Volatilisation and transformation behavior of sodium species at high temperature and its influence on ash fusion temperatures

Zhundong coalfield is one of the superhuge coalfields newly mined in the Xinjiang Autonomous Region of China. However, operational practices have shown that power plants frequently encounter ash deposition problems during combustion of Zhundong coals because of the volatilisation of sodium species. To address such an issue, in this paper, volatilisation levels of sodium species of different coal ash samples in different conditions were studied. The effects of atmosphere and ratio between the major elements in coal ash were also studied to find the resolvent for prohibiting volatilisation. The results show that two Zhundong coals both with a higher sodium content but different calcium and silicon content induce different ash fusion temperatures (AFTs). The volatilisation levels of Wucaiwan (WCW) coal ashes which have higher calcium but lower silicon content were much higher than XHS coal ashes during ashing and AFTs test process. Further studies found that most of the sodium species in XHS coal ash which have a higher silica content transform to Na6Ca2Al6Si6O24(SO4)2 at high temperature. However, fewer sodium aluminosilicate but greater amount of Ca3Al2Si3O12 was found in WCW coal ash of 1200 °C. By analyzing the coal ash compositions, it was found that the volatilisation of sodium species was insignificant for the samples with a SiO2/Na2O ratio of more than 20 or with a (SiO2 + Al2O3)/Na2O ratio of more than 25. Calcium species promoted the volatilisation of sodium species, and the volatilisation was insignificant for the samples with a SiO2/CaO ratio of more than 4.