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

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.