Effects of Water-Soluble Sodium Compounds on the Microstructure and Combustion Performance of Shengli Lignite

Different water-soluble sodium compounds (NaCl, Na₂CO₃, and NaOH) were used to treat Shengli lignite, and the resulting effects on the microstructure and combustion performance of the coal were investigated. The results showed that Na₂CO₃ and NaOH had a significant impact on combustion performance of lignite, while NaCl did not. The Na₂CO₃-treated lignite showed two distinct weight-loss temperature regions, and after NaOH treatment, the main combustion peak of the sample moved to the high temperature. This indicates that both Na₂CO₃ and NaOH can inhibit the combustion of lignite, with the latter showing a greater effect. The FT-IR/XPS results revealed that Na⁺ interacted with the oxygen-containing functional groups in lignite to form a "-COONa" structure during the Na₂CO₃ and NaOH treatments. It is deduced that the inhibitory effect on combustion of lignite may be attributed to the stability of the "-COONa" structure, and the relative amount is directly correlated with the inhibitory effect. The XRD/Raman analysis indicated that the stability of the aromatic structure containing "-COOH" increased with the number of "-COONa" structures formed. Additionally, experiments with carboxyl-containing compounds further demonstrated that the number of oxygen-containing functional groups combined with Na was the main reason for the differences in the combustion performance of treated lignite.