Effect of Kinetic–Diffusion Competition on Apparent Oxidation Kinetics of Coal: Insights from Experiment and Particle-Resolved Simulation

Apparent kinetic parameters are widely used to characterize coal spontaneous combustion, yet their values are strongly affected by particle size. This raises a critical issue that estimations from laboratory-scale coal samples cannot be directly linked to engineering-scale coarse fragments. Here, temperature-programmed oxidation experiments are conducted for coal particles with sizes of 3–7 mm, and a validated particle-resolved model is developed to identify how particle size regulates the rate-controlling mechanism. Besides, the Damköhler number (Da) is introduced to quantify the competition between kinetic and intraparticle oxygen diffusion during coal spontaneous combustion. Results suggest that increasing particle size enhances the spatial heterogeneity of oxygen concentration and temperature fields, thereby altering the apparent kinetic response derived from sample-averaged oxygen consumption. As particle size increases from 3 to 7 mm, the apparent activation energy increases by approximately 14% and 7% in the low- and high-temperature stages, respectively, while the corresponding apparent pre-exponential factor increases by approximately 17% and 33%. Such particle-size dependence can be quantitatively interpreted by the Da-governed kinetic–diffusion competition. Across the examined particle sizes, the apparent activation energy follows a unified logarithmic dependence on Da, while a compensation relationship is observed between apparent activation energy and pre-exponential factor. These findings establish a Da-based framework for interpreting transport-induced distortions in apparent kinetic parameters and provide a mechanistic basis for transferring coal oxidation kinetics across particle sizes.