140 CHAPTER IV OXIDATIVE AND STORAGE STABILITY STUDIES OF BIODIESEL USING COMMERCIALLY AVAILABLE ANTIOXIDANTS 1. Introduction The word “antioxidant” has become increasingly popular in modern society as it gains publicity through mass media coverage of its health benefits. The dictionary definition of antioxidant is rather straightforward but a traditional annotation would define antioxidant as “a substance that opposes oxidation or inhibits reactions promoted by oxygen or peroxides, many of these substances (as the tocopherols) being used as preservatives in various products (as in fats, oils, food products, and soaps for retarding the development of rancidity, in gasoline and other petroleum products for retarding gum formation and other undesirable changes, and in rubber for retarding aging)”. A more biologically relevant definition of antioxidants is “synthetic or natural substances added to products to prevent or delay their deterioration by action of oxygen in air [1]. Biodiesel, defined as fatty acid mono-alkyl esters made from vegetable oil or animal fat, is an alternative fuel for combustion in compression–ignition (diesel) engines. Several recent reviews have reported on the technical characteristics of biodiesel. In short, biodiesel is made from domestically renewable feedstock, is environmentally innocuous, is relatively safe to handle (high flash points), and has an energy content, specific gravity, kinematic viscosity (KV), and cetane number (CN) comparable to petroleum middle distillate fuels (petro diesel) [2, 3]. With a production of almost one million tons in Europe, fatty acid methyl ester (FAME) more generally called biodiesel, has become a fast growing renewable liquid biofuel within the European Community. Just like vegetable oils or fats, fatty acid methyl esters undergo degradation over time, mainly influenced by temperature and oxygen. Degradation products of biodiesel, such as insoluble gums and sediments, or the formation of organic acids and aldehyde may cause engine and injection problems [3 - 5].
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140
CHAPTER IV
OXIDATIVE AND STORAGE STABILITY STUDIES
OF BIODIESEL USING COMMERCIALLY
AVAILABLE ANTIOXIDANTS
1. Introduction
The word “antioxidant” has become increasingly popular in modern society as it gains
publicity through mass media coverage of its health benefits. The dictionary definition of
antioxidant is rather straightforward but a traditional annotation would define antioxidant as
“a substance that opposes oxidation or inhibits reactions promoted by oxygen or peroxides,
many of these substances (as the tocopherols) being used as preservatives in various products
(as in fats, oils, food products, and soaps for retarding the development of rancidity, in
gasoline and other petroleum products for retarding gum formation and other undesirable
changes, and in rubber for retarding aging)”. A more biologically relevant definition of
antioxidants is “synthetic or natural substances added to products to prevent or delay their
deterioration by action of oxygen in air [1].
Biodiesel, defined as fatty acid mono-alkyl esters made from vegetable oil or animal
fat, is an alternative fuel for combustion in compression–ignition (diesel) engines. Several
recent reviews have reported on the technical characteristics of biodiesel. In short, biodiesel
is made from domestically renewable feedstock, is environmentally innocuous, is relatively
safe to handle (high flash points), and has an energy content, specific gravity, kinematic
viscosity (KV), and cetane number (CN) comparable to petroleum middle distillate fuels
(petro diesel) [2, 3].
With a production of almost one million tons in Europe, fatty acid methyl ester
(FAME) more generally called biodiesel, has become a fast growing renewable liquid biofuel
within the European Community. Just like vegetable oils or fats, fatty acid methyl esters
undergo degradation over time, mainly influenced by temperature and oxygen. Degradation
products of biodiesel, such as insoluble gums and sediments, or the formation of organic
acids and aldehyde may cause engine and injection problems [3 - 5].
141
The bis-allylic configurations, where the central methylene group is activated by the
two double bonds (i.e., -CH=CH-CH2-CH=CH-), react with oxygen via the autoxidation
mechanism, with the radical chain reaction steps of initiation, propagation, chain branching,
and termination. During these reaction steps, several products can be formed, such as
peroxides and hydro peroxides, low molecular weight organic acids, aldehydes and keto
compounds, alcohols, as well as high molecular weight species (dimers, trimers, and cyclic
acids) via polymerization mechanisms. The use of antioxidant additives can help slow the
degradation process and improve fuel stability up to a point [6 - 9]. Fuel properties degrade
during long-term storage as follows:
(i) oxidation or autoxidation from contact with ambient air; (ii) thermal or thermal-oxidative
decomposition from excess heat; (iii) hydrolysis from contact with water or moisture in tanks
and fuel lines; or (iv) microbial contamination from migration of dust particles or water
droplets containing bacteria or fungi into the fuel [10]. Monitoring the effects of autoxidation
on biodiesel fuel quality during long-term storage presents a significant concern for biodiesel
producers, suppliers, and consumers [11].
1.1. Antioxidants Assay Studies
Assays for antioxidant protection against oxidative damage generally depend on
measurements of decreases in a marker of oxidation. Many terms have been used by different
researchers to describe antioxidant capacity including total antioxidant capacity, efficiency, power,
parameter, potential, potency, and activity. The “activity” of a chemical would be meaningless
without the context of specific reaction conditions such as pressure, temperature, reaction media,
co-reactants, and reference points. Because the “antioxidant activity” measured by an individual
assay reflects only the chemical reactivity under the specific conditions applied in that assay, it is
inappropriate and misleading to generalize the data as indicators of “total antioxidant activity”. The
other terms listed above are more independent of specific reactions and have similar chemical
meanings. To remain consistent, we use “capacity” to refer to the results obtained by different
assays. Oxidant-specific terms such as “peroxyl radical scavenging capacity”, “superoxide
scavenging capacity”, “ferric ion reducing capacity” and the like would be more appropriate to
describe the results from specific assays than the loosely defined terms “total antioxidant capacity”
and the like [1].
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On the basis of the chemical reactions involved, major antioxidant capacity assays can be
roughly divided into two categories: (1) hydrogen atom transfer (HAT) reaction based assays and
(2) single electron transfer (ET) reaction based assays. The ET-based assays involve one redox
reaction with the oxidant (also used as the probe for monitoring the reaction) as an indicator of the
reaction endpoint. Most HAT-based assays monitor competitive reaction kinetics, and the
quantitation is derived from the kinetic curves. HAT-based methods generally are composed of a
synthetic free radical generator, an oxidizable molecular probe, and an antioxidant. HAT- and ET-
based assays are intended to measure the radical (or oxidant) scavenging capacity, instead of the
preventive antioxidant capacity of a sample. Because the relative reaction rates of antioxidants (or
substrates) against oxidants, particularly peroxyl radicals, are the key parameters for sacrificial
antioxidant capacity, autoxidation and its inhibition kinetics are analyzed before in-depth analysis of
the individual assays [12, 13].
Mechanism Antioxidant assays
Assays involving hydrogen atom transfer
reactions
ROO* + AH→ ROOH + A*
ROO* + LH →ROOH + L*
• ORAC (oxygen radical absorbance
capacity)
• TRAP (total radical trapping antioxidant
parameter)
• ABTS method
• Crocin bleaching assay
• IOU (inhibited oxygen uptake)
• Inhibition of linoleic acid oxidation
• Inhibition of LDL oxidation
Assays by electron-transfer reaction
M(n) + e (from AH) → AH*+ + M(n - 1)
• FRAP (ferric ion reducing antioxidant
parameter)
• TEAC (Trolox equivalent antioxidant
capacity)
• SOS method
• DPPH (diphenyl-1-picrylhydrazyl)
• Copper(II) reduction capacity
• Total phenols assay by Folin-Ciocalteu
reagent
Other assays • TOSC (total oxidant scavenging capacity)
• Inhibition of Briggs-Rauscher oscillation
reaction
• Chemiluminescence
• Electrochemiluminescence
Table 4.1: Mechanism of antioxidant assays
143
Antioxidants with free radical scavenging activities may therefore be relevant in the
prevention and therapeutics of diseases where free radicals are implicated. WHO has
recommended the use of natural antioxidants that can delay or inhibit the oxidation of lipids
or other molecules by inhibiting the initiation or propagation of oxidative chain reactions.
Antioxidants are substances that when present at low concentrations, compared to those of
the oxidisable substrate significantly delays or inhibits the oxidation of the substrate [9]. An
important role of antioxidants is to suppress free radical mediated oxidation by inhibiting the
formation of free radicals by scavenging radicals. Radical scavenging action is dependent on
both the reactivity and concentration of the antioxidant. Research on the role of antioxidants
in biology focused earlier on their use in preventing the oxidation of unsaturated fats, which
is the cause of rancidity [14].
1.2. Biodiesel Storage and oxidation stability studies
The parameter of oxidation stability has been fixed at a minimum limit of a
6-hour induction period at 110 °C [15]. The method adopted for determination of the
oxidation stability is the so called Rancimat method which is commonly used in the vegetable
oil sector. Especially high contents of unsaturated fatty acids, which are very sensitive to
oxidative degradation, lead to very low values for the induction period. Thus, even the
conditions of fuel storage directly affect the quality of the product. Several studies showed
that the quality of biodiesel over a longer period of storage strongly depends on the tank
material as well as on contact to air or light. Increase in viscosities and acid values and
decreases in induction periods have been observed [16] during such storage. Although there
are numerous publications on the effect of natural and synthetic antioxidants on the stability
of oils and fats used as food and feed, little is available on the effect of antioxidants on the
behavior of FAME used as biodiesel. To retard oxidative degradation and to guarantee a
specific stability, it becomes necessary to find appropriate additives for biodiesel. Simkovsky
et al studied the effect of different antioxidants on the induction period of rapeseed oil methyl
esters at different temperatures but did not find significant improvements [17]. Schober et al
tested the influence of the antioxidant TBHQ on the peroxide value of soybean oil methyl
esters during storage and found good improvement of stability [18]. Canakci et al described
the effect of the antioxidants TBHQ and α-tocopherol on fuel properties of methyl soyate and
found beneficial effects on retarding oxidative degradation of the sample [19]. Das et al
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described effect of commercial antioxidants used in kharanja biodiesel for storage stability
[20]. Most recently Karavalakis et al described the effect of synthetic phenolic antioxidants
used for storage stability and oxidative stability. The storage stability of different biodiesel
blends with automotive diesel treated with various phenolic antioxidants has also been
investigated over a storage time of 10 weeks [21].
In the previous studies, numerous methods for assessing the oxidation status of
biodiesel have been investigated, including acid value, density, and kinematic viscosity. The
peroxide value may not be suitable because, after an initial increase, it decreases due to
secondary oxidation reactions, although the decrease likely affects only samples oxidized
beyond what may normally be expected. Thus there is the possibility of the fuel having
undergone relatively extensive oxidation but displaying an acceptable peroxide value. The
peroxide value is also not included in biodiesel standards. Acid value and kinematic
viscosity, however are two facile indicators for rapid assessment of biodiesel fuel quality as
they continuously increase with deteriorating fuel quality [22].
In this chapter, oxidative and storage stability of biodiesel was investigated using
commercially available antioxidants. The experimental details of antioxidant assay (scavenging
activity test) for five commercially available antioxidants such as BHA, BHT, GA, TBHQ and PY
are provided. FRAP (ferric reducing ability of plasma) radical scavenging activity, ABTS (2,2,-