Empty Fruit Bunch (EFB) resulted from oil palm plantations and mills can be converted into ash through open combustion. The EFB ash then treated by simple calcination and used as a heterogeneous catalyst for biodiesel production. The characteristics of EFB ash were identified based on its elemental composition, porous structure, and active site size. The effectivity of the EFB ash as a catalyst was tested in a transesterification reaction of Refined Bleached Deodorized Palm Oil (RBDPO) with excess methanol (30 %-w) in various catalyst loads (in%-wt). The lab-scale experiments were conducted in a three-neck glass reactor, which was put on the hot plate stirrer at 450 rpm. The EFB ash performed the best as a catalyst by attaining optimal conversion at 65 °C for 1 h with a 16 %-wt of catalyst load. In this condition, most of the standard quality of biodiesel were complied with total glycerol under 0.24% and ester methyl contents up to 98.9 %. The characteristics tests showed that the properties and active side of the EFB ash are excellent after calcination at 600 for 5 h. The recyclability test of EFB ash as a catalyst showed high performance in two repetition cycles, each showing an increase in the yield of biodiesel, which was 92.21 % in cycle 2 and 91.23 % in cycle 3.
1. Introduction
Biodiesel production is driven by the increasing demand for renewable energy. This condition was regarding climate change, the requirement to reduce dependence on fossil fuels, and government support. The biodiesel market is undergoing significant growth and transformative developments that are transforming the global energy and chemical industries (
Marguerite Chauvin, 2023). Advancements in technology in biodiesel production are contributing to the growth of the global biodiesel market (
Bhawarkar, 2023;
Marguerite Chauvin, 2023). Some improvements in biodiesel production technology have been made in order to increase efficiency and decrease production costs. It brings biodiesel more competitive than other kinds of biofuel and traditional fossil fuels (
Chen et al., 2018;
Harahap et al., 2019;
Kumar et al., 2021).
Transesterification is the most popular process in producing biodiesel. Transesterification is a three-step process where a triglyceride is converted into diglycerides, followed by monoglycerides, in sequential order. These monoglycerides upon reaction with methanol result in the formation of Fatty Acid Methyl Ester (FAME), and crude glycerol. Triglycerides are readily trans-esterified in the presence of an alkaline catalyst at atmospheric pressure and at a temperature of approximately 60–70 °C with an excess of methanol (
Amurugam, 2022;
Hundie et al., 2022;
Meriatna et al., 2021). Transesterification commonly occurs either in the presence of a homogeneous or heterogeneous catalyst. The significance test in this study was conducted to determine whether the results of treating variations in the catalyst ratio and its effect on yield, conversion, and biodiesel purity were statistically significant. There is a strong correlation between the yield of biodiesel as a response and some of the operating conditions, the catalyst concentration, methanol-to-oil molar ratio, and reaction temperature (
Arita et al., 2023;
Gülüm et al., 2020;
Thangaraj et al., 2019).
The majority of commercial biodiesel production facilities use homogenous, alkaline catalysts. Strong alkali catalysts such as sodium hydroxide (NaOH), potassium hydroxide (KOH), potassium methoxide (CH
3ONa), and potassium methoxide (CH
3OK) are most frequently used for biodiesel production. Some of the drawbacks of homogeneous catalysts are in terms of corrosion-related (
Krishnan et al., 2021), water formation, and reusability (
Shan et al., 2018). Therefore, heterogeneous catalysts are then considered as another option to bring significant effects in the production of more beneficial biodiesel.
A number of researchers have pointed out the many advantages of using heterogeneous catalysts. They provide ease of separation and high reusability (
Foroutan et al., 2021). The solid catalyst can be prepared easily by optimizing the carbon surface with acid or base (
Salmasi et al., 2020). Generally, essential homogeneous catalysts such as potassium hydroxide and sodium hydroxide are the main catalysts that are often used because they have the advantage of high catalytic activity in a short reaction time and low reaction conditions (
Bambase et al., 2021;
Ibrahim et al., 2019). The heterogeneous catalysts are easy to recover from the reaction mixture, are capable of withstanding aqueous treatment steps, and are amenable to modification to obtain high activity (
Krishnan et al., 2021), selective, and longer catalyst lifetimes (
Thangaraj et al., 2019).
Indonesia is the world's largest producer of palm oil. This is supported by the large area of oil palm planting, which always expands every year. Based on data from the Indonesian Palm Oil Association (GAPKI), in 2022 the palm oil industry contributed US$39.07 billion with a total oil plantation areas of 16.38 million hectares. In 2023, Indonesian production of palm oil was estimated at 49 million tons, of which about half was exported. Accounting for around 59 % of the total global export of palm oil, the Indonesian palm oil industry commands around 40 % of global market of vegetable oils, outcompeting other three main rival vegetable oils (
Besalicto, 2024).
The operations of oil palm plantations and processing of crude palm oil (CPO) always leave biomass which is often also referred to as palm oil industry waste. The sterilized palm's fresh fruit bunch (FFB) will be stripped, resulting in a huge quantity of empty fruit bunch (EFB). Then, the EFB is usually sent for incineration or composting (
Cheah et al., 2023). The solid wastes may consist of empty fruit bunches, mesocarp fruit fibres (MF), and palm kernel shells (PKS). The p ’combustion produces ash residue (EFB ash) containing inorganic compounds, especially 30–40 % potassium in crystal form, which is easily converted into potassium oxide (K
2O) by burning and calcining at moderate temperatures (
Lim et al., 2020). Some studies show that EFB has a relatively high content of potassium. Potassium is already reactive in its free form. When it is in contact with oxygen, it readily bonds with the O-atom and forms potassium oxide. According to
Okoye et al. (2019) potassium is the key elemental composition found in the calcined ash.
Moreover, many researchers proved that potassium salt in biomass is promising excellent properties in many applications, such as for feedstock of power plants (
Novianti et al., 2015), raw materials for fertilizer or constructions materials (
Abu Aisheh, 2023;
Samadhi et al., 2020), gasification (
Lahijani and Zainal, 2011), glycerol carbonate synthesis (
Okoye et al., 2019), biomass combustion (
Chao et al., 2018;
Deng et al., 2018), and as catalyst for pyrolysis (
Zhou et al., 2018), and catalyst for interesterification process (
Ibrahim et al., 2019;
Wong et al., 2020). The active group K
2O found in EFB ash, is alkaline. It will react with methanol to form a methoxide ion compound, which acts as a nucleophile or active catalyst to convert fat or oil become a methyl ester (
Rezki et al., 2020).
The use of EFB ash as a catalyst in biodiesel production has been carried out by several researchers with various raw materials, such as palm oil (
Ishfaq et al., 2022;
Putra et al., 2017), waste oil (
Foroutan et al., 2021;
Guan et al., 2009;
Hamza et al., 2021;
Thushari and Babel, 2018), rapeseed oil (
Musil et al., 2018), hazelnut oil (
Bilgin and Gulum, 2018), sunflower oil (
Jalalmanesh et al., 2021;
Salmasi et al., 2020), jathropa curcas (
Yaakob et al., 2012), rice bran oil (
Taslim et al., 2018). The biodiesel produced shows that yield varies from 50.5 % to 99 % form the reactions with various catalyst load. This study investigates the performance and effectivity of EFB ash as a catalyst for transesterification of RBDPO. Many studies on the EFB ash utilization as a catalyst for producing biodiesel have been carried out widely. However, it still requires several stages of complex catalyst preparation.
Generally, the palm-based ash used is obtained from combustion at high temperatures as a result of biomass boilers. In this condition, some of the constituents can be lost which may affect the ash's active properties and catalytic performance. Some researchers have reported the performance of EFB ash in transesterification reactions, but most still cannot comply with the biodiesel quality standards, especially the purity of methyl esters. In this work, the EFB ash was treated with simple preparation stages and at lower burning temperatures to keep the higher composition of potassium compounds within. Furthermore, EFB ash can be promoted as a potentially efficient and affordable catalyst for biodiesel production because it can be generated from palm oil solid waste. The concept of circular bioeconomy is important to solve the challenges faced by various industries, especially palm oil, its implementation will contribute to the environmental sustainability and economic sustainability of the entire palm oil industry supply chain (
Cheah et al., 2023).. The result of current work may trigger the economic circular of the palm industries and, specifically in Indonesia, may potentially reduce dependence on catalyst imports.
2. Materials and methods
Palm empty fruit bunches were taken from the PT. Perkebunan Nusantara VII, South Sumatera, Indonesia. It is a state-owned company engaged in the plantations of rubber, oil palm, sugarcane, and tea. The stages of catalyst preparation are shown in
Fig. 1. Palm EFB was burned with an open combustion system at a temperature of 500–600 °C. The closed loop control was chosen for the combustion, because it can compensate for variations in combustion efficiency and ensure that the desired temperature is maintained. The ash is collected at the bottom or base of the combustion plate. It was dark grey, dusty, and still contained many impurities in various size. EFB ash is separated/filtered and then dried in the open air for 24 h. The gravimetric filter is chosen for this case, where the contaminated sample put through a control filter and a sample filter. In this method, two pre-weighed filters were arranged one on top of the other, in a single filter holder then filter a sample. The ash then goes through a screening process for uniformity of 100 mesh sizes.
2.1. Characterization of EFB ash
The EFB ash identified its catalytic properties through several chemical physical parameters, including density (using a spring balance and a measuring cylinder), pore size (Barrett-Joyner-Halenda, BJH), surface structure/morphology (using Phenom Pro-X G6 Desktop Scanning Electron Microscope, SEM), and analysis of elemental composition and constituent compounds (using X-MET handheld X-Ray Fluorescence, XRF analyzer). The parameters mentioned earlier are also used by many researchers (
Changmai et al., 2020;
ShenavaeiZare et al., 2021;
Sitepu et al., 2022) to identify EFB ash as a catalyst. While, the micro-structure of materials (crystalline forms) is closely related to pore size and some other chemical properties (
Abu Aisheh, 2023).
2.2. Preparation for transesterification
One crucial part of the preparation of catalysts from EFB ash is calcination. In this process, the EFB ash was heated to 700 °C for the purpose of removing volatile substances, oxidizing a portion of mass, or rendering them friable (
Wang et al., 2020). It was taken for 5 h. The calcination is also directed to affect the crystal structure and pore size of the catalyst.
The calcined EFB ash was previously mixed in methanol. The mixture of the catalyst and methanol is heated at 40 °C and gently mixed for 30 min. A glass flask and magnetic stirrer were used for the mixing process, and the stirring speed was 100 rpm. The weight ratio of ash to methanol is 1:4. The number of catalysts added in the reaction process follows a variation in a catalyst weight ratio to RBDPO, which varies from 12, 14, 16, 18, to 20 %-wt.
2.3. Transesterification of RBDPO to produce biodiesel
The raw material (RBDPO) is placed into a three-neck glass reactor on a hot plate with a magnetic stirrer at 65 °C, with the stirring speed set at 450 rpm for 1 h. A mixture of EFB ash and methanol catalysts is introduced according to the weight ratio, which varies following process conditions. A reflux cooling system is installed to maintain a stable temperature throughout the reaction. After the reaction is complete, the biodiesel product is poured into a separatory funnel that has been covered with filter paper and left for 24 h to form 3 layers (methyl ester, glycerol, and a solid catalyst). To remove the remaining catalyst and glycerol in the methyl ester, washing was carried out using distilled water repeatedly until a clear water layer was obtained. Then the methyl ester is dried and purified from the washing residue by heating it for 1 hour at 100 °C.
2.4. Analysis of biodiesel quality
The quality of biodiesel produced from the transesterification process with EFB ash is then analyzed in several of the quality parameters as shown in
Table 1. For chemcial composition in samples, we used Gas Chromatography- Mass Spectrometry (GC–MS) type of Shimadzu QP 2010 SE with FID and MS detector.
Table 1. Method for analysis of biodiesel.
| Parameter | Units | Method | Standard value |
|---|
| Density | kg/m3 | EN ISO 3675 | 820–845 |
| viscosity | mm2/s | ASTM D445 | 1.9–6.0 |
| acid value | g KOH/g | ASTM D664 | 0.50 |
| total glycerol | %-wt | ASTM 6584 | 0.240 |
| Ester Methyl content | % | EN 14,078 | 96.5 (min) |
Each experiment for processing EFB, catalyst preparation, and transesterification reaction was conducted with 2–3 repetitions. Furthermore, for each sample that required quality analysis, such as total glycerol and methyl ester content, the repetition was done twice. Meanwhile, for measurement data such as density, viscosity, acid number, and composition analysis from GC–MS, 1 to 2 repetitions were carried out because the measuring instruments used were ensured to have maximum accuracy and all equipment had been well calibrated. The data collected has analysed using a t-test: paired two samples for means, both on the relationship of changes in catalyst ratio to yield, conversion (total glycerol) and methyl ester content (biodiesel purity).
3. Results and discussion
3.1. Characteristics of EFB ash
The catalytic properties of heterogeneous catalysts are highly dependent on the constituent components of the catalyst. Before the catalyst was tested in the transesterification reaction, a catalyst characterization of EFB ash was carried out using XRF analysis.
As the results of the analysis shown in
Fig.2 indicate, it is known that several elements and oxide compounds make up the catalyst. The most dominant elements are potassium, silica, calcium, chlorine, phosphorus, sulfur, and iron.
The dominant component of EFB catalyst is potassium, which generally comes from K
2CO
3 compounds. At the time of burning, empty fruit bunches will produce CO
2 and K
2O. From the results of the analysis using the titration method, it was found that the K
2CO
3 content in the EFB ash was 48.97 %, while the K
2O from the XRF analysis was 35.65 %, meaning that the remaining 13.32 % was CO
2 gas. Some studies show that EFB has a relatively high content of potassium. Potassium is already reactive in its free form. When it is in contact with oxygen, it readily bonds with the O-atom and forms potassium oxide. According to
Okoye et al. (2019) potassium is the key elemental composition found in the calcined ash. Potassium burns in oxygen to make potassium oxide. The combustion of EFB at a temperature of 500–600 °C produces ash containing higher potassium oxide (K
2O) levels reaching 55 %-wt, which is higher than silica levels. Potassium oxide (K
2O) is an ionic compound of potassium and oxygen called alkali metal oxide. K
2O is highly reactive and can react vigorously.
The pore size and pore volume of the ash as a catalyst is important to observed. It is very dependent on the characterization of the chemical composition and physical structure of the EFB ash. The measurements of the pore size of the active site and pore volume was using the Barrett-Joyner-Halenda (BJH) technology, which can be seen in
Table 2.
Table 2. Analysis results in Barret–Joiner–Halenda (BJH).
| Analysis parameters | Units | Results |
|---|
| Specific surface area (BJH Adsorption) | m2/g | 0.129 |
| Specific surface area (BJH Desorption) | m2/g | 0.041 |
| BJH adsorption cumulative micropore volume | cc/g | 0.001 |
| BJH desorption cumulative micropore volume | cc/g | 0.0003 |
| BJH adsorption pore radius | nm | 13.330 |
| BJH desorption pore radius | nm | 13.349 |
The pore diameters of EFB ash during the adsorption and desorption processes were 13.33 and 13.349 nm, the micropore volume was 7.67 × 10−4 cc/g for adsorption, and desorption is smaller, namely only 2.76 × 10−4 cc/g, with a specific surface area of 0.12942 m2/g at the time of adsorption and 0.0412897 m2/g for the desorption process. From the data, it can be interpreted that the adsorption process can take place in a volume that is quite large compared to the volume of the product desorption process coming out of the pore of the active catalyst site.
The surface area of EFB ash in this study was very small at 0.1294 mm2/g. It is due to the bottom ash sample from EFB combustion which has a relatively large and diverse particle size. The measurement results showed the average ash particle size was 110–190 microns. To obtain a high specific surface area, ash samples need to be further reduced in particle size through grinding mechanisms to a size of 45 – 50 microns. However, although it has a small surface area, EFB ash can still show good catalytic performance. The methyl ester resulted in a purity that meets high quality and yield requirements. This achievement is contributed by the active zone of potassium oxide, which microstructurally forms an evenly open alkali metal surface. It triggered all parts of the catalyst to be in an active state. Meanwhile, the other elements in palm ash (e.g. SiO2) may encourage the formation of metal combinations that provide higher catalytic performance or are considered catalyst support.
Scanning Electron Microscope (SEM) images in
Fig. 2 illustrate the surface morphology of the EFB as has a catalyst. This image was captured at 10,000x resolution and magnification. The texture of the catalyst shows agglomerated particles arranged with uniform distribution. The pattern indicates that strong, compact particles were detected for the structure of potassium and its oxide. The surface structure of the catalyst appears smooth and homogeneous, indicating high crystallinity. These morphological characteristics are identical to some reported biomass ash-based catalyst characteristics test results (
Lau et al., 2019;
Lim et al., 2020;
Rezki et al., 2020).
As also seen in
Fig. 3, the EFB ash shows a crystalline structure contributed by the domination of potassium elements. The crystallinity index of the catalyst affects the reaction speed at which the oil can be converted into methyl esters. The calcination of EFB ash has promoted the development of mesoporous structures on the catalyst. The mesoporous structure has provided a high surface area capacity and porosity. The calcination and activation process could also prevent agglomeration and sintering effects which assisted in forming the desired catalytic porous structure (
Lim et al., 2020).
In detecting the micro structural of the materials surface, Phenom Pro-X G6 Desktop SEM is equipped with a Light optical navigation camera so that it can show color images, and distinguish the dominant elements analyzed by generating synthetic spectrum. A colored representation is accessible for larger areas. This section can quickly detect new composition and observe sample orientation. Generally, the color pink indicates the dominance of potassium, purple for magnesium, green for phosphorus, and others.
Fig. 3 on the left side shows the surface of ash in the combined map, while the right image shows the cut out of the map. X-ray analytical mapping give information about how composition varies over the sample surface. The EDS signal is useful to identify the elements and their concentrations.
As seen in
Fig. 3, the surface structure of the catalyst appears smooth and homogeneous, indicating high crystallinity. The high crystallinity of the catalyst kept it free from various impurities and physical properties, such as higher catalytic properties and better stability at high temperatures. The degree of crystallinity of the catalyst affects the reaction speed at which the oil is converted into methyl esters.
The elemental compositions of the catalyst samples showed an appreciable amount of O, K, Mg, Ca, C, P, Cl, and Fe (as seen in
Table 3). The relatively high potassium is suitable for a catalyst of transesterification reaction. The presence of a high amount of oxygen suggests the existence of these elements in oxides forms.
Table 3. The elemental analysis of EFB ash using Energy Dispersive X – Ray Spectroscopy (EDS).
| Element symbol | Element name | Weight Conc. |
|---|
| O | Oxygen | 41.511 |
| K | Potassium | 22.332 |
| Mg | Magnesium | 9.210 |
| Ca | Calcium | 8.184 |
| C | Carbon | 6.105 |
| P | Phosphorus | 5.981 |
| Cl | Chlorine | 2.752 |
| Fe | Iron | 2.572 |
| Si | Silicon | 1.013 |
| Na | Sodium | 0.354 |
The comparison of the palm-based ash chemical composition in this study with previous ones as presented in
Table 4. It is known that the ash burning of palm biomass, especially EFB, provides a different composition and morphological structure. This condition is strongly influenced by the combustion technique and the conditions and properties of the ash used in the process or for characterization purposes. EFB ash shows a more unified structure and fewer pores than the EFB char and its uniform distribution of the potassium elements in EFB ash (
Arfiana et al., 2021).
Table 4. Chemical composition of EFB ash.
The ash analyzed in this study showed a greater proportion of potassium oxide due to the controlled EFB combustion temperature (500–600 °C). In contrast, ash samples studied by many researchers before are generally obtained from combustion at high temperatures, such as in boilers fueled by palm biomass. Generally, the ash's composition contains dominant SiO
2, followed by alumina oxides. The calcination of EFB ash led to increase the degree of crystallinity of K
2O. It also can enhance crystals growth of a compound, wherein the crystals growth will increase with increasing temperature of heating to form a complete crystalline structure at optimum temperature (
Husin et al., 2018).
