Investigation and enhancement on fuel stability characteristics of biodiesel dosed with various synthetic antioxidants

Abstract

Nearly 350 species of biodiesel feedstocks have been identified by many researchers for the past few decades. Unlike petroleum diesel, the biodiesel degrades rapidly and deterioration of its quality occurred while contacting with atmospheric oxygen. This main drawback of instability of fuel properties limited the commercial use of biodiesel in the global fuel market. To inhibit this oxidative degradation of biodiesel, the antioxidants are used. Comparing to other antioxidants, the synthetic antioxidants (phenolic) are more efficient. This study investigates the effects of commercially available and cheap synthetic antioxidants (TBHQ – tert-butylhydroxyquinone, PY – pyrogallol, PG – propyl gallate, BHT – butylated hydroxytoluene, and BHA – butylated hydroxyanisole) at 1000 ppm concentration on the fuel stability of Calophyllum inophyllum biodiesel. The discrepancy in antioxidant activity has been characterized using Fourier transform infrared spectroscopy by analyzing the O–H and C–H molecular chains prevalence in the infrared spectrum region of 3000–3700 cm⁻¹ and 2800–3000 cm⁻¹. TBHQ at 1000 ppm dosed with C. inophyllum biodiesel improves the oxidation stability by 42.56%, storage stability by 36.57%, and thermal stability by 41.02% when compared to those of pure biodiesel (B100) without any antioxidant. The rank of antioxidants effectiveness with pure biodiesel is obtained as TBHQ > PG > PY > BHT > BHA.

Keywords

Antioxidant biodiesel Fourier transform infrared oxidation stability storage stability thermal stability

Introduction

Biodiesel instability refers to the deviation in its fuel composition and properties due to oxidation reaction. In such case, the properties of biodiesel could change from the prescribed quality standard, which will greatly affect its applicability and adaptability in diesel engine. Engine performance is greatly altered due to the deterioration of fuel. Biodiesel is much more instable when compared to diesel.¹

Biodiesel is composed of various saturated and unsaturated fatty acid esters. Also, the oxygen prevalence makes it more aggressive to react with the surface of metal. The degradation of biodiesel could lead to the formation of various undesirable compounds, which not only affect the biodiesel properties, but also cause engine performance problems including filter plugging, injector coking, piston ring sticking, engine lubricant degradation, and dilution.² Degradation of biodiesel leads to the formation of aldehyde, ketone, acids, and insoluble sediments resulted in the variation of fuel properties like acid number, density, viscosity, and flash point.³ These property discrepancies lead to the engine performance and durability degradation. For instance, amplified viscosity can influence fuel atomization problems,⁴ thereby increasing plugging of fuel filter, choking of fuel injector, sticking of moving parts, etc. Due to this instability uniqueness of biodiesel, it is could not be effectively stored for prolonged duration without any deviation in its properties.

‘Fuel stability’ is the resistance offered by a fuel to the degradation processes, which could change fuel properties by forming undesirable products. Biodiesel properties can degrade by one or more of the mechanisms like auto-oxidation, hydrolysis, microbial contamination, and thermal-oxidative decomposition.⁵ Fuel stability (FS) is generally conversed as three types depending on the above mechanisms of degradation as follows:

Oxidation stability (OS) – It describes the relative susceptibility of the fuel to degradation by oxidation and also refers to the tendency of the fuel to react with oxygen at temperatures nearer atmospheric condition. These reactions are much slower than the reaction occurring at higher temperatures. The degree of oxidative degradation experienced by biodiesel proceeding to combustion in a diesel engine will be affected by many factors including biodiesel production method, the nature of fuel additives and impurities, lipid feedstock, storage and handling conditions as well as by conditions within the fuel delivery system and fuel tank.⁵

Storage stability (SS) – It addresses the general stability of the fuel while it is in long-term storage. SS involves the issues of water contamination and microbial growth. Water can promote microbial growth, lead to the formation of emulsions as well as hydrolysis or hydrolytic oxidation. The term ‘OS’ is more common and is distinguished from the term ‘SS’ since oxidation of fuel may occur not only during storage condition but also during production and end-use conditions.⁵

Thermal stability (TS) – It refers to resistance to degradation contributed by greatly elevated temperatures, much higher than the atmospheric conditions and is applicable to biodiesel usage since high fuel temperatures may occur at conditions encountered in fuel injection systems, as fuel is re-circulated through the injection system and stored back in the fuel tank.⁵

The fuel stability of biodiesel could be specified through some significant fuel properties as (i) induction period (IP) – It indicates the oxidation reaction initiation by determining the conductivity of fuel. Higher the IP, higher the OS.⁶ (ii) Kinematic viscosity (KV) – Oxidation products like peroxides and hydro peroxides are higher in molecular weight, which leads to increased viscosity of fuel. Higher the KV, lower the SS.⁷ (iii) Acid value (AV) – Further oxidation of peroxides and hydro peroxides leads to the formation of organic acids, which is measured by AV. Higher the AV, lower the SS.⁸ (iv) Onset temperature (T_ON) – Higher temperature at which the biodiesel starts to degrade by accelerated oxidation is measured by T_ON. Higher T_ON value signifies higher TS.⁹

Antioxidants inhibit the oxidation process and are well recognized to control the biodiesel oxidation. The oxidation chain reaction is thus broken up, while the antioxidant is added.¹⁰ In the biodiesel, antioxidants can be present naturally (in the parent oil) or can be added intentionally. There are two types of antioxidants as chain breakers and hydro peroxide decomposers. Biodiesel development work has been wholly limited to the chain breaking type; the two common types are amine and phenolic, of which almost all fatty oil and ester work has been limited to phenolic types.¹⁰ Tocopherols are such natural antioxidants. Synthetic types are available as additives which can improve the stability of biodiesel. By various stability test methods for biodiesel, the research results have consistently shown superior stability for synthetic antioxidants dosage when compared to tocopherols.¹⁰ Some of the most significant phenolic synthetic antioxidants used in the biodiesel stability studies are as tert-butylhydroxyquinone (TBHQ), 2,5-di-tert-butyl-1,4-dihydroxybenzene (DTBHQ), pyrogallol (PY), butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and propyl gallate (PG).

Many recent studies concluded that the most effective chain breakers are synthetic antioxidants like PY, PG, TBHQ, BHT, and BHA are contributing more in the improvement of fuel stability of biodiesel. Kurechi et al.¹¹ in their study on the oxidation products of TBHQ antioxidant observed that during oxidation reaction TBHQ might produce chemicals which had higher effectiveness than TBHQ itself, which could contribute to the higher effectiveness and antioxidant activity of TBHQ. Many studies have been done on the effects of antioxidants on fuel stability of various biodiesel.

Maia et al.¹² investigated the effects of BHA, BHT, and TBHQ on the SS and OS of soybean biodiesel and observed that TBHQ and BHA presented higher efficiency in the prevention of the oxidative process of the biofuel B100 while comparing BHT-dosed sample. Yang et al.¹³ studied the influences of TBHQ, PG, PY, BHA, BHT at 3000 ppm concentration on the OS of soybean biodiesel and found that the synthetic antioxidants are more effective in the rank order as TBHQ > PY > PG >BHA >BHT. Ryu¹⁴ also investigated the OS of soybean biodiesel dosed with TBHQ, PG, BHA, and BHT and recorded that the antioxidants at 1000 ppm was effective. They observed the rank order of efficiency as TBHQ > PG >BHA >BHT. Tang et al.¹⁵ studied the effectiveness of TBHQ, PG, PY, BHA, BHT on soybean biodiesel and cottonseed biodiesel and observed that all antioxidants at 1000 ppm were effective towards fuel stability with the order as TBHQ > BHA > PY > BHT > PG for soybean biodiesel and TBHQ > PY > PG for cottonseed biodiesel.

Ileri and Gunnur¹⁶ observed that TBHQ has the strongest beneficial effect on OS of Canola biodiesel at 1000 ppm concentration. Verma et al.¹⁷ found that TBHQ was an effective antioxidant than PG, PY, BHA, and BHT to resist the oxidation process of biodiesel obtained from vegetable oils. Buosi et al.¹⁸ studied the effectiveness of antioxidants TBHQ, BHA, and BHT on the OS of soybean biodiesel and concluded that TBHQ is the most effective antioxidant. Yang et al.¹⁹ investigated the OS and SS of camelina biodiesel dosed with TBHQ, PG, BHA, and BHT and found the effectiveness order as TBHQ > PG >BHT >BHA. The best condition is due to the fact that TBHQ has two hydroxyl groups in its molecule restoring, in this way, two free radicals; meanwhile, BHT and BHA having a single hydroxyl group in its molecule.¹²

Fourier transform infrared (FTIR) spectroscopy uses the mathematical process (Fourier transform) to translate the raw data (interferogram) into the actual spectrum. FTIR method is used to obtain the infrared spectrum of transmission or absorption of a fuel sample. FTIR identifies the presence of organic and inorganic compounds in the sample. Depending on the infrared absorption frequency range 600–4000 cm⁻¹, the specific molecular groups prevailing in the sample will be determined through spectrum data in the automated software of spectroscopy. In this paper, FTIR has been used as a novel approach to characterize the variability in fuel stability of various biodiesel/antioxidant samples. FTIR spectroscopy has been projected as an effective, reliable, and quick method for the evaluation of biodiesel fuel stability and is also proposed as a well-organized correlating tool for OS results obtained from Rancimat instrument.²⁰ Oxidation-induction breakage structure of biodiesel obtained from waste cooking oil was analyzed using spectrum regions obtained from FTIR spectroscopy.²¹

Torres Rodríguez et al. used FTIR for monitoring the production of biodiesel and for checking its purity scale. They investigated the purity of jatropha and soybean biodiesel, which were esterified by using a base catalyst (cesium impregnated sodium zirconate).²² The quality of conversion of free fatty acids to methyl ester was monitored using FTIR spectra range of 1437 to 1464 cm⁻¹.²³ Infrared peak intensities of esters visualized from FTIR spectrum data were used to characterize the biodiesel (esters) obtained from waste cooking oil and fish oil.²⁴ Mirghani et al.²⁵ recorded the wavelength regions 1500–1700 cm⁻¹ and 3075–3700 cm⁻¹ to identify the moisture content in palm oil methyl ester blends.

Previous studies focused only on the OS of biodiesel without considering the corresponding storage and TS, which are also the significant biodiesel quality indicators. This study aims at determining the best synthetic antioxidant with superior fuel stability comprising all oxidation, storage, and TS along with a novel technique of FTIR inference.

Materials and methodology

Fuel

Transesterification or alcoholysis is the replacement of alcohol from an ester by another in a process similar to hydrolysis, except than alcohol is used instead of water.²⁶ As Calophyllum inophyllum (CI) oil is highly acidic, two steps of acid–base catalyzed transesterification have been used to extract biodiesel. The determined fatty acid composition is listed in Table 1. Initially the oil was taken to esterification process, which is heated at about 95°C for 1 h to remove moisture prevailing in it. Then methanol (50% v/v oil) and sulfuric acid (1% v/v oil) were added to the oil at 55–60°C for 2.5 h accompanied by 350 r/min stirring speed. After reaction completion, the oil was poured into separation funnel to remove the excess impurities, sulfuric acid and methanol in the upper layer of the oil. Then the lower layer of the esterified oil was removed and heated at 95°C for 1 h to remove the water and methanol. By repeating this process, the AV of esterified oil has been reduced to less than 4 mg KOH/g oil. Then the esterified oil has been involved in transesterification or alcoholysis process, in which the obtained oil was added with methanol (25% v/v oil) along with catalyst sodium hydroxide (0.9% w/w oil) and heated at 66°C for 2 h stirred at 350 r/min. In this transesterification process, triglyceride molecules of oil have been converted to methyl ester (biodiesel) and glycerol (by-product). The glycerol has been removed from the lower layer of separation funnel after 12 h of settling. Then the upper layer methyl ester has been involved in post-treatment process as washing, by which the biodiesel added with distilled water (50% v/v oil) and maintained at 60°C. Distilled water was sprayed over biodiesel surface and stirred softly. By discarding the lower layer, the pure biodiesel has been obtained, which is dried using rotary evaporator to free from methanol and water.

Table 1.

Fatty acid composition of crude oil.

Fatty acid	Systematic name	Formula	Structure	Wt%
Palmitic	Hexadecanoic	C₁₆H₃₂O₂	C16:0	14.7
Palmitoleic	cis-9-Hexadecenoic	C₁₆H₃₀O₂	C16:1	0.3
Stearic	Octadecanoic	C₁₈H₃₆O₂	C18:0	13.2
Oleic	cis-9-Octadecenoic	C₁₈H₃₄O₂	C18:1	46.1
Linoleic	cis-9, cis-12-octadecadienoic	C₁₈H₃₂O₂	C18:2	24.7
Linolenic	cis-9-cis-12-Octadecatrienoic	C₁₈H₃₀O₂	C18:3	0.2
Arachidic	Eicosanoic	C₂₀H₄₀O₂	C20:0	0.8

Antioxidant reagents

Synthetic antioxidants as PY, PG, TBHQ, BHT, and BHA were chosen due to their molecular structure (Figure 1) and the effectiveness report from other studies. The reagents were purchased from Ranbaxy and LOBA chemical organics. Since the antioxidants were effective at 1000 ppm concentration with biodiesel samples,¹⁰ the same constant concentration scale has been implemented in this study, which was measured using microbalance instrument (Shimadzu AUW220D Microbalance) as shown in Figure 2. Mixing has been done by using ultrasonicator (Life-care ENUP-500 ultrasonic) as shown in Figure 3. Ultrasonication is a process of dispersion of wide range of fine particles in a solvent with the help of ultrasonic waves which has a frequency of 20,000 Hz. The ultrasonic waves produce cavities which creates a bubble–particle–solvent interaction. This interaction completely disperses the antioxidant which is of nano size into the biodiesel which is highly viscous. The biodiesel sample has been taken in a 50 ml Eppendorf tube which was placed in a beaker containing water. The measured amount of antioxidant has been added to the tube and the ultrasonication was initiated. The ultrasonication has been done for 10 min under pulse action for every 20 s to reduce the generation of heat inside the tube. All the antioxidants were completely soluble in biodiesel at all concentrations. The following test fuel blends were prepared and denoted as: (i) 100% biodiesel – B100, (ii) 100% biodiesel + 1000 ppm PY – B100A3, (iii) 100% biodiesel + 1000 ppm PG – B100C3, (iv) 100% biodiesel +1000 ppm TBHQ – B100D3, (v) 100% biodiesel + 1000 ppm BHT – B100E3, (vi) 100% biodiesel + 1000 ppm BHA – B100F3. The properties of the fuel blends should be measured based on the standard methodology of the American Society for Testing and Materials (ASTM),²⁷ which were measured using laboratory equipments available in Thermal Laboratory, Government College of Technology, Coimbatore, India, and are listed in Table 2.

Figure 1.

Molecular structures of synthetic antioxidant additives (Source: Ileri and Gunnur¹⁶).

Figure 2.

Microbalance.

Figure 3.

Ultrasonicator.

Table 2.

Properties of fuel blends.

Properties/Samples	Kinematic viscosity @ 40°C	Density @ 15°C	Calorific value	Flash point	Cloud point	Pour point	Cetane index
Units	cSt	kg/m³	MJ/kg	°C	°C	°C	CCI
Diesel	3.9	833	43.35	63	6.5	−3	54
B100	4.78	873	38.33	140	13.2	4.3	57
B100A3	4.79	876	38.3	142	13.3	4.5	56.36
B100C3	4.79	875	38.31	141	13.2	4.3	56.46
B100D3	4.79	874	38.28	142	13.3	4.3	56.55
B100E3	4.83	877	38.31	142	13.4	4.4	56.12
B100F3	4.83	878	38.31	141	13.3	4.4	55.93

OS determination

Rancimat method depicts the OS of biodiesel through accelerated oxidation reaction based on the standard EN14214. Metrohm 873 Biodiesel Rancimat apparatus induce the oxidation reaction in biodiesel by heating the sample (3 g) at constant temperature (110°C) and passing air stream at 10 l/h. The vapors coming out of the heated sample carrying water soluble carboxylic acids (secondary oxidation products) is passed into the flask containing 50 ml distilled water with conductivity cell (Figure 4). Accumulation of oxidation products in the distilled water leads to increase in conductivity of water, which is indicated by the rise in conductivity curve. With respect to time, the conductivity curve increases gradually regarding accelerated oxidation, similar to exponential growth curve. This curve decreases after reaching a maximum value. Automated Rancimat software evaluates the IP by analyzing the maximum second derivative curve (Figure 5) with respect to conductivity and time.

Figure 4.

Rancimat method.

Figure 5.

Induction period of B100 at 110°C.

SS determination

SS is investigated based on the standard ASTM D4625, so that the antioxidant-dosed biodiesel samples were stored in air tight aluminum containers for a time period of 100 days. The containers are maintained at constant temperature of 40°C in an incubator.^10,21 Stability analysis was done by regular monitoring of AV and KV for an equal time interval of 10 days.

TS determination

TS term addresses the biodiesel resistance to oxidation at higher temperatures. The oxidation phenomenon at lower temperature is different from that of higher temperatures. It is explained by Dies Alder reaction, which insists the rapid oxidation reaction of fatty acid chains at the temperature above 250°C.²⁰ At elevated temperatures, the natural phenolic antioxidants present in biodiesel also deteriorate at faster rate, which contributed to further increase in oxidation process.²⁸ The effect of TS is significant when the biodiesel comes in contact with the higher temperature due to engine heat, which may deprive the biodiesel physio-chemical properties.^10,29 There are no sufficient studies have been done concerning TS of biodiesel by previous researchers.³⁰ TS is studied in this paper using thermo-gravimetric analyzer – TGA 2050. There is no specific standard for fuel TS analysis, but the influences of antioxidants could be investigated from ‘Onset Temperature’ parameter.⁹

FTIR spectroscopy

As mentioned earlier, FTIR is an effective method for characterizing the oxidation variability by correlating fuel stability results with the spectrum regions of specific molecular compounds of samples in the absorption frequency in the wavelength range of 600–4000 cm⁻¹.³¹ In the present research, FTIR analysis has been done by using Perkin Elmer spectrum instrument (Figure 6).

Figure 6.

Perkin Elmer spectrum instrument.

A drop of biodiesel sample has to be placed on the spectrum plate. Then by closing the cap, the infrared is allowed to pass through the spectrum plate. The molecules prevailing in the sample absorbs the light and transmits out the plate horizontally. The amount of absorption of wavelength was determined by the automated software and visualized through FTIR graph as percentage of infrared wavelength transmittance (%T)³² as shown in Figure 7.

Figure 7.

FTIR spectrum of B100.

Figure 7 illustrates FTIR spectrum plotted between wave number (cm⁻¹) vs. transmittance percentage (%T) based on the amount of light absorbed by specific molecules present in the CIME (B100) sample. The wave number and transmittance percentage were marked for C–O (600–1400 cm⁻¹), C = O (1500–1800 cm⁻¹), C–H (2700–3000 cm⁻¹), and O–H (3000–3700 cm⁻¹) bonds.³⁰ Here C–O and C = O signifies the presence of ester or ether group in the sample. It could be identified by comparing the FTIR spectrum of biodiesel sample at two conditions, i.e. before oxidation of sample and after oxidation of sample tested in Rancimat instrument. If there is change in the %T of C–O and C = O after oxidation of sample, it implies the presence of ester group, i.e. the confirmation of biodiesel character as shown in Figure 8.³³

Figure 8.

FTIR spectrum of B100 before and after oxidation reaction.

From Figure 8, it can be seen that drastic changes were found in the transmittance percentage C–O and C = O bonds portraying that the sample contains ester molecules, thereby confirming the character of biodiesel. The change in O–H bond from 78.11%T to 98.34%T signifies that most of the O–H molecules present in the sample before Rancimat gets contributed to the oxidation reaction. Higher %T represents the lower presence of molecules in the sample.

O–H and C–H bonds play the key role in the oxidation rate of any sample. If the presence of O–H is higher, the oxidation rate is faster due to the contact of sample with oxygen in atmosphere, which enhances the oxidation reaction. If the C–H molecule is higher, it leads to the consumption of oxygen by C–H for the formation of carbon dioxide that resulted in longer duration for degradation of the sample. It could be clearly visible from Figure 9.

Figure 9.

FTIR spectrum of B100 and diesel (HSD).

Figure 9 represents the FTIR spectrum for diesel (HSD) and CIME (B100) before oxidation reaction. Generally, diesel has the higher fuel stability than that of biodiesel. It could be clearly noted that %T for diesel (99.34%T) is higher than that of CIME (78.11%T), signifying that higher O–H prevalence is found in biodiesel. Also much lower %T for diesel (1.68%T) was found at C–H bond, whereas for B100 it was 27.85%T, signifying that higher C–H was found in diesel. Hence, higher C–H and lower O–H help the diesel fuel to possess superior fuel stability by resisting oxidation reaction more.

Therefore, by viewing the %T levels of all antioxidant-dosed biodiesel samples, the higher fuel stability character of specific antioxidant could be inferred through O–H and C–H bonds. Hence, FTIR is used as a novel approach in this research to characterize the oxidation variability of antioxidant samples.

Results and discussion

OS analysis

As indicated by EN14214 standard specifications, a biodiesel should display 6 h of IP at 110°C while testing in Rancimat instrument. Generally, CI biodiesel (CIME) shows better OS by possessing higher IP than other renowned biodiesel like soybean, jatropha, and karanja.³⁴ Pure biodiesel (B100) shows IP of 8.47 h at 110°C,³³ thereby satisfying with EN14214 standard of IP > 6 h at 110°C. CI biodiesel shows 24.57% higher OS when 20% biodiesel added with 10% pentanol in our previous OS analysis.³³ CI biodiesel was studied for OS variation with three different additives like 2-ethylhexyl nitrate, N-phenyl-1,4-phenylenediamine and N,N-diphenyl-1,4-phenylenediamine (DPPD) at 1000 ppm concentration and found that DPPD was effective among the other additives.³⁵ By dosing synthetic antioxidants at 1000 ppm concentration, the OS has been enhanced in a beneficial approach. All antioxidants show higher IP than that of pure biodiesel (Figure 10). TBHQ at 1000 ppm (B100D3) increased the OS of pure biodiesel blend (B100) by 42.56%.

Figure 10.

IPs (hour) at 110°C for various antioxidants with pure biodiesel (B100).

The efficacy of antioxidants on the OS is in the rank order of TBHQ (48.58 h) > PG (37.5 h) > PY (36.17 h) > BHT (20.65 h) > BHA (20.12 h). TBHQ antioxidant shows an average of 41.11% higher OS than that of other antioxidants. Generally, the antioxidant activity depends on the phenolic group position in the aromatic ring particularly 1,2 or 1,4 positions. Hydroxyl group of antioxidants deliver protons facilitating decrement in the radical formation and propagation, which delays the rate of biodiesel oxidation.^10,36 Due to the molecular structures, TBHQ, PG, PY are more effective than BHT and BHA. The presence of two hydroxyl group in TBHQ, PG, and PY surpass the effectiveness of BHA and BHT having one hydroxyl group (Figure 1). TBHQ, PY, and PG provide more active sites for the formation of complex between sample’s free radical and that of antioxidants, thereby increasing the stability of methyl ester chain.^37,38 Due to low volatility of BHT and BHA, the additives may lose its activity during the early stage of Rancimat test leading to comparatively low OS.^39,40

CI biodiesel has been investigated and found to have improvement in OS by adding BHT and 4-methyl-6-tert-butyphenol with 30% biodiesel/diesel blend.⁴¹ By adding 15% antioxidant extracted from pongamia leaf with 20% CI biodiesel, IP was found to increase from 5 h to 14 h at 110°C.⁴² In this study, TBHQ-dosed sample shows higher OS than the other antioxidant samples.

SS analysis

Antioxidants/biodiesel blends stored in aluminum containers are regulated for KV and AV change at an equal interval of 10 days based on the standard ASTM D4625. Figure 11 shows the variation of AV and KV for all samples over the storage time period of 100 days, respectively. The reason for using aluminum containers in this study is the resistance of aluminum to the catalytic effect on biodiesel oxidation reaction.³⁷ AV and KV values increase with increase in storage time period. When the oxidation of biodiesel takes place, the formation of products like polymetric compounds and peroxides leads to the accumulation of gums and sediments, which increases the KV values. Likewise the prevailing peroxides and hydroperoxides gets oxidized further and converted into acids, which increases the AV of the sample with storage duration.³⁹

Figure 11.

Variation of KV and AV for biodiesel dosed with various antioxidants.

B100 shows radical increment in both AV and KV after storage period of about 40 days, whereas the dosage of antioxidants lessens the increasing rate of AV and KV by resisting the oxidation reaction and thereby reducing the peroxides formation rate. At the end of 100 days, TBHQ blended biodiesel B100D3 shows the lowest values of AV and KV, which signifies the better SS of B100D3. The effectiveness of antioxidants was in the rank order as TBHQ > PG > PY > BHT > BHA. Similar trend of antioxidant effectiveness order (TBHQ > PG > BHT > BHA) has been observed by Yang et al.¹⁹ while evaluating camelina biodiesel. The reason behind this characterization is similar to OS characterization. Due to the presence of one hydroxyl group in BHA and BHT, the hindrance in sample could hold back the electron release and reduce the resistance to oxidation. Meanwhile, TBHQ, PG, and PY containing two hydroxyl groups increase the antioxidant activity by providing more sites for free radicals than BHA and BHT.¹⁹

TS analysis

Thermo-gravimetric analysis (TGA) has been extensively used to investigate the biodiesel thermal oxidation degree.⁴³ TGA has the advantage of requirement of very small quantity of biodiesel sample for analysis and also there is no need of any pre-treatment of sample for this test.^44,45 In this study, the antioxidants effect has been evaluated by determining onset temperature (T_ON) from TGA 2050 thermo-gravimetric analyzer. Biodiesel sample of 8 mg quantity was taken in a mini sample holder, which is purged with oxygen at a rate of 10°C/min and heated to 500°C. At elevated temperature, the biodiesel gets thermally oxidized and the secondary oxidation products were removed by the oxygen flow, thereby the sample loses its weight and indicates the result through TGA curve graph (Figure 12).

Figure 12.

TGA curve for TBHQ-dosed sample B100D3.

The graph consists of three phases.⁹ In the first phase, minimal weight change was displayed during the induction, the graphs shows that 1% weight loss of antioxidant/biodiesel sample occurs around 150°C. During second phase, a rapid change in weight was displayed after 30 min. Maximum degradation (T_{MAX DGTN}) occurs at the temperature of 263.80°C at which the sample weight rate decreased to the maximum level. Tertiary phase displays slower weight reduction and the curve starts flattering at around 420°C, after which there was no occurrence of sample conversion.

A horizontal baseline was extrapolated at 1% weight degradation. The intercept of tangent of the downward falling curve and the extrapolated line was indicated as onset temperature (T_ON). Similarly, the intercept of flattering line and tangent of curve is defined as offset temperature (T_OFF). As mentioned earlier, T_ON is used to denote the resistance to thermal degradation of biodiesel. Figure 13 shows the variation of TS of various antioxidants-dosed biodiesel through T_ON. Higher T_ON value of sample indicates better TS. Pure biodiesel shows the lowest onset temperature (T_ON) of about 118.58°C, shows the rapid thermal degradation due to the presence of higher oxygen molecules, which induces quick oxidation at elevated temperature. The effectiveness order of TS in the form of onset temperature (T_ON) is TBHQ (201.05°C) > PG (176.6°C) > PY (163.24°C) > BHT (140.46°C) > BHA (136.65°C). Biodiesel dosed with TBHQ (B100D3) escalated the TS of biodiesel by 41.02% than that of B100. The higher resistance of TBHQ to thermal oxidative reaction could be clearly revealed by viewing the presence of higher C–H bonds (Figure 14), while comparing other antioxidants in the FTIR spectrum region.

Figure 13.

Onset temperature values of pure biodiesel and antioxidant-dosed samples.

Figure 14.

FTIR spectrum of antioxidant-dosed biodiesel samples.

FTIR inference

The reason for higher fuel stability character of TBHQ dosage with CI biodiesel (CIME) is figured out effectively using FTIR technique. The antioxidant activity rank of synthetic antioxidants is determined from the fuel stability analysis as follows

TBHQ > PG > PY > BHT > BHA

As mentioned in the previous chapter, O–H and C–H bonds play the key role in the oxidation rate of any biodiesel sample. Higher prevalence of O–H molecules in the sample amplifies the oxidation rate of sample and gets degraded rapidly, whereas the higher prevalence of C–H resist the oxidation rate by increasing the oxidation duration by consuming oxygen for the formation of carbon dioxide. Hence, by analyzing the quantity of the presence of C–H molecules, the reason for higher stability of a specific sample could be portrayed. Lower %T represents the higher presence of molecules. The quantity of C–H molecules prevalence is depicted from the percentage of transmittance of IR light in 2700–3000 cm⁻¹ wavelength range. Figure 14 represents the FTIR spectrum plotted between wave number (cm⁻¹) vs. transmittance percentage (%T) based on the amount of light absorbed by C–H molecules present in the samples. The order of %T of C–H bonds in the biodiesel samples is

TBHQ (8.79%T) < PG (12.57%T) < PY (13.43%T) < BHT (15.99%T) < BHA (24.17%T)

It is observed that %T is lower for B100D3 (TBHQ sample), which insists the higher presence of C–H molecules. Hence, the higher C–H molecules in TBHQ contribute to the prolonged resistance to biodiesel oxidation in the antioxidant activity rank of TBHQ > PG > PY > BHT > BHA. The results of the effects of the antioxidants on fuel stability of CI biodiesel are effectively correlated with the FTIR spectrum data. Hence, FTIR is successfully utilized for the investigation of effectiveness rank of various antioxidants.

Conclusion

Biodiesel has inferior fuel stability due to higher presence of oxygen molecules and unsaturation degree of feedstock oil. Five synthetic antioxidants such as TBHQ, PG, PY, BHA, BHT at 1000 ppm concentration dosed with CI biodiesel have been tested in this research. The following conclusions were drawn:

OS of pure biodiesel was escalated by 42.56% from originally 8 h to 48.58 h (at 110°C) of IP by dosing TBHQ at 1000 ppm.

SS was assessed by monitoring AV and KV using ASTM D4625 aging method. Accordingly, B100D3 shows 36.57% higher SS.

TS was studied by an effective tool (TGA), by which onset temperature (T_ON) was measured for all antioxidant/biodiesel samples. TBHQ-dosed sample displays 41.02% higher T_ON than that of other antioxidants.

The antioxidant activity has been characterized by FTIR spectroscopy by analyzing the prevalence of C–H molecular chains in the spectrum region of 2800–3000 cm⁻¹. Based on the FTIR spectrum data, it was found that B100D3 shows an average of 41.11% higher OS than the biodiesel without any antioxidant due to the prevalence of higher C–H bond chains (8.79%T) in its molecular composition which confines the biodiesel not to get oxidized at higher rate.

In the scope of the present study, antioxidants were found to increase the fuel stability of CI biodiesel in the effectiveness order as TBHQ > PG > PY > BHT > BHA, among which TBHQ dosed at 1000 ppm is identified as the most beneficial antioxidant for the fuel stability improvement.

Footnotes

Acknowledgments

We thank VV College of Engineering, India, for their support to this research.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Mohamed Shameer P completed his PhD degree (Full Time) in Mechanical Engineering from Anna University Chennai, Tamil Nadu, India in 2017. He has been awarded Research Excellence Award – 2017 by MT Research and Educational Service, New Delhi, India. He is a former Senior Research Fellow (SRF), awarded by Council of Scientific and Industrial Research (CSIR), New Delhi, India. He received his post-graduate degree (Full Time) in Thermal Engineering with distinction from Anna University Chennai, Tamil Nadu, India in 2015. In 2009, he joined Mechanical Engineering (B.E – Full Time) and graduated as University rank holder with Distinction in Anna University Chennai, Tamil Nadu, India. He is currently working as Assistant Professor in the Department of Mechanical Engineering at V V College of Engineering, Tirunelveli, Tamil Nadu, India. He has published 23 research articles under scopus indexed journals related to his research area including 7 Elsevier and 7 Springer papers with total Thomson Reuter impact factors of 51.688. His research interests include renewable energy, biodiesel, fuel stability and combustion.

Mohamed Nishath P completed his under-graduate in Mechanical Engineering from the Anna University Chennai, Tamil Nadu, India in 2017. He is currently pursuing post-graduation in Manufacturing Engineering from the Anna University Chennai, Tamil Nadu, India. He is also working as research fellow in biodiesel research laboratory. His research interests include biodiesel, fuel stability and combustion analysis.

References

Khalife

Tabatabae

Demirbas

et al . Impacts of additives on performance and emission characteristics of diesel engines during steady state operation. Prog Energy Combust Sci 2017; 59: 32–78.

Zhou

Xiong

Gong

et al . Analysis of the oxidative degradation of biodiesel blends using FTIR, UV–Vis, TGA and TD-DES methods. Fuel 2017; 202: 23–28.

Rizwanul Fattah

Masjuki

Liaquat

et al . Impact of various biodiesel fuels obtained from edible and non-edible oils on engine exhaust gas and noise emissions. Renewable Sustainable Energy Rev 2013; 18: 552–567.

Knothe

Dunn

RO.

Dependence of oil stability index of fatty compounds on their structure and concentration and presence of metals. J Am Oil Chem Soc 2003; 80: 1021–1026.

Pullen

Saeed

An overview of biodiesel oxidation stability. Renewable Sustainable Energy Rev 2012; 16: 5924–5960.

Wexler

Polymerization of drying oils. Chem Rev 1964; 64: 591–611.

Xin

Imahara

Saka

Oxidation stability of biodiesel fuel as prepared by supercritical methanol. Fuel 2008; 87: 1807–1813.

Dos Santos

VML

Da Silva

JAB

Stragevitch

et al . Thermo chemistry of biodiesel oxidation reactions: a DFT study. Fuel 2011; 90: 811–817.

Wan Nik

Ani

Masjuki

HH.

Thermal stability evaluation of palm oil as energy transport media. Energy Convers Manage 2005; 46: 2198–2215.

10.

Dunn

RO.

Antioxidants for improving storage stability of biodiesel. Biofuels Bioprod Bioref 2008; 2: 304–318.

11.

Kurechi

Aizawa

Kunugi

Studies on the antioxidants XVIII: oxidation product of tertiary butyl hydroquinone (TBHQ) (I). J Am Oil Chem Soc 1983; 60: 1878–1882.

12.

Maia

ECR

Borsato

Moreira

et al . Study of the biodiesel B100 oxidative stability in mixture with antioxidants. Fuel Process Technol 2011; 92: 1750–1755.

13.

Yang

Hollebone

Wang

et al . Factors affecting oxidation stability of commercially available biodiesel products. Fuel Process Technol 2013; 106: 366–375.

14.

Ryu

The characteristics of performance and exhaust emissions of a diesel engine using a biodiesel with antioxidants. Bioresour Technol 2010; 101: 78–82.

15.

Tang

Wang

Salley

et al . The effect of natural and synthetic antioxidants on the oxidative stability of biodiesel. J Am Oil Chem Soc 2008; 85: 373–382.

16.

Ileri

Gunnur

Effects of antioxidant additives on engine performance and exhaust emissions of a diesel engine fueled with canola oil methyl ester–diesel blend. Energy Convers Manage 2013; 76: 145–154.

17.

Verma

Sharma

Dwivedi

Investigation of metals and antioxidants on stability characteristics of biodiesel. Mater Today Proc 2015; 2: 3196–3202.

18.

Buosi

da Silva

Spacino

et al . Oxidative stability of biodiesel from soybean oil: comparison between synthetic and natural antioxidants. Fuel 2016; 181: 759–764.

19.

Yang

Corscadden

et al . Improvement on oxidation and storage stability of biodiesel derived from an emerging feedstock camelina. Fuel Process Technol 2017; 157: 90–98.

20.

Saluja

Kumar

Sham

Stability of biodiesel – a review. Renewable Sustainable Energy Rev 2016; 62: 866–881.

21.

Furlan

Wetzel

Johnson

et al . Investigating the oxidation of biodiesel from used vegetable oil by FTIR spectroscopy: used vegetable oil biodiesel oxidation study by FTIR. Spectrosc Lett 2010; 43: 580–585.

22.

Torres Rodríguez

Romero Ibarra

Ibarra

et al . Biodiesel production from soybean and Jatropha oils using cesium impregnated sodium zirconate as a heterogeneous base catalyst. Renewable Energy 2016; 93: 323–331.

23.

Reyman

Saiz Bermejo

Ramirez Uceda

et al . A new FTIR method to monitor transesterification in biodiesel production by ultrasonication. Environ Chem Lett 2014; 12: 235–240.

24.

Mothe

Cesar de Castro

Mothe

MG.

Characterization by TG/DTG/DSC and FTIR of frying and fish oil residues to obtain biodiesel. J Therm Anal Calorim 2011; 106: 811–817.

25.

Mirghani

MES

Kabbashi

Alam

et al . Rapid method for the determination of moisture content in biodiesel using FTIR spectroscopy. J Am Oil Chem Soc 2011; 88: 1897–1904.

26.

Mandana

Esmail

Meisam

An overview of the recent advances in the application of metal oxide nanocatalysts for biofuel production. In: Rai, Mahendra, da Silva and Silvio Silvério (Eds.) Nanotechnology for bioenergy and biofuel production. Cham: Springer, pp. 255–299.

27.

Esmail

Hanif

Mostafa

et al . Experimental investigation of low-level water in waste-oil produced biodiesel-diesel fuel blend. Energy 2017; 121(27): 331–340.

28.

Jain

Sharma

MP.

Stability of biodiesel and its blends: a review. Renewable Sustainable Energy Rev 2010; 14: 667–678.

29.

Jain

Sharma

MP.

Thermal stability of biodiesel and its blends: a review. Renewable Sustainable Energy Rev 2011; 15: 438–448.

30.

Mohamed Shameer

Ramesh

FTIR evaluation on the fuel stability of Calophyllum inophyllum biodiesel: influence of tert-butyl hydroquinone (TBHQ) antioxidant. J Mech Sci Technol 2017; 31: 3611–3617.

31.

Mohamed Shameer

Ramesh

FTIR assessment and investigation of synthetic antioxidant on the fuel stability of Calophyllum inophyllum biodiesel. Fuel 2017; 209: 411–416.

32.

Mohamed Shameer

Ramesh

Influence of antioxidants on fuel stability of Calophyllum inophyllum biodiesel and RSM-based optimization of engine characteristics at varying injection timing and compression ratio. J Brazil Soc Mech Sci Technol 2017.

33.

Mohamed Shameer

Ramesh

Sakthivel

et al . Experimental evaluation on oxidation stability of biodiesel/diesel blends with alcohol addition by Rancimat instrument and FTIR spectroscopy. J Mech Sci Technol 31: 455–463.

34.

Atabani

Cesar

ADS.

Calophyllum inophyllum L. – a prospective non-edible biodiesel feedstock. Study of biodiesel production, properties, fatty acid composition, blending and engine performance. Renewable Sustainable Energy Rev 2014; 37: 644–655.

35.

Rashed

Kalam

Masjuki

et al . Improving oxidation stability and NO_x reduction of biodiesel blends using aromatic and synthetic antioxidant in a light duty diesel engine. Ind Crops Prod 2016; 89: 273–284.

36.

Shahidi

Wanasundara

PK.

Phenolic antioxidants. Crit Rev Food Sci Nutr 1992; 32: 67–103.

37.

ZahiraYaakob

Narayanan

Surya Unni

et al . A review on the oxidation stability of biodiesel. Renewable Sustainable Energy Rev 2014; 35: 136–153.

38.

Karavalakis

Hilari

Givalou

et al . Storage stability and ageing effect of biodiesel blends treated with different antioxidants. Energy 2011; 36: 369–374.

39.

McCormick

Ratcliff

Moens

et al . Several factors affecting the stability of biodiesel in standard accelerated tests. Fuel Process Technol 2007; 88: 651–657.

40.

de Guzman

Tang

Salley

et al . Synergistic effects of antioxidants on the oxidative stability of soybean oil- and poultry fat-based biodiesel. J Am Oil Chem Soc 2009; 86: 459–467.

41.

Rashedul

Masjuki

Kalam

et al . Effect of antioxidant on the oxidation stability and combustion–performance–emission characteristics of a diesel engine fueled with diesel–biodiesel blend. Energy Convers Manage 2015; 106: 849–858.

42.

Ramalingam

Govindasamy

Ezhumalai

et al . Effect of leaf extract from Pongamia pinnata on the oxidation stability, performance and emission characteristics of calophyllum biodiesel. Fuel 2016; 180: 263–269.

43.

Bezerra Mota Gomes Arruda

Arruda Rodrigues

Duarte Arruda

et al . Chromatography, spectroscopy and thermal analysis of oil and biodiesel of sesame (Sesamum indicum) – an alternative for the Brazilian Northeast. Ind Crops Prod 2016; 91: 264–271.

44.

Vega-Lizama

Díaz-Ballote

Hernandez-Mezquita

et al . Thermogravimetric analysis as a rapid and simple method to determine the degradation degree of soy biodiesel. Fuel 2015; 156: 158–162.

45.

Christensen

McCormick

RL.

Long-term storage stability of biodiesel and biodiesel blends. Fuel Process Technol 2014; 128: 339–348.