Release characteristics of different alkali metal salts in Zhundong coal under O₂/CO₂ combustion atmosphere based on FES technology

The O₂/CO₂ combustion technology is a clean coal-fired power generation technology. However, a large amount of alkali metals released during the combustion of high-alkali coal is usually an important substance that causes ash deposition, slagging and corrosion on the heating surface of boiler, which seriously affect the safe and stable operation of equipment. It is of great significance to study the release characteristics of alkali metal salts with different occurrences in the Zhundong coal under the O₂/CO₂ combustion atmosphere for the clean and efficient utilization of coal. Based on flame emission spectroscopy (FES), the release characteristics of different alkali metal salts in the O₂/N₂ and O₂/CO₂ atmospheres during pulverized coal combustion were studied in a metal wire mesh reactor. The results show that the combustion of volatile compounds produces a bright flame under the O₂/N₂ atmosphere. The combustion intensity and flame temperature of the pulverized coal in the O₂/CO₂ atmosphere decrease compared with that in the O₂/N₂ atmosphere, the ignition time is delayed, the combustion time of volatiles and coke is prolonged, and the release of Na(g) is inhibited. For NaAc, Na₂CO₃ and Na₂SO₄, the O₂/CO₂ atmosphere delays the decreasing time of mass concentration, so that the Na(g) in the pulverized coal enters a stable release stage from the beginning, and the mass concentration variation rate of Na(g) does not fluctuate much. However, the mass concentration variation rate of NaCl will turn negative about 4 seconds after release, resulting in a consistent decrease in the mass concentration of Na(g). At the same time, the CO₂ atmosphere also causes the conversion of NaAc, NaCl and other types of salts to form Na₂CO₃, which is more difficult to release, and further inhibits the release process of Na(g). The main reason that the mass release rate of Na₂CO₃ also decreases under the O₂/CO₂ atmosphere is that CO₂ inhibits the decomposition process of Na₂CO₃ to Na₂O. The Na₂SO₄ is more difficult to release itself, so the main reason for the reduction of its mass release rate is due to the reduction of fuel combustion temperature and intensity.