Environmental Engineering Reference
In-Depth Information
The cetane number of a fatty acid chain increases with increasing chain length and increasing
saturation. Thus, methyl stearate has a high cetane number (> 90) and methyl linolenate has a low
cetane number (~25). The cetane number of methyl stearate is also higher than that of methyl laurate
(~65). The cetane number of a mixture (e.g., biodiesel) approximately correlates with the cetane
numbers of the individual components proportionally taking their amounts into consideration.
Minimum cetane numbers in biodiesel standards are 47 in ASTM D6751 (United States) and 51 in
EN 14214 (Europe). Most biodiesel fuels possess cetane numbers in the range of the high 40s to
lower 60s.
The high viscosity of vegetable oils, approximately an order of magnitude greater than that of
petroleum-based diesel fuel (petrodiesel), is the major reason why these feedstocks are transesterified
to biodiesel. The high viscosity of vegetable oils, influencing penetration and atomization of the fuel
in the combustion chamber, leads to operational problems such as engine deposits. Biodiesel fuels
possess viscosity values closer to those of petrodiesel. Thus, kinematic viscosity is prescribed in
biodiesel standards with the ranges being 1.9-6 mm
2
/s (ASTM D6751) and 3.5-5 mm
2
/s (EN 14214).
Most biodiesel fuels exhibit kinematic viscosity in the range of 4.0-5 mm
2
/s. Again, compound
structure significantly influences viscosity. Viscosity increases with chain length and decreasing
cis
-unsaturation. The kinematic viscosity of methyl laurate is 2.43 mm
2
is that of methyl palmitate
is 4.38 mm
2
is methyl stearate is 5.85 mm
2
is methyl oleate is 4.51 mm
2
is methyl linoleate is 3.65
mm
2
is and methyl linolenate is 3.14 mm
2
is (Knothe and Steidley 2005). Thus, the kinematic
viscosity of a biodiesel fuel depends on its fatty acid profile.
Oxidative stability is one of the major technical issues affecting the commercial use of biodiesel.
Oxidation of fatty acid chains is a complex reaction, consisting initially of the formation of
hydroperoxides followed by secondary reactions during which products such as acids, aldehydes,
ketones, hydrocarbons, etc., can be formed. Unsaturated fatty acid chains, especially the
polyunsaturated species (i.e., esters of linoleic and linolenic acids) are susceptible to oxidation.
Relative rates of oxidation given in the literature (Frankel 2005) are 1 for oleates, 41 for linoleates,
and 98 for linolenates. Small amounts of unsaturated fatty esters probably affect oxidative stability
more than their small amounts indicate. Oxidative stability is addressed in biodiesel standards
primarily by the corresponding specification which prescribes the use of a Rancimat instrument.
This instrument permits an accelerated test to be conducted with the goal of judging the oxidative
stability of a sample. The lower the induction time by this method, the less oxidatively stable the
sample. Minimum induction times prescribed in biodiesel standards by this test are 3 h (ASTM
D6751) and 6 h (EN 14214). However, an antioxidant will almost always be needed to achieve these
specifications because the induction time of methyl oleate is 2.79 h, that of methyl linoleate is 0.94 h,
and that of methyl linolenate is 0 h. Methyl esters of saturated fatty acids possess induction times
greater than 24 h (Knothe 2008). No typical oxidative stability times for specific biodiesel fuels can
be given because oxidative stability is strongly influenced by factors such as storage conditions and
the presence of extraneous materials, including antioxidants. However, to achieve the mentioned
minimal times in standards, the use of antioxidants is almost always necessary as mentioned above.
It may be noted that the European biodiesel standard EN 14214 contains some specifications that
can also be related to the phenomenon of oxidative stability. These specifications are the iodine
value, a crude measure of total unsaturation of a sample, a maximum of 12% for linolenic acid
methyl esters and a maximum of 1% for esters of fatty acids with more than three double bonds.
In addition to oxidative stability, cold flow is another major technical issue that affects the
commercial use of biodiesel. Almost all biodiesel fuels have relatively poor cold-flow properties, as
demonstrated by relatively high cloud points. The cloud point (i.e., the temperature at which the first
solids are visible in a sample upon cooling) for the methyl esters of soybean oil is approximately
0°C and only slightly lower, approximately -3°C, for the methyl esters of rapeseed (canola) oil. The
esters of biodiesel fuels with high amounts of longer-chain saturated fatty esters may have even
higher cloud points. For example, the cloud point of the methyl esters of palm oil which contains
approximately 40% methyl palmitate, is 15°C or even higher. The melting points of fatty esters
