Introduction

The escalating demand for sustainable energy sources coupled with the environmental consequences of fossil fuels has accelerated the global pursuit of renewable resources1. Biomass pyrolysis, a process that converts renewable biomass into bio-oil and intermediate chemicals, has emerged as a promising avenue for sustainable energy production2,3,4. Pyrolysis, a thermal decomposition process conducted in oxygen-limited conditions, offers significant economic and environmental benefits by recovering valuable products and ~ 70% of the energy content5,6. Upgraded bio-oils hold promise as second-generation biofuels for transportation and stationary applications and a source of oxygen-containing chemicals7. Commercial-scale bio-oil production for district heating applications has already been established in countries like Finland and the Netherlands8. While bio-oil presents a promising alternative, its utilization is hindered by the presence of macromolecular aromatic compounds, particularly polycyclic aromatic hydrocarbons (PAHs), which pose a significant risk to the ecological environment9,10. PAHs are known for their toxicity, carcinogenicity, mutagenicity, and teratogenicity5,11. Several PAH compounds, including benz[a]anthracene, benzo[a]pyrene, and dibenz[a, h]anthracene, are classified as probable human carcinogens by the International Agency for Research on Cancer (IARC). Benz[a]pyrene (BaP), in particular, is a widely used toxicity indicator due to its potent carcinogenic effects and environmental prevalence. The presence of PAHs in liquid fuels is a significant concern due to their potential emissions and pollution. These compounds are considered priority pollutants by the US Environmental Protection Agency (USEPA) and the European Environment Agency (EEA)12,13. Catalytic pyrolysis, which involves using catalysts to enhance the process, offers a potential solution for improving bio-oil quality and reducing PAH formation14,15.

Previous studies have demonstrated the techno-economic feasibility of fast pyrolysis with inexpensive catalysts. Optimal bio-oil yields are typically achieved at temperatures between 400 and 600 °C across various reactor types16. A wide range of catalysts, including zeolites (ZSM-5, Na-ZSM-5, H-ZSM-5), mesoporous materials (MCM-41, SBA-15, Al-MCM-48), acids, bases, and inorganic salt compounds (NaCl, KCl, K2CO3, MgCl2, NaOH, KOH, FeCl3, ZnCl2), metal oxides (Al2O3, CaO, Fe2O3, TiO2, ZnO), and others, have been investigated to enhance bio-oil characteristics17. Yanjun Hu performed pyrolysis on sewage sludge utilizing a tubular reactor at (850 °C) and reduced (450 °C) temperatures, employing catalysts such as CaO, Na2CO3, and Fe2O3. The findings indicated that while non-catalyzed pyrolysis resulted in increased PAH concentrations with temperature, catalysts significantly decreased PAH levels in bio-oil samples at higher temperatures. This reduction indicates that catalysts significantly reduce harmful compound formation in pyrolysis9. Günay Özbay’s study further emphasized the role of various catalyst types in mitigating PAH formation, particularly when using ZnO as well as differing catalyst loadings (5, 10, 15, and 20 wt%) on the yields of pyrolysis products and the characteristics of bio-oil17. Zhao analyzed the formation characteristics of polycyclic aromatic hydrocarbons (PAHs) during the pyrolysis of bituminous coal and biomass and found that PAHs content was higher in coal than biomass. The results indicate that PAHs emissions are influenced by temperature and that increasing oxygen content during pyrolysis reduces PAHs emissions18.

Chao Li’s study showed that oxygen during the catalytic pyrolysis of lignin generally affects polycyclic aromatic hydrocarbons (PAHs), but this effect depends on the type of PAHs and the reaction conditions. PAHs with simpler structures and side groups, such as toluene, xylene, and other aromatic hydrocarbons or phenolics with side chains, were more susceptible to oxidation19.

Vanadium pentoxide (V2O5) has emerged as a significant catalyst in the catalytic pyrolysis of biomass, presenting promising opportunities for producing high-quality bio-oils and valuable chemicals.

Kantarelis investigated the influence of silica-supported nickel and vanadium on the yields and composition of products derived from the steam pyrolysis of biomass. The findings indicate that catalysts containing vanadia are crucial in facilitating selective deoxygenation reactions, thereby enhancing the formation of lighter liquid products characterized by decreased oxygen content throughout the pyrolysis procedure20.

By transforming renewable biomass into economically valuable bio-oil and chemicals, V2O5-based catalysts significantly aid in diminishing reliance on fossil fuels and alleviating greenhouse gas emissions21,22. The economic viability of V2O5 catalysts is contingent upon various factors, including the cost of the catalyst, its stability, and the feasibility of regeneration processes. Although V2O5 is typically more cost-effective than noble metal catalysts, the advancement of economically efficient synthesis and regeneration techniques is imperative for their implementation on a large scale22,23.

Therefore, considering the factors mentioned above, as well as the affordability and availability of bagasse and vanadium pentoxide catalysts, this research investigates the impact of the metal oxide catalyst V2O5 and catalyst loading on the production of 16 priority pollutant PAHs in bio-oil during large-scale biomass pyrolysis in a catalytic auger reactor.

Materials and methods

Biomass and catalysts

Sugarcane bagasse was procured from the Karun Agro-industry in Khuzestan Province, Iran, which produces 600 K tons of bagasse annually as agricultural waste. The preliminary moisture content of bagasse was determined to be 55%. The bagasse underwent an initial air-drying process conducted under ambient environmental conditions for three days, succeeded by an oven-drying phase at a temperature of 60 °C for 8 h, ultimately achieving a moisture content of 8%. Subsequently, the dried bagasse was ground using a hammer mill and sieved to obtain particles within the 1–4 mm size range, resulting in a final moisture content of 4.9% ± 0.1% by weight. Vanadium pentoxide (V2O5) powder was acquired from Farayand Sabz Company. Its purity, as determined by X-ray fluorescence (XRF) analysis according to ASTM E 1621-13, was found to be 91.9 wt%24. The catalyst had a pore volume of 0.08 cm³/g and a pore size of 3.5 nm.

Pyrolysis experiments

Figure 1 illustrates a large-scale auger reactor system designed for bagasse pyrolysis. The setup includes an electric motor, auger reactor chamber, hopper screw feeder, an electrical furnace, a condenser, bio-oil chamber, and biochar collection chamber.

Fig. 1
figure 1

Large-scale reactor designed for the catalytic pyrolysis of bagasse (a). Schematic of pyrolysis process (b).

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The reactor chamber, constructed of stainless steel (ASTM A-316), houses a screw conveyor (diameter: 7.5 cm, pitch: 5 cm) for transporting the solid fuel. The reaction tube, measuring a total length of 150 cm, is heated by electrical bands along a 110 cm segment. Downstream of the furnace are two separate containers for collecting the pyrolysis products. Biochar, moved by the rotating screw, is gathered in one container, while bio-oil is stored in another. A continuous flow of nitrogen gas at a rate of 2.5 L/min maintains an inert atmosphere within the reactor chamber during pyrolysis. The vapor phase exits through the upper outlet and passes through water-cooled condensers (4–8 °C). Bio-oil is gathered while the non-condensable gases are released25.

The pyrolysis process was conducted in a controlled reactor, where the furnace temperatures were maintained at 400, 500, and 600 °C. The heating method utilized electrical elements to achieve and sustain these temperatures, ensuring uniform heat distribution throughout the reactor. Temperature monitoring was performed using three thermocouples placed within the reactor to provide accurate readings. Before feeding the bagasse feedstock, the reactor was heated to the target temperatures of 400, 500, and 600 °C. Once the desired temperature was reached, the bagasse was fed into the reactor at a 400 g/h fixed rate. The residence time of the biomass within the reactor was set at 120 s. Various catalyst-bagasse ratios, including 0:100, 10:90, 20:80, and 30:70% w/w, were investigated to assess their impact on the pyrolysis outcomes.

Preparation of pyrolytic oil sample

The pyrolytic oil samples collected from the condenser were fully soluble in organic solvents, including acetone and acetonitrile. Before processing, the samples were treated with ultrasonic waves to ensure homogeneity. Subsequently, each sample’s appropriate volume (250–500 µL) was transferred to a 10 mL volumetric flask, and acetone was added to reach the desired volume. The vials were then placed in an ultrasonic bath, and finally, 1.0 µL of the sample was injected into the Gas Chromatography-Flame Ionization Detector (GC-FID) instrument.

Gas chromatography condition

The separation and quantification of 16 US EPA PAHs were performed using a Varian CP-3800 gas chromatograph equipped with a GC-FID and a CP-Sil8 fused-silica capillary column (30 m × 0.32 mm, with a film thickness of 0.25 μm). The following temperature program was used:

  • Initial temperature: 120 °C for 3 min.

  • Ramp: 5 °C/min to 260 °C.

  • Hold: 1 min at 260 °C.

  • Ramp: 20 °C/min to 300 °C.

  • Hold: 5 min at 300 °C.

Ultrapure nitrogen gas was utilized as the carrier gas, maintained at a pressure of 6.0 psi. The injector temperature was set at 280 °C (with a split ratio of 5), and the FID temperature was adjusted to 300 °C. Each sample was injected into the GC, and the average peak areas were employed for the quantification of each analyte.

Results and discussion

PAHs formation in biomass pyrolysis

Polycyclic aromatic hydrocarbons (PAHs) are significant byproducts of the pyrolysis process, formed through intricate reactions. This analysis examines the formation of PAHs at three critical pyrolysis temperatures: 400 °C, 500 °C, and 600 °C with varying concentrations of the catalyst vanadium pentoxide (V2O5). Each temperature regime exhibits distinct reaction dynamics and yields characteristic PAH profiles. Table 1 presents the concentrations of 16 PAHs at different temperatures and V2O5 percentages.

Table 1 PAHs concentration (mg/L) at different ration of catalyst and temperatures.
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As shown in Table 1, low molecular weight (LMW) PAHs, including naphthalene (Nap), acenaphthylene (AcPy), acenaphthene (Acp), fluorene (Flu), and phenanthrene (PA), were the predominant PAHs in all samples. Conversely, high molecular weight (HMW) PAHs consisting of five or six aromatic rings such as benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(a)pyrene (BaP), and dibenz(a, h)anthracene (DBA), were not detected in most bio-crude samples, likely due to their concentrations were lower than the limit of detection (Nd). This observation aligns with previous studies on PAH analysis in bio-oil derived from biomass pyrolysis12,26.

Effect of temperature on PAHs

At 400 °C, pyrolysis initiates the decomposition of bagasse, a process slightly facilitated by adding V2O5. Without the catalyst, the total PAH concentration reaches 3009 mg/L, primarily consisting of acenaphthene (1124 mg/L) and fluorene (973 mg/L). Introducing V2O5 at concentrations of 10–30% (w/w) reduces the total PAH yield to 2360–2532 mg/L, likely due to the cracking reaction becoming the major reaction in the chamber compared to recombination of the pyrolysis products. However, at 30% V2O5, the PAH concentration increases back to 2532 mg/L, suggesting that while V2O5 promotes some organic breakdown at this temperature, it is not efficient in reducing PAH formation. At this stage of pyrolysis, the primary effect of the catalyst is suppressed, possibly due to a series of recombination reactions of hydrocarbon fragments27.

At the moderate temperature (500 °C) the substrate (mainly lignin) breaks down and produces macro-molecules such as PAHs. In the absence of V2O5, the PAHs concentration increases to 5342 mg/L, emphasizing the enhanced thermal energy’s role in promoting cyclization and ring growth processes28. The addition of V2O5 significantly impacts the PAHs yield, especially at 20% concentration, where the total PAHs content decreases to 2923 mg/L. This notable reduction illustrates the catalyst’s high efficacy at this temperature and catalyst rate, which indicates suitable conditions at this temperature.

At the highest temperature studied (600 °C), pyrolysis reaches its peak regarding thermal decomposition potential. The total PAHs concentration without V2O5 reaches 5405 mg/L, reflecting significant molecular breakdown and cyclization. The catalytic effect of V2O5 is evident, though more complex. At 10% V2O5, PAHs yield decreases to 4202 mg/L, indicating that the presence of V2O5 as a catalyst significantly enhances the cracking of PAHs in bio-oil. This concentration provides enough active sites for the catalytic reactions without overwhelming the system. A balanced catalyst concentration enables efficient bond cleavage in larger PAHs, facilitating their breakdown into smaller fragments. Higher catalyst concentrations may lead to an enhanced aggregation of V2O5 entities, thereby obstructing accessibility to reactive sites and disrupting the equilibrium within the catalytic framework29. Denison reported the mechanism involves the adsorption of PAHs onto the catalyst surface, followed by electron transfer from the aromatic ring to the metal center, forming aromatic radicals. These radicals initiate a cascade of reactions that convert PAHs into less harmful products30. However, a 30% V2O5 concentration shows a total PAHs content of 6326 mg/L, which, while substantial, is lower than at 500 °C. This suggests that secondary decomposition processes begin to dominate at such high temperatures. Larger PAHs may break down into smaller or non-PAH compounds, reducing the yield. Therefore, while V2O5 still promotes PAH formation, the extreme thermal conditions cause a balance of both PAH formation and further degradation9.

As the temperature increases from 400 to 600 °C (without catalyst), the concentration of PAHs in bio-oil rises from 3009 to 5405 mg/L, respectively. This trend aligns with previous studies31. Figure 2 illustrates the comparison of pyrolysis temperatures and V2O5 rates on PAH formation.

Fig. 2
figure 2

Trend of total PAHs in metal oxide catalytic pyrolysis of bagasse.

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This analysis highlights the importance of temperature and catalyst concentration in pyrolysis. The varying yields of PAHs at high and low temperatures may be attributed to different intermediate products within the system9. At 0% V₂O₅, the pyrolysis process is primarily temperature-dependent. In this case, the production of PAHs is likely to increase, as it relies entirely on breaking chemical bonds in the feedstock without any catalytic assistance. When the V2O3 concentration is raised to 10%, a reduction in PAHs production occurs. This indicates that even a small amount of catalyst positively influences reaction facilitation and improves bio-oil quality. This stage can be viewed as the initial point for enhanced catalytic activity. At 20% V₂O₅, a notable decrease in PAHs production is observed. This concentration seems more suitable, as the catalyst participates more effectively in the reactions. Also, the high concentration of PAHs is attributed to the higher cellulose content in bagasse compared to other biomass sources26.

Effect of V2O5 on PAHs

The results indicated that both temperature and catalyst rate are significant parameters affecting the yield of PAHs.The distribution of polycyclic aromatic hydrocarbons produced from both catalytic (with V2O5) and thermal (non-catalytic) pyrolysis of bagasse is illustrated in Table 1; Fig. 2. The presence of V2O5 especially, at ratios 10 and 20%, can significantly enhance the degradation of PAHs during the pyrolysis of bagasse, leading to a reduction in their formation and improving the overall efficiency of the process. Vanadium pentoxide (V2O5) plays a pivotal role in catalytic pyrolysis, influencing the complex mechanisms involved in PAH formation. This analysis delves into the detailed mechanisms at various V2O5 concentrations, providing insights into how the catalyst modulates PAH formation at different pyrolysis temperatures. Relevant experimental data supports the theoretical mechanisms proposed.

V2O5 powder acts as an active catalyst by providing sites for the initial adsorption and decomposition of organic molecules within bagasse. At lower temperatures (e.g., 400 °C), the catalyst undergoes an activation phase, where organic substrates adsorb onto the V2O5 surfaces. The minimum amount of ∑PAHs is formed at 400 °C and 20% V2O5 catalyst9.

V2O5 also facilitates hydrogen abstraction from organic molecules. At temperature of 500 °C and higher V2O5 concentrations (30%), the catalyst becomes highly effective in removing hydrogen atoms, creating highly reactive sites on the molecule. These sites then react more readily with other hydrocarbons or radicals in the system, forming PAHs. The hydrogen abstraction mechanism is central to the growth of aromatic structures, as it drives the formation of reactive intermediates necessary for cyclization. This is evidenced by the substantial increase in total PAH concentration to 6498 mg/L at 500 °C with 30% V2O5 28.

Secondary reactions and Over-Cracking

At 600 °C, while the V2O5 catalyst continues to promote the Hydrogen Abstraction-Acetylene Addition (HACA) mechanism and other PAH formation pathways, the high thermal energy introduces secondary decomposition processes. Over-cracking becomes more prevalent, particularly at high V2O5 concentrations (30%). In these conditions, the generated PAHs can break down into smaller, non-aromatic hydrocarbons or gases, reducing the net yield of detectable PAHs. Excessive V2O5 facilitates these reactions, wherein the highly active catalyst and thermal energy combine to degrade further larger PAHs rather than simply promoting their formation. This is supported by the experimental data showing a decrease in total PAH concentration from 6498 mg/L at 500 °C to 6326 mg/L at 600 °C with 30% V2O5, indicating secondary decomposition. Naphthalene (2158 mg/l) and Fluorene (1533 mg/l) had the greatest effect in increasing the amount of ∑16PAHs at 600 °C and high catalyst concentrations (30%). Also, the main decrease in total PAHs compared to the without catalyst (5405 mg/l) at this temperature occurred in the catalyst of 20%, which reduced up to 56% (2393 mg/l). Based on the results above, it is apparent that the addition of metal oxide catalysts with appropriate loading could be beneficial for controlling PAH formation. The results agreed with previous studies16,17,32.

Conclusion

The catalytic role of V2O5 in the pyrolysis of bagasse significantly influences the formation of PAHs. Adding the catalyst (10% and 20%) at temperatures of 400, 500, and 600 °C has reduced the total ∑16PAH concentration compared to non-catalytic conditions. The minimum amount of ∑PAHs is formed at 400 °C with a 20% V2O5 catalyst (2206 mg/L). Naphthalene, Acenaphthylene, Acenaphthene, Fluorene, Phenanthrene, and Anthracene play a significant role in the variation of PAHs at different temperatures and catalyst loadings. Also, at 500 °C, the combination of temperature and 20% V2O5 reaches significant efficiency in PAHs reduction. At different temperatures there are different substrates at the pyrolysis chamber to be catalyzed by V2O5. As a result, the effect of catalyst and temperature can be either in the same or opposite directions in the production or elimination of PAHs. Understanding these detailed mechanisms, supported by experimental data, provides essential insights for optimizing pyrolysis conditions for targeted PAH production and other industrial applications.