Study on fusibility of coal ash rich in sodium and sulfur by synthetic ash under different atmospheres

- Ash fusibility is influenced by different atmospheres due to Na2O and SO3 in coal ash.
- Na2SO4 was only formed under the oxidizing atmosphere and increased AFTs.
- The formation of Na2SO4 depended on SiO2 and Al2O3 content in coal ash.
- The formation condition of Na2SO4 was established to prevent ash-related issues.

Coal ash fusibility is the key parameter related with ash fouling and slagging in boiler and gasifier. The influences of major compositions in coal ash on fusibility are well known, but the ash rich in sodium and sulfur, such as Zhundong coal, is lack of study. Meanwhile, the sodium and sulfur in Zhundong coal lead to severe slagging and ash deposition during thermal utilization. In order to reveal the behavior of sodium and sulfur in ash and its effect on ash fusibility under different atmospheres, two synthetic ashes rich in sodium and sulfur were prepared. Fe2O3 was not included in order to shield its effect on ash fusibility by atmosphere. The results show that ash fusion temperatures (AFTs) of coal ash rich in sodium and sulfur under oxidizing atmosphere are higher than that under reducing atmosphere. The mineral transformation experiments reveal that sodium has an obvious effect on decreasing AFTs under reducing atmosphere due to the formation of fusible Na-containing aluminosilicates. However, under oxidizing atmosphere, a majority of sodium transformed into sodium sulfate (Na2SO4), which was separated out from the slags above 1250 °C, leading to a decrease of sodium content in coal ash. The formation of sodium sulfate under the oxidizing atmosphere also depends on the total content of SiO2 and Al2O3 in coal ash. The formation condition of sodium sulfate was further proposed in a ternary diagram. The conclusions supply a theoretical support to evaluate the fusibility of ash rich in sodium and sulfur properly, and also help to prevent slagging and ash deposition by sodium sulfate.