Effect of phosphorus-based additives on ash fusion characteristics of high-sodium coal under gasification condition

Abstract

Ash fusion characteristics and melting mechanism of two high-sodium coals including Luan (LA) and Hongshaquan (HSQ) with addition of ammonium dihydrogen phosphate and tricalcium phosphate under gasification condition were investigated to reveal reactions between phosphorus-based additives and high-sodium coal ash in detail. Methods such as ash fusion test, X-ray diffraction, and high temperature heating stage microscope were used for that purpose. High-sodium coal ashes have low ash fusion temperatures (AFTs) due to their high SiO2/Al2O3 ratio and alkaline and alkaline-earth contents. Low-temperature eutectics are formed between nepheline and akermanite, as well as anorthite and akermanite at 1210 °C and 1274 °C, respectively. Phosphorus has a higher affinity to alkaline and alkaline-earth metals for the higher ionic potential of acidic components than basic components. As a result, high melting point phosphates like Ca9MgNa(PO4)7, Ca9Fe(PO4)7, and Na3Ca6(PO4)5 are generated in coal ash with phosphorus-based additives. Additionally, when PO43-/Na molar ratio increases, contents of quartz and phosphates are also increased, which enhance the formation of skeleton structure with high AFTs. However, the formation of low-temperature eutectic mixtures of Fe-Na silicates accelerates the melting process of coal ash, which weak the enhanced AFTs of high-sodium coal by phosphorus-based additives. Furthermore, the melting mechanism of high-sodium coal ash with and without phosphorous-based additives follows the “melting-dissolution” mechanism.