Improving Stability of Biodiesel from 20% Free Fatty Acid Palm Oil with Tert-butylhydroquinone at Various Concentrations for 52 Weeks of Storage

Abstract

Overcoming the oxidation stability of biodiesel has been a significant challenge, especially after an extended storage period. To test a major factor affecting biodiesel quality, eight different conditions consisting of water at a concentration of 500 ppm and tert-butylhydroquinone (TBHQ) concentrations of 500, 1000, and 2000 ppm, in combination, were added to palm biodiesel, with no-water-added treatment as the control. Samples were kept in dark storage and air-limited at room temperature for 52 weeks with an initial carbon residue of 0.05 wt%. Every sample was periodically taken for property examination, which included the percentage of fatty acid methyl ester (FAME), iodine value (IV), kinematic viscosity (KV), acid value (AV), and oxidation stability. The properties of the samples with 500 ppm of water-added biodiesel exhibited the most significant degradation, even though oxidation stability (starting from 43.37 h) remained higher than 10.00 h after 32 weeks. The IV dropped 48.43% from 49.92 to 25.56 g I₂/100 g. The KV increased 6.14% from 4.56 to 4.84 cSt. The AV rose from 0.45 to 1.09 mg KOH/g. Biodiesel with 2000 ppm TBHQ added was stable for 22 weeks, with all properties under standard values. However, biodiesel in the same condition but with water contamination, its stability was reduced to 16 weeks.

1. Introduction

Global population growth is expected to increase energy demand [1] rapidly. Within 30 years, an approximately 80% increase in energy consumption will be reached [2]. An unforeseen energy shortage crisis might occur in the next few decades. Energy security enhances the continuous development of renewable energy [1]. Electricity will eventually play a key role in most appliances [2]. However, diesel generators still require diesel to produce electricity. Thus, biodiesel is an alternative energy source for these engines, which converts unused oil or oil waste into fuel [3,4]. Biodiesel also supports several countries’ bioeconomy and waste utilization [5]. Therefore, ensuring biodiesel stability is necessary over a prolonged storage period to overcome the combustion problem in the engine caused by low-quality biodiesel [6,7]. Biodiesel is an alternative fuel that can be produced from renewable sources. Vegetable oil, animal fat, and waste cooking oil are typical feedstocks for biodiesel production [6,8]. The biodiesel, or mono-alkyl ester of fatty acids alkyl ester, is biodegradable. Sulfur and aromatic-free releases are lower in biodiesel than in petroleum diesel fuel [9]. However, biodiesel usually has low oxidation stability or a low ability to resist oxidation [10]. The oxidation stability of biodiesel affects some correlated properties, such as the kinematic viscosity (KV), acid value (AV), iodine value (IV), and carbon residue, which affect the performance of commercial biodiesel applications in automobile engines [11].

Biodiesel stability mostly depends on the structure of the fatty acid composition [12]. Unsaturated fatty acid content plays an essential role through its allylic and bis-allylic methylene moieties adjacent to the double bond on the chain [13]. These reactive sites are readily exposed to free radicals such as (in descending order of importance) the superoxide anion radical, singlet oxygen, the hydroxyl radical, the nitrogen radical, and the perhydroxyl radical; they are also chemically changed from alkyl ester to acids, esters, aldehydes, and ketones [14,15]. Palm biodiesel has drawn much attention for its use as feedstock for biodiesel production due to its similar properties to petroleum diesel [16]. Furthermore, palm yields the highest conversion when chemical equilibrium is reached compared to other feedstocks due to its high triglycerides content and low unsaturated fatty acid [17,18]. However, a significant challenge of using biodiesel is its low oxidation stability, especially after being stored for an extended period, such as for commercial uses [19,20]. TBHQ is one of the most effective antioxidants among different feedstocks, especially high-free fatty acid (FFA) [14,21,22,23,24]. Biodiesel blends have been studied to improve their oxidation stability. However, antioxidants are crucial in maintaining stability over the mixing formula [25]. Previously, our study indicated that adding 1000 ppm TBHQ extended the biodiesel stability of jatropha and sunflower oils, a high content of unsaturated fatty acids oil, within 22 weeks of storage [26]. There is little research on how biodiesel oxidation stability holds up throughout a long storage period. Thus, the question that has to be answered is whether adding TBHQ to high-free fatty acid biodiesel improves its oxidation stability after over a year of storage. Even though the turn-over at the gas station for Thailand’s 3000 L underground tank is only a few days, the vehicle fuel plastic tank is about 4 weeks. However, waste oil, which becomes high-free fatty acid biodiesel, might require more storage time to collect the necessary volume. Therefore, we examined the impact of varying amounts of changes in water contamination and different concentrations of added TBHQ on 20% FFA palm biodiesel.

The biodiesel properties might be unexpectedly changed for the used cooking oil or waste oil where the raw material contains high-free fatty acid. It is known that if the FFA content is higher than 1%, two-step esterification and transesterification are used [27]. This study formulated 20% FFA raw material by mixing 20% palm fatty acid distillate (PFAD) and 80% palm stearin. The 20% FFA palm oil was converted to high-quality biodiesel (FAME > 96.5%) using the two-step reaction. The palm biodiesel was then kept for 52 weeks, and samples were occasionally taken for quality checking. The antioxidants, TBHQ (tert-butylhydroquinone), at various concentrations were added to mimic the solution for long-term storage. On top of that, water as an oxidative inducer was added to biodiesel before storage to simulate the expected contamination of biodiesel over the storage time. This study combines the effects of high FFA raw materials, water contamination, antioxidant concentrations, and long storage time on biodiesel quality and stability.

2. Materials and Methods

2.1. Chemicals

Palm stearin (PS) and palm fatty acid distillate (PFAD) were obtained from Patum Vegetable Oil Company Limited (Pathum Thani, Thailand). Potassium hydroxide (Ajax Finechem, Sydney, NSW, Australia), sulfuric acid (Qrec reagent, Auckland, New Zealand), methanol (JT Baker, Lake Charles, LA, USA), and tert-butylhydroquinone (TBHQ) (Fluka, St. Gallen, Buchs, Switzerland) were of analytical grade.

2.2. Biodiesel Production

Biodiesel samples were prepared by mixing PS (at a free fatty acid (FFA) content of 0.01% by weight) and palm fatty acid distillate (PFAD; at an FFA content of 80% by weight) to make the initial FFA of 20% and initial triglyceride of 80%. All biodiesel samples were produced using two-step production of esterification and transesterification.

2.2.1. Esterification

Sulfuric acid was used as a catalyst at 2 wt% (% by weight) of triglyceride plus 4 wt% of FFA. The amount of methanol used was based on calculating a 3:1 molar ratio of methanol to triglyceride and a 2:1 molar ratio of methanol to FFA. The biodiesel production conditions were 60 °C, 500 rpm, and 1.5 h reaction time (SLR, SCHOTT, Mainz, Germany). The crude product was neutralized by washing with distilled water and then dried by heating (SLR, SCHOTT, Mainz, Germany) at 105 °C for 45 min. Please note that the molecular weights of methanol, FFA, and triglycerides were 32, 268, and 841 g/mole.

2.2.2. Transesterification

The catalyst, 1% potassium hydroxide, was used based on the weight of the mixture obtained from the previous esterification step. The molar ratio of methanol to the mixture was 6:1, and the production conditions were performed at 60 °C, 500 rpm, and 1 h. After the reaction, all the glycerol was removed using a separation funnel. Then, the crude product was washed with distilled water and dried at 105 °C for 45 min. The initial kinematic viscosity, oxidation stability, acid value, iodine value, and carbon residue were measured.

Please note that the two-step esterification and transesterification mean 2 reactions to produce biodiesel. The first reaction was esterification, which aimed to convert most FFA to FAME. The follow-up reaction was transesterification to convert triglyceride to FAME.

2.3. Storage Conditions

The biodiesel was divided into eight samples. TBHQ was added to the first four samples at 0, 500, 1000, and 2000 ppm concentrations. The second set of four treatments contained 500 ppm water (0.05% weight per weight) added with TBHQ at 0, 500, 1000, and 2000 ppm concentrations. All samples were stored in the dark for 52 weeks at room temperature (approximately 30 ± 2 °C). During the storage, the samples were taken every 4 weeks (1 month) for up to 24 weeks (6 months), then at 36 (9 months) and 52 weeks (12 months) to check all parameters (triplication). However, 2 more samples were analyzed at 18 and 22 weeks for the critical acid value and FAME. On the other hand, carbon residue was analyzed every 8 weeks (2 months) until the biodiesel quality was out of standard ranges.

2.4. Sample Analysis

Kinematic viscosity (KV, cSt) was determined using a Cole-Pamar JKO1 at 40 °C according to ASTM D445 [28]. The 6.4 mL sample was placed into a viscometer (Chemiscience, Bangkok, Thailand) and heated to 40 °C for 10 min. The sample’s moving time was recorded from the initial to the final point. The viscosity was then calculated [29].

Biodiesel stability was determined as the induction period (h) using a Rancimat 873 at 110 °C according to EN 15751 [28]. Different amounts of TBHQ (500, 1000, and 2000 ppm) were added to the samples. The samples were kept at the storage conditions and then were taken periodically for property analysis.

FFA and acid value (AV, mg KOH/g) were measured using a Metrohm model 873 (Herisau, Switzerland) according to ASTM D664 [28]. The biodiesel sample of 3 mg was mixed with 30 mL of 2-propanol. The mixture was titrated with 0.1 M NaOH using phenolphthalein as an indicator. AV was then calculated.

The iodine value (IV, g I₂/100 g) was analyzed using auto titration (Metrohm model 848; Bangkok, Thailand) according to ASTM D664 [28]. One gram of the sample was mixed with 20 mL 98% (w/w) acetic acid. Then, 0.1 M iodine monochloride (Panreac Química SLU, Castellar del Vallès, Spain) and 3% (w/w) magnesium acetate (QReC, Auckland, New Zealand) were added to the mixture. The mixture was kept dark for 5 min, and 10% (w/w) was added. The mixture was titrated with a 0.1 M thiosulfate solution (APS Finnechem, Melbourne, Australia).

Fatty acid methyl esters analysis: 50 mg of biodiesel samples were mixed with 1 mL of 10 mg/mL methyl heptadecanoate as the internal standard. The sample was analyzed in gas chromatography (GC-2010PLUS Shimadzu, Kyoto, Japan) according to EN 14103 [28]. The flame ionization detector (FID) and the column (DB-Wax), sized 30 m × 0.32 mL × 0.25 µm with 380 °C—an injection temperature at 260 °C and a split ratio of 25—were used. Helium was used as the carrier gas. The percentage of FAME was calculated using Equation (1). All experiments were carried out in triplicate.

$$\mathrm{C}={\displaystyle \frac{\left(\sum \mathrm{A}\right)-{\mathrm{A}}_{\mathrm{EI}}}{{\mathrm{A}}_{\mathrm{EI}}}}\times {\displaystyle \frac{{\mathrm{C}}_{\mathrm{EI}}\times {\mathrm{V}}_{\mathrm{EI}}}{\mathrm{m}}}\times 100$$

(1)

where C is the FAME percentage, ∑A is the total area of methyl ester of C₁₄ to C₂₄, A_EI is methyl heptadecanoate area, C_EI is methyl heptadecanoate concentration (mg/mL), V_EI is a volume of methyl heptadecanoate, and m is biodiesel weight (mg).

Carbon residue was measured using the Conradson carbon residue apparatus according to ASTM D4530 [28]. Ten grams of distilled biodiesel was transferred to Conradson’s carbon residue apparatus and burned (medium fire) until the white smog occurred (approximately 10 min). Then, the fire was turned high and burned until the white smog disappeared (approximately 13 min). After that, the fire was turned off, and the sample was cooled down and weighed. Carbon residue is calculated using Equation (2).

$$\%\mathrm{Carbon} \mathrm{residue}={\displaystyle \frac{\left(\mathrm{A}\ast 100\right)}{\mathrm{w}}}$$

(2)

where A is the mass of residue (g), and w is the mass of sample (g).

3. Results and Discussion

3.1. Effects of Added Antioxidant on Biodiesel Quality

The addition of antioxidants, TBHQ, improves biodiesel stability by inhibiting the chain reaction, resulting in no secondary oxidation products [25,30].

Table 1. Different commercial antioxidants are commonly used to enhance biodiesel stability [31,32,33].

Antioxidant	Usage Details	Minimum Concentration (ppm) Required by the EU Standard
THBQ (tert-butylhydroquinone)	More effective in vegetable oil or animal fats with low-free fatty acids feedstocks. Additionally, it can be employed in storage environments, like high temperatures or air insufflation.	500
PG (propyl gallate)	Moderate to high antioxidant activity for biodiesel	500
PY (pyrogallol)	High antioxidant activity on biodiesel with high-free fatty acid feedstock like animal fats	500
BHT (butylated hydroxytoluene)	More effective in animal fats	1000
BHA (butylated hydroxyanisole)	More effective in animal fats	2000

compares different antioxidants commonly used in biodiesel [31,32,33]. The antioxidants stabilize the biodiesel’s physical and chemical properties. When compared within 12 weeks, the concentrations of TBHQ did not affect the biodiesel properties, which were acid value, viscosity, iodine value, and FAME. All antioxidant-added biodiesel could maintain its properties above the standard, as seen in Figure 1. It is noted that the FAME of non-TBHQ-added biodiesel was the most degraded, which was 97.22% and 81.43% at 12 and 52 weeks (Table S1 in Supplement Material) (standard value of >96.5%). However, after 12 weeks, the differences in biodiesel properties were noticed. Non-TBHQ-added biodiesel’s viscosity (with water added) increased from 4.58 cSt to above 4.80 cSt after 52 weeks but still under 5.0 cSt (Figure 1a), the same as the iodine value (Figure 1d).

TBHQ could control oxidative reactions, exhibiting the more extended stability of biodiesel. The KV increased from 4.56 to 4.84 cSt (500 ppm TBHQ) to 4.73 cSt (1000 ppm TBHQ) and to 4.71 cSt (2000 ppm TBHQ). Similar to AV and IV, biodiesel with 2000 ppm TBHQ had the lowest change after 52 weeks of storage. The final values of AV and IV were 0.61 mg KOH/g (Figure 1b) and 33.17 g I₂/100 g (Figure 1d). Higher concentrations of TBHQ could control oxidative reactions better, exhibiting lower KV, AV, and higher IV than biodiesel without TBHQ added.

Meanwhile, the FAME of 500, 1000, and 2000 ppm TBHQ biodiesels at 52 weeks was 85.79, 86.58, and 87.55%, respectively (Figure 1c and Table S4 in Supplement Material). The results showed that FAME had the most sensitive biodiesel property to oxidation reactions in non-added and added TBHQ. Different TBHQ concentrations slightly impacted biodiesel’s properties. Adding 2000 ppm, TBHQ maintained FAME above the standard for 22 weeks, whereas the samples with less added TBHQ did not meet the FAME standard after 16 weeks. The feedstock type was the primary factor impacting FAME in biodiesel, depending on its composition and percentage of free fatty acids. For example, Pongamia (or Karanja), a high-free fatty acid biodiesel, maintained its FAME above the standard until 36 weeks [34]. The oxidation reaction of biodiesel leads to an increase in acid value, density, and viscosity but a decrease in FAME and iodine value. TBHQ provides H⁺ to peroxide radicals, creating a stable product that inhibits the autoxidation reaction and has been proven to be a superior antioxidant [26,35,36].

The characteristics of palm biodiesel are given in

Table 2. Properties of palm biodiesel.

Property	Unit	Test Method [28]	Standard Limit	Palm Biodiesel
Acid value	mg KOH/g	ASTM D664	<0.5	0.45
Carbon residue value	wt%	ASTM D4530	<0.3	0.05
Kinematic viscosity (at 40 °C)	cSt	ASTM D445	3.5–5	4.56
Oxidative stability (at 110 °C)	h	EN 15751	>10	43.37
Iodine value	g I2/100 g sample	EN 14111	<120	42.92
Flash point	°C	ASTM D 93	>52	165
Pour point	°C	ASTM D 97	Not specified	19
Ester content	wt%	EN 14103	>96.5	98.88
Saturated				59.67
Unsaturated				39.21

. All properties of palm biodiesel were in the range of standard values except the pour point, which was not specified in the Thai specification [28]. Oxidation stability was high at 43.37 h due to 80% palm stearin in raw material containing high saturated fatty acid. A low IV of 42.92 g I₂/100 g confirmed biodiesel’s high saturated fatty acid ratio. The total FAME was 98.88%, higher than the standard value of >96.5%. According to commercial biodiesel standards worldwide, this biodiesel is of high quality [28]. No precipitate was formed after being left at room temperature for a few days. Therefore, intermediates, i.e., mono-glyceride, di-glyceride, etc., and total glycerides and steryl glucoside, were not examined. The low carbon residue confirmed low intermediates and total glycerol.

3.2. Effects of Antioxidants on Water-Contaminated Biodiesel

Water contamination must be less than 500 ppm (ASTM D6751), which is required by the Department of Energy Business, Ministry of Energy, Thailand [28]. Biodiesel naturally absorbs water well, leading to bacterial contamination and ester hydrolysis [37]. Therefore, this study mimicked the worse quality of biodiesel by adding 500 ppm water into biodiesel to investigate the effect of antioxidant concentrations. Different concentrations of TBHQ were added to 500 ppm water-contaminated biodiesel to examine changes in biodiesel properties. Figure 2 shows the variation in biodiesel properties with various doses of antioxidants (0, 500, 1000, and 2000 ppm) with and without 500 ppm contamination (data in the Supplementary Materials). The kinematic viscosity (KV), acid value (AV), iodine value (IV), and fatty acid methyl ester (FAME) values were determined. At 12 weeks, the FAME of all conditions was higher than 96.5%, except for the sample with water contamination (96.11%). Dehydrated biodiesel provided 2.05% higher FAME than water-contaminated biodiesel, and the oxidation stability was also 0.43 h higher.

The noticeable decreases in all properties were seen, especially for the non-added TBHQ samples and worse for non-added TBHQ with water contamination. After being kept for 52 weeks, the acid value of non-TBHQ-added biodiesel with and without water contamination increased from 0.45 to 1.09 and 1.01 mg KOH/g. The acid values of 500, 1000, and 2000 ppm TBHQ biodiesels with and without water contamination were 0.96, 0.71, and 0.68; and 0.65, 0.66, and 0.61 mg KOH/g after 52 weeks, respectively. For 18 weeks, all samples were under 0.5 mg KOH/g (standard value). Effects of water contamination were more prominent after storage for 36 weeks to 52 weeks.

Water accelerates hydrolysis reactions and bacterial growth but does not affect the significant properties of biodiesel [38]. However, slight changes in all biodiesel properties between TBHQ and non-TBHQ-added water-contaminated samples were found except at 2000 TBHQ added (Figure 2). A moderately polar antioxidant like TBHQ delays biodiesel oxidation, producing fewer peroxides [39]. Generally, the donated proton from the phenolic structure reacts with free radicals, regenerating the acyl glycerol molecule and eventually inhibiting the free radical oxidation [40].

3.3. Evaluation of Storage Stability

Effect of Antioxidant

The primary factor impacting the oxidation activity of biodiesel is the double bonds present in the fatty acids of raw materials. As per the EN 15751 standard [28], the oxidation stability of biodiesel, measured by Rancimat in terms of induction time, must be at least 10 h according to the Thai commercial biodiesel standard [28], as calculated by Equation (3). The POLYUNSAT is %unsaturated fatty acid, and IP is the induction period related to biodiesel oxidation stability. During storage, it was expected that the viscosity of the methyl ester would increase due to the formation of more polar, oxygen-containing molecules and oxidized polymeric compounds, which would lead to the formation of gums and sediment that would clog filters [10].

$$\mathrm{IP}=49{\left[\mathrm{POLYUNSAT}\right]}^{-0.5}$$

(3)

The oxidation kinetics of biodiesel after adding different antioxidants was widely discussed [41], and the minimum concentrations required by EU are shown in Table 1. However, TBHQ was used in this study. Interestingly, the kinematic viscosity of all samples was within 5.0 cSt after 52 weeks of storage, even though it slightly changed from 4.56 (initial value) to the maximum of 4.84 cSt (500 ppm TBHQ added without water contamination), as shown in Figure 3 and the Supplementary Materials. It is possible that after oxidation, not only are long-chain molecules formed, but so are shorter molecules, such as fatty acids, aldehydes, alcohols, carboxylic acids, oligomers, etc. Therefore, the kinetic viscosity did not increase as high as expected [19]. The iodine value was reduced from 49.92 (initial value) to 25.56 I₂/100 g (no TBHQ added with water contamination). The iodine value was measured based on the number of double bonds in the fatty acid structure [42]. The result shows the potential of an oxidation reaction at the double bonds of the biodiesel molecules and the effect of water contamination on the oxidation reaction. The IV data in the Supplementary Materials or Figure 3 show fewer IV values of the samples with water contamination, both TBHQ added and none. However, the IV of all samples was under 120 g I₂/100 g sample.

It was evident that adding TBHQ to biodiesel stabilizes its oxidation stability, leading to its properties being maintained. However, 2000 ppm TBHQ eventually maintained the FAME of biodiesel until week 18 without water contamination and week 16 with water contamination. Samples of 1000 and 2000 ppm TBHQ had the highest IV values after 52 weeks, showing the antioxidant’s effect in reducing the oxidation reaction of biodiesel. Concerning the AV, TBHQ at all concentrations (500, 1000, and 2000 ppm) maintained biodiesel quality at 0.5 mg KOH/g up to 22 without water contamination. With water contamination, the TBHQ at all concentrations could maintain AV for only 20 weeks.

In contrast, no effect of TBHQ on the KV of biodiesel with and without water contamination was observed in this study, as all samples were in the range of 3.5 to 5.0 cSt for 52 weeks of storage. This trend is similar to that of Obadiah et al. (2012) [35]. TBHQ delays the increase in AV and viscosity and the decrease in IV and FAME due to its ability to inhibit the chain reaction of oxidation.

3.4. Effects of Water Contamination on Biodiesel Oxidation Stability over the Storage

The effect of storage time on oxidation stability in terms of the induction period measured by Rancimat (EN 15751) is shown in Figure 4. The oxidation stability of biodiesel samples with and without added water decreased with storage time due to more prolonged oxidation. The initial induction period was 43.37 h. After being kept for 52 weeks, changes in biodiesel properties, precisely oxidation stability exhibited as induction time, were three-step changes. Firstly, slight changes in biodiesel properties were found at the beginning of the storage until 8 weeks, dramatically changing from 8 to 16 weeks. Secondly, oxidation stability was maintained from 16 to 24 weeks and dropped rapidly between 24 and 36 weeks. Thirdly, it was retained from 36 to 52 weeks. The same pattern was found in biodiesel induction time with and without water contamination (Figure 4). The rapid decrease in the oxidation stability resulted from the chain reaction at the double bonds. Once the double bonds disappear, the chance of an oxidation reaction decreases or is maintained. A short-chained carboxyl group is one of the significant oxidation reaction products from peroxides (primary oxidation products) and aldehydes. Therefore, it affected the AV of biodiesel. This oxidation mechanism is a well-established concept in lipid chemistry [43].

As expected, the changes in oxidation stability correspond to KV and AV (in Supplementary Materials), which were maintained for 8 weeks, but then both values were gradually reduced toward 52 weeks. Differently, IV and FAME slightly reduced from the beginning until 52 weeks, but the values were within the standard for 8 weeks. No maintained period was observed. The results from Figure 3 and Figure 4 show that oxidation stability is related to KV and AV. It can be concluded that palm biodiesel in this study (from 20% FFA) could be stored for 8 weeks with quality maintained, which is the case for vehicle plastic fuel tank turn-over time. In Thailand, the turn-over time for gas stations is much shorter: only 2–3 days (3000 L underground tanks).

3.5. Carbon Residue Analysis

Early findings were that the FAME of palm biodiesel with 2000 ppm TBHQ (no water contamination) was higher than 96.5% after storage for 22 weeks (Figure 3 or Supplementary Materials). It can be concluded that the maximum time storage of palm biodiesel is 22 weeks. However, with water contamination and 2000 ppm TBHQ added, FAME was higher than 96.5% only for 16 weeks. Therefore, the carbon residue was measured for 24 weeks to cover the FAME limit, and untreated palm biodiesel was the only sample showing the worst possible values in this investigation. As shown in Figure 5, the carbon residue content increased with storage time by 0.05 wt%, 0.12 wt%, 0.09 wt%, and 0.13 wt% for 0, 8, 16, and 24 weeks, respectively. In other words, the carbon residue increased dramatically after 8 weeks and remained almost constant until 24 weeks. The amount of carbon residue keeps increasing for long-term biodiesel storage due to the secondary oxidation products. This results in more char after the combustion and clogging problems are generally found due to the high carbon residue of blended fuel in the engine [44,45,46]. It can cause other operational issues, such as blockage of nozzles and corrosion and cracking of components [44,46].

4. Conclusions

This study investigated the changes in the properties of palm biodiesel during extended storage times. Twenty percent of palm fatty acid distillate and water were added to accelerate biodiesel stability. In the meantime, TBHQ was added to prevent the oxidative reaction. Palm biodiesel could be stored for up to 16 weeks, and its kinematic viscosity, iodine value, oxidation stability, fatty acid methyl ester, and acid value met the specifications. Meanwhile, water contamination significantly contributed to the degradation of all tested samples during the 12–36 week storage period. Interestingly, the most significant change in biodiesel properties was found in the 500 ppm water-contaminated biodiesel without THBQ addition. The IV dropped by 48.43%, KV increased by 6.14%, and its AV rose to 2.4 times its original properties. However, adding TBHQ could delay the oxidation reaction effectively in the biodiesel samples. At a dosage of 2000 ppm, the palm biodiesel sample met the specifications for 22 weeks of storage. Biodiesel with 2000 ppm added could be stored and used for commercial uses within the accepted property changes. However, the combustion efficiency must be investigated further to ensure engine safety.

Besides the production processes and antioxidants, biodiesel feedstocks play a crucial role in biodiesel stability, affecting its quality [47,48,49]. Biodiesel feedstocks containing the least amount of unsaturated fatty acids, particularly C18:2, are recommended for biodiesel production. There is room for improvement by optimizing the feedstock (production volume and quality) and production process to increase or maintain biodiesel stability for as long a storage period as petroleum diesel.