Enhancing pyrolysis efficiency of oil sludge with coconut shell insights into hydrocarbon recovery and heavy metal stabilization

Abstract

This study investigates the effect of coconut shell (CS) addition on the pyrolysis behavior of oil sludge (OS) through co-pyrolysis experiments using bottom sludge from an oil refinery in Sichuan Province, China. Changes in pyrolysis-derived oil composition, residue elemental characteristics, and heavy metal stabilization were analyzed. Thermogravimetric analysis and kinetic modeling were performed to elucidate reaction mechanisms. The results indicate that CS exerts a pronounced synergistic effect on OS pyrolysis. Specifically: (1) The yield of light oil fractions increased significantly. The C18 hydrocarbon content in the recovered oil decreased, promoting macromolecular decomposition during OS pyrolysis. (2) The concentrations of hydrogen (·H) and hydroxyl radicals (·OH) increased, enhancing the cracking of heavy aromatic hydrocarbons and saturated compounds. The carbon content and fixed carbon in coconut shell and oil sludge (OCC) residue increased by 40.10% and 487.77%, respectively, compared to OS alone. (3) Heavy metal immobilization improved markedly. The leaching concentration of Pb in the residue decreased by 68.37% with CS addition. Furthermore, at a CS: OS ratio of 1:1, the second-stage weight loss rate increased by 2.7 wt%/min, while the activation energy of the third-stage reaction decreased from 8.30 kJ/mol to 3.30 kJ/mol. These findings provide theoretical insights into heat and mass transfer mechanisms and support the engineering application of biomass-assisted pyrolysis for oil sludge treatment.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-25675-0.

Introduction

Sludge and heavy oil in oil storage tanks at refineries precipitate and accumulate at the bottom, forming etank bottom oil sludge (OS). This sludge is characterized by high solid content, elevated oil content, and considerable resource value¹. Pyrolysis utilizes high temperatures—typically ≤ 800 °C in current industry practice—to induce physical and chemical reactions such as volatilization, cracking, and condensation of hydrocarbons in OS². This method is widely applied to the resource utilization of OS, aiming to maximize the recovery of oil components while ensuring that the oil content in pyrolysis residues meets industry standards^3–5. However, traditional pyrolysis technology presents bottlenecks, including high energy consumption, low light oil yield, and a high risk of secondary pollution. Moreover, unrecovered oil and heavy metals remaining in the pyrolysis residue may cause environmental hazards.

Coconut shell, a renewable energy source, has attracted significant interest due to its capacity to generate a substantial number of free radicals (such as ·H and ·OH) under high-temperature conditions. This property accelerates the decomposition of oil components⁶, enhances oil yield, and increases the content of light fractions⁷, indicating the potential to improve OS pyrolysis efficiency^8,9. Zhao et al.¹⁰ studied the effects of several biomass materials, including rice husks, walnut shells, sawdust, and apricot shells, on OS pyrolysis. Their results showed that biomass addition mitigated non-uniform heat transfer, improved dehydration efficiency, and increased oil recovery rates. Furthermore, biomass is recognized as an optimal carburant and an effective co-pyrolysis agent with sludge, significantly enhancing the pore structure and enriching the functional groups in sludge-derived carbon^11,12. Wang et al.¹³ found that co-pyrolysis of rice husk with OS markedly increased the mesoporosity of the resulting carbon, with the specific surface area rising from 1100 to 2575 m²//g, thereby generating more oxygen-containing functional groups for adsorption^14,15.

To examine the synergistic effects of coconut shell (CS) on OS pyrolysis behavior, the pyrolysis characteristics of OS, CS, and a coconut shell and oil sludge mixture (OCC) were examined using OS sourced from a refinery in Sichuan Province. This study investigates the pyrolysis recovery of oil and petroleum hydrocarbon components, as well as elemental analysis and heavy metal leaching in the recovered oil. The effects of adding CS on the leaching concentrations of heavy metals in OS pyrolysis residue were also analyzed. Additionally, thermogravimetric (TG) analysis was used to examine pyrolysis kinetics, clarifying the influence of CS addition on the pyrolysis reaction rate of OS. The findings aim to provide a theoretical reference for the co-decomposition of biomass and OS.

Experimental materials and methods

Experimental materials

OS was obtained from a refinery in Sichuan Province. The material appeared black and emitted a strong, irritating odor. Figure 1a depicts the oil sludge prior to drying, and Fig. 1b shows the sludge after low-temperature drying at 60 °C for 72 h.

Fig. 1.

Appearance of raw materials: (a) Oil sludge; (b) low-temperature dried oil sludge.

The compositional analysis is summarized in Table 1. OS was oven-dried at low temperature for 72 h to remove moisture, after which large particles of sand and gravel were separated by screening. The dried OS was then further ground and passed through a 60-mesh sieve for subsequent use.

Table 1.

Analysis of related components of tank bottom sludge samples.

Water, oil, solid component wt%			Four-component analysis wt%
Water content	Oil condition content	Solid rate	Saturates	Aromatics	Colloid	Asphaltene
24.70	20.92	50.38	25.3 ± 0.12	32.5 ± 0.05	27.6 ± 0.13	14.6 ± 0.08

Commercially sourced CS was oven-dried at 60 °C to minimize water content and subsequently ground. After sieving through a 200-mesh, a homogeneous powder was obtained and stored for later use. The mixed raw materials and coconut shell-oily sludge SEM are shown in Fig. S1. The three components of the sample after adding coconut shell (CS) were separated and measured, and the results are shown in Table S1.The relatively high oil content indicates a high potential for resource utilization.

Main reagents and equipment

The following instruments and reagents were utilized in this study: an Electronic Analysis Balance (FA2204C, Shanghai Tianmei Balance Instrument Co., Ltd.); an Electric Blast Drying Oven (101-1AB, Tianjin Taisite Instrument Co., Ltd.); a Multi-Stage Temperature Control Tube Furnace (BIF-1200 C, Anhui Beiyike Equipment Technology Co., Ltd.); a Synchronous Thermal Analyzer (STA 449 F1, Netzsch, Germany); a Vario EL Cube Elemental Analyzer (Elementar, Germany); an X-ray Fluorescence Spectrometer (ARL PERFORM’X); and a Plasma Spectrometer (iCAP 7000, Thermo Fisher Scientific, USA).The purpose of the experimental instrument is described in Table S2.

Experimental methods

Analysis of oil, water, and solid components

The azeotropic distillation method, in accordance with ASTM D95-13 (Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation), was used to determine oil, water, and solid fractions. Organic components—including saturates, aromatics, resins, and asphaltenes—were quantified following the SY/T 5119-2008 standard for the analysis of soluble organic matter in rocks and crude oil group components.

Heavy metal speciation and leaching experiment

A 0.1 mol/L acetic acid solution (pH = 2.88) was utilized as the extractant at a solid-liquid ratio of 1:20. After soaking, the suspension was shaken at 150 rpm for 18 h at room temperature. The supernatant was filtered through a 0.22 μm membrane, acidified to 50 mL with 0.5% HNO₃, and stored at 4 °C for subsequent analysis. Heavy metal speciation w‘as characterized using the continuous BCR three-step extraction method¹⁶.

Thermogravimetric analysis

Samples of CS, OS, and OCC mixtures (CS: OS = 2:1, 1:1, 1:2; mass 8.0–10.0 mg) were analyzed at an atmosphere of N₂ with a heating rate of 10 °C/min, with a maximum temperature of 1000 °C.

Pyrolysis reaction

Samples were placed in a high-temperature-resistant porcelain boat and inserted into a tube furnace. Nitrogen (N₂) was introduced at 200 mL/min to purge the tube, creating an inert atmosphere. The temperature was increased from 20 to 500 °C or 800 °C at a heating rate of 10 °C/min, then held for 60 min.

The experimental apparatus followed the sequence: inert environment establishment, temperature-controlled pyrolysis, and multiphase product collection. The schematic diagram of the device is shown in Fig. 2. Samples were placed in a boat crucible, then heated under high-purity nitrogen (purity ≥ 99.99%) to maintain anaerobic conditions, inhibit heavy metal volatilization, and promote the release of light components. Pyrolysis vapors were rapidly condensed for efficient recovery of pyrolysis oil, and uncondensed gases were collected via a vacuum pump into a gas collection bag.

Fig. 2.

Pyrolysis collection device schematic diagram.

Pyrolysis kinetics analysis method

Kinetic parameters for the three stages of OS pyrolysis were calculated using the differential method based on TG curve. According to the Arrhenius equation, the kinetics of pyrolysis are described by Eq. (1), and the reaction rate is given by Eq. (2):

1

2

where T is the pyrolysis temperature (K), A is the pre-exponential factor (s^− 1), E is the apparent activation energy (kJ/mol), β is the heating rate (K/min), and R is the universal gas constant (8.314 × 10^− 3 kJ mol^− 1 K^− 1). The OS pyrolysis reaction was assumed to follow first-order kinetics (n = 1). Substituting Eq. (2) into Eq. (1) yields Eq. (3):

3

After integration and application of the Hancock empirical equation, Eq. (4) is obtained:

4

Analysis and discussion of the results

Effect of pyrolysis temperature and CS on the recovery of oil components from tank bottom sludge

OCC with a mass ratio of CS: OS = 1:1 were subjected to pyrolysis at 500 °C and 800 °C. The pyrolysis oil samples were designated as OS-500, OS-800, OCC-500, and OCC-800. Five hydrocarbon fractions were analyzed: kerosene (C₁₁–C₁₃), diesel oil (C₁₄–C₁₈), heavy diesel oil (C₁₉–C₂₆), lubricating oil (C₂₇–C₄₀), and residual oil (> C₄₀). The relative concentrations of these fractions are shown in Fig. 3, Figure S2 and Table S3. Increasing the pyrolysis temperature from 500 °C to 800 °C enhanced the decomposition of macromolecules in OS, increasing the yield of lighter components (C₁₈ and below) and reducing the fractions of C₁₉–C₂₆ and C₂₇ and above. In OCC, the fraction below C₁₃ increased, while the C₁₄–C₁₈ and C₁₉ and above decreased. At a fixed temperature, the addition of CS increased the production of light hydrocarbons (C₁₈ and below) and reduced the content of heavier components (C₁₉ and above) in OCC. Therefore, the incorporation of CS into OS significantly promoted the formation of light hydrocarbons.

Fig. 3.

Relative concentration distribution of recovered oil components from OS and OCC at different temperatures.

Ultimate analysis of pyrolysis residue

Proximate and ultimate analysis of Raw materials and co-pyrolysis residues

Elemental analysis results are shown in Fig. 4. OS contained 30.82 ± 0.27% carbon (C), 4.08 ± 0.01% hydrogen (H), 7.14 ± 0.02% sulfur (S), and 24.77 ± 2.12% oxygen (O). CS contained 46.9 ± 0.2% C, 4.7 ± 0.01% H, 0.1 ± 0.02% S, and 35.1 ± 2.24% O. The industrial analysis demonstrated that CS addition significantly increased the carbon content in the OS pyrolysis residue, while decreasing hydrogen, sulfur, and oxygen levels. Notably, the carbon and fixed carbon contents in the OCC residue were 40.1% and 487.77% higher, respectively, than those in OS alone. The H/C molar ratio reflects oil saturation¹⁷, whereas the O/C ratio is indicative of oxygen-containing structures related to oil stability. The H/C and O/C ratios in CS were 1.20 and 0.56 respectively, indicating the potential to generate free radicals such as hydrogen (H) and hydroxyl (OH) during pyrolysis. These free radicals facilitate the cracking of heavy aromatic hydrocarbons and saturated compounds and inhibit recombination. Therefore, CS addition enhances thermal decomposition during pyrolysis.

Fig. 4.

Proximate analysis and ultimate analysis of raw materials and co-pyrolysis residues.

Distribution of main elements on material surface before and after co-pyrolysis

Energy dispersive spectroscopy (EDS) was used to characterize the distribution of elements on the solid surface of OCC before and after pyrolysis (Fig. 5). After pyrolysis, the distributions of C, O, Fe, Al, and Ca were relatively uniform, and the combined relative content of C and O exceeded 80%. Fe content on the solid surface remained high. The pyrolysis residue showed effective fixation of metal constituents, consistent with the elemental analysis and with previously reported results¹⁸.

Fig. 5.

Distribution of main elements before and after pyrolysis. (a) EDS element distribution on the surface of OCC before pyrolysis. (b) EDS element distribution on the surface of OCC after pyrolysis.

Changes of typical heavy metals in residue

Heavy metal leaching concentrations in OS and pyrolysis residues at 800 °C (designated OSC-800 and OCC-800) were quantified according to GB 5085.3–2007 “Hazardous Waste Identification Standard—Leaching Toxicity Identification” and the US TCLP-1311 method. The results are shown in Fig. 6. Plumbum (Pb), cadmium (Cd), zinc (Zn), and nickel (Ni) were detected in the OS leachate; chromium (Cr) and copper (Cu) were not detected. Pb, Cd, and Ni leaching concentrations (7.76 mg/L, 2.22 mg/L, and 6.59 mg/L, respectively) exceeded the limits in GB 5085.3–2007. In OSC-800, Cd, Zn, and Ni concentrations decreased to 5.46 mg/L, 0.89 mg/L, and 1.98 mg/L. Elevated temperature altered the forms of some metals in the solid phase, lowering their leaching concentrations. In OCC-800, Pb, Cd, and Ni concentrations were 1.46 mg/L, 0.37 mg/L, and 0.18 mg/L, respectively; Pb content decreased by 68.37% and fell below the GB 5085.3–2007 limit. These results indicate that carbonization effectively reduces heavy metal leaching in the residue after OS pyrolysis, achieving values below regulatory limits.

Fig. 6.

Illustrates the effect of CS addition on heavy metal leaching in OS. (*a concentration limit: heavy metal threshold per GB 5085.3–2007 ‘Hazardous Waste Identification Standard Leaching Toxicity Identification.’ *b concentration limit: maximum permissible limit per US TCLP-1311. ND: below detection limit).

The different chemical forms of heavy metals (Pb, Cd, Ni, Zn) in OS were analyzed using the BCR three-step sequential extraction method: F1 (water-soluble and exchangeable, HAc extraction), F2 (reducible, NH₂OH·HCl extraction), F3 (oxidizable, H₂O₂ extraction), and F4 (residual). The mobility and environmental risk for each metal decrease in the order F1 > F2 > F3 > F4, with F4 posing no risk. The distribution of these forms in OS, OSC-800, and OCC-800 is shown in Fig. 7 and Table S4. Zn in OS was primarily in the F1 form (42%), making it readily leachable. After pyrolysis (OSC-800), Pb and Ni shifted to F2 and F3 forms; Cd and Zn showed little change. Co-pyrolysis with CS (OCC-800) led Pb, Ni, and Cd to transform into more stable forms. Pb (F1 and F2) converted fully to F3 and F4; F3 and F4 comprised 86% of Ni, and F4 comprised 45% of Cd. This stabilization was less pronounced for Zn. These findings show that CS addition markedly increased the stability of Pb, Ni, and Cd in the residue, with varying efficiencies for different metals.

Fig. 7.

BCR leaching results of OS, OSC-800 and OCC-800 (a) Pb, (b) Cd, (c) Ni, (d) Zn.

Pyrolysis process and kinetic analysis

Pyrolysis curve analysis

Pyrolysis experiments were conducted separately for CS, OS, and their mixtures (OCC). The relationships between the TG and derivative thermogravimetric (DTG) curves are presented in Fig. 8.

Fig. 8.

Pyrolysis curves of OS, CS and OCC.

As CS was dried before pyrolysis, the effect of water volatilization was negligible. Figure 8a shows that CS weight loss occurs in two main stages, as indicated by changes in the DTG slope. The first stage, from room temperature to 393.7 °C, accounts for nearly 50% weight loss, primarily due to the decomposition of hemicellulose, cellulose, and a portion of lignin, generating volatiles such as CH₄, CO₂, CO, and tar¹⁹. The second stage, above 393.7 °C, is characterized by continued carbonization of lignin, with a weight loss rate of 7.59%, mainly from residual lignin decomposition up to 500 °C. The final residue constitutes 36.58% of the original mass.

Figure 8b shows the TG and DTG curves for OS, which indicate three stages of weight loss. The first stage (room temperature to 239.5 °C) is attributed to the volatilization of free water and some light fractions, resulting in minimal weight loss. The second stage (239.5–533.4 °C) is dominated by the volatilization of bound water and light components, as well as the cracking of complex petroleum hydrocarbons, yielding lighter hydrocarbons (oil and gas). Chemical bond cleavage in this stage leads to 30.15% weight loss. The third stage (above 550 °C) is characterized by further cracking and carbonization of complex organics, resulting in a residue containing both carbon and inorganics, with a final mass of 43.66%.

Figure 8c displays TG curves for mixtures of CS and OS at mass ratios of 1:0, 1:2, 1:1, 2:1, and 0:1, highlighting differences in OS pyrolysis characteristics before and after CS addition²⁰. All ratios display similar weight loss trends divided into three stages. The mixed samples (OCC) exhibit a plateau in the first stage, and CS addition increases the onset temperature of the second stage for OS. This is attributed to the release of CO and CO₂, produced during hemicellulose decomposition at higher temperatures in CS. Compared to OS alone, the weight loss rate in this stage remains largely unchanged. Figure 8(d) demonstrates that adding CS substantially increases both the pyrolysis rate and weight loss during the second stage of OS pyrolysis. As the CS: OS ratio increases, the pyrolysis rate is notably enhanced, particularly when shifting from 1:2 to 1:1. At 693.5 °C, the residue content decreases from 60.86% to 36.89%, representing a reduction of 23.97%. This indicates that CS addition promotes OS decomposition. Furthermore, CS facilitates the cracking of heavy components in the third stage, reducing residual content from 43.66% (CS: OS = 0:1) to 30.80% (CS: OS = 1:1). OS continued to undergo pyrolysis above 800 °C after CS addition, likely due to secondary cracking or polymerization of hydrocarbons and dehydrogenation leading to ring structure formation. As temperature increases, polycyclic aromatic hydrocarbons further condense to form coke²⁰, resulting in a higher residue rate for OS compared to CS. Tar generated at lower temperatures in OS undergoes secondary pyrolysis at higher temperatures, volatilizing as small molecules. CS addition accelerates this secondary pyrolysis, releasing more volatiles²². At a CS: OS ratio of 1:1, the final residue is significantly lower than with CS: OS = 1:0, indicating a synergistic effect of CS on OS pyrolysis²³.

Pyrolysis kinetics analysis

The kinetic fitting results and parameters are summarized in Table 2. All fitting correlation coefficients (R²) exceed 0.90, indicating that the first-order reaction model accurately describes the pyrolysis of CS and OS. The kinetic analysis excluded the free water volatilization stage; the apparent activation energy (E) for each stage was obtained from the fittings.

Table 2.

Fitting of pyrolysis kinetic parameters for different CS and OS proportions.

Description of sample	Temperature range (℃)	Phase	Fitting curve	Correlation coefficient R2	E/(kJ/mol)
CS: OS = 0:1	25.3-239.5	1	y = 0.678-689.90x	0.9989	5.74
	239.5-533.4	2	y=-0.430-206.95x	0.9661	1.72
	533.4-806.8	3	y = 0.846-998.20x	0.9842	8.30
CS: OS = 1:2	25.8-239.3	1	y = 2.567-1308.47x	0.9981	10.88
	239.3-443.6	2	y=-0.216-219.42x	0.9932	1.82
	443.6-717.5	3	y = 0.457-638.80x	0.9972	5.31
CS: OS = 1:1	25.6-228.5	1	y = 3.386-1423.48x	0.9944	11.83
	228.5-398.4	2	y = 0.264-216.04x	0.9904	1.80
	398.4-670.2	3	y = 0.554-396.63x	0.9882	3.30
CS: OS = 2:1	25.7-235.7	1	y = 4.098-1652.77x	0.9906	13.74
	235.7-352.9	2	y = 0.248-197.07x	0.9985	1.64
	352.9-625.4	3	y = 0.302-222.97x	0.9908	1.85
CS: OS = 1:0	25.7-393.7	1	y = 5.372-2014.54x	0.9907	16.75
	393.7-498.4	2	y = 0.095-75.06x	0.9046	0.62

Table 2 shows that for OS (CS: OS = 0:1), the activation energy in the third stage is higher than in the second. This increase is due to the pyrolysis of heavy oil and decomposition of long-chain polymers in the third stage, which require more energy than the lighter fractions and bound water volatilization in the second stage.

For CS (CS: OS = 1:0), the activation energy in the first stage is much higher than in the second. The first stage is characterized by hemicellulose and cellulose pyrolysis, whereas the second stage is dominated by lignin decomposition. Because lignin and hemicellulose are amorphous. Cellulose has a highly ordered crystalline structure, and destroying it requires more energy. Amorphous polymers decompose more easily upon heating because they lack the ordered lattice structure of crystalline or semi-crystalline materials. This finding explains the increase in activation energy in the first stage as CS content rises in OS samples.

As the CS proportion increases (CS: OS from 0:1 to 1:1), the activation energy in the third stage declines from 8.3 kJ/mol to 3.30 kJ/mol. This decrease indicates that CS addition facilitates OS pyrolysis in the third stage²², likely due to metal oxides in CS that promote cracking of heavy fractions and complex polymers, thereby lowering the required activation energy. Together with Figs. 3, 4, 5 and 6, these results demonstrate that incorporating CS increases the pyrolysis rate of OS, reduces the activation energy for heavy component decomposition, and accelerates residual solid phase breakdown²⁴.

Conclusions and suggestions

1. For OCC with a CS: OS mass ratio of 1:1, increasing the pyrolysis temperature from 500 °C to 800 °C increases the concentration of hydrocarbon components below C₁₃ in the recovered pyrolysis oil, whereas the concentrations of C₁₄–C₁₈ and C₁₉ and above decrease. Higher temperature and CS addition facilitate the decomposition of macromolecules during OS pyrolysis, resulting in more light hydrocarbon components.

2. The H/C and O/C ratios in CS are higher than those in OS. During pyrolysis of OS with CS, the concentration of hydrogen (·H) and hydroxyl (·OH) free radicals increases. These free radicals promote the cracking of heavy aromatic and saturated compounds and suppress recombination reactions. The addition of CS substantially increases the carbon content in OS pyrolysis residue and simultaneously reduces the hydrogen, sulfur, and oxygen contents. The carbon and fixed carbon contents in OCC pyrolysis residue increased by 40.1% and 487.77%, respectively, compared with the highest content observed in OS. Moreover, the distribution of carbon, oxygen, iron, aluminum, and calcium on the solid surface after OCC pyrolysis is relatively uniform, with the relative content of carbon and oxygen exceeding 80%, and iron being relatively abundant. The solid phase products from CS and OS co-pyrolysis show effective fixation of metal components.

3. CS reduces the leaching concentrations of heavy metals in the residue following OS pyrolysis. The leaching concentrations of Pb, Cd, and Ni in OS and OCC pyrolysis residues decreased from 5.46 mg/L, 0.89 mg/L, and 1.98 mg/L to 1.46 mg/L, 0.37 mg/L, and 0.18 mg/L, respectively. The leaching concentration of Pb in OCC was 68.37% lower than in OS and fell below the standard limits specified in GB 5085.3–2007 and TCLP-1311. CS significantly enhanced the stability of Pb, Ni, and Cd in the OS residue, with variable abilities to transform different heavy metals into a stable state.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Maoren WANG (FirstAuthor, Corresponding Author): Conceptualization, Methodology, Software, Investigation, Formal Analysis, Writing-Original Draft, Data Curation; Maoqi LIAO: Data Curation, Writing-Original Draft; Xin BAI: Visualization, Investigation; Yucheng Liu: Resources, Supervision; All authors reviewed the manuscript.

Funding

Funded by China University of Petroleum (Beijing) Karamay Campus, (XQZX20240026), Jointly funded by the Karamay City Science and Technology Program Project (XQZX20240055).

Data availability

All data generated or analysed during this study are included in this published article [and its supplementary information files.

Declarations

Competing interests

The authors declare no competing interests.