A fractal char particle combustion model was developed to study the char combustion mechanism and improve the prediction accuracy of the char combustion characteristics. This model considered the coupling effects of the secondary reactions and gas diffusion within the char pores. The apparent reaction order of char combustion changed with the particle temperature

the O2 partial pressure at the particle surface and the particle pore structure parameters instead of keeping constant 1. The primary CO/CO2 ratio was obtained by theoretical derivation. The combustion characteristics of eleven different chars were analyzed by a thermogravimetric analyzer (TGA) with their apparent activation energies and pre-exponential factors measured which were used to solve the undetermined model parameters. These chars were also combusted in a drop tube furnace (DTF) and the char samples were collected at the outlet of the DTF. The conversions of these char samples were measured by TGA based on the ash conservation method. The combustion processes of these chars were simulated by this model. The predicted char conversions by this model were matched very well with the measured data and the accuracy was significantly improved compared with the intrinsic model

proving that this model had good accuracy over a wide range of char particles from lignite to anthracite. The study also showed that the apparent reaction order of char combustion first decreased and then gradually stabilized in the char combustion process.