Release and transformation of sodium during combustion of Zhundong coals

• Water-soluble sodium is the predominant chemical form in Zhundong coals.
• More sodium of HCl-soluble form is generated with increasing combustion temperature.
• Tendency of ash deposition is closely related to occurrence mode of sodium.
• NaCl(g) is the main form of sodium volatized from Zhundong coals between 600 and 800 °C.

Zhundong coalfield is one of the super-huge coalfields newly discovered in Xinjiang Autonomous Region of China. Zhundong coal is characterized by high content of sodium. This study deals with the release and transformation of sodium during combustion of two Zhundong coals. The volatilization of sodium and evolution of its occurrence modes in resultant ash were investigated in a laboratory-scale reactor. Experimental results show that sodium is the principal alkali metal and water-soluble sodium is the predominant chemical form in Zhundong coals. Combustion temperature has a pronounced effect on the release and transformation of sodium. The fraction of water-soluble sodium in residual ash decreases significantly and more HCl-soluble form is generated with the increase of combustion temperature. The tendency of ash deposition during coal combustion is closely related to occurrence mode of alkali metals. In addition, halite (NaCl) is clearly present in ash of Zijin coal at low temperature, while most sodium in form of NaCl is released into gas phase and the remainder is transformed into aluminosilicates with increasing temperature. Chlorine has a significant influence on transformation of sodium and NaCl(g) is the main form of sodium volatized from coal between 600 and 800 °C.