Biomedical Engineering Reference
In-Depth Information
Table 9. Physical characterisations of biodiesel from different feedstocks
Cloud
point( )
Pour
point( )
Iodine
value
Cetane
number
Heat value
(kJ kg -1 )
Viscosity
mm 2 s -1
Feedstock
Diesel
6-12
47
45343
2.7
-15.0
-33.0
Tallow
35-48
-
40054
51.15
-
-
Sunflower
110-143
37.1
39575
37.1
7.2
-15.0
Soybean
117-143
37.9
39623
32.6
-3.9
-12.2
Safflower
126-152
41.3
38519
31.3
18.3
-6.7
Rapeseed
94-120
37.6
39709
37.0
-3.9
-31.7
Peanut
80-106
41.8
39782
39.6
12.8
-6.7
Palm
35-61
42
-
-
-
-
Cotton
90-119
41.8
39468
33.5
1.7
-15.0
Recycle oil
-
46.5
41132
38.5
12
2
3.5. Chain Length and Degree of Saturation
While diesel fuel typically contains hundreds of compounds, biodiesel, regardless of the
feedstock, has similar chemistry. As discussed earlier, the feedstock oil or fat consists of
triglycerides. The triglyceride has a glycerin backbone of three carbons. Attached to each
carbon is a long chain fatty acid. The variation comes in the length and saturation of the long
chain. In the common types of fatty acids in the various types of oils and fats used for
biodiesel, the chains range from 12 to 22 carbons in length. The degree of saturation refers to
the amount of hydrogens bonded to each carbon along the chain. A saturated chain has the
maximum number of hydrogens on the carbons. Therefore, the amount of unsaturation is
shown by less than maximum number of hydrogen which results in double bonds. One double
bond is considered monounsaturated and two or more double bonds on one chain are
polyunsaturated. Fatty acids are often characterized by the number of carbon to double bonds.
For instance, 16:1 indicates a carbon chain length of 16 and 1 double bond
(monounsaturated). Different feedstocks are made of differing combinations of saturated,
monounsaturated, and polyunsaturated fatty acids. According to the Department of Energy,
the “'perfect' biodiesel would be made only from monounsaturated fatty acids.” [1] Figure 11
shows the compositions of common feedstocks.
Table 10. Fuel properties as a function of fuel composition in diesel engines [4]
As shown by Table 10, while it would be ideal to have a moderate fuel composed of
monounsaturated fatty acids, the actual composition of oils is not this simple. It is therefore
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