COAs was obtained from Jinshu Ecological Camellia Professional Cooperative, Shangyou County, Ganzhou City, Jiangxi Province. HNO₃, of domestic analytical pure, was obtained from Xilong Scientific Co., Ltd. SiC spheres with a diameter of 5 mm were purchased from Xinli Wear-Resistant Materials Co., Ltd. The WVO from a fried shop was filtered and then stored in a 4-°C refrigerator with shading for further experiments. The WVO was 1.0 ml, and n-hexane 5 ml and NaOH–methanol solution (0.5M) 2.5 ml were added, and the solution was refluxed in a 70°C water bath for 10 min. Then, the solution was cooled to room temperature. Then, it was transferred to a graduated test tube, and deionized water was added to make it 20 ml for ultrasonic extraction for 10 min, and the supernatant was taken for GC–MS analysis after centrifugation. The predominant fatty acid composition of WVO is shown in Table 1.

Gas chromatography–mass spectrometry (7890B-7000D, Agilent Technologies, United States, 2017) was used to analyze the bio-oil obtained from the pyrolysis of WVO. The analytical conditions were consistent with the previous study (Wu et al., 2020). The GC–MS conditions are as follows: Column: HP-PONA (50 m × 0.200 mm × 0.50 μm); Carrier gas: helium; Vaporizer temperature: 280°C; Ion source temperature: 230°C; Injection mode: split (50:1); Target temperature: 180°C, Heating rate: 5°C/min, and Hold time: 2 min; Target temperature: 280°C, Heating rate: 8°C/min, and Hold time: 3 min.

The FTIR results of untreated COAs biochar and pretreated COAs biochar are shown in Figure 2. Untreated COAs biochar and pretreated COAs biochar show a similar peak shape. In particular, 790, 830, and 870 cm⁻¹ are caused by aromatic C–H bending. C–H around 1,419 cm⁻¹, C=C around 1,577 cm⁻¹, and the C=O bond at 2,346 cm⁻¹ may be caused by the presence of the aldehyde or ketone. The band strength at 3,446 cm⁻¹ is related to the tensile vibration of the O–H group. The increase of the liquid-to-solid ratio results in a slight increase of the group peak, and its peak value decreases with the increase of carbonization temperature (Wang et al., 2020; Zhang et al., 2021). The group signal peak in biochar after HNO₃ treatment is clearer than that in untreated biochar. However, C=C, C=O, O-H, and other groups were obviously missing when the carbonization temperature reaches 800°C; thus, high temperature promotes the decomposition of functional groups (Zhang et al., 2021). This scenario indicates that the high temperature is harmful to the groups. This result is consistent with the study of Zhang et al. (2020).

The COAs biochar has a lamellar structure and rough surface, as shown in Figure 3. This feature is caused by the COAs structure itself and the devolatilization and breakdown of biomass during carbonization (Shi, 2018; Duan et al., 2020). The increase of the liquid-to-solid ratio and carbonization temperature results in the increase of the lamellar structure and appearance of pore structure. This result is consistent with the N₂ adsorption/desorption isotherm. Acid solution has a corrosive effect. It can destroy and dissolve the cellulose and other substances in the biomass and remove some metals. The removal of metals creates new pores, which is conducive to the volatilization analysis. Moreover, the evaporation degree of volatile substances increases with the increase in carbonization temperature, resulting in the increase in surface area and pore structure formation (Nikkhah et al., 2020).

The results in Table 2 show that HNO₃ pretreatment significantly affects the specific surface area of the COAs biochar catalyst. The untreated biochar has a very low BET surface area (0.58 m²/g). However, the BET surface area increases significantly with the decrease of the solid-to-liquid ratio. Moreover, the specific surface area of the 1:3–600°C biochar is as high as 392.65 m²/g, which is approximately 677 times larger than that of the untreated biochar. Furthermore, 1:3–600°C biochar has a relatively high micropore area of 323.34 m²/g. These findings may be caused by the removal of alkali and alkaline earth metals and some inorganic substances in biomass by HNO₃ pretreatment (Wang et al., 2020). The larger the specific surface area of the biochar, the more the contact surface area and active sites can be provided in the catalytic system (Yang et al., 2020).

The effects of the solid-to-liquid ratio (M/V) (untreated, 1:1, 1:2, and 1:3) and carbonization temperature (800, 700, 600, and 500°C) on bio-oil constituents were studied. The feedstock-to-catalyst ratio was 8:1, and the catalytic temperature was kept at 450°C.

The distribution of pyrolysis products under different biochar catalyst conditions is shown in Figure 4. The main components of bio-oils were divided into five categories, as shown in Figure 4B: C₂₀₊ hydrocarbons, green diesel (C₁₃–C₂₀), green gasoline (C₁₀–C₁₂), green naphtha (C₅–C₉), and oxy-compound (Khodadadi et al., 2020). The content of green naphtha was significantly higher than that of untreated COAs biochar with the increase of the liquid-to-solid ratio. Compared with that obtained under the untreated biochar catalyzed condition, the selectivity of green naphtha in the liquid product obtained under the catalyzed condition of 1:3–600°C biochar greatly increased from 36.01 to 67.17%. The reason is that the active groups, surface structure levels, and BET surface area of biochar prepared from COAs pretreated with HNO₃ were increased, which was beneficial to the catalytic performance of biochar; this scenario enhanced catalytic cracking of decarboxylation and decarbonylation of long-chain fatty acids, alcohols, and esters in the process of catalytic pyrolysis and accelerated the Diels–Alder reaction to convert alkenes to aromatics (Duan et al., 2020). Moreover, the content of aromatics was increased, and the content of oxy-compound in bio-oil was decreased. The presence of oxy-compound in bio-oil can cause it to be chemically unstable and corrosive and reduce its calorific value (Nikkhah et al., 2020). As shown in Figure 4A, compared with those under untreated biochar condition, the content of the monocyclic aromatic hydrocarbon (MAH) in the bio-oil under the catalyzed condition of 1:3–600°C biochar increased significantly, and the selectivity of MAH increased from 46.08 to 66.86%. Moreover, the content of polycyclic aromatic hydrocarbon (PAH) was reduced, and the selectivity dropped from 37.26 to 15.90%. When the solid-to-liquid ratio was 1:2 and 1:3, the MAH content first increased and then decreased with the increase of carbonization temperature, and the PAH content first decreased and then increased. The increase of carbonization temperature resulted in the increase of the release of volatile substances in biomass, formation of the pore structure, and increase in the specific surface area. Moreover, the microporous structure inhibited the formation of high molecular weight aromatic hydrocarbons. However, the pore structure of biochar was destroyed when the carbonization temperature was excessively high, and the inhibitory effect was reduced (Zhang et al., 2021). Increasing MAH content and reducing PAH content can be considered the advantages of HNO₃ pretreatment for biochar preparation. According to the results of catalytic pyrolysis experiments, when 1:3–600°C COAs biochar was used as a catalyst, the content of MAH is the highest, and the content of PAH is the lowest, and the content of green naphtha is also the highest in terms of carbon number classification. Therefore, 1:3–600°C COAs biochar was used in the following experiments.

The solid-to-liquid ratio of the pretreatment was 1:3, the catalytic temperature was fixed at 450°C, and the carbonization temperature was 600°C to study the effects of the feedstock-to-catalyst ratio (catalyst free, 8:1, 4:1, 2:1, and 1:1) on the bio-oil composition and yield. The bio-oil yields at different feedstock-to-catalyst ratios are shown in Figure 5A. The pyrolysis of WVO without catalyst had the highest bio-oil yield (82.50%). The bio-oil yield decreased with the increase of catalyst amount. Therefore, the increase of catalyst amount provides abundant catalytic active sites, prolongs the catalytic reaction time, promotes the secondary fracture of pyrolysis steam, and forms considerable noncondensable vapor, leading to the decline of bio-oil yield from 82.50 to 41.25% (Jin et al., 2018).

The composition of bio-oil collected at different feedstock-to-catalyst ratios are shown in Figure 5B. The area percentage of oxy-compound decreased significantly from 66.00% (catalyst-free) to 2.96% (8:1). The oxy-compound in the bio-oil produced without catalysis mainly consists of acids, alcohols, esters, and ketones. The selectivity of green naphtha, green gasoline, and green diesel was all lower than 15.00%, which limits its use in industry. The small amount of oxy-compound in the bio-oil produced by the catalyst was mainly alcohols, esters, and ketones. The area percentage of acids, alcohols, and esters decreased obviously with catalysis. The contents of green naphtha and gasoline were significantly increased with the use of the catalyst. However, the selectivity of C₅–C₉ was significantly inhibited, and the selectivity of C₁₃–C₂₀, C₂₀₊ hydrocarbons, and oxy-compound increased with the increase of catalyst dosage (more than 1 g). Long-chain fatty acids, alcohols, and esters were catalyzed to decarboxylation and decarbonylation and dehydrogenation/hydrogenation to form short-chain hydrocarbons, which then formed olefins or further cyclization and aromatization to alicyclic compounds and aromatic hydrocarbons (Lam et al., 2016). As a result, the area percentage of oxy-compound and hydrocarbons decreased considerably, whereas the area percentage of PAH and MAH increased considerably. The area percentage of hydrocarbons decreased and reached the lowest of 9.76% at 4:1 with the decrease of the feedstock-to-catalyst ratio. The further decrease of the feedstock-to-catalyst ratio slightly increased the hydrocarbon content. Olefins can be converted to aromatic hydrocarbons by the Diels–Alder reaction, a kind of a cycloaddition reaction (Lam et al., 2016). The area percentage of MAH increased from 5.10% (catalyst-free) to 69.21% (4:1). The area percentage of PAH increased from 0.00% (without catalyst) to 20.30% (2:1). High catalyst usage can hinder the passage rate of volatile substances, thus prolonging the catalytic reaction time. This scenario leads to the secondary fracture reaction and repolymerization reaction. As a result, the hydrocarbon content decreased, and the contents of MAH and PAH increased (Balasundram et al., 2018). PAHs have harmful biological effects, such as toxicity, mutagenicity, carcinogenicity, and adverse environmental effects. They are also precursors of carbon deposits, which greatly affect catalyst activity, and the high oxygen content of bio-oil affects the bio-oil quality (Zhang et al., 2014; Sun et al., 2021). It is worth noting that when the feedstock-to-catalyst ratio was 8:1 to 4:1, the increase rate of MAH and total aromatics was lower than that of the catalyst (Ding et al., 2018). Therefore, it is not economically feasible to increase the aromatics content by simply increasing the amount of the catalyst. Therefore, by analyzing the yield and composition of bio-oil, the feedstock-to-catalyst ratio of 8:1 was chosen for subsequent experiments.

The effects of catalytic temperature (600, 550, 500, 450, and 400°C) on the composition and yield of bio-oil were studied. The catalyst was 1:3–600°C biochar, and the feedstock-to-catalyst ratio was 8:1. The bio-oil components and yield distributions at different catalytic temperatures are shown in Figure 6.

As shown in Figure 6A, the yield of bio-oil decreased with the increase of catalytic temperature, and the maximum yield was 50.00 wt% at 400°C. Pyrolysis is an endothermic reaction, which can be promoted by a high temperature. High catalytic temperature (the temperature in the current study exceeded 400°C) can provide much energy, promoting long-chain compound fracture and small molecule compound formation. The condensable part of the small molecule compounds after condensation formed bio-oil. However, the secondary pyrolysis of the pyrolysis vapor was promoted when the catalytic temperature was further increased, and considerable noncondensable vapor was generated, leading to the bio-oil yield decrease (Wang et al., 2018). This scenario was also the main reason for the decrease of bio-oil yield as the catalytic temperature increased from 400 to 600°C.

The results showed that when the catalytic temperature increased from 400 to 600°C, the oxy-compound content decreased from 2.63 to 0.00%. The high catalytic temperature promoted the deoxidation reaction and reduced the oxy-compound content. The area percentage of the hydrocarbon decreased from 15.15% (400°C) to 6.03% (550°C). The area percentage of the MAH increased from 66.86% (450°C) to 78.82% (600°C). The high catalytic temperature also promoted PAH formation, and the area percentage of PAH increased from 10.15% (400°C) to 16.07% (550°C); the total aromatics content was up to 93.27% when the catalytic temperature was 550°C. This occurrence was due to the increase of temperature, which caused short-chain alkene oligomerization to produce mixtures of long-chain alkenes and alkanes; aromatic hydrocarbons were produced by aromatization, alkylation, and isomerization of long-chain alkenes and alkanes (Wu et al., 2020). Therefore, high temperature promotes deoxidation and aromatization at the expense of oxy-compound and olefins, and this finding is consistent with the results of Duan et al. (2020). However, the area percentage of MAH and hydrocarbons showed an opposite trend to PAH when the catalytic temperature was further increased to 600°C, indicating that the high catalytic temperature reduced PAH production. As shown in Figure 6B, the green naphtha content increased from 67.17 to 76.32% when the catalytic temperature was increased from 450 to 600°C, and the area percentage of C₁₀₊ hydrocarbons decreased significantly. This result was attributed to the catalytic fast pyrolysis process. The high catalytic temperature promoted the cracking reaction of long-chain fractions (C₁₀₊ hydrocarbons) (Duan et al., 2020). When the catalytic temperature was 600°C, the area percentage of MAH was up to 78.82%, and the area percentage of green naphtha was 76.32%, and also the oxy-compound content was the lowest, so the best catalytic temperature was 600°C.