Stability of biodiesel – A review

Renewable and Sustainable Energy Reviews 62 (2016) 866–881

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Stability of biodiesel – A review Rajesh Kumar Saluja a,n, Vineet Kumar b, Radhey Sham c a b c

IKG Punjab Technical University, Jalandhar, India Department of Mechanical Engineering, Rayat and Bahra Institute of Eng. and Biotech., Kharar, Punjab, India Department of Mechanical Engineering, Chandigarh Engineering College, Landran, Mohali, Punjab, India

art ic l e i nf o

a b s t r a c t

Article history: Received 12 August 2015 Received in revised form 19 February 2016 Accepted 2 May 2016

Biodiesel is the common name given to ethyl or methyl esters of long chain fatty acids obtained from vegetable oils or animal fats. Biodiesel is renewable, non toxic, biodegradable and usually contains no sulfur or aromatic compounds. The drawbacks of biodiesel are that it costs more than petroleum based diesel, softens and deteriorates certain elastomers and rubber compounds that are used in parts of fuel injection system such as fuel and pump seals. Another very important problem associated with the biodiesel is its storage as biodiesel is vulnerable to oxidation due to environmental factors such as air, moisture light etc. During oxidation, biodiesel breaks into unwanted smaller chain compounds such as aldehydes, small chain esters etc. beyond tolerable limits. Thus the oxidation process deteriorates fuel quality which can cause problems such as choking of injector and fuel filter and formation of deposits in various components of the fuel system including combustion chamber. Therefore it is essential to conduct the stability analysis of the biodiesel. A lot of work has been published on the stability of biodiesels. This paper discusses in detail about the types, causes and the effects of instability, about various tests and standards used for analyzing stability, various parameters and values used to measure and quantify stability, effects of various external agents such as antioxidants on the Stability. It also discusses the recent trends in the ongoing research in this field. It is the critical study of the previous works done on the stability of the biodiesel. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Biodiesel Stability Oxidation stability Storage stability Antioxidants

Contents 1. 2. 3. 4.

5.

6.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stability of biodiesel – definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The cause of instability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stability-types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Oxidation stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Measurement of oxidation stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Storage stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Thermal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters for measuring oxidation stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Iodine value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Peroxide value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Acid value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Kinematic viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. Oxidizability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors effecting stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Fatty acid structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Correspondence to: Amity School of Engineering and Technology, 580, Palam Vihar road, Bijwasan, New Delhi-61, India. Tel.: þ91 9968882824. E-mail address: [email protected] (R.K. Saluja).

http://dx.doi.org/10.1016/j.rser.2016.05.001 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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6.2. Relative antioxidant content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Metals contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Blends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Scope of future study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction There is more than one reason to look for alternative to the conventional petroleum fuels. The crude oil reserves are limited and demand for fuels is ever increasing. Not only the transport sector, but also agricultural and industrial sectors are dependent on the fuel supply. This leads to the sharp rise in the prices of crude oil. Since 1999–2000 to 2014–15, the crude oil prices have seen a steep rise from just above 20$/barrel to near 140$/barrel and then back to around 60$ [1]. Therefore when the world is on the verge of facing energy crises as well as environmental crises, researchers all over the world are keenly looking the alternative options so as to reduce this high dependability on conventional fuels. Diesel fuel is mainly used in transportation sector. The demand for diesel in India is 5 times that of the petrol. Therefore it is urgent to find the alternatives for the diesel. During the last three decades, the vegetable oils have been the focus of the interest of the researchers as the alternative to the conventional fuels. Experiments around the world are being performed on vegetable oils and their derivatives (biodiesels) obtained from various crops like sunflower, soybean, linseed, palm, coconut, rape, peanut, mustard, karanja, jatropha, neem, caster, cotton, etc. The competency of vegetable oils as a CI engine fuel comes from the facts that vegetable oils have good heating value and they give out almost no sulfur or aromatic polycyclic compounds. The two properties of vegetable oils viz. cetane numbers and heating values are comparable to those of the conventional diesel. Also the carbon cycle is completed by their burning as they are derived from plants [2–4]. Though the vegetable oils have the potential of being the substitute of diesel, the use of straight vegetable oils has certain problems associated with them. The viscosity of the vegetable oils is very high (usually 32–40 mm2 s  1 at 38 °C) as compared to that of the diesel (3–4 mm2 s  1 at 38 °C). The normal injection as used in diesel engine cannot be used with vegetable oils due to their different atomization characteristics from diesel [5]. Therefore if the straight vegetable oils are to be used as fuels, there is requirement of engine modifications and preheating arrangement [6,7]. Due to this reason, the straight vegetable oils are decomposed by pyrolysis or converted into biodiesel by transesterification reaction [8–13], before using as fuel for diesel engines. There are many significant advantages of biodiesel over conventional diesel. It is derived from renewable resources, hence leads to less dependency on the conventional fuel. It has higher flash point (150–180 °C) compared to 70 °C of the conventional diesel, leading to safer handling and storage. It has good lubricity, is biodegradable, and causes reduction in exhaust emissions (except NOx). The drawbacks of using biodiesel as CI engine fuels are its lower energy content leading to lower engine power and speed, higher viscosity, higher pour point, higher cloud point, injector coking, engine compatibility, Higher NOx emissions, high engine wear and high price [14]. Another problem associated with the biodiesel is that it is sensitive to oxidation when exposed to atmosphere (air, light and moisture etc). During oxidation, the biodiesel breaks into unwanted smaller chain compounds such as aldehydes, small

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chain esters etc. beyond tolerable limits. In other words, biodiesel becomes chemically unstable. The oxidation process deteriorates fuel quality which can cause problems such as choking of injector and fuel filter and formation of deposits in various components of the fuel system including combustion chamber [15–17]. Therefore it becomes essential to conduct the stability analysis of the biodiesel. A lot of work has been published [1–119] on the stability of biodiesels. Various factors that affect the stability are air, heat, light, antioxidants, minerals, peroxides, material of the storage container etc. These factors have been investigated in most of the studies. This paper reviews the previous work done by the researchers and summarizes the results of their studies. This paper also enlists various methods and standards that are used to measure the oxidation stability. This work discusses in detail about the types, causes and the effects of instability; various parameters and values used to measure and quantify stability; effects of various external agents such as antioxidants and metal contaminants on the stability of biodiesel.

2. Stability of biodiesel – definition According to Westbrook [18], “stability of the biodiesel is its ability to resist the physical and chemical changes caused by interaction with the environment”. Biodiesel is susceptible to oxidation, contaminants and interaction with light and temperature. The interactions of fatty acid chains present in the biodiesel with the oxygen makes the fuel unstable. Apart from this, the reactions of alkenes, dienes and compounds containing nitrogen, sulfur and oxygen also play a part in the oxidation phenomenon. Depending upon the amount and type of unstable matter, the effects of the oxidation can be the change in color of biodiesel, deposit formation and other changes which decrease the fuel clarity and cleanliness [18,19].

3. The cause of instability During the process of transesterification, if the reaction of fatty acid is carried out with methanol, the result is methyl ester of that fatty acid and if it is implemented with ethanol, ethyl ester is formed. During the transesterification process, the fatty acid chain remains unchanged; therefore the oxidation chemistry of biodiesel is similar to that of the fatty acid or oil from which it is derived. It becomes very important to study and understand the chemical composition and structure of the fatty acids and their corresponding methyl or ethyl esters for the proper understanding of the phenomenon of autoxidation. On the basis of the carbon bonds present, Fatty acids can be classified into two types: (i) Saturated fatty acids, where no double bonds between two carbon atoms are present (ii) Unsaturated fatty acids, where carbon-carbon double bonds are present

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Fig. 1. Methylene interrupted Structure.

Fig. 2. Conjugate structure.

Examples of saturated fatty acids are butyric acid, myristic acid, palmitic acid and stearic acid whereas unsaturated fatty acids are oleic acid, elaidic acid and linoleic acids. Further, when double bonds are present, unsaturated fatty acids can be classified as:

Fig. 3. Structure of methyl linoleate.

(i) Monounsaturated (MUFA); containing one C–C double bond (ii) Polyunsaturated fatty acids (PUFA) ; containing more than one C–C double bond separated by a single methylene group [20,21] Linoleic and linolenic are PUFA that contain two and three double bonds respectively whereas arachidonic acid is example of PUFA having four carbon-carbon double bonds. The factors promoting auto-oxidation of the fatty acids are (i) Polyunsaturation [20–22] (ii) The position of the C–C double bond [22–25], and (iii) Number of bis-allylic sites [26–28] Most of the naturally occurring fatty oils have polyunsaturation in the form of methylene interrupted configuration that has more than one cis double bonds separated by a single methylene (– CH2–) unit. This is shown in Fig. 1 for the Linolenic acid. This configuration is quite unstable as compared to the conjugated arrangement for its isomer shown in Fig. 2 [23] Polyunsaturation is one of the main reasons of autoxidation in the fatty acids and their corresponding esters. [21,22,27,28]. The reason for the stability of conjugated arrangement is the fact that the delocalization of the pi electrons provides partial stabilization to the molecule [24]. Most of the fatty oils derived from plants contain methylene-interrupted polyunsaturated fatty acids [15,25] and hence are more prone to oxidation. Also the carbon atoms at the bis-allylic sites are more sensitive to the oxidation attack. We can consider the example of polyunsaturated methyl esters such as methyl linoleate as shown in Fig. 3 with chemical formula C19H34O2 (acronym C18:2) and methyl linolenate (Fig. 4) with chemical formula C19H32O2 (acronym C18:3) [26]. In both of the esters, the double bonds are prone to oxidation. Counting from the carboxylic acid end, the carbon chain of Methyl linoleate has carbon–carbon double bonds at the 9th and 12th position respectively. Due to his arrangement, one bis-allylic site is created at the 11th Carbon. The hydrogen radical can be easily extracted from these bis-allylic sites during the initial stage of oxidation which is called ‘Initiation’. Therefore the bisallylic positions are more likely to undergo autoxidation than the corresponding allylic sites. In methyl linolente, the double bonds are present at Δ9, Δ12, and Δ15 positions due to which the two bis-allylic sites are created at C-11 and C-14 positions and these are even more sensitive to autoxidation. Polyunsaturation in the fatty acids and their esters is measured using various structure indices; “Iodine value” being the most important of them [27]. Allylic position equivalent (APE) and bisallylic position equivalent (BAPE) are two other important

Fig. 4. Structure of methyl linolenate.

structure indices [27,28]. APE and BAPE measure the number of single and double allylic carbon sites respectively in the fatty oil or its corresponding ester having methylene-interrupted polyunsaturation. Both APE and BAPE correlate well with the oxygen stability index (OSI) [28]. Pullen and Saeed [29] conducted the experimental studies to measure the APE, BAPE and oxidizability of biodiesel prepared from palm, olive, soyabean, and jatropha oil. They concluded that BAPE value is more significant for oxidation. Yang et. al. [30] also confirmed the increase in oxidation stability with decreasing APE and BAPE. Zuleta et. al. [31] also developed correlation between BAPE and oxidation stability.

4. Stability-types Stability of the biodiesel is classified into three types: (1) Oxidation stability, (2) Storage stability and (3) Thermal stability. During the oxidation process, the peroxides and hydroperoxides are formed which further form shorter-chain compounds such as ketones, aldehydes, alcohols and low molecular weight acids [28,32]. Storage stability pertains to the degradation of the biodiesel and its interaction with the light, air, metal, moisture and other conditions during storage [22]. Thermal stability deals with the oxidation at high temperature (cooking temperature) that causes increase in weight of oil and fat [33].

R.K. Saluja et al. / Renewable and Sustainable Energy Reviews 62 (2016) 866–881

R* þ O2-ROO*

4.1. Oxidation stability

*

Biodiesel is nonresistant to oxidation when exposed to air, light and moisture etc. The quality of fuel is ultimately affected due to oxidation. Therefore, the oxidation stability studies have been the integral part of biodiesel characterization and research. The oxidation stability standards are included in the European biodiesel standards EN 14213 and EN 14214 and American standards D6751. The phenomenon of oxidation is generally studied as either primary or secondary oxidation process [34–37,44,45] or vinyl polymerization process in which there is formation of oligomers of the fatty esters [37,38]. The initial products of oxidation are peroxides and hydroperoxides which further degrade into shorter chain products such as aldehydes, alcohols, ketones, and low molecular weight acids [39–41]. There may also be the formation of highermolecular-weight products due to tendency of double bonds to polymerization-type reactions. This leads to formation of insoluble compounds and increase in the viscosity of biodiesel which can cause choking of fuel lines and pumps. According to one study [42], these polymers are insoluble in the petrodiesel-biodiesel blends. The primary oxidation is further classified into three reactions [43] (i) Initiation, (ii) Propagation and (iii) Termination During the initiation reaction, the carbon free radical is formed [22,44]. The diatomic oxygen present in the free radical causes it to form peroxy radical which is reactive enough to further form the carbon free radical and hydroperoxide (ROOH) by extracting hydrogen atom from carbon atom of the chain. This carbon radical, thus formed, again reacts with diatomic oxygen and is known as propagation. This reaction ends with the formation of stable products by reaction of two carbon free radicals known as termination reaction. The three reactions are explained as follows: Initiation reaction: during the initiation reaction as shown in Eq. (1), hydrogen gets removed from the carbon atom of the biodiesel (RH) by an initiator radical (I*) and a carbon free radical (R*) is formed. RH þI*-R* þ IH

(1)

These initiator radicals may be formed due to any of the following: (a) Thermal dissociation of hydroperoxides (ROOH) ROOH-RO* þOH*

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(2)

(b) Metal catalyzed decomposition of hydroperoxides ROOHþM2 þ -RO* þOH* þM3 þ

(3)

ROOHþM3 þ -ROO* þH þ þM2 þ

(4)

(c) Photo-oxidation Propagation Reaction: during the propagation reaction, the carbon free radical R* formed at the initiation stage is highly reactive and quickly reacts with a bi-radical oxygen in the air and forms peroxyl radical (ROO*). This peroxyl radical further separates a hydrogen atom from a biodiesel molecule and forms a hydroperoxide and another carbon free radical.

ROO þRH-ROOHþR

(5) *

(6)

It is notable that during this reaction, a single radical may lead to the formation of up to 100 new radicals due to which, the decomposition occurs at an exponential rate and byproducts are formed quickly [44]. Termination Reaction: finally, the primary reaction comes to an end when two radicals combine together and form a stable product as shown in Eqs. (7) and (8) R þ R →R−RðStable ProductsÞ

ð7Þ

ROO þ R -ROORðStable ProductsÞ

ð8Þ

The above reactions (1–8) represent peroxidation chain mechanism. In these chain reactions, the hydrogen atoms attached to non-allylic carbons are stable than hydrogen atoms attached to allylic carbons due to the reason that the pi electron system in the adjacent olefin group gives resonance stability to those hydrogen atoms [45]. In secondary oxidation, the hydroperoxides further break up to form aldehydes such as propanals, hexenals, and heptenals along with formic acid, aliphatic alcohols and formate esters [46]. There is also formation of shorter chain fatty acids which cause increase in the acidity of the biodiesel [35]. The oxidation reaction goes on until the reactive sites or available oxygen is completely depleted. The time period from the start of oxidation after which the oxidation rate suddenly increases is known as Induction period [47]. During this period, the concentration of ROOH remains very low. As the induction period ends, there is a sudden rise in the hydroperoxides levels. Induction period is a very important parameter which determines the oxidation stability of the vegetable oils or biodiesels. It is usually measured by Rancimat test or by Oxidation Stability Index (OSI). The induction period of vegetable oils is usually more than that of their corresponding esters. Oxidation induction period for sunflower oil is nearly three times as that of its fatty acid monoalkyl esters (biodiesel) [48]. 4.2. Measurement of oxidation stability Oxidation stability is an important specification of biodiesel and therefore has been included in both American as well as European biodiesel standards. American standards ASTM D6751 and European biodiesel standards EN 14213 and EN 14214 specify Rancimat test method to determine oxidation stability. This method measures the induction period and is quite similar to OSI method [49]. Table 1 shows various specifications of induction period for measurement of biodiesel oxidation stability in biodiesel standards of various countries. Table 2 lists various standard test methods prescribed in various International standards available for testing of Biodiesel being used in testing the stability Table 1 Specifications related to induction period in biodiesel standards of various countries. S.No Biodiesel standard

Country/ Region

Test Methods

Limits (Induction time, h (minimum))

1. 2. 3.

USA Europe Australia

EN 14112 EN 14112 EN 14112

3 6 6

Brazil South Africa India

EN 14112 EN 14112 EN 14112

6 6 6

4. 5. 6

ASTM D6751 EN 14214 Fuel standard (Biodiesel), 2003 ANP 255 SANS 1935 IS 15607

870

Table 2 Various international standards for testing of oxidation stability [94–103]. S.no. Test method

Title

Description

Reference

This test method specifies the procedure of measurement of the oxidation stability of biodiesel (B100) as per ASTM D6751 and blends of biodiesel with middle distillate petroleum fuels, including B6 to B20 blends as specified in ASTM D7467 under specified oxidizing conditions at 95 °C. This test method covers the measurement of the inherent stability of middle distillate petroleum fuels under specified oxidizing conditions at 95 °C. This method is not applicable to fuels containing residual oil or significant amounts of components derived from non-petroleum sources. Cellulose ester filters are substituted with glass fiber filters in order to use this method for biodiesels. It is the accelerated test method that measures storage stability of middle distillate fuels. The test conditions are initial boiling point above 175 °C and 90% (v/v) recovery point below 370 °C. It is not applicable to fuels containing residual components of a non-petroleum source. The predictive test can also be used to measure the storage stability of biodiesel. Due to long storage period of 4 weeks to 24 weeks, the test method is not suitable for quality control testing, but it does provide a tool for research on storage properties of biodiesel. This test method covers relative stability of middle distillate fuels under high temperature aging conditions with limited air exposure. The test method can be useful for investigation of operational problems related to fuel thermal stability. This European Standard specifies a method for the determination of the oxidation stability of fatty acid methyl esters (FAME) at 110 °C. This standard method is used for volatility of biodiesel and the longer stability of blends. In this method, the reaction tube length is increased and the minimum analysis time is also increased to 20 h. This standard lays guidelines for determination of the oxidative stability of fats and oils under extreme conditions of high temperature and air flow, which increase oxidation. It does not allow determination of the stability of fats and oils at ambient temperatures, allows a comparison of the efficacy of antioxidants added to fats and oils. It is a quick test method and involves a small sample. It measures the time taken for a pressure drop of 10% of the maximum pressure. It is quite effective method and repeatability is high. Very economic Method. Has been used on biodiesels effectively. This American test method is equivalent to the Europeon.

[94]

1.

ASTM D 7462-11 Oxidation stability of biodiesel and biodiesel blends

Oxidation stability

2.

ASTM D2274

Stability of middle distillate petroleum fuels

Oxidation stability

3.

ISO 12205

Stability of middle distillate petroleum fuels

Oxidation stability

4.

ASTM D4625

Standard Test method for middle distillate fuel storage Storage stability stability at 43 °C (110 °F)

5.

ASTM D 6468

High temperature stability of middle distillate fuels

Thermal stability

6.

EN 14112

Oxidation stability

7.

EN15751

Determination of oxidation stability (accelerated oxidation test) of pure FAME Determination of oxidation stability of biodiesel and biodiesel blends with diesel.

8.

ISO 6886:2006

Determination of oxidative stability (accelerated oxidation test)

Thermal stability

9.

prEN16091

Determination of oxidation stability by rapid small scale oxidation method

Storage stability/ thermal stability

10

ASTM D7545

Determination of oxidation stability by rapid small scale oxidation method

Storage/thermal stability

Storage stability

[95]

[96]

[97]

[98]

[99] [100]

[101]

[102]

[103]

R.K. Saluja et al. / Renewable and Sustainable Energy Reviews 62 (2016) 866–881

Stability measured

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Numerous experiments have been conducted to measure oxidation of the lipids and the corresponding methyl or ethyl esters. One such old and popular method to determine oxidation stability of the biodiesel is finding its iodine value (IV). IV and oxidation stability both depend on saturation. The IV is an indicator of total unsaturation of a vegetable oil or biodiesel. It is measured as amount of iodine in grams per 100 g of sample that can be added to the double bonds of any fatty acid or oil chain [56]. Some of the studies [25,27,56,57,58] also point out that it may also indicate their tendency of polymerization and deposit formation. Biodiesel standards EN 14214 and EN 14213 have specified the IV of 120 and 130 respectively. However, the problem with IV is that its value can be same for different fatty acid structures [15]. Results of some of the works [57,58] that used mixture of vegetable oils with different IV as fuel for engine performance tests could not justify their low IV. Bondioli and Folegatti, [59] in their investigations on biodiesels with a different IV, could not establish any clear relation between the IV and oxidative stability. Schaal's oven test is another older method used for determining the stability of the vegetable oils. It is an accelerated method in which the sample (50 g) of oil is put in loosely sealed glass that is kept in the oven under the controlled temperature of 63 °C [60]. The quality of sample is periodically monitored till the end point is achieved. The end point of the test was initially found by smell and taste analysis (organoleptic analysis) [61]. Later on, in a modified process, the rapid increase in peroxide value (PV) [62] or sudden weight gain of the sample [63] were used to indicate the end point of test. The sudden increase in PV or commencement of a rapid weight gain is considered as an indicator of beginning of addition of oxygen into the oil. The other tests for determining the oxidation stability include the oxygen bomb test, active oxygen method (AOM), photochemical luminescence method, pressurized differential scanning calorimetry (P-DSC), the Rancimat method and modified Rancimat apparatus method. Table 3. lists various methods available for determining the oxidation stability. In the oxygen bomb test the oil sample is oxidized at about 100 °C at high pressure and then the pressure drop is recorded with time. From this the induction period can be easily found. Liang and Schwarzer [62] conducted investigations on samples of lard and tallow oil for its stability analysis. Four accelerated test methods used by them were Schaal oven test, AOM, the Rancimat method and the oxygen bomb test. It was concluded that while determining the stability of animal fats and the effect of the antioxidants on them, any single accelerated test method cannot be reliable and Rancimat test is least reliable. Therefore use of more than one test method wes recommended. Though, the results of AOM and Shaal oven test could be correlated. Rancimat method (EN 14112) [64] is the most common accelerated method that is used for determining oxidative stability of biodiesels and is mentioned in biodiesel standards EN 14214 and ASTM standard D6751. As per the biodiesel standard EN 14214, the test should be conducted at temperature of 110 °C and minimum induction period should be 6 hours whereas the induction time of 3 h is required as per ASTM standard D6751. In this test, the biodiesel sample is heated in a sealed tube up to 110 °C and the temperature is kept constant. The air is made to pass through the reaction tube. The primary products formed due to oxidation are continuously mixed with distilled water in another container and the conductivity of this solution is monitored continuously. There is a steep rise in the conductivity of the absorption solution as soon as the secondary products of oxidation (volatile carboxylic acids) are formed (as shown in Fig. 5). The time duration of steep rise in conductivity from the beginning of the test is known as Induction time or Induction period and is a strong indicator of the oxidation stability of the biodiesel sample.

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George Karavalakis et. al. [65] conducted the modified Rancimat stability test on nine different types of available biodiesel samples and their 168 blends in different proportions. A strong inverse relationship was obtained between total unsaturation of biodiesels and the oxidation stability. Rancimat method under controlled conditions (100 °C, 20 L air/h) was also used in another study by Velasco et. al. [66] to study the oxidation phenomenon in dried microencapsulated oils (DMOs). Their studies also associated the Rancimat response to the oxidation in the DMOs. Another study [67] evaluated the OSI of Castor oil FAME using the quick method of “thermal and air contact degradation”. The samples were brought in contact with pure oxygen at high temperatures ranging from 120–160 °C) and high pressure (700 KPa). The results were obtained in quite a little time of 30 minutes as compared to that of 10 hours under normal conditions. Differential scanning calorimetery was used under nonisothermal conditions by Simon et.al. [68] while investigating oxidation of rapeseed and sunflower oils. Oxidograph was utilized under isothermal conditions for different temperatures and Temperature dependence of induction period was formulated in Arrhenius- like equation: Z Ti dT β¼ ð9Þ T o A expB=T In the above equation, β is the coefficient of temperature rise (scan), To and Ti are the initial temperature and the temperature at the end of induction period respectively, A and B are constants and T is the absolute temperature. Xin et.al. [69] used Rancimat method to conduct stability analysis on safflower biodiesel between the temperature range of 100–120 °C with and without Propyl gallate (PG) antioxidant. They also observed that the antioxidant concentration improves the induction period of biodiesel whereas the temperature rise reduces the Induction period. 4.3. Storage stability Storage stability of biodiesel is the resistance offered by the biodiesel to the chemical and physical changes taking place due to the environmental factors and interaction during the long term storage [73]. Storage stability pertains to the degradation of the biodiesel and its interaction with the light, air, metal, moisture and other conditions during storage. The visible effects of oxidation during storage can be either deposit formation or the change in color of the biodiesel. The color of the biodiesel also changes from yellow to brown and starts smelling like that of paint [74,75]. The factors responsible for storage instability are moisture or water, particulate solids present in the sample, degradation products, light, presence of metals and microbial slimes [74–76]. Bouaid et al. [76] prepared the biodiesel form Brassica Carinata oil and conducted the storage stability studies by storing the samples for 12 months. The studies reflected that the B. Carinata methyl ester was very stable and there was no quick increment in viscosity, acid value and peroxide value if the biodiesel is not allowed to come in contact of moisture and oxygen. Bondioli et. al. [77] proposed a quick test, which if performed just after production, could predict the storage stability of the biodiesel after 12 months. They conducted the experiments (ASTM D4625) on 8 biodiesel samples at 80 °C for 24 h to show the accelerated storage behavior. It was found that the modified Rancimat test, showed good correlation with long term storage tests in terms of repeatability and significance. It was also found that the sample that had lowest long term storage stability under actual conditions was first to degrade during the accelerated tests.

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Table 3 Various oxidation stability tests for oils and fatty acids [25,27,48–63,82,105,110] [25,27,57–72,93,105,110]. Standard/ Test method

Parameter measured

Standard values

Units

Conditions of test

Performance evaluation

Usage

Economical aspects

Limitations

1

Iodine Value

EN 14214, EN 14213

Iodine value

120 g Iodine/ 100 gm

g Iodine/100 g

2

Schaal oven test

Organoleptic analysis Peroxide value

–

–

–

– Reagent strength, – Indicates the potential of fatty acid to be sample weight oxidized and reaction time – Indicator of level of should be polyunsaturation constant – Repeatability possible only if exact conditions of the test followed – Simple test – 68.2 °C – Requires minimal temperature laboratory equipment – Sample is placed in oven for a fix time

Has been widely used for oils and fats and their esters

– Low cost

Many fatty acids compositions give the same IV Other factors can influence fat stability Not much reliable method

Used for calculating shelf life of oils, fats and fat containing foods (Storage stability of biodiesel)

– Test requires – Test requires large sample large sample – Test is of – Test is of longer duration (4–8 days) longer duration (4– 8 days)

– Temperature of – Corealtes well with Rancimat's test for PV 100-140 °C greater than 100 – Continous supply of oxygen

This method is being used but faster methods supplementing

Not economic because of its time consuming procedures

Weight gain AOCS Cd 12–57 Time required to reach PV of 100 Milli equivalents/kg

– –

Millimoles/kg or (Milli equivalents/ kg) – h

Oxygen bomb test

ASTM- D525

Sudden drop of oxygen pressure with time

–

Min

High pressure of 750 kPa Temperature- 135 °C

Extraction of fat is not required Correlates well with Rancimat test

Used for fats and finished products

–

5

Rancimat test

EN 14112

Induction time

3 or 6

h

80–160 °C (110 °C for biodiesel) Sample-60 ml Gas flow- 20 l/h

This is accelerated method which gives accurate results which have good repeatability

Most widely used method Used for oil and fat containing prpoducts including biodiesel

Quick and economic method

6

High-pressure Differential Scanning Calorimetery (DSC)

Modification of Oxidation InducASTM D5483 tion Time (OIT) Oxidation temperature (OT)

–

OIT – h OT- Kelvin (K)

120 °C temperature 500 psi pressure

This method further accelerates the stability measurement Requires small sample

Apart from stability, it can be useful in determining properties of oil such as specific heat, melting point etc.

Specialized equipment is needed. Less economic

7

Photochemilluminesence method

–

h

–

It measures temperature dependent photoluminescence

–

–

3

Active oxygen method

4

Induction time

still the are it

– Time consuming procedure – Frequent titrations required – Results may have large deviation – maintenance of high pressures is difficult – Oxidograph may require modification in design Not much suitable for fat containing foods and oils Sometimes the oxidation of oils is different at high temperatures. [70] Reproducability of P-DSC results may become difficult with addition of antioxidants as OSI increases Sometimes slight modification in the set up may be required – Very little data has been reported

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S.NO Test method

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calibration curves that help in the prediction of the induction period of the samples. In petroOXY method, the analysis of both volatile and non volatile products can be done which is advantage over Rancimat test as only the analysis of Volatile products can be performed. This method has been described in the standards ASTM D7545 and EN 16091 Table 4. shows the description of these alternate and recent methods of stability measurements. Lesser literature is available on TGA and UAOM [86,87]. 4.4. Thermal stability

Fig. 5. Determination of Induction period. [67].

Obadia et al. [78] in their studies concluded that Acid number and kinematic viscosity are the good measure for storage stability of biodiesel. Bouaid et al. [79] analyzed biodiesel made from brassica carinata, sunflower oil and waste frying oil stored under various conditions for thirty months. Increase in the parameters; AV, PV, viscosity, and insoluble impurities, was recorded with time for all biodiesel samples, whereas IV decreased with time. Westbrook [80] conducted ASTM D4625 test for soy biodiesel and its blends. In ASTM D4625 test, the samples are stored at a temperature of 43 °C for 12 weeks. The parameters measured during the studies were acidity, amount of sediments formed and change in viscosity. It was found in the investigations that the value of acid number, total insolubles, kinematic viscosity and isooctane insoluble material increased with time. It was also noticed that after 4 weeks during the test, the value of acidity and insoluble increased to high levels for the most unstable of the samples. McCormick et al. [81] conducted the storage stability tests as per EN 14112 and ASTM D2274 accelerated tests and D4625 long term storage tests on various B100 samples and B5 and B20 blends. Their tests were based on D2274 method and measured induction time. Their results supported the pre existing concept that the induction period of the blends depends on the induction period of the pure biodiesel and is not dependent on diesel fuel sulfur level, aromatic content, or stability. In another study by Das et al. [82], the various samples of Karanja oil methyl esters (KOME) were stored and analyzed under different conditions for 180 days. Their observations also showed increment in peroxide value and viscosity with time. Also the higher concentration of antioxidants had positive effects on the samples. In a recent study, Boulifi et al. [83] produced biodiesel from rice bran oil and conducted experiments during the long term storage period of 24 months. They developed correlations for variations of acid value, iodine value, peroxide value and kinematic viscosity as the function of time which can be used for the prediction of storage time of Rice bran oil. In recent years, various alternative methods have been used successfully by various researchers [84–89]. These are Near Infrared Spectroscopy (NIRS) [84], Ultrasonic Accelerated Oxidation Method (UAOM) [85], Fourier Transform Infrared (FTIR) spectroscopy [86], Thermogravimetric Analysis (TGA) [87], and petroOXY method [88,89]. These methods are quick, efficient and reliable. The results of these analyses are well correlated with those of the Rancimat method. NIRS has gained popularity in the past decade as the quickest and effective method of stability measurement [84]. NIRS, UAOM and FTIR involve development of

Thermal stability refers to the resistance offered by biodiesel to oxidation at high temperatures (4 250 °C). The phenomenon of oxidation at high temperatures is different from the oxidation at low temperatures. As discussed previously, the conjugate structure is more stable than methylene interrupted structure. At high temperatures, methylene-interrupted structure isomerizes to conjugated structure which is more stable. After the commencement of Isomerization, cyclohexane ring is formed due to reaction of a conjugated diene group from one acid chain (conjugated diolefin) with a single olefinic group from another fatty acid chain (mono olefin group) [72,90]. This reaction, known as Diels Alder reaction (Fig. 6), plays a prominent role above the temperature of 250 °C. The products formed during Diels Alder reaction are dimmers [15,91] but trimers can also be formed in addition if the dimer reacts with a conjugated diene from other molecule [15]. Thermal polymerization results in rapid reduction of total unsaturation of the fatty acid chain. The reason cited for this in various studies is that all the three olefin groups combine to form a single group. The high temperature in biodiesel also deteriorates the natural antioxidants at a faster rate due to which the oxidation process further increases [15]. The effect of thermal oxidation process is of importance only when biodiesel comes in contact with the engine heat. Not much work has been done which relates thermal stability to the storage stability. [92]

5. Parameters for measuring oxidation stability 5.1. Iodine value The IV is an indicator of total unsaturation of a vegetable oil or biodiesel. It is measured as amount of iodine in grams per 100 g of sample that can be added to the double bonds of any fatty acid or oil chain [56,58]. Some of the studies [25,27,56–58] also point out that it may also indicate their tendency of polymerization and deposit formation. IV is the property that has been related to the suitability of biodiesel for long term engine usage [104]. Iodine values of 120 and 130 have been specified in the European standards EN 14214 and EN 14213 respectively. The standard method for determining IV is EN 14111. However, IV cannot be relied for assessing the unsaturation because lots of fatty acids compositions give the same IV [25,27]. This is so because the IV is just the measure of double bonds present and does not account for the position of double bonds. Iodine value is also not a good index to assess oxidation stability. 5.2. Peroxide value The peroxide value is defined as the amount of peroxide oxygen per 1 kg of fat or oil. Peroxide value indicates the amount of hydroperoxide that initiates oxidation and reacts with antioxidant [105]. The lower is the peroxide value; the higher is the oxidation stability in biodiesel. It indicates the extent of primary oxidation. During the initial stages of oxidation process, the peroxides increase but towards the

874

Table 4 Recent alternate methods to measure stability of biodiesel.

1

2.

3.

4.

5.

Procedure

Advantages

Simple method, faster than Rancimat method, found to be a very reliable method The results of NIRS correlate well with those of the Rancimat method. The results are very accurate with very less prediction error for the biodiesels without antioxidants Ultrasonic – accelerated oxidaSono-degradation in the biodiesel sample is induced with the help of Simple fast and accurate method tion method ultrasonic homogenizer and then the degradation is monitored using Equipment easily available in the most of UV–vis spectrometry by checking the absorbance every 2 minutes at labortaries 270 nm. The absorbance vs time curve is plotted and the point of sudden change of curvature of the curve is noted Fourier Transform Infrared (FTIR) The biodiesel samples are heated at a const temp of 110-120 °C in an Measures Thermal stability Spectroscopy oven, The heated samples are collected and IR spectrometery is per- Fast and economic method Does not require the use of solvents or toxic formed. The spectra of the samples of biodiesel and the parent reagents vegetable oil are compared by subtractive IR spectroscopy Benefecial in characterization of biodiesel The results are in agreement with the Rancimat Thermogravimetric Analysis Thermoanalyser is used to record thermogram of the samples with (TGA) rate of heating of 10 °C/min from low temperature of 10 °C to 700 °C. test The thermogravimetric data is used to understand the kinetics of oxidation PetroOXY method 5 mL of the sample are kept at room temperature and then pressur- Faster than Rancimat test ized in a pure oxygen atmosphere at 700 kPa and the temperature and It provides the complete analysis of volatile as pressure are increased to 110 °C and 910 kPa. The end point is the time well as non volatile products of oxidation Has a good repeatability and reproducibility at which the pressure drop is 10% of the maximum pressure Correlates well with the EN 14112 test results

Near infrared spectroscopy (NIRS) Spectrometer is used to acquire the near infrared spectra of biodiesel samples at room temperature and between the wave number range 12,000–4000 cm  1 with spectral resolution of 16 cm  1. The data is analyzed and calibration models developed using suitable technique

Limitations

Refrence

The prediction error increases to twice for the biodiesel with the antioxidants but the range still remains acceptable

[84]

–

[85]

–

[86]

Little data has been reported

[87]

–

[88,89]

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S.NO. Method

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Table 7 Variation of Kinematic viscosity (cSt at 40 °C) over a period of 6 months for various biodiesels.

Fig. 6. Diels–Alder reaction. Table 5 Variation of peroxide value (mg/kg) over a period of 6 months for various biodiesels. Methyl ester

0 months

3 months

6 months

Reference

JME KOME MOME SME^ SME$ RBOME

4.16 4.9 4.7 3.8 3.8 3.10

21.21 20.21 21.02 36.5 55 13.6

38.26 23.9 22 – – 21.64

[152], [82], [116], [145], [145], [83],

KOME: karanja oil methyl ester, MOME: mahua oil methyl ester, SME^: soyabean methyl ester in presence of carbon steel, SME$: soyabean methyl ester in presence of Galvanized steel, RBOME: rice bran oil methyl ester.

Table 6 Variation of acid value (mg KOH/g) over a period of 6 months for various biodiesels.

JME JME RME CME SME PME COME RSME RBOME

0 months

3 months

6 months

Refrence

0.15 0.44 0.12 0.19 0.11 2.54 0.85 0.19 0.09

0.3 0.78 0.54 0.2 0.3 2.87 1.18 0.26 0.14

0.45 1.1 0.84 0.32 0.46 – – 0.5 0.17

[152], [153], [154], [154], [154], [118], [118], [111], [83],

CME: castor oil methyl ester, PME: palm oil methyl ester, COME: coconut oil methyl ester, RSME: Rapeseed oil methyl ester.

end of oxidation, they oxidize into aldehydes and ketones and therefore the peroxide levels fall in the later part of the oxidation [32–42]. Peroxide value is an important indicator of oxidation stability even though it is not listed in ASTM standard D6751. But cetane number, that depends on the concentration of hydroperoxides is listed in ASTM Specification D6751. Increasing peroxide value may reduce ignition delay of the fuel [106,107]. However, significant degradation due to oxidation decomposition of hydrogen peroxide decreases the peroxide value, and ultimately the cetane number reduces, which leads to prolonged ignition delay [108–110]. For this reason PV is not so suitable evaluation factor of stability of biodiesel, and is usually not a part of the biodiesel standards. Table 5. shows the variation of peroxide values before and after oxidation over a period of 6 months.

5.3. Acid value The acid value of the fat or oil indicates the quantity of free fatty acids in it [111]. Quantitatively, it is the amount of potassium hydroxide (KOH) required to neutralize 1 g of the sample and indicates the number of carboxylic acidic groups present in it [112]. Acid value of vegetable oil plays an important role in transesterification and should be less than 2.0 mg KOH/g [112] before transesterification.

JME JME RME CME SME PME COME KOME MOME RBOME

0 months

3 months

6 months

reference

4.38 6.1 4.6 7.3 4.3 4.92 3.67 5.01 4.85 4.14

5.00 7.1 5.95 7.45 5.6 5.92 4.72 5.19 5.05 4.2

5.63 8.1 6.5 7.9 6.05 – – 5.41 5.15 4.3

[152], [153], [154], [154], [154], [118], [118], [82], [116], [83],

Kinematic viscosity and the acid number are two convenient parameters to assess the quality of biodiesel, because they are constantly increasing with deteriorating fuel quality [113]. The acid number is measured using ASTM D664 and is included in standard ASTM D675. It is also contained in European standard 14214 and can be found using method EN 14104 [114]. Table 6. Shows variation of acid value for various biodiesels over a period of six months. 5.4. Kinematic viscosity Straight vegetable oils cannot be used as fuels due to their high viscosity. To convert them into usable fuels, reduction in viscosity is necessary. The viscosity can be reduced either by blending oils with diesel or in the form of their alkyl esters. The Kinematic viscosity continuously increases with decreasing fuel quality during storage. Therefore, Kinematic viscosity is considered a good indicator for assessing quality of biodiesel. The viscosity is influenced by certain factors. (i) Number of carbon atoms [57], (ii) Degree of saturation [57], and (iii) Double bond configuration; cis or trans [115]. Both, the number of carbon atoms and the degree of saturation, tend to increase the viscosity [57]. The position of double bonds has little effect on viscosity but ‘trans’ configuration has higher viscosity than ‘cis’ configuration [115]. During oxidation processes, usually ‘cis’ to ‘trans’ isomerism takes place. This isomerism, accompanied with formation of products of higher molecular weight, lead to increase in viscosity during oxidation [115]. During oxidation, increase in viscosity takes place only when the peroxides reach a particular value [79,116]. According to a study [79], there may be a relation between increase in viscosity and IV. The results of another study [116] indicated a slight increase in viscosity of mahua oil methyl ester during storage of 12 months. Out of the samples that were stored under different conditions, the most effected were samples kept inside a room open to air. The variation of Kinematic viscosity and specific gravity during storage over a period of 6 months has been shown in Tables 7 and 8 respectively. 5.5. Oxidizability Oxidizability (OD) has been discussed in some of the works [45,80,117] as:

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OD¼0.02 (Ox) þ 1(Ll)þ 2(Lln)

(10)

where Ox, Ll and Lln refer to fraction of Oleic acid, Linoleic acid and Linolenic acid content respectively. These works show that the oxidizability reduces with increase in the induction period. Table 9 shows various parameters for measurement of stability discussed above and their specified values.

Table 9 Specifications of various stability parameters [45,56,79,80,105,111,112,116,117]. Property

Test methods

Units

ASTM 6751

EN 14213

EN 4214

Induction period (at 110 °C) Iodine value

EN 14112

hr

3 (min)

4 (min)

6 (min)

EN 14111

–

130 (max) 120(max)

Peroxide value Acid values

– ASTM D664

g Iodine/ 100 g – mg KOH/ g

– 0.50 max

– –

– 0.50 max

6. Factors effecting stability 6.1. Fatty acid structure Polyunsaturation, the position of the carbon double bond and number of bis-allylic sites [20–30] are very important factors promoting the oxidation in fatty acids. M shahabuddin et. al. [118] studied the effect of the percentage of the unsaturated fatty acid and the presence of long chain double bonded hydrocarbon on the stability of the biodiesel. They concluded that properties of biodiesel degraded quickly with the storage time with increasing long chains and higher unsaturation. Raghuveer and Hammond [119] observed in their experiments that there is reduction in stability of fats during oxidation between 37 °C and 50 °C due to “randomization by transesterification”. They also observed that if the unsaturated fatty acids are concentrated at the 2-position of triacylglycerols, there is increase in stability. In another study, Miyashita et al. [120] observed that rate of autoxidation of polyunsaturated triacylglycerols containing linoleate (L) and linolenate (Ln) increased with increasing total unsaturation. In yet another study [39], the rates of oxidation for oleates (methyl, ethyl esters), linoleates, and linolenates were found to be 1:41:98. The structure of the fatty acid and the amount of unsaturation also influence ignition delay which affects the performance and causes the increase in exhaust emissions [121,122]. 6.2. Relative antioxidant content After transesterification, during distillation and purification process, the natural antioxidants such as tocopherols, sterols and tocotrienols get destroyed due to which the biodiesel becomes prone to oxidation. OSI of undistilled biodiesel from palm oil biodiesel (PME) is greater than 25 h, whereas the same for distilled PME is approximately 3.5 h. This is due to a fact that the ingradients such as carotenes and α–tocopherols which are found in crude Palm oil get washed away upon distillation after transesterification[123]. The addition of appropriate antioxidants improves oxidation stability of biodiesel and also reduces the NOx formation during combustion [24,64,112,123–131]. The reason may be that the highly reactive hydrogen present in the antioxidant gets easily Table 8 Variation of Specific gravity over a period of 6 months for various biodiesels.

JBD JME RME CME SME PME COME RBOME

0 months

3 months

6 months

Reference

0.85 0.887 0.888 0.887 0.887 0.843 0.844 0.879

– 0.888 0.890 0.887 0.888 0.847 0.856 0.881

1.9 0.889 0.891 0.888 0.889 – – 0.884

[153], [154], [154], [154], [154], [118], [118], [83],

Kinematic viscosity (at 40 °C) Linolenic acid content FAME contentZ 4 double bonds Oil stability index (OSI) (at 110 °C)

EN 14104 ASTM D445 EN ISO 3104 EN 14103

mm2/s

1.9–6.0

3.5–5

3.5–5

% m/m

–

–

12 (max)

–

% m/m

–

1 (max)

1 (max)

AOCS method Cd 12b-92

(hr)

–

–

–

separated by the peroxy radical than the fatty oil or ester hydrogen [123]. Antioxidants can be classified as: (a) phenolic-type and (b) aminic-type [123]. They can also be categorized as natural or synthetic antioxidants. The various natural and Synthetic antioxidants are shown in Table 10. Isbell et al. [64] studied the role of antioxidants on the stability of several vegetable oils and their corresponding FAMEs at 110 °C. It was observed that tocopherol increased the oxidation stability of the vegetable oils and their corresponding esters, except the meadowfoam oil. Their results also explained that C5 double bond was the most stable than the others with respect to the olefin position). Das et al. [82] observed that the effectiveness of antioxidants on Karanja oil methyl ester was in the order of Propyl galate (PG) 4butylated hydroxyanisole (BHA) 4butyl-4hydroxytoluene (BHT). Liang et al. [124] conducted stability studies on palm oil biodiesel using natural antioxidant (α–tocopherol) and synthetic antioxidants (BHT and TBHQ). They found that the synthetic antioxidants improved the stability better than the natural ones. The antioxidants used were effective in order of TBHQ 4BHT 4 α– tocopherol. Another study [125] also confirmed the effectiveness of natural antioxidants in improving oxidation stability of the biodiesel. Another study by Dunn [126] evaluated effects of α-tocopherol and TBHQ antioxidants on SME using P-DSC accelerated tests. Samples with antioxidants were found to be more stable than those without antioxidants. Results also indicated that TBHQ is more effective than 7α-tocopherol. Mittelbach and Schober [127] found that the effectiveness of any antioxidant is dependent on the fatty acid from which biodiesel is derived. Chaithongdee et al. [128] concluded from their studies on various antioxidants that PG was most effective antioxidant for Jatropha methyl Ester (JME). In another study, Canakci et al. [129], conducted storage stability tests on SME and confirmed the positive effect of TBHQ on Peroxide Value of SME. Simkovsky and Ecker [130] tested various antioxidants of low concentrations of 300 ppm using active oxygen method (AOM) on RME at different temperatures and their tests results showed a little improvement in induction period for the low concentrations of antioxidants. Rios et. al. [132] evaluated the effect of three antioxidants; BHT, hydrogenated cardanol, alkyl hydrogenated cardanol on thermo oxidative stability of soybean biodiesel keeping antioxidant

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Table 10 Various antioxidants used for improving the stability. S.No

TYPE

Name

Abbrevation

Type

Nature

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Natural

α-tocopherol β-carotene Butylated hydroxyanisole Butyl-4-hydroxytoluene Tert-butyl-hydroquinone 2,5-di-tert-butyl-hydroquinone Propyl gallate Pyrogallol Hydrogenated cardanol Alkyl hydrogenated cardanol Gallic Acid IONOX 220 Vulkanox ZKF Baynox

α-T β-C BHA BHT TBHQ DTBHQ PG PY HC AHC GA – – –

Phenolic Phenolic Phenolic Phenolic Phenolic Phenolic Phenolic Phenolic phenolic phenolic Phenolic Phenolic Phenolic Phenolic

Chain inhibitor Chain inhibitor Chain inhibitor Chain inhibitor Chain inhibitor Chain inhibitor Chain inhibitor Oxygen absorber Oxygen absorber Oxygen absorber Reducing agent Reducing agent Reducing agent Reducing agent

Synthetic

Table 11 Effect of antioxidants on various methyl esters [32,124,128,133,134,139–141]. (Test method EN 14112). S.No.

Antioxidant

1 2

Nil α–tocopherol

3

BHT

4

BHA

5

DTBHQ

6

TBHQ

7

8

PG

PY

Concentration (ppm)

– 250 500 1000 50 250 500 1000 250 500 1000 250 500 1000 50 250 500 1000 250 500 1000 250 500 1000

Approximate induction period (h) SME

PME

RME

CSME

JME

KOME

3.5 4 4.5 4.7 – 4.5 5.3 6.6 4.8 5.8 6.8 4.8 5.7 6.5 – 5 7.3 11.5 7 8.9 10.4 9 10.8 11.8

3.52 – – 6.17 6.42 – – – – – – – – – 8.85 – – – – – – – – –

4.5 – – – – – – – – – – 5 7 9.4 – – – – – – – – – –

6.5 6.5 6.4 6 – 7 7.7 9.1 7.4 7.9 9.1 10.3 13 18.5

4.21 – – – – 4 6.5 – 7.5 9 – – – – – 8.12 12.23 – 26.35 32.98 – 11.2 17.5 –

1.82 – – – – – 3.87 5.9 – 6.32 8.29 – – – – – 3.6 4.94

concentration 200, 300 and 400 ppm. AHC showed the best results for thermo oxidative stability followed by HC and BHT. Yang et. al. [30] conducted experiments on the biodiesel samples prepared from tallow oil, soyabean oil and canola oil to study the effect of antioxidants on the biodiesel samples. It was found in their studies that PY and TBHQ were the most effective antioxidants. Agarwal and Khurana [133] tested Karanja oil (Pongamia pinnata) methyl esters samples for storage stability tests using antioxidants in various concentrations (300–1000 ppm). The effectiveness of PY was the best followed by followed by PG and BHA. Similar conclusion was made in the studies by Obadia et al. [78] PY increased IP of Pongammia biodiesel from 0.33 hr to 34.35 hr after it was added in concentration of 3000 ppm. In another study, Dunn [134] conducted non-isothermal P-DSC analyses in static and dynamic (positive air – purge) modes to study the effect of five different antioxidants on SME and its blends under controlled conditions. His results confirmed the better effectiveness of scientific antioxidants (PG, BHT and BHA) over natural antioxidant α- tocopherol in improving the induction

13.5 19 30 10 14 21 13 18.8 26.5

7.71 13.79 – 14.65 22.49

period. The study also recommended BHA or TBHQ for improving storage stability even at high concentrations (up to 3000 ppm). BHT was suitable at relatively low concentrations of (up to 210 ppm) while PG was not soluble in the biodiesel blends due to some physical compatibility problems with SME. Fattah et al. [135] used BHA and BHT to study their effect on the stability of Palm oil biodiesel and its blend with diesel (B20) and recorded the rise in the induction period of B20 blend to 20.6 h. Ileri and kocar [136] studied the effect of various antioxidants on canola oil methyl ester and diesel blends. Their results indicated that TBHQ was more effective with B20 blend than BHA and BHT for concentrations above 750 ppm and the induction period increased from 6.9 h without antioxidants to 38.7 h with 1000 ppm concentration. Balaji and Cheralathan [137] have studied the effect of αtocopherol acetate on the Neem oil methyl ester. In their experiment they have shown that with 400 ppm concentration the Neem oil methyl ester could attain the induction period of 6 h which is 2.13 h without using antioxidants. In another study [138],

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Balaji and Cheralathan determined the effectiveness of BHT to the Neem oil methyl ester and reported similar results. Table 11 shows the effect of various antioxidants on various methyl esters as reported in various works discussed above.

their pure form was in the order of castor oil biodiesel 4palm oil methyl ester4 Jatropha methyl ester. For the blends, JME75: COME25 (Jatropha biodiesel 75% and Castor oil biodiesel 25%) fulfilled the IP criteria.

6.3. Metals contaminants 7. Scope of future study It has been found in the various works [15,25,142–146] that the presence of metals Iron, copper, Nickel, cobalt, manganese etc., even in small traces, accelerates the oxidation process in the biodiesel. This happens because the metals enhance the catalytic reaction of the initiation of the oxidation [142]. Sarin et al. [142] investigated the effect of above 5 mentioned metals on the induction period of the Jatropha Biodiesel. They found that the metals showed the same effect on the induction period irrespective of their concentrations. Another study by Jain et al. [141] on Jatropha biodiesel also obtained the similar results. Copper had the maximum catalytic effect than the other metals [141,142]. In another study, Jain et al. [143] concluded that the relative catalytic effect of the various metals with Jatropha biodiesel were in the order of Cu4 Co 4Mn 4Ni4 Fe. Yang et al. [30] confirmed the strong catalyst effect of copper and found in their studies that lead was also a strong catalyst to metal oxidation. Therefore the storage tanks of the above mentioned materials accelerate the pace of oxidation. Hu et al. [144] found in their experiments that aluminum and stainless steel were more resistant to corrosion than copper and carbon steel. They corrosion rates for copper and carbon steel were 0.02334 mm/year and 0.01819 mm/year respectively and for aluminum, and stainless steel were found to be 0.00324, and 0.00087 mm/year. Various metals and alloys that may be available as an alternative to the reactive metals include aluminum, carbon steel [145,146], and stainless steel; aluminum being most suitable [15]. In one of the recent studies, Fernandes et al. [145] found the carbon and galvanized steel to be very compatible to the biodiesel during the storage period of 56 days and that the further addition of TBHQ increased the stability. Non metals such as Viton, Teflon, Fluorinated plastics and Nylon are also resistant to oxidation [147]. 6.4. Blends Park et al. [148] conducted investigations to study the effect of blending on oxidation stability. The investigation was done by blending three different types of biodiesels made from Palm, Rapeseed and Soyabean methyl esters. SME has the lowest (IP less than 6 h) and PME has maximum oxidation stability of the three biodiesels. Addition of RME and PME to SME improved the oxidation stability of SME. Sarin et al. [149] conducted tests on the various blends of biodiesels from pongamia, Jatropha and palm oil. They also found that blending improves the oxidation stability of pongammia biodiesel when blended with palm oil or JME. However blending can increase the precipitation formation in some cases [150] which may lead to poor engine performance. Serrano et al. [151] studied the effect of blending on the oxidation stability of biodiesel. Various blends were prepared from six methyl esters (soyabean, sunflower, coconut, palm oil and babacu oils). Their results showed that unsaturation had strongest influence on the IP of the blends. They also developed the mathematical equation to show that dependency of IP on polyunsaturation. IP ¼ 0:27ðSÞ þ 0:13ðM Þ–ð0:19ÞP

ð11Þ

where (S), (M) and (P) are wt% of saturated, monounsaturated and polyunsaturated fatty acids present in the blend. Zuleta et al. [31] evaluated the stability of biodiesels from palm, jatropha and castor oils and their blends using standard EN 14112 and ASTM D6371 tests. They found that stability of biodiesels in

Various stability test methods have been discussed in this review. Rancimat is the standard and most widely used method to measure the stability of biodiesel. In the past few years new methods faster than Rancimat method have been developed and tested on some of biodiesels [85–89] viz. NIRS, FTIR, UAOM, and TGA. Still, little literature is available on UAOM and TGA. These techniques are fast, efficient and may help in the choosing and optimizing the additives for the better stability of biodiesel. There is tremendous scope of further study in these techniques and development of the new and efficient models for better stability predictions.

8. Conclusion The Stability of biodiesel is an important characteristic that affects the usage of biodiesel. It is the ability to resist the physical and chemical changes caused by interaction with the environment. The factors promoting auto-oxidation of biodiesel are (i) polyunsaturation (ii) the position of C–C double bond and (iii) number of bis-allylic sites. Conjugate arrangement of double bonds in carbon chain is more stable than the methylene interrupted structure. Out of the various structure indices which are measure of the polyunsaturation, BAPE is most important. Unsaturation also has the strongest influence on the Induction period of the blends of biodiesel. This study also discussed the various tests available for measurement of stability of fatty acids, oils, food products and biodiesel. Rancimat test is a unanimous choice for measurement of oxidation stability and has been prescribed in various American and Europeon standards. NIRS has gained popularity in the past decade as the quickest and effective method of stability measurement due to its advantages over Rancimat test. Antioxidants are very effective in increasing the stability of biodiesel and reduction in NOx. PY and PG have been reported as the best antioxidants in major studies. This study also point out that Aluminum and Stainless steel are most suitable for containers of biodiesel. There is further scope of study in the UAOM and thermogravimetric analysis methods of stability testing.

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