Minor Seed Oils - Bioenergy Crop Plants - page 642

From the fitted line equation y = -0.027 x + 41.63, the projected decrease in heating value for
every 1% increase in unsaturation is 0.027 units (MJ/kg).
25.4.1.3 cetane number
Cetane number (CN) indicates the ignition quality of a fuel, i.e., how fast a fuel takes part in
combustion. In other words, it quantifies the activation energy to start the reaction. Higher CN
implies lower activation energy and therefore quick reaction. Hence, the time required for a fuel to
start the combustion reduces, which is the ignition delay period. Therefore, higher CN indicates a
shorter ignition delay period. Delay period is the time interval between the start of fuel injection and
the start of combustion. When the delay period is reduced, the fuel accumulated during the delay
period also reduces and therefore a small amount of fuel can take part in combustion. In contrast,
lower CN fuel has a higher activation energy and longer ignition delay period. A longer ignition
delay period accumulates more fuel and higher premixed combustion, which in turn produces more
oxides of nitrogen.
CN is a derived property that is influenced by several other physical and chemical properties.
In the case of biodiesel, CN is influenced by chain length, alcohol moiety, degree of unsaturation,
position of double bond, and cis or trans structure (Knothe 2005). CN increases with chain length
and decreases with degree of saturation (Knothe 2005). Gerhard Knothe states that
• CN increases with chain length and decreases with the number of double bonds or
unsaturation.
• CN of fatty esters generally increases with the number of methylene groups (CH 2 ) in
the chain of the fatty compound, the number of CH 2 groups in the ester moiety, and the
increasing saturation of the fatty compound.
• For the methyl esters, the CN were found to increase in a nonlinear relationship with
molecular weight (chain length).
From Table 25.5, it can be observed that MOME has a higher cetane number compared with
other biodiesel fuels. The higher CN of MOME can be attributed to the higher carbon number, i.e.,
stearic ester (C18:0). On the other hand, the ROME has a lower CN than that of other biodiesel fuels.
This is because the CN decreases as the number of double bonds (unsaturation) increases. The influ-
ence of unsaturation on CN is depicted in Figure 25.7.
From the fitted line equation y = -0.373 x + 80.27, it can be noticed that a reduction of 0.373 units
in CN could be predicted for each percentage increase in unsaturation.
25.4.1.4 Iodine value
Iodine value (IV) that can be considered as a direct measure of unsaturation may be defined as
the amount of iodine (in grams) necessary to saturate a 100-g oil sample. IV is included in the
European biodiesel standards. It purportedly addresses the issue of oxidation stability because the
IV is a measure of total unsaturation of fatty materials measured in grams of iodine per 100 g of
a sample when formally added to the double bonds. Another idea behind the use of the IV is that
it indicates the propensity of the oil or fat to polymerize and form engine deposits (Graboski et al.
1998). The engine manufacturers have always been aware of the iodine number which expresses the
number of double bonds. The limit was set at 120 in EN14214 and 135 in EN14213 (Prankl 2002).
From Table 25.5, it can be noticed that ROME has an IV of 160 and MOME has a value of 65.
The influence of fatty acid ester composition on IV for biodiesel samples was studied. From Table
25.6, the correlation analysis exhibits a strong positive correlation between IV and percentage of
unsaturation. This was expected because from the definition, IV could be considered as a direct
measure of unsaturation. Figure 25.8 depicts the effect of unsaturation on IV.
From the fitted line equation y = 2.544 x - 69.26, every 1% increase in unsaturation may result in
an increase of 2.544 units (g iodine/100 g oil) in IV.