What Do You Need to
Know?
As described previously, biodiesel
lowers emissions, raises the fuel’s cetane number
and flash point, and has excellent lubricity. It
also has the powerful advantage that it can be used
directly in diesel engines without modifications. In
spite of these advantages, there are some things you
should know about biodiesel.
The most serious obstacle to the
commercialization of biodiesel in the United States
is its high cost. Alternate uses for soybean oil
and even animal fats and recycled greases keep their
price at a level where they cannot compete directly
against diesel fuel from petroleum. Government
subsidies of some type will be necessary for the
industry to develop. These subsidies, primarily in
the form of tax waivers, have been responsible for
the rapid growth in biodiesel in Europe. In
Germany, the price of biodiesel is actually less
than for petroleum diesel fuel as shown in the
photograph below.
In Minnesota, a state
mandate requiring the use of 2% biodiesel in all
fuel sold for use in engines by 2005 is another type
of subsidy. This subsidy removes price as an issue
since the fuel suppliers will have to pay whatever
is necessary to get the biodiesel they need for
blending.
There are other
issues about biodiesel that consumers should be
aware of, including: cold flow problems, stability,
energy content, elevated NOx levels, and material
compatibility. These issues will be discussed below.
Cold Flow
At low temperatures, biodiesel will
gel or crystallize into a solid mass that cannot be
filtered or pumped. The engine cannot run at these
temperatures. This is not a new problem for diesel
engine operators. Petroleum-based diesel fuel also
gels but at temperatures that are lower than for
biodiesel.
Table 1 shows data for the cold flow
properties of biodiesel and blends of biodiesel with
diesel fuel. The cloud point is the temperature at
which crystals first start to form in the fuel and
the pour point is the lowest temperature at which
the fuel will still pour from a container. The cold
filter plugging point (CFPP) is the lowest
temperature at which at certain volume of fuel can
be drawn through a metal screen filter. It usually
correlates well with the lowest temperature that an
engine will operate. A more complete discussion of
these measures can be reached by
clicking here .
Table 1 shows that soybean oil based
biodiesel will start to gel at about 32 Table 1 also
shows that even small amounts of biodiesel can raise
the temperatures at which gelling will occur. It can
also be seen that the differences between the cloud
and pour points are much larger for petroleum diesel
fuels than they are for biodiesel fuels. For
example, the difference between the cloud point and
the pour point for the yellow grease is only 3
degrees F while the difference for #2 diesel fuel is
34 degrees F. This difference reflects that fact
that biodiesel consists of a relatively few
compounds that all solidify at about the same
temperature. When crystallization starts for
biodiesel the entire fuel tends to gel at the same
time. In contrast, petroleum diesel fuel is a
mixture of hundreds of different compounds that
solidify at very different temperatures. So, even if
some compounds crystallize at a relatively high
temperature, many other compounds will stay liquid
to a much lower temperature.
|
|
Cloud Point (oF) |
Pour Point (oF) |
CFPP(oF) |
|
Soy Methyl Esters (soy) |
32 |
25 |
22 |
|
Yellow Grease Methyl
Esters (YG) |
48 |
45 |
48 |
|
#2 low Sulfur Diesel Fuel
(#2) |
4 |
-30 |
1 |
|
2% Soy in #2 |
6 |
-25 |
1 |
|
5% Soy in #2 |
8 |
-20 |
-1 |
|
2% YG in #2 |
6 |
-25 |
0 |
|
5% YG in #2 |
9 |
-20 |
1 |
|
50% #2
/ 50% #1
(50/50) |
-6 |
-45 |
-12 |
|
2% Soy in 50/50 |
-6 |
-40 |
-14 |
|
5% Soy in 50/50 |
-6 |
-40 |
-14 |
|
2% YG in 50/50 |
-6 |
-40 |
-12 |
|
5% YG in 50/50 |
-2 |
-30 |
-10 |
|
#1 Low Sulfur Diesel Fuel
(#1) |
-54 |
-70 |
< -30 |
|
2% Soy in #1 |
-45 |
-60 |
< -30 |
|
5% Soy in #1 |
-32 |
-55 |
< -30 |
|
2% YG in #1 |
-38 |
-60 |
< -30 |
|
5% YG in #1 |
-19 |
-60 |
-26 |
Table 1. Cold Flow
Characteristics of Biodiesel Blends [Williams
Laboratory Services].
More cold flow
data are available and can be accessed by
clicking here
Fuel energy content
As can be seen in the table below, biodiesel
has a lower energy content (lower heating value)
than No. 2 diesel fuel. On a weight basis, the
energy level is 12.5% less. Since biodiesel is more
dense than the diesel fuel, the energy content is
only 8% less on a per gallon basis.
|
|
Btu/lb |
Btu/gal |
|
No. 2 Diesel |
18,300 |
129,050 |
|
Biodiesel |
16,000 |
118,170 |
|
|
(12.5% less) |
(8% less) |
Since
diesel engines will inject equal volumes of fuel,
most diesel engine operators see a power loss of
about 8%. In some cases, the power loss may be even
less than this. Biodiesel's higher viscosity can
decrease the amount of fuel that leaks past the
plungers in the diesel fuel injection pump.
Tests have shown that the actual efficiency at which
the energy in the fuel is converted to power is the
same for biodiesel and petroleum-based diesel fuel.
Therefore, the brake specific fuel consumption (BSFC),
the parameter most often used by engine
manufacturers to characterize fuel economy will be
at least 12.5% higher for biodiesel.
Other issues relating to biodiesel
production and use:
|