The percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to the ash formation process in coal combustion, and its validity is examined by comparison with the experimental results obtained using the drop tube furnace facility (DTF). The characteristics of a burning coal particle, such as particle diameter and specific surface area, are investigated in detail. Newlands and Plateau coals with different fuel ratios and ash contents are tested. The ambient temperature is set at 850 or 1400 degreesC, at which temperature fluidized-bed or pulverized coal combustion occurs. The relationship between particle temperature and conversion of coal required in the percolation model is obtained by performing a numerical simulation of a combustion field in the DTF. The results show that the characteristics of the burning coal particle obtained through the computations of the percolation model are generally in agreement with the experimental data. The particle diameter of Newlands coal with a higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustion-dominant process. For both Newlands and Plateau coals, compared to the particle diameter of the lower ambient temperature case of 850 degreesC, that of the higher ambient temperature case of 1400 degreesC becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward. These behaviors can be explained in terms of swelling due to devolatilization and ash agglomeration.