O X I D A T Ive Stability of Biodiesel: Gerhard Knothe

Ch6.4.
1(Biodies)(122-126)(Co#1) 6/6/05 3:45 PM Page 122
6.4
Oxidative Stability of Biodiesel
6.4.1
Literature Overview
Gerhard Knothe
Biodiesel is susceptible to oxidation upon exposure to air. The oxidation process

ultimately affects fuel quality. Accordingly, the oxidative stability of biodiesel has
been the subject of considerable research (1–20). An oxidative stability specifica-
tion was included in the European biodiesel standards EN 14213 and EN 14214
(see Appendix B). The method to be used for assessing oxidative stability utilizes
a Rancimat apparatus. This method is very similar to the Oil Stability Index (OSI)
method (21). The following chapter on the “Stability of Biodiesel” (BIOSTAB)
project in Europe details the development of the oxidative stability parameter in
the European biodiesel standards using the Rancimat test [see also (13,15,17)].
This chapter provides a brief overview of results reported in the literature on
oxidative stability.
Biodiesel is also potentially subject to hydrolytic degradation, caused by the
presence of water. This is largely a housekeeping issue although the presence of
substances such as mono- and diglycerides (intermediates in the transesterification
reaction) or glycerol which can emulsify water, can play a major role (4).
The reason for autoxidation is the presence of double bonds in the chains of
many fatty compounds. The autoxidation of unsaturated fatty compounds proceeds
at different rates depending on the number and position of the double bonds (22).
The CH2 positions allylic to the double bonds in the fatty acid chains are those sus-
ceptible to oxidation. The b i s-allylic positions in common polyunsaturated fatty
acids (PUFA) such as linoleic acid (double bonds at Δ9 and Δ12, giving one bis-
allylic position at C-11) and linolenic acid (double bonds at Δ9, Δ12, and Δ15 giv-
ing two bis-allylic positions at C-11 and C-14) are even more prone to autoxida-
tion than allylic positions. Relative rates of oxidation given in the literature [(22)
and references therein] are 1 for oleates (methyl, ethyl esters), 41 for linoleates,
and 98 for linolenates. This is essential because most biodiesel fuels contain signif-
icant amounts of esters of oleic, linoleic, or linolenic acids, which influence the
oxidative stability of the fuels. The species formed during the oxidation process
cause the fuel to eventually deteriorate. Small amounts of more highly unsaturated
fatty compounds had a disproportionately strong effect on oxidative stability using
the OSI method (16).
Copyright © 2005 AOCS Press

Ch6.4.1(Biodies)(122-126)(Co#1) 6/6/05 3:45 PM Page 123
Initially, hydroperoxides are formed during oxidation, with aldehydes, acids,

and other oxygenates constituting oxidation products further along the reaction
chain (22). However, the double bonds may also be prone to polymerization-type
reactions so that higher-molecular-weight products, leading to an increase in vis-
cosity, can occur. This may lead to the formation of insoluble species, which can
clog fuel lines and pumps. One study (13) reports that polymers formed during
storage under controlled conditions are soluble in biodiesel due to its polar nature
and are insoluble only when mixing the biodiesel with petrodiesel.
The issue of oxidative stability affects biodiesel primarily during extended
storage. The influence of parameters, such as the presence of air, heat, light traces
of metal, antioxidants, peroxides as well as the nature of the storage container, was
investigated in most of the aforementioned studies. Summarizing the findings from
these studies, the presence of air, light, or the presence of metals as well as elevat-
ed temperatures facilitate oxidation. Studies performed with the automated OSI
method confirmed the catalyzing effect of metals on oxidation, with copper show-
ing the strongest catalyzing effect; however, the influence of the compound struc-
ture of the fatty esters, especially unsaturation, was even greater (16). Numerous
other methods, including wet-chemical ones such as acid value (AV), peroxide
value (PV), and pressurized differential scanning calorimetry (P-DSC), have been
applied in oxidation studies of biodiesel.
Long-term storage tests on biodiesel were conducted. Viscosity, PV, AV, and
density increased in biodiesel stored for 2 yr, and heat of combustion decreased
(6). Viscosity and AV, which can be strongly correlated (11), changed dramatically
over 1 yr with changes in the Rancimat induction period depending on the feed-
stock (15); however, even in storage tests for 90 d, significant increases in viscosity,
PV, free fatty acid, anisidine value (AnV), and ultraviolet absorption were found
(2). Biodiesel from different sources stored for 170–200 d at 20–22°C did not
exceed viscosity and AV specifications but induction time decreased, with expo-
sure to light and air having the most effect (12).
The PV is less suitable for monitoring oxidation because it tends to increase and
then decrease upon further oxidation due to the formation of secondary oxidation
products (9,11,15). When the PV reached a plateau of ~350 meq/kg ester during
biodiesel [soybean oil methyl esters (SME)] oxidation, AV and viscosity continued to
increase monotonically (9). In addition to viscosity, the AV has good potential as a
parameter for monitoring biodiesel quality during storage (14). P-DSC can be used
for determining the oxidative stability of biodiesel with and without antioxidants (10).
Stability tests developed for petrodiesel fuels were reportedly not suitable for
biodiesel or biodiesel blends with petrodiesel (8,11), although appropriate modifi-
cation may render them more useful (8). However, another study (15) states that
the petrodiesel method ASTM D4625 [Standard Test Method for Distillate Fuel
Storage Stability at 43°C (110°F)] is suitable but relatively slow.
Vegetable oils usually contain naturally occurring antioxidants such as toco-
pherols. Therefore, unrefined vegetable oils that still contain their natural levels of

antioxidants usually have improved oxidative stability compared with refined oils
(1) but do not meet other fuel requirements. Natural antioxidants were also deliber-
ately added to biodiesel to investigate their antioxidant behavior. In addition to nat-
ural antioxidants, a variety of synthetic antioxidants exists. Many of them are sub-
stituted phenols such as butylated hydroxytoluene (BHT; 2,6-di-tert -butyl-4-
methylphenol), butylated hydroxyanisole [BHA;(3)-t-butyl-4-hydroxyanisole] tert-
butylhydroquinone (TBHQ; 2-t e r t-butylhydroquinone), pyrogallol (1,2,3-trihy-
droxybenzene), and propyl gallate (3,4,5-trihydroxybenzoic acid propyl ester).
These synthetic antioxidants were also investigated for their effect on biodiesel.
Different synthetic antioxidants have different effects on biodiesel, depending
on the feedstock (18,19) without affecting properties such as viscosity, cold-filter
plugging point, density, and others. In another study, different antioxidants studied
by the AOM method had little or no effect (7). TBHQ and α-tocopherol retarded
SME oxidation (14). A high-performance liquid chromatography method for
detecting antioxidants in biodiesel was also developed (20).
A European standard (pr EN 14112) was established for potential inclusion of
an oxidative stability parameter in the European biodiesel standard EN 14214. The
biodiesel standard EN 14214 calls for determining oxidative stability at 110°C
with a minimum induction time of 6 h by the Rancimat method. The Rancimat
method is nearly identical to the OSI method, which is an AOCS method. The
ASTM standard D6751 currently does not include any specification of this kind.
Another parameter that was originally included in some biodiesel standards for
addressing the issue of oxidative stability is the iodine value (IV). The IV is a mea-
sure of total unsaturation of a fatty material measured in g iodine/100 g of sample
when formally adding iodine to the double bonds. The IV of a vegetable oil or ani-
mal fat is almost identical to that of the corresponding methyl esters (see tables in
Appendix B). However, the IV of alkyl esters decreases with higher alcohols.
The idea behind the use of IV is that it would indicate the propensity of an oil
or fat to oxidize, but it may also indicate the propensity of the oil or fat to polymer-
ize and form engine deposits. Thus, an IV of 120 was specified in EN 14214 and
130 in EN 14213. This would largely exclude vegetable oils such as soybean and
sunflower as biodiesel feedstock.
However, the IV of a mixture of fatty compounds, as found in oils and fats, does
not take into consideratin that an infinite number of fatty acid profiles can yield the
same IV (23). Different fatty acid structures can also give the same IV (23). Other,
new structural indices are likely more suitable than the IV (23). Engine performance
tests with a mixture of vegetable oils of different IV did not yield results that would
have justified a low IV (24,25). No relation between the IV and oxidative stability
was observed in another investigation on biodiesel with a wide range of IV (4).
Thus the IV was not included in biodiesel standards in the United States and
Australia; it is limited to 140 in the provisional South African standard (which
would permit sunflower and soybean oils), and the provisional Brazilian standards
require only that it be noted (see Appendix B).

References
1. Du Plessis, L.M., Plant Oils as Diesel Fuel Extenders: Stability Tests and Specifications on
Different Grades of Sunflower Seed and Soyabean Oils, CHEMSA 8: 150–154 (1982).
2. Du Plessis, L.M., J.B.M. de Villiers, and W.H. van der Walt, Stability Studies on Methyl
and Ethyl Fatty Acid Esters of Sunflowerseed Oil, J. Am. Oil Chem. Soc. 62: 748–752
(1985).
3. Bondioli, P., A. Gasparoli, A. Lanzani, E. Fedeli, S. Veronese, and M. Sala, Storage
Stability of Biodiesel, J. Am. Oil Chem. Soc. 72: 699–702 (1995).
4. Bondioli, P., and L. Folegatti, Evaluating the Oxidation Stability of Biodiesel. An
Experimental Contribution, Riv. Ital. Sostanze Grasse 73: 349–353 (1996).
5. Simkovsky, N.M., and A. Ecker, Influence of Light and Contents of Tocopherol on the
Oxidative Stability of Fatty Acid Methyl Esters, [Einfluß von Licht und Tocopherolgehalt
auf die Oxidationsstabilität von Fettsäuremethylestern.] Fett/Lipid 100: 534–538 (1998).
6. Thompson, J.C., C.L. Peterson, D.L. Reece, and S.M. Beck, Two-Year Storage Study with
Methyl and Ethyl Esters of Rapeseed, Trans. ASAE 41: 931–939 (1998).
7. Simkovsky, N.M., and A. Ecker, Effect of Antioxidants on the Oxidative Stability of
Rapeseed Oil Methyl Esters, Erdoel Erdgas Kohle 115: 317–318 (1999).
8. Stavinoha, L., and S. Howell, Potential Analytical Methods for Stability Testing of
Biodiesel and Biodiesel Blends, SAE Technical Paper Series 1999-01-3520, SAE,
Warrendale, PA, 1999.
9. Canakci, M., A. Monyem, and J. Van Gerpen, Accelerated Oxidation Processes in
Biodiesel, Trans. ASAE 42: 1565–1572 (1999).
10. Dunn, R.O., Analysis of Oxidative Stability of Methyl Soyate by Pressurized-Differential
Scanning Calorimetry, Trans. ASAE 43: 1203–1208 (2000).
11. Monyem, A., M. Canakci, and J.H. Van Gerpen, Investigation of Biodiesel Thermal
Stability Under Simulated In-Use Conditions, Appl. Eng. Agric. 16: 373–378 (2000).
12. Mittelbach, M., and S. Gangl, Long Storage Stability of Biodiesel Made from Rapeseed and
Used Frying Oil, J. Am. Oil Chem. Soc. 78: 573–577 (2001).
13. Bondioli, P., A. Gasparoli, L. Della Bella, and S. Tagliabue, Evaluation of Biodiesel Storage
Stability Using Reference Methods, Eur. J. Lipid Sci. Technol. 104: 777–784 (2002).
14. Dunn, R.O., Effect of Oxidation Under Accelerated Conditions on Fuel Properties of
Methyl Soyate (Biodiesel), J. Am. Oil Chem. Soc. 79: 915–920 (2002).
15. Bondioli, P., A. Gasparoli, L. Della Bella, S. Tagliabue, and G. Toso, Biodiesel Stability
Under Commercial Storage Conditions over One Year, Eur. J. Lipid Sci. Technol. 105:
735–741 (2003).
16. Knothe, G., and R.O. Dunn, Dependence of Oil Stability Index of Fatty Compounds on
Their Structure and Concentration and Presence of Metals, J. Am. Oil Chem. Soc. 80:
1021–1026 (2003).
17. Lacoste, F., and L. Lagardere, Quality Parameters Evolution During Biodiesel Oxidation
Using Rancimat Test, Eur. J. Lipid Sci. Technol. 105: 149–155 (2003).
18. Mittelbach, M., and S. Schober, The Influence of Antioxidants on the Oxidation Stability of
Biodiesel, J. Am. Oil Chem. Soc. 80: 817–823 (2003).
19. Schober, S., and M. Mittelbach, The Impact of Antioxidants on Biodiesel Oxidation
Stability, Eur. J. Lipid Sci. Technol. 106: 382–389 (2004).
20. Tagliabue, S., A. Gasparoli, L. Della Bella, and P. Bondioli, Quali-Quantitative
Determination of Synthetic Antioxidants in Biodiesel, Riv. Ital. Sostanze Grasse 80: 37–40
(2004).

21. AOCS Official Method Cd 12b-92, Oil Stability Index (OSI), AOCS, Champaign, IL, 1999.
22. Frankel, E.N., Lipid Oxidation, The Oily Press, Dundee, Scotland, 1998.
23. Knothe, G., Structure Indices in FA Chemistry. How Relevant Is the Iodine Value? J. Am.
Oil Chem. Soc. 79: 847–854 (2002).
24. Prankl, H., M. Wörgetter, and J. Rathbauer, Technical Performance of Vegetable Oil
Methyl Esters with a High Iodine Number, Proceedings of the 4th Biomass Conference of
the Americas, Oakland, CA, 1999, pp. 805–810.
25. Prankl, H., and M. Wörgetter, Influence of the Iodine Number of Biodiesel to the Engine
Performance, Proceedings of the 3rd Liquid Fuel Conference, Liquid Fuels and Industrial
Products from Renewable Resources, edited by J.S. Cundiff. E.E. Gavett, C. Hansen, C.
Peterson, M.A. Sanderson, H. Shapouri, and D.L. VanDyne, ASAE, Warrendale, PA, 1996,
pp. 191–196.

O X I D A T Ive Stability of Biodiesel: Gerhard Knothe

Uploaded by

Copyright:

Available Formats

O X I D A T Ive Stability of Biodiesel: Gerhard Knothe

Uploaded by

Document Information

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

O X I D A T Ive Stability of Biodiesel: Gerhard Knothe

Uploaded by

Copyright:

Available Formats

Ch6.4.

1(Biodies)(122-126)(Co#1) 6/6/05 3:45 PM Page 122

Oxidative Stability of Biodiesel

Biodiesel is susceptible to oxidation upon exposure to air. The oxidation process

Copyright © 2005 AOCS Press

Initially, hydroperoxides are formed during oxidation, with aldehydes, acids,

Copyright © 2005 AOCS Press

Copyright © 2005 AOCS Press

Copyright © 2005 AOCS Press

Copyright © 2005 AOCS Press

You might also like