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ARTIC AR TICLE LE IN PR PRESS ESS

Energy 33 (2008) 1646–1653

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Review

Feasibility of edible oil vs. non-edible oil vs. waste edible oil as biodiesel feedstock 

M.M. Gui, K.T. Lee , S. Bhatia School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia

a r t i c l e

i n f o

 Article history: Received 2 January 2008 Keywords: Biodiesel Edible oil Waste edible oil Non-edible oil

a b s t r a c t

Biodiesel has high potential as a new and renewable energy source in the future, as a substitution fuel for petroleum-derived diesel and can be used in existing diesel engine without modification. Currently, more than 95% of the world biodiesel is produced from edible oil which is easily available on large scale from the agricultural industry. However, continuous and large-scale production of biodiesel from edible oil without proper proper plannin planning g may cause negative negative impact impact to the world, such as depletio depletion n of food supply supply leading leading to econom economic ic imbala imbalance nce.. A poss possibl ible e solut solutionto ionto over overcom come e this this probl problem em is to use use non-ed non-edibl ible e oil or wast waste e edible edible oil (WEO). In this context, context, the the next question question that that comes in mind would would be if the use of non-edible non-edible oil overcomes overcomes the short-comings of using edible oil. Apart from that, if WEO were to be used, is it sufficient to meet the demand demand of biodiese biodiesel. l. All these issues issues will be address addressed ed in this paper by discuss discussing ing the advanta advantages ges and disadva disadvanta ntages ges of using using edible edible oil vs. non-edib non-edible le vs. WEO as feeds feedstock tock for biodiese biodiesell productio production. n. The discuss discussion ion will cover various aspects ranging from oil composition, oil yield, economics, cultivation requirements, land availability and also the resources availability. Finally, a proposed solution will be presented. &  2008 Elsevier Ltd. All rights reserved.

Contents

1. 2.

3.

4. 5. 6.

Intr Introd oduc ucti tion. on. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1646 646 Char Charac acte teri rist stic ic of edi edible ble and and non non-e -edi dibl ble e oil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1647 647 2.1 2.1. Oil Oil yiel yield d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 1647 7 2.2. 2.2. Oil Oil compo composi sitio tion n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 1647 7 2.3. 2.3. Proper Propertie tiess of biodie biodiese sell from diff differe erent nt feeds feedstoc tock. k. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1648 2.4. 2.4. Cult Cultiv ivat atio ion n requ requir irem emen ents. ts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1649 2.5. 2.5. Cost Cost of of plan planta tati tion on.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1649 2.6. 2.6. Curren Currentt techniq technique uess avail availabl able e for conve convertin rting g non-ed non-edible ible oil oil to biodies biodiesel el . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1650 1650 Biod Biodie iese sell from from was waste te edi edible ble oil oil (WEO (WEO)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1650 650 3.1 3.1. Waste Waste edib edible le oil (WEO) (WEO):: avail availabi ability lity,, economi economicc value value and and proper propertie tiess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1650 1650 3.2. 3.2. Curren Currentt techniq technique uess avail availabl able e in conver convertin ting g WEO into into biodie biodiesel sel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1650 1650 Curr Curren entt scena scenari rio o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1651 651 Reco Recomm mmen enda dati tion. on. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1652 652 Conc Conclu lusi sion on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1652 652 Ackn Acknow owle ledg dgme ment ntss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1652 652 Refe Refere renc nces es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1652 652

1. Introduc Introduction tion

In the past few decades, fossil fuels mainly petroleum, natural gas and coal have been playing an important role as the major



Corresponding Corresponding author. Tel.: +604 5996467; fax: +604 594101 5941013. 3. E-mail address:  [email protected]  [email protected] y (K.T. Lee).

0360-5442/$- see front front matter  &  2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2008.06.002 doi:10.1016/j.energy.2008.06.002

energy resources worldwide. However, these energy resources are non-renew non-renewable able and are projected projected to be exhausted exhausted in the near future. future. The situation situation has worsened with the escalating escalating energy consumptio consumption n wor worldwi ldwide de due to rapid rapid populatio population n growth growth and econom economic ic devel developm opment ent.. This This has caused caused the price price of cru crude de petroleum to hit a record high of USD (US dollar) 90 per barrel in October 2007 and still rising. Therefore, there is an urgent need to find a new energy resource that is renewable, clean, reliable

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M.M. Gui et al. / Energy 33 (2008) 1646–1653

and yet economically feasible as a substitution to the current fossil fuels. In this context, recently, biodiesel derived from vegetable oil has been shown to be a potential alternative replacing petroleum-derived diesel oil for diesel engine. Biodiesel is mono alkyl ester derived from oils (plant or animal) which have characteristics similar to petroleum-derived diesel oil. Currently, about 84% the world biodiesel production is met by rapeseed oil. The remaining portion is from sunflower oil (13%), palm oil (1%) and soybean oil and others (2%) [1]. Since more than 95% of the biodiesel is made from edible oil, there are many claims that a lot of problems may arise. By converting edible oils into biodiesel, food resources are actually being converted into automotive fuels. It is believed that large-scale production of  biodiesel from edible oils may bring global imbalance to the food supply and demand market. Recently, environmentalists have started to debate on the negative impact of biodiesel production from edible oil on our planet especially deforestation and destruction of ecosystem [2]. They claimed that the expansion of oil crop plantations for biodiesel production on a large scale has caused deforestation in countries such as Malaysia, Indonesia and Brazil since more and more forest has been cleared for plantation purposes. Furthermore, the line between food and fuel economies is blurred as both of the fields are competing for the same oil resources. In other words, biodiesel is competing limited land availability with food industry for plantation of oil crop. Arable land that would otherwise have been used to grow food would instead be used to grow fuel [3]. In fact, this trend is already being observed in certain part of this world. There has been significant expansion in the plantation of oil crops for biodiesel in the past few years in order to fulfill the continuous increasing demand of  biodiesel. Fig. 1   shows the trend in global vegetable oil ending stocks due to the production of biodiesel in the years 1991–2005 [4]. Although there is continuous increase in the production of  vegetable oil; however, the ending stocks of vegetable oils are continuously decreasing due to increasing production of biodiesel. Eventually, with the implementation of biodiesel as a substitute fuel for petroleum-derived diesel oil, this may lead to the depletion of edible-oil supply worldwide. In order to overcome this devastating phenomenon, suggestions and research have been made/conducted to produce biodiesel by using alternative or greener oil resources such as non-edible oils. In fact in India, nearly half a dozen states have set aside a total of 1.72 million hectares of land for jatropha cultivation and small quantities of jatropha biodiesel are already being sold to the public sector oil companies. In this context, the next question that comes in mind would be if the use of nonedible oil overcomes the short-comings of using edible oil. Or rather it just simply diverts the issue and not solving it completely

14

4000

   ) 12    %    (    S 10    K    C    O 8    T    S 6    G    N    I 4    D    N    E 2

3500

as plantations for non-edible oils still requires large plantation land areas. Alternatively, can waste edible oil (WEO) be used instead, and is it sufficient to meet the demand of biodiesel. All these issues will be addressed in this paper by discussing the advantages and disadvantages of using edible oil vs. non-edible vs. WEO as feedstock for biodiesel production. The discussion will cover various aspects ranging from oil composition, oil yield, economics, cultivation requirements, land availability and also the resources availability. The non-edible oils that were included in this study are jatropha, rubber seed, castor ( Ricinus communis  L.), sea mango (Cerbera odollam or   Cerbera manghas), and  Pongamia  pinnata (abbreviated hereafter as  P. pinnata). Finally, at the end of  this paper, authors’ point of view to overcome this issue will be presented.

2. Characteristic of edible and non-edible oil  2.1. Oil yield

The oil yield from the crops itself is always the key factor to decide the suitability of a feedstock for biodiesel production. Oil crops with higher oil yield are more preferable in the biodiesel industry because it can reduce the production cost. Generally the cost of raw materials accounts about 70–80% of the total production cost of biodiesel.  Table 1 shows the oil yield in terms of kg/ha and wt% and also the price for various types of edible and non-edible oils in the world. It is clear that higher oil yield always corresponds with lower cost. Some of the costs of the non-edible oils cannot be obtained as they are currently not traded in the open market. From Table 1 we can see that palm oil was found to give the highest oil yield with 5000 kg oil per hectare; this value is far higher than other oils which are only in the range of hundreds to 2000kg oil per hectare. On the other hand, among the various non-edible oils shown in Table 1, jatropha was found to give the highest yield. This is followed by  P. pinnata  and castor. However, the oil yield in  P. pinnata  is not stable, depending on many factors such as plantation and oil extraction technique of the oil crops.  2.2. Oil composition

Another important criteria to determine the suitability of oil as a raw material for the production of biodiesel is the composition of the oil itself. The composition of oil will subsequently determine the properties of the biodiesel obtained. The effect of  oil composition on the properties of the biodiesel produced will  Table 1 Oil yield for major non-edible and edible oil sources

3000    S

Type of oil

   R    E

2500    T    I VEGETABLE ENDING STOCK BIODIESEL PRODUCTION

   L

2000    N

   O    I 1500    L    L    I 1000    M 500

0

0

   2    9    3    9  4    9    5    9    6    9    7    9    8    9    9    0    0    0  1    0    2    0    3    0  4    0    5    9  1   9    9    9    9   1    9   1    9   1    9   1    9   1    9   1    9    2    0    2    0    2    0    2    0    2    0    2    0   1   1   1 YEAR

Fig. 1.  Global vegetable oil ending stock and biodiesel production.

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Non-edible oil  Jatropha [5,6]

Rubber seed [7] Castor [5,9] Pongamia pinnata  [8] Sea mango [9] Edible oil Soybean  [5,11] Palm [5,12] Rapeseed  [5,13]

Oil yield (kg oil/ha)

Oil yield (wt%)

Prices (USD/ton)

1590

N/A

80–120 1188 225–2250 N/A

Seed: 35–40, kernel: 50–60 40–50 53 30–40 54

N/A N/A N/A N/A

375 5000 1000

20 20 37–50

684 478 683

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be discussed in the subsequent section. The composition for various types of edible and non-edible oils is shown in   Table 2. From this table, it was observed that the major oil composition in both non-edible and edible oils is generally similar except for castor oil. The major fatty acids content in both non-edible and edible oils are oleic, linoleic, stearic and palmitic acid. The fatty acids in the oils are further categorized into saturated and unsaturated fatty acids. The former include stearic, palmitic and dihydroxystearic acid, while the latter include oleic, linoleic, ricinoleic, palmitoleic, linolenic and eicosenoic acid. Castor oil has the most unique composition with approximately 89.5% ricinoleic acid. Ricinoleic acid is also known as castor oil acid, an unsaturated fatty acid which is soluble in most organic solvents. Apart from the content of fatty acids in the oil, edible oils such as soybean, rapeseed and palm oil also contain valuable nutrients that should not be ignored. For soybean oil, it has a high content of  protein (35–40%), which contains all the essential amino acids necessary for human growth and can sustain health at all stages of  development [18]. Apart from that, soybean oil and rapeseed oil also has high content of linoleic acid, with 51% and 22.3%, respectively. Linoleic acid is also known as polyunsaturated fatty acid and is a member of the essential fatty acid group called omega-6 fatty acids which is also an essential dietary compound required for all mammals. This type of fat is a well-known healthy fat that can help to lower the risk of heart disease   [19]. On the other hand, although palm oil has a high saturated acid content, it is a rich source of polynutrients such as beta-carotene, alphacarotene, vitamin-E, lycopene, tocotrienols and other carotenoids. These polynutrients can act as antioxidants that help in reducing the risks of certain types of cancer [20]. Thus, with proper removal of the saturated fatty acid in palm oil during the oil refining process, palm oil can become a high-quality edible oil with high nutrient content.

In contrast to edible oil, non-edible oils like jatropha, castor, P. pinnata, rubber seed and sea mango are not suitable for human consumption due to the presence to toxic compounds in the oil. The main toxin compounds in jatropha plants are curcin and purgative which are found mainly in the seeds, fruits and sap [21]. The toxic content in the seed is transferred into the oil after the extraction process. The phytotoxin in castor plant is ricin, a watersoluble protein which is concentrated in the seeds [22], while the toxin in rubber seed oil is cyanogenic glucoside that yields poisonous prussic acid (HCN) due to enzymatic reaction [23]. On the other hand, sea mango tree is a well-known ‘suicide tree’ because of its highly poisonous nature. The leaves and fruits of sea mango contain the potent cardiac substance (a glycoside) called cerberin, which is extremely poisonous if ingested [24]. The seed of   P. pinnata  contains pongam oil which is bitter and non-edible with disagreeable taste due to the presence of flavonoid constituent, pongamiin and karajiin. This seed is usually used as fish poison [25].

 2.3. Properties of biodiesel from different feedstock

Apart from the cost of the raw materials, another important factor to consider is the properties of the biodiesel obtained from various types of oil. The properties of biodiesel vary accordingly to the fatty acid composition in the feedstock oil which is used to produce biodiesel. The properties of biodiesel have to be comparable or better than petroleum-derived diesel oil in order to ensure that it can be used in diesel engine without any modification. The properties include flash point, viscosity, cetane number, cloud point, pour point, calorific value, acid value, ash content and cold flow properties.   Table 3   shows some of the important physical and chemical properties of biodiesel produced

 Table 2 Oil composition of various non-edible and edible oil

Fatty acid composition (%)

Molecular formula

Oleic Linoleic Palmitic Stearic Linolenic Eicosenoic Ricinoleic Dihydroxystearic Palmitoleic Others

C18H34O2 C18H32O2 C16H32O2 C18H36O2 C18H30O2 C20H38O2 C18H34O3 C18H36O4 C16H30O2 –

Non-edible oil

     

Edible oil

 Jatropha [6]

Rubber seed [7]

Castor  [14]

43.1 34.3 14.2 6.9 – – – – – 1.4

24.6 39.6 10.2 8.7 16.3 – – – – –

3.0 4.2 1.0 1.0 0.3 0.3 89.5 0.7 – –

 

Pongamia  pinnata [8]

Sea mango [9]

Soybean [15]

Palm [16]

44.5–71.3 10.8–18.3 3.7–7.9 2.4–8.9 – 9.5–12.4 – – – –

54.2 16.3 20.2 6.9 – – – – – 2.4

23.0 51.0 10.0 4.0 7.0 – – – – –

40.0 10.0 45.0 5.0 – – – – – –

 

Rapeseed [17] 64.1 22.3 3.5 0.9 – – – – 0.1 9.1

 Table 3 Physical and chemical properties of biodiesel from different oil sources as compared to petroleum-derived diesel

Parameters

Non-edible oil

Viscosity (Cst) at 40 C Specific gravity Calorific value (MJ/kg) Flash Point ( C) Cloud point ( C) Pour point ( C) Ash content (wt%) Acid value (mg KOH/g) 1

1

1

1

Edible oil

 Jatropha [26]

Rubber seed [7]

Castor  [27]

4.80 – 39.23 135 – 2 0.012 0.400

5.81 0.874 36.50 130 4 – – 0.118

– 0.960 39.50 260 12 32 0.020 –

 

Petroleumderived diesel

Pongamia  pinnata [9]

Rapeseed [7]

Palm [28]

4.80 – – 150 – – 0.005 0.620

4.50 0.882 37.00 170 4 12 – –

4.42 0.860–0.90 – 182 15 15 0.020 0.080

 

Soybean [7]

4.08 0.885 39.76 69 2 3 – –

2.60 0.850 42 68 – 20 0.010 –

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from different oil sources as compared to petroleum-derived diesel oil. Among all the properties listed in Table 3, cold flow properties (pour point and cloud point) are among the most important properties that are usually looked upon. Cold flow properties basically concern the changes of biodiesel properties such as crystallization, gelling or viscosity increase due to temperature changes that might adversely affect the operability of the vehicles. These properties are reflected by the values of cloud point and pour point. These properties will be the deciding factor if the biodiesel produced can be used in cold climate countries. This is important because currently the largest demand of biodiesel is in the European countries. The cold flow properties of biodiesel are determined by the types of fatty acid in the feedstock oil. Higher percentage of unsaturated fatty acid in the feedstock oil will result in biodiesel having better cold flow properties. For example, biodiesel from palm oil has poor cold flow properties because it has high content of saturated fatty acid (about 50%). Meanwhile biodiesel from rapeseed oil which has high content of unsaturated fatty acid has the best cold flow properties  [29]. Fatty acid composition in the oil is also an important factor that determines the storage stability of biodiesel. Bouaid et al. [30] has conducted a research for a duration of 30 months to study the long storage stability of biodiesel by using the allylic position equivalent (APE) and bis-allylic equivalent (BAPE) concept. In this concept, the oxidative stability is obtained based on the relative rates of oxidation of these positions in unsaturated fatty acid as well as their amounts. They have reported that the oxidative stability of the oil may be more strongly influenced by the presence of small amounts of more highly unsaturated fatty compounds than by increasing amounts thereof. Two important factors affecting the degradation of biodiesel were also observed in their study, which are water content and air exposure. However, the effect of the presence of unsaturated fatty acid on the storage stability of biodiesel can be avoided by taking proper precaution during the storage such as limiting contact to oxygen and exposure to light and moisture.

 2.4. Cultivation requirements

With the increasing human population worldwide, effective land utilization has now become an important issue particularly in developed countries. Proper allocation of land for specific uses such as for agricultural, commercial, domestic and forest reserve is required to ensure efficient utilization and sustainability. If nonedible oils were to be used as feedstock for biodiesel production, more lands would have to be converted to plantations growing these non-edible oils. Therefore, in this section, the requirements for non-edible oil crop cultivation will be discussed in terms of  ecological requirements. Cultivation for edible oils will not be discussed in this paper as it has been well reported. Non-edible oil crops such as jatropha, castor and   P. pinnata have unique ecological requirements and botanical features that make it suitable to be cultivated in lands that are not suitable for food crops. It was reported that jatropha plant can grow almost anywhere, even on gravely, sandy and saline soils [31]. Due to its characteristic, jatropha can be easily cultivated without intensive care and very minimal efforts are required to sustain its growth. It has a healthy life cycle of 30–50 years, which eliminates the need for yearly re-plantation but yet can still sustain reasonably high yield even with minimum irrigation. Castor is another plant that is easily grown and is resistant to drought. It has similar ecological requirements with jatropha. Castor is currently cultivated on commercialized scale for the seeds and oils which are used in textile and printing industry, in the manufacturing of high-grade lubricants and as traditional

 

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medicine   [32]. Castor plantation can be found currently in many countries such as United States, India and Australia with different cultivation cultures. Irrigation is a usual practice in United States; but it is planted as dry crop in India. Due to the similarities between jatropha and castor (ecological requirements such as whether and soil properties), an Indian expert in nonedible oil crops plantation, Lele   [33]   has suggested a more economical and attractive way for the plantation of jatropha and castor plant; that is by intercropped the castor plant during the earlier plantation of jatropha. According to that report, there is little income from the plantation of jatropha alone for first 2–3 years. Therefore, castor can be intercropped to obtain income for that initial duration as castor can give higher production yield in a shorter period than jatropha. This would help to improve economical viability of both jatropha and castor plantation on a commercial scale. On the other hand,   P. pinnata   is native to humid and subtropical environments, thrives in areas having an annual rainfall ranging from 500 to 2500mm. It is one of the few nitrogen fixing trees (NFTS) that produce seeds with a significant oil content.   P. pinnata   is currently cultivated mainly for ornamental purposes due to its large canopy and showy flowers. Pongamia tree can be planted in degraded lands, wastelands, or fallow lands and is highly tolerant to salinity. However, highest growth rate and yield of oil are observed on well-drained soils with assured moisture [34]. Moreover, with its unique role as an NFTS plant,  P. pinnata  can be cultivated on land which has been exhausted of nutrients due to long duration of plantation. It can thus play an important role to help to improve the soil quality so that the exhausted land can be reused for agricultural purpose in future. Sea mango is another type of plant that is cultivated for ornamental purpose. It is naturally distributed at coastal habitats and is often associated with mangrove forests [24]. Nevertheless, sea mango trees are rarely cultivated on large scale or for commercial purposes. Among all the non-edible oils discussed in this study, rubber seed oil can be obtained without much cultivation effort and land area requirement as rubber seed can be obtained easily and abundantly from rubber estates. Rubber trees are currently planted for latex production in tropical countries such as South East Asia and India. Rubber tree requires a fairly deep and well-drained acidic soil. Rubber tree can be grown in a wide variety of soils ranging from clay foam to sandy foam, however, it will grow adequately in impoverished soil with good nutrient supply and balanced manuring program, based on the nutrient status of the soil [23].

 2.5. Cost of plantation

The estimated cost of plantation in USD per hectare and USD per kg oil for edible and non-edible oil crops is shown in Table 4. The difference in the plantation cost is due to different operating cost required during the plantation process such as cost of  fertilizer, herbicides and insecticides. Generally, the cost of  plantation in terms of per kg oil for non-edible oil crops is lower than the plantation cost for edible oil crops, with an exception for palm oil. High requirement on soil nutrient and good irrigation system during the cultivation of edible oil crops such as soybean, rapeseed and oil palm has led to higher plantation cost. However, in the case of palm oil, this factor is offset by the high oil yield. Therefore, this factor allows palm oil to be very economically competitive despite its high cost of plantation per hectare. On the other hand, the low plantation cost for castor and   P. pinnata compared to the rest is basically because these two plants require very minimum fertilizer and irrigation.

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Nevertheless, recently, with the increase in world demand for biodiesel, WEO can actually be a rather promising alternative as biodiesel feedstock.

 Table 4 Plantation cost of several oil crops in USD/ha

Oil crops

Non-edible oil  Jatropha [33] Rubber seed Castor [35,36] Pongamia pinnata  [37] Sea mango (estimated value)  [38] Edible oil Soybean  [39] Palm (estimated value) [12] Rapeseed  [40]

Plantation cost

 

(USD/ha)

(USD/kg oil)

620 N/A 140–160 310 360–690

0.39 N/A 0.12–0.14 0.25 N/A

615 950 336

1.64 0.19 0.34

While conducting this study, the cost of plantation for sea mango is unavailable because there is no large-scale plantation of  sea mango for commercial purpose. Thus, the cost of plantation for sea mango was estimated by referring to the cost of plantation for mango due to the ecological and biological similarity of these two plants. Apart from that, the plantation cost for rubber is negligible because rubber seeds can be obtained abundantly as by-products from existing rubber estate.  2.6. Current techniques available for converting non-edible oil to biodiesel

The technology for converting edible oil to biodiesel has been well established, however, the main concern for converting nonedible oil into biodiesel is always associated with the high free fatty acid (FFA) content. For the production of biodiesel from  jatropha, P. pinnata and rubber seed oil, high FFA content in the oil has caused conventional transesterification reaction especially the alkaline-catalyzed process not feasible. The FFA will react with alkaline catalyst to produce soap that inhibits the separation of  ester and glycerin. A two-step transesterification process is reported by several researchers as the best method to produce biodiesel from non-edible oil. At the initial step, the FFA content of  oil is reduced by acid-catalyzed esterification process; meanwhile, at the second step, an alkaline-catalyzed process is used to convert oil and methanol to methyl esters and glycerol. This twostep process was found to be very effective with the yield of  biodiesel in the overall process reaching up to above 90% [8–10]. However, the disadvantage of this process is the requirement of  two steps, leading to higher production cost as compared to conventional process. 3. Biodiesel from waste edible oil (WEO)

Waste edible oil is oil-based substance consisting of animal and/or vegetable matter that has been used in cooking or preparation of foods and is not longer suitable for human consumption. Previously, WEO was used as an ingredient in animal feed. However, EU has banned the use of waste cooking oil as animal feed due to the concern on animal health and the subsequent food chain [42]. However, due to the large amount of  WEO generated annually, the disposal of WEO has somehow become a problematic issue in most countries. WEO cannot be discharged into drains or sewers because this will lead to blockages and odor or vermin problems and may also pollute watercourses leading to problems for wildlife. It is also prohibited and will cause problems if it is dumped in municipal solid waste landfill and municipal sewage treatment plant  [41,42].

 3.1. Waste edible oil (WEO): availability, economic value and  properties

The amount of WEO generated from every country worldwide is huge and varies accordingly to the amount of edible oil consumed. Annually, a total of more than 15 million tons of WEO is generated from selected countries in the world as shown in Table 5. It should be noted that the worldwide WEO is way much larger than that value. Therefore, WEO is readily available feedstock that can be used for biodiesel production. The cost for WEO is generally lower than fresh edible oil as a major fraction contributing to the cost lies in the collection and purifying processes. For comparison purposes, the costs of soybean and WEO are given. Currently, WEO from soybean is sold as yellow grease that has a market value of approximately $1.09/US gallon and is expected to rise to $1.21/US gallon by the year 2013. On the other hand, the price of soybean in the market is about $2.22/US gallon in year 2004 and is expected to rise to $2.47/US gallon in 2013 [45]. Generally, the physical and chemical properties of WEO are almost similar to fresh edible oil and are different from source to source. For example, the chemical and physical properties of palm WEO might be different from rapeseed WEO due to the different oil composition. Apart from that, the water content and FFA content in WEO are relatively higher than fresh edible oil as a result of frying process. During the frying process, edible oil is heated in the presence of air and light at temperatures of  160–200 C for a relatively long period of time. Some common physical changes observed in edible oil after frying are: increase in viscosity and specific heat, change in surface tension, color and higher tendency of fat formation. All these changes are due to three common reactions during the frying process: thermolytic, oxidative and hydrolytic [47]. 1

 3.2. Current techniques available in converting WEO into biodiesel

Despite the changes in WEO chemical and physical properties as compared to fresh edible oil, both of the oil feedstocks can still be converted to biodiesel using similar method; that is via catalytic reaction using alkali catalyst, acid catalyst and enzyme or via non-catalytic reaction in supercritical transesterification reaction. Nevertheless, the high content of FFAs and water in WEO has caused alkaline-catalyzed transesterification reaction less efficient. A more suitable method would be via non-catalytic supercritical reaction or the two-step transesterification method that consists of acid-catalyzed reaction followed by alkalicatalyzed reaction.   Table 6   summarizes the recently introduced transesterification techniques to convert WEO into biodiesel as reported in the literature.  Table 5 Quantity of waste edible oil (WEO) in various countries worldwide

Country

Quantity (million tons/year)

China [43] Malaysia  [44] United States [45] Taiwan [46] European  [47] Canada  [47]  Japan [48]

4.5 0.5 10.0 0.07 0.7–1.0 0.12 0.45–0.57

         

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 Table 6 Current techniques available to convert WEO into biodiesel

Technique

Description

Biodiesel yield (%)

Advantages

Disadvantages

Source

Acid-catalyzed transesterification Solid base catalyzed transesterification

Canola WEO transesterified with methanol catalyzed by sulfuric acid Transesterification of WEO with methanol catalyzed by calcium oxide as solid base catalyst

99% at 4h reaction 99% at 2h reaction

High biodiesel yield

Acid catalyst causes corrosive to equipment Formation of calcium soap at initial stage of  transesterification

Zheng et al. [49]

Acid-alkaline catalyzed transesterification

Step 1: FFA esterified with methanol catalyzed by ferric sulfate as acid catalyst Step 2: Transesterification with methanol catalyzed by potassium hydroxide

Overall: 97.02% at 5 h reaction

High biodiesel yield

Involved two-step processes. Higher production cost might be required

Wang et al. [51]

Silica gel pretreatment followed by alkaline-catalyzed transesterification

Step 1: Palm WEO was pretreated by silica gel as absorbent to remove FFA content Step 2: Transesterification of WEO with methanol using NaOH as alkaline catalyst

99%

High biodiesel yield

Involved two-step processes. Thus, higher production cost might be required

Loh et al. [44]

Lipase-catalyzed transesterification

Lipase was immobilized on hydrotalcite and catalyzed transesterification reaction at room temperature

95% at 105h reaction

Long reaction time Might not be feasible for largescale production

Yagiz et al. [52]

High biodiesel yield in short reaction time

Easy recovery of  catalyst

Characteristic of  biodiesel obtained similar to biodiesel from fresh edible oil

The characteristics of the biodiesel produced from WEO are indeed similar to those obtained from their respective fresh edible oil. Loh et al. [44], who had conducted research in production of  biodiesel from WEO, have proved that the biodiesel derived from palm WEO has characteristic similar to biodiesel derived from fresh palm oil if proper pretreatment process is conducted before transesterification process. In their study, silica gel has been used as absorbent to reduce the FFA content and peroxide value, while the biodiesel is produced by using conventional transesterification process. The characteristics of both biodiesel from palm WEO and crude palm oil such as density, viscosity, sulfur content, pour point, flash point and gross heat of combustion are found to be similar.

4. Current scenario

Enable transesterification in room temperature

Kouzu et al.  [50]

3000

   N 2800    O    T 2600    /    )    A    I    S 2400    Y    A    L 2200    A    M 2000    T    I    G    G 1800    N    I    R    ( 1600    E    C    I 1400    R    P 1200 1000

Currently, the global biodiesel market is led by Europe and US. In the year 2005, the total consumption of biodiesel reached 3.32million tons/year; which is 3.07million tons/year in Europe and 0.25 million tons/year in US. These values are only 2% of the total amount of diesel consumed for transportation in Europe and 0.5–1% in US. In a bid to further boost the use of biofuels in European transport, EU leaders have committed to raise the share of biofuels in transport to 5.75% and 20% in the year 2010 and 2020, respectively. Therefore, the demand for biodiesel in Europe alone is expected to increase to more than 10 million tons/year in the year 2010 and 68 million tons/year in 2020 [53]. At the moment, soybean and rapeseed oil are the widely used biodiesel feedstock in EU and US market. This is not only because of its large availability in these two regions, but also due to the properties of the biodiesel produced. It was found that biodiesel produced from rapeseed and soybean oil has good cold flow properties making it suitable to be used during cold weather in these two regions especially during winter. Nevertheless, the relatively low yield (and also high cost) of these two oils will not be sufficient to meet the increasing demand of biodiesel in the future. Therefore, in the recent years, more and more biodiesel producers have been targeting on palm oil. With the highest yield per hectare, palm oil is currently the world’s cheapest and largest edible oil that is being consumed. The low cost of palm oil makes

Jan-04

Jan-05

Jan-06

Jan-07

Fig. 2.  Prices of crude palm oil in 2004–2007.

it a very suitable candidate to be converted to biodiesel. In the recent years, more and more biodiesel production plants using palm oil as the feedstock have been set up in Thailand, Malaysia and Indonesia to take advantage of the low cost of palm oil. Currently, in Malaysia alone, five biodiesel plants are in operation and another five are coming on stream in the near future. Nevertheless, it was reported that about 90 biodiesel plant licences has been issued in early 2007 in order to reach the targeted production rate of as high as 3 million tons annually [54]. However, since palm oil is also the world’s largest edible oil, the competition of using palm oil as a source of edible oil vs. biodiesel has caused the price of crude palm oil to rise significantly in the last 1 year as shown in Fig. 2 [12]. This trend is a clear indication that edible oil might not be the best candidate as the feedstock for biodiesel production. If the price of palm oil continues to soar higher, it is just a matter of time, consumers who use palm oil as a source of edible oil will protest. Apart from that, edible oils such as soybean, rapeseed and palm oil are food sources that have valuable nutrients that should not be ignored. These nutrients are beneficial for human health and are therefore

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more worthwhile to be used as food stock for human rather than as engine fuel.

5. Recommendation

It is clear that the demand of biodiesel will increase significantly in the future and although edible oils, mainly palm oil might be the cheapest feedstock for biodiesel production, but it may not be a sustainable source. This justifies the need to find a reliable, economical and sustainable feedstock for biodiesel production. The data presented in the early section clearly indicate that there are many other non-edible oils that can be the alternative feedstock for biodiesel production. Although the yield of oil from these non-edible oil plants is lower than oil palm, they are still comparable with rapeseed and soybean, two of the main biodiesel feedstock at the moment. Apart from that, advancement in research has also shown that the quality of  biodiesel obtained from different feedstocks is no longer an issue as additive can be added to improve the cold properties of  biodiesel produced from oils with high content of saturated fatty acid. Furthermore, most of the non-edible plants can be grown in wasteland and infertile land which otherwise would not have much use. This would not only allow wasteland utilization but at the same time would also be used to produce oil crops for biodiesel production without the need to compete with food crops for the limited arable land. Taking all these factors into consideration, non-edible oils definitely have the advantage over edible oils as biodiesel feedstock. At the same time, we should always keep in mind that WEOs are readily available feedstock for biodiesel production. Production of biodiesel from WEO will avoid the competition of the same oil resources for food and fuel. Apart from that, it will also overcome the WEO disposal problem. The statistic presented in the earlier part of this paper showed that on the basis of total amount of WEO in certain countries in this world, it is sufficient to meet the current world demand of biodiesel. Apart from that, recent development in research has also proved that the quality of  biodiesel produced from various sources of WEO can meet international biodiesel standard, provided proper pre-treatment steps are taken. Therefore, WEO should be used as the primary feedstock for biodiesel production throughout the world. However, in order to ensure that this ideology can be implemented successfully, WEO should not only be collected from the industries and bulks users but also every individual person and every single household in this world would be required to play their role. This is not something impossible, as it has been proved in Kyoto, Japan, that the collection and conversion of WEO from household within the city to biodiesel is sufficient to supply fuel for local buses. The important factor here would be the awareness level among the citizens of the city so that they would realize the importance of  their role in making the world a better place to live in. The cost involved would mainly be the collection process. However, if the truck used for the collection of WEO runs on the biodiesel produced, this would keep the collecting cost at minimal level, and would make this whole process very sustainable. On top of that, fresh edible oils and non-edible oils should be used as supplements to meet the remaining demand for biodiesel. Economically, palm oil, being the worlds’ cheapest oil, would be the most suitable candidate as the feedstock. Nevertheless, the competition between palm oil as a source of fuel and food supply has caused the price of palm oil to increase sharply within the last 1 year. This incident actually serves as a reminder that the world should never be overdependent on a single resource, neither as a food resource nor as fuel resource. The worlds’ dependence on crude petroleum would serve as another perfect example.

Therefore, the feedstock for biodiesel should come from a diversified oil sources, either edible or non-edible depending on geographical locations. An ideal solution would be an equal share contributed by edible oil and non-edible oil. Fertile agricultural land should remain for edible oil cultivation while wasteland or fallow land should be planted with non-edible oil crops such as castor, jatropha and   P. pinnata   that has simpler ecological requirements. This will allow optimum utilization of limited land areas especially in developed countries. Diversified resources for biodiesel feedstock will also ensure that the quality of biodiesel obtained is suitable within that particular region. For example, biodiesel produced from palm oil has poor cold flow properties and is therefore not suitable for cold weather countries; however, it can be used as a very good feedstock for hot weather countries such as in the South East Asia, India, Brazil and South America countries. On the other hand, biodiesel with good cold flow properties produced from rapeseed oil, jatropha and castor can be specifically used in cold weather countries especially North America and European countries. 6. Conclusion

The demand for biodiesel worldwide is expected to increase sharply in the near future. Competition of edible oil sources as food vs. fuel makes edible oil not an ideal feedstock for biodiesel production. Instead, WEO should be made the primary source for biodiesel feedstock due to its abundant availability. Fresh edible and non-edible oils can then be used to supplement the shortfall of WEO as feedstocks. This arrangement will ensure that biodiesel can be produced from a variety of feedstocks. Ultimately, this will make biodiesel a sustainable resource replacing petroleumderived diesel oil without significantly affecting the global food economy.  Acknowledgments

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