Melting behavior and heterogeneous reaction of Na-bearing deposits at high temperature during high-alkali coal combustion

The pure chemicals were used to simulate the composition of typical ash deposits on heat exchanger surfaces during high-alkali coal combustion, and the thermal mechanical analysis (TMA), TG-DSC analysis and high-temperature calcination were performed to investigate the high temperature melting characteristics of the deposits with varying content of Na₂SO₄. Subsequently, XRD and SEM-EDS techniques were applied to investigate the reaction products and detailed mechanism. The results indicate that the melting characteristic temperature of deposits decreases significantly after blending with Na₂SO₄, and the transformation of Na₂SO₄ is closely related to the blending ratio. When the blending ratio is below 20% (mass ratio), most Na₂SO₄ is converted to CaSO₄ and sodium aluminosilicate, while Na₂SO₄ would react with CaSO₄ and generate sulfate double salts when the blending ratio is over 40%. Moreover, the particles of Na₂SO₄ enriched deposits begin to adhere at 800℃. When the temperature is elevated to 900-950℃, the low-temperature eutectics are easily formed in the presence of nepheline and albite, and the eutectics and Na-Ca sulfate double salts are gradually molten. The large amount of co-melting of akermanite and Ca-bearing minerals was observed at temperatures of 1200-1250℃, and the minerals are completely melted at temperatures over 1300℃.