Experimental study on the diffusion–kinetics interaction in heterogeneous reaction of coal

The diffusion of oxygen (O₂) plays an important role in the heterogeneous oxidation of coal and biomass, but is inadequately understood. This work aims to study the influence of intra-, inter-particle and external O₂ diffusions on the high-temperature heterogeneous oxidation using the TG-FTIR technique and two bituminous coals as example. Results show that coal sample of higher reactivity and smaller pore surface area is more sensitive to the O₂ diffusion. Specifically, increasing the size of particle, the reduced intra-particle (Knudsen) diffusion can reduce the conversion rate by 10–50%. While increasing the size of sample, the effective inter-particle diffusion shows a linear decrease. Comparatively, the influences of inter-particle and external diffusion in the TG scale (<5 mm) are weaker. For large TG samples (>10 mg) and low heating rates (2 K min⁻¹), the influence of thermal diffusion is strong enough to cause a thermal leap for the oxidation. Kinetic analysis using nth-order model-fitting method predicts the apparent activation energy (E) decreases with increasing reactivity. However, both model-free and Kissinger’s methods show E increases with increasing reactivity, against the physical definition of E. This work may help understand the diffusion–kinetics interaction in the fuel conversion and smoldering fire of coal and biomass.