The Reaction Kinetics of Heavy Oil Combustion and Oxidation: Transition from Diffusion Limitation to Chemical...

The combustion and oxidation behavior of heavy oil plays a critical role in thermal recovery processes such as in situ combustion, where the reaction kinetics directly affects combustion front stability and energy efficiency. Owing to the complex composition of heavy oil, its combustion process exhibits pronounced multistage characteristics and continuously evolving kinetic regimes, which require systematic identification based on consistent experimental data. In this study, the nonisothermal combustion and oxidation behavior of a heavy oil from the Daqing oilfield was investigated under air atmosphere using combined thermogravimetric and differential scanning calorimetric (TGA-DSC) analysis. An integrated kinetic analysis framework combining model-free isoconversional methods (Ozawa-Flynn-Wall and Kissinger-Akahira-Sunose) with multistep mechanistic modeling was employed to quantify the evolution of apparent activation energy and identify the kinetic control mechanisms as a function of conversion. The results indicate that the combustion process can be divided into light-component release/volatilization, heavy component pyrolysis, coke formation, and deep coke oxidation stages. The apparent activation energy exhibits a pronounced nonlinear variation with conversion, ranging from 17.02 to 59.90 kJ·mol^-1. Isoconversional analysis combined with mechanistic modeling reveals that the reaction is predominantly controlled by three-dimensional diffusion (D3 model) at intermediate conversions (α = 0.4-0.7), while a transition to chemically controlled kinetics (R3 model) occurs at higher conversions (α ≥ 0.8).The proposed framework provides a robust approach for identifying kinetic control mechanism transitions during complex combustion processes and can be extended to other heavy oils, oil sands, and asphaltene-rich systems.