Copyrolysis of Waste Paint Oil and Algae Biomass to Produce High-Energy Biochar: Physicochemical Characterization and Solid-Fuel Performance

Abstract

This study investigates the copyrolysis of waste paint oil (WPO) and algae biomass as an integrated waste-to-energy pathway for producing high-energy biochar while mitigating the environmental risks associated with WPO disposal. Copyrolysis experiments were conducted in a laboratory-scale fixed-bed reactor using WPO–algae weight ratios of 1 : 0.5 to 1 : 1.5 under an inert nitrogen atmosphere (150 mL/min), with temperatures ranging from 400 to 600°C, a heating rate of 10°C/min, and a residence time of 60 min. The physicochemical properties of WPO (density 0.88–0.89 g/mL; kinematic viscosity 4.5–5.0 mm²/s) were controlled to ensure feedstock consistency. The resulting biochars exhibited markedly improved solid-fuel characteristics compared to single-feed pyrolysis. Fixed carbon content increased from 65.3 to 72.8 wt % (ASTM D3172), exceeding the baseline value of 62.4 wt %, while volatile matter decreased to 16.2–21.5 wt %. The higher heating value (HHV) increased systematically from 29.7 to 32.5 MJ/kg (ASTM D5865), reflecting enhanced carbonization and aromatic structure development. Ultimate analysis confirmed increased carbon content (72.4–76.0 wt %) and reduced H/C ratios (0.58–0.70), indicative of synergistic aromatization during copyrolysis. Structural and thermal analyses supported these findings. FTIR spectra revealed intensified aromatic C=C stretching bands at ~1600 cm^–1, with peak intensities increasing by up to 2.1× at higher algae ratios. GC–MS analysis of volatile fractions identified stable polyaromatic compounds, including naphthalene (18–25%) and phenanthrene (12–19%) within the C₉–C₂₄ range. Thermogravimetric analysis (TGA/DTG) demonstrated enhanced thermal stability, with onset decomposition temperatures exceeding 420°C and three distinct degradation stages: moisture removal (30–150°C), devolatilization (150–450°C; 25–35 wt % mass loss), and fixed carbon oxidation (450–750°C). Activation energy values determined using the Kissinger–Akahira–Sunose method increased from 125 to 142 kJ/mol, confirming the formation of stronger aromatic carbon networks. Fixed-bed combustion tests conducted at 850–950°C (λ = 1.2–1.4) validated the fuel performance of the co-pyrolyzed biochars, yielding ignition temperatures of 360–375°C, reactivity indices (Rᵢ) of 0.85–1.42 min^–1, and favorable ash fusion characteristics (IDT/HT/FT: 780/790/1120–1280°C; ASTM D1857) with moderate slagging indices (R_s = 4.2–6.8). Gaseous emissions remained low, with CO concentrations of 120–180 ppm and NO_x levels of 85–110 ppm. The biochars also exhibited well-developed hierarchical porosity and BET surface areas of 110–130 m²/g. Overall, the results demonstrate that copyrolysis of waste paint oil and algae biomass is an effective industrial waste valorization strategy, producing thermally stable, high-energy biochar suitable for 30–50% co-firing blends with sub-bituminous coal, while simultaneously addressing hazardous waste management challenges.