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Investigations on Gradual and Accelerated Oxidative Stability of Karanja Biodiesel and Biodiesel-Diesel Blends C K Suraj, Anand Krishnasamy, and T Sundararajan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01678 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019
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Energy & Fuels
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Investigations on Gradual and Accelerated Oxidative Stability of Karanja Biodiesel and
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Biodiesel-Diesel Blends
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C. K. Suraj, K. Anand* and T. Sundararajan
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Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, India
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* Corresponding author email id:
[email protected] 6
Abstract
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One of the major limitations that hinders widespread application of biodiesel in automotive
8
engines is its poor oxidative stability, which in turn depends upon methyl ester constituents of
9
biodiesel as well the storage conditions. Hence, a relative assessment of the oxidative stability of
10
biodiesels across the different parts of the world is rather difficult. In the present work, oxidative
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stability of biodiesel based on ASTM D4625 accelerated oxidative stability test is compared with
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that of gradual oxidation under two different long term storage conditions, viz. open to air and
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sunlight and closed to air and sunlight. Neat Karanja biodiesel and its blends with diesel at 25,
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50 and 75 percent by volume are used for the present study. The important physicochemical
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properties of fuel samples are measured at regular time intervals to evaluate the rate of oxidation
16
and the extent of fuel quality degradation. The results obtained show that neat Karanja biodiesel
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stored under open to air and sunlight has the highest rate of oxidation and fuel quality
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degradation with 32% increase in kinematic viscosity, 1.5% increase in density and 3% decrease
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in calorific value. The acid value of all the tested fuel samples increased beyond ASTM and EN
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standard specification limits within the first three months of the storage period. The peroxide
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value showed a steep increase during the first six months storage period, and decreased
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afterwards. The effects of adding TBHQ antioxidant in Karanja biodiesel at varying
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concentrations are also evaluated based on the measured Rancimat induction period, and it is
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observed that 250ppm of TBHQ is required to meet EN 41214 standard specifications for
25
biodiesel. No correlations are found to exist between the properties of fuel samples stored under
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gradual and accelerated oxidation conditions. However, adopting ASTM D4625 standard test
27
method to evaluate the storage stability of biodiesel avoids any ambiguity owing to the variations
28
in the storage and the ambient conditions across the different parts of the world.
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Keywords: Karanja Biodiesel, Stability, Storage Conditions, Properties, Oxidation
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1. Introduction
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The demand for energy for the transportation sector is increasing at an alarming rate [1].
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Biodiesels have gained attention as a promising alternative to fossil fuels because of their similar
34
fuel properties and renewable nature [2]. Biodiesel is composed of saturated and unsaturated
35
fatty acid esters of different chain lengths and degrees of unsaturation [3]. The presence of
36
unsaturated fatty acid esters make biodiesel susceptible to oxidation [4]. When biodiesel is
37
exposed to atmospheric conditions, molecular oxygen attacks bis-allylic/allylic positions and
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initiates the auto-oxidation process [5]. The oxidation rate of biodiesel has no direct correlation
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with the total number of double bonds; rather it is related to the total number and position of
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allylic and bis-allylic carbon atoms [6]. The auto-oxidation of biodiesel results in the formation
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of unstable primary oxidation products such as peroxides and hydroperoxide, which in turn leads
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to several secondary oxidation products including aldehydes, acids, ketones, alcohol, gums, and
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sediments. Alternatively, the polymerisation reaction could also take place with a fatty acid
44
molecule to form dimers and trimers [7]. Along with polymerisation, biodiesel oxidation leads to
45
changes in the fuel composition and thereby, fuel quality degradation [8]. An increase in
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biodiesel viscosity owing to oxidation results in poor spray and evaporation characteristics
47
leading to improper fuel-air mixing and lower combustion efficiency [9]. An increase in acid
48
number upon long term storage enhances the corrosive nature of biodiesel and causes wear in the
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fuel handling system [10]. Biodiesel oxidation also results in the formation of insoluble gums
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and sediments that increase fuel stickiness, cause injector coking and fuel filter plugging [11,12].
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Biodiesel produced from the production plants has to be transported to the end-user where it is
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consumed for heating or transportation purpose. The lead time between the production and
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consumption of biodiesel results in ageing and fuel quality degradation. The ageing may
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accelerate based on fuel handling methods during the transit process. Hence, it is imperative to
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know the long term storage stability of biodiesel in order to ensure appropriate fuel quality to the
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end-user. The rate of oxidation of biodiesel is affected by multiple factors including composition,
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temperature, light, availability of oxygen, presence of metal ions and the presence of natural
58
antioxidants [13].
59
There are many established accelerated fuel stability measurement methods currently in use, viz.
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ASTM D2274, ASTM D4625, active oxygen method and oxidation stability index (OSI) [14,15].
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In these methods, biodiesel is exposed to higher temperature, higher pressure or enhanced 2 ACS Paragon Plus Environment
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Energy & Fuels
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oxygen supply in order to accelerate the auto-oxidation process. [16][17]. As the temperature
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increases, the rate of oxidation is found to increase exponentially [18]. The relationship between
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temperature and oxidation rate depends upon the activation energies of the rate-controlling steps
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in the oxidation reactions. Hence, the above-said methods provide qualitative trends on biodiesel
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stability rather than a quantitative one. Among these methods, the fuel oxidation rate is lower in
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ASTM D4625 [19] wherein the tests are carried out at ambient pressure and at 43°C without any
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enhanced oxygen supply. This method is not designed for evaluating the fuel storage stability at
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room temperature, rather it is intended to shorten the duration of storage stability compared to
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those carried out under ambient conditions. The existing studies on accelerated oxidation
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stability of biodiesel based on ASTM D4625 test method are briefly discussed next.
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One of the foremost storage stability studies on biodiesel using ASTM D4625 method is done by
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Bondioli et al. [20]. After a storage period of 24 weeks, the properties of eight different biodiesel
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samples that are tested are found to exceed the International Standard Specifications for
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biodiesel. Westbrook [21] compared the different accelerated stability methods to evaluate the
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stability of soybean biodiesels and concluded that a single parameter, viz. insoluble, acid value
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or viscosity measurement, is not sufficient to express the stability of biodiesel. McCormick [22]
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utilised the 43°C method to compare the storage stability of biodiesel and biodiesel-diesel blends
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and concluded that the stability of biodiesel-diesel blends could be predicted based on the
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stabililty of neat biodiesel. Christensen [4] studied the long term storage stability of fresh
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biodiesel and aged biodiesel stabilised with antioxidant at 43°C and found that the anti-oxidant
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type and peroxide values are important factors that affect the fuel degradation when the induction
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periods are similar. Yang et al. [23] attempted to compare the stability of Camelina biodiesel
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based on 43°C test method for 12 weeks with that of 80°C method done for 24 hours as
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suggested by Bondioli [24]. The data obtained from these two methods do not exhibit any
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correlation for peroxide value and induction period. Obadiah [25] studied the effects of anti-
87
oxidants on the storage stability of Pongamia biodiesel for 12 weeks at 43°C.
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Based on the literature survey, it is observed that there are limited studies available on the
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oxidative stability of biodiesel under accelerated ASTM D4625 test conditions. However, there
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are no attempts to compare the oxidative stability of biodiesel under gradual and accelerated
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conditions. Further, there are no studies available that compare and correlate the oxidative
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stability of biodiesel and biodiesel-diesel blends under gradual and accelerated conditions based 3 ACS Paragon Plus Environment
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on ASTM D4625 method. For petroleum diesel, one week of storage stability at 43°C is widely
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accepted as equivalent to 4 weeks of storage stability at 17°C [26]. Such a relationship does not
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exist for biodiesel and biodiesel-diesel blends. The present study attempts to compare and
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correlate the gradual and accelerated oxidative stability of Karanja biodiesel and Karanja
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biodiesel-diesel blends. Further, the stability of neat Karanja biodiesel stabilised with TBHQ
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antioxidant is also evaluated in terms of the Rancimat induction period under gradual and
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accelerated ASTM D4625 method.
100 101
2. Materials and Methods
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2.1 Materials
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In the present study, non-edible Karanja oil which is procured from a local supplier is used to
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produce biodiesel. Potassium hydroxide in the form of pellets of a minimum assay of 85.0% and
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sulphuric acid of a minimum assay of 87.0% are procured from Thermo Fisher Scientific India
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Pvt. Ltd. The antioxidant tert-butyl hydroquinone (TBHQ) is purchased from Spectro Chemicals
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Pvt. Ltd., while the analytical grade methanol of minimum assay of 99.0% is procured from
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Finar Ltd.
109 110
2.2 Biodiesel Production
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The free fatty acid (FFA) content of Karanja oil measured by ASTM D974 method is found to be
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12.3% w/w. Owing to a higher FFA content of Karanja oil, two-stage acid-base
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transesterification is carried out to produce biodiesel. In the first stage, the acid-catalysed
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pretreatment process is carried out to reduce the FFA content of Karanja oil to below 2% w/w.
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In this process, sulphuric acid is used as a catalyst at a concentration of 1.5% w/w to oil along
116
with methanol to oil molar ratio of 8:1. After the reaction, the mixture is allowed to settle down
117
in a conical flask for 5 hours before removing the top layer consisting of unreacted methanol,
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catalyst and water produced during the reaction. The FFA content of the pretreated oil is found to
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be 1.14% w/w, which enables the base-catalysed transesterification process that is carried out
120
using potassium hydroxide as a catalyst. The catalyst concentration is fixed with a base value of
121
0.7% w/w to oil, along with the concentration required to neutralise the FFA of Karanja oil [27].
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Upon completion of the reaction, the mixture is allowed to settle down in a conical flask for 8
123
hours to allow complete separation of glycerol and biodiesel. Both acid- and base-catalysed 4 ACS Paragon Plus Environment
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reactions are carried out for 90 min at 55±1°C. The separated raw biodiesel is subjected to water
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washing using heated deionised water to remove dissolved soap, traces of glycerol and unreacted
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alcohol. The pH level of the washed water is monitored by using a litmus paper. Any traces of
127
methanol and water left in the biodiesel are removed by heating it to 105°C for 10 minutes.
128 129
2.3 Biodiesel Fuel Characterisation
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The important engine fuel properties of Karanja biodiesel are measured following ASTM D6751
131
standard test methods. The fatty acid methyl ester (FAME) composition of biodiesel is analysed
132
based on EN-14103 test method by using a gas chromatograph (Nucon 5765). Methyl
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heptadecanoate is used as an internal standard to find out the ester content in biodiesel. The ester
134
content (C), expressed as a mass fraction in percentage, is calculated using Eq. (1).
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Ester content
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(1)
is the total peak area of the methyl esters;
is the peak area corresponding to methyl
137
heptadecanoate;
is the concentration of the methyl heptadecanoate solution (mg/mL);
138
the volume of the methyl heptadecanoate solution (mL);
139
experiments are repeated thrice and the average of those is taken as the final ester content which
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is around 96.9% with a standard deviation of 1.03%.
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A FAME reference standard is used for identification of biodiesel constituents by comparing
142
their specific retention times. The higher heating values of fuel samples are measured by using a
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digital bomb calorimeter (IKA C2000) and a Stabinger Viscometer (Anton Paar SVM 3000) is
144
used to measure the density and viscosity. ASTM D974 colour titration method using p-
145
naphtholbenzene as an indicator is used to measure the acid number (AN) of the fuel samples.
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ASTM D3703 titration method is used to determine the peroxide value (PV). The Rancimat
147
Induction period of biodiesel is measured using a biodiesel Rancimat apparatus (Metrohm 892).
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The measurement uncertainty is calculated by repeating a representative experiment 10 times
149
[28]. Based on this, the measurement uncertainty for viscosity, density, calorific value, acid
150
value, peroxide value and induction period are estimated to be 0.25, 0.22, 1.24, 2.36, 1.09 and
151
2.36% respectively.
152 153 5 ACS Paragon Plus Environment
is
is mass of the sample (mg). The
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2.4 Sample Preparation and Storage Conditions
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The Karanja biodiesel-diesel blends are prepared with 25, 50 and 75 percent by volume of
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Karanja biodiesel in diesel which are mixed with the help of a magnetic stirrer for 30 minutes at
157
500 rev/min. The test fuel samples of 150mL are stored in Borosil glass bottles of 200mL
158
capacity. Borosil glass bottles are used to avoid any possible catalytic effect of storage container
159
material in the fuel oxidation reaction during the long term storage. All the fuel samples are
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stored under three different storage conditions, viz. at 43°C according to ASTM D4625 standard,
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open to air and sunlight under ambient temperature conditions, and closed to air and sunlight
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under ambient temperature conditions (by keeping the fuel samples in the storage container
163
closed with airtight lid in a dark room). The same procedure is followed to store Karanja
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biodiesel samples stabilised with TBHQ antioxidant.
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2.5 Oxidised Biodiesel Fuel Properties
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The peroxide value, acid value, kinematic viscosity, density, calorific value and Rancimat
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induction period of fuel samples are measured at regular intervals to evaluate their storage
169
stability. For the samples stored at 43°C, the fuel properties are measured once in three weeks for
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20 weeks. For the samples stored at room temperature, the fuel properties are measured once in
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three months for one year period. The composition of neat Karanja biodiesel stored under three
172
different storage conditions are also measured at the end of the storage period to examine the
173
variations in methyl ester composition.
174 175
3. Results and Discussion
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The results obtained from the present work to compare the gradual and accelerated oxidation of
177
Karanja biodiesel and biodiesel-diesel blends are presented and discussed next.
178 179
3.1 Fresh Biodiesel Fuel Properties
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The measured composition of Karanja biodiesel includes ~75% unsaturated fatty acid methyl
181
ester constituents which are expected to result in poor oxidative stability. The various
182
unsaturated fatty acid methyl esters present in Karanja biodiesel include oleic acid (C18:1),
183
linoleic acid (C18:2), linolenic acid (C18:3) and gondoic acid (C20:1) with 54.59, 17.22, 3.10
184
and 1.20 weight % respectively. The saturated fatty acid methyl esters identified in Karanja 6 ACS Paragon Plus Environment
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Energy & Fuels
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biodiesel include palmitic acid (C16:0), stearic acid (C18:0), arachidic acid (C20:0), behenic acid
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(C22:0) and lignoceric acid (C24:0) with 8.68, 6.48, 1.66, 5.49 and 1.58 weight % respectively.
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The measured properties of Karanja biodiesel in comparison to diesel fuel properties and ASTM
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D6751 and EN 14214 standard specification limits for biodiesel are provided in Table 1. It is
189
observed that the density and viscosity of biodiesel are higher than that of diesel. The higher
190
viscosity and density of biodiesel are found to result in poor fuel spray characteristics in terms of
191
longer spray penetration, narrow spray cone angle and a higher Sauter mean diameter[29]. The
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calorific value of biodiesel is lower than diesel by ~14% which would result in a higher brake
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specific fuel consumption with longer injection duration for the same engine power output. The
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measured fuel properties of Karanja biodiesel are found to be within ASTM and EN standard
195
specification limits except that the induction period is well below the EN standard limits.
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Table 1. Measured properties of Karanja biodiesel and diesel along with ASTM and EN
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standard specification limits for biodiesel.
199
Biodiesel Diesel Viscosity (cSt) 5.10 2.90 3 Density (kg/m ) 889.90 825.50 Calorific Value (MJ/kg) 38.40 44.80 Acid Value (mg KOH/g oil) 0.35 --Peroxide Value (meq/kg) 12.30 --Rancimat Induction 4.14 --Period (h) Ester content (%) 96.90 --*min-Minimum limit, max-Maximum limit
ASTM D6751 1.90-6.00 ----0.50 (max) ---
EN 14214 3.50-5.00 900 (max) --0.50 (max) ---
3.00 (min)
8.00 (min)
---
96.50(min)
200 201
3.2 Biodiesel Composition
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As discussed, the composition of neat Karanja biodiesel stored under three different storage
203
conditions are measured at the end of the storage period. A comparison of the composition of
204
neat Karanja biodiesel stored under three different conditions with those of fresh biodiesel is
205
shown in Table 2. It is observed that the total ester content reduces for all three biodiesel
206
samples, indicating the conversion of esters into other primary and secondary oxidation products.
207
The reduction in the ester content is highest for the samples stored under open to air and sunlight,
208
followed by the samples stored at 43°C. Thus, the extent of fuel quality degradation is expected 7 ACS Paragon Plus Environment
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209
to be higher for the Karanja biodiesel stored under air and sunlight. It is interesting to note that
210
all the unsaturated methyl ester constituents undergo a significant decrease in their concentration
211
upon storage, with a maximum decrease observed for the samples stored under air and sunlight
212
conditions.
213 214
Table 2. Variations in neat Karanja biodiesel composition at the end of storage period. Storage Conditions Fresh Biodiesel Stored at 43°C Closed to sunlight and air Open to sunlight and air
Composition weight (%) C16:0 C18:0 8.68 6.48 8.12 6.31
Ester content C18:1 C18:2 C18:3 C20:0 C20:1 C22:0 C24:0 (%) 54.59 17.22 3.10 1.66 1.20 5.49 1.58 96.9 42.43 8.06 --2.05 0.74 3.32 0.94 72.11
7.99
7.31
49.34
14.28
---
4.10
0.92
4.04
1.14
89.15
8.66
6.67
40.93
5.88
---
1.61
0.97
3.05
0.91
68.43
215 216
3.3 Peroxide Value
217
The fuel peroxide value is an indication of the extent of the primary oxidation process during
218
storage [30]. The effects of storage conditions on the peroxide value of the test fuel samples are
219
shown in Figure 1. It is observed that for the samples stored at 43°C and exposed to sunlight and
220
air, the peroxide value increases with storage time, reaches a peak value, and decreases thereon.
221
During the initial period of biodiesel storage, the rate of primary oxidation is higher than that of
222
secondary oxidation and thus, peroxide value increases with time. The reduction in peroxide
223
value after reaching a maximum value implies that the peroxides decompose to form other stable
224
secondary oxidation products like aldehydes, ketones, alcohols, polymers, etc. [4]. Thus, during
225
this period, the rate of secondary oxidation is higher than that of primary oxidation. The peak
226
peroxide value is attained after 3 months and 6 months of storage respectively for the samples
227
stored at 43°C and exposed to sunlight and air conditions. The peak peroxide value of the
228
samples exposed to sunlight and air is significantly higher than that stored at 43°C. For the
229
samples stored at 43°C, secondary oxidation reactions happen at a much earlier stage leading to
230
an early decrease in peroxide value compared to those stored under room temperature. Based on
231
these trends, it can be concluded that both sunlight and temperature promote biodiesel oxidation.
232
At the same time, the photo-oxidation in the presence of sunlight increases the rate of primary 8 ACS Paragon Plus Environment
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oxidation reaction, while a higher ambient temperature increases the rate of secondary oxidation
234
reaction leading to an early decomposition of peroxides in case of samples stored at 43°C.
235 236
Figure 1. Effects of storage conditions on the peroxide value of Karanja biodiesel and
237
biodiesel-diesel blends.
238
For the samples closed to sunlight and air, the peroxide value increases steadily throughout the
239
storage period and attain a value of 198 meq/kg at the end of one year storage period for neat
240
Karanja biodiesel. It is interesting to note that the magnitude of peroxide values of the neat
241
Karanja biodiesel stored under sunlight and air and closed to sunlight and air are almost similar
242
at the end of one year storage period. The above observation substantiates that the biodiesel
243
peroxide value alone is not a true representative of the extent of fuel oxidation. As expected, neat
244
Karanja biodiesel exhibits the highest peroxide value under all the three storage conditions. At a
245
particular storage condition, the differences in peroxide value among the biodiesel-diesel blends
246
are minimal which is not expected and can be attributed to a possible catalytic effect by any of
247
the constituents or additives present in diesel. Similar trends of the higher rate of increase in 9 ACS Paragon Plus Environment
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peroxide value with Karanja biodiesel-diesel blends is also observed by Tennison and Anand
249
[31].
250 251
3.4 Acid Value
252
The hydroperoxides formed from polyunsaturated fatty acids are highly unstable and undergo
253
further oxidation at suitable conditions to form monomeric and polymeric secondary oxidation
254
products along with numerous low-molecular-weight products. The secondary oxidation
255
products are also unstable; aldehydes, in particular, are very reactive. They are easily oxidised
256
into peroxy acids, which further decompose to form other products. Formic acid and other low-
257
molecular-weight fatty acids are the end products of oxidation [32][33]. Thus, the magnitude of
258
acid value is one of the indicators of the extent of the secondary oxidation process during the
259
long term storage of biodiesel [34]. The effects of storage conditions on the acid value of
260
Karanja biodiesel and biodiesel-diesel blends is shown in Figure 2.
261 262
Figure 2. Effect of storage conditions on the acid value of Karanja biodiesel and biodiesel-
263
diesel blends.
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As expected, because of the secondary oxidation process and also by hydrolysis of methyl esters,
265
the acid value increases for all the test fuel samples at all storage conditions. It is observed that
266
all the fuel samples fail to meet ASTM standard within the first two months of storage period at
267
43°C. As discussed earlier, the rate of secondary oxidation is higher for the samples stored at
268
43°C, resulting in a higher rate of increase in acid value. At ambient temperature conditions, all
269
the fuel samples fail to meet ASTM standard limit within first six months of storage period. The
270
catalytic effect of the constituents present in diesel fuel is evident for the biodiesel-diesel blends,
271
wherein an increase in acid value is observed with all the biodiesel-diesel blends.
272 273
3.5 Kinematic Viscosity
274
Owing to a higher kinematic viscosity of biodiesel compared to diesel, it is recommended to
275
utilise biodiesel-diesel blends to overcome the limitations due to poor fuel spray characteristics
276
[35]. The oxidation of biodiesel leads to isomerisation of double bonds (usually cis to trans) and
277
polymerisation, which leads to an increase in kinematic viscosity. The effects of storage
278
conditions on the kinematic viscosity of Karanja biodiesel and biodiesel-diesel blends is shown
279
in Figure 3. The fuel samples stored at 43°C showed the highest rate of increase in kinematic
280
viscosity as compared to those stored under other conditions. An increase in temperature during
281
the storage period is found to accelerate secondary oxidation and polymerisation reactions,
282
leading to a higher rate of increase in kinematic viscosity. Thus, commercial utilisation of
283
Karanja biodiesel in tropical countries is expected to pose problems related to lower shelf life
284
and fuel quality degradation. The samples stored under sunlight and air show the next highest
285
rate of fuel quality degradation. Exposure to sunlight results in photooxidation reactions which
286
accelerate the oxidation process by accelerating free radical production [36]. In case of the fuel
287
samples stored at 43°C, neat Karanja biodiesel crosses the maximum limit set by the ASTM
288
standard specifications within four months of storage, while all the biodiesel-diesel blends meet
289
the specification limits even after five months of storage period. For the fuel samples exposed to
290
sunlight and air, neat Karanja biodiesel crosses the maximum limit set by ASTM standard
291
specfications within six months, and there is a maximum increase of 32% after one year storage
292
period. All the biodiesel-diesel blends stored under air and light conditions meet the standard
293
specification limits even after one year storage period. For the samples stored under dark and
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closed to air conditions, ASTM standard specification limits for kinematic viscosity are met even
295
after one year storage period.
296 297
Figure 3. Effect of storage conditions on the kinematic viscosity of Karanja biodiesel and
298
biodiesel-diesel blends.
299 300
3.6 Density
301
The density of Karanja biodiesel is higher than that of diesel by ~8% (refer Table 1). The effects
302
of storage conditions on the density of Karanja biodiesel and biodiesel-diesel blends are shown
303
in Figure 4. As discussed earlier, the oxidation reactions are followed by polymerisation
304
reactions during the long term storage of biodiesel, wherein smaller molecules are combined to
305
form high molecular weight compounds. The polymerisation reactions result in close packing of
306
the constituent monomer molecules and thus increases the density of biodiesel during storage
307
[37]. It is observed that the density increases with the storage period for all the fuel samples
308
stored under three different storage conditions. The fuel samples stored at 43°C showed the 12 ACS Paragon Plus Environment
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highest rate of change, followed by those exposed to sunlight and air. The changes in density for
310
the fuel samples stored under closed to air and light conditions are insignificant. The densities of
311
all the test fuel samples are well within EN standard specifications for biodiesel even after one
312
year storage period under all the test conditions.
313 314
Figure 4. Effect of storage conditions on the density of Karanja biodiesel and biodiesel-
315
diesel blends.
316 317
3.7 Calorific Value
318
The effects of storage conditions on the calorific value of Karanja biodiesel and biodiesel-diesel
319
blends are shown in Figure 5. It is observed that the calorific value of test fuel samples decreases
320
by around 2-3% after the storage period under all the storage conditions. During the autoxidation
321
process, oxygen from ambient air is consumed to form oxidative products. However, these
322
products on combustion do not substantially increase the heat of reaction; but the mass of the
323
overall system increases due to the addition of oxygen [31]. This, in turn, is reflected as a slight 13 ACS Paragon Plus Environment
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reduction in the calorific value of the oxidised fuel samples. Among the test fuel samples, neat
325
Karanja biodiesel stored under sunlight and air showed highest rate of decrease in calorific value;
326
however, it is within 3%. For the other fuel samples, the changes in calorific value are well
327
within 2%.
328 329
Figure 5. Effect of storage conditions on the calorific value of Karanja biodiesel and
330
biodiesel-diesel blends.
331 332
3.8 Storage Stability of neat Karanja Biodiesel Stabilised with Antioxidant
333
The Rancimat induction period is taken as a measure of oxidative stability of biodiesel [38]. The
334
measured Rancimat induction period of neat Karanja biodiesel, without the addition of anti-
335
oxidant, is found to be 4.14 h, which is well below EN 14214 specifications. It is well
336
established in the literature that the biodiesel with a higher degree of unsaturation shows fuel
337
quality degradation on long term storage [39]. The Karanja biodiesel includes ~75% unsaturated
338
methyl ester constituents (refer Table 1), and thus exhibits a higher rate of fuel quality 14 ACS Paragon Plus Environment
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degradation under different storage conditions. It is intended to improve the oxidative stability of
340
Karanja biodiesel by adding a commercial antioxidant tert-butyl hydroquinone (TBHQ). The
341
effects of adding TBHQ at various concentrations to Karanja biodiesel on the Rancimat
342
induction period are shown in Figure 6. It is observed that around 250 ppm of TBHQ is required
343
to be added to Karanja biodiesel to meet EN 14214 specifications for the induction period.
344
345 346
Figure 6. Effects of varying TBHQ concentration on the induction period of biodiesel
347 348
Further, the long term stability of neat Karanja biodiesel with the addition of TBHQ is examined
349
under three different storage conditions. The Rancimat induction period is taken as the duration
350
at which the rate of production of volatile acids reaches an inflection point. The biodiesel in the
351
Rancimat apparatus undergoes primary oxidation, producing hydroperoxides; and proceeds to
352
secondary oxidation, wherein decomposition of hydroperoxide produces volatile components
353
along with carbonyl compounds, alcohol and olefins [32]. Non-volatile secondary products
354
undergo further decomposition and act as an additional source of volatile compounds [33]. The
355
biodiesel stored under different conditions has inherent volatile acids due to the autoxidation
356
undergone before their introduction to the Rancimat apparatus. However, fresh fuel has
357
significantly low volatile acids. The presence of volatile acids from the initial time of
358
introduction to the apparatus accelerates the production of volatile acids. Hence, this
359
phenomenon is observed as the reduction in the Rancimat Induction period with an increase in
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360
storage time. The effects of storage conditions on the Rancimat induction period of Karanja
361
biodiesel with varying TBHQ concentrations are shown in Figure 7.
362
363 364
Figure 7. Effects of storage conditions on the induction period of Karanja biodiesel
365
stabilised with antioxidant.
366 367
For the Karanja biodiesel stored without TBHQ, the degradation rate in terms of induction period
368
over storage time is highest for the fuel sample stored at 43°C, followed by the fuel sample
369
stored under sunlight and air conditions. The sample stored in the dark and closed to air
370
conditions shows better oxidation stability as expected. With an increase in TBHQ concentration,
371
the induction period increases for all the test fuel samples at the three different storage 16 ACS Paragon Plus Environment
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conditions. It is interesting to note that as the TBHQ concentration increases, the effect of
373
sunlight supersedes the effect of temperature during the initial period of storage. The relative
374
importance of photo-oxidation during the initial period of storage is evident based on the above
375
observation. This could also be due to the fact that TBHQ is more effective to extend the
376
oxidative stability at higher temperature conditions, but is not effective to retard the catalytic
377
effect of UV radiation during the initial period of storage. It is observed that for the samples
378
stored under sunlight and air, as well as those stored under dark and closed to air, 250 ppm of
379
TBHQ could help to meet ASTM standard specifications even after 100 days of storage period.
380
However, for the samples stored at 43°C, the rate of degradation is higher and thus, a higher
381
concentration of TBHQ of up to 1000 ppm is required to meet ASTM standard specifications
382
after 100 days of storage period.
383 384
4. Conclusions
385
In the present work, the effects of storage conditions on the long term oxidative stability of neat
386
Karanja biodiesel and biodiesel-diesel blends are evaluated by monitoring peroxide value, acid
387
number, kinematic viscosity, density, and calorific value at regular intervals. Further, the methyl
388
ester constituents of neat Karanja biodiesel are also monitored during the storage period. The
389
results obtained show that the concentration of unsaturated methyl esters reduces upon storage,
390
indicating the conversion of esters into primary and secondary oxidation products. The peroxide
391
value initially increases and then decreases for all the tested fuels owing to its decomposition
392
into secondary oxidation products. The peroxide value is highest for the samples stored under
393
sunlight and air, however, after one year storage period the magnitude of peroxide value is
394
similar for the samples stored under sunlight and air and those stored under closed to sunlight
395
and air conditions. Thus, peroxide value alone is not a true representative of the extent of fuel
396
oxidation. The acid value of all the fuel samples increased beyond ASTM and EN standard
397
specifications within the first three months of storage period. It is concluded that the photo-
398
oxidation in the presence of sunlight increases the rate of primary oxidation reaction, while a
399
higher storage temperature increases the rate of the secondary oxidation reaction. The kinematic
400
viscosity of neat Karanja biodiesel stored under sunlight and air exceeds the ASTM standard
401
specification limits within the first three months of storage period. During the storage period, the
402
density of the fuel samples is increased while the calorific value is decreased. However, the 17 ACS Paragon Plus Environment
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403
changes are well within 2% and 3% respectively. For the neat Karanja biodiesel stored at 43°C, a
404
higher value of TBHQ up to 1000 ppm is required to meet ASTM standard specification for
405
induction period for 100 days, while 250 ppm is found to be sufficient for the samples stored at
406
ambient temperature.
407
Overall, there are no correlations that exist between the biodiesel fuel properties stored under
408
gradual and accelerated oxidation conditions. However, adopting ASTM D4625 standard test
409
method to evaluate the storage stability of biodiesel avoids any ambiguity owing to the variations
410
in the storage and the ambient conditions across the different parts of the world. The trends of the
411
fuel property variations with storage period under gradual and accelerated storage conditions
412
need to be examined with different biodiesels which would be taken up as the future work.
413 414
Acknowledgements
415
The authors wish to acknowledge the National Centre for Combustion Research and
416
Development, Indian Institute of Technology Madras, for providing access to fuel property
417
measurement test facilities.
418 419
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