The combustion characteristics of the Xiaolongtan lignite and Jiaozuo anthracite chars in O₂ /CO₂ were investigated by using a thermogravimetric analyzer.On the basis of the key relationship between the reaction rate and the pore surface area of coal chars,the effective reaction surface area of coal chars during the combustion was revised in a small increment of time,and a stochastic differential equation is solved by means of differential increment analysis method.Finally,a modified discrete random pore model was constructed to analyze the combustion characteristics of the Xiaolongtan lignite and Jiaozuo anthracite chars in O₂ /CO₂ at different temperatures.By comparing the modified model with the existing discrete random pore model,it is found that the modified model has a better prediction results during the whole combustion process of coal chars,while the values predicted by the existing discrete random pore model show a larger deviation from the experimental data at the later combustion stage.