biofuels biodiesel: additives
Are significant improvements in the quality of biodiesel and its blends possible with the use
of quality multifunctional diesel fuel additives, antioxidants and cold flow additives, in areas
of concern such as fuel system corrosion?
Biodiesel:
The good, the bad…
and additives
by Robert Quigley
Europe’s biodiesel
market
The use of biodiesel as a road
transportation fuel is increasing
throughout Europe as member
states seek to comply with the
European Directive
2003/30/EC. Figure 1 illustrates
the estimated volume of
biodiesel that will be required to
meet the directive’s target of
5.75% biofuel consumption by
the end of the decade.
Pure biodiesel (B100)
continues to be used,
particularly by fleets. However,
blending of up to 5% biodiesel
into mineral fuel is now
widespread, with Germany
being the leading consumer. As
long as the biodiesel
component complies with the
EN 14214 standard, the
resulting blend is permitted
16000
14000
12000
10000
8000
6000
4000
2000
0
2005
Figure 1
70 february 2007
2007
within the EN 590 specification
for diesel fuel. Current CEN
(European Committee for
Standardisation) discussions
are focused on permitting the
proportion of biodiesel in blends
to rise to 10%. As a result of this
widespread presence of
biodiesel, some multifunctional
diesel fuel additives have been
specifically designed to perform
in biodiesel and its blends.
Performance
concerns
Substituting biodiesel for
conventional fossil fuels is
widely considered to have
societal benefits, such as
reducing greenhouse gas
emissions and supporting rural
agricultural economies.
However, there are potential
problems when
using it in
vehicles.
Biodiesel blends
can show an
increase in water
separation and
fuel foaming,
compared to pure
fossil fuels. In
addition, blending
with low-stability
biodiesel can lead
to fuel system
2010
problems, such as
higher levels of
Dr. Robert Quigley is a Product Manager within The
Lubrizol Corporation’s fuels, refinery and oilfield
additives group. His responsibilities cover diesel and
gasoline additives in the European market area. Dr.
Quigley has worked in the field of fuel additive
chemistry since obtaining his D. Phil. degree in
transition metal chemistry from the University of
Sussex, UK, in 1992. He has been granted five
patents and has authored a number of papers on
subjects including additive performance in biodiesel
and diesel fuel lubricity improver technology.
injector deposits and corrosion
arising from the generation of
low- molecular weight acids.
OEMs and fuel injection
equipment manufacturers have
reported this type of damage in
the field. In addition, advanced
direct injection engines may
lose significant power when
poor quality biodiesel blends are
used. An effective performance
diesel fuel additive can help
guard against such problems.
Other well understood issues
include reduced oxidation
stability and cold filtration
properties, when compared to
standard mineral diesel fuels
(Figure 2).
Additives
Quality multifunctional diesel
fuel additives have reduced
many of the problems
encountered with biodiesel
Figure 2
biofuels international
biofuels biodiesel: additives
blends, such as fuel system
corrosion, water separation and
increased fuel foaming. Injector
fouling tests show that
multifunctional additives can
reduce the injector deposit
levels generated when biofuel
blends are used. Specialised
flow improvers are also
available to address the
challenging low-temperature
operability profile of many
biodiesel fuels, while specific
antioxidants can help to stabilise
the fuel against the degradation
that may lead to fuel system
deposits and corrosion.
The RME used in the blend did
not contain antioxidant
additives. The results of keep
clean testing for dispersant
treated and base blends are
shown in Figure 4. These
indicate greater power loss with
the unstabilised B10 fuel
compared to the pure mineral
fuel. Treating the B10 blend with
a dispersant at a treat rate
typically used in the market
reduced power loss by 70%
compared to the untreated fuel.
4.0%
oxidation of the FAME to lowmolecular weight acids. As
illustrated in Figure 6, use of a
multifunctional diesel fuel
additive formulated with an
effective corrosion inhibitor
can dramatically reduce the
corrosive
30
tendency of
24
biodiesel fuels. 25
17.5
13
10
3.6%
3.5%
3.0%
5
B5
B30
Mineral Fuel - RME blend
Figure 3
The impact an RME B10 blend
has on nozzle fouling and
power loss was also assessed
in the Peugeot DW-10 engine.
biofuels international
1.1%
0
0
3
5
7
10
Standing Time (min)
0.0%
0.0%
B0
0
0
1.0%
0.5%
B0
1
1.5%
50
45
40
35
30
25
20
15
10
5
0
22
15
2.0%
A key and frequently raised
concern about using biodiesels
is their impact on injector
cleanliness and the potential for
serious injector coking and
nozzle fouling. The standard
CEC (Co-ordinating European
Council for development of
performance tests) 10-hour
XUD-9 test procedure was used
to compare the performance of
untreated and dispersant
additive treated rapeseed
methyl ester (RME) mineral
diesel blends. For comparative
purposes, the same petroleum
diesel fuel was evaluated in the
test. These results are shown in
Figure 3. The results show that
using a dispersant at a treat rate
typically used in the market is
effective in fuels containing fatty
acid methyl ester (FAME), and
there are significant
improvements in injector
cleanliness compared to base
fuel.
One of the most significant
current concerns with the use of
biodiesel fuels is their reduced
oxidation stability compared to
standard mineral diesel fuels. It
20
2.5%
Fuel Injector
Cleanliness
Oxidation stability
B10 Base
B10 + Dispersant
Figure 5
B5 Base
Additive
Test Fuel
Figure 4
is well known
that the
70
unsaturated
60
fatty acids
50
found in
40
30
typical
20
feedstocks,
10
such as rape
0
0.0
1.0
2.0
3.0
and soy, are
Multifunctional Additive Relative Treat
the cause of
this
Figure 6
susceptibility. These
unsaturated fatty acids may
Foaming
react with atmospheric oxygen,
Blending FAME into
forming peroxides and result in
conventional mineral diesel fuel
a variety of problematic
can worsen the foaming
degradation byproducts,
tendency, which becomes
including corrosive, lowevident when filling a vehicle’s
molecular weight acids and biofuel tank at a service station.
polymers. These byproducts are
Testing carried out with the
the principal cause of sludge
industry BNPe laboratory bench
and lacquer in diesel fuel
test has shown significant
injection systems, and they also
increases in foam decay time as
contribute to fuel filter plugging.
the proportion of RME is
For this reason, vehicle and fuel
increased from 5% to 30%
injection equipment
(Figure 7). However, the same
manufacturers have expressed
testing shows that pure B100
concern about the careful
does not foam. Treatment of B5
control of oxidation stability.
to B30 blends with
In Europe, biodiesel oxidation
multifunctional
120
diesel fuel
100
additives
80
containing an
Base
60
antifoam
+ MFDA
40
component
20
shows excellent
0
control of this
B0
B5
B30
B100
increased
Biodiesel
Content
foaming tendency. Figure 7
90
Impact on water
separation and
corrosion
It is well known that blending
biodiesel into mineral fuel
results in reduced water
separation capability, which
can cause problems such as
fuel filter blocking, increased
fuel system corrosion and
microbial contamination. For
example, Figure 5 shows
results from ASTM D1094
water separation testing of a
base B5 blend sampled from
the German market. The
extremely poor water
separation properties of the
fuel are evident. In
comparison, the presence of
multifunctional diesel fuel
additive provides excellent
emulsion
control.
Another
Base concern is the
Disp possibility that
biodiesel and
its blends can
cause
B100
increased
corrosion of
the vehicle fuel injection
system. In addition to the risks
from higher levels of water
entrainment in the fuel,
corrosion also can result from
80
february 2007 71
biofuels biodiesel: additives
stability is controlled through the
Rancimat test in the EN 14214
specification, with a minimum
induction time requirement of
six hours. Use of the Rancimat
method is also being explored
in the United States, where a
lower three-hour induction time
is being considered for the
predominantly soy-based
methyl esters produced there.
Biodiesels that do not meet
the minimum induction time
standard need to be stabilized
with an antioxidant additive.
Although an untreated biodiesel
fuel made from a feedstock
such as rape may meet the
European specification when it
is produced, its oxidative
stability can rapidly degrade
during distribution and storage
(Figure 8), making use of an
antioxidant advisable.
stability. However, because the
cost of TBHQ is approximately
ten times more than BHT, it is
not feasible on a net treat cost
basis. Several developmental
antioxidants show effective
performance at lower net treat
costs than BHT.
20
18.7
18
16
14
12.9
12
10
8.0
7.6
8
6
4.5
4
2
0
None
BHT
TBHQ
Antioxidant
Figure 9
Cold-weather
performance
All biodiesels are not created
equal when it
comes to cold6
6
weather
5
4.5
operability. As
4
with diesel
3
2.5
fuels,
2
biodiesels and
1
blends with
0
0
10
20
30
40
50 mineral fuel
Time after production (days)
can form
solids as
Figure 8
ambient temperatures drop.
Biodiesels with a higher
Selection of an appropriate
saturated fatty ester content
antioxidant additive is critical
form more solids and begin to
because many have been found
form precipitates at significantly
to be ineffective in FAMEs.
higher temperatures than those
Some antioxidants, such as tertcontaining higher amounts of
butyl hydroquinone (TBHQ),
unsaturated fatty esters. For
may be very effective but are
example, biodiesel produced
not feasible commercially due to
from rapeseed has the best lowextremely high cost. Betatemperature properties of all
hydroxytoluene (BHT) has a
major classes of vegetable oil
reasonably good impact on
feedstock. Soybean-derived
oxidation stability, so it is
biodiesel has intermediate lowtypically used in the biodiesel
temperature performance, and
industry, even though treat rates
biodiesel made from palm and
tend to be high.
coconut oils, which are
Improved antioxidants that
PP (ºC)
CFPP (ºC)
provide a better balance
20
between cost and performance
15
may be needed. Figure 9
10
5
compares Rancimat induction
0
times for RME, both untreated
Rape
Soy
Palm
-5
and treated, with several
-10
antioxidants. BHT improves the
-15
-20
induction time over the
Biodiesel Feedstock
specification minimum, while
Figure
10
TBHQ has a dramatic impact on
7
72 february 2007
extremely high in saturated fatty
acids, behaves extremely poorly
at low temperatures. These
trends are illustrated in Figure
10, where pour and cold filter
plugging points are charted for
some significant biodiesel
feedstock types.
The
higher the
percentage
of biodiesel
in a blend
with mineral
fuel, the
poorer the
lowtemperature
properties
are likely to be. However,
biodiesel blends of B5 or less
typically exhibit cold-weather
performance similar to the base
diesel fuel.
Flow improver additives are
available that will improve the
cold-weather
0
0.0%
performance of -2
many biodiesel -4
-3
types and their
-6
blends. As with -8
traditional cold
-10
flow additives
-12
for diesel fuel,
-14
the additive
-16
must be
matched to the
Figure 11
composition of the
biodiesel. A particular additive
might work well in RME but
have no effect on soy methyl
ester (SME), or vice versa.
In Europe, where B100 is
routinely used in engines, the
most severe measure of lowtemperature operability is
meeting a Cold Filter Plugging
Point (CFPP) for B100 of less
than -20° C. Cold flow additives
for RME, with treat rates in the
range of 1500 to 3000 ppm,
exist to meet this specification.
Different sources of RME may
require varying
amounts of
additive to reach
the specification.
This situation is
analogous to that
Tallow
seen in diesel
fuels, where base
fuel properties
determine the
amount of
DEV 1
DEV 2
additive required.
It is also possible to improve
the cold-weather operability of
soy-derived biodiesel with
specialised flow improver
additives (Figure 11). Compared
to rapeseed-derived methyl
esters, these fuels are typically
harder to treat for improved lowtemperature performance and,
accordingly, additive treat rates
of the specialized additives
required are significantly higher
than those needed for RMEs.
It is important to use caution
when choosing a flow improver
additive for biodiesel-mineral
fuel blends. Some additive
chemistries can have negative
effects when blended into
biodiesel blends in which the
diesel portion was previously
treated with a traditional diesel
cold flow additive. Testing can
be performed to avoid this
scenario.
0.3%
0.6%
1.8%
-7
-9
-15
Flow Improver %Weight
Conclusions
Significant improvements in the
quality of biodiesel and its
blends are possible with the use
of quality multifunctional diesel
fuel additives, antioxidants and
cold flow additives in areas of
concern such as detergency,
water separation, fuel system
corrosion, foaming and
oxidation, and during coldweather operation. Such quality
improvements cost-effectively
reassure consumers of the
performance of these fuels,
thereby increasing their
acceptance in the wider
marketplace. Q
For further information on this
topic, please contact:
[email protected].
This article was written by Robert Quigley,
Diesel and Gasoline Additives Product
Manager, Lubrizol Limited
biofuels international