11 degradation. Table 1 shows the comparison of fossil diesel fuel with vegetable
oil characteristic [17]. Table 1. Comparison of fossil diesel fuel with vegetable oil characteristics
Oil Density at 20
o
C, kgliter
Kinematics viscosity at
20
o
C, cSt ∆Hc,
MJkg Cetane
Number Cloud
point,
o
C. Pour point,
o
C.
Coconut 0,915
30 37,10
40 – 42 28
23 – 26 Palm
0,915 60
36,90 38 – 40
31 23 – 40
Jatropha Curcas 0,920
77 38,00
23 – 41 2
-3 Peanut
0,914 85
39,30 30 – 41
9 -3
Soybean 0,920
61 37,30
30 – 38 -4
-20 Sunflower
0,925 58
37,75 29 – 37
-5 -16
Diesel 0,830
6 43,80
50 -9
-16 Source : Soerawidjaja, T., H., 2006 [17]
It is clear that the problems with substituting vegetable oil for diesel fuel are mostly associated with their high viscosities, low volatilities and polyunsaturated
character. Consequently, long term operation on neat vegetable oils or on mixture of vegetable oils with fossil diesel fuel, inevitably would result in an engine
breakdown. These problems can be solved by either adapting the engine to the fuel or by adapting the fuel to the engine. Four methods widely used to reduce the
high viscosity of vegetable oils to enable their use in common diesel engines without operational problems are pyrolysis, micro emulsification, dilution, and
transesterification [18], but only the transesterification reaction can lead to the products commonly known as biodiesel, i.e., Alkyl esters of oil and fats [19, 20].
Pyrolysis denotes thermal decomposition reaction, usually brought about in the absence of oxygen. The cetane number of plant oils is increased by pyrolysis,
and the concentrations of sulfur, water and sediment for the resulting product are acceptable. However, according to modern standards, the viscosity of the fuels is
considered as too high, ash and carbon residue far exceed the values for fossil diesel, and the cold flow properties of paralyzed vegetable oils are poor [21]. The
equipment for thermal cracking and pyrolysis is expensive for modest
12 throughputs. In addition, while the products are chemically similar to
petroleum-derived gasoline and diesel fuel, the removal of oxygen during the thermal processing also removes any environmental benefits of using an
oxygenated fuel. It produced some low value materials and, sometimes, more gasoline than diesel fuel [20].
Micro emulsification is the formation of thermodynamically stable dispersion of two usually not miscible liquids, brought about by one or more
surfactants. Drop diameters in micro emulsions typically range from 100 to 1000A [18]. Various investigators have studied micro emulsification of vegetable
oils with methanol, ethanol or 1-butanol [16]. They arrived at the conclusion that micro emulsions of vegetable oils and alcohols cannot be recommended for long-
term use in diesel engines for similar reasons applied to neat vegetable oils. The fuels produced are prone to incomplete combustion, the formation of carbon
deposits, and an increase in the viscosity of the lubricating oil. Moreover, micro emulsions display considerably lower volumetric heating values as compared to
hydrocarbon-based diesel fuel due to their high alcohol contents [18], and they have been assessed insufficient in terms of cetane number and cold temperature
behavior. Dilution of vegetable oils can be accomplished with such materials as diesel
fuels, a solvent or ethanol. Most studies concluded that vegetable oilpetrodiesel blends are not suitable for long-term fuelling of direct injection diesel engines.
The studies results, yielding engine problems similar to those found for combustion of neat vegetable oils [18, 19].
Another alternative to improve suitability of vegetable oil for diesel engine is by hydrogenating the oil using typical process of commonly found in crude oil
refinery. Koyama et al. 2006 Koyama A., Iki H., Iguchi Y. Applicability of Hydrogenated Palm Oil for Automotive Fuels. 16
th
Saudi Arabia – Japan Joint Symposium. Dhahran. Saudi Arabia. November 5 – 6. 2006 had tried to directly
hydrogenate palm oil at temperature range of 240
o
C to 360
o
C and pressure of 6 to 10 MPa with catalyst normally used in hydrodesulphurization . They suggested
that the product has a better property in term of oxidation stability than palm oil methyl ester. The relatively better in oxidation stability would make it potentially
13 possible for blending with fossil diesel at higher ratio than currently suggested
by the Wide World Fuel Chapter organization of 5. However, this fuel had relatively higher Cold Flow Plugging Point parameter CFPP 20
o
C than that of FAME CFPP 12
o
C, which prohibits its use in winter season and has problems in cold start up. Although this method may offer a great opportunity to substitute the
diesel oil, large-scale demonstration of such a finding followed by full commercialization would take some time. On the other hand, transesterification
of vegetable oil has reached full commercialization and it would be an ideal choice for diesel oil substitute.
Transesterification [22], also called alcoholysis, is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis, except
alcohol is used instead of water. The result of fatty acid transesterification is Fatty Acid Methyl Esters FAME, generally known as biodiesel. Several vegetable oils
methyl esters VOME characteristic is shown in Table 2 [17]. The resulting biodiesel is quite similar to conventional diesel fuel in its main characteristics.
Biodiesel is compatible with conventional diesel and the two can be blended in any proportion. Therefore, transesterification with lower alcohols, however, has
turned out to be an ideal modification, so that the term “biodiesel” is now only used to denote products obtained by this technology.
Table 2. Vegetable Oil Methyl Esters VOME characteristics
Methyl Ester Density at
15
o
C, kgliter
