Thermal decomposition kinetics analysis of the oil sludge using model-based method and model-free method.

• Application of an innovative combination model for the pyrolysis of OS is proposed. • The high heating rate is beneficial for the pyrolysis of OS. • A three-step thermal decomposition model is proposed to describe pyrolysis process. • Results will be useful for optimization of OS industrial pyrolysis and treatment. Oil sludge (OS) is one of the main waste in the petrochemical industry, and serious consequences can result from the improper disposal of this waste. Pyrolysis is an effective method to dispose of OS and to recover the high heating value combustibles from it. The pyrolysis of three OS samples was carried out in a nitrogen atmosphere at heating rates of 5 °C/min, 10 °C/min, and 20 °C/min, respectively. The TG-DTG/DSC-DDSC curves show that the thermal decomposition of OS has been divided into 5 stages, and each stage has unique thermal decomposition characteristics. Two model-free methods were used to calculate the apparent activation energy at a conversion rate from 0.05 to 0.95, and based on 10 solid-state mechanisms of thermal decomposition, the model-based method was used to fit the thermal decomposition mechanisms exposed to the three heating rates. The results show that the high heating rate is beneficial for the pyrolysis of OS. The apparent activation energy varies irregularly with the conversion rate, and the apparent activation energy error calculated by the FWO method is smaller. For the three samples, OS-1, OS-2 and OS-3, the average apparent activation energies calculated by the FWO method are 40.39 kJ/mol, 38.01 kJ/mol, and 85.53 kJ/mol, respectively. The mechanism function of OS thermal decomposition changed, and the process of thermal decomposition should be described by a three-step thermal decomposition model. During 140.48–331.88 °C, the fitting correlation coefficients of the two-dimensional phase interfacial reaction mechanism under the three heating rates were all above 0.99. During 400.00–556.57 °C, the fitting result of the three-dimensional nucleation and growth mechanism is better than fitting results of other models. During 583.82–676.60 °C, the two-dimensional diffusion mechanism has a good fitting regression. Significantly, model-based method and model-free method provide effective and reliable thermal decomposition kinetics parameters and models as a theoretical support for optimization of OS industrial pyrolysis and treatment of OS. [ABSTRACT FROM AUTHOR]