The fatty oil hydroperoxides ultimately decompose to form hexenals,
heptenals, propanal, pentane, and 2,4-heptadienal.22–24 Increased acidity is
always a result of the oxidation of fatty oils and biodiesel leading to the
formation of shorter chain fatty acids.3,5,25–27
As hydroperoxides decompose, oxidation linking of fatty acid chains can occur to form species with higher molecular weights, known as oxidation polymerization. One of the results of polymer formation is an increase in the oil viscosity. Increased levels of polyunsaturated fatty acid chains increase oxidation polymerization in fatty oils. Safflower oil, which is high in linoleic (18 : 2) acid was found to exhibit a much greater increase in viscosity than
safflower oil high in oleic acid (18 : 1) during air oxidation at 250 uC.28
The increase in viscosity indicates the presence of higher molecular weight materials in the oils.
Relative rates of oxidation are 1 for oleates, 41 for linoleates, and 98 for
linolenates.29,30 This demonstrates that the OS decreases with increasing
content of polyunsaturated methyl esters.
4.2.2
Measurement of Oxidation Stability
The OS of biodiesel was studied using the Rancimat equipment model 743 according to the EN 14112 and the Indian IS 15607 specifications (Figure 4.2). In the Rancimat method, oxidation is induced by passing a stream of air at a
rate of 10 L h21through the biodiesel sample (3 g), at a constant temperature
of 110uC. The vapors released during the oxidation process, together with air,
were passed into a flask containing 50 mL of demineralized water, containing an electrode for measuring conductivity. The electrode was connected to a measuring and recording device. It indicates the end of the IP when the conductivity begins to increase rapidly. This accelerated increase is caused by the dissociation of volatile carboxylic acids produced during the oxidation
Figure 4.2 Principles of measurement for the Rancimat test method (EN 14112 and
process and absorbed in the water. The conductivity of this measuring solution is recorded continuously and an oxidation curve is obtained whose point of inflection is known as the IP. The biodiesel standards EN 14214 and IS 15607
require OS at 110uC with a minimum IP of 6 h by the Rancimat method (EN
14112) and the ASTM standard D-6751 recently introduced a limit of 3 h for OS by the Rancimat test.
The OS measured using the Rancimat apparatus has been found to be similar to the oil stability index (OSI) of the American Oil Chemists’ Society
(AOCS) Cd 12b-93 method.31 Under the project stability of biodiesel
(BIOSTAB), samples of methyl esters from canola, sunflower oil, used frying oil and tallow were investigated to determine which method is best used to
determine stability parameters.32
Spectroscopic methods such as nuclear magnetic resonance (NMR) and ultraviolet–visible (UV–visible) have also been used in analyzing the products
of lipid oxidation.33,34 Fourier transform infrared (FT-IR) spectroscopy has
also been applied to the analysis of biodiesel degradation products resulting
from accelerated oxidation in the presence of 2-ethylhexyl nitrate.35Based on
the peaks of the carbonyl, C–O, and O–H moieties, several concomitant reaction mechanisms were proposed during oxidative degradation of biodiesel, including reverse transesterification, perester formation with ensuing second- ary products, hydrolysis, and formation of various carbonyl compounds.
4.2.3
Thermal Oxidation Stability
Oxidation can also occur due to exposure of the oil to high-temperature
(cooking temperature) conditions.7,36–41At high temperatures, the methylene-
interrupted polyunsaturated olefin structure begins to isomerize to a more stable conjugated structure, then a conjugated diene group from one fatty acid chain can react with a single olefin group from another fatty acid chain to form
a cyclohexene ring.19,39,42This reaction between a conjugated di-olefin and a
mono-olefin group is known as the Diels–Alder reaction, and becomes
important at temperatures of 250–300uC or more and the products formed are
called dimers.43–45 This reaction results in the formation of carbonyl
compounds such as aldehydes (formed from hydroperoxides) or high- molecular-weight polymers (formed from peroxide radicals) which increase the viscosity of biodiesel. Many authors have reported on the effect of temperature on the stability of biodiesel. The influence of the temperature on the OSI of biodiesel has been investigated and it has been reported that increasing the temperature accelerates oxidation, which decreases the OSI of
the fatty acid methyl ester (FAME) contents.7,37,40,46
4.2.4
Storage Stability
Long-term storage tests on biodiesel have been conducted. Storage stability is the ability of liquid fuel to resist change in its physical and chemical
characteristics brought about by its interaction with its environment.47 Therefore the storage problem is accelerated by storage conditions which may include exposure to air and/or light, excess temperature and the presence
of metals in the storage container of the biodiesel.37,48 The resistance of
biodiesel to oxidation degradation during storage is an important issue for the viability and sustainability of such alternative fuels.
As discussed above, the oxidation of unsaturated esters in biodiesel occurs by contact with atmospheric air and other pro-oxidizing conditions during long-term storage and impairs the fuel quality, therefore affecting the engine performance. Researchers have performed long-term storage tests on biodiesel quality and investigated the influence on the physical properties of the fuel
with respect to time.5,9,49–53It was reported that the viscosity, density, peroxide
value, and acid value of biodiesel increases but the heat of combustion
decreases when it is stored for two years.5,49,50,54The viscosity and acid value
changed dramatically over one year with variation in Rancimat IP depending on the feedstock but during 90-day storage tests, significant increases in viscosity, peroxide value, free fatty acid content, anisidine value and UV
absorption were observed.5,7,27Biodiesel from different sources stored for 170–
200 days at 20–22uC did not exceed viscosity and acid value specifications but
a decrease in the induction time with exposure to light and air, has been found
to have a significant effect.25
The long-term storage stability of biodiesel from high oleic sunflower oil and used frying oil showed that the acid value, density and viscosity increased with
increasing storage time while the iodine value decreased.49 The researchers
reported the deterioration of rapeseed oil methyl esters under different storage conditions including changes in acidity, peroxide value and viscosity, and found that the acid value, peroxide value and viscosity increased with
time.6,49,55 Polyunsaturated content has the largest impact on biodiesel
stability in terms of increasing insoluble product formation and reduction in
IP.56The storage stability of poultry fat and diesel fuel mixtures was studied
with respect to specific gravity, dynamic viscosity, sedimentation accumulation and separation (layering) including corrosive effects of the fuels on various
metals.50 It was observed that the viscosity and specific gravity of these
biofuels changed very little over a storage period of one year.
Long-term storage stability of biodiesel produced from karanja oil was also investigated and it was reported that the OS of karanja oil methyl ester decreased, i.e., the peroxide value and viscosity increased with increasing
storage time.51 The samples stored under ‘open to air inside the room’ and
‘exposed to metal and air’ conditions had high peroxide values and viscosities compared to other conditions, hence were more susceptible to oxidation degradation.
The presence of certain metals such as Cu, Fe, Ni, Sn and brass (a copper-
rich alloy) can increase the oxidizability of fatty oils.57 The presence of Cu,
even at 70 ppm in rapeseed oil greatly increased its oxidizability. Copper has also been found to reduce the OSI of methyl oleate more than either Fe or Ni.
Iron is a very effective hydroperoxide decomposer and its effect on rapeseed oil
methyl esters was more pronounced at 40uC than at 20 uC.5
The influence of metal contaminants on the OS of jatropha, pongamia and palm biodiesel was investigated and it was reported that even small concentrations of metal contaminants showed nearly the same influence on
OS as large amounts.9–11 Copper showed the strongest detrimental and
catalytic effect on the OS. Different transition metals—Fe, Ni, Mn, Co and Cu—commonly found in metal containers were blended, as metal naphthe- nates, with varying concentrations (ppm) in biodiesel samples. Metal naphthenates were selected, being highly soluble in biodiesel. The metal concentration in metal naphthenates was checked using the ASTM D-4951 test method, using inductively coupled plasma atomic emission spectroscopy. The concentrations of Co, Mn, Fe, Cu and Ni in their naphthenates were 5.21, 5.20, 3.91, 6.80, and 4.99%, respectively. The samples were further diluted in biodiesel to the desired concentration. The concentration of carboxylic acids in the metal naphthenates was practically none (,1%); therefore, their significance in biodiesel will be insignificant as naphthenates were blended at the ppm level.
Figures 4.3–4.5 show that the presence of these metals depressed the OS of biodiesel, as measured by the IP. The presence of metals in biodiesel resulted in acceleration of free radical oxidation due to a metal-mediated initiation
reactions.9–11Copper had the strongest catalytic effect and other metals—Fe,
Ni, Mn and Co—also had a strong negative influence on the OS. The strong catalytic effect of Cu is due to its relative high pro-oxidant effect. Figures 4.3– 4.5 shows that for all the metal contaminants, IP values became almost constant as the concentration of the metal increased. This proves that the influence of metals was catalytic, as even small concentrations of metals had nearly the same effect on the OS as large amounts. The dependence of the OS
on the type of metal confirms that the Rancimat method is a suitable lab test to correlate long-term stability.
Besides oxidation caused by exposure to air, biodiesel is also potentially subject to hydrolytic degradation due to the presence of water. This is a largely housekeeping issue although the presence of, for example, mono- and diglycerides (intermediates in the transesterification reaction) or glycerol
which can emulsify water can also play a major role.58
It can be summarized that the nature and amount of the fatty acid chains found in biodiesel determine its OS. However, various other factors like the presence of air, light, peroxides, elevated temperatures and metals (present in the storage container of the biodiesel) can have catalytic or inhibitory effects on the OS.
Figure 4.4 The effect of metal contamination on the OS of pongamia methyl ester.