Integrated two-step co-pyrolysis under several low-cost natural catalysts to produce aromatic-rich liquid fuel from mixed municipal solid waste

Abstract

The world’s increasing human population and industrial activities have resulted in an enormous rise in energy consumption throughout the years. Substantial attention has been given to the impending energy crisis caused by the depletion of fossil fuel supplies and their contribution to environmental degradation. As a result, it is necessary to investigate and make use of nonfossil energy sources for the purpose of maintaining demand stability as well as creating a sustainable green environment. Pyrolysis is a reliable method to convert municipal solid waste materials into useful energy. Hence, the co-pyrolysis of unsegregated municipal solid waste was investigated in this study using an integrated two-step pyrolysis process with a double reactor, supported by various natural catalysts, such as zeolite, dolomite, and kaolin, at 550°C for 210 minutes as constant variables—an approach that has not been reported previously. To determine the physical and chemical properties, liquid fuel was subjected to ASTM and gas chromatography–mass spectroscopy analyses, and the impact of each catalyst on its characteristics was also examined. The aromatic fraction was prominent in the liquid fuel yields produced using kaolin and zeolite catalysts (57.4% and 46.1% peak area, respectively). Meanwhile, the highest yield of liquid fuel was obtained using dolomite as the catalyst. The viscosity and density of liquid fuel with dolomite, kaolin, and zeolite were 10.83, 4.25, and 4.04 mm²/second and 0.88, 0.89, and 1.01 g/cm³, respectively. Conversely, the corresponding calorific values for zeolite, kaolin, and dolomite were 41.37, 41.09, and 41.19 MJ/kg, respectively. The physical characteristics of the liquid fuel are comparable to those of common fuels such as petrol-88, which is utilized in Indonesia as a vehicle fuel.

Graphical Abstract

Open in new tabDownload slide

1. Introduction

The increasing amounts of municipal solid waste (MSW) have drawn attention to several environmental problems, such as climate change, which could have serious effects on living things. The open dumping strategy that is being employed in most countries, particularly in emerging ones, is no longer appropriate due to the exponential growth in waste, and the practice is severely constrained by the requirement for expensive land areas and high expenses. Anticipating a sustainable MSW management strategy is therefore crucial. One strategy that can be employed is the conversion of MSW into usable energy. Pyrolysis and gasification are two methods of thermal degradation that can be used to transform MSW into liquid fuel (LF), syngas, and solid fuel (char) [1, 2], and subsequently, industrial and transportation sectors can use LF and syngas as fuel sources [3, 4]. Between the two methods, pyrolysis yielding three products is a workable solution for managing MSW, as it simultaneously converts MSW into liquid, gas, and solid fuels [5]. Based on the dominant product type, pyrolysis is classified into three main types: slow pyrolysis, fast pyrolysis, and advanced pyrolysis, which includes flash, vacuum, microwave, plasma, and solar pyrolysis. This classification is based on the process conditions and the heating rate, which influence the yield and quality of the produced fuels [6, 7]. Based on the feed material, the pyrolysis process can be classified into pyrolysis (single material) and co-pyrolysis (mixed material) processes. Single-stage, two-stage, and multi-stage processes are classifications based on the operating system of the pyrolysis process [8–10]. The location of the catalyst and feedstock determines how a single-stage, two-stage, or multi-stage pyrolysis process differs from each other. It is referred to as single-stage pyrolysis if the catalyst and feedstock are combined. If the catalyst and feedstock are not combined (separated), the pyrolysis process can involve two or more stages [11–13].

Furthermore, the addition of a catalyst during the pyrolysis process is a viable solution to address issues related to thermal degradation, such as the need for large amounts of energy [14]. Generally, a catalyst can enhance the reaction rate and activation energy, improve the quantity and quality of the product, and increase thermal efficiency without consuming or altering itself during the reaction process [15]. However, the most significant advantage of using some porous catalysts is their selective power, which enables the process to be directed towards desired hydrocarbon products [16, 17]. The acidity and texture of a solid catalyst are key parameters in breaking long hydrocarbon chains into shorter ones, although other factors such as thermal stability, inhibitory capacity, and cost need to be considered. The incorporation of aluminum into the silica framework generates a charge imbalance that mainly contributes to the catalyst’s acidic properties. Porosity, surface area, and particle size determine the access power of polymer molecules to the internal acid sites. Catalysts with high acidity levels exhibit larger pores or wider external surfaces [18–20]. Therefore, the Si/Al ratio strongly determines the acidity level in the solid catalyst pores and has a different effect on the pyrolysis product. A low Si/Al ratio provides a high acidity level, which in turn leads to high yields in the pyrolysis product [21], similar to the results reported by Matuszewska et al. [22].

In conclusion, catalysts containing silica and alumina, such as dolomite, zeolite, limestone, and clay, can be utilized in the pyrolysis process, and their use is highly recommended. Catalysts can be used directly in the pyrolysis process (in situ), by mixing them with the raw material to be converted, or they can be added separately in a second reactor (ex situ), a process called two-stage pyrolysis [13, 23]. The comparison between pyrolysis processes with and without a catalyst is detailed in Table 1. Generally, in pyrolysis, synthetic catalysts, such as Y-zeolite, β-zeolite, MoO₃, Ni–Mo, HZSM-5, Al(OH)₃, Ni, CeO₂, Al₂O₃, alumina, CeO₂, Rh, SiO₂, Li, Na, Na₂CO₃, K₂CO₃, ZnCl₂, Ni/SiO₂-N, Zeolite, and ZrO₂, were used.

The use of β-zeolite, Y-zeolite, and m-Ni–Mo catalysts in catalytic cracking at 500°C using a batch reactor for a mixture of plastic waste has produced LF with high heating value (HHV) containing paraffinic, olefinic, aromatic, and cyclic compounds. The presence of a catalyst in the pyrolysis process greatly affects the concentration of hydrogen, isobutene, and isopentane in the gas. It has been found that among various synthetic catalysts, β-zeolite exhibits the highest yields of LF [25]. Similarly, in a study carried out by López et al. [26, 27], a ZSM-5 zeolite catalyst was used in the pyrolysis of mixed plastic waste in a batch reactor at 440°C for 30 minutes. Zeolite shows highly effective characteristics to produce LF containing aromatic compounds and syngas rich in C₃–C₄ fractions. Furthermore, it has also been reported that among several types, Y-zeolite and β-zeolite catalysts have better performance than other synthetic catalysts in plastic waste pyrolysis processes [28, 29]. Moreover, the use of NaHCO₃ and AgNO₃ catalysts reduces chlorine in the syngas product from plastic waste and increases the amount of LF produced [30].

In 2013, Miskolczi et al. [31] performed a catalytic pyrolysis test on MSW and municipal plastic waste (MPW) using Y-zeolite, β-zeolite, equilibrium FCC, MoO₃, Ni–Mo, HZSM-5, and Al(OH)₃ catalysts at 500°C and 600°C using a batch reactor. The results showed that the viscosity of LF decreased when using catalysts, specifically β-zeolite and MoO₃. On the other hand, LF also contained K, S, P, Cl, Ca, Zn, Fe, Cr, Br, and Sb impurities, and the concentrations of K, S, P, Cl, and Br impurities decreased the pyrolysis process using a catalyst. The catalyst has a significant positive effect on the quality of LF produced, and the HZSM-5 catalyst is more effective in MSW degradation. Similarly, its use increases the volatile fraction and reduces the reaction time for the degradation process [31]. The efficiency of catalysts with MPW raw materials is higher than that with MSW, which varies in type and size. The effect of various materials in MSW may be the reason for the low efficiency of the catalyst [32]. A mixture of plastic waste [low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP)] and paper produces LF containing oxygen-containing compounds, such as alcohols, aldehydes, ketones, carboxylic acids, and phenols, and these compounds can be reduced using ZSM-5 catalysts in pyrolysis [33]. The degradation of coal materials using catalytic processes has been proven by Amin et al. [34], where the use of MoS₂ catalysts increased the amount of LF from 18% to 33% by weight and decreased the amount of syngas from 27% to 12% by weight.

It is important to note that the performance of natural catalysts may not be as effective as synthetic catalysts. However, studies showed that natural catalysts, such as dolomite, zeolite, and kaolin, can still provide good yields of LF and syngas, with the added benefit of being cost-effective and environmentally friendly. Additionally, the use of abundant and widely available natural catalysts, such as zeolite, dolomite, and kaolin, as a substitute for synthetic catalysts can also help reduce production costs for catalyst procurement. The effectiveness of using natural clay and dolomite catalysts has been proven by Kyaw and Hmwe [35] in the pyrolysis process of various plastic waste mixtures using a fixed bed reactor. The maximum LF yield of 67.06% by weight was obtained at a temperature of 220°C–370°C for 1.5 hours. The obtained syngas yield ranged from 19.92% to 28.52% by weight, with a fuel density of 0.7016 g/ml and a kinematic viscosity of 2.26 mm²/second, with dominant chemical compositions of alkane hydrocarbon. Lee et al. [36] performed pyrolysis of polystyrene plastic waste at 400°C using natural zeolite clinoptilolite for LF production with a semi-batch reactor. Then, the yields were compared with the pyrolysis process using synthetic catalysts, such as HZSM-5 and silica-alumina, and with thermal degradation processes. According to the test results, natural zeolite clinoptilolite was as effective as synthetic catalyst HZSM-5 for LF production, with dominant C₅–C₁₂ carbon chains. These results showed that natural zeolite clinoptilolite catalysts have reliable selectivity for LF production with a uniform distribution of hydrocarbon compound chains [36]. Similar results were obtained using natural kaolin catalysts at a temperature of 450°C using a batch reactor. The presence of the catalyst also reduced the reaction time of the degradation process, showing that the addition of catalysts to the pyrolysis process can increase energy efficiency [37, 38].

The use of natural zeolite catalysts thermally activated at 500°C enhanced the degradation of unsegregated MSW (UMSW). Pyrolysis of unprocessed raw materials with a catalyst resulted in a 40% increase in LF yield compared to that without a catalyst. The dominant compositions obtained in the LF product are paraffins and olefins. This shows that the addition of zeolite in the degradation process has broken down long hydrocarbon chains and produced more uniform hydrocarbon compounds. Based on the distribution of hydrocarbons in LF, it is confirmed that the natural kaolin catalyst performs better than the natural zeolite catalyst when pyrolyzed in a fixed bed reactor at 400°C for 1 hour. The natural kaolin catalyst produced a light fraction hydrocarbon compound (C₅–C₁₂) with a peak area of 65.4%, while the natural zeolite catalyst produced a light fraction with a peak area of 51.6% [39, 40]. This success demonstrates the use of kaolin as an effective catalyst for the long-term production of LF at a low cost, as it is readily available in large quantities.

The literature review [39, 40] clearly showed that MSW, specifically plastic waste (single material), has great potential for conversion into LF, which is a substitute for conventional LF. Meanwhile, natural catalysts have great potential for use in pyrolysis as a substitute for synthetic catalysts. However, in developing and densely populated countries, it is very difficult to separate waste with various types of components into single material due to the large amount and contamination. Therefore, this study used the integrated two-step pyrolysis (ITSP) process in a double reactor on UMSW (eight types of waste), with very inadequate and restricted knowledge regarding co-pyrolysis using ITSP and UMSW.

2. Materials and methods

2.1 Materials

In this study, UMSW was obtained as the raw material from a temporary waste disposal site located in the Isola village, Bandung City, West Java, Indonesia, where glass and metal waste were manually separated. The UMSW separated from the glass and metal waste was then exposed to the open environment for a week to undergo a drying process by sunlight and then separated into the individual components, as shown in Fig. 1. The sorting process was performed on eight types of wastes, but for this study, they were grouped based on plastic, biomass, rubber, textile, and paper wastes. Subsequently, plastic waste consisted of HDPE, LDPE, PP, and polyethylene terephthalate (PET), each with a weight percentage of 13%, and biomass waste consisted of branches, twigs, and leaves, with a weight percentage of 34%. Paper waste consisted of printing paper, cardboard, and bookbinding, with a weight percentage of 9%. Rubber waste consisted of sandals and motorcycle inner tubes, with a weight percentage of 3%. Meanwhile, textile waste consisted of used clothes and scraps from tailors, with a weight percentage of 2%. To obtain UMSW with sizes (3–5 cm) proportional to the ITSP reactor, the UMSW was mechanically chopped using a diesel-powered machine and then stored in containers to prepare for testing.

In this two-stage co-pyrolysis process, three types of natural catalysts were used: kaolin, dolomite, and zeolite, purchased directly from local traders (online trading: Lazada) with the price of 0.4, 0.2, and 0.3 USD/kg, respectively. These three catalysts were then activated by heating in an oven at 150°C for 3 hours. The activation aimed to reduce the moisture content and increase the surface area and pores [41]. For study purposes, the catalysts were manually formed into 20- to 25-mm balls. The three catalysts prepared were then tested using scanning electron microscopy (Hitachi Scanning Electron Microscope SU3500) and X-ray fluorescence spectrometry (EDAX Orbis Micro-XRF Spectrometer) to observe the surface characteristics and composition.

2.2 Pyrolysis experiments and product analysis

In this study, the experiment was carried out using an improvised reactor, in which a main reactor was equipped with an ITSP double reactor (secondary reactor) attached to the inner wall of main reactor and can be dismantled [42]. The main reactor was made of a stainless-steel cylinder material with an inner diameter of 150 mm, a thickness of 10 mm, and a height of 470 mm. The secondary reactor was also made of the same material with an inner diameter of 26.6 mm and a height of 410 mm. To ensure oxygen-free conditions, nitrogen gas was introduced into the reactor before the experiment. In this study, an electric furnace heater with a heating rate of 7°C–8°C/minute was used as a reactor heater to achieve a maximum reactor temperature of 800°C (Fig. 2). In the experiment, 500 g of UMSW was placed in the main reactor and 500 g of spherical catalysts was placed in the ITSP cabin. Furthermore, the electric furnace heater was turned on by pressing the switch button of the heating equipment and setting the desired temperature [43]. From the initial conditions, the pyrolysis process took place at atmospheric pressure by opening the flow valve to the condenser, and when the experimental temperature was reached, the valve was closed and held for the specified reaction time. When the reaction time was reached, the valve on the pyrolysis reactor was opened to allow the pyrolytic vapor to flow into the second reactor for the catalytic cracking process. Subsequently, the catalytic gas flowed into the first condenser at a temperature between 25°C and 30°C (water tank) and then into the secondary condenser at a temperature between 5°C and 10°C (a refrigerator). The condense liquid (LF) was collected in a sample bottle, and syngas was collected in a Tedlar gas sampling bag.

The percentage mass yield for each co-pyrolysis product was calculated using Equations (1) and (2), where the weight percent of LF and char products was obtained by dividing the mass of each pyrolytic product by the mass of the feedstock and then multiplying by 100. While the percentage of syngas mass was calculated using the law of mass equilibrium [44],

Furthermore, both samples were sent to the laboratory for the analysis of chemical composition and physical properties. During this time, the remaining material was collected for the mass balance calculation of the product. In this study, two-stage and single-stage pyrolysis processes were performed with and without a catalyst at 550°C for 210 minutes. The same procedure and equipment were used for the single-stage pyrolysis test without a catalyst. The composition of LF was analyzed using gas chromatography (GC, Shimadzu 2010 Plus) connected to a mass spectrometer (MS, QP 2010 Ultra). The syngas was equipped with a Rxi-5ms column (30 m × 0.25 mm × 0.5 m), and helium with a flow rate of 1 ml/minute was used as the carrier gas. The syngas oven was held at 70°C for 1 minute, then the temperature was ramped up at a rate of 9°C/minute to 300°C and held for 3 minutes, and a split ratio of 1:200 was used in this analysis. The mass spectrum scanning range was m/z 35–500, and the identified hydrocarbon compounds from GC–MS analysis were then matched with the Wiley 7 library based on peak area presence output [43].

The LF products were also subjected to physical property testing using ASTM standards to obtain density, viscosity, flash point, cloud point, water, and solid content values, and the results were compared to conventional fuel properties, namely diesel-48 and gasoline-88. Furthermore, viscosity (ASTM D-445) was determined using a Cannon-Fenske viscometer for transparent liquids, and density (ASTM D-4052) was measured using an Anton Paar DMA 4100 density meter. Meanwhile, the flash point (ASTM D-92) was determined using an open-cup tester, the solid content (ASTM D-4007) was tested using a conical centrifuge tube, and the calorific value (ASTM-D5865) was measured using a bomb calorimeter [40].

3. Results and discussion

3.1 Characteristics of natural catalysts

Based on Fig. 3 and Table 2, in terms of morphology and particle structure characterization, dolomite has a more regular and hexagonal shape compared to natural kaolin and zeolite catalysts. On the other hand, zeolite and kaolin have an irregular and clumped structure due to pore collapse, and this structure is possibly a mixture of hexagonal and tetrahedral shapes. This signifies that kaolin and zeolite have a smaller surface area compared to dolomite, and the surface difference affects the level of catalytic activity in the pyrolysis process and the ability in terms of selectivity of pyrolysis products [45]. Table 2 presents the chemical composition of the natural catalysts used. Based on the silica and alumina content, kaolin has the highest acidity level, followed by dolomite and zeolite, and this high acidity level represents high catalytic activity.

3.2 LF productivity

Figure 4 shows the two-stage pyrolysis products from UMSW using the ITSP reactor with three types of natural catalysts activated at 550°C for 210 minutes. Subsequently, dolomite produced the highest yield of LF, followed by kaolin and zeolite.

This indicates that dolomite catalyst has a proportional cracking ability with UMSW, while kaolin produced the highest syngas yield, proving its higher cracking ability compared to dolomite and zeolite. The low Si/Al ratio (Table 2) demonstrates that the acidity level at the active site has contributed to a higher proportion on syngas [46, 47]. Compared to LF produced through co-pyrolysis without a catalyst, the use of natural catalysts—dolomite, kaolin, and zeolite—increased the amount of LF by 20.15%, 10.45%, and 3.35%, respectively. Among these three natural catalysts, zeolite has the lowest Si/Al ratio, contributing less to the formation of LF. This indicates that the use of natural zeolite-based catalysts was not suitable for mixed waste feedstock, and it is maybe more appropriate for single waste streams, such as plastics waste or biomass [48].

Moreover, according to Abnisa and Wan Daud [49], the addition of specific catalysts, such as zeolite or dolomite, increased the production of LF significantly when biomass and plastic waste were co-pyrolyzed. The gas-to-char ratio, on the other hand, tended to rise in the absence of the catalyst. Similar outcomes were obtained in this investigation, where catalysts based on the Si/Al ratio demonstrated an impact on raising the yield of LF and decreasing the yield of char. In contrast to catalysts containing Al, such kaolin, the catalysts utilized in this study, such as zeolite, appeared to have a less substantial impact on enhancing the LF yield. This may be because the acidity of the catalyst plays an important role in supporting the cracking of large molecules into smaller compounds, thereby increasing the LF yield. This suggests that catalysts containing Al₂O₃ are more effective in improving the cracking efficiency and reducing the char yield [49]. The same result was also reported by Hakeem et al. [50], who showed an increase in LF productivity using kaolin and a decrease in LF yield when using natural zeolite [51].

Based on the experimental data and graphical analysis, it is evident that the contribution of zeolites to conversion is not significant, with char yield being almost the same as without a catalyst. This indicates that the role of Si (which is abundant in zeolites) in enhancing cracking efficiency is quite limited. Zeolites with a high Si/Al ratio are known to possess low acidity, making these catalysts less effective in supporting cracking reactions. Cracking of large hydrocarbons is highly dependent on the acidity of the catalyst, which plays a crucial role in breaking down larger molecules into smaller compounds. Furthermore, elements such as Mg and Ca found in dolomite do not show a significant contribution to increasing cracking efficiency. Although dolomite is often used as a basic catalyst, it tends to be more effective in pyrolysis processes but less optimal for cracking heavy hydrocarbons. This could be due to the basic nature of dolomite, which favors pyrolysis but is less effective for cracking reactions that require acidic catalysts [52, 53].

By contrast, the combination of Si and Al in kaolin appears to be more effective compared to other natural catalysts. This may be attributed to the presence of Al₂O₃ (alumina) in kaolin, which is known to have stronger acidic active sites. Al plays a critical role in increasing the acidity of the catalyst, directly contributing to improved cracking efficiency. Alumina-based catalysts are known to exhibit better cracking activity than catalysts dominated by Si or dolomite. Several studies have shown that zeolites with a high Si/Al ratio are more suitable for adsorption rather than cracking due to their low acidity. Other research has also demonstrated that Al₂O₃-based catalysts are more active and stable in cracking reactions [54, 55].

When comparing the LF yield from single-stage catalytic pyrolysis (uniformly blended UMSW–catalyst) or direct catalytic (DC) pyrolysis with non-catalytic (NC) pyrolysis, an increase of 4.5% in LF yield was observed. The presence of a catalyst enhances the cracking ability of the feedstock. However, the increase in mass yield remains relatively small. This can be attributed to the uniform blending process between UMSW and the catalyst, which may hinder the catalyst’s reactivity [56]. This indicates that the layered placement of UMSW and the catalyst needs further investigation. Similar results have been reported by several researchers, who observed higher LF yields in catalytic pyrolysis [57]. Additionally, the presence of catalysts such as zeolite has been shown to produce bio-oil with higher concentrations of valuable hydrocarbons, making the process more efficient and the end product more commercially viable [58]. Figure 4 also shows that the remaining amount of product was still relatively high. The high residue level can be attributed to the temperature or insufficient reaction time where the cracking process has not been completed [59]. Other factors may include unwashed UMSW left in the reactor. Overall, the presence of natural catalysts in the pyrolysis process of UMSW has not significantly increased the yield of LF products. The contaminated nature of UMSW was found to be the reason for reduced catalytic activity towards UMSW. Therefore, there was a need to give more attention to investigating the washed UMSW. Nevertheless, the LF results achieved are satisfactory in terms of using natural catalysts to support the co-pyrolysis process. Special attention should be given to the use of the ITSP reactor in this study, where the placement of an integrated bed catalyst in the main reactor or thermal degradation reactor has improved the efficiency of the co-pyrolysis process by using only one heat source to operate. However, the temperature of the secondary reactor cannot be changed.

3.3 LF selectivity

Selective activity is the ability of a catalyst to produce uniform hydrocarbon products during co-pyrolysis. Figure 5 shows the effect of different types of natural catalysts on the distribution of paraffin, isoparaffin, olefin, naphthene, and aromatics (PIONA) and hydrocarbon compounds in LF. The distribution of hydrocarbon compounds can be classified into three categories of LFs, namely gasoline (C₅–C₁₂), diesel (C₁₃–C₂₀), and heavy oil (C>20) [60]. The presence of natural catalysts in the co-pyrolysis process of UMSW has increased the light hydrocarbon fraction in LF compared with the process without catalyst. This indicates that the natural catalysts placed in the ITSP reactor have improved the secondary cracking process and broken down the long hydrocarbon chains into shorter ones. The C₅–C₁₂ fraction has increased from 64.5% peak area (without catalyst) to 78.0%, 77.1%, and 69.6% peak area, respectively, for kaolin, zeolite, and dolomite (Fig. 6).

Kaolin and zeolite show the best performance compared to dolomite, as indicated by the amount of C₅–C₁₂ hydrocarbon fraction present in LF, and this shows that kaolin and zeolite have better selectivity than dolomite. The less uniform pore structure of dolomite can be considered a reason for its lower selectivity as a catalyst [61, 62]. This can be demonstrated by the lower yield of aromatic hydrocarbon compounds obtained when using natural dolomite as a catalyst. However, the presence of natural catalysts in the co-pyrolysis process in the ITSP reactor reduced the concentration of aromatic hydrocarbon compounds in LF, although these compounds were still dominant. The reduction of aromatic compounds affects the calorific value, and although the change is not significant, in large quantities, this factor needs to be considered. The decrease in aromatic compounds in LF can affect its calorific value. Aromatic hydrocarbons typically have higher energy content due to their stable ring structures and high hydrogen-to-carbon ratios. Thus, a reduction in aromatic content can lead to a lower calorific value of the bio-oil, potentially making it less efficient as a fuel [63, 64].

Furthermore, dolomite has the most significant role in reducing aromatic compounds, followed by zeolite. The reduction of aromatic compounds is caused by the secondary cracking effect in the ITSP reactor, which breaks down long hydrocarbon chains and causes a shift reaction of aromatic compounds into olefin and paraffin compounds. The high variable temperature bed catalyst is the main contributor to the reduction of aromatic compounds. As reported by Jing et al. [65] and Saha et al. [66], the temperature and reaction time reduce the olefin hydrocarbon compounds and increase the amount of lower chain-length hydrocarbon compounds.

Unfortunately, LF produced from UMSW using the ITSP reactor contains undesirable hydrocarbon compounds, such as oxygenated and acidic compounds (Fig. 7). These compounds affect the calorific value because oxygen is an element that does not have energy. Consequently, the higher the fraction of oxygenated compounds in LF, the greater the decrease in its calorific value. Zeolite produced LF with the lowest level oxygenated compounds, while dolomite and kaolin produced LF with slightly higher levels of oxygenated compounds. The formation of oxygenated compounds cannot be avoided because they are contributed by biomass and PET waste components [67, 68].

3.4 Physical properties of LF

Several characteristics are used to describe the quality of LFs, including viscosity, density, flash point, calorific value, and several other criteria. The viscosity represents the fluidity of the fuel and is related to the frictional resistance that occurs, and it is very important as it relates to the fuel’s ability to be pumped into the combustion chamber. Density describes the ratio of mass to volume, representing the fuel’s phase. Flash point is the temperature at which the vapor above the LF will ignite quickly (flash/explosion) when exposed to flame, while fire point is the temperature at which the vapor above the LF will continuously ignite when exposed to a flame.

The calorific value represents the amount of heat generated during the combustion process of a certain amount of fuel. The calorific value of oil-based fuels generally ranges from 42.5 to 46.0 MJ/kg [69, 70]. Table 3 shows that the viscosity and density of LF produced using natural catalysts are comparable to those of conventional diesel fuels. Meanwhile, the calorific values presented in Table 3 are very compatible with the calorific values of gasoline-88 and diesel-48 and are in accordance with the reported results [37]. LF produced using the ITSP reactor with kaolin as a natural catalyst has higher physical properties compared to other catalysts. This indicates that LF from UMSW can be used as a substitute for conventional fuels. Therefore, energy and environmental crises can be avoided. However, the high solid content in LF produced through the dolomite catalyst has resulted in high LF viscosity.

Additionally, Table 3 shows that the LF content and solid content of the samples that were put through several physical property tests ranged from 54 to 57 weight percent and 12 to 24 weight percent, respectively. This suggests that in the presence of the catalyst, some of the LF product was transformed into syngas, which resulted in a lower LF yield compared to the process without the catalyst. It is quite feasible to increase the pyrolysis yield by lowering the reaction time and temperature during the process. The impurities in the UMSW and the potential for catalyst particles to remain in the pyrolytic vapor were responsible for the high solid content in the pyrolysis results.

4. Conclusions

Pyrolysis technology showed significant potential in recovering liquid oil from UMSW through thermal and catalytic degradation processes. Catalytic pyrolysis has advantages over thermal degradation in terms of energy consumption, reaction time, and oil yield. The selection of catalysts plays a dominant role in determining reaction temperature, time, and product distribution. Pyrolysis of UMSW with ITSP reactors and natural catalysts has proven to be a successful method for converting it into valuable products. Thus, UMSW can be relied upon for biofuel production, given its abundant and continuous supply, indicating a solution for sustainable MSW management. Among the catalysts, dolomite was the most effective catalyst for LF production, with kaolin and zeolite being second and third, respectively. Conversely, kaolin yields the highest selectivity for gasoline fractions rich in aromatic compounds. On the other hand, zeolite tends to decrease the mass yield of LF and increase the yield of syngas product. The presence of the natural catalyst did not affect LF with HHV. The overall results indicate that natural catalysts can be considered for the catalytic cracking process of UMSW, and the natural catalysts provide comparable results to synthetic catalysts. Additionally, although the presence of catalysts still results in relatively low yield improvements, the use of natural catalysts offers substantial economic advantages. These include low cost (USD 0.4/kg and USD 0.03/kg for kaolin and dolomite, respectively), environmental friendliness, easy availability, reusability, and large abundance. Hence, to improve the quality and quantity of pyrolysis products, however, it is important to pay close attention to optimizing process parameters, catalyst particle size, and experiments with washed UMSW.

Author contributions

Indra Mamad Gandidi (Conceptualization [equal], Methodology [equal], Software [equal]), Yusep Sukrawan (Data curation [equal], Writing—original draft [equal]), Iwa Kuntadi (Investigation [equal], Visualization [equal]), Nugroho Agung Pambudi (Supervision [equal]), and Arinal Hamni (Software [equal], Validation [equal], Writing—review & editing [equal])

Conflict of interest statement

None declared.

Funding

This study was supported by Universitas Pendidikan Indonesia and the Ministry of Education, Culture, Research, and Technology.

Data availability

The data supporting the findings of this research are available from the corresponding author, who is the lead researcher and responsible party for the study, upon reasonable request. The authors acknowledge the importance of transparency and reproducibility in research and are committed to sharing the study data with the scientific community. Researchers interested in accessing the data are encouraged to contact the corresponding author to discuss the details and terms of data sharing.

References