|
What
are Biodiesel's Advantages?
In a previous discussion it was
noted that biodiesel is renewable, nontoxic, and
biodegradable. Depending on the audience, these may
or may not be strong advantages. While biodiesel is
definitely renewable, the fact that it cannot
displace a significant fraction of our current
petroleum-based fuel consumption means that it does
not really allow us to make much progress toward a
sustainable energy supply. Nontoxicity and
biodegradability are useful characteristics but they
are only significant when the fuel is used in its
pure form (B100) as is common in Germany and
Austria. For the 20% and lower blends that are
common in the United States, the diesel fuel portion
of the blend determines the toxicity and
biodegradability. Biodiesel does provide a
reduction in net CO2 emissions. Although
the amount of CO2 emitted from the
exhaust pipe per kilowatt of power is essentially
the same as for petroleum diesel fuel, the carbon
was originally removed from the atmosphere so there
is little net change in atmospheric carbon dioxide.
Biodiesel's primary advantages lie in
its effect on cetane number, emissions, its flash
point, and its lubricity.
Cetane
Number
The cetane number is an indication of
a fuels readiness to autoignite after it has been
injected into the diesel engine. It might be
helpful for some readers to refer to the section of
this course that describes
diesel combustion, and to the section that
discusses
ignition indices of diesel fuels.
Diesel fuel for use in on-highway engines is
required to have a cetane number of 40 or higher.
Since higher cetane number translates into higher
fuel cost, most refiners keep the cetane number of
their diesel fuels between 40 and 45.
Table 1 shows the cetane numbers for some
typical esters of vegetable oils. The cetane
numbers are generally between 46 and 60 depending on
the feedstocks used to make the biodiesel. Methyl
esters tend to be slightly below ethyl and higher
esters. Biodiesel from saturated feedstocks such as
animal fat and recycled restaurant greases will be
higher than the esters of oils high in
polyunsaturates such as soybean oil.
The usual effect of a high cetane
number is to shorten the ignition delay period
between when the diesel fuel is injected and when it
actually ignites. With conventional diesel fuel,
the effect of shortening this period is to decrease
the amount of fuel that is prepared to burn so that
when autoignition actually occurs, the combustion
event will be less severe. This results in a lower
rate of pressure rise and less engine noise. Most
research indicates that this gradual start of
combustion also helps to decrease NOx emissions.
The research shows that biodiesel's higher cetane
number does shorten the ignition delay and
biodiesel's lower volatility also tends to reduce
the rate at which fuel is prepared to burn during
the ignition delay period. These two factors
contribute to a more gradual start of combustion
than occurs with diesel fuel. This more gradual
start of combustion would be expected to cause a
lower level of NOx emissions but, in fact, the
opposite occurs. Oxides of nitrogen are generally
found to increase with the use of biodiesel. The
reasons for this will be discussed in more detail in
the section on emissions.
|
Heat of
Combustion MJ/kg
|
Cetane No.
|
|
Methyl Soybean
|
39.8
|
46.2
|
|
Ethyl Soybean
|
40.0
|
48.2
|
|
Butyl Soybean
|
40.7
|
51.7
|
|
Methyl Sunflower
|
39.8
|
47.0
|
|
Methyl Peanut
|
-
|
54.0
|
|
Methyl Rapeseed
|
40.1
|
-
|
|
Ethyl Rapeseed
|
41.4
|
-
|
|
No. 2 Diesel
|
45.3
|
47.0
|
Table 1. Cetane Number and
Energy Content for Biodiesel Fuels
Bagby, M.O. and Freedman, B.,
“Seed Oils for Diesel Fuels: Sources and
Properties,” ASAE Paper 871583, 1987.
Wagner, L.E., Clark, S.J. and Schrock, M.D.,
“Effects of Soybean Oil Esters on the
Performance, Lubricating Oil, and Wear of Diesel
Engines,” Society of Automotive Engineers Paper
No. 841385, 1984.
Freedman, B. and Pryde, E.H., “Fatty Esters from
Vegetable Oils for Use as A Diesel Fuel,”
Vegetable Oil Fuels, Proceedings of the
International Conference on Plant and Vegetable
Oils as Fuels, American Society of Agricultural
Engineers, Fargo, North Dakota, Aug. 2-4, 1982.
Figure 1. Black Smoke from Diesel
Exhaust pipe.
Here is the primary reason why we
worry about emissions from diesel engines - black
smoke. Although modern diesel engines are very
clean, older diesels and diesels needing maintenance
are notorious for emitting plumes of dense black
smoke. Biodiesels’ greatest advantage is its ability
to reduce this smoke.
Table 2 shows that biodiesel
can dramatically reduce particulate matter
emissions. Unburned hydrocarbons and carbon monoxide
are also reduced although they are not usually a
problem with diesel engines.
|
Test Engine
|
Test Fuel
|
Transient Emissions,
g/hp-hr
|
|
HC
|
CO
|
NOx
|
PM
|
|
Cummins N-14
|
B100
|
0.01
|
0.41
|
5.17
|
0.076
|
|
Cummins N-14
|
B20
|
0.19
|
0.64
|
4.76
|
0.102
|
|
Cummins N-14
|
2-D
|
0.23
|
0.75
|
4.57
|
0.106
|
|
DDC Series 50
|
B100
|
0.01
|
0.92
|
5.01
|
0.052
|
|
DDC Series 50
|
B20
|
0.06
|
1.38
|
4.66
|
0.088
|
|
DDC Series 50
|
2-D
|
0.06
|
1.49
|
4.50
|
0.102
|
|
Cummins B5.9
|
B100
|
0.08
|
1.27
|
4.90
|
0.081
|
|
Cummins B5.9
|
B20
|
0.21
|
1.61
|
4.79
|
0.109
|
|
Cummins B5.9
|
2-D
|
0.31
|
2.05
|
4.70
|
0.128
|
Table 2. Emissions on 3 engines with
biodiesel
From: Sharp, C.A., S.A. Howell, and
J. Jobe, "The Effect of Biodiesel Fuels on Transient
Emissions from Modern Diesel Engines, Part I
Regulated Emissions and Performance," SAE Paper
2000-01-1967, 2000.
Table 2 also shows how NOx
can increase with the use of biodiesel. The
reason for the NOx increase is still an
area of active research, but it is at least
partially due to injection timing advances
associated with property differences between
biodiesel and petroleum.
Flashpoint
The flashpoint of a fuel is the
temperature at which the vapors above the fuel
become flammable. Petroleum based diesel fuels
have flash points of 50oC to 80oC
so they are considered to be intrinsically safe.
Biodiesel has a flash point that is considerably
higher than petroleum-based diesel fuel (above
160oC). This means that the fire
hazard associated with transportation, storage,
and utilization of biodiesel is much less than
with other commonly used fuels.
Flashpoint can also be used as a
measure of whether the production process has
been successful in removing the residual alcohol
from the fuel. Figure 2 shows how the
flashpoint decreases rapidly when a small amount
of alcohol is left in the biodiesel. As 1%
methanol in the biodiesel will drop the
flashpoint below ambient where the fuel needs to
be considered as hazardous as gasoline.
Figure 2. The effect of
alcohol on the flashpoint of biodiesel
Howell, S., Research Director for the
National Biodiesel Board,
personal communication, June 1996.
Lubricity
Lubricity can be defined as: "The
property of a lubricant that causes a difference in
friction under conditions of boundary lubrication
when all the known factors except the lubricant
itself are the same. The lower the friction the
higher the lubricity." [Kajdas, C., S.S.K. Harvey,
and E. Wilusz, Encyclopedia of Tribology,
Elsevier, New York, 1990.]
Lubricity is actually a very
difficult property to characterize. In spite of the
definition's attempt to separate the lubricity as a
fluid property, it is also strongly dependent on the
method used to measure it and on the characteristics
of the solid surfaces being lubricated. For
example, as friction occurs, small particles of the
solid material may be removed and entrained in the
lubricant. In some applications, these particles
will be swept away by a flow of lubricant while in
others, the particles stay in the vicinity of the
surface contact. Particles that are present in the
area of surface contact may act very differently.
In some cases, they may act as an abrasive to
increase wear while in other cases, the particles
may shield the surface from further wear. When
trying to characterize lubricity, it is important to
use a measurement technique that correlates well
with the actual lubrication situation. In the case
of diesel fuel, the fuel acts as a lubricant for the
finely fitting parts in the diesel fuel injection
system. While all diesel fuel injection systems
depend on the fuel to act as a lubricant, rotary
pump style injection systems seem to be the most
sensitive to fuel lubricity.
The need for diesel fuel lubricity
has been recognized for many years. Most early
concern focused on the use of #1 diesel fuel in
place of #2 diesel fuel under cold weather
conditions. Higher wear rates with #1 diesel fuel
would be aggravated by #1 diesel fuel's lower
viscosity. However, when the U.S. Environmental
Protection Agency mandated that the sulfur content
of on-highway diesel fuel be lowered from 5000 ppm
to 500 ppm in 1993, fuel lubricity captured national
attention. There is still disagreement about what
specififuel changes are caused by the sulfur
reduction that result in lubricity reduction. Some
have suggested that sulfur compounds themselves
provide lubricity, others have suggested that
nitrogen compounds or naphthenic hydrocarbons are
responsible. In any case, there is general
agreement that the severe hydrotreating process used
by petroleum refineries to remove sulfur results in
lower fuel lubricity. Recent regulations by the
EPA to further lower the sulfur content of diesel
fuel to 15 ppm is expected to make the fuel
lubricity even worse. The addition of small amounts
of biodiesel (0.25% to 2%) to diesel fuel has a
dramatic effect on the lubricity of that fuel. Pure
biodiesel and high level blends have excellent
lubricity.
There are two methods that are
commonly used to measure lubricity, the Scuffing
Load Ball On Cylinder
Lubricity Evaluator (SLBOCLE - ASTM D 6078-99) and
the High Frequency Reciprocating Rig (HFRR - ASTM D
6079-99). The
apparatus used for the SLBOCLE test is shown in
Figure 3. This trest involves placing a steel ball
bearing against a
rotating steel ring whose lower edge is immersed in
the test fluid. Weight is gradually applied to the
ball until a "scuff" mark is seen on the rotating
ring.
The tangential force is also
measured and the point of scuffing is indicated by a
large increase in the friction coefficient. The EMA
has indicated that a weight of 3150 grams is
representative of an acceptable lubricity level.
The higher this number, the better the fuel
lubricity.
The HFRR
test also uses a steel ball but in this case the
ball is held against a stationary disk and the ball
is reciprocated back and forth across the disk with
a frequency of 50 Hertz. The applied load is 200g
and the test duration is 75 minutes. The wear scar
produced on the disk is measured and a scar diameter
of less than 450 micron is considered to be
acceptable.
Measurements of Lubricity
Figure 3.
SLBOCLE for #2 Diesel 2004 Tier 2 Fuel,
Biodiesel, and Biodiesel Blends
Schumacher and Adams [10th
Biennial Bioenergy Conference – Bioenergy 2002,
Boise, Idaho, Sept. 22-26, 2002] have measured
the effect of low-level blends of soybean-based
biodiesel on biodiesel that has been produced to
meet 15 ppm sulfur levels. Figure 3 shows
SLBOCLE results for No. 2 diesel fuel with small
amounts of biodiesel. As little as 1% biodiesel
could change the diesel fuel from an
unacceptable level to an acceptable level.
Figure 4. SLBOCLE for #1 Diesel Fuel,
Biodiesel, and Biodiesel Blends
Figure 4 shows
the same effect for No. 1 diesel fuel that has
also been treated to lower the sulfur contain to
less than 15 ppm. In this case, the lubricity
of the original No. 1 diesel fuel was so low
that even 2% biodiesel was not able to bring the
lubricity back up to the acceptable level of
3150 grams. However, the lubricity was greatly
improved and it is unlikely that the engine
would suffer damage from short term use at a
lubricity level of 2880 grams.
|