Effects of Sodium Compounds on Sintering Characteristics and Microstructural Evolution of Coal Ash during Thermal Treatment

The sodium content in sodium-rich coal significantly influences the sintering characteristics of coal ash during the combustion processes. To investigate the effects of sodium compounds on the sintering characteristics of coal ash and the structural changes in silicate crystals, these experiments were conducted. In this paper, the sintering characteristics of Wucaiwan coal (WCW) ashes and five typical synthetic coal ashes were experimentally investigated based on a sieve experiment. The migration of mineral components during the combustion process was investigated using inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) analyses. The study demonstrates that (1) for nonvolatile sodium species, the initial sintering temperature of coal ash exhibits a positive correlation with the active sodium generation temperature, Na₂CO₃ and Na₂SO₄ establishing a threshold at 800 and 900 °C, respectively, through its thermal decomposition behavior; (2) sodium speciation governs the sintering characteristic, while NaCl and organic sodium induce early stage Ca–Si reactions at 700 °C, their low retention limits sintering enhancement, whereas sulfate’s influence manifests predominantly above 900 °C; (3) with insoluble sodium causing minimal disruption compared to volatile species in the silicon structure; and (4) different silicon structures formed are caused by different sodium compounds.