sold in the state contain part biodiesel, requiring a contentof at least 2% biodiesel

October,2008

ASTM published new Biodiesel Blend Specications Standards

November,2008

The current version of the European Standard EN 14214 waspublished and supersedes EN 14214:2003

1022 L. Lin et al. / Applied Enin Augsburg, Germany. The engine model has been fueled nothingbut peanut oil. In remembrance of this event, August 10 has beendeclared International Biodiesel Day. Dr. Diesel was visionary.Away back in 1912, he has long predicated that the use of vegeta-ble oils for engine fuels would one day become as important aspetroleum and the coaltar products of the present time.

His usage continued until 1920s before fossil-based diesel al-most completely eliminated vegetable oils in the market due tocheaper price, higher availability and government subsidies. Thediesel engine has been modied to run on the lower viscositypetroleum fuel, now known as diesel. Nevertheless, Diesels ideason agriculture and his invention provided the foundation for a soci-ety fueled with clean, renewable, locally grown fuel [8].

In 1970s, fossil fuels supply shortage and security hadprompted new interest in developing vegetable oils as alternativeenergy. However, the altered diesel engine is no longer suitablefor high viscosity and low volatility vegetable oils to be applied di-rectly. Some operational problems were reported due to the highviscosity of vegetable oils compared to fossil-based diesel, whichresults in poor atomization of the fuel in the fuel spray and oftenleads to deposits and coking of the injectors, combustion chamberand valves. Renement has to be made in order to turn those veg-etable oils into quality fuel. Attempts to overcome these problemsincluded pyrolysis, blending [9,10] and microemulsication [11].Yet, problems were still found with carbon deposition and contam-ination [12,13].

Transesterication of a vegetable oil was conducted as early as1853 by scientists E. Duffy and J. Patrick, many years before therst diesel engine became functional. The transesterication pro-cess can convert the vegetable oils to their alkyl esters and reducethe viscosity to diesel fuel level, which produced biodiesel withproperties that were similar to petroleum-based diesel fuel. There-fore, it has become the most viable process to transform the vege-table oils to be used in existing engines without modications.A Belgian scientist, G. Chavanne was granted a patent for a Pro-cedure for the transformation of vegetable oils for their uses asfuels in 1937. The concept of what is known as biodiesel todaywas proposed for the rst time [14]. After that, a Brazilian scientist,Expedito Parente, applied for the rst patent of the industrial pro-cess for biodiesel in 1977 [15]. Meanwhile research for the produc-tion and rening of biodiesel using sunower oil was initiated inSouth Africa in 1979. By 1983, the process for producing fuel-quality, engine-tested biodiesel was completed and publishedinternationally [16]. An Austrian company, Gaskoks, obtained thetechnology from the South African Agricultural Engineers, se-quently established the rst biodiesel pilot plant and the rstindustrial-scale plant in 1987 and 1989, respectively.

However, increasing subsidization in petroleummarket had im-peded any other signicant breakthrough being achieved. Not untilin the late 1990s that growing concerns about the environmentsustainability and decreasing cost differential had driven thegrowth in commercial production of biodiesel [7]. During that per-iod, biodiesel plants were opened in many European countries,including the Czech Republic, Germany and Sweden. Francelaunched the local production of biodiesel fuel known as diesterfrom rapeseed oil. By 1998, the Austrian Biofuels Institute hadidentied 21 countries with commercial biodiesel projects. Duringthe same period, nations in other parts of the world also saw localproduction of biodiesel starting up: In 2004, the government ofPhilippines had made it compulsory for the incorporation of 1%of coconut biodiesel blend in diesel fuel for use in governmentvehicles. In 2005, Minnesota became the rst US state to mandatethat all diesel fuel sold in the state contain part of biodiesel, requir-ing a content of at least 2% biodiesel.

For strengthening the quality control requirements of engineand equipment manufacturers, and allowing further companiesto issue biodiesel engine warranties for the use of biodiesel fuels,a series of biodiesel standards were issued in succession, such asDIN 51606 (Germany), EN 14214 (Europe), ASTM D6751 (USAand Canada). Other countries have also established or are planningto adopt similar standards for the use of biodiesel as a motor fuel.Those standards have been periodically revised and updated. In2008, the current version of EN 14214 and ASTM D6751 were pub-lished respectively, and superseded previous standards [17].

Now, biodiesel blend fuel is available at many normal servicestations across Europe and US. With the quick development of bio-diesel industry, biodiesel is playing a more and more importantrole in globe primary energy.

2.2. Feedstocks of biodiesel

A variety of biolipids can be used to produce biodiesel. Asshown in Fig. 3, the feedstock of biodiesel depends greatly on cli-mate, local soil conditions and availability; consequently differentregions are focusing their efforts on different types of oil. Typicalraw materials of biodiesel are rapeseed oil, canola oil, soybeanoil, sunower oil and palm oil. Beef and sheep tallow and poultryoil from animal sources and waste cooking oil are also sources ofraw materials. There are various other biodiesel sources: jatropha,almond, barley, camelina (Camelina sativa), coconut, copra, sh oil,groundnut, karanja (Pongamia glabra), laurel, oat, poppy seed, okraseed, rice bran, sesame, sorghum and wheat [1821].

The common fatty acids, which exist in biolipid, are shown inTables 2 and 3. The physical and chemical fuel properties of bio-diesel basically depend on the fatty acids distribution of the tri-glyceride used in the production. The fatty acid distributions ofsome feedstocks commonly used in biodiesel production are

88 (2011) 10201031shown in Table 2.In Table 4, some critical fuel properties of biodiesel (methyl es-

ter, ME) fuels produced from different feedstocks are shown,

L. Lin et al. / Applied Energy 88 (2011) 10201031 1023respectively. In this section, the fuel properties of diesel and fattyacid alkyl mono-esters are compared to each other. It is easy to ndthat biodiesel has better fuel properties than diesel, such as highercetane number, higher ash point, and better lubrication.

However, rst generation biodiesel is mainly produced fromfood-grade oils. According to some researches, feedstock acquisi-tion currently accounts for over 75% of biodiesel production ex-penses, which is a serious threat to the economic viability of the

Table 2The chemical structures of common fatty acids.

Fatty acid Chemical structure

Lauric (12:0) CH3(CH2)10COOHMyristic (14:0) CH3(CH2)12COOHPalmitic (16:0) CH3(CH2)14COOHStearic (18:0) CH3(CH2)16COOHOleic (18:1) CH3(CH2)7CH@CH(CH2)7COOHLinoleic (18:2) CH3(CH2)4CH@CHCH2CH@CH(CH2)7COOHLinolenic (18:3) CH3CH2CH@CHCH2CH@CHCH2CH@CH(CH2)7COOHArachidic (20:0) CH3(CH2)18COOHBehenic (22:0) CH3(CH2)20COOHErucic (22:1) CH3(CH2)7CH@CH(CH2)11COOH

Table 3The fatty acid distributions of some biodiesel feedstocks.

Feedstock Fatty acids (% w/w)

12:0 14:0 16:0 18

Sunower 6.08 3.Rapeseed 3.49 0.Soybean 10.58 4.Palm 1 42.8 4.Peanut 0.3 12.3 4.Coconut 46.5 19.2 9.8 3Soybean soapstock 17.2 4.Used frying oil 12 Tallow 36 2432 20Lard 12 2830 12

Fig. 3. FAME aroubiodiesel industry as depicted in Fig. 4 [36]. Accordingly, the endcost of the biodiesel mainly depends on the price of feedstock.

The international market prices of main oil crops were shown inTable 5. With vegetable oil price soaring high in recent years, thecost of producing biodiesel will keep raising. Biodiesel will loseits competitive advantage due to high price. Furthermore, prob-

nd the world.lems associated with the impacts on food security and land changehave also arisen.

Base on the above, scientists are developing a new generation ofbiodiesel to help avoid such problems. One potential solution tothis problem is employment of alternative feedstocks of varyingtype, quality, and cost. These feedstocks may include soapstocks,acid oils, tall oils, used cooking oils, and waste restaurant greases,various animal fats, non-food vegetable oils, and oils obtained fromtrees andmicroorganisms such as algae. Additionally, genetic mod-ication is also being used to introduce favorable traits into biodie-sel crops, such as higher yields or the ability to grow on non-arableland [3845].

In all new generation feedstocks of biodiesel, microalgae arethe most promising one. Like plants, microalgae use sunlight, car-bon dioxide and water to produce oils but they do so more ef-ciently than crop plants. The process of making biodiesel fuel

References

:0 18:1 18:2 18:3

26 16.93 73.73 [22]85 64.40 22.30 8.23 [22]76 22.52 52.34 8.19 [23]5 40.5 10.1 0.2 [24]6 53.6 29 0.1 [24]

6.9 2.2 [24]4 15.7 55.6 7.1 [25]

53 33 1 [26]25 3743 23 [27,28]18 450 713 [27,28]

Table 4Some properties of diesel and biodiesel produced from different feedstocks.

Fuel Kin. viscosity (mm2/s, at 40 C) Density (g/cm3, at 21 C) Cetane number Flash point (C) Cloud point (C) Pour point (C) References

Diesel 2.04.5 0.8200.860 51.0 55 18 25 [29]Soybean ME 4.08 0.884 50.9 131 0.5 4 [30,25]Rapeseed ME 4.83 0.882 52.9 155 4 10.8 [30,31]Palm ME 4.71 0.864 57.3 135 16 12 [32,33]Sunower ME 4.60 0.880 49.0 183 1 7 [30,34]Jatropha ME 4.4 0.875 57.1 163 4 [33]Tallow ME 5.00 0.877 58.8 150 12 9 [30,34,35]Soapstock ME 4.30 0.885 51.3 169 6 [25]

Fig. 4. General cost breakdown for production of biodiesel [36].

Table 5Price of main oil crops (US$/tonne) [37].

Date Soybean Sunower Groundnut Palm Rapeseed Coconut

1998 483 560 801 486 482 7481999 356 413 744 309 359 5392000 336 428 685 235 372 3232001 412 587 659 329 451 3882002 534 592 1139 421 588 4492003 633 663 1178 481 670 6302004 545 703 1102 392 660 6362005 573 635 931 416 770 5832006 771 846 1219 655 852 8122007 1327 1639 2018 1058 1410 13062008 826 837 1339 633 868 7352009 919 937 1255 749 912 784

Fig. 5. Simplied systems block diagram of the algae to biodiesel process (from Ref. [46]).

Table 6The oil productivity of different crops (from Ref. [48]).

Oil crops Productivity (gallons per acre per year)

Corn 18Soybeans 48Safower 83Sunower 102Rapeseed 127Oil palm 635Microalgae 500015,000

1024 L. Lin et al. / Applied Energy 88 (2011) 10201031

from microalgae involves several steps. Fig. 5 is a simplied blockdiagram of the entire system operation, which includes growth,harvest, extraction and conversion four major steps.

Different algae species produce different amounts of oil. Somespecies of algae are ideally suited to biodiesel production due totheir high oil content in some species, topping out near 50%. AsTable 6 shows, oil productivity of many microalgae greatly exceedsthe oil productivity of the best producing oil crops [4750].

Meanwhile, microalgae are the fastest-growing photosynthesiz-ing organisms. They can complete an entire growing cycle everyfew days. Approximately 46 tons of oil/hectare/year can be pro-duced from diatom algae. The production of algae to harvest oilfor biodiesel has not been undertaken on a commercial scale, butworking feasibility studies have been conducted to arrive at theabove number. Specially bred mustard varieties can produce rea-sonably high oil yields and have the added benet that the mealleft over after the oil has been pressed out can act as an effectiveand biodegradable pesticide [51]. Furthermore, algae can be grownalmost anywhere, even on sewage or salt water, and does not re-quire fertile land or food crops, and processing requires less energythan the algae provides [52].

2.3. Biodiesel conversion technologies

As mentioned above, diesel engine is not suitable for high vis-

Aimed at problems in dilution and micro-emulsion methods,some new techniques were applied to solve the problems encoun-tered with the high fuel viscosity. These methods include pyrolysis,transesterication and supercritical methanol. The comparison ofmain biodiesel preparation technologies was shown in Table 7.

Amongst the four techniques, transesterication is the mostpromising solution to the high viscosity problem. Now, transeste-rication is widely available technique for industrialized biodieselproduction due to its high conversion efciency and low cost.

In transesterication, the triglyceride can be transformed intomonoester. Due to the transesterication in the ester exchangeprocess, the viscosity of vegetable oil is reduced and heat valuesmaintained. The cetane number increases because the molecularchain is cut into 1/3.

Transesterication is the chemical reaction between triglycer-

Comparison of main preparation technologies.

L. Lin et al. / Applied Energy 88 (2011) 10201031 1025Technologies Advantage Disadvantage

Dilution or micro-emulsion [5861]

Simple process 1. High viscosity

2. Bad volatility3. Bad stability

Pyrolysis [62,63] 1. Simple process 1. High temperature is required2. No-polluting 2. Equipment is expensive

3. Low purity

Transesterication[64,65]

1. Fuel propertiesis closer to diesel

1. Low free fatty acid and watercontent are required (for basecatalyst)

2. High conversionefciency

2. Pollutants will be producedbecause products must beneutralized and washed

3. Low cost 3. Accompanied by side reactions4. It is suitable forindustrializedproduction

4. Difcult reaction productsseperation

Supercriticalmethanol[66,67]

1. No catalyst 1. High temperature and pressureare required

2. Short reactiontime

2. Equipment cost is high

3. High conversion 3. High energy consumptioncosity, low volatility and polyunsaturated character vegetable oilsto be applied directly [53]. Renement has to be made in orderto turn those vegetable oils into quality fuel. Conventional meth-ods of the application of vegetable oil in diesel engines are directmixing and micro-emulsion. These two physical methods do notrequire any chemical process and can lower the viscosity of vege-table oil, but they cannot solve the problem of carbon deposits andlube pollution, and the high temperature pyrolysis cracking is hardto be controlled by its reactant at high temperature. The most rel-evant process parameters in these kinds of operation are reactiontemperature, ratio of alcohol to vegetable oil, amount of catalyst,mixing intensity (RPM), catalyst, and the raw oils used [5457].

Table 74. Goodadaptabilityides and short-chain alcohol in the presence of a catalyst to pro-duce mono-esters. The long- and branched-chain triglyceridemolecules are transformed to mono-esters and glycerin [42]. Com-monly-used short-chain alcohols are methanol, ethanol, propanoland butanol. Methanol is used commercially because of its lowprice [40].

Because this process is a reversible reaction, the output of bio-diesel will be directly inuenced by the proportion of reactants, thetype and the dosage of the catalyst, and the reaction conditions.

From the principle of reversible reaction, it follows that a higherusage of methanol leads to a higher output of biodiesel. However,the higher density of methanol can cause a polycondensation reac-tion; as a result, it will reduce the effective concentration of meth-anol, and cause difculties for the separation of biodiesel.Furthermore, more methanols are associated with higher costs.In the process of batch reaction or continuous reaction activatedby an alkalescence catalyst, a 6:1 mol ratio has been used widely[6871].

There are three common kinds of catalysts in the ester reaction:lipase catalysts, acid catalysts, and alkali catalysts. Each catalysthas its own advantages and disadvantages in the whole reactionprocess.

The transesterication is typically catalyzed by lipases such asCandida antartica, Candida rugasa, Pseudomonas cepacia, immobi-lized lipase (Lipozyme RMIM), Pseudomonas spp. or Rhizomucarmiehei. The yield of biodiesel from this process can vary dependingon the type of enzyme used [72]. As the catalyst, enzyme is re-stricted to rigorous reaction condition and activity lose of lipase,etc., it cannot be used on the large commercial production untilnow.

In catalytic transesterication using homogeneous acid catalyst,the reaction is catalyzed by sulphuric [73,74], hydrochloric [74] orsulphonic acids [75]. Homogeneous acid catalysts are less sensitive

Table 8Comparison of homogeneously and heterogeneously catalyzed transesterication(from Ref. [72]).

Factors Homogeneous catalysis Heterogeneouscatalysis

Reaction rate Fast and high conversion Moderate conversionPost-treatment Catalyst cannot be recovered,

must be neutralized leading towaste chemical production

Catalyst can berecovered

Processingmethodology

Limited used of Continuous x bedcontinuous methodologyoperation possible

Presence ofwater/free

Sensitive Not sensitivefatty acidsCatalyst reuse Not possible PossibleCost Comparatively costly Potentially cheaper

ergyto FFAs and can simultaneously conduct esterication and transe-sterication. However, acid-catalyzed transesterication has beenlargely ignored mainly because they are slower and necessitatehigher reaction temperatures.

Most of the commercial biodiesel is produced from plant oilsusing very effective homogeneous alkali catalysts such as sodiumor potassium hydroxides, carbonates or alkoxides [72,75,76]. Thespeed of the alkali catalyzing process is higher than that in the acidactivating process. This, together with the good corrosion resis-tance properties, promoted the alkali catalysts to be widely usedin industry. However, the alkali catalyzing process is very sensitiveto the presence of water and free fatty acids and needs lots ofmethanol. When the raw feedstocks have a high percentage of freefatty acids or water, the alkali catalyst will react with the free fattyacids to form soaps. The water can hydrolyze the triglycerides intodiglycerides and form more free fatty acids. Both of the above reac-tions are undesirable and reduce the yield of the biodiesel product.Moreover, since the alkali catalysts must be neutralized, giving riseto wastewater that cannot be reutilized, and glycerol is obtained asan aqueous solution of relatively low purity [7779].

These problems can be alleviated by using heterogeneoustransesterication catalysts. A comparison between homogeneousand heterogeneous catalysis is summarized in Table 8. The advan-tage of heterogeneous catalyst usage is its fast and easy separationfrom the reaction mixture without requiring the use of neutraliza-tion agent. Furthermore such type of catalyst could be regeneratedand reused, and it has a less corrosive character, leading to safer,cheaper and more environment-friendly operation [80]. Therefore,there is an increasing interest in the possibility of replacing thehomogeneous alkaline hydroxides, carbonates or metal alkoxidesby heterogeneous solid catalysts insoluble in methanol [8185].

Many different types of solid catalysts were applied in transe-sterication of vegetable oils to produce biodiesel, such as alkaliearth metal oxides and transition metal oxides [8692]. Despitethe solid phase catalytic methods are intensively studied, theindustrial applications are limited. The advantages of solid cata-lysts are easy separation and simple post treatments. Nevertheless,heterogeneous catalytic methods are usually mass transfer resis-tant, time consuming and inefcient [93].

One of the ways to minimize the mass transfer limitation forheterogeneous catalysts in liquid phase reactions is using catalystsupports. Effective factors on catalytic activity of solid catalystsare specic surface area, pore size, pore volume and active site con-centration on the surface of catalyst. Supports can provide highersurface area through the existence of pores where metal particlescan be anchored [94]. The use of catalyst supports, such as alumina[95,96], zinc oxide [97], silica [98] and zirconium oxide [99], couldimprove the mass transfer limitation of the three phase reaction.Furthermore, by anchoring metal oxides inside pores, catalyst sup-ports could prevent active phases from sintering in the reactionmedium [91].

In recent years, with the development of nano technology, nanotechnology is applied more and more in catalytic eld. Becausenanocatalysts have high specic surface and high catalysis activi-ties, they may provide a possible way to solve the above problems.Therefore, they have become the focus of recent research[100,101].

Most earlier studies have focused on the use of nanoparticle-based catalysts for the transesterication of the triglycerides intobiodiesel. The very high activity and surface reactivity of nanocatalysts were considered to be related to the high surface/vol-ume ratio of nano-sized catalyst particles [102105]. Someresearchers also tried to add magnetic substance in nano catalyst,

1026 L. Lin et al. / Applied Enin order to enhance separation efciency. This nano catalystachieved favorable effects due to assembly of magnet and cata-lytic activity [106].Recently, the CNTs (carbon nanotubes) showed various newperformances in catalysis and nd more and more application asnew kinds of high performance catalyst due to its high specic sur-face area and pore structure [107,108]. Sulfonated-multiwalledcarbon nanotubes (s-MWCNTs) were used in the synthesis of bio-diesel frommethanol and oleic acid. The result showed that the so-lid acid catalyst, s-MWCNTs, has a high catalytic activity forbiodiesel production from cheap raw feedstocks with high concen-trations of FFAs [109].

Besides catalysts, studies on biodiesel synthesis have focused ondevelopment of process intensication technologies, for exampleultrasonic and micro wave technology. Because oils and methanolare not completely miscible, the mixing efciency was stated asone of the most important factors affecting the yield of the transe-sterication [110]. Low frequency ultrasonic irradiation is widelyused in industry for emulsication of immiscible liquids. Previ-ously, productions of biodiesel from vegetable oils with short-chain alcohols (ethanol, propanol, and butanol) under ultrasonicirradiation were investigated. The results shown that low fre-quency ultrasound is an efcient, time saving and economicallyfunctional, offering a lot of advantages over the classical procedure.Ultrasounds can be a valuable tool for the transesterication, aim-ing to prepare the biodiesel fuel at industrial scale [111114].

The same to ultrasonic technology, microwave technology hasalso been developed for biodiesel synthesis. Because the mixtureof vegetable oil, methanol, and alcohol contains both polar and io-nic components, microwave irradiation can play an active role inheating reactants to the required temperature quickly and ef-ciently. Breccia et al. [115] studied the transesterication of com-mercial seed oils with methanol under microwave irradiation. Inthe presence of a variety of catalysts, yields greater than 97% wereachieved with reaction times of less than 2 min. Other studiedshown that the preparation of biodiesel using microwave heatingproved to be more energy efcient than the conventional synthesisas well [116118].

Apart from the aforementioned technologies, someother processintensication technologies have also been applied in transesteri-cation, including static mixers [119], micro-channel [120,121],oscillatory ow [122] and cavitational [123]. These technologiescan enhance reaction rate, reduce molar ratio of alcohol to oil andenergy input by intensication of mass transfer and heat transfer,thus achieve continuous product in the reactor. Some of these tech-nologies have already been commercialized successfully [124].

3. Key drivers and challenges of biodiesel industry development

3.1. Security of energy supply

Security of energy supply means that energy can be adequately,affordably and reliably supplied. For most countries, the primaryreason for joining the biodiesel bandwagon is energy security[125]. In other words, one of the driving forces behind the develop-ment of biodiesel is to hope for a reduction in dependence on fos-sil-based oil. For the foreseeable future, fossil-based oil willcontinue to dominate world energy supply, but its production costsare rising, and supply is dominated by a few major producers,many of them in the volatile Middle East. Although fossil-basedoil prices are currently relatively low, they are likely to be volatilein the short-term and to rise in the longer term [126128]. Depen-dence on imported fossil-based oil will pose serious threat to na-tional security of energy supply. Hence, a number of nationalgovernments have used targeted policies to increase the produc-tion and use of a broadening range of biological resources for fuel.

88 (2011) 10201031The US National Renewable Energy Laboratory (NREL) statesthat energy security is the number one driving force behind theUS biofuels program and a White House Energy Security for the

ergy21st Century paper makes it clear that energy security is a majorreason for promoting biodiesel. The EU commission president, JoseManuel Barroso, speaking at a recent EU biofuels conference,stressed that properly managed biofuels have the potential to rein-force the EUs security of supply through diversication of energysources [129].

Coupled with increased demand and strong growth in India andChina, Asia will emerge as the epicenter for the global energy land-scape. The International Energy Agencys (IEA) report projects thatAsias energy demand will expand by 76% between 2007 and 2030[130]. As home to 60% of the global population, Asias future energyneeds are predicted to escalate in response primarily to increase indemand from the transport sector. At the same time, Asias energysecurity is considered one of the most fragile in the world becausethe region is heavily dependent upon imported oil to satisfy the de-mand of its transport sector. Therefore, the development of renew-able energy technologies and policy, particularly those thatpromote the expansion of biofuels production, is believed to beone of the paths to achieving energy security.

However, energy security improvements through biodieseldevelopment must be based on differences in feedstock resourcesand energy consumption mix in different countries. Hence, it isimportant for the governments of different countries to implementpolicies aimed at incorporating biodiesel into their respective na-tional energy mix.

3.2. Environmental effects

The surge of interest in biodiesels has highlighted a number ofenvironmental effects associated with its use. Biodiesel proponentsargue that unlike fossil fuels which release carbon dioxide that hasbeen stored for millions of years beneath the earths surface, bio-diesel produced from biomass have the potential to be carbonneutral over their life cycles as their combustion only returns tothe atmosphere the carbon dioxide absorbed from the air by feed-stock crops through photosynthesis. It thus has the potential to re-place fossil-based fuels and contribute to the mitigation of GHGemissions [131,132]. According to the EPAs Renewable Fuel Stan-dards Program Regulatory Impact Analysis, released in February2010, biodiesel from soy oil results, on average, in a 57% reductionin greenhouse gases compared to fossil diesel, and biodiesel pro-duced from waste grease results in an 86% reduction [133].

Ensuring that biodiesel is a credible source of low-carbon en-ergy that deliver greenhouse gas savings compared with fossil fuelsis a key component of many countries efforts to set standardsworldwide for lowering emissions in the future. Europes aim tocut GHG emission by one-fth by 2020, partly through demandingthat one in 10 vehicles are fueled by biofuels, will spark a surge indemand for biodiesel. The president of USA, Barack Obama, an-nounced that the Federal Government will reduce its GHG pollu-tion by 28% by 2020. Meanwhile, the Chinese governmentannounced the action to control greenhouse gas emissions targets,the decision by 2020 carbon dioxide emissions per unit of GDPthan in 2005 dropped 4045%, and decided to help Africa countriesto develop their clean energy projects. The Japanese governmenthas a GHG emission reduction target of 6080% by 2050 from itscurrent level [134]. India, Brazil, South Africa and other countrieshave also set their GHG reduction target and development programof substitute energy in the future. All of these provide biodieselindustry many unprecedented opportunities of development.

However, the impact of biodiesel on environment widely varies,i.e., it may not necessarily be positive, or as positive as is often ini-tially assumed. In other words, biodiesel is not always offering

L. Lin et al. / Applied Enemissions reductions compared to fossil fuels. Consequently, wehave to nd a way to address the GHG balance of using biodieselfor fuel. A useful tool for addressing GHG balance is the Life CycleAssessment (LCA), which has been applied to different biofuels,with varying results. LCA of GHG balance is complex, plantingand harvesting of crops (including fertilizer and pesticide use, irri-gation technology, and soil treatment); processing the feedstockinto biodiesel; transporting the feedstock and nal fuel; storing,distributing, and retailing biodiesel can all have a considerableinuence on results [135137].

Apart from the aforementioned problems, the development ofbiodiesel industry may directly or indirectly cause other negativeeffects on environment. In order to grow the oil crops necessaryto produce biodiesel, additional land must be brought into produc-tion. This has led to pristine rainforests being cleared for the sakeof monoculture plantations. Rainforests are one of the worlds larg-est carbon sinks. The clearing of rainforest and peatland for oilcrops results in a sudden release of large amounts of carbon diox-ide. In addition, loss of biodiversity is also another issue that ariseswhen rainforests are being cleared [138].

Moreover, the clearing of rainforest even cause climate changein some regions. For example, in recent months, Southeast Asia issuffering from drought. One possible cause is that many forestshave been cleared and substituted for the energy crops in manySoutheast Asia countries. Because forests can hold some of that ex-cess water in rainy season and release it in the dry season. Rightnow, due to large-scale deforestation, the water storage and con-servation capability of soil has been very much weakened. Mean-time, we also should pay close attention to haze caused by theforest res in some developing countries, where farmers use reto clear land for agriculture uses.

3.3. Food security, land use changes and water source

As populations grow over the next four decades, demand forfood, and thus water, will continue to rise across the world. How-ever, food prices have risen dramatically around the world, eventriggering riots in West Africa. In the past there was no signicantcorrelation between biodiesel and food prices, but since 2002 thecorrelation has strengthened. Increases in producing biodieselsource material (such as soybean and rapeseed) costs by at least50% over the past few years have harmed the comparative advan-tage and competitiveness of individual countries. While growthwithin the biodiesel sector can contribute to increases in the priceof soybean oil and other biodiesel source material, the competi-tiveness of the sector can be adversely affected by these verysame prices changes, as well as other economic factors. Theseemerging trends suggest that food and energy markets are likelyto be more strongly linked in future such that spikes and uc-tuations in the prices of energy lead to corresponding changes infood prices.

As a result, much of the literature on crop-based biodiesel pro-duction focuses on the potential impacts on food security as well asland use changes and water source. On the plus side, biodiesel pro-duction has helped some farmers and workers boost their incomesand increase employment in the agricultural sector, especially fordeveloping countries. But, as with many other types of agriculture,other workers and farmers have experienced inadequate workingconditions. In some countries, local communities have reportedlylost control ofor even been evicted fromtheir land to makeway for biodiesel production. Farmers may switch from producingfood crops to producing biodiesel crops to make more money, evenif the new crops are not edible. The law of supply and demand pre-dicts that if fewer farmers are producing food the price of food willrise. Such practices can reduce food availability and may consignfood and feed production to less productive land, thus reducing

88 (2011) 10201031 1027yields and food security, and raising food prices. Meantime, highercommodity prices will have negative consequences for netfood-importers. And for low income food decit countries, food

of the global community, such exible GHGs emissions reduction

ergyimport bills will rise, precipitously in some cases. In 2008, a FAOanalysis found global expenditures on imported foodstuffs in2007 rose by about 29% above the record of the previous year.The bulk of the increase was accounted for by rising prices ofimported vegetable oils commodity groups that feature heavilyin biodiesel production.

Furthermore, boosting biodiesel production by growing moreoil crops without considering the quality and availability of waterby region could put a signicant strain on water resources in someparts of the world, especially in developing countries [139]. Thereis very high demand for access to water for irrigation, cooking, anddrinking. Because the production of biodiesel is very water inten-sive, agricultural shifts to growing biodiesel crops could changethe availability of clean water and greatly increase pressure onwater resources in those areas. As a result, those areas will faceserious challenges to meet the predicted increase in demand forfood produce, let alone sustain any further growth prompted byexpanding biodiesel production [140,141].

4. Policy and government incentives

In general, the energy policy may include international treaties,legislation on commercial energy activities (trading, transport,storage, etc.), incentives for investment, guidelines for energy pro-duction, conversion, and use (efciency and emission standards),taxation and other public techniques, energy-related researchand development, energy economy, general international tradeagreements and marketing energy diversity [142]. Current energypolicies also address environmental issues including environmen-tal friendly technologies to increase energy supplies and encouragecleaner, more efcient energy use, air pollution, greenhouse effect(mainly reducing carbon dioxide emissions), global warming andclimate change [143146].

The energy policy will directly inuence the development ofbiodiesel industry. As a policymaker, government play an essentialrole in determining the course, and crucially, the scale, of biodieseldevelopment, in particular by means of the proper incentives suchas tax exemptions, price controls, targets and direct subsidies. Akey question is the long-term economic, social and environmentalsustainability of bio-energy policies. Therefore, comprehensiveassessments will be required to improve their ability to identifythe proper energy policies which are likely to avoid potential con-icts with food production, land change, and provide most benets[18,147,148].

Now, there are many incentives that can be offered by a govern-ment to spur the development of biodiesel industry and maintainits sustainability, such as crop plantation in abandoned and fal-lowed agricultural lands, implementation of carbon tax, subsidiz-ing the cultivation of non-food crops, and exemption from the oiltax [149,150].

Furthermore, some governments have mandated the use ofbiodiesel in recent years. For instance, the establishment of thedirective on the promotion of the use of biofuels for transportin EU (Directive 2003/30/EC) mandates an increasing share ofbiofuels from 2% of total fuel supply in 2005 to 5.75% of totalsupply in 2010 (based on energy content). In other large coun-tries, like USA and Canada, governments have also implementedsimilar directives, which triggered a huge demand for biodiesel[151,152].

Additionally, as of right now, general public awareness in bio-diesel industry still remains low. Majority of the public is eitherignorant or has limited knowledge on the biodiesel issues. There-fore, governments should enhance the promotion of biodiesel

1028 L. Lin et al. / Applied Enindustry and increase public awareness on the biodiesel industry,which will help the governments biodiesel policies to garner suf-cient public support [7].target, to cultivate a ground which will further promote the useof renewable fuels.

Acknowledgements

The nancial support from National Natural Science Foundationof China (30940058), National Science Foundation for Post-doctoralScientistsofChina (20100471383), JiangsuPlannedProjects for Post-doctoral Research Funds(1001035B), Natural science foundation forcolleges and universities in Jiangsu Province (09KJD4800001),Jiangsu key lab of mechanical clean energy and application foun-dation (QK09006), Jiangsu University research foundation foryoung Scholars (08JDG039) and Yancheng agricultural science andtechnology development program (YK2009081) are gratefullyacknowledged.

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[1] BP. BP statistical review of world energy; 2009.Finally, while governments are focusing on the ways to improvebiodiesel production and consumption, they have to give enoughattention to unresolved issues like rainforest depletion, food pricesincrease. Worldwide, deforestation accounts for an estimated 20%of greenhouse gas emissions. And much of the forest now beingcleared for palm oil is peatland, with marshy soils that are crucialholders of methane, a greenhouse gas even more potent than car-bon dioxide. Without taking into account this, their policies mighthave detrimental effects on climate change. In this situation, gov-ernments should advocate biofule companies to use feedstocksfrom crops grown on degraded lands, waste products, plantationscertied as sustainable, and clarify limits on fuels from sensitiveareas like forests and partly drained peatlands. Meanwhile, agricul-tural subsidies should be paid to domestic farmers by their govern-ments that guarantee them a xed income on traditional food crop.Furthermore, developed countries should pay poor ones to saveforests. Through these ways, plantation can keep balance withforest.

5. Conclusions

Biodiesel will form a small but very important part of global en-ergy supply in the coming decades. Under the appropriate condi-tions, increasing biodiesels share of the energy mix cancontribute to meet important global needs such as reducing GHGemissions, enhancing energy security and, particularly in develop-ing countries, promoting sustainable rural development [5].

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L. Lin et al. / Applied Energy 88 (2011) 10201031 1031

Opportunities and challenges for biodiesel fuelIntroductionProfile of biodieselHistorical background and development of biodieselFeedstocks of biodieselBiodiesel conversion technologies

Key drivers and challenges of biodiesel industry developmentSecurity of energy supplyEnvironmental effectsFood security, land use changes and water source

Policy and government incentivesConclusionsAcknowledgementsReferences