Effect of Sodium on the Structure and Reactivity of the Chars Formed under N2 and CO2 Atmospheres

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-N-2 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-N-2. 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 N-2 and CO2 atmospheres was compared, char-CO2 is much more reactive than char N-2 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.