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It is not
necessary to be a chemist to
understand where biodiesel
comes from and how it is
used. However, it is useful
to review some of the
fundamental chemical
principles that are behind
biodiesel so that its
properties can be
understood.
All vegetable oil and
animal fats consist
primarily of triglyceride
molecules as shown
schematically below.
R1,
R2, and R3
represent the
hydrocarbon chain of the
fatty acid elements of
the triglyceride. Note
that there is a
three-carbon chain
called the glycerol
backbone that runs along
the left side of the
molecule. Extending
away from this backbone
are the three long fatty
acid chains.
In their free form, the
fatty acids have the
configuration shown
below.
where R
is a hydrocarbon chain
of greater than 10
carbon atoms
The properties of the triglyceride and the
biodiesel fuel will be
determined by the
amounts of each fatty
acid that are present in
the molecules.
Fatty acids are
designated by two
numbers: the first
number denotes the total
number of carbon atoms
in the fatty acid and
the second is the number
of double bonds. For
example, 18:1 designates
oleic acid which has 18
carbon atoms and one
double bond. Table 1
shows the fatty acid
compositions of a number
of common vegetable oils
and animal fats.
Table
1 - Composition of
Various Oils and
Fats.
|
Oil or fat
|
14:0
|
16:0
|
18:0
|
18:1
|
18:2
|
18:3
|
20:0
|
22:1
|
|
Soybean
|
|
6-10
|
2-5
|
20-30
|
50-60
|
5-11
|
|
|
|
Corn
|
1-2
|
8-12
|
2-5
|
19-49
|
34-62
|
trace
|
|
|
|
Peanut
|
|
8-9
|
2-3
|
50-65
|
20-30
|
|
|
|
|
Olive
|
|
9-10
|
2-3
|
73-84
|
10-12
|
trace
|
|
|
|
Cottonseed
|
0-2
|
20-25
|
1-2
|
23-35
|
40-50
|
trace
|
|
|
|
Hi linoleic
Safflower
|
|
5.9
|
1.5
|
8.8
|
83.8
|
|
|
|
|
Hi Oleic
Safflower
|
|
4.8
|
1.4
|
74.1
|
19.7
|
|
|
|
|
Hi Oleic
Rapeseed
|
|
4.3
|
1.3
|
59.9
|
21.1
|
13.2
|
|
|
|
Hi Erucic
Rapeseed
|
|
3.0
|
0.8
|
13.1
|
14.1
|
9.7
|
7.4
|
50.7
|
|
Butter
|
7-10
|
24-26
|
10-13
|
28-31
|
1-2.5
|
.2-.5
|
|
|
|
Lard
|
1-2
|
28-30
|
12-18
|
40-50
|
7-13
|
0-1
|
|
|
|
Tallow
|
3-6
|
24-32
|
20-25
|
37-43
|
2-3
|
|
|
|
|
Linseed Oil
|
|
4-7
|
2-4
|
25-40
|
35-40
|
25-60
|
|
|
|
Tung Oil |
|
3-4 |
0-1 |
4-15 |
|
75-90* |
|
|
|
Yellow grease |
1.27 |
17.44 |
12.38 |
54.67 |
7.96 |
0.69 |
0.25 |
0.52 |
Peterson, C.L.,
"Vegetable Oil as a
Diesel Fuel: Status and
Research Priorities,"
ASAE Transactions, V.
29, No. 5, Sep.-Oct.
1986, pp. 1413-1422.
Linstromberg, W.W.,
Organic Chemistry,
Second Edition, D.C.
Heath and Company,
Lexington, Mass., 1970.
Tat,
M.E., and J. H. Van
Gerpen, "Fuel Property
Effects on Biodiesel,"
ASAE Paper No. 036034,
American Society of
Agricultural Engineering
Annual Meeting, Las
Vegas, NV. July 27-30,
2003.
*
The
dominant fatty acid in
tung oil is a conjugated
isomer of linolenic acid
called eleostearic acid.
The three double bonds
in eleostearic acid are
located at 9:10, 11:12,
and 13:14 instead of at
9:10, 12:13 and 15:16 as
in linolenic acid.
The names of the fatty
acids given in Table 1
are as follows:
|
14:0
|
Myristic Acid
(tetradecanoic
acid)
|
|
16:0
|
Palmitic Acid
(hexadecanoic
acid)
|
|
18:0
|
Stearic Acid
(octadecanoic
acid)
|
|
18:1
|
Oleic Acid
|
|
18:2
|
Linoleic Acid
|
|
18:3
|
Linolenic
Acid
|
|
20:0
|
Arachidic
Acid (eicosanoic
acid)
|
|
22:1 |
Erucic Acid |
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