Effect of sodium and calcium on the ash fusion temperatures of Zhundong coal

The effect of sodium and calcium on the ash fusion temperatures (AFTs) of Zhundong coal was investigated in this study using an ash fusibility tester combined with X-ray diffraction technology. The results indicate that the AFTs of Zhundong coal are closely related to the content of sodium and calcium in coal ash compounds. The pretreatment of the raw coal with water leaching and hydrochloric acid leaching can remarkably enhance the AFTs, which is attributed to the removal of inherent alkali metal species in coal. The AFTs decrease first and then increase not only with the increment of Na2CO3 loading but also with the increment of CaO addition, and the minimum AFTs were, respectively, reached at 15 mass% Na2CO3 loading and 10 mass% CaO addition. The transformation of hematite during ashing of Zhundong coal with different Na2CO3 loadings gives much contribution to the reduction of the AFTs. Meanwhile, the formation of KSi3AlO8 and muscovite accounts for the rising AFTs beyond the loading amount of 15 mass% Na2CO3. For Zhundong coal-added CaO, the drop of AFTs maybe results from the formation of low-melting iron species and eutectic salts, whereas the growing AFTs may be ascribed to the overdosed lime with high melting point and the formation of Ca2SiO4 during ashing.