The enhanced SO3 formation by alkali-metal sulfates from ash in the post-flame region during the combustion of high-alkali coal.

High alkali-metal sulfate contents in ash from high-alkali coal are a result of the alkali metals' strong sulfur-capturing capacity. In this work, the effects of sulfates in ash on SO3 formation were investigated by adding alkali-metal sulfates (Na2SO4 and K2SO4) to ash and performing experiments to simulate SO3 formation. The results show that Na2SO4 and K2SO4 addition significantly increased SO3 formation and the formation rate increased with increasing temperature. The formed SO3 concentration increased by 6.8 ppm (adding Na2SO4) and 6.3 ppm (adding K2SO4) at 1000 °C. These increases are the result of SO3 release from sulfate during the formation of aluminosilicates such as NaAlSi3O8 (albite), NaAlSiO4 (nepheline), KAlSiO4 (kalsilite), and KAlSi3O8 (feldspar) with the SiO2 and Al2O3 in the ash. This was confirmed by X-ray diffraction (XRD) and thermodynamic calculation. In addition, increasing the SO2 concentration increased the SO3 concentration and decreased the SO3 conversion ratio. Graphical abstract Note: This data is mandatory.