View PDF VersionPrevious ArticleNext Article
 
DOI: 10.1039/C5RA23963J (Paper) RSC Adv., 2016, 6, 8198-8210

Evaluation of the characteristics of non-oxidative biodiesels: a FAME composition, thermogravimetric and IR analysis

I. Shancita*, H. H. Masjuki, M. A. Kalam*, S. S. Reham, A. M. Ruhul and I. M. Monirul
Centre for Energy Sciences, Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail: shancitaislam@gmail.com; kalam@um.edu.my; Fax: +603 79675317; Tel: +603 79674448

Received 13th November 2015 , Accepted 23rd December 2015

First published on 5th January 2016

Abstract

This experiment evaluates the effects of non-oxidative biodiesel (low oxygen content biodiesels) characteristics and their engine performances. Biodiesels produced from different feedstocks typically contains 10% to 15% oxygen by weight, which enhances the combustion quality and reduces the emissions of hydrocarbons (HCs) and carbon monoxide (CO). However, it produces a higher amount of nitrogen oxides (NOx) due to an increasing number of combustion products, resulting in a higher cylinder temperature. In addition, lean air–fuel mixtures can contribute to higher NOx emissions because biodiesel is more oxygenated than diesel. In this study, biodiesels produced from different feedstocks by a transesterification process were used to reduce the oxygen content by dipping an iron bar in the biodiesels, which absorbs oxygen and gets oxidized. Then, the oil characteristics, such as the percentage of saturated and unsaturated fatty acids, thermal degradation, stability and existing functional groups, were analyzed using fatty acid methyl ester (FAME) composition analysis, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy analysis of neat biodiesel and non-oxidative biodiesel. Herein, Pongamia and Moringa biodiesels, containing normal and reduced weight percentages of oxygen, were evaluated to improve the quality and stability of biodiesels used in the diesel engine, which will also reduce the NOx emissions. Non-oxidative biodiesels had some positive effect on their properties, which can further reduce the NOx emissions. Herein, non-oxidative Pongamia and Moringa had quite similar characteristics and the former was observed to perform better in the reduction of NOx and other emissions as well.

1. Introduction

Nowadays, biodiesel is one of the most suitable alternative energy providers for industrial, transportation and domestic consumption as they are a renewable, eco-friendly, readily available, biodegradable and non-toxic resource. Biodiesel can be used in its pure form or mixed with diesel in any volume percentage; moreover, it is feasible to be used in the diesel engine without any modification of the engine.1,2 Although biodiesel emits almost zero sulfur oxides, and has 14.2% less HC, 26.8% less PM and 9.8% less CO2 emissions than diesel fuel, its negative impact on NOx emissions raises several questions regarding environmental pollution.3,4 Biodiesel is typically produced by a transesterification reaction, in which triglycerides of vegetable oils or animal fats are reacted with short chain alcohol, such as methanol or ethanol, to produce the corresponding mono-alkyl esters and glycerin. Biodiesel is mainly a mixture of different fatty acid methyl esters (FAME) containing long chain, highly concentrated mono and poly unsaturated compounds.5,6

Typically, neat biodiesel consists of 11–15 wt% oxygen, which is the main cause for the improvement of combustion efficiency of biodiesel. On the other hand, it degrades the biodiesel stability and also makes it less efficient than fossil fuel and cannot be used in airplanes. Moreover, it increases the emissions of NOx although it decreases HC and CO emissions. This results in products of combustion, triggered by the higher oxygen content, which increases the temperature of the combustion chamber, thus improving combustion efficiency and consequently promoting NOx formation.4,7,8 In this study, the oxygen percentage in biodiesel, termed as a non-oxidative biodiesel, is decreased in an optimum amount with an aim to reduce NOx emissions. These reformed biodiesels were obtained from low temperature oxidation process in an iron–oxygen bath. The iron bar absorbed the oxygen molecule from the biodiesels. These biodiesels were characterized by FAME composition, TGA, DSC and FT-IR analysis.

Thermal degradation, FAME composition and IR characteristics of biodiesel are important quality assessment sources for biodiesel to be commercially used in a diesel engine. Thermal characteristics give the information about thermal stability and volatility of a biodiesel by measuring heat capacity, enthalpy, activation energy and melting point.2,7 Thermogravimetric analysis (TGA) is considered as a thermoanalytical technique, which measures the change in mass of a substance as a function of increasing temperature at a constant rate. The loss in mass of a substance with the increase in temperature may be caused by decomposition, oxidation or vaporization. A derivative of the weight loss curve is normally used to obtain the decomposition or vaporization temperature for pure as well as mixed compounds. On the other hand, differential scanning calorimetry (DSC) is used together with TGA and also is a thermoanalytical technique. It typically measures the energy required for the temperature increase of the substance studied. DSC curves provide important information about enthalpy of boiling, melting, oxidation and decomposition. An endothermic weight loss is accounted for endothermic decomposition or boiling and exothermic weight loss is attributed for exothermic oxidation or decomposition of the substance.9,10 Many studies used thermogravimetric analysis for the measurement of stability and production rate of biodiesels. Jain et al.11 reported the effect of antioxidants on the thermal degradation of Jatropha curcas biodiesel. By varying the concentration and type of antioxidants, various thermodynamic properties of biodiesel, such as activation energy (Ea), onset temperature (Ton) and offset temperature (Toff), were measured by TGA analysis. This study helped to improve the thermal stability of the biodiesel thermal by selecting the suitable additives in an engine fuel. On the other hand, Vega-Lizama et al.1 conducted their research on the measurement of thermal degradation degree of soy biodiesel using the residual mass obtained from the decomposition curve. The obtained results showed that TGA analysis is an efficient method to determine the oxidation degree of biodiesel without knowing the biodiesel oxidation process. Moreover, Niu et al.2 studied the thermal degradation of different biodiesels produced from different feedstocks through transesterification process by TGA analysis. Moreover, the combustion characteristics of palm and rapeseed biodiesels by thermogravimetric analysis was studied by Yuan et al. in the furnace of a thermogravimetry-differential scanning calorimetry (TGA-DSC) thermal analyzer instead of a diesel engine.7 This study was focused on comparing the suitability of non-oxidative biodiesels with neat biodiesels and diesel, which will affect the engine performance and exhaust emissions characteristics.

2. Materials and methods

2.1 Materials

Crude Moringa and Pongamia oils were collected from India. The characteristics of these crude oils are listed in Table 3. The diesel fuel (B0) was purchased from PETRONAS. Other materials, reagents, and chemicals, such as methanol, H2SO4, KOH, and Na2SO4, were obtained from LGC Scientific Sdn. Bhd. (Malaysia).

2.2 Production of Pongamia and Moringa biodiesels

The production of Pongamia and Moringa biodiesels from their corresponding crude oils was performed in the energy laboratory of the University of Malaya. A 1 L batch reactor equipped with a flux condenser, magnetic stirrer, thermometer, and sampling outlet were used to produce biodiesels. Crude Moringa oil was transesterified to produce biodiesel from it as the acid value was less than 4 mg KOH per g. On the other hand, for Pongamia biodiesel production, to reduce the high acid value of feedstocks, a two-step process, involving esterification and transesterification, was performed. First, Moringa oil was reacted with 25% (v/v oil) methanol (6[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio) and 1% (w/w oil) potassium hydroxide (KOH) and maintained at 60 °C and 600 rpm for 2 h. After the reaction, the mixture was deposited in a separation funnel for 12 h to separate glycerol from the produced biodiesel. The lower layer containing glycerol and impurities was drained. In the post-treatment process, the methyl ester formed from the previous process was washed with hot distilled water at 60 °C to remove glycerol and impurities. The upper layer was poured into a control rotary evaporator (IKA) to remove water and excess methanol from methyl ester, whereas the lower layer was drained. Methyl ester was dried using Na2SO4. The produced biodiesel was filtered using a qualitative filter paper to obtain the final product. For the Pongamia oil, in the esterification process, the molar ratio of methanol was maintained at 12[thin space (1/6-em)]:[thin space (1/6-em)]1 (50% v/v oil), and 1% (v/v oil) sulfuric acid (H2SO4) was added to the pre-heated oils at 60 °C and 600 rpm for 3 h in a glass reactor to refine the crude oils. After reaction completion, the product was transferred to a separation funnel, in which the esterified oil (lower layer) was separated from the upper layer. The upper layer included excess alcohol, sulfuric acid, and impurities. The lower layer was then loaded into a control rotary evaporator (IKA) and heated at 60 °C under vacuum conditions for 1 h to remove methanol and water from the esterified oil. Then, during transesterification, the esterified oils were reacted with 25% (v/v oil) methanol and 1% (m/m oil) potassium hydroxide (KOH) and maintained at 60 °C and 600 rpm for 2 h. After reaction completion, the produced biodiesels were deposited in a separation funnel for 12 h to separate glycerol from the biodiesels. The upper layer was washed three times with hot distilled water. The formed methyl ester was poured into a control rotary evaporator (IKA) to remove water and excess methanol and then dried using Na2SO4. The lower layer containing impurities and glycerol was drained. The produced methyl ester was filtered with a qualitative filter paper to obtain the final products as biodiesel.

2.3 Oxygen reducing process from biodiesels

The oxygen reducing process was conducted using a commercial grade iron bar of length 3 cm, width 1.5 cm and thickness 0.3 cm, which was dipped in the produced biodiesel. The iron bar was dipped in 10–20 mL biodiesels at 60 °C and 100 rpm after cleaning. All the biodiesels with iron bars were kept in airtight glass bottle at room temperature for 6 weeks. All the iron bars were weighted before and after the test period by a Scaltec SBA 31 (0.0001 g resolution). A change of weight of iron bars was observed for the 1.04% and 0.7% Pongamia and Moringa biodiesels, respectively. The iron bars were oxidized by reacting with oxygen from the biodiesels, which was the main cause for reduced oxygen weight percentage in non-oxidative biodiesels than neat biodiesels.

2.4 Iron oxidation and elemental composition

The oxidation of iron bars due to the dipping in biodiesels was observed using a SEM (scanning electron microscope) model Hitachi TM3030 at 150× magnification with an acceleration voltage of the microscope of 5 kV while operating. Elemental composition of normal and oxidized iron bars has been analyzed by a Bruker Quantax 70 EDX (energy dispersive system) at 150× magnification attached with a microscope. Each bar was scanned at three different spots and their average result is reported in this article.

2.5 Fatty acid methyl ester (FAME) composition

The fatty acid methyl ester composition of neat and non-oxidative Pongamia and Moringa biodiesels was measured using gas chromatography (GC) analysis in an Agilent 7890A model equipped with a flame ionization detector with a HP-INNOWax column (30 m × 0.25 mm, 0.25 μm of thickness). The flow rate of carrier gas helium was 3.5 mL min−1. The oven temperature was maintained at 50 °C for 5 min initially and then programmed to increase at a rate of 20 °C min−1 to 210 °C and maintained for 18 min. Then, another increase was made to 230 °C at a rate of 20 °C min−1, which was maintained for 13 min. Both the injector and detector temperature was maintained at 250 °C. A sample was injected at a spilt ratio of 50[thin space (1/6-em)]:[thin space (1/6-em)]1 of 0.3 μL volume.

2.6 Property analysis of neat and non-oxidative biodiesels

Biodiesels can be characterized by several important physicochemical properties that determine the quality and suitability of the biodiesels in order to be used in the unmodified diesel engine as a blend with diesel or in a pure form. In this study, some key characterization properties of neat and non-oxidative coconut, Pongamia and Moringa biodiesels are measured and compared, which include kinematic viscosity, density, calorific value, cetane number, oxidation stability and flash point. These physicochemical properties were measured according to the international standard specifications of ASTMD6751 or EN14214 and then compared between neat and non-oxidative biodiesels and also with conventional diesel.

2.7 Thermogravimetric (TGA) analysis

The TGA analysis test of neat, non-oxidative biodiesels and diesel was performed using a TGA Q500 V20.13 Build 39 thermal analyzer at a constant heating rate of 50 °C min−1 under a nitrogen atmosphere at a flow rate of 40 mL min−1. About 10–20 mg of samples was used in a 40 μL platinum pan at temperature interval from 4.0 °C to 950 °C.

2.8 Differential scanning calorimetry (DSC) analysis

The DSC analysis test for neat, non-oxidative biodiesels and diesel was performed using an instrument named DSC Q200 V24.11 Build 124 DSC analytical module in a standard cell under inert (nitrogen) atmosphere with gas flow rate of 50 mL min−1 and at a 10 °C min−1 heating rate to 130 °C to heat the samples. DSC cell was loaded with one sample and one reference pan. About 10 mg of sample was placed in a sealed aluminum pan and one identical empty pan was set as a reference to perform the test. For heating scans, samples were cooled rapidly and held isothermally for 2 min at −60 °C and then heated to 130 °C and then the samples were held isothermally for 2 min at 130 °C, then cooled to −80 °C for cooling scans.

2.9 Fourier transform infrared (FTIR) spectroscopy analysis

The FTIR spectroscopy analysis of neat, non-oxidative biodiesels and diesel was done in a Perkin Elmer biodiesel FAME analyzer connected with an MIR TGS detector. The range of spectrum was 4000–450 cm−1 and the resolution and scans were 4 cm−1 and 16 scans, respectively. This spectrum was processed by e-spectrum software.

3. Results and discussions

3.1 Oxidation of iron bars to reduce the oxygen content of biodiesels

Fig. 1(a)–(c) present the SEM images of normal, oxidized iron bars from Pongamia and Moringa biodiesel, respectively. From the SEM images, we can observe the change in the iron bars from normal to oxidized mode due to reacting with oxygen in biodiesels, which in turns confirmed that the iron bars were oxidized. Moreover, it can be comprehended from the image that the iron bar from Pongamia biodiesel was more oxidized than the iron bar from Moringa biodiesel. As a result, there was higher reduction of oxygen content from Pongamia biodiesel than of Moringa. As the biodiesels were kept in an airtight opaque bottle, they were examined before and after the oxygen reduction process.
image file: c5ra23963j-f1.tif
Fig. 1 SEM images of (a) normal iron, (b) oxidized iron (Pongamia) and (c) oxidized iron (Moringa).
3.1.1 Elemental composition analysis of normal and oxidized iron bars. The elemental composition of normal and oxidized iron bars was analyzed to observe the main components of the bars before and after the oxidation process. Results were obtained from three different spots and the atomic percentage of elements are presented in Fig. 2. The elements presented in the bars were oxygen, iron and carbon. From the Fig. 2(a)–(c), it can be seen that the atomic percentage of oxygen of normal and oxidized iron bars was different and the iron bar from Pongamia biodiesel had higher oxygen molecule content than the iron bar from Moringa biodiesel. However, the elemental composition also confirmed the oxidation of iron bars due to the reaction with oxygen from the biodiesels as the oxygen percentage increases from normal to oxidized mode and therefore, higher oxygen reduction from Pongamia biodiesel is obtained.
image file: c5ra23963j-f2.tif
Fig. 2 Elemental composition of (a) normal iron, (b) oxidized iron (Pongamia) and (c) oxidized iron (Moringa).

3.2 FAME composition of neat and non-oxidative biodiesels

In Table 1, the FAME composition of neat and non-oxidative Pongamia and Moringa biodiesels are presented. The saturated and unsaturated fatty acid composition of both the neat and non-oxidative biodiesels is compared to determine the most suitable one for better performance.
Table 1 Fatty acid composition of neat and non-oxidative Pongamia and Moringa biodiesels
FAME Structure Molecular weight Formula Neat biodiesels Non-oxidative biodiesels
Pongamia (wt%) Moringa (wt%) Pongamia (wt%) Moringa (wt%)
Methyl hexanoate 6[thin space (1/6-em)]:[thin space (1/6-em)]0 130.18 CH3(CH2)4COOCH3 — <0.1 <0.1 <0.1
Methyl octanoate 8[thin space (1/6-em)]:[thin space (1/6-em)]0 158.24 CH3(CH2)6COOCH3 <0.1 <0.1 <0.1 <0.1
Methyl decanoate 10[thin space (1/6-em)]:[thin space (1/6-em)]0 186.29 CH3(CH2)8COOCH3 <0.1 <0.1 <0.1 <0.1
Methyl laurate 12[thin space (1/6-em)]:[thin space (1/6-em)]0 214.34 CH3(CH2)10COOCH3 <0.1 <0.1 0.1 <0.1
Methyl myristate 14[thin space (1/6-em)]:[thin space (1/6-em)]0 242.39 CH3(CH2)12COOCH3 <0.1 <0.1 0.1 0.1
Methyl palmitate 16[thin space (1/6-em)]:[thin space (1/6-em)]0 270.45 CH3(CH2)14COOCH3 9.7 10.8 10.2 10.9
Methyl palmitoleate 16[thin space (1/6-em)]:[thin space (1/6-em)]1 268.43 CH3(CH2)5CH[double bond, length as m-dash]CH(CH2)7COOCH3 <0.1 0.1 <0.1 0.2
Methyl heptadecanoate 17[thin space (1/6-em)]:[thin space (1/6-em)]0 284.48 CH3(CH2)15COOCH3 — — 0.1 0.1
Methyl(Z)-heptadec-10-enoate 17[thin space (1/6-em)]:[thin space (1/6-em)]1 282.46 CH3(CH2)5CH[double bond, length as m-dash]CH(CH2)8COOCH3 — — <0.1 0.1
Methyl stearate 18[thin space (1/6-em)]:[thin space (1/6-em)]0 298.50 CH3(CH2)16COOCH3 6.8 4.4 6.9 4.0
Methyl oleate 18[thin space (1/6-em)]:[thin space (1/6-em)]1 296.49 CH3(CH2)7CH[double bond, length as m-dash]CH(CH2)7COOCH3 50.9 25.2 50.8 32.1
Methyl linoleate 18[thin space (1/6-em)]:[thin space (1/6-em)]2 294.47 CH3(CH2)4CH[double bond, length as m-dash]CHCH2CH[double bond, length as m-dash]CH(CH2)7COOCH3 18.2 52.1 17.0 44.1
Methyl linolenate 18[thin space (1/6-em)]:[thin space (1/6-em)]3 292.46 CH3CH2CH[double bond, length as m-dash]CHCH2CH[double bond, length as m-dash]CHCH2CH[double bond, length as m-dash]CH(CH2)7COOCH3 4.0 6.0 3.6 4.6
Methyl archidate 20[thin space (1/6-em)]:[thin space (1/6-em)]0 326.56 CH3(CH2)18COOCH3 1.6 0.4 1.7 0.4
Methyl eiosenoate 20[thin space (1/6-em)]:[thin space (1/6-em)]1 324.54 CH3(CH2)7CH[double bond, length as m-dash]CH(CH2)9COOCH3 1.2 0.3 1.3 0.2
Methyl behenate 22[thin space (1/6-em)]:[thin space (1/6-em)]0 354.61 CH3(CH2)20COOH 5.6 0.4 5.6 0.4
Methyl erucate 22[thin space (1/6-em)]:[thin space (1/6-em)]1 352.59 CH3(CH2)7CH[double bond, length as m-dash]CH(CH2)11COOH — 0.2 — —
Methyl lignocerate 24[thin space (1/6-em)]:[thin space (1/6-em)]0 382.66 CH3(CH2)22COOH 1.6 0.1 1.6 0.2
Other   0.4 — 1.0 2.6
  Saturation   25.3 16.1 26.3 16.1
Mono-unsaturated   52.1 25.8 52.1 32.6
Poly-unsaturated   22.2 58.1 20.6 48.7
Total   99.6 100 99 97.4

The monounsaturated fatty acid was same for both the neat and non-oxidative Pongamia biodiesels, whereas it increased for Moringa biodiesel from the neat to non-oxidative mode. These biodiesels consisted mainly of oleic acid C 18[thin space (1/6-em)]:[thin space (1/6-em)]1 (50.9%) and (50.8%) and eicosenoic acid C 20[thin space (1/6-em)]:[thin space (1/6-em)]1 (1.2%) and (1.3%) for neat and non-oxidative Pongamia biodiesel. Moreover, the Moringa biodiesel consisted of oleic acid C 18[thin space (1/6-em)]:[thin space (1/6-em)]1 and eicosenoic acid C 20[thin space (1/6-em)]:[thin space (1/6-em)]1 as (25.2% and 32.1%) and (0.3% and 0.2%) for neat and non-oxidative mode, respectively. Other unsaturation percentage was completed by polyunsaturated fatty acid, which was due to the presence of linoleic C 18[thin space (1/6-em)]:[thin space (1/6-em)]2 and linolenic acid C 18[thin space (1/6-em)]:[thin space (1/6-em)]3. For both Pongamia and Moringa biodiesel, the percentage of linoleic (18.2% to 17.0% and 52.1% to 44.1%) and linolenic (4.0% to 3.6% and 6.0% to 4.6%) acid was decreased from neat to non-oxidative mode, which resulted in the total decrease in poly-unsaturation of both biodiesels from neat to non-oxidative mode. High monounsaturated fatty acid resulted in high oxidation stability of Pongamia biodiesel, which was also confirmed from the test. Moreover, low amount of polyunsaturated fatty acid develops higher cetane number and also produces lower emissions of NOx.12–15 On the other hand, the saturated fatty acid composition was same for Moringa but increases for Pongamia biodiesel from neat to non-oxidative mode. The highest saturated fatty acid was palmitic C 16[thin space (1/6-em)]:[thin space (1/6-em)]0 (9.7% to 10.2% and 10.8% to 10.9%), followed by stearic C 18[thin space (1/6-em)]:[thin space (1/6-em)]0 (6.8% to 6.9% and 4.4% to 4.0%), behenic C 22[thin space (1/6-em)]:[thin space (1/6-em)]0 (5.6% to 5.6% and 0.4% to 0.4%) and arachidic C 20[thin space (1/6-em)]:[thin space (1/6-em)]0 (1.6% to 1.7% and 0.4% to 0.4%) for Pongamia and Moringa biodiesel from neat to non-oxidative mode. Biodiesels containing highly saturated fatty acids have low quality cold flow properties and also they have high melting points.12,14 In this study, both the biodiesels had low amount of saturated fatty acids and high amount of monounsaturated fatty acid with a higher value in Pongamia biodiesel for both neat and non-oxidative mode. These results are comparable with the previous literatures.16–22

3.3 Elemental composition of neat and non-oxidative biodiesels

Table 2 presents the elemental composition of neat and non-oxidative Pongamia and Moringa biodiesels. In general, biodiesel contains carbon, hydrogen and oxygen, whereas diesel contains only carbon and hydrogen. Although high oxygen content in biodiesel helps to reduce CO and HC emissions by the complete combustion process that influences the oxidation of unburned hydrocarbons, which in turns increase the NOx emissions by increasing the combustion chamber temperature. On the other hand, for reducing the NOx emissions produced from biodiesels, non-oxidative Pongamia and Moringa biodiesels containing optimum amount of oxygen helps to reduce the CO and HC emissions as well as decreases the NOx emissions unlike the typical biodiesels. From the table, it can be seen that neat Pongamia biodiesel contains higher oxygen and hydrogen content, whereas had lower carbon content than that of the Moringa biodiesel. Therefore, Pongamia biodiesel had more complete combustion characteristics, which also increase the NOx emissions. The reduction of oxygen weight percentage observed was 1.9% and 1.4% for Pongamia and Moringa biodiesels, respectively. Moreover, the carbon percentage increases for both biodiesels from neat to non-oxidative mode, which can be due to the iron bar that had lower carbon percentage from normal to oxidized mode. From the table, it can be concluded that non-oxidative Moringa had the lowest oxygen content hence id more suitable for NOx reduction, although the reduction percentage was higher for Pongamia biodiesel.
Table 2 Elemental composition of neat and non-oxidative Pongamia and Moringa biodiesels
Wt% Test method Neat biodiesels Non-oxidative biodiesels Diesel
Pongamia Moringa Pongamia Moringa
Carbon (C) ASTM D5291 74.0 75.8 76.1 77.1 85.2
Hydrogen (H) ASTM D5291 12.4 12.3 12.2 12.4 14.8
Oxygen (O) ASTM D5291 13.6 11.9 11.7 10.5 0
C/H — 5.97 6.16 6.24 6.22 5.76
Empirical formula — C6.17H12.3O0.85 C6.32H12.2O0.74 C6.34H12.1O0.73 C6.43H12.3O0.66 C7.1H14.68

3.4 Characterization of neat and non-oxidative biodiesels

The major physicochemical properties of neat and non-oxidative Pongamia and Moringa biodiesels are listed in Table 3 to compare with no. 2 diesel (B0). All the properties were compared with the ASTM D6751 standards. Cetane number was determined three times using fatty acid methyl ester composition with several empirical equations23–26 presented below:
 
CN = 46.3 + (5458/SV) − (0.225 × IV) (1)
 
SV = ∑(560 × Ai)/Mwi (2)
 
IV = ∑(254 × Ai × D)/Mwi (3)
where Ai is the weight percentage of each fatty acid component, D is the number of double bonds in each fatty acid, Mwi is the molecular mass of each fatty acid component, SV is the saponification value and IV is the iodine value.
Table 3 Physicochemical properties of crude oil, neat and non-oxidative Pongamia and Moringa biodiesels
Property Units Equipment Accuracy ASTM D6751 B100 Crude oil Neat biodiesels Non-oxidative biodiesels ASTM D975 diesel
Test method Limits b Pongamia Moringa Pongamia Moringa Pongamia Moringa Limits c Results
a Limit according to EN-14214.b Ref. 27 and 28.c Ref. 27.
Kinematic viscosity at 40 °C mm2 s−1 SVM 3000 0.1% ASTM D445 1.9–6 44.17 33.26 5.13 4.43 6.50 4.33 1.3–4.1 3.85
Density at 15 °C kg m−3 SVM 3000 ±0.1 kg m−3 ASTM D1298 860–894 941.2 921.3 894.7 883.2 909.5 885.2 850 839.1
Oxidation stability h 873 Rancimat ±0.01 h EN ISO 14112 3 h mina 13.47 6.27 6.70 4.99 11.98 0.88 — 19.89
Calorific value MJ kg−1 C2000 basic calorimeter ±0.001 MJ kg−1 ASTM D240 — 38.85 39.60 38.19 39.97 39.45 39.90 42–46 45.67
Cetane number —     ASTM D613 47 min — — 56.13 45.97 56.60 49.28 40–55 48

3.4.1 Kinematic viscosity. In Table 3, there is fluctuation in the change of the property for neat and non-oxidative biodiesels. The kinematic viscosity decreases for Moringa biodiesel from neat to non-oxidative, but increases for Pongamia biodiesel. However, both the biodiesels had higher viscosity than diesel for both neat and non-oxidative mode. Moreover, in case of crude oil, Pongamia had higher viscosity than Moringa. As viscosity influences the fuel injection system specially the spray atomization, penetration of the injected jet, air–fuel mixture combustion quality at low temperatures, it is very important for combustion quality. Biodiesels have higher viscosity than diesel fuel because of the presence of electronegative oxygen, which makes the biodiesels more polar than diesel.28–30 Hence, the reduction of oxygen from biodiesels can improve the viscosity.
3.4.2 Density. The changes in density are almost similar to the changes in kinematic viscosity except in the case of Moringa biodiesel, as shown in Table 3. The density increases for Pongamia and Moringa biodiesels from neat to non-oxidative mode. However, all the neat and non-oxidative biodiesels had a higher density than diesel. Among them, Moringa biodiesel had lower density range for both neat and non-oxidative mode. Crude Pongamia oil had higher density than Moringa similar as the neat and non-oxidative biodiesels. Density is an important parameter for fuel property as it affects the fuel atomization and combustion as well as other engine properties are also related to fuel density such as heating value, cetane number and viscosity. The density of biodiesel is normally higher than diesel as the density of biodiesel depends on its fatty acid composition, molar mass, water content and purity.28,29
3.4.3 Oxidation stability. The oxidation stability increases for Pongamia biodiesel from neat to non-oxidative mode, whereas decreases for Moringa biodiesel from neat to non-oxidative mode, which was under the minimum requirement. This decrease indicates the lower oxidation stability of Moringa biodiesel from neat to non-oxidative mode. However, diesel had the highest oxidation stability among all crude oils, neat and non-oxidative biodiesels. Moreover, non-oxidative Pongamia biodiesel had higher oxidation stability after diesel among all other neat and non-oxidative biodiesels due to high monounsaturated fatty acid. Oxidation stability affects the biodiesel quality by oxidizing it during storage for distribution or in the fuel system. Biodiesels are fatty acid methyl ester, which oxidize automatically to form aldehydes, ketones and resins and makes the fuel useless to run the engine. The oxidization rate of biodiesel depends on temperature, fatty acid composition, reaction catalyst, radiation intensity and light and it can be delayed by adding different antioxidants.28,29
3.4.4 Calorific value. It can be inferred from the Table 3 that for Moringa biodiesel, the calorific value decreases from neat to non-oxidative mode but increases for Pongamia biodiesel. However, among all the biodiesels, Moringa had the highest calorific value for both neat and non-oxidative mode. Moreover, all the biodiesels and crude oils had lower calorific value than diesel. Biodiesels have lower heating value than diesel fuel because of the deviation in the hydrogen and carbon content and presence of high oxygen molecule that decreases the heating value by about 10–13% in biodiesel than diesel. Heating value of biodiesel increases with its number of carbon atoms and decreases with its number of double bonds.28,29 Therefore, it can be shown that in Pongamia biodiesel, the calorific value increases with increasing carbon molecules and decreasing oxygen molecules.
3.4.5 Cetane number. The cetane number for all biodiesels was increased from neat to non-oxidative mode, as seen in Table 3. However, Pongamia biodiesel had higher cetane number than Moringa for both neat and non-oxidative mode. Moreover, all the biodiesels had higher cetane number than diesel except neat Moringa and also non-oxidative Moringa had almost similar value as diesel. Cetane number is defined as the dimensionless measure of the ignition quality of diesel fuel during combustion of a compression ignition engine. It describes the fuel quality based on the ease of self-ignition of the particular fuel. Ignition delay (ID) period of diesel fuel is also indicated by it during the ignition period of injected diesel fuel in the combustion chamber. The higher ignition time lag indicates the lower cetane number and vice versa. The range of cetane number in ASTM D6751 standards is based on the two experimental measured fuels named hexadecane (C16H34) with cetane number 100 and 2,2,4,4,6,8,8-heptamethylnonane with cetane number 15. The first one is an easily ignited fuel and the other one is highly resistive to ignite. Typically, biodiesels have higher cetane number than diesel fuel. 41–56 is the usual range of cetane number for no. 2 diesel and it should not be higher than 65.28,29 Therefore, in that case, Moringa biodiesel had the more suitable cetane number but as indicated earlier that lower polyunsaturated fatty acid indicates high cetane number and thus a low level of NOx emissions, so non-oxidative Pongamia can perform better than Moringa biodiesel. Moreover, all the neat and non-oxidative Pongamia and Moringa biodiesels satisfied the requirements, which ensures the suitability of use of the biodiesels in the diesel engine without any modification by blending with diesel or in a pure form.

By analyzing all the properties, it can be observed that all the neat and non-oxidative Pongamia and Moringa biodiesels satisfied the requirements according to the ASTM D6751 standards, which ensure the suitability of use of those biodiesels in a diesel engine without any modification by blending with diesel or in a pure form.

3.5 Thermogravimetric analysis of neat and non-oxidative biodiesels

Thermogravimetric analysis of neat, non-oxidative biodiesels and diesel were conducted to understand the decomposition, thermal degradation behavior and volatility of those biodiesels.

In Fig. 3, the TGA curves of neat, non-oxidative Pongamia, Moringa biodiesels and diesel were compared. In this figure, all the curves had the similar trend. First, the TGA curves achieve ascending trend due to buoyancy and molecular adsorption effect but not in a distinct way and all of the neat and non-oxidative biodiesels and diesel were decomposed in only one step, which was clearly observed.2 The decomposition temperature decreases from neat to non-oxidative biodiesels. Neat Pongamia and Moringa biodiesels had almost a similar thermal stability range. Pongamia and Moringa biodiesels were thermally stable up to 170.16 °C and 172.20 °C, respectively. However, non-oxidative Pongamia and Moringa biodiesels were thermally stable up to 158.28 °C and 171.78 °C, respectively. On the other hand, diesel was thermally stable up to 129.04 °C. Hence, Moringa biodiesel is thermally more stable than Pongamia biodiesel and diesel at both neat and non-oxidative mode. Then, with the increasing temperature, the TGA curves were descending due to volatilization of weak chemical bonds and small molecules. Around 99% weight loss took place at the temperature ranges from 95 °C to 279 °C, 107 °C to 317 °C and 61 °C to 302 °C for neat Pongamia, Moringa biodiesels and diesel, respectively. On the other hand, for non-oxidative biodiesels, the 99% weight loss range increases compared to the neat biodiesels. Then, the final thermal degradation was observed with the 0.09%, 0.006% and 0.02024% carbon residue at the end of decomposition for neat Pongamia, Moringa biodiesels and diesel, respectively. However, in case of non-oxidative biodiesels, there were negative percentage of carbon residue observed at the end of the decomposition.2,31,32


image file: c5ra23963j-f3.tif
Fig. 3 TGA curves of neat, non-oxidative Pongamia, Moringa biodiesels and diesel at 50 °C min−1 heating rate.

In Fig. 4, the derivative of thermogravimetric curves (DTG) are also presented. This curves showed the temperature at its peak point where the maximum rate of change of thermal decomposition (dm/dTmax) took place for all of the neat and non-oxidative biodiesels and diesel. From the figure, it can be seen that the peaks of the DTG curves occurred at 177 °C, 179 °C and 203 °C for neat Pongamia, Moringa biodiesels and diesel, respectively. For non-oxidative Moringa biodiesel, the peak was increased but it decreased for Pongamia biodiesel. Therefore, Moringa biodiesel had the highest temperature for the maximum rate of change of thermal decomposition (dm/dTmax) for both neat and non-oxidative mode, but lower than that of diesel.10,33


image file: c5ra23963j-f4.tif
Fig. 4 DTG curves of neat, non-oxidative Pongamia, Moringa biodiesels and diesel at 50 °C min−1 heating rate.

From the two figures, it can be concluded that almost all the biodiesels and diesel had the similar TGA and DTG curves and thermal characteristics with slight variations. In Table 4, all the results are summarized.

Table 4 Thermal characteristics (TGA) of neat and non-oxidative Pongamia, Moringa biodiesels and diesel
Parameters Neat biodiesels Non-oxidative biodiesels Diesel
Pongamia Moringa Pongamia Moringa
Product weight (mg) 18.07 19.72 14.04 20.56 19.89
Decomposition temperature (°C) 170.16 172.20 158.28 171.78 129.04
Volatility (%) Not found Not found Not found Not found Not found
1st weight loss (%mg per mg) 99.3 99.4 101.8 99.67 99.5
1st weight loss temperature range (°C) 95–279 107–317 57–444 95–353 61–302
Lost weight at 1st weight loss (mg) 17.95 19.60 14.29 20.49 19.79
Residue (%mg per mg) 0.08 0.006 −2.48 −0.3379 0.02024
Weight at residue (mg) 0.0156 0.0013 −0.3481 −0.06946 0.004
Peak temperature (°C) 177 179 169 180 203

3.6 DSC analysis of neat and non-oxidative biodiesels

Differential scanning calorimetry (DSC) is one of the measurement methods for determining the thermal stability of biodiesels. It determines the differential heat flow of biodiesel samples either endothermic or exothermic in respect with a reference as a function of temperature. In other words, DSC measures the energy absorbed or released by a sample as it is heated or cooled.34 In the DSC curves, heat flow (W g−1) was plotted against temperature (°C). In the curves, there were variations in transitions and in the inert nitrogen atmosphere, the transitions were endothermic, which means that the molecules of the samples were absorbing heat from surrounding. The melting point and the corresponding enthalpy for those temperatures can be determined from the heating curves and from the cooling curves crystallization point and corresponding enthalpy can be obtained. There was no glass transition found for all of the biodiesel samples.35–39

In Fig. 5(a) and (b), DSC curves of neat Pongamia biodiesel for heating and cooling are shown, respectively. It represents all the parameters that are important to analysis the curves. The curves showed that there was no glass transition phase and only melting and freezing phases were visible. Therefore, it was clear from the curves that in heating curve, only endothermic and in cooling curve only exothermic reactions occurred. Moreover, the enthalpy change for melting and freezing was also presented in the curves, including the reaction temperature range and heat flow range.


image file: c5ra23963j-f5.tif
Fig. 5 DSC curve analysis of Pongamia biodiesel at 10 °C min−1 heating rate for (a) heating scans (b) cooling scans.

In Fig. 6 and 7, the DSC curves for heating and cooling are presented for neat, non-oxidative Pongamia, Moringa biodiesels and diesel to compare their changes. For diesel and both neat and non-oxidative biodiesels, only cracking reactions occurred and there is no distillation region for both diesel and neat, non-oxidative biodiesels as per the figure. From the heating curves, the melting point of both neat and non-oxidative Pongamia was higher than both neat and non-oxidative Moringa biodiesels but lower than that of diesel. However, the melting point decreases for Moringa biodiesel from neat to non-oxidative mode but increases for Pongamia biodiesel. On the other hand, the enthalpy of melting decreases for all of the biodiesels from neat to non-oxidative mode. It was the highest for both neat and non-oxidative Moringa biodiesels and the lowest for diesel.


image file: c5ra23963j-f6.tif
Fig. 6 DSC heating curves of neat, non-oxidative Pongamia, Moringa biodiesels and diesel at 10 °C min−1.

image file: c5ra23963j-f7.tif
Fig. 7 DSC cooling curves of neat, non-oxidative Pongamia, Moringa biodiesels and diesel at 10 °C min−1.

On the other hand, from the cooling curves, it is clearly comprehended that diesel had the highest and Moringa had the lowest crystallization temperature for both neat and non-oxidative mode, among all the biodiesels. However, the crystallization temperature decreases from neat to non-oxidative mode for both Pongamia and Moringa biodiesels. Moreover, for the enthalpy of crystallization, it was highest for Moringa biodiesel for both neat and non-oxidative mode and lowest for diesel among all other biodiesels. In addition, the enthalpy of crystallization decreases for all of the biodiesels from neat to non-oxidative mode.

In Table 5, all the results for heating and cooling curves of DSC for diesel and neat, non-oxidative Pongamia and Moringa biodiesels are summarized to clearly understand the comparison among them.

Table 5 Thermal characteristics (DSC) of neat and non-oxidative Pongamia, Moringa biodiesels and diesel
Parameter Neat biodiesels Non-oxidative biodiesels Diesel
Pongamia Moringa Pongamia Moringa
Product mass (mg) 17.7 14 11.2 5.8 15.8
Melting temperature (Tm) (°C) −32.95 −48.49 −32.31 −49.52 −8.36
Onset temperature for melting (°C) −39.02 −52.59 −36.84 −53.62 −39.03
Enthalpy of melting (ΔH) (J g−1) 34.40 43.58 29.16 41.53 22.82
Temperature range of melting (°C) −47.27 to −22.91 −70.81 to −33.34 −54.78 to −20.24 −70.90 to −25.62 −57.89 to −19.60
Heat flow range for melting (W g−1) −0.31 to −0.25 −0.09 to −0.16 −0.14 to −0.19 −0.04 to −0.11 −0.26–0.28
Crystallization temperature (Tc) (°C) −47.14 −58.65 −53.97 −60.32 −16.49
Onset temperature for crystallization (°C) −47.00 −58.24 −51.43 −59.02 1.67
Enthalpy of crystallization (ΔH) (J g−1) 40.35 40.81 28.68 36.73 21.04
Temperature range of crystallization (°C) −65.30 to −36.48 −74.33 to −46.69 −70.46 to −42.03 −73.02 to −48.12 −58.33–12.70
Heat flow range for crystallization (W g−1) 0.2674–0.2669 0.09–0.18 0.13–0.22 0.11–0.16 0.28–0.29

3.7 Fourier transform infrared spectroscopy (FT-IR) analysis of neat and non-oxidative biodiesels

In this study, the FT-IR characteristics of all neat and non-oxidative biodiesels are presented to compare with diesel fuel and also to confirm that the functional groups presented in non-oxidative biodiesels were similar to neat biodiesels and diesel. This proofs the suitability of those biodiesels in the diesel engine without any modification.

In Fig. 8 and 9, the IR spectrum of neat and non-oxidative Pongamia and Moringa biodiesels are shown in comparison to diesel fuel. Table 6 summarized all the frequency range, functional groups, absorbance peaks and percent transmittance (%T) for all neat and non-oxidative biodiesels along with diesel fuel. There is slight difference in the functional group present between neat and non-oxidative biodiesels. All the neat and non-oxidative biodiesels contained esters and the absence of any broad peaks of O–H stretching vibration of carboxylic acids in the region of 2500–3300 cm−1 indicates the absence of moisture in the biodiesels and diesel. Moreover, the %T decreases from neat to non-oxidative mode for all biodiesels.19,31,40


image file: c5ra23963j-f8.tif
Fig. 8 FT-IR spectrum of neat Pongamia and Moringa biodiesels compared with diesel.

image file: c5ra23963j-f9.tif
Fig. 9 FT-IR spectrum of non-oxidative Pongamia and Moringa biodiesels compared with diesel.
Table 6 IR characteristics region for neat, non-oxidative biodiesels with diesel fuel
Frequency range (cm−1) Bond type Functional group Neat biodiesels Non-oxidative biodiesels Diesel
Pongamia Moringa Pongamia Moringa
2850–3000 C–H stretching Alkanes Present (2922.97 cm−1, 65.92 %T); (2853.55 cm−1, 74.08 %T) Present (2923.49 cm−1, 67.48 %T); (2853.98 cm−1, 75.26 %T) Present (2923.07 cm−1, 61.90 %T); (2853.62 cm−1, 69.8 %T) Present (2923.56 cm−1, 64.06 %T); (2854.07 cm−1, 71.82 %T) Present (2954.89 cm−1, 81.75 %T); (2922.9 cm−1, 64.65 %T); (2853.41 cm−1, 75.18 %T)
1735–1750 C[double bond, length as m-dash]O stretching Esters Present (1741.66 cm−1, 66.58 %T) Present (1741.53 cm−1, 63.84 %T) Present (1740.96 cm−1, 64.33 %T) Present (1741.66 cm−1, 61.03 %T) Absent
1350–1480 –C–H bending Alkanes Present (1462.97 cm−1, 85.31 %T); (1435.92 cm−1, 83.55 %T) Present (1461.95 cm−1, 85.47 %T); (1435.71 cm−1, 83.09 %T) Present (1436.12 cm−1, 77.95 %T) Present (1435.64 cm−1, 79.04 %T) Present (1462.24 cm−1, 92.94 %T); (1377.22 cm−1, 88.7 %T)
1000–1300 C–O stretching Esters Present (1195.72 cm−1, 79.93 %T); (1168.30 cm−1, 75.44 %T) Present (1244.40 cm−1, 84.92 %T); (1195.63 cm−1, 78.25 %T); (1169.46 cm−1, 74.28 %T); (1120.07 cm−1, 90.71 %T) Present (1169.09 cm−1, 70.48 %T) Present (1169.62 cm−1, 71.69 %T) Absent
700–725 C–H rock Alkanes Absent Present (722.52 cm−1, 84.14 %T) Present (722.82 cm−1, 82.33 %T) Present (722.64 cm−1, 81.10 %T) Present (722.04 cm−1, 95.87 %T)
2500–3300 O–H stretching Carboxylic acids Absent Absent Absent Absent Absent

All the non-oxidative biodiesels satisfied the minimum requirements and had some improvements in their properties with very few exceptions to be used as an alternative fuel mixed with diesel instead of neat biodiesels. This new form of biodiesels will affect the performance of engine by reducing the NOx emissions as well as maintain the other performance and exhaust emissions similar to neat biodiesels. However, the non-oxidative biodiesels is an alternative fuel source of the future for commercial use in large scale.

4. Conclusions

In this study, non-oxidative biodiesels were characterized by fuel physicochemical property, FAME composition, thermogravimetric and IR analysis and were compared with neat biodiesels and diesel fuel to determine the suitability of the biodiesels to be used in the diesel engine according to stability and quality. By considering and optimizing all the properties, it can be said that non-oxidative Pongamia has high suitability to be used in a diesel engine, although it has higher viscosity and density than its neat biodiesel; moreover, it had an improved oxidation stability and calorific value. Further research can be carried out to further improve those properties. The findings of this study are summarized here:

• The oxidation of iron bars was higher for Pongamia biodiesel than for Moringa biodiesel.

• The oxygen reduction percentage was higher for Pongamia than for Moringa biodiesel.

• The kinematic viscosity of Moringa biodiesel was decreased due to reduction of oxygen content but it increases for Pongamia biodiesel. Density increases for both Pongamia and Moringa biodiesels for the reduction of oxygen content.

• Non-oxidative Pongamia had the highest oxidation stability after diesel among other biodiesels.

• Pongamia biodiesel had increased calorific value due to the reduction of oxygen.

• Non-oxidative Pongamia had the highest cetane number among other neat and non-oxidative biodiesels and diesel. Lower polyunsaturated fatty acid indicates high cetane number and thus low level of NOx emissions. Hence, non-oxidative Pongamia can reduce more NOx compared to Moringa biodiesel.

• TGA and DSC analysis confirmed the thermal, oxidation and storage stability for both the non-oxidative biodiesels compared with neat biodiesels and diesel. For all biodiesels and diesel, no volatile characteristics were found.

• FT-IR analysis confirmed the suitability of all non-oxidative biodiesels to be used with diesel fuel replacing the neat biodiesels by defining the esters content and transmittance rate of those biodiesels.

Acknowledgements

The authors would like to acknowledge the University of Malaya for financial support through High Impact Research grant UM.C/HIR/MOHE/ENG/07. Thanks to Mr Solaiman and Mr Ismail for their help in oxygen reduction process and thermogravimetric analysis test, respectively.

References

  1. T. Vega-Lizama, L. Díaz-Ballote, E. Hernández-Mézquita, F. May-Crespo, P. Castro-Borges, A. Castillo-Atoche, G. González-García and L. Maldonado, Fuel, 2015, 156, 158–162 CrossRef CAS.
  2. H. Li, S.-l. Niu, C.-m. Lu and S.-q. Cheng, Energy Convers. Manage., 2015, 98, 81–88 CrossRef CAS.
  3. M. M. Conceiçao, V. J. Fernandes, A. S. Araújo, M. F. Farias, I. M. Santos and A. G. Souza, Energy Fuels, 2007, 21, 1522–1527 CrossRef.
  4. J. Gardya, A. Hassanpoura, X. Laia, A. Cunliffeb and M. Rehana, in International Conference on Environment and Renewable Energy (ICERE), At Cité Internationale Universitaire de Paris, 17 Boulevard Jourdan, Paris, France, 2014 Search PubMed.
  5. S. Jain and M. Sharma, Renewable Sustainable Energy Rev., 2011, 15, 438–448 CrossRef CAS.
  6. G. Çaylı and S. Küsefoğlu, Fuel Process. Technol., 2008, 89, 118–122 CrossRef.
  7. Y. Lai, B. Wang, X. Chen, Y. Yuan, L. Zhong, X. Qiao, Y. Zhang, M. Yuan, J. Shu and P. Wang, Biotechnology, 2015, 14, 9–15 CrossRef.
  8. S. M. A. Rahman, H. H. Masjuki, M. A. Kalam, M. J. Abedin, A. Sanjid and H. Sajjad, Energy Convers. Manage., 2013, 76, 362–367 CrossRef CAS.
  9. P. Varanasi, B. Knierim, E. Bosneaga, L. Sun, M. Auer, P. Sarkar and S. Singh, Quantifying Bio-Engineering: The Importance of Biophysics in Biofuel Research, INTECH Open Access Publisher, 2011 Search PubMed.
  10. P. Chand, V. Reddy, J. G. Verkade, T. Wang and D. A. Grewell, in Agricultural and Biosystems Engineering Conference Papers, Posters and Presentations, 2008 Search PubMed.
  11. S. Jain and M. Sharma, Fuel, 2012, 93, 252–257 CrossRef.
  12. M. Salaheldeen, M. Aroua, A. Mariod, S. F. Cheng and M. A. Abdelrahman, Ind. Crops Prod., 2014, 61, 49–61 CrossRef CAS.
  13. B. R. Moser, Fuel, 2012, 92, 231–238 CrossRef CAS.
  14. A. Karmakar, S. Karmakar and S. Mukherjee, Bioresour. Technol., 2010, 101, 7201–7210 CrossRef CAS PubMed.
  15. S. L. Nettles-Anderson and D. B. Olsen, Survey of straight vegetable oil composition impact on combustion properties 0148–7191, SAE Technical Paper, 2009 Search PubMed.
  16. A. E. Atabani, M. Mofijur, H. H. Masjuki, I. A. Badruddin, M. A. Kalam and W. T. Chong, Ind. Crops Prod., 2014, 60, 130–137 CrossRef CAS.
  17. U. Rashid, F. Anwar, B. R. Moser and G. Knothe, Bioresour. Technol., 2008, 99, 8175–8179 CrossRef CAS PubMed.
  18. M. Mofijur, H. H. Masjuki, M. A. Kalam, A. E. Atabani, M. I. Arbab, S. F. Cheng and S. W. Gouk, Energy Convers. Manage., 2014, 82, 169–176 CrossRef CAS.
  19. M. N. Nabi, S. M. N. Hoque and M. S. Akhter, Fuel Process. Technol., 2009, 90, 1080–1086 CrossRef CAS.
  20. M. Naik, L. C. Meher, S. N. Naik and L. M. Das, Biomass Bioenergy, 2008, 32, 354–357 CrossRef CAS.
  21. P. T. Scott, L. Pregelj, N. Chen, J. S. Hadler, M. A. Djordjevic and P. M. Gresshoff, BioEnergy Res., 2008, 1, 2–11 CrossRef.
  22. N. Mukta and Y. Sreevalli, Ind. Crops Prod., 2010, 31, 1–12 CrossRef.
  23. M. M. Rahman, M. H. Hassan, M. A. Kalam, A. E. Atabani, L. A. Memon and S. A. Rahman, J. Cleaner Prod., 2014, 65, 304–310 CrossRef CAS.
  24. S. M. A. Rahman, H. H. Masjuki, M. A. Kalam, M. J. Abedin, A. Sanjid and M. M. Rahman, Renewable Energy, 2014, 68, 644–650 CrossRef CAS.
  25. K. Krisnangkura, J. Am. Oil Chem. Soc., 1986, 63, 552–553 CrossRef CAS.
  26. P. Devan and N. Mahalakshmi, Fuel Process. Technol., 2009, 90, 513–519 CrossRef CAS.
  27. A. E. Atabani, A. S. Silitonga, I. A. Badruddin, T. M. I. Mahlia, H. H. Masjuki and S. Mekhilef, Renewable Sustainable Energy Rev., 2012, 16, 2070–2093 CrossRef.
  28. A. E. Atabani, A. S. Silitonga, H. C. Ong, T. M. I. Mahlia, H. H. Masjuki, I. A. Badruddin and H. Fayaz, Renewable Sustainable Energy Rev., 2013, 18, 211–245 CrossRef CAS.
  29. I. Barabás and I.-A. Todoruţ, in Biodiesel-Quality, Emissions and By-Products, 2011, pp. 3–28 Search PubMed.
  30. T. M. Y. khan, A. E. Atabani, I. A. Badruddin, A. Badarudin, M. S. Khayoon and S. Triwahyono, Renewable Sustainable Energy Rev., 2014, 37, 840–851 CrossRef CAS.
  31. M. Farooq, A. Ramli and D. Subbarao, J. Cleaner Prod., 2013, 59, 131–140 CrossRef CAS.
  32. C. S. Madankar, A. K. Dalai and S. N. Naik, Ind. Crops Prod., 2013, 44, 139–144 CrossRef CAS.
  33. H. Kandala, Masters thesis, 2009, 93.
  34. J. Leifeld, Org. Geochem., 2007, 38, 112–127 CrossRef CAS.
  35. R. O. Dunn, J. Am. Oil Chem. Soc., 1999, 76, 109–115 CrossRef CAS.
  36. G. Knothe and R. O. Dunn, J. Am. Oil Chem. Soc., 2009, 86, 843–856 CrossRef CAS.
  37. E. Topa, MS thesis, Middle East Technical University, Turkey, 2010.
  38. S. Jain and M. Sharma, Renewable Sustainable Energy Rev., 2010, 14, 1937–1947 CrossRef CAS.
  39. M. M. Conceição, R. A. Candeia, F. C. Silva, A. F. Bezerra, V. J. Fernandes and A. G. Souza, Renewable Sustainable Energy Rev., 2007, 11, 964–975 CrossRef.
  40. M. Ndana, J. Grace, F. Baba and U. Mohammed, International Journal of Science, Environment and Technology, 2013, 2, 1116–1121 Search PubMed.
This journal is © The Royal Society of Chemistry 2016
Click here to see how this site uses Cookies. View our privacy policy here.