3.1 Physicochemical Properties of Volcanic Ash
3.1.1 Evaluation BET for Volcanic Ash Characterisation
Before being used for the pyrolysis process, volcanic ash was first analyzed using the SAA instrument to determine the specific surface area, pore volume, and pore diameter. Two samples of inactive and activated volcanic ash (VA) were tested with a size of 100 mesh and less than 0.1 gram at a temperature of 77.35 K for 193.65 minutes and 201.283 minutes, respectively using N2 as adsorbate. Since this method is based on N2 adsorption, there are limitations associated with monolayer when applied to surface area measurements, as discussed by several authors [21]. The Brunauer–Emmett–Teller (BET method /equation) was used to compare and evaluate the determination of the surface area. The specific surface area calculation model is the BET equation based on the physical absorption theory. Table 1 shows the textural properties of the volcanic ash of Mount Tangkuban Perahu.
Table 1
Textural properties of the volcanic ash of Mount Tangkuban Perahu
Sample | Spesific surface area (m2/g) | Pore volume | Pore size |
| SAA | BET method | (mm3/g) | (nm) |
Inactive VA | 1.1188 | 1.3993 | 0.0106 | 7.2826 |
Activated VA | 3.8475 | 4.8125 | 0.0140 | 18.9600 |
In Table 1, there is a slight difference between the specific surface area measured and the specific surface area calculated using the BET method. This was possible due to the effect of the uneven distribution of nitrogen adsorbate on some of the volcanic ash pores during the test, as discussed by Ambroz et al., 2018 [21]. The surface area is expressed by the number of pores in each sample area, where the greater the value of the surface area of volcanic ash, the greater the number of pores in the unit area of volcanic ash. The larger surface area of the VA catalyst provides a large contact area between the reactant molecules and the VA catalyst. The size of the contact area will directly affect the overall catalytic process which increases the LF produced. In Table 1, it can also be seen that the surface area of activated VA is 3.8475 m2/g, greater than the inactive VA of 1.1188 m2/g. It can be interpreted that the spread of adsorbate (N2 gas) on activated VA is better than that of inactivated VA. The activation process in VA can cause the number of active sites in VA as a catalyst to be better, which means that VA will absorb more reactants and produce more products in a reaction process. Vargas et al., 2021 reported that catalytic pyrolysis using a catalyst from volcanic ash is suitable for plastic pyrolysis and tends to produce higher pyrolysis products and low carbon residues [17].
The tested total pore volume values of < 0.1 mL/g of each sample are listed in Table 1. The surface area is closely related to the pore volume of the catalyst. The larger the pore volume, the larger the surface area of the catalyst. The pore volume of activated VA catalyst is greater than the pore volume of inactive VA catalyst of 0.0140 mL/g and 0.0106 mL/g, respectively. This means that inactive VA catalyst has a smaller space capacity than activated VA catalyst. Logically, this difference is in consequence of the amount of nitrogen gas adsorbed in filling the VA pores. The more nitrogen trapped in the VA pores, the larger the total pore volume. Several researchers stated that based on its chemical properties in the activation process of immersion with HCl solution, dealumination will produce an optimal aluminum content in VA and make it more stable at high temperatures. The dealumination process also causes Al to break up to form AlCl3. Al as a more electropositive electron donor leads to Cl2- ions attracting electrons from Al. Since Al is more electropositive, Al will be more easily attacked by hydrocarbon compounds induced to a larger pore volume and affect more reactants being adsorbed [25, 26]. Kumar et al, 2007 also concluded that the detailed characterization of the catalyst indicated that the size and volume of the pores affected the catalytic activity [27].
In this study, it was found that the pore diameters of the two volcanic ash samples were in the nano order. In Table 1. Volcanic ash as a catalyst can be categorized as a mesoporous pore size (2–50 nm) according to the most frequently used IUPAC pore size classification [21, 28]. This pore diameter is closely related to the ability of the pores to absorb a gas/fluid which will affect the density and porosity values of volcanic ash as a catalyst. The large size and number of pores construct space for the fluid to flow, resulting in greater adsorption capacity. Owing to the pore diameter is also directly proportional to the pore volume, the larger the pore diameter in volcanic ash, the larger the pore volume and the greater the adsorption capacity [27, 29]. The mesoporous size of such as majority of catalysts is suitable to be utilized for long-chain hydrocarbon cracking or short-chain hydrocarbon formation. Thus the two volcanic ash samples (inactivated and activated) are worthy to be used as a catalyst, as also reported by Vargas et al., 2021 and Munoz et al., 2019 [17, 29].
The pore size distribution curves were evaluated from the adsorption-desorption isotherm using the Brunauer–Emmett–Teller (BET) method as presented in Figs. 2 and 3. Based on the IUPAC classification, the shape of BET isotherms can be characterized as type V isotherm at over relative pressure (P/Po) between 0.4-1.00 atm which indicates the formation of a multilayer. Type V isotherm indicates that the porous material exposed to nitrogen gas belongs to the mesoporous category [21, 28]. Adsorption-desorption curve of inactivated and activated VA catalysts has a visible difference in the resulting multilayer formation. Activated VA catalyst produces a clearer multilayer (further distance) compared to inactivated volcanic ash. This multilayer induces a better absorption capacity of the VA catalyst.
Referring to the results of this experiment and discussion, the use of volcanic ash as a catalyst has the opportunity for further research. Considering that volcanic ash has a light and porous structure that is able to induce high adsorption capabilities. The adsorption ability of volcanic ash with its SiO2 content can be synthesized into a zeolite catalyst for pyrolysis, as previously disclosed by previous authors [17].
3.1.2 Morphological Characterization of Volcanic Ash
Analysis of surface structure and morphology as well as compound composition of inactive and activated VA was carried out using SEM-EDS. The VA surface morphology analysis was observed from SEM, while the elemental composition was searched from the EDS diffractogram spectrum [29]. The results of the VA SEM micrographs with a magnification of 5000 times are presented in Fig. 4 and Fig. 5.
Figure 4 shows the round morphology of inactive VA, still clumped, quite regular, and relatively smaller in structure than activated VA. While Fig. 5 shows an irregular morphology and lamellar pattern with a wider structure than inactive VA. It might be the physical destruction due to HCl treatment which removed some of the crystalline impurities in the VA [30]. This indicates that the smaller the particle size, the larger the pore volume, making it more reactive and easier for a permeate compound to pass. Hence VA as a catalyst may have a good permeation rate to facilitate the adsorption of cracking hydrocarbon compounds in the catalytic pyrolysis process.
Table 2
EDS chemical data of activated volcanic ash catalyst
Element (% m/m) | Compound (% m/m) |
O | 50.58 | | |
Al | 2.95 | Al2O3 | 5.57 |
Si | 38.32 | SiO2 | 81.97 |
Ti | 1.36 | TiO2 | 2.27 |
Mo | 6.79 | MoO3 | 10.18 |
The EDS spectrum diffractogram of activated VA is presented in Fig. 6. According to the EDS spectrum with the highest peak results, the activated VA basically has a high atomic value is silicon then successively followed by oxygen, aluminium, and others. It means that a small grain of activated VA in the sample test is silicon oxide (SiO2). The silica and oxide atomic mass of the EDS spectrum were found to be 38.32% and 50.58%, respectively.
The results of the EDS spectrum analysis from activated VA samples are shown in Table 2. SiO2 and Al2O3 as important components in the selection of catalysts are listed in mass percentage. Munoz et al., 2019 reported that the SiO2/Al2O3 ratio of activated VA is between 2 and 4 and stated the volcanic ash resulted in a safe and recyclable catalyst [29]. Dealumination of VA with activation through acid treatment will increase the ratio of Si/Al. Syamsiro et al, 2014 stated that the formation of coke during catalytic degradation will decrease due to an increase in the Si/Al ratio. The stability of the crystal structure will increase with the increase of the Si/Al ratio [30]. The high content of SiO2 and Al2O3 (Si/Al ratio) in this activated VA (Table 3) makes VA suitable to be used as a better catalyst for the pyrolysis process.
3.2 Effect of activated VA catalyst on LF yield
The LF yields were obtained from the thermo reactor and catalytic reforming of styrofoam and LDPE, respectively, without further heating. The condensation of the cracking products from the outlet of the catalytic reformer was collected as LF, while the incondensable gas was allowed to escape into the ambient air. The incondensable gas is calculated based on the mass balance of the LF and the residual char collected at the complete pyrolysis time, tf as presented in Table 3 and Table 4.
Table 3
Yield of styrofoam pyrolysis products
Catalyst (g) | Temperature (oC) | Pyrolysis time (min.) | Yield (% w/w) |
LF | Char | Incondensable gas |
Without Catalyst | 0 | 345 | 170 | 52.50 | 2.66 | 44.84 |
Inactivated VA | 20 | 345 | 200 | 56.00 | 0.38 | 43.62 |
50 | 360 | 130 | 69.00 | 5.01 | 25.99 |
70 | 360 | 130 | 66.00 | 4.96 | 29.04 |
Activated VA | 20 | 345 | 140 | 59.50 | 0.11 | 40.39 |
50 | 375 | 120 | 77.00 | 1.66 | 21.34 |
70 | 355 | 120 | 26.00 | 18.80 | 55.20 |
In this work, pyrolysis and catalytic reforming of styrofoam and LDPE without the use of a catalyst were also carried out as a comparison to determine the LF yield. As can be seen in Tables 3 and 4, the presence of a VA catalyst led to an increase in the LF yield produced. It was found that the yield of LF in the pyrolysis process using an activated catalyst was greater than without a catalyst. Vargas et al., 2021 reported that a higher gas yield of 76.42 ± 4.95% was produced from catalytic pyrolysis using a synthetic catalyst from VA [17]. These results show that styrofoam pyrolysis produces relatively higher yields than LDPE in the range of 52–77% and 27–52%, respectively. This is due to the LDPE structure being composed of a long linear polymer chain, while styrofoam is made of styrene monomers, high temperatures require a decomposed long carbon chain [7, 31].
Table 4
Yield of LDPE pyrolysis products
Catalyst (g) | Temperature (oC) | Pyrolysis time (min.) | Yield (% w/w) |
LF | Char | Incondensable gas |
Without Catalyst | 0 | 345 | 170 | 27.00 | 9.75 | 63.25 |
Inactivated VA | 20 | 345 | 200 | 30.00 | 3.15 | 66.85 |
50 | 360 | 130 | 29.00 | 7.51 | 63.49 |
70 | 360 | 130 | 49.00 | 7.11 | 43.89 |
Activated VA | 20 | 345 | 140 | 30.00 | 10.68 | 59.32 |
50 | 375 | 120 | 42.00 | 5.05 | 52.95 |
70 | 355 | 120 | 52.00 | 7.20 | 40.80 |
The cracking of heavy hydrocarbons into light hydrocarbons was increased by the existence of a VA catalyst in the catalytic reformer [30]. In addition, the pyrolysis process at low to moderate temperatures produces more LF than the gaseous product. The use of a large enough condenser capacity enforces more condensation of condensable gaseous to produce more LF as also reported in previous research studies [8, 24]. The more amount of VA catalyst used during the pyrolysis operation, the higher the yield of LF produced. Surface area, pore volume, and pore diameter can affect the pyrolysis rate as described above. The smaller LF yield of styrofoam pyrolysis with an active catalyst of 70 g is the exception to the results of this study. Leakage due to improper packing door loading in the thermal reactor caused the release of some gaseous products without going through the condensation process.
3.3 Pyrolysis Liquid Fuel (LF) Characteristic
The density and moreover the viscosity of the pyrolysis liquid fuel play an important role in the evaluation of fuel performance. It is an accepted fact that the molecular size of the molecules contained in the fuel oil affects its density and viscosity. The large molecular size contained in the fuel oil results in higher viscosity, higher density, and lower volatility. Density and viscosity greatly affect the spraying process during combustion. Higher viscosity and density tend to form larger droplet sizes which in turn lead to poor atomization [8, 32, 33]. The average LF density and viscosity of styrofoam and LDPE pyrolysis were calculated from Table 5 and Table 6. The average LF density was 0.8793 kg/L and 0.7558 g/mL, respectively. Meanwhile, the average viscosity of LF is 1.6438 cP and 1.5624 cP, respectively. In this case, the use of a catalyst did not give a significant change both to the density and viscosity of LF. This could be due to the similarity in the content of aliphatic hydrocarbons (alkanes and alkenes) as major substances in LF [24]. However, the density and viscosity of the pyrolysis oil are still in the range of density and viscosity of commercial kerosene of 0.79–0.89 g/mL and 0.294–3.340 cP, respectively [31, 32]. It was found that the LF viscosity from LDPE pyrolysis had a lower value than the LF viscosity from styrofoam pyrolysis. This was possible in the presence of an activated VA that could degrade double bonds and the length of the hydrocarbon chain into lower molecular weight in addition to the more aromatics contained in LF [24]. Lower fuel viscosity is required to obtain better fuel atomization in the formation of a smaller droplet size, which in turn results in good combustion [32].
Table 5
Liquid fuel (LF) combustion properties of styrofoam pyrolysis products
Catalyst (g) | Density (g/mL) | Viscosity (cP) | HHV (MJ/kg) | Flash point (oC) | Auto ignition point (oC) |
Without Catalyst | 0 | 0.8754 | 1.5700 | 40.6245 | 36 | 215 |
Inactivated VA | 20 | 0.8828 | 1.5567 | 40.6188 | 41 | 230 |
50 | 0.8773 | 1.6440 | 40.6395 | 40 | 220 |
70 | 0.8732 | 1.6236 | 40.6328 | 34 | 230 |
Activated VA | 20 | 0.8877 | 1.6714 | 40.6452 | 35 | 230 |
50 | 0.8732 | 1.7480 | 40.6642 | 38 | 210 |
70 | 0.8856 | 1.6932 | 40.6524 | 38 | 206 |
The flash point and auto ignition point of LF are related to its density and viscosity, the lower content of hydrocarbon fraction (< C20) and higher volatile aromatic content of LF will decrease its density and viscosity. The flash point and auto ignition point of LF tends to decrease as the density and viscosity decrease [24, 30, 31]. This is in line with the results of this work, that the use of an amount of volcanic ash catalysts produces LF with relatively low density and viscosity, which in turn reduces the flash point and auto ignition point. The lowest flash point and automatic ignition point of the LF of 38 oC and 195 oC were measured from activated VA pyrolysis. The use of activated VA allows more volatile hydrocarbon compounds in the LF. Generally, classifications of liquid fuel related to flammable liquids and combustible liquids have a flash point below 37.8 oC and equal to or above 37.8 oC, respectively. Meanwhile, kerosene has a flash point of 38–52°C and an automatic ignition point (temperature) of 210°C [31].
Table 6
Liquid fuel (LF) combustion properties of LDPE pyrolysis products
Catalyst (g) | Density (g/mL) | Viscosity (cP) | HHV (MJ/kg) | Flash point (oC) | Auto ignition point (oC) |
Without Catalyst | 0 | 0.7446 | 1.5519 | 40.6446 | 46 | 225 |
Inactivated VA | 20 | 0.7488 | 1.4877 | 40.6245 | 49 | 215 |
50 | 0.7568 | 1.5514 | 40.6367 | 54 | 200 |
70 | 0.7453 | 1.5921 | 40.6528 | 40 | 197 |
Activated VA | 20 | 0.8023 | 1.5739 | 40.6248 | 37 | 225 |
50 | 0.7457 | 1.5549 | 40.6424 | 38 | 205 |
70 | 0.7473 | 1.6251 | 40.6609 | 38 | 195 |
The energy content of pyrolitic fuel can be predicted using widely used equations related to the physical properties of density and viscosity [31, 32]. A developed equation of relationships between higher heating value (HHV) and viscosity, density, or flash point was employed to the energy content of LF. It was found in this work that the HHV value of the LF is at about 40 MJ/kg with no significant deviation compared to the values of the bomb calorimeter measurement. There is no significant difference in HHV between the LF generated from pyrolysis provided with or without VA catalyst. This may be due to the similarity of hydrocarbon compounds but different chemical structures in each liquid fuel (LF) produced. This is presumably indicated by the lower thermal properties of the flash point and automatic ignition of the liquid fuel. Al-Salem et al., 2017 mention in their literature review that an energy content of 39.5 MJ/kg could be obtained from LDPE pyrolysis using a micro batch reactor with a ferromagnetic wire [7]. A higher heating value of 45 MJ/kg and a viscosity of 2.35 cP from pyrolysis of polystyrene in Microwave power of 450W and polymer to activated carbon ratio of 10:1 was reported by Pratibha et al., 2018. Conventional fuel heating values of gasoline, kerosene, and diesel of 43.4–46.5 MJ/kg, 43.0-46.2 MJ/kg, and 42.8–45.8 MJ/kg, respectively were used as comparisons [5, 7]. It can be concluded that the calorific value of liquid fuel resulting from the pyrolysis of LDPE and styrofoam with and without a VA catalyst is close to the fuel grade criteria for kerosene or diesel.
In this research, GC-MS analysis was performed on pyrolysis LF of styrofoam and LDPE with 50 g of active catalyst. There were 15 peaks and 30 peaks, respectively as the main GC-MS spectrum. The largest areas of the GC-MS pyrolysis LF spectrum analysis results are presented in Table 7 and Table 8.
Table 7
Mass Spectrum of styrofoam pyrolysis liquid fuel
Ret. Time (min.) | Peak area | Fraction, % | Formula | Compound |
3.205 | 2040670 | 7.843 | C7H8 | Metilbenzene |
3.726 | 2395174 | 9.206 | C8H10 | Ethylbenzene |
4.062 | 309868 | 1.191 | C9H12 | Isopropilbenzene |
4.841 | 18297320 | 70.323 | C8 H8 | Styrene |
5.566 | 1928073 | 7.410 | C9H10 | Isopropenylbenzene |
The GC-MS spectrum results showed the highest peaks of 5 and 6 for LF styrofoam and LDPE which contained various compounds, respectively. Styrene (aromatic) is the dominant compound in styrofoam liquid fuel (70.32%), with 1-Octene being the most plenteous compound in LDPE liquid fuel (25.83%). This is understandable, considering that styrene is the basic compound that forms styrofoam [34]. Whereas LDPE contains a side chain of ethylene monomer (C2H4) of a certain length. Therefore, the GC-MS results show that liquid oil from Pyrolysis LDPE is a complex hydrocarbon of paraffin (alkane), olefin (alkene), naphtha (cycloalkane), and aromatic class [35]. It was observed that LF from LDPE composed of many different hydrocarbons. The significant compounds include 1-octene, n-propilcyclohexane, n-Heptane, 2-propilheptane, and n-nonane. These results are in accordance with several reports of researchers, that using a VA catalyst (type of catalyst) is an important determinant of the LF chemical composition of plastic pyrolysis, although with several variations of the main hydrocarbon compounds [7, 20, 23, 24, 34, 35].
The GC-MS characterization results also represent carbon atoms from C5 to C15 as presented in Table 9. The difference in the number of carbon atoms in the LF pyrolysis liquid fuel is influenced by the presence of the cracking process by the VA activated catalyst used in a thermo-catalytic reactor. Syamsiro et al, 2014 reported that the presence of a catalyst in sequential pyrolysis and catalytic reforming increased the yield of light LF (C5-C12) depending on the catalyst used [30]. LF pyrolysis containing gasoline hydrocarbons with a higher range (C5-C12) as the highest amount of 74.63% styrene from pyrolysis with a catalytic reformer was reported in a recent study by Suhartono et al, 2023 [36].
Table 8
Mass Spectrum of LDPE pyrolysis liquid fuel
Ret. Time (min.) | Peak area | Fraction,% | Formula | Compound |
2,319 | 2499261 | 7.957 | C7H16 | n-Heptane | |
2,376 | 2070025 | 6.590 | C9H20 | n-nonane | |
2,427 | 2285605 | 7.277 | C10H22 | n-dekane/2-propilheptane | |
2,520 | 3477444 | 11.071 | C9H18 | n-propilcyclohexane | |
2,573 | 8113618 | 25.831 | C10H20 | 1-octene | |
High quality conventional hydrocarbon fuels are generally based on the number of carbon chains. Gasoline consists of a number of hydrocarbons with a chain of carbon atoms between C5 to C8 [8]. Meanwhile, the liquid fuel fraction as kerosene ranges from C9-C14 hydrocarbons [37].
Table 9
Range of carbon atoms in liquid fuels from pyrolysis Styrofoam and LDPE
Liquid Fuel | Hydrocarbons range | Composition, % |
Styrofoam | C5 - C9 | 92.49 |
C9 - C14 | 6.71 |
C15 - C17 | 0.80 |
LDPE | C5 - C9 | 38.55 |
C9 - C14 | 61.27 |
C15 - C17 | 0.18 |
Based on the composition of the results of the GC-MS analysis of the LF sample, it was observed that the fraction of LF from styrofoam pyrolysis has the largest composition is in the range of C5–C9 carbon atoms of 92.49%. Accordingly, LF styrofoam pyrolysis is on par to gasoline fuel, with higher quality compared to commercial gasoline from the perspective of the low percentage of aliphatic hydrocarbons alkanes (paraffin) and alkenes (olefins) (> C11). This is in accordance with the results of previous research studies [8, 13, 15, 24]. Whereas the LF fraction of LDPE pyrolysis was observed to have the largest composition in the range of C9–C14 carbon atoms of 61.27% as a high aliphatic proportion. These results agree with reports of some previous researchers. Suhartono et al, 2022 in their previous research reported that the LF from LDPF pyrolysis is the closest hydrocarbon composition to kerosene. 1-Tetradecanol (C14H30O) as the largest fraction with the highest percentage of hydrocarbon composition > C20 of 34.28% and C15-C19 of 19.85% [24]. The chemical composition of hydrocarbon within carbon atoms ranging from C7 to C29 was also reported by Felix and Eki, 2022. This LDPE liquid fuel consists of paraffin (27.5%) and olefin (50%) as the main substances [35].