Theoretical analysis and experimental verification of diminishing the diffusion influence on determination of char oxidation kinetics by thermo-gravimetric analysis.

In this paper, theoretical analysis and experimental studies are carried out to analyze the intrinsic kinetic properties of coal char oxidation based on a thermo-gravimetric analyzer (TGA). A one-dimensional oxidation model of the char particles' layer is established to consider various diffusion resistances, including external, inter-particle layer, and intra-particle diffusion in the crucible during TGA experiments. The model is validated (1) with the effective factor results calculated by the Thiele modulus formula; (2) with the present TGA experimental data. Then, the oxidation process of coal char in TGA under different crucible loading positions, particle thicknesses, oxygen concentrations, and heating rates, is inverted by the model. The results show that the external diffusion in the crucible significantly affects the char oxidation process. The external diffusion resistance increases with increasing the stagnation zone. The inter-particle layer resistance enhances with thickening particle thickness. The influence of diffusion resistance can be diminished as much as possible by filling the bottom of the crucible with inertia alumina particles. Further non-isothermal TGA experiments and kinetic analysis of a bituminous coal char confirm that the activation energy obtained by the optimized method (168.3 kJ/mol) is much higher than that obtained by the conventional method (134.1 kJ/mol). • Model considering external, inter-particle, intra-particle diffusion is developed. • Factors affecting the diffusion processes in the TGA experiment are investigated. • Inert particle filling is an effective method to avoid crucible external diffusion. • A much higher active energy of 168.3 kJ/mol for a bituminous char is obtained. [ABSTRACT FROM AUTHOR]