Release and transformation behaviors of sodium during combustion of high alkali residual carbon

In this study, the physical and chemical properties of three typical Zhundong coal gasification fly ashes were investigated by sequential chemical extraction, ICP-AES, XRD, SEM-EDX and particle size analysis. Furthermore, the release and transformation behaviors of Na during combustion (850-950 °C) of gasification fly ashes were studied in a vertical tube furnace. The results illustrate that alkali metals in gasification fly ashes are more than those in Zhundong raw coals due to the condensation and deposition of gaseous alkali metals. For gasification fly ashes, water soluble Na is the main occurrence of Na, water soluble K is the predominant occurrence of K, Na exists mainly as NaCl, Na2SO4 and sodium aluminosilicates. Higher temperature, higher O2 concentration and longer reaction time lead to higher release rate of Na, the transformation between different sodium occurrences is also enhanced. Moreover, the release and transformation of Na are inhibited in O2/CO2 atmosphere, compared with O2/N2 atmosphere. The total Na content, the four occurrences of Na content, and the release rate of Na vary greatly with the particle size of gasification fly ash. Surface of gasification fly ash is covered with a layer of oxides and sulfates of Ca, therefore, the Na in gasification fly ash performs weaker release and transformation behaviors than Na in Zhundong raw coal during combustion.