The characteristics of oxy-fuel combustion with recycled flue gas are different with air combustion in several aspects, such as char structure and reactivity. A high concentration of CO2 has important implications on char characteristics. The present study is a further effort to extend our knowledge of the effect of sodium on the char crystalline structure and combustion reactivity in the presence of a high concentration of CO2. Coal samples include a bituminous coal and a lignite coal. H- and Na-form coal samples are prepared separately. The following conclusions are drawn. The char–CO2 yields are obviously less than the char–N2 yields, especially for lignite coal. The gasification reaction is a very important factor in a CO2 atmosphere. When the pair of H- and Na-form coal samples are compared, experimental results show about 5 and 3% of the decrease in the char–CO2 yield was observed for Na-doped lignite and bituminous coal reacting in a CO2 atmosphere, respectively. During the pyrolysis and gasification of the bituminous coal, the Brunauer–Emmett–Teller (BET) surface area and total pore volume of char–CO2 are much larger than those of char–N2. When char combustion was under a reaction-controlled regime during thermogravimetric analysis, the char reactivity indexes of the four chars from the bituminous coal were almost the same. When the pair of chars formed in N2 and CO2 atmospheres was compared, char–CO2 is much more reactive than char–N2 for both the H- and Na-form lignites. The higher reactivity of char–CO2 implies that the catalysis of the inorganic matter, including the added Na, as well as higher surface area played a significant role in char reactivity.