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.

Industrial disposal processes for treatment of polychlorinated dibenzo-p-dioxins and dibenzofurans in municipal solid waste incineration fly ash.

Hazardous waste disposal is a serious environmental concern in China. Therefore, in this study, industrial trials were conducted in a low-temperature thermal degradation facility, a tunnel kiln, and a shaft kiln to effectively treat dioxins in municipal solid waste incineration (MSWI) fly ash. The results indicated that the low-temperature thermal degradation facility efficiently decomposed polychlorinated dibenzo-p-dioxins and dibenzofurans in the MSWI fly ash. Additionally, the concentrations of dioxins in the treated fly ash and exhaust gas were lower than the suggested standard limits and the degradation ratio of dioxins was ∼99%. Therefore, treated fly ash characterized by acceptable dioxin risks could be utilized for the production of non-fired building materials. The results from the tunnel kiln indicated complete decomposition of the dioxins in the firing and insulating sections. However, the addition of fly ash in the tunnel kiln increased the concentration of dioxins in the flue gas. This can be primarily attributed to the heterogeneous catalytic synthesis reaction in the low-temperature section of the tunnel kiln. The results from the shaft kiln indicated degradation of at least 22% of the dioxins in the ash. The dioxin concentration in the flue gas was lower than the national standard while that in the clinker was within a reasonable limit. Furthermore, the environmental risks were significantly reduced at fly ash addition ratios lower than 3%.

Enhanced effects of ash and slag on SO3 formation in the post-flame region.

The effects of slag, fly ash (formed in boiler above 1500 °C), and experimental ash (formed in muffle furnace at 815 °C) on the formation of sulfur trioxide (SO3) were studied in a fixed bed rector. The results showed that the slag had the best catalytic effect on SO3 formation, the effect of fly ash was second, and the effect of experimental ash was the worst. The reason may be that the forms of iron in different samples were different. Iron in the experimental ash all existed in the form of Fe2O3. Iron in the fly ash mainly existed in the form of composite iron oxides, such as Fe0.3Mg0.7SiO3, Ca3Fe2(SiO4)3, and MgFe2O4. Iron in the slag also mainly existed in the form of composite iron oxides, such as CaFe2O4, MgFe2O4, and CaMgO0.88Fe0.12SiO4. The different forms of iron had different effects on SO3 formation. Composite iron oxides could produce more oxygen vacancies owing to lattice defects. This likely promoted the migration and regeneration of lattice oxygen and thus better promoted the formation of SO3 than Fe2O3. Moreover, MgFe2O4 and Ca3Fe2(SiO4)3 could better promote SO3 formation than CaMgO0.88Fe0.12SiO4 and Fe0.3Mg0.7SiO3. In addition, increasing the SO2 concentration and O2 concentration increased the SO3 concentration but increasing the SO2 concentration decreased the SO3 formation ratio.